COLLECTION FOR MYANMAR GEOLOGY STUDENTS AND LEARNERS-1

Original Articlerge_145 1..29
Geology of the High Sulfidation Copper Deposits,
Monywa Mine, Myanmar
Andrew H. G. Mitchell,1
Win Myint,2
Kyi Lynn,2
Myint Thein Htay,2
Maw Oo2
and
Thein Zaw2
1
IMHL, U Wisara Road, Kamayut, Yangon, Myanmar and 2
Myanmar Ivanhoe Copper Company Limited, Mine Site, Monywa,
Myanmar
Abstract
The 50 km2
Monywa copper district lies near the Chindwin River within the northward continuation of the
Sunda-Andaman magmatic arc through western Myanmar. There are four deposits; Sabetaung, Sabetaung
South, Kyisintaung, and the much larger Letpadaung 7 km to the southeast. Following exploration drilling
which began in 1959, production of copper concentrates from a small open pit started at Sabetaung in 1983.
Since 1997, when resources totaled 7 million tonnes contained copper in 2 billion tonnes ore, a heap leach–
electro-winning operation has produced over 400,000 t copper cathode from Sabetaung and Sabetaung South.
Ore is hosted by mid-Miocene andesite or dacite porphyry intrusions, and by early mid-Miocene sandstone
and overlying volcaniclastics including eruptive diatreme facies which the porphyries intrude. District-wide
rhyolite dykes and domes with marginal breccias probably post-date andesite porphyries in the mine area and
lack ore-grade copper. Host rocks to mineralization are altered to phyllic and advanced argillic hydrothermal
assemblages within an outer chlorite zone; hypogene alunite is most abundant at Letpadaung and Kyisintaung.
Most mineralization is structurally-controlled with digenite-chalcocite in breccia dykes, in steeply dipping
NE-trending sheeted veins, and in stockwork and low-angle sulfide veins. A high-grade pipe at Sabetaung
grades up to 30% Cu, and much of the ore at Sabetaung South is in a NE-trending zone of mega-breccia and
stockworked sandstone. The hydrothermal alteration, together with replacement quartz, alunite and barite in
breccia dykes and veins, the virtual absence of vein quartz, and the presence of chalcopyrite and bornite only
as rare veins and as inclusions within the abundant pyrite, indicate that the deposits are high sulfidation.
Regional uplift, resistance to erosion and leaching of the altered and mineralized rocks have resulted in porous
limonite-stained leached caps over 200 m thick forming the Letpadaung and Kyisintaung hills. The barren caps
pass abruptly downwards at the water table into the highest grade ore at the top of the supergene enrichment
zone, within which copper grade, supergene kaolinite and cubic alunite decrease, and pyrite increases with
depth; in contrast, marcasite is mostly shallow. Much of the copper to depths exceeding 200 m below the water
table occurs as supergene digenite-chalcocite and minor covellite. Disseminated chalcocite is mostly near-
surface and hence almost certainly supergene. We infer that during prolonged uplift at all four deposits,
oxidation of residual pyrite at the water table generated enough acid to leach all the copper from earlier
supergene-enriched ore; below the water table the resulting acid sulfate solutions partly replaced enargite,
covellite, chalcopyrite, bornite and pyrite with supergene chalcocite. Undeformed upward-fining cross-
bedded conglomerates and sands of the ancestral Chindwin River floodplain overlie the margins of the
Received 25 January 2010. Accepted for publication 17 August 2010.
Correspondening author: A. H. G. MITCHELL, IMHL, 234/A-1, U Wisara Road, Kamayut Township, Yangon, Myanmar. Email:
imhle@myanmar.com.mm
Present address: 20 Dale Close, Oxford, OX1 1TU, UK.
doi: 10.1111/j.1751-3928.2010.00145.x Resource Geology Vol. 61, No. 1: 1–29
© 2010 The Authors
Resource Geology © 2010 The Society of Resource Geology 1

Sabetaung deposits, form a major aquifer up to 40 m thick, and are a potential host for exotic copper
mineralization. A mid-Miocene pluton is inferred to underlie the Monywa deposits, but the possibility of
porphyry-type mineralization within the district is at best highly speculative.
Keywords: heap leachable ore, high sulfidation, Miocene breccia dykes, Monywa copper mine, Myanmar,
supergene enrichment.
1. Introduction
The Monywa copper district is 115 km west-
northwest of Mandalay in an elevated flood plain west
of the Chindwin River in western Myanmar (Fig. 1).
The district is on the northern margin of Myanmar’s
dry zone; annual rainfall is about 800 mm confined to
the May–October period. Maximum daily tempera-
tures from March to June can exceed 45°C.
Four major high sulfidation deposits of Miocene age
define the district: the almost contiguous Sabetaung,
Sabetaung South and Kyisintaung deposits and the
much larger Letpadaung 7 km to the southeast (Fig. 2).
Production is from Sabetaung and the adjacent smaller
Sabetaung South, now within a single open pit. Kyis-
intaung lies beneath a prominent hill immediately west
of Sabetaung, and Letpadaung underlies an isolated
6 km2
hill bisected by a northeast-trending valley
(Figs 2, 3). Together these deposits comprise the north-
ernmost major copper resource in the 7000 km long
Banda-Sunda-Myanmar arc; with pre-mining com-
bined resources totaling 2 billion tonnes ore with over
7 million tonnes contained copper, they constitute a
giant deposit. In Southeast Asia only the undeveloped
high sulfidation Tampakan deposit in the Philippines
has a larger copper resource.
The Monywa deposits differ from most high sulfida-
tion systems (e.g. Heald et al., 1987; Hedenquist et al.,
2000) in lacking economically significant enargite and
gold mineralization, and their classification as high sul-
fidation epithermal became widely accepted only in the
late 1990s. This followed a quarter of a century during
which they were widely but not unanimously inter-
preted as supergene-enriched porphyry-type deposits.
Despite their size the geology of deposits is not widely
known, and since mining began the only published
accounts are those by Kyaw Win and Kirwin (1998)
drawing on D.J. Kirwin (1995, unpublished data),
and by Mitchell et al. (2008). Here we describe the
geology based largely on recent systematic mapping of
levels in the Sabetaung–Sabetaung South pit, on mid-
1990s feasibility study reports, on logging and selective
re-logging of core drilled between 1973 and 2008, and
on results of regional mineral exploration and geologi-
cal mapping.
Elevations in the mine area are reported as sea level
plus 500 m, or m RL (meters relative level). The Chind-
win flood plain (Fig. 2) is around 590 m RL, Let-
padaung and Kyisintaung hills rise to 832 m RL and
775 m RL, respectively, Sabetaung was a 655 m RL hill
prior to mining, and Sabetaung South a low rise.
2. Regional geological setting
2.1 Wuntho-Popa magmatic arc and
Central belt
The Monywa deposits lie on the eastern flank of the
460 km long Wuntho-Popa or western Myanmar
Fig. 1 Location map for Monywa copper district,
showing position within Myanmar and within Burma
plate (shaded), plate boundary from Steckler et al.
(2008).
A. H. G. Mitchell et al.
2
© 2010 The Authors
Resource Geology © 2010 The Society of Resource Geology

magmatic arc, within the regional Central belt or
Central basin (Fig. 4) of Myanmar. The arc, the north-
ern continuation of the Sunda-Andaman arc, is a
northerly-trending geanticlinal uplift which exposes
Mesozoic intrusions and their host rocks in four inliers.
The 160 km long Wuntho-Banmauk inlier is in the
north, the much smaller Okkan inlier is west of the
Monywa deposits (Fig. 2), and Salingyi (Barber, 1936;
United Nations, 1979a) and Shinmataung are to the
south. The inliers consist largely of dioritic to grano-
dioritic plutons of early Upper Cretaceous age, and
Tertiary dykes and stocks, all intrusive into pre-Albian
basalts and a locally exposed greenstone, amphibolitic
and gneissic basement. The Wuntho-Banmauk inlier
includes the Kanzachaung batholith (United Nations,
1978a). There are three extinct andesitic strato-
volcanoes (Fig. 4) of which the southernmost is Mt.
Popa where late Cenozoic volcanic rocks (Stephenson
& Marshall, 1984) include potash-rich andesites with
mid-Miocene Ar-Ar ages (Yang, 2008).
In the Okkan inlier west of Monywa, Mesozoic rocks
consist of basalt breccias and pillow lavas (Fig. 5a)
intruded by Cretaceous diorites and biotite granodior-
ites. These are overlain unconformably to the east by a
latest Oligocene and Miocene succession equivalent to
the regional Upper Pegu Group. The succession con-
sists of a basal conglomerate with local limestone
overlain by the Powintaung Sandstones forming a
west-facing scarp. Above are shales, cross-bedded
sandstone, and local basalt breccia, with minor inter-
bedded andesitic tuff in the upper part, comprising
the Magyigon Formation, which includes debris ﬂow
deposits with rhyolite blocks west of Letpadaung. In
and within 5 km of the copper deposits rhyolite dykes
and domes intrude the Magyigon Formation and older
rocks. Gently folded Lower Irrawaddian sandstone and
overlying Upper Irrawaddian gravel beds, of Pliocene
age, occupy a sinuous anticline northwest of the Okkan
inlier. Immediately north of Sabetaung in the Lower
Chindwin District there are spectacular low-aspect
Fig. 2 Geological map, Monywa copper deposits and surroundings, simpliﬁed from Myint Naing Win and Myint Thein
Htay (2006, unpublished data). Ticked line is approximate western limit of Kangon Sands, black line is motor road. Eastern
part of Okkan inlier is Jurassic and Cretaceous at western edge of map. Location on Figures 1 and 4.
Monywa copper deposits
© 2010 The Authors

basaltic craters with volcanic aprons which include
boulders of garnet-hornblende rock described by
Chhibber (1934a). The basalts are shoshonitic and
include absarokite (Yang, 2008).
2.2 Chindwin-Minbu basins and
Indo-Myanmar (Indo-Burman) Ranges
West of the Wuntho-Popa magmatic arc the Chindwin
and Minbu basins (Fig. 4) contain 15-km thick syncli-
norial unconformity-bound sedimentary successions
of Upper Albian to Cenomanian, Senonian to Danian,
mid-Palaeocene to Miocene, and Pliocene age. The
mid-Palaeocene and younger rocks comprise the Ter-
tiary fore-arc basin (Fig. 6) of the magmatic arc (Win
Swe et al., 1972, unpublished data); the late Oligocene
to Pliocene part of the succession extends eastwards
across the arc into the Shwebo basin. The Kabaw Fault
is a zone with local W-directed thrusts near the western
margin of the Chindwin-Minbu basins.
In the oil and gas ﬁelds of the Minbu basin (Fig. 4)
100 km southwest of Monywa, there are Oligocene to
early Miocene extensional faults and ENE-directed
thrusts which offset Pliocene strata. The thrusts have
been related to stresses associated with dextral strike-
slip movement on the Sagaing Fault to the east (Pivnik
et al., 1998).
The Indo-Myanmar Ranges (Figs 4, 6), west of the
Chindwin-Minbu basins, record the early subduction
history of the Wuntho-Popa arc. They consist of two
belts (United Nations, 1979b, Mitchell et al., 2010) sepa-
rated by an east-dipping thrust. The Eastern belt con-
sists of Upper Triassic ﬂysch and associated ophiolitic
rocks underlain by mica schists. In the Western belt,
highly-deformed Senonian pelagic limestones, mud-
stones and turbidite sandstones are overlain by Pale-
ocene and Eocene marine clastics with upright open
folds. The Tertiary rocks pass westwards into the sedi-
mentary sequence of the Chittagong–Tripura fold belt
(CTFB) and the Bengal (Alam et al., 2003) or Ganges-
Brahmaputra (Steckler et al., 2008) delta basin in Bang-
ladesh and India.
2.3 Myanmar plate
The Central belt and Indo-Burman Ranges form part of
the small Myanmar (formerly Burma) plate (Fig. 1) of
Curray et al. (1979). The western boundary of the plate
(Figs 1, 6), the northern continuation of the Sunda
Trench, is an eastward-dipping seismic zone (Steckler
et al., 2008), which lies about 100 km beneath the
Monywa segment of the Wuntho-Popa magmatic arc;
present day convergence is oblique. The subducting
plate beneath the Bay of Bengal and probably that
beneath the Monywa deposits (Fig. 6) is oceanic, but
the crust beneath the CTFB and Bengal delta to the
north and west could be either oceanic or thinned con-
tinental crust of the Indian margin.
Fig. 3 View of mine looking southeast from hill above Kyisintaung deposit, across Sabetaung (left foreground) and Sabe-
taung South (centre) pits to leached cap above Letpadaung deposit (hills in right background). Leach pads are behind
Sabetaung South pit.
4
© 2010 The Authors

The eastern boundary of the Myanmar plate is the
Sagaing dextral fault (Figs 4, 6). South of latitude 23°N,
the fault forms the western boundary of the Shan-Thai
block, part of the Asian plate. The block margin is
intruded by Cretaceous and Tertiary granites and vol-
canic rocks, possibly the northern continuation of the
Wuntho-Popa arc offset on the Sagaing Fault. Total
northward movement of the Myanmar plate relative to
Asia has been variously estimated at 350 to 1100 km.
The Naga Thrust (Fig. 4) can be related to collision of
the Myanmar plate with the southeastern passive
margin of India southeast of the Foreland Spur.
2.4 Mineralization in the magmatic arc
Beyond the Monywa district, mineralization in the
magmatic arc is largely confined to the large Banmauk-
Wuntho inlier (Fig. 4). Here there are artisanal
workings on mesothermal quartz-gold veins in grano-
diorites (Chhibber, 1934b) and in the basaltic and
greenstone host rocks to the intrusions (United
Nations, 1978a; Mitchell et al., 1999); porphyry copper-
gold prospects of which the most promising is at Shan-
galon near Wuntho; quartz-molybdenite veins also at
Shangalon (United Nations, 1978b); and some high sul-
fidation gold prospects. Low sulfidation epithermal
quartz-gold veins occur within Neogene sedimentary
rocks south of Shangalon, and there are traces of epi-
thermal gold in quartz-alunite-replaced Neogene vol-
canics near Mt. Popa. Scattered artisanal workings of
placer gold with platinum group metals extend south-
wards from the Hukawng Valley in northernmost
Myanmar to immediately north of the Monywa copper
deposits.
Near the Monywa deposits, regional exploration in
1995–97 (O. Radislao et al., 1996, 1977, unpublished
data) identified gold-silver mineralization within
Miocene sedimentary rocks intruded by rhyolitic sills
and domes with peperite margins. The rhyolites with
associated mineralization are found at Kyaukmyet, first
described by Kelterborn (1925, in Chhibber, 1934a); the
small Shwebontha-Nache Taung hills; and Taungzone
(Fig. 2). At Kyaukmyet colloform-banded chalcedonic
quartz veins and spectacular stockworks of comb-
textured open-centre veinlets occur within silicified
rocks (Figs 2, 5b). Maximum gold values in 6000 m of
diamond drilling were around 3 g t-1
with Ag : Au
ratios of 300, accompanied by high zinc; antimony (up
to 569 ppm) and arsenic (up to 6370 ppm) do not cor-
relate with gold. Higher gold values from trenches
suggest significant supergene enrichment. Vein tex-
tures at Kyaukmyet (D.J. Kirwin, 1994, unpublished
data) and minor adularia reported by M. Simpson
and L. Zhang (1996, unpublished data) imply low
sulfidation epithermal systems. Quartz veins with Pb
Zn Cu sulfides but no gold occur in an ENE-trending
discontinuous zone extending for a kilometer through
andesite porphyry west of Kyisintaung.
Fig. 4 Sketch map of northern and central Myanmar
showing location of Monywa copper deposits relative
to main structural features. CTFB, Chittagong-Tripura
fold belt. Indian continent: FS, Foreland Spur. Indo-
Burman Ranges: EB, Eastern belt; J, Jade mines uplift;
WB, Western belt. Strato-volcanoes: L, Mt. Loimye; P,
Popa; T, Taungthonlon. Structures: NT, Naga Thrust;
SF, Sagaing Fault; SZ, subduction zone; TTU, 22°N
uplift. Basins: CB, Chindwin; HB, Hukawng; PB,
Pathein; SB, Shwebo; SMB, Salin (Minbu). Mesozoic
inliers with magmatic arc rocks (dark grey): M, Mabein
granodiorite; SA, Salingyi; SH, Shinmataung; W, B,
Wuntho-Banmauk. Ma, Mandalay, Ya, Yangon. Modi-
fied from Mitchell et al. (2008).
© 2010 The Authors

3. Exploration history and resource
Copper at Myanmar was extracted from shallow
underground workings and smelted long before the
British occupation in 1885. Barber (1936) noted veins of
malachite and chalcanthite in rhyolites at Letpadaung,
and Chhibber (1934a) reported similar occurrences at
Kyisintaung, his Hill 937. Until 2008, adits and shafts
from c.1905 (D.J. Kirwin, 1994, unpublished data) could
be seen at the base of slope at Letpadaung. The ﬁrst
exploration drilling was undertaken at Letpadaung,
Sabetaung and Kyisintaung in 1959–60 by predecessors
of the Geological Survey and Mineral Exploration
Department (DGSE) and totaled 5830 m; a further 54
holes were drilled by 1972. The ﬁrst ground and air-
borne geophysical surveys in the area were carried out
in 1969, and in 1971 a UK-Myanmar government
Colombo Plan survey discovered a major induced
polarization anomaly south of Sabetaung (T. Green-
wood and J. H. Tooms, 1973, unpublished data). Drill-
ing of this anomaly by DGSE in 1972 led to discovery of
the Sabetaung South deposit.
From 1972 to 1976, a Japan-Myanmar (Colombo
Plan) project drilled a total of 134 wireline holes, most
of them vertical and at Kyisintaung, demonstrated an
economic resource at Sabetaung South, and completed
a 92 m adit in Sabetaung (MMAJ-OTCA 1973, 1974a, b,
Mincorp and Minproc, 1996, unpublished data).
MMAJ-OTCA applied stratigraphic concepts to the
pre-intrusion rocks, recognized that Kyisintaung and
Fig. 5 Photographs of representa-
tive rock types near and in the
copper district (a) basalt pillow
lava, Jurassic (?), Okkan inlier;
(b) low-temperature quartz-gold
veins in sandstone and rhyolite,
Kyaukmyet; (c) Sabetaung South
Sandstone with laminated mud-
stone block and mudstone rip-up
clasts; (d) Mine Pyroclastics drill
core, top to left, reverse-graded
bed with clasts of quartz-pyrite
replaced rock overlain by silty
laminated tuff with spherical
concretions.
Fig. 6 Schematic west-east regional
cross-section, Indian Shield
through Indo-Burman (Indo–
Myanmar) Ranges (IBR) and
Wuntho-Popa arc (WPA) with
Monywa copper deposits. GB,
Ganges-Brahmaputra. Partly
re-labelled from Steckler et al.
(2008), location on Figure 1.
6
© 2010 The Authors

Sabetaung were epithermal deposits with mineraliza-
tion in NE-trending breccia dykes, and inferred the
presence of mushroom-shaped bodies of zoned quartz,
alunite, and kaolinite alteration analogous to those at
the Kasuga gold mine in Japan.
Exploration at Letpadaung resumed from 1974–78
when a United Nations-executed project with the
DGSE undertook a resistivity IP and Turam survey,
identifying deep strong anomalies in the northeastern
third of the hill, east of the central valley. The project
drilled 66 vertical wireline holes, encountering
chalcocite-covellite ore in steep breccia dykes beneath a
leached cap up to 200 m thick (United Nations, 1978c)
and giving a resource of 152 million tonnes at 0.70% Cu
(Watts et al., 1995, unpublished data). By 1986 a further
52 vertical core holes drilled largely by DGSE brought
the total holes at Letpadaung to 147, or 31,286 m.
From 1978 to 1983 the Bor Copper Institute of Yugo-
slavia undertook a feasibility study on Sabetaung and
Kyisintaung, constructed a small commercial-scale
plant, and drilled 68 holes. The government-owned
No. (1) Mining Enterprise (ME-1) then developed an
open pit at Sabetaung and extracted about 14.4 million
tonnes of ore between 1983 and 1996, containing
138,000 tonnes copper-in-concentrate.
Exploration at Sabetaung–Kyisintaung was resumed
in 1994, and by 1996 Ivanhoe Myanmar Holdings had
completed 143 diamond holes, 81 at Kyisintaung, 44 at
Sabetaung, and 18 at Sabetaung South, bringing the
total holes drilled at Sabetaung-Kyisintaung to over
400. At this time resources for Kyisintaung were 391
million tonnes at 0.31% Cu with a 0.15% cut-off. In 1998
an ME-1–Ivanhoe joint venture, the Myanmar Ivanhoe
Copper Company Limited (MICCL), began open-pit
mining at Sabetaung with annual production of 25,000
tonnes copper cathode from a heap leach–electro-
winning process. Later a second pit was opened at
Sabetaung South and in 2008 the two pits became one.
In 2005, published resources at Sabetaung-Sabetaung
South were 213 million tonnes @ 0.26% Cu with a
0.14% cut-off.
At Letpadaung, a feasibility study was completed
early in 1997 by Minproc and MRDI (1997, unpub-
lished data) for Ivanhoe. This included diamond core
drilling of 91,871 m in 304 angled exploration holes,
initially on a 70 m ¥ 140 m grid with later infill drilling
to give a 70 m ¥ 70 m grid in the northeastern part of
Letpadaung hill. The drilling indicated a published
measured, indicated and inferred resource at Let-
padaung of 1478 million tonnes averaging 0.37% Cu at
a 0.10% cut-off. An air-borne radiometric and magnetic
survey over the Monywa district in 1997 showed that
the deposits had no detectable magnetic signature.
The deposits were regarded as transitional between
porphyry-type and Kuroko-type by P. H. Krisl (1975,
unpublished data), Goossens (1978), Bender (1983) and
ESCAP (1996), and as supergene-enriched porphyry-
type by United Nations (1978c) and R. H. Sillitoe (in
United Nations, 1978c). Following D. J. Kirwin’s (1995,
unpublished data) identification of the Monywa depos-
its as a deep-level example of high sulfidation epither-
mal mineralization, detailed petrographic work by M.
Simpson et al. (1996, unpublished data), T. M. Leach
(1996, unpublished data) and others confirmed the acid
sulfate nature of the hydrothermal systems. P. H.
Krisl’s (1975, unpublished data) interpretation may
have resulted from a misunderstanding of the Japanese
term “massive sulfide” for the Monywa (MMAJ-
OTCA, 1974a, b) and other pyrite-rich high sulfidation
epithermal deposits, also known as acid sulfate, high
sulfide, pyritic, quartz-alunite, enargite-gold, and
quartzite deposits.
Since 2000, exploration and evaluation have included
preliminary geological mapping of Letpadaung, the
Sabetaung and Sabetaung South pits and Kyisintaung
(Marjoribanks, R., 2004, unpublished data); drilling in
2004, 2005 and 2008 of 156 exploration and condemna-
tion holes at Sabetaung and Sabetaung South for a total
meterage of 17,775 m; some mineralization and alter-
ation studies on all four deposits; and bench mapping
carried out as mining advanced.
Of the 1997 resource with over 7 million tonnes con-
tained copper, MICCL have produced more than
400,000 t from ore averaging 0.56% Cu.
4. Host rocks to the copper
mineralization
Mineralization at Monywa is hosted (Fig. 7) by dykes
and sills of porphyritic biotite andesite, quartz andes-
ite and minor dacite, and by the folded Magyigon
Formation which the porphyritic rocks intrude; rhyo-
lite dykes are only weakly mineralized. The Magyigon
in the mine area consists of the Mine Pyroclastics and
the Sabetaung South Sandstone. The Sabetaung South
pit is mostly in sandstone, and Sabetaung in Mine
Pyroclastics and andesite porphyry intrusions; the
Kyisintaung deposit is in andesite porphyry (Fig. 8)
and Letpadaung in quartz andesite porphyry and
Mine Pyroclastics.
The Sabetaung South Sandstone is more than 250 m
thick and can be correlated with the Mid-Miocene
© 2010 The Authors

Obogon Formation of the Minbu basin oil fields to the
southwest. The Sandstone consists of felspathic quartz
arenite (D. R. Mason, 2004, unpublished data) with
interbedded mudstone and includes high-angle cross-
bedded sandstones, sandstones with flaser bedding
and ripple marks, and abundant mud-pebble conglom-
erates with rip-up clasts (Fig. 5c), implying an estua-
rine depositional environment (Dr Thura Oo, oral
commun., 2009); minor conglomerates with volcanic
clasts also occur. Within the Sandstone a well-bedded
basalt breccia or hyaloclastite up to 60 m thick consists
of aphyric green-grey chloritised amygdaloidal
pebbles and grains with a calcareous matrix and calcite
veinlets. On its northwestern side the sandstone is cut
by a mineralized breccia zone which separates the
sandstone from Mine Pyroclastics and rhyolites to the
northeast.
The Mine Pyroclastics at Sabetaung–Sabetaung South
(Fig. 9a) consist of a lower structureless unit, overlain
by about 60 m of mostly fine-grained well-bedded tuffs
with distinctive concentrically zoned spherical concre-
tions (Fig. 5d), the pisolites of MMAJ-OTCA (1974a, b).
The tuffs dip NW at up to 60° and pass up into layered
but poorly-sorted coarse andesitic volcaniclastics with
high-angle cross-bedding locally forming large-scale
channel-fill deposits. Disrupted mudstone dykes occur
and tuffisite water escape structures are common.
Pebble and cobble clasts of andesite porphyry, totally
replaced by fine-grained quartz with minor sericite
and up to 30% pyrite, are concentrated in layers and
help to define bedding in the coarser beds (Fig. 5d). The
clasts, mostly angular but including some well-
rounded pebbles, were termed “mineralization of
the first period” by MMA-OTCAJ (1973, 1974a). At
Fig. 7 Schematic stratigraphic co-
lumn, Monywa copper deposits,
succession under Sabetaung
South Sandstone is inferred from
regional geology west and south
of copper district.
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© 2010 The Authors

Letpadaung the Mine Pyroclastics are most abundant
in the northeast and are locally intersected down to
130 m sub-surface where R. Majoribanks (2004, unpub-
lished data) described arkosic grits of pyroclastic
origin. Thin concretion-bearing pyroclastic beds within
sandstones and mudstones up to 10 km from the mine
resemble those in the Mine Pyroclastics.
Andesite and quartz-andesite porphyries are the
predominant intrusive rocks, occupying much of the
Sabetaung pit (Figs 8, 9b), and Kyisintaung hill; dacites
predominate at Letpadaung (M. Sheehan, 2007, unpub-
lished data). Most intrusions are coarsely plagiophyric
with biotite, lesser hornblende and minor pyroxene
phenocrysts, and rod-like rutile crystals; plagioclase
phenocrysts are zoned, often crowded, poorly sorted
and up to 12 mm long. At Sabetaung xenoliths of
microdiorite are widespread; local sandstone, aphyric
andesite and rare diorite xenoliths also occur. Rafts or
screens of sandstone within andesite porphyry are
present in drill core from Letpadaung (United Nations,
Fig. 8 Simpliﬁed bedrock geology map of the Sabetaung-Kyisintaung area, location on Figure 2. From outcrop and drilling
through Kangon Sands cover, white where no bedrock data. Hill contour intervals 25 m, lowest bench level in combined
Sabetaung-Sabetaung South pit is 405 m RL.
© 2010 The Authors

1978c) and Kyisintaung (F. B. Lazo, 2006, unpublished
data). The andesite porphyries occur as tabular dykes
up to 10 m wide, larger bodies are less regular (Fig. 9c)
with variable texture. In northwest Sabetaung South
fine-grained andesite porphyries with sparse phenoc-
rysts intrude hornfelsed sandstone and are cut by
coarsely porphyritic dykes. At Letpadaung R. Marjor-
ibanks (2004, unpublished data) has shown that thick
sills in pyroclastics can best explain earlier observa-
tions (in M. Sheehan, 2007, unpublished data) on the
porphyry-pyroclastic relationships. A 20 m thick
andesite porphyry sill intrudes pyroclastics northeast
of the Sabetaung pit. Andesite porphyries also form
Kyadwintaung hill southwest of Kyisintaung (Fig. 8),
and the upper part of Taungkamauk (775 m RL) where
limonite-veined andesite porphyry is probably a sill or
dome more than 200 m thick. Andesite porphyry with
only supergene kaolinite alteration intrudes the sedi-
mentary rocks 6 km west of Letpadaung (Fig. 2), but is
absent elsewhere in the Wuntho-Popa arc.
Flow-banded biotite rhyolites (Fig. 9d) with con-
spicuous vertical joints and a rhyolite breccia at their
margin intrude Mine Pyroclastics in the saddle
between Sabetaung and Sabetaung South, locally
intrude mineralized breccia, and are cut by rare breccia
dykes and sulfide fracture fills. Irregular dacite dykes
(Fig. 9e) occur in the same area. Rhyolites form a prob-
able sill tens of meters thick around 735 m RL on the
southern side of Kyisintaung.
A 5.8 Ma K/Ar age was reported by MMAJ-OTCA
(1974a, b) on an andesite porphyry dyke from Kyis-
intaung. More recently, Dr S. L. Chen (written comm.,
2009) obtained a mid Miocene U-Pb zircon age of
13.5 Ϯ 0.2 Ma on a dacite dyke intruding the Mine
Pyroclastics (Fig. 9e) in Sabetaung South. Kyaw
Win and D. J. Kirwin (1998, unpublished data)
reported K/Ar ages on sericite of 13 Ma at Kyisin-
taung and 19 Ma at Letpadaung. The 13 Ma alteration
age is compatible with the 13.5 Ma determination on
the dacite dyke.
Fig. 9 Photographs showing ore
host rocks and alteration. (a)
Mine Pyroclastics (brown)
dipping northwest and faulted
against megabreccia and andesite
porphyry (grey) with Sabetaung
South Sandstone in foreground,
looking west; (b) chlorite-epidote
altered andesite porphyry, 545 m
RL, eastern Sabetaung; (c) andes-
ite porphyry dyke (light grey) in
sandstone with chalcocite vein
(right), all argillised, 535 m RL,
Sabetaung South; (d) banded
rhyolite dyke in contact with
andesite breccia, pit saddle area;
(e) chlorite-altered dacite dyke
(dark grey) intruding Mine Pyro-
clastics, 545 m RL, western Sabe-
taung South; (f) ASTER image
Monywa copper district, showing
alunite (red) and kaolinite
(green); K, Kyisintaung; L,
Letpadaung; S, Sabetaung and
Sabetaung South deposits; Mo,
Monywa town (from Earthscan,
2006, unpublished data).
10
© 2010 The Authors

5. Hydrothermal alteration
Our understanding of alteration at Monywa is based on
recent pit mapping together with, and partly con-
trolled, by results of mid-1990s XRD petrographic and
mineralogical work on drill core carried out during
feasibility studies, on subsequent petrographic studies,
and on XRD and spectral analyses on individual depos-
its by consultants and research workers including S.
Pontual (2001, unpublished data), Maung Maung
Naing (2003, unpublished data) and M. Sheehan (2007,
unpublished data).
In the combined pit (Fig. 10) four main hypogene
alteration zones or assemblages are mapped. From
oldest to youngest these are a widespread outer chlo-
rite, a localised specular haematite, a quartz-white
mica-pyrite or phyllic, and a quartz-pyrite or quartz-
pyrite-alunite zone also local in occurrence. Let-
padaung and Kyisintaung have more widespread
replacement quartz and alunite and less sericite than
Sabetaung and Sabetaung South. An Advanced Space-
borne Thermal Emission and Reﬂectance Radiometer
(ASTER) satellite image of the Monywa area (Earth-
scan, 2006, unpublished data) shows much stronger
Fig. 10 Sketch map showing
alteration zones, Sabetaung-
Sabetaung South, location on
Figure 8. Data outside pit are
largely from recent drill holes
shown by circles; chl, chlorite;
hm, haematite; mag, magnetite;
py, pyrite; qtz, quartz; ser, seric-
ite. H, high grade zone; WWF,
West Wall fault; XF, Cross-fault.
Red on blue in Sabetaung South
indicates zone of mega-breccia
and quartz-replaced sandstone.
Red line in southwestern Sabe-
taung is quartz-pyrite replace-
ment “vein”.
© 2010 The Authors

alunite alteration at Letpadaung and Kyisintaung than
at Sabetaung and Sabetaung South (Fig. 9f).
5.1 Sabetaung and Sabetaung South
A major feature of the alteration at Sabetaung and to a
lesser extent Sabetaung South is a supergene kaolinite
or argillic “blanket” (Fig. 10) which obscures the nature
of the hypogene alteration in much of the upper and
middle levels of the pit. Clay veins in the northeastern
wall at Sabetaung are probably dickite.
Chlorite alteration affects andesite porphyry, pyro-
clastics and basalt breccia in the upper levels of the pit
and beyond the pit rim for distances locally exceeding
500 m, and to drilled depths around the rim of at least
200 m. The chlorite mostly replaces groundmass,
biotite and hornblende and less commonly cores of
plagioclase phenocrysts. It is sometimes accompanied
by calcite or galena veinlets and by specular haematite
either in veinlets or replacing biotite. Sericite or illite
often occurs with chlorite (D. R. Mason, 2004, unpub-
lished data), commonly as rims to chloritised cores of
feldspar phenocrysts. Epidote is found only in a small
area between the pits, as grains in the andesite por-
phyry groundmass accompanied by chlorite (Fig. 9b).
In most chloritised andesite porphyry, phenocryst
biotite is unaltered, brown-pink and lusterless, or
replaced by white mica. Chloritised rocks are grey
when fresh but rapidly develop a greenish black
coating on exposure. Chlorite-altered andesite por-
phyry with disseminated magnetite forms a competent
rock; chloritised porphyry with pyrite is usually argil-
lised. Both host scattered veins of pyrite with or
without chalcocite and a silicified and pyritic selvedge
best seen where oxidized.
Specular haematite is present in two main areas at
depth in the Sabetaung pit (Fig. 10), surrounded by
phyllic alteration and in the northwest of Sabetaung
South. It occurs as grains disseminated in the andesite
porphyry groundmass, replacing biotite, and partly
replacing plagioclase phenocrysts; mined surfaces
have a dark purple colour. The haematite is accompa-
nied by magnetite with or without sericite or chlorite,
but with less replacement quartz than in the phyllic
alteration; pyrite can occur as sparse grains or veinlets.
A zone of specular haematite-chlorite alteration on the
525 m RL in the south of Sabetaung hosts high-grade
veins, and in a few drill holes around the combined pit
rim specular haematite accompanies chlorite.
Quartz-sericite-pyrite or phyllic alteration pre-
dominates below about 470 m in Sabetaung and in the
deepest part of Sabetaung South. In Sabetaung, where
host rocks are andesite porphyry and pyroclastics,
feldspar phenocrysts are replaced by sericite and by
either pyrophyllite or supergene kaolinite, biotite is
altered to brown, pink or yellow mica and the ground-
mass to quartz and minor clay and sericite. Pyrite
occurs as coarse ragged and very fine dissemination in
groundmass, as replacement grains in biotite and as
stockwork veinlets, and increases with depth where it
exceeds 5% of the rock. Destruction of porphyry
texture varies from near-total to negligible on a meter
scale. In sandstone at Sabetaung South, sericite com-
prises 20 to 50% of the rock, replacing lithic fragments,
some crystals and groundmass (D.R. Mason, 2004,
unpublished data). In much of the Sabetaung pit wall,
phyllic alteration passes upwards and outwards with
disappearance of visible sericite, and decrease in
quartz and pyrite, into kaolinised rocks.
A quartz-sericite-pyrite-alunite assemblage is wide-
spread within the phyllic alteration at Sabetaung.
Petrographic work on a drill core here (M. Simpson
et al., 1996, unpublished data) showed replacement
quartz and sericite throughout the 200 m core, with
sporadic diaspore and rare alunite. In more recent
advanced spectral determination (ASD) results on 100
samples from the 455 m RL at Sabetaung, white mica is
the most abundant mineral in all samples, with rare
alunite, kaolinite and pyrophyllite and minor diaspore.
However, among 59 ASD samples on the 435 m RL
bench, bladed alunite is the most common mineral
together with illite or sericite. Pit mapping shows that
while some alunite replaces feldspars adjacent to veins,
most occurs within veins and breccia dykes, suggesting
that the bladed alunite in bench wall samples occurs in
hydrothermal veinlets within sericite alteration.
Quartz-pyrite together with quartz-alunite-pyrite
replacement of andesite porphyry and pyroclastics
form a yellow to black irregular carrot-shaped body up
to 40 m wide at the top (Fig. 11), and a 2-m wide
replacement vein, both in the Sabetaung pit. A granular
mass of replacement quartz and pyrite is cut by breccia
dykes with a quartz-pyrite mosaic matrix, by pyritic
veins with alunite and barite in vugs, by veins of
massive pyrite, and by veins or dykes of fine-grained
quartz-pyrite. Alunite also replaces some rock frag-
ments and feldspar phenocrysts. In these bodies
copper content is mostly below 0.05%, gold up to
0.3 ppm and pyrite can comprise over 30% of the rock.
However, quartz-pyrite-replaced stockworked sand-
stone hosts some of the mineralization in Sabetaung
South (Fig. 12).
12
© 2010 The Authors

5.2 Letpadaung and Kyisintaung
Data on alteration in Letpadaung is almost entirely
from drill core within the ore body beneath the north-
eastern half of Letpadaung hill. Here M. Simpson et al.
(1996, unpublished data) reported results of XRD, pet-
rographic and mineralogic work on 121 samples from
17 angled drill holes over vertical intervals of up to
400 m. All samples are from quartz andesite porphyry
or pyroclastics and have a groundmass replaced by
anhedral quartz. Tabular or bladed alunite is present in
almost all samples, diaspore is uncommon; pyrophyl-
lite with alunite occurs in a third of the samples. In
three of the longer (over 400 m) drill cores, alunite is
present in all samples in and below the leached caps,
and sericite or sericite-illite accompany alunite in half
of the samples.
More than 422 short-wavelength infrared spectrom-
eter (SWIR) measurements on 5100 m of Letpadaung
core (S. Pontual, 2001, unpublished data) demonstrated
the predominance of alunite and pyrophyllite with
major variations in their relative abundance. M.
Sheehan (2007, unpublished data) noted that at Let-
padaung alunite is most abundant in hydrothermal
breccias, where it replaces phenocrysts, occurs in blades
up to 1 cm across, and is surrounded by illite, and that
vuggy silica is very scarce. There is no obvious system-
atic variation in alteration assemblages with depth.
From their petrographic work on drill core, largely
from Letpadaung, M. Simpson et al. (1996, unpublished
Fig. 11 Cross-section, Sabetaung
pit, selected drill holes shown,
location on Figure 10. Veins and
fracture ﬁlls are schematic,
breccia dykes shown only where
recorded in drill core. RL is rela-
tive level in meters.
Fig. 12 Cross-section, Sabetaung South pit, location on Figure 10. RL is relative level in meters.
© 2010 The Authors

data) and T. M. Leach (1996, unpublished data) inferred
initial leaching of most components other than silica,
and recrystallization as quartz of magmatic silica in the
porphyry groundmass. This was followed by deposi-
tion in sequence of quartz, rutile, diaspore, alunite and
sulfides; pyrophyllite; and late kaolinite; most sericite
was later than alunite and sulfides. M. Simpson et al.
(1996, unpublished data) visualized a deposit-scale
zoning, with a core of quartz-alunite-diaspore, best
developed in breccias, grading out to marginal zones
where pyrophyllite fills phenocryst sites and floods the
groundmass; beyond this were sericite-illite and rare
chlorite. R. Marjoribanks (2004, unpublished data)
identified a massive silica-pyrite ore zone from drill
core beneath part of Letpadaung (Fig. 13); M. Simpson
et al.’s work suggests this zone includes advanced
argillic minerals.
At Kyisintaung, M. Simpson et al. (1996, un-
published data) described alteration from two drill
cores as early replacement quartz associated with
tabular alunite, usually with later pyrophyllite, and
up to 15% pyrite; sericite is absent. ASD determina-
tions on 121 samples from the same drill holes
(Ivanhoe Mines, written comm., 2006) showed alunite
as the most abundant mineral in 81 samples and
pyrophyllite in 25; in most samples these are the two
predominant minerals. A few samples have predomi-
nant kaolinite, diaspore or paragonite. A quartz-pyrite
replacement body similar to those in Sabetaung is
exposed on the western side of Kyisintaung. In
general Kyisintaung resembles Letpadaung rather
than Sabetaung in the abundance of tabular alunite,
very little sericite and absence of reported specular
haematite.
Fig. 13 Northeastern Letpadaung
showing approximate area of
grid drilling. (a) geological inter-
pretation map; (b) interpreted
northwest-southeast cross-
section, RL is relative level in
meters, from R. Marjoribanks
(2004, unpublished data).
14
© 2010 The Authors

5.3 Alteration zones related to mineralization
Alteration associated with mineralized structures is
best documented at Letpadaung where S. Pontual
(2001, unpublished data) and M. Sheehan (2007,
unpublished data) found alunite and pyrophyllite to be
most abundant in or close to breccia dykes. In Sabe-
taung, alteration zones related to mineralization are
seen only at shallow depths, where individual breccia
dykes or thick veins in andesite porphyry are sur-
rounded by a bleached supergene argillic zone up to a
few meters wide passing outwards into chlorite. On a
larger scale, the 130-m wide sheeted vein zone in
mostly phyllic alteration at Sabetaung is bordered at
upper levels by chlorite alteration.
6. Mineralization styles
Copper sulfides are largely confined to planar struc-
tures, both breccia dykes and veins (Figs 11, 12). Their
geometry is known in most detail from Sabetaung and
Sabetaung South, although more petrographic and
mineralogical data are available on Letpadaung. The
breccia dykes and veins occur in all lithologies, but are
rare in the rhyolites, pyroclastics and andesites in the
saddle between Sabetaung and Sabetaung South. In
Sabetaung South the most productive ore body is a
sandstone stockwork and breccia zone. Ore grade gen-
erally decreases with depth from a maximum immedi-
ately beneath the water table or leached cap, but drill
intercepts above 1% Cu occur in the deepest drill inter-
cepts in Sabetaung (170 m RL) and Letpadaung (50 m
RL). In overall shape each of the ore bodies resembles
an upward-flaring funnel or inverted cone.
6.1 Breccia dykes
Breccia dykes are widespread in all four deposits,
although forming not more than 2 or 3% of total rock
volume. Most are steeply-dipping parallel-sided
tabular bodies (Fig. 14a, b) with sharp planar margins
but a few bifurcate upwards or are deflected along a
low-angle structure for a few meters. Some continue
vertically for over 60 m horizontally and vertically in
pit walls (Fig. 14c), but others can be discontinuous,
and pass up or down into a vertical fracture. In Sabe-
taung widths of breccia dykes are up to 2 m in upper
and outer parts of the pit but mostly less than 20 cm
near the pit bottom (Fig. 14b). No bench wall data are
available from upper levels above the pit centres, but
we infer an overall vertical rather than lateral variation
in dyke width. At Letpadaung estimated true widths of
breccia dykes in drill core can exceed 5 m; an early,
1996 map of Letpadaung shows sinuous breccia dykes
over 20 m wide but these widths are conceptual. In
Sabetaung pit walls non-tabular intrusive breccias are
rare.
Clasts are up to cobble or rarely block size and many
are from the immediate wall rock (Fig. 14a). Most
andesite porphyry clasts are angular and retain a por-
phyry texture (Fig. 14d), but all breccia dykes, even
those with jigsaw-fit texture, have few to abundant
well-rounded clasts replaced by finely crystalline
quartz and pyrite with or without sericite, and with
pyritic fractures. Angular to rounded pebbles, cobbles
or blocks of fine or coarse-grained massive pyrite also
occur (Fig. 14d, e).
Breccia dykes have either a mud matrix or variably
silicified sulfide matrix. Mud (or argillised rock flour)
matrix dykes are most abundant in Sabetaung South
Sandstone and Mine Pyroclastics, and are probably
phreato-magmatic and hence diatremes. Some sulfide-
matrix breccia dykes are probably entirely hydrother-
mal and were emplaced along fractures, but we believe
others, particularly the wider and more continuous
dykes, resulted from ascent of hydrothermal fluid
within earlier mud-matrix dykes. Most sulfide-matrix
breccias appear to be clast-supported with a matrix of
pyrite, or pyrite plus digenite-chalcocite and covellite.
Where supergene kaolinite alteration is intense, above
about 500 m RL at Sabetaung, the sulfide matrix
becomes a sulfidic clay.
6.2 Veins and fracture fills
In Sabetaung-Sabetaung South sulfide veins, which
include fracture fills, are planar structures with widths
which on average are greater at shallow depths; where
veins and breccia dykes intersect, the breccia dykes are
usually, but not invariably, later. Thick shallow veins
usually dip steeply, are up to 2 m wide and may
decrease dramatically in width within a few tens of
meters of the base of oxidation; black sooty chalcocite
renders them highly conspicuous. Those with abun-
dant pyrite often include a central sulfide-matrix
hydrothermal breccia. Spaced stockwork veins are
intersecting mineralized fractures, randomly oriented
in three dimensions, and up to tens of centimeters
apart. Sheeted veins are close-spaced parallel struc-
tures with vertical or steep easterly dips, each up to a
few centimeters wide; they are largely confined to and
visually dominate a 130-m wide zone in Sabetaung
(Figs 11, 15a) where they trend N 40°E parallel to the
© 2010 The Authors

long axis of the pit. The sheeted veins are mostly well-
mineralized but in the west wall within the phyllic
zone consist almost entirely of pyrite and have distinct
silicified selvedges up to several centimeters wide.
Through-going low-angle veins dip at less than 40°,
can persist for over 50 m and are mostly only a few
centimeters wide. A typical bench wall shows a spaced
stockwork of planar fracture fills, each at least a few
meters in length, and sparse low-angle veins. At Let-
padaung, M. Sheehan (2007, unpublished data)
reported that vein widths are up to 1.5 m but that most
are less than 10 cm; M. Simpson et al. (1996, unpub-
lished data) noted that some breccia dykes at Let-
padaung pass into veins.
6.3 Mega-breccia and stockworked sandstone
in Sabetaung South
The mega-breccia is a NNE-trending zone more than
500 m long and includes part of the main ore body in
Sabetaung South (Fig. 8). It has sharp but irregular
margins, and is characterized by blocks of wall rock in
a weakly lithified to friable clay matrix. In the south-
west wall of the pit the mega-breccia is more than 20 m
wide with meter-scale blocks of silicified pyritic sand-
stone and rare slabs of carbonized wood. To the north-
east the width decreases to a few m and the breccia lies
between stockworked sandstone and Mine Pyroclas-
tics or rhyolite. Here cobble clasts are largely of
Fig. 14 Photographs showing breccia dykes and veins. (a) 80 cm mud-matrix breccia dyke in argillised sandstone, east side
Sabetaung South; (b) Sabetaung 415 m RL, sheeted vein zone with breccia dyke unusually wide for this level—clasts and
matrix are pyritic and silicified, below 0.1% Cu; (c) NW-trending 1.2 m breccia dyke (dark grey, right of figure) with
chalcocite-pyrite matrix in argillised sandstone and andesite porphyry, above 535 m RL, southern Sabetaung South; (d)
close-up of sulfide-matrix breccia in (c) showing clasts of argillised andesite, sandstone and rare rounded phyllic altered
porphyry in pyrite-chalcocite-covellite matrix; (e) breccia dyke, Letpadaung core, with angular quartz-replaced andesite
porphyry and sericitised fragments in hypogene crystalline covellite matrix.
16
© 2010 The Authors

silicified pyritic sandstone, silicified andesite porphyry
and rare mica schist; rhyolite blocks occur adjacent to
rhyolite wall rock and there are scattered rafts of black
sulfidic ore.
6.4 Disseminated chalcocite and the
high-grade zone in Sabetaung
Disseminated copper sulfides are almost entirely of
chalcocite and economically significant amounts are
mostly shallow, extending downwards for a few tens of
meters from the base of oxidation. Here chalcocite
replaces pyrite or marcasite which itself replaces phe-
nocrysts. Disseminated chalcocite also occurs in or
around the currently inaccessible high-grade zone in
the east wall of Sabetaung (Mining Geology Section,
2005, unpublished data). The high-grade zone is a ver-
tical pipe-like body about 2000 m2
in area with an
average grade of 15% Cu on the 495 m RL (Figs 8, 11)
decreasing to around 2% Cu in drill core at 270 m RL.
It consists of sulfide-matrix breccia dykes with
chalcocite-covellite veins. The andesite porphyry host
rock at 455 m RL is black and friable with phenocrysts
replaced by chalcocite, probably after pyrite; super-
gene leaching has transformed part of this to a grey
“residual” silica sand.
7. Copper sulfides, associated minerals
and metal values
7.1 Minerals in breccias and veins
In early mineralogic work on United Nations drill core
from Letpadaung, Win Htein (1978, unpublished data)
reported that enargite formed coarse aggregates and
replaced chalcopyrite and pyrite, and that neodigenite
and covellite successively replaced enargite, the
Fig. 15 Photographs showing vein mineralization. (a) steep sheeted sulfide veins in quartz-alunite-sericite altered andesite
porphyry, stockwork veinlets not visible in photograph, 465 m RL, northeastern Sabetaung; (b) hexagonal blades of pale
translucent pink alunite overgrown by enargite, Letpadaung (from M. Simpson et al., 1996, unpublished data); (c) fracture
fill showing chalcocite, minor covellite, pyrite and barite, Sabetaung 415 m RL; (d) banded cm wide veins of pyrite with
covellite centres (blue) alternating with anhedral quartz, Sabetaung 415 m RL.
© 2010 The Authors

neodigenite retaining the optical features of enargite.
R. H. Sillitoe (in United Nations, 1978c) found that
most of the copper at Letpadaung occurred as chalcoc-
ite, but that the main hypogene copper sulfides were
enargite and covellite, in ore with over 10% pyrite.
However, among 25 Letpadaung samples with copper
sulfides, D. J. Kirwin (1994, unpublished data) reported
enargite in only two, suggesting possible over-
estimation by earlier workers. Metcon Research (1995,
unpublished data) showed that chalcocite replaces
pyrite, chalcopyrite and bornite at Letpadaung, and
that digenite and covellite can replace all these miner-
als. From mineralogical studies on drill core from Sabe-
taung, Kyisintaung and Letpadaung, E. Iasillo (1995,
unpublished data) argued that all chalcocite, whether
sooty and coating pyrite, or grey and orthorhombic,
and also covellite, are supergene, and that chalcocite
replaces enargite and chalcopyrite. K. Wenrich (1996,
unpublished data) suggested that most chalcocite at
Letpadaung might be djurleite, and also identified
spionkopite and yarrowite. E. Iasillo (1995, unpub-
lished data) and Metcon Research (1995, unpublished
data) reported traces of molybdenite in Kyisintaung
core.
The most comprehensive mineralogic study, by M.
Simpson et al. (1996, unpublished data), includes
description of 34 polished thin sections of drill core in
which pyrite, commonly forming 6 to 15% of the
section, is accompanied by more than a trace of copper
sulfides. Twenty-seven of the samples are from Let-
padaung, where chalcocite occurs in fifteen, digenite in
seven, and digenite-chalcocite, mostly very minor, in
eight; enargite or less common luzonite is present in
eight samples, replacing pyrite, covellite and alunite
(Fig. 15b), and chalcopyrite overgrowing pyrite occurs
in three.
In M. Simpson et al.’s (1996, unpublished data) few
samples from Kyisintaung with copper sulfides, three
have up to 6% chalcocite, accompanied by enargite or
covellite. Of their six samples from Sabetaung-
Sabetaung South, four contain chalcopyrite and bornite
totaling 5 to 10%, partly replaced by a few percent of
digenite and chalcocite; three of these samples are from
one drill core, suggesting intercepts with a single
unusually chalcopyrite-rich structure.
Among all 34 of their polished sections, M. Simpson
et al. (1996, unpublished data) reported hypogene chal-
cocite and digenite in only one, where they occur as
coarse blades replacing enargite. In the 33 other
samples supergene chalcocite or digenite partly replace
all sulfides including pyrite and covellite, and mostly
very minor chalcopyrite, bornite, and tennantite. In
two of these samples supergene chalcocite comprises
35% of the section, replacing enargite, the only primary
copper sulfide. Coarse covellite in two samples is
regarded as hypogene; this and other covellite over-
grows pyrite, a relationship said to be uncommon in
deposits elsewhere (Chavez, 2000). Chalcopyrite,
bornite, tennantite and pyrrhotite occur as inclusions
within pyrite.
M. Sheehan (2007, unpublished data) found that in
Letpadaung veins, covellite is the main hypogene
copper mineral and that chalcocite or covellite is either
intergrown with quartz and pyrite or occurs in vein
centres bordered by pyrite, and is later than alunite;
some pyrite may replace pyrrhotite. He reported little
enargite. Chalcocite and covellite in breccia matrix and
vugs in Letpadaung are often coarsely crystalline with
very coarse pyrite, and the covellite-chalcocite ratio
shows some increase with depth.
In the combined Sabetaung–Sabetaung South pit
walls sulfide matrix breccia dykes and veins have
similar gross features. Anhedral to euhedral coarse to
fine-grained pyrite usually predominates, forming
vein margins and crusts or rims on rock clasts in brec-
cias. At Sabetaung the pyrite is coated or crusted with
chalcocite, either steel grey and crystalline or soft,
black and “sooty”. Spaces between clasts may be
partly filled with alunite, pyrophyllite, and barite.
Covellite is very minor relative to chalcocite-digenite.
In bench walls preferred breakage along the structur-
ally weak centre line of fracture fills exposes extensive
surfaces of pyrite, grey or sooty chalcocite, or rarely
covellite.
In Sabetaung, barite occurs at all levels as elongate
euhedral crystals (Fig. 15c) within fracture fills and as
crystals up to 8-cm long in vugs within breccia. Rarely
fine-grained marcasite replaces coarse crystalline
covellite. Alunite in veins and breccia is mostly tabular
but cubic alunite within vugs can be seen in hand
specimens from the 415 m RL. The sheeted vein zone
near this level includes rare compound veins in
which centimeter-wide bands of anhedral grey quartz
alternate with thinner pyrite-covellite-alunite veins
(Fig. 15d), but in general quartz veins are absent in the
Monywa deposits.
In the main ore zone in Sabetaung South, chalcocite
and pyrite occur as fracture-fills cutting through the
breccia and forming stockwork in sandstone and as
pods of massive steel grey crystalline chalcocite with
enargite and dispersed coarse pyrite. Chalcocite and
pyrite also occur in irregular vugs together with
18
© 2010 The Authors

enargite, minor covellite, and barite, alunite and pyro-
phyllite. Marcasite occurs as later veinlets.
7.2 Mineral paragenesis
M. Simpson et al. (1996, unpublished data) defined the
mineralization sequence in veins and breccia as pyrite
in veins and dissemination, overgrown by chalcopy-
rite, itself overgrown by bornite, tenanntite and covel-
lite; enargite; coarsely crystalline digenite and
chalcocite. They noted that mineralization followed
formation of replacement quartz and quartz–alunite,
and preceded sequential formation of sericite, pyro-
phyllite, barite, and kaolinite; they emphasized the
overgrowth of pink alunite by enargite (Fig. 15b), a
relationship common in high sulfidation deposits (e.g.
Muntean & Einaudi, 2001). Maung Maung Naing (2003,
unpublished data) suggested that digenite, chalcocite
and enargite in veins and vugs were coeval.
7.3 Metal values other than copper
Gold determinations are largely from Letpadaung
where in 100 drill core samples United Nations (1978c)
reported a maximum value of 0.5 ppm with most
samples below 0.03 ppm Au; silver averaged below
1 ppm. Ivanhoe reported average values of 0.024 ppm
Au and 0.58 ppm Ag in more than 400 core samples, all
from Letpadaung. A maximum gold value of 0.51 ppm
and average of 0.185 ppm Au were reported from 28
core samples from Kyisintaung; silver at Kyisintaung
averaged 27 ppm (Maung Maung Naing, 2003, unpub-
lished data). Assays on nine samples from the quartz-
pyrite body at Sabetaung gave a maximum gold value
of 0.31 ppm, average 0.08 ppm Au and 3.8 ppm Ag.
Among other metals at Letpadaung, a background
As value of 185 ppm in soil samples was reported by
United Nations (1978c) and D. J. Kirwin (1994, unpub-
lished data) obtained average values of 100 ppm As on
25 core samples. Arsenic in 28 drill core samples from
Kyisintaung in 1995 averaged 85 ppm; the same
samples assayed 4 ppm Mo. Barium is strongly anoma-
lous in these 28 samples, averages 680 ppm Ba, and
correlates with copper; three samples from Sabetaung
averaged 1800 ppm Ba. In 67 samples from a Let-
padaung drill core, zinc ranged from 13 to 470 ppm,
increasing with depth; the average Pb value in D. J.
Kirwin’s (1994, unpublished data) 25 core samples was
100 ppm. Many early multi-element determination are
from around the oxide-sulfide boundary at Sabetaung
(Thet Lwin and Hla Win Zaw, 1993, unpublished data)
and are omitted here.
8. Structural controls on mineralization
8.1 Dyke trends and mineralization
Breccia dykes in Sabetaung mostly trend approxi-
mately N 40°E, parallel to the sheeted veins (Fig. 15a),
but those in the southwest of the pit and some in the
northwest wall trend northwesterly. In Sabetaung
South most breccia dykes, and almost all those in the
southeast wall, trend northwest (e.g. Fig. 14c). In any
one area almost all breccia dykes have roughly the
same trend, but high-angle intersections do occur.
Most andesite porphyry and rhyolite dykes have the
same orientation as breccia dykes in the same area.
At Kyisintaung breccia dykes and veins reportedly
trend N 35°E and N 15°E (MinCorp and Minproc, 1996,
unpublished data); results of later drilling confirmed a
broadly NE trend (F.B. Lazo et al., 2006, unpublished
data). Surface mapping and drill intercepts in Let-
padaung indicated slightly sinuous predominantly
northeasterly trends to breccia dykes (Fig. 16) but addi-
tional preferred directions were also identified
(Minproc Limited, 1998, unpublished data).
8.2 Folding, faulting and mineralization
The Sabetaung South Sandstone, exposed in a low-
angle NE-trending anticline, is part of the Magyigon
Formation which outside the mine area shows upright
folds with local steep limbs. Within the deposits,
Fig. 16 Conceptual 1996 map of Letpadaung showing
mineralized veins and post-mineral porphyry intru-
sions. Line north of Nache Taung is motor road.
© 2010 The Authors

breccia dykes and magmatic dykes with few excep-
tions are within 10° of vertical, even where host rock
bedding is inclined at 30° or more. We therefore infer
that folding of the Magyigon Formation either accom-
panied or preceded mineralization.
Faults can be defined only from lithological offsets
although NE-trending faults have been inferred from
drilling along the northwest and southeast margins of
the Kyisintaung andesite porphyry (F. B. Lazo, 2006,
unpublished data). In Sabetaung South the West Wall
Fault or reverse fault (Figs 10, 12) is almost certainly
pre-mineral, but the NE-trending cross-fault which
drops hanging-wall sandstone 30 m relative to foot-
wall basalt breccia, post-dates mineralization. At Let-
padaung, faults identified during the 1990s feasibility
study include a dominant NW-trending set said to
downthrow to the northeast; faults believed to control
erosion of the central valley were inferred from topo-
graphic features. In northeastern Letpadaung, R. Mar-
joribanks (2004, unpublished data) mapped a series of
steep easterly-trending faults (Fig. 13a, b) defined by
vertical displacements of up to 200 m in the contact
between gently-dipping pyroclastics and an underly-
ing andesite porphyry sill.
R. H. Sillitoe (in United Nations, 1978c) considered
that the mineralization at Monywa took place in a
strato-volcano, while Bender (1983) favoured a caldera
margin. More recently, R. Marjoribanks (2004, unpub-
lished data) inferred that at Letpadaung hydrothermal
fluids ascended along the E-trending faults to the base
of the pyroclastics, where they ponded and mineral-
ized the underlying andesite porphyry during inter-
mittent pressure release. However, in Sabetaung South
numerous veins cut bedding in the host rocks without
detectable offset, suggesting that here ascent of miner-
alizing fluids may not have been focused in faults. Fault
zones identified from breccia and clay intercepts in
2008 drill core (Geology Section, MICCL, 2009, unpub-
lished data) are not mineralized.
Steep easterly dips to most of the sheeted veins at
Sabetaung (Figs 11, 15a), first reported by MinCorp
and Minproc (1996, unpublished data), imply minor
rotation of vertical structures and hence post-mineral
relative uplift to the southeast.
8.3 Mineralization and the regional stress field
Many porphyry and high sulfidation deposits form
following a protracted period of arc magmatism, sug-
gesting that the mineralization is dependent on the
state of stress or change of stress within the arc. Most
arcs are under extensional stress through pull of the
subducting oceanic plate, although compression can
accompany either oceanward advance of the overrid-
ing plate and decrease in slab dip, or collision com-
monly with a submarine ridge. Takada (1994) and
Richards (2005) argued that compression causes
magma ponding and development of base of crust sills,
and that with stress relaxation, magma ascends to form
upper crustal chambers from which volatile and metal-
rich magma rises as stocks or dykes. For giant Cu-Mo
breccia deposits in Central Chile, Stern and Skewes
(2005) infer compressive stress during mineralization;
this inhibits dyke formation in the upper brittle crust,
allowing brines and vapour exsolved from shallow
magma chambers to deposit sulfides as they ascend
and cool, rather than being carried to the surface and
lost from erupting magma.
In the combined Sabetaung–Sabetaung South pit the
northwesterly trend of many of the larger breccia dykes
and adjacent andesite porphyry dykes implies a least
compressional principal stress oriented northeast.
However, a northwesterly minimum compressive
stress is implied by the N 40°E trending sheeted veins
and associated breccia dykes in Sabetaung. The N 40°E
structures could be explained if western Myanmar
during mineralization were a mega-shear, bounded by
the dextral Sagaing Fault in the east and a parallel fault in
the Bengal basin, provided that shearing was inter-
rupted to allow development of the northwesterly-
trending structures. On a regional scale, NNW-trending
thrusts in the Minbu basin southwest of Monywa have
been related to the collision of the Burma plate with
India in the Naga Hills (Pivnik et al., 1998).Alternatively,
the thrusts, and folding of the Pegu Group including the
Mine Pyroclastics, could speculatively be explained by a
decrease in dip of the subduction zone landward of the
NNW-trending segment of the trench (Fig. 1). The 22°N
uplift between the Chindwin and Salin basins lies
directly west of the Monywa deposits (Fig. 4), but
whether the uplift and magmatism at Monywa have a
common cause is unclear.
9. Post-mineral dykes and diatremes
9.1 Late- and post-mineral andesite
porphyry dykes
Post-mineral andesite porphyry dykes within mineral-
ized host rocks were first reported from Letpadaung
drill core (United Nations, 1978c, R. H. Sillitoe, in
United Nations, 1978c), where chloritised or fresh por-
phyry intrusions with lustrous biotite phenocrysts
20
© 2010 The Authors

were regarded as products of a distinct magmatic
event later than mineralization (R. Marjoribanks, 2004,
unpublished data) and hence as diluents of ore. Andes-
ite porphyry dykes with unaltered biotite, kaolinised
margins and chloritised cores intruding limonitic and
locally alunite-altered rhyolite on upper benches of the
leached cap at Kyisintaung have also been regarded as
post-mineral.
At Sabetaung and Sabetaung South unaltered or
weakly altered andesite porphyry dykes are absent in
the deeper parts of the pits which are mostly mineral-
ized and pass upwards and outwards into chloritised
andesite porphyry with fresh biotite phenocrysts. Very
rarely, mineralized fracture fills terminate against a
dyke which like its host rocks shows pervasive phyllic
alteration. In the upper and outer argillised levels of
northwestern Sabetaung, pyritic andesite porphyry
bodies with bleached margins occur within more sul-
fidic dark grey porphyry, but are cut by mineralized
breccia dykes and their boundaries with sulfidic rocks
are transitional, not chilled margins (Fig. 17a). Evi-
dence for post-mineral porphyry is seen only at the
northwest wall of Sabetaung South where an unaltered
andesite dyke intrudes mineralized breccia.
9.2 “Diatreme” in Sabetaung South
Intrusive breccia dykes or diatremes younger than
mineralization have been inferred or at least suspected
from surface and pit mapping at Monywa since at least
the mid 1990s, although none have been identified in
any of the core drilling. R. Marjoribanks (2004, unpub-
lished data) mapped a post-mineral diatreme, a
NE-trending body with highly irregular margins, in
the then partly covered northwest wall of Sabetaung
South. New bench cuts through Marjoribank’s
diatreme, southwest of the rhyolites (Fig. 8), show
stratified Mine Pyroclastics; the mineralized mega-
breccia zone (Section 6.3); and a body of brecciated
pyroclastics which could be post-mineral.
Fig. 17 Photographs of supergene zone, leached cap and Kangon Sands. (a) northwestern Sabetaung 515 m RL, kaolinised
andesite porphyry showing bleached zone with jarositic surface and texture-destroyed margins transitional to sulfide veins
and breccia and cut by breccia dyke; (b) breccia dyke in bench wall with marcasite and crystalline and sooty chalcocite within
argillised andesite porphyry (light grey), chlorite alteration at right, base of oxide (above dyke) around 553 m RL, southeastern
Sabetaung; (c) looking west across Sabetaung, Kyisintaung (K) leached cap in background, base of scree (sc) overlain by
western limit of Kangon Sands (ks) around 580 m RL, oxide (ox) base around 570 m RL in pyroclastics and andesite porphyry
dykes; alteration: ch, chlorite; ka, argillic (kaolinite); qp, quartz-pyrite; s, sericite-quartz-pyrite; (d) Sabetaung saddle looking
east, Kangon Sands cycles 2 and 3 overlying oxidized chlorite-altered andesite porphyry with limonite veins after pyrite: ox,
base of oxide; w, waste dump.
© 2010 The Authors

10. The supergene enrichment–argillic
zones and leached caps
In the Monywa deposits sulfide-bearing rocks are over-
lain abruptly by an oxidized leached cap, best pre-
served at Kyisintaung and Letpadaung. The base of the
cap coincides with the present water table indicating
that leaching is more or less in equilibrium with uplift.
Underlying mineralized structures have undergone
supergene digenite-chalcocite enrichment, and their
host rocks pervasive argillisation, both of which
decrease with depth.
10.1 Supergene enrichment and kaolinisation
Supergene enrichment has been recognized at
Monywa since the 1970s and up to 75% of total
copper has been regarded as supergene (e.g. M.
Simpson et al., 2006, unpublished data, M. Sheehan,
2007, unpublished data). Immediately below the base
of the oxides at Sabetaung South and Sabetaung,
abundant chalcocite or digenite-chalcocite with or
without covellite occur in both the wide steep veins
(Fig. 17b) and in sulfide-matrix breccia dykes, some-
times with disseminated chalcocite in host rocks.
Copper grade and widths of the mineralized struc-
tures decrease with depth from the base of oxidation,
rapidly at first and then more slowly, implying that
much of this copper is supergene. In the central part
of the Sabetaung pit the upper levels of the enriched
zone provided substantial tonnages of high-grade (2%
Cu or above) ore during early mining. In addition, in
many drill cores from the pit rim (Geology Section,
MICCL, 2009, unpublished data) pyrite or rarely
marcasite-rich veins or breccias within chlorite-
altered andesite porphyry are mineralized, with
grades commonly above 1% Cu, in only the first 5 to
20 m below the base of oxidation. This supergene
copper was evidently leached from the same or
nearby veins oxidizing at the water table. At Kyisin-
taung and Letpadaung, as at Sabetaung, averaged
assays on drill cores indicate maximum copper grades
immediately beneath the leached caps, and at Let-
padaung supergene enrichment to the depths of
530 m or 20 m RL is inferred. Copper enrichment
partly through replacement of pyrite can explain the
upward decrease in residual pyrite towards the base
of the leached cap, at which it falls abruptly to zero.
Since marcasite is replaced more readily than pyrite
by supergene chalcocite (Chavez, 2000), the marcasite
at shallow depths may have facilitated enrichment.
The mid 1990s feasibility studies at Monywa con-
cluded that black sooty chalcocite is supergene but that
crystalline grey chalcocite and crystalline covellite may
be hypogene. In some other deposits, for example, the
Tyrone and Santa Rita porphyry copper deposits in
New Mexico “steel glance” chalcocite, in addition to
sooty chalcocite and to covellite, have been described
as supergene (Cook & Porter, 2005). Nevertheless, the
mineralogic reports on Monywa show that most
copper occurs as digenite-chalcocite; that most of this
is supergene and replaces enargite, hypogene and early
supergene covellite and minor chalcopyrite, bornite
and other copper sulfides as well as pyrite; but that
significant amounts of hypogene digenite-chalcocite
are also present.
Copper enrichment and pyrite depletion are accom-
panied by strong kaolinisation (Figs 10, 17a) superim-
posed on earlier hypogene alteration, resulting in
structurally incompetent ore. At Sabetaung and Sabe-
taung South the kaolinisation is intense and pervasive
down to below 500 m RL, and characterized in andes-
ite porphyry and pyroclastics by replacement of phe-
nocrysts and some of the groundmass by kaolinite with
or without cubic alunite, and in sandstone by kaolini-
sation of detrital felspar and matrix. M. Simpson et al.
(1996, unpublished data) reported cubic alunite
throughout the 70 m of a drill core beneath the base of
oxide at Sabetaung, and Metcon Research (1995,
unpublished data) noted the common replacement of
sericite by kaolinite in Kyisintaung and Sabetaung core.
In the upper and outer parts of the combined pit, white
argillic zones up to tens of meters wide within chlorite
alteration narrow downwards and are clearly super-
gene. At Letpadaung and Kyisintaung, argillisation is
less intense than in Sabetaung and Sabetaung South,
and the ratio of incompetent to competent ore is much
lower. Nevertheless, at Letpadaung M. Simpson et al.
(1996, unpublished data) reported abundant kaolinite
in the upper 100 m of drill cores beneath the base of
oxide.
10.2 Leached caps
The leached cap is more than 200 m thick beneath the
hill tops at Kyisintaung (Fig. 17c) and Letpadaung. At
Sabetaung the pre-mine leached cap was less than 70 m
thick, and in the north and east of Sabetaung it was
absent (Fig. 17d). At Kyisintaung and Letpadaung,
andesite porphyry or pyroclastics have been leached to
a white to grey porous but erosionally resistant rock
consisting of hypogene quartz, tabular alunite and
22
© 2010 The Authors

pyrophyllite, and of cubic alunite and kaolinite, with
minor limonite on fractures and in breccia dykes, and
with jarositic surface stain in benches and drill roads.
Much of kaolinite and also cubic alunite replacing pla-
gioclase probably formed in the supergene enrichment
zone during uplift. The pre-mining low topographic
relief and the high proportion of incompetent ore at
Sabetaung–Sabetaung South suggest that these depos-
its had less hypogene quartz than Kyisintaung, and
consequently were easily eroded by the migrating
Chindwin River. Supergene chalcedonic silica may
also be significant in increasing resistance to erosion.
Around the combined Sabetaung pit the base of the
leached cap at about 560 m RL, is mostly 20 to 25 m
below the level of the plain (Fig. 17c), but around
600 m RL, locally rising to 700 m RL, beneath Kyisin-
taung. In detail the base at Sabetaung-Sabetaung South
is highly irregular within a 10 m vertical interval, but in
most places can be defined within a few tens of centi-
meters. It is least distinct in drill-holes in chloritised
andesite porphyry, where limonitic fractures occur in a
transition zone for 10 or 15 m below totally oxidized
rock (Fig. 17c). At Letpadaung the oxide base is mostly
between 570 and 580 m RL under the central valley but
10 to 20 m lower beneath the hills.
The upward change within high-grade chalcocite-
rich breccia dykes as they pass through the water table
into the overlying oxidized cap can be seen in some
southeastern benches at Sabetaung. Here chalcocite,
much of it supergene, is oxidized to red haematite,
with local haematitic gossan at the base; red haematite
continues upwards for 10 or 20 m surrounded by
brown or orange jarositic limonite. Oxide copper min-
erals analogous to those above some porphyry copper
deposits (Chavez, 2000) are confined to thin crusts at
the base of the water-saturated Kangon Sands cover
described below, and to old workings near the base of
slope at Letpadaung.
10.3 Leaching and enrichment process
The supergene leaching and enrichment process at
Monywa was similar in principle to that described
from enriched porphyry copper deposits (Blanchard,
1968; Anderson, 1982; Chavez, 2000), whereby residual
pyrite is oxidized and supergene and residual hypo-
gene copper sulfides go into solution as they are
elevated through the water table. The highly efficient
leaching at Monywa, with copper values in the leached
caps mostly well below 150 ppm, reflects the high
pyrite content, high pyrite : copper ratio, and the
non-reactive nature of the hypogene (acid)-altered host
rocks.
M. Simpson et al. (1996, unpublished data) explained
the argillic alteration or kaolinisation at Letpadaung by
descent of steam-heated meteoric water from above the
water table, during the waning stages of hydrothermal
activity. However, kaolinisation must have post-dated
all hypogene veining, which required brittle struc-
tures. The decrease in intensity of kaolinisation with
depth below the present water table suggests that it is
related to the base of oxidation and not to a palaeo-
surface which was perhaps 1 km above the present
surface. We infer that since the late Miocene persistent
regional uplift has resulted in elevation of mineralized
rocks allowing continuous leaching.
Within the leached caps, the bladed alunite and
pyrophyllite with goethite and jarositic veins and brec-
cias and local haematite near the base can be explained
by uplift and oxidation of mineralized rocks which
have passed through the supergene enrichment zone.
At the water table residual pyrite was evidently suffi-
cient to generate enough acid to remove all the
digenite-chalcocite and any residual copper sulfides,
producing acid cupric sulfate. This replaced with chal-
cocite, all or most of the other copper sulphides, and
some of the pyrite in the underlying supergene enrich-
ment zone, most intensively within the upper hundred
meters. The process is probably self-adjusting: given
constant uplift and consumption of acid in argillisation
of host rock, more hypogene digenite-chalcocite avail-
able for oxidation at the water table consumes more
acid, leaving less available to effect supergene enrich-
ment; less hypogene chalcocite at the water table con-
sumes less acid, allowing replacement of more of the
residual sulfides by chalcocite. Although residual
copper sulfides include more enargite than chalcopy-
rite, bornite, and tennantite, arsenic values in Let-
padaung core average only 100 ppm; therefore either
enargite was not a major constituent of the hypogene
ore, or arsenic liberated during enargite replacement
by chalcocite was highly mobile. Survival of bornite, a
sulfide readily replaced by chalcocite, may reflect its
occurrence mostly as inclusions in pyrite.
Meter-scale intercepts of quartz-replaced sponge-
textured sulfide-free andesite porphyry with leached-
out felspar phenocrysts occur to depths which at
Letpadaung exceed 600 m. This has been explained as
residual silica from hypogene acid leaching (e.g. R.
Marjoribanks, 2004, unpublished data), and equated
with the vuggy silica described from many high sulfi-
dation deposits (e.g. Arribas, 1995). Alternatively, at
© 2010 The Authors

Letpadaung groundmass silicification may have been
followed by replacement of phenocrysts by anhydrite,
subsequently removed together with sulfides, by
ground water.
11. Post-mineral cover and exotic
copper potential
Around the Sabetaung South and part of the Sabetaung
pit the bedrock surface is overlain by the Kangon Sands
of MMAJ-OTCA (1974a, b), a more or less horizontal
formation which extends eastwards towards the
Chindwin River and southwards to Letpadaung
(Fig. 2). Prior to mining, the formation covered almost
all of Sabetaung South and the flanks of a 50 m high hill
at Sabetaung.
Along the southern, eastern and northern margin of
the combined pit the formation consists of two main
fining-upward sedimentary cycles. The southern cycle,
with a basal conglomerate around 580 m RL in the
southern rim of Sabetaung South, overlies oxidized
bedrock. Northwards the conglomerate cuts down to
around 565 m RL and is overlain by 10 m of silt-mud
alternating beds. This cycle is truncated in the eastern-
most rim of Sabetaung by the northern cycle (Fig. 17d)
with a quartz-pebble basal conglomerate up to 2 m
thick which cuts down to the north to around 548 m
RL, below the base of oxidation. The conglomerate
passes up into a northward-thickening cross-bedded
sand. Overlying black “cotton” soil thickens north-
wards but continues southwards across the southern
cycle. In the northeast of Sabetaung the northern cycle
is at least 40 m thick, and forms a major aquifer, in
which the elevation of the water table, at around 563 m
RL, is similar to the dry season level of the Chindwin
River. Towards Kyisintaung the conglomeratic base
rises rapidly to 580 m RL, and overlies scree which
covers oxidized bedrock (Fig. 17c). The cycles are inter-
preted as Quaternary fluvial deposits of the migrating
Chindwin River channel and flood plain.
The sands and basal conglomerate of the northern
cycle have a potential for exotic copper deposits,
carried laterally in solution from the oxidizing sulfide
surface at the topographically higher base of the
leached caps. The basal conglomerate in the northeast
rim of Sabetaung has an orange limonite cement,
locally hosts layered crusts with the upward sequence
cuprite-chrysocolla-azurite developed on underlying
chalcocite ore and contains up to 1% Cu. However,
here and elsewhere around the pit rim, elevated copper
values in the conglomerate may be derived from early
1980s waste dumps up-slope. Unequivocal exotic
copper mineralization therefore has not yet been found
at Monywa.
12. Geological environment during
hypogene mineralization
The very few research studies carried out since large-
scale mining began in 1997 permit only the most super-
ficial discussion of the geological environment
immediately before and during mineralization.
12.1 Mine Pyroclastics, breccia dykes and
the palaeo-surface
Within the Mine Pyroclastics the presence of abundant
clasts of pyrite and of quartz-pyrite replaced and
phyllic-altered rock indicate explosive hydrothermal as
well as pyroclastic eruptions, requiring emplacement of
andesitic magma below the present exposure level
(Fig. 18a). The source of the pyrite and hydrothermally
altered clasts lies somewhere beneath the Pyroclastics.
Folding of the Sabetaung South Sandstones and
Mine Pyroclastics was followed by intrusion of the
sub-volcanic andesite or dacite dykes and sills present
in all the deposits, and by subsequent emplacement
of breccia dykes, mineralization and hydrothermal
alteration (Fig. 18b). Some of the breccia dykes are
diatremes, perhaps resulting from intrusion of magma
into over-pressured sediments; many of these breccias
formed preferred structures for ascent of hydrothermal
fluids. The association of pyrophyllite with quartz
implies a temperature of at least 285°C (Muntean et al.,
1990) and is widespread in the ore bodies and overly-
ing leached caps at Monywa. Assuming near-
hydrostatic pressure, the boiling-depth curve implies a
palaeo-water table a kilometer or more above the
pyrophyllite-quartz assemblage in the leached caps.
We infer that the eroded rock between the caps and
palaeo-water table was similar either to the underlying
Mine Pyroclastics or to clastic sedimentary rocks
within which tuffs are interbedded outside the mine
area. Therefore even if the breccia dykes failed to vent
as diatreme and hydrothermal eruptions, it is unlikely
that mineralization at Monywa took place within or
beneath a strato-volcano.
Many epithermal deposits resemble Monywa in the
association of mineralization with hydrothermal brec-
cias and reworked products of explosive hydrothermal
or diatreme eruptions. For example, at Sari Gunay in
Iran (Richards et al., 2006), explosive eruption of
24
© 2010 The Authors

fragmentals, intrusion of “hydrovolcanic” breccias and
overprinting by hydrothermal breccias with jigsaw tex-
tures, is similar to the sequence inferred at Monywa,
and at Kelian in Indonesia (Davies et al., 2008) the wide-
spread “accretionary lapilli” in explosively-erupted
rocks are identical to the spherules characteristic of the
Mine Pyroclastics at Monywa. However, both Kelian
and Sari Gunay are low sulfidation deposits, and there
is no reason to suppose that the genesis of the distinc-
tive breccia dykes and pre-mineral Mine Pyroclastics at
Monywa was dependent on the sulfidation state of the
hydrothermal fluid.
12.2 Hydrothermal fluid and vertical zonation
of mineralization
The close spatial association of the Monywa mineral-
ization with andesitic to dacitic porphyry intrusions
implies a genetic relationship, and we infer that a few
kilometers beneath each of the four deposits the dykes
are rooted in a cupola or salient in the roof of a large
dioritic pluton inferred to underlie the mineral district
(Fig. 19). Crystallisation of the dykes and progressive
downward growth of the cupola roof inhibited escape
of volatiles, resulting in a low-density fluid-saturated
cap or column above the underlying magma. Episodic
hydraulic fracturing with formation of breccia dykes
and veins followed increases in hydrostatic pressure
within the low-density column to above lithostatic.
Following the proposals of Muntean and Einaudi
(2001) and Pudack et al. (2009) for the generation of
high sulfidation epithermal gold deposits in Chile, we
speculate that at Monywa the magmatic fluid was
supercritical, and contracted from vapour to liquid on
rising and cooling. Acid sulfate alteration of wall rock
resulting from condensation of magmatic SO2 gas in
water probably accompanied mineralization in veins
and breccias, as argued by Masterman et al. (2005) for
zoned alteration around high sulfidation veins in the
Collahuasi District in Chile.
The Monywa deposits differ from many high sulfi-
dation epithermal deposits in the scarcity of large
bodies of replacement quartz or quartz-alunite and in
the absence of associated near-economic gold grades.
This suggests either a deep erosion level in a formerly
vertically zoned epithermal deposit, or more probably
exposure of a deep epithermal or sub-epithermal
system (Fig. 20) in which hypogene digenite,
Fig. 18 Cartoon cross-sections
showing two stages in evolution
of Sabetaung or Letpadaung
deposit. (a) Eruption of phreato-
magmatic dykes and hydrother-
mal breccia and surface
reworking to form Mine Pyro-
clastics, (b) subsidence with
further accumulation of erupted
material and sediments, folding,
andesitic magmatism, emplace-
ment of breccia dykes, and
copper mineralization. BA, Basalt
breccia; MB, Mesozoic basement;
MP, Mine Pyroclastics; SH, shale;
SS, sandstone. Thick wavy line in
(b) is present erosion level, grey
shading is mineralization.
© 2010 The Authors

chalcocite, enargite and covellite were not necessarily
overlain by gold or enargite-gold mineralization
(Kyaw Win & Kirwin, 1998). Mineralization at palaeo-
depths below the epithermal range is supported by
some similarities to the high sulfidation ore body in the
western part of the super-giant Chuquicamata deposit
in Chile, where a central enargite zone passing
outwards into digenite and then covellite extends
vertically for more than a kilometer (e.g. Faunes et al.,
2005). However, a close analogy with Chuquicamata
would require that much of the digenite-chalcocite at
Monywa is hypogene, rather than supergene as
inferred from the mineralogical studies by M. Simpson
et al. (1996, unpublished data) and others and summa-
rized above.
Many mineral districts with one or more high sulfi-
dation epithermal copper-gold deposits include
slightly older porphyry copper or copper-gold systems
at similar or greater palaeo-depths (e.g. Arribas, 1995).
The dependence of mineralization style on the depth of
cupolas in the roof of an underlying pluton proposed
by Proffett (2009) provides a possible mechanism for
generating the two deposit types in one district. M.
Simpson et al. (1996, unpublished data) inferred por-
phyry copper mineralization beneath Letpadaung from
the presence of saline fluid inclusions in quartz vein
fragments within breccias. It could also be argued that
within the Monywa district porphyry deposits might
exist at shallow depth beneath Kangon Sands or allu-
vium as a result of post-mineral tilting and erosion.
Alternatively, it may be that only very weak porphyry
copper systems developed at Monywa, and were sub-
sequently overprinted by the high sulfidation mineral-
ization. This could explain the chalcopyrite-bornite
veins encountered in some drill holes, for example at
Sabetaung.
Fig. 19 Schematic northwest-
southeast cross-section through
Monywa copper district showing
highly speculative geology at
depth and possible intrusion(s)
beneath cover. Porphyry copper
mineralization could occur above
the cupolas in pluton roof.
Fig. 20 Zonations in metals and
copper sulfides in high sulfida-
tion systems, from D.J. Kirwin
(1994, unpublished data) modi-
fied from T.M. Leach (1996,
unpublished data).
26
© 2010 The Authors

Two high sulfidation deposits with some features
similar to Monywa but undoubtedly generated at shal-
lower epithermal depths are the Quaternary gold-
(copper) deposit at Chinkuashih in Taiwan (Tan, 1991)
and the Cretaceous copper-gold deposit at Zijinshan in
eastern China (So et al., 1998). At Chinkuashih gold and
minor enargite mineralization occurs within veins and
hydrothermal breccias in silicified intrusive dacites
and in sandstones over an 800 m vertical interval, with
eruptive dacites at the top. At Zijinshan, sheeted quartz
porphyry dykes and their granite host rocks are cut by
parallel or sheeted mineralized hydrothermal breccia
veins. In upper levels the breccias are within quartz-
replaced rock and host ore-grade gold; below, breccias
within a larger alunite zone host covellite, digenite and
enargite ore. Neither Chinkuashih nor Zijinshan are
underlain by a porphyry ore body.
13. Distinctive features of the Monywa
copper district
We summarise below some characteristic features of
the Monywa deposits.
Host rocks to the mineralization are distinctive in
having as the uppermost stratigraphic unit a sequence
of interbedded pyroclastics and eruptive diatreme
breccia deposits which was folded prior to intrusion of
andesitic porphyries and local rhyolites, subsequent
emplacement of breccia dykes, and mineralization.
There were therefore two episodes of breccia or
diatreme emplacement at Monywa, with copper min-
eralization confined to the second episode in which
breccias may or may not have erupted.
The mineralization style differs from that in most
high sulfidation deposits in the abundance of hydro-
thermal breccia dykes of which more than 200 have
been mapped on any one of the upper levels in the
Sabetaung-Sabetaung South pit walls and almost all of
which have one of two orientations, roughly at right
angles. The main hypogene ore minerals, with early
enargite overgrowing alunite and replacing pyrite,
later more abundant digenite-chalcocite and minor
covellite, are typical of high sulfidation deposits but
distinguished by a scarcity of large massive quartz-
pyrite replacement bodies and by the related absence
of economic or near-economic gold values.
A major feature of the deposits is the very strong
supergene leaching, with removal of almost all copper
in the leached cap. This highly efficient supergene
process at Monywa is best explained by an exception-
ally high pyrite content and by uninterrupted uplift,
compatible with the coincidence of the present water
table with the base of the leached caps. Development of
a highly porous but rigid leached cap to form a sea-
sonal reservoir for oxygenated meteoric water may
have contributed to efficient leaching, and was depen-
dent on hypogene quartz replacement of the porphyry
groundmass and perhaps also on deposition of super-
gene chalcedonic silica.
Descent of acid sulfate solutions generated at the
water table resulted in the supergene enrichment and
argillisation which decrease, probably exponentially,
with depth from a maximum immediately beneath the
very well-defined base of oxidation. Extensive replace-
ment of enargite, replacement of covellite, minor chal-
copyrite and bornite and partial replacement of
abundant pyrite and of marcasite by supergene chal-
cocite and digenite to depths of several hundred
meters were partly dependent on the weak neutraliza-
tion potential of the hypogene advanced argillic and
phyllic alteration below the water table. This replace-
ment, the presence of hypogene digenite-chalcocite,
and relative scarcity of enargite explain the amenability
of the Monywa ore to heap leaching and hence to pro-
duction of copper metal through solvent extraction and
electro-winning.
Acknowledgements
We thank Douglas Kirwin for encouragement, discus-
sion, arrangement of ASD determinations, detailed
comments on the draft, and provision of Figure 14e,
and Win Kyaw for advice on the history of the deposits.
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