The document provides information on the geology and mineralization of the Henty Gold Mine in Tasmania. It summarizes that the mine contains four major zones of gold mineralization within altered volcanic rocks, with the deepest zone being Zone 96 which contains reserves of 526,000 tonnes at 26.2 g/t gold. Regional geology of the area places the mine within the Cambrian Mount Read Volcanic Belt, known for other base and precious metal deposits. Gold mineralization at Henty is hosted within quartz phyric volcanic and volcaniclastic rocks of the Tyndall Group formation.

1. Geology and Mineralisation of the Henty Gold Mine, Tasmania
P De Mark1
and T Callaghan2
I TRODUCTIO
The Henty Gold Mine is located approximately 30 km north of
Queenstown, at the head waters of the Henty River on the west
coast of Ta mania at 41°50'S, 145°40'E on the Queenstown (SK
55-5) 1:250000 and Sophia (8014) I: 100 000 map sheets (Figure
1). The mine is accessed from Queenstown along the A I0
Highway and Anthony's Road. The town of Queenstown is
located 260 km north west of Hobart on the A I0 Highway, and
I. Mine Geologist, Henty Gold Mine, PO Box 231, QlIeenstown Tas
7467.
2. Exploration Geologist, Henty Gold Mine, PO Box 231, QlIeenstown
Tas 7467.
approximately 40 km equidistant from the towns of Strahan,
Zeehan and Rosebery.
The Henty Gold Mine encompasses four major zones of gold
mineralisation trending along strike and down plunge within a
package of highly altered volcanic rocks. The shallowest zone is
termed the Sill Zone, followed by the Intermediate Zone
(currently undergoing exploration), Zone 96 (the main focus of
mining), and Mt Julia, also currently in the exploration stage.
Henty is owned and operated by Goldfields (Tasmania) Ltd, and
is the first mine to open in Tasmania since 1989 and the first gold
mine to open since Beaconsfielcl last century.
The Zone 96 1996 reserves are 526 000 tonnes at 26.2 g/t Au,
containing 442 678 ounces of gold. Annual gold production is
estimated to average 90 000 ounces over a period of 4.5 years.
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FIG 1 - Location map of Henty Gold Mine and regional geology. After Corbett and Lees, 1987.
Mining Geology Conference Launceston, 10 - 14 November 1997 25

2. P De MARK and T CALLAGHAN
During the 1996-1997 financial year, Henty produced 74 930
tonnes at 21.1 g/t Au, recovering 48 377 ounces.
Exploration and mining history
Exploration in the Henty area was initiated by Renison Ltd in
1966. In 1973, samples from a costean over an IP and soil
geochemical anomaly at the current portal pad returned 2.4 m of
semi-massive sulphide mineralisation at 1.8 per cent Cu, 1.76 per
cent Pb, 0.2 per cent Zn, and 37.89 per cent FeS2. At this time
samples were not assayed for Au. Two diamond drill holes into
the costean intersected only a narrow sulphide lens.
Goldfields Exploration Pty Ltd took over the exploration
license in 1982 and drilled three diamond holes the following
year, with two of them again intersecting thin massive sulphide
mineralisation. Routine gold assaying began at this time and
returned high gold values for the massive sulphide intersection
and another silica-pyrite alteration zone. Wider exploration
around the site of the Henty Gold Mine renewed in 1984 when
EL 9/66 became due for reduction. The area was reinterpreted
and previous drill holes were assayed for Au. HFZ05, one of the
first diamond drill holes sited at the portal pad, returned an
intersection of 6.7 m at 7 g/t Au.
Exploration intensified on the basis of these results, and from
1984 to 1988 a further 38 diamond holes were drilled,
identifying a shallow gold resource between 50 and 150 m below
the surface over a strike length of 650 m, known as the Sill Zone.
The initial resource estimate of the Sill Zone in 1987 was 500
000 tonnes at 10 g/t Au. However, the irregular spatial
distribution and style of mineralisation observed in drill holes
required further work to prove geological continuity and
sufficient tonnes and grade to economically develop the deposit.
In November 1988 development commenced on an 800 m long
exploration decline and a 200 m long ore drive into the Sill Zone
to further define mineralisation. Results from a 25 m by 25 m
spaced diamond drilling program and methodical sampling of the
ore drive delineated insufficient mineable reserves to justify
further development of the Sill Zone. However, continued
diamond drilling located a deeper target down plunge of the Sill
Zone mineralisation at 350 to 550 m below the surface. In
September 1989, diamond drill hole HP096A intersected 7.5 m
at 107.1 g/t Au. During the next three years the Zone 96 area,
named in honour of the bonanza drill hole, was drilled to 40 m
by 30 m spacing, identifying a mineable reserve of 526 000
tonnes at 26.2 g/t Au.
Following a feasibility study undertaken between 1990 and
1992, Goldfields decided to drill a shaft to access the deeper
mineralisation. Excavation of the decline leading to the
underground shaft collar and drilling of the internal shaft
commenced in 1993. Henty officially opened in July 1996 after
a three-year pre-production period and an investment of $53
million. Because of delays encountered in accessing the Zone 96
orebody, mining of the Sill Zone during 1995 - 1996 provided
approximately 22 000 tonnes at 7.09 g/t Au for the
commissioning of the gold plant. Development driving from the
bottom of the shaft intersected the Zone 96 orebody in October
1996.
A 250 m spaced diamond drilling program in late- I995
identified Henty-style mineralisation at Mount Julia
approximately 1.3 km south of the current mine workings. A
150 m by 75 m spaced infill drilling program is presently under
way and is expected to be completed by the end of 1997.
Drilling of the Intermediate Zone, located between the Sill Zone
and Zone 96 orebodies, has been on-going since late-1996.
Should infill drilling programs in these areas prove to be
successful, an incline to the Intermediate Zone and a decline to
Mt Julia from Zone 96 will be considered.
REGIONAL GEOLOGY
Henty lies within the Cambrian Mount Read Volcanic Belt, a rich
mineral zone that hosts the Hellyer, Que River, Rosebery,
Hercules and Mt Lyell ore deposits. The Mount Read Volcanics
form a 20 km wide arcuate belt of submarine and subaerial
rhyolitic to basaltic lavas, intrusions, and volcaniclastics with a
strike length of over 200 km, lying along the eastern margin of
the Dundas Trough. They are bound to the east by Precambrian
basement rocks of the Tyennan Region, interfinger Cambrian
volcano-sedimentary rocks of the Dundas Group to the west and
are overlain by Late Cambrian to Early Ordovician siliciclastics
of the Owen Conglomerate.
Stratigraphy
Corbett (1992), Corbett and Lees (1987) and White and McPhie
(1996) have documented the stratigraphy of the Mount Read
Volcanics and form the basis of the following summary. The
Cambrian Mount Read Volcanics may be divided into four major
lithostratigraphic units comprising the Central Volcanic
Sequences, the Eastern quartz-porphyritic sequence, the Western
volcano-sedimentary sequences (comprising the Yolande River
sequence, the Dundas Group, and the Mount Charter Group), and
the Tyndall Group (Figure I). The NNE trending Henty Fault
Zone was active during the deposition of the Tyndall Group
rocks, causing rapid facies changes in its vicinity, and divides
these sequences into two distinctly differing stratigraphic and
structural domains.
Southeast of the Henty Fault, volcaniclastics, mudstones, and
sandstones of the Western volcano-sedimentary sequences
overlie or interfinger with the Central Volcanic Sequences. The
Central Volcanic Sequences comprises mainly rhyolitic to dacitic
lavas, pyroclastics, and intrusives, and is characterised by
potassic rhyolites. The Eastern quartz-porphyritic sequence
mainly comprise quartz porphyritic volcanics, intrusives, and
volcaniclastics intertingering with the Central Volcanic
Sequences. The Tyndall Group mainly comprises rhyolitic
volcanics and volcaniclastic breccia, conglomerate, sandstone,
and mudstone and overlie both the Western volcano-sedimentary
sequences and the Central Volcanic Sequences, in places
unconformably. The Mount Read Volcanics are overlain by the
Owen Conglomerate, a thick sequence of shallow marine to
fluvial conglomerate derived from the rapid uplift of the
Precambrian Tyennan basement rocks. The Newton Creek
Sandstone locally occupies the base of the Owen Conglomerate
and unconformably overlies or intertingers with the Tyndall
Group.
Northwest of the Henty Fault, the Central Volcanic Sequences
comprises lavas and pyroclastics dominated by dacites and
andesites. The Central Volcanic Sequences is unconformably
overlain by felsic tuft's, siltstones, greywackes, slates, mudstones,
and conglomerates of the Dundas Group.
Regional structural setting
The NNE trending Henty Fault, a Late Cambrian structure
running for over 60 km between Mount Charter and Mount Read,
dominates the structure in the centre of the Mount Read
Volcanics (Figure I). The Henty Fault dips at 70° to the west
and diverges into the North and South Henty Faults at its
southern extremity near Mount Read. The Great Lyell Fault, a
major Cambrian structure reactivated in the Devonian, trends
south of and parallel to the Henty Fault and possibly intersects it
near the Henty Gold Mine. The Henty and Great Lyell Faults
formed the western margin of an active graben during deposition
of the Owen Conglomerate. Other important structures in the
area are Devonian NNW trending broad, open folds and later
NW trending folds and faults, and a strong steeply dipping,
NNW trending regional foliation.
26 launceston, 10·14 November 1997 Mining Geology Conference

3. GEOLOGY AND MINERALISATION OF THE HENTY GOLD MINE
FIG 3 - Cross section through Henty Gold Mine at 54 900 mN.
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MINE GEOLOGY
Mineralisation of the Mt Read Volcanic Belt
Henty is contemporary with the high-grade Cambrian Pb-Zn-Cu-
Au-Ag VHMS deposits at Hellyer, Que River, Rosebery and
Hercules and the Cu-Au Mt Lyell deposit in the Mt Read
Volcanic Belt. The high-grade gold deposits at Henty are a new
style of mineralisation in the Mount Read Volcanics, and are the
only economic deposits hosted in Tyndall Group rocks. Henty is
a remarkably Au-rich volcanogenic gold deposit at nearly 30 g1t
Au, compared to an average of 2.8 g/t Au for other Tasmanian
volcanogenic deposits and an average of 1.8 g/t Au for deposits
world wide (Halley and Roberts, in press).
Pb isotope ratios from samples taken from gold-rich alteration
types at Henty, including quartz infill in Devonian tension
gashes, are highly uniform and lie in the same field as the other
Cambrian Mount Read Volcanic deposits (Halley and Roberts, in
press).
tratigraphy
The Henty mine lease covers rocks of the Central Volcanic
Sequences, the Henty Fault Sequences, and Tyndall Group rocks
of the Mount Read Volcanics and the overlying Owen
Conglomerate (Figures 2 and 3). Near the mine, the Henty Fault
splays into the North and South Henty Faults, dividing theFIG 2 - Plan of Henty Gold Mine and local geology.
AMO
78000N
AMO
64000N
Mining Geology Conference Launceston. 10 - 14 November 1997 27

4. P De MARK and T CALLAGHAN
geology into segments to the east and west of the faults, and a
package between the splays. Gold mineralisation is hosted in
Tyndall Group rocks to the east of the Henty Fault.
Rocks to the west of the Henty Fault comprise dominantly
pink, feldspar phyric dacitic lavas and coarse grained crystal-rich
volcanics of the Central Volcanic Sequences. Numerous
chloritised, fine grained tholeiitic dykes of the Henty Dyke
Swarm intrude rocks of the Central Volcanic Sequences.
The Henty Fault Sequences lie between the North and South
Henty Faults and comprise carbonaceous black shales, mafic to
ultramafic volcanics, and quartz phyric volcaniclastics.
Rocks to the east of the Henty Fault comprise quartz phyric
volcanics of the Tyndall Group and siliciclastics of the Newton
Creek Sandstone of the Owen Conglomerate. Minor altered
dacitic volcaniclastics and lavas of the Central Volcanic
Sequences also occur east of the Henty Fault in the southern area
of the lease.
White and McPhie (1996) have divided the Tyndall Group
rocks into the overlying Zig Zag Hill Formation and the
underlying Comstock Formation, which broadly correspond to
the Upper and Lower Tyndall Groups of Corbett (1974, 1988,
1992). They have further subdivided the Comstock Formation
into the upper Mount Julia Member and the lower Lynchford
Member, which hosts the Henty orebodies.
In the mine area, the Lynchford Member comprises green to
red, massive coarse grained crystal-rich feldspar phyric
volcaniclastic sandstone with lesser siltstones and matrix
supported lithic breccias and minor interbedded cherts and
cream, pink, or purple carbonates. Original textures are still
discernible despite subsequent hydrothermal alteration and
deformation.
Basal units of the Mount Julia Member comprise massive
quartz crystal-rich volcaniclastics and autobrecciated and flow
banded rhyolites which thicken rapidly from north to south,
coinciding with a swing in the strike of lithology from 340° to
020°, subparallel to the Henty Fault. The basal units of the
Mount Julia Member are present at Mt Julia but are not present at
the Henty Mine.
The middle and upper portions of the Mount Julia Member
comprise polymictic volcaniclastic greywacke, graded mass
flows, quartz crystal-rich volcaniclastic sandstones and epiclastic
shales, overlain by massive crystal-rich volcaniclastic sandstones
with minor matrix supported diamictic breccias.
The overlying Zig Zag Hill Formation comprises polymictic
volcaniclastic conglomerate, coarse grained quartz crystal
sandstones, and laminated siltstones.
Abundant quartz-feldspar to quartz porphyritic rhyolites occur
throughout the Tyndall Group, along with pepperitic and
autoclastic sills, dykes, and flows. This rhyolite complex
intruded the Comstock Formation lithologies and formed
subaqueous extrusives and lava domes with associated
autoclastic deposits at the top of the Comstock Formation and
within the Zig Zag Hill Formation. The autoclastic deposits were
extensively reworked and formed a large localised sediment
source for the Zig Zag Hill Formation.
Structure
The Henty orebodies are hosted east of the Henty Fault (Figure
3), on the steeply west dipping overturned western limb of a
shallowly south plunging asymmetric syncline trending into the
Henty Fault. The orebodies plunge at 45° to the south between
the Sill Zone and Zone 96, and shallow at depth towards Mt
Julia. The mineralised zone at Zone 96 is up to 300 m long, 30
m wide, and 200 m high. The lithologies and regional foliation
strike at 340° with a steep westerly dip, and swing parallel to the
Henty Fault as they approach the structure. Two generations of
cleavages (S I and S2) overprint the orebodies, with the earlier SI
cleavage dragged into the later Devonian S2 cleavage.
The structure of the Henty Gold Mine is dominated by the
steeply west dipping Henty Fault Zone, which splays into the
North and South Henty Faults in the middle of the lease. The 20
m wide South Henty Fault forms the upper boundary to
mineralisation, and truncates it to the south. At depth the
vertically dipping orebodies become increasingly separated from
the westerly dipping fault. Adjacent to the South Henty Fault,
foliation ·intensifies and lithologies become brecciated and
mylonitised. The orebodies are disrupted by numerous north-
south trending, steeply west dipping brittle-ductile faults with
displacements of up to a few metres.
Alteration
Nearly all of the stratigraphic units of the Tyndall Group present
at the Henty Gold Mine have undergone hydrothermal alteration.
The most intense quartz-sericite-sulphide alteration and gold
mineralisation has affected the Lynchford Member of the
Comstock Formation, adjacent to the Henty Fault, and is referred
to as 'A-Zone' type alteration. A Zone alteration types include
MA, MZ, MV, MQ, MP, and CB (Figure 3). The main
mineralised zone comprises MQ, MV, and MZ, and are the
alteration types followed in underground ore drives.
From west to east, the alteration types are as follows:
MA (sericite-quartz ± pyrite ± carbonate ± feldspar± fuchsite
schist) - The MA alteration type ranges between 15 and 60 m
thick and lies between the Henty Fault to the west and A Zone
type alteration to the east. It is an orange, brown, to green
moderately to strongly altered, highly foliated to mylonitic rock.
The original lithology is believed to be coarse epiclastic mass
flows and sandstones (Taheri and Green, 1991).
MZ (quartz-sericite-suLphide schist)- is a black, fine grained,
sheared and brecciated rock containing quartz, sericite, pyrite,
local carbonate and minor chlorite, feldspar, chalcopyrite,
sphalerite and galena. The MZ contains less silica and sericite
than the MQ and MV afteration types and is less strongly
foliated, and has a higher sulphide content at about five per cent.
MZ is volumetrically the most abundant alteration type in the
mineralised zone, ranging between 2 and 30 m thick and is
present stratigraphically above and below the MQ and MV
alteration types. It contains discontinuous lenses of MQ and MY.
Petrographic studies support a felsic epiclastic origin for the MZ,
MV, and MQ alteration types (Taheri and Green, 1991). The MZ
averages 0.6 glt Au.
MV (quartz-sericite-carbonate-suphide schist)- is a yellow-
green, fine grained, highly foliated rock containing quartz,
sericite, pyrite and local carbonate and minor chlorite, feldspar,
chalcopyrite, sphalerite, and galena and rare purple fluorite. The
total sulphide content is less than one per cent. MV is the second
most volumetrically abundant alteration type in the mineralised
zone, followed by MQ and MP. The MV forms I to 15 m thick
lenses and commonly envelops the MQ alteration type. It
extends for tens of metres beyond the most northerly lenses of
MQ and is more extensively developed on the hangingwall side
of the MQ, where it is associated with elevated gold values and
base metal mineralisation. The MV averages 4.0 g/t Au.
MQ (massive quartz-suLphide-native goLd) - is a grey, cream,
or pink massive to recrystallised brecciated quartz rock with
minor muscovite, sericite, pyrite, carbonate and chalcopyrite,
with lesser galena and sphalerite and rare gold and bismuth
metal. Si02 forms 90 - 95 per cent of the rock mass and the total
sulphide content is about two per cent. It forms lenses 2 to 50 m
long and up to 8 m wide. RemobiJised sulphides, gold, and
carbonates till late horizontal tension gashes in the MQ formed
during Devonian deformation. The MQ averages 55 g/t Au.
Thicker zones of MQ contain higher gold values.
MP (massive pyrite-carbonate-quartz±goLd) - is a bronze-
black massive pyritic rock containing 40 to 80 per cent pyrite
28 Launceston. 10 - 14 November 1997 Mining Geology Conference

5. with interstitial carbonate and quartz. It forms discontinuous
lenses about 10 to 20 cm thick and a few metres long and is
hosted in the MZ. It has undergone extensive fracturing,
brecciation, and recrystallisation.
CB (massive carbonate) - The CB alteration type forms the
hangingwall of A Zone type alteration and occurs as white to
pink laterally discontinuous lenses up to 5 m thick containing
carbonate, sericite, sulphides, albite and red jasper fragments,
with a banded to stylolitic texture. The massive carbonate beds
are commonly interbedded with feldspar phyric sandstones and
siltstones of the Lynchford Member. It is possible that the CB is
an original lithology rather than an alteration product.
AS (albite-silica alteration) - occurs to the east of the A Zone
alteration and overprints volcaniclastics of the Lynchford
Member and pervades up through the Mount Julia Member and
the overlying Zig Zag Hill Formation. The alteration occurs as
an irregular pervasive flood of massive white or orange fine
grained silica and albite, completely destroying original textures
of the volcaniclastics.
Mineralisation
Gold at the Henty Mine is present as both free gold and gold-rich
electrum associated with chalcopyrite and galena in the main
mineralised zone (MQ, MV, MZ). The electrum forms irregular
grains ranging in size from I J..lm to about 15 mm. Initial studies
(Taheri and Green, 1991) show that most grains are on the order
of 5 to 15 ~lm and are found in microfractures, pyrite grain
boundaries, cleavages in galena, or as inclusions in quartz,
carbonate, pyrite, chalcopyrite and galena. The presence of gold
along grain boundaries or microfractures indicates remobilisation
of gold and later recrystallisation. Gold is also found in
subhorizontal quartz tension gash veins in MQ associated with
Devonian deformation.
Ore genesis
The initial Cambrian system formed sub seafloor by similar
processes to VMS systems but was probably low in base metals
and had a gold content of about 2 to 3 g/t Au. Devonian
deformation remobilised the gold to form the current high grade
gold deposit.
One model (eg Halley and Roberts, in press) suggests that
Henty is a Middle to Late Cambrian volcanogenic gold deposit,
formed in the actively filling shallow water Dundas Trough, at
the footwall of the early Henty Fault. The deposit is laterally and
vertically zoned away from the Henty Fault, with proximal sub-
sea floor quartz-sericite alteration and Cu, Au, Pb, and Bi
mineralisation and distal10w level Zn, Pb, carbonate, jasper, and
GEOLOGY AND MINERALISATION OF THE HENTY GOLD MINE
massive pyrite lenses at or below the sea floor. This
interpretation has mineralisation synchronous with deposition of
the Lynchford Member and positioned within 10 to 20 m of the
sea floor.
A second interpretation is that the sea floor position was
higher in the stratigraphy during mineralisation, possibly at the
top of the Tyndall Group. Hangingwall carbonate horizons may
be replacement rather than primary features, and Na stripped
from the volcanic rocks in the A zone may have remobilised into
the overlying intense albite-silica alteration."
Both models suggest that subsequent Devonian deformation
repeatedly fractured and veined the brittle MQ lenses and
intensely foliated the more ductile, sericitic MV lenses. Au,
pyrite, chalcopyrite, and galena sourced from the MV and MZ
lithologies locally remobilised on the scale of a few metres, and
recrystallised into late brittle fractures in the MQ.
ACKNOWLEDGEMENTS
Thanks to S Dunham for the opportunity to write this paper and
for subsequent discussions regarding the history and geology of
the Henty Mine. Thanks also to W Edgar and D Glover of Henty
Gold Mine, Dr R Berry at the University of Tasmania, and Kim
Denwer and Michael Vicary of RGC Exploration for reviewing
drafts of the paper.
REFERENCES
Corbett, K D, Reid, K 0, Corbet!, E B, Green, G R, Wells, K and
Sheppard, N W, 1974. The Mount Read Volcanics and Cambrian-
Ordovician relationships at Queenstown, Tasmania, Journal of the
Geological Society (!{Australia, 21:173-186.
Corbet!, K D and Lees, T C, 1987. Stratigraphic and structural
relationships and evidence for Cambrian deformation at the western
margin of the Mt Read Volcanics, Tasmania, Australian Journal of
Earth Sciences, 34:45-67.
Corbett, K D, 1988. Geological compilation map of the Mt Read
Volcanics and associated rocks, Hellyer to South Darwin Peaks,
Tasmanian Department of Mines, Mt Read Volcanics Project Map 2.
COl'bet!, K D, 1992. Stratigraphic-volcanic setting of massive sulfide
deposits in the Cambrian Mount Read Volcanics, Tasmania,
Economic Geology, 87:564-586.
Halley, S Wand Roberts, R H, in press. Henty: A shallow-water, gold-
rich VMS deposit in western Tasmania.
Taheri, J and Green, G R, 1991. The origin of the gold mineralisation at
the Henty Prospect, Division of Mines and Mineral Resources.
White, M J and McPhie, J, 1996. Stratigraphy and paleovolcanology of
the Cambrian Tyndall Group, Mt Read Volcanics, western Tasmania,
Australian Journal (!{Earth Sciences, 43:147-159.
Mining Geology Conference Launceston, 10- 14 November 1997 29

6. 30 Launceslon, 10 - 14 November 1997 Mining Geology Conference