2. GENERAL CHARACTERISTICS OF
PORPHYRY Cu-Mo ORE DEPOSIT
Porphyry deposits are
large, low to medium
grade deposit in which
primary ore minerals are
dominantly structurally
controlled and which are
spatially and genetically
related to felsic to
intermediate porphyritic
intrusions.
Fig 1:
BINGHAM PORPHYRY Cu MINE, UTHA, USA.
3. SPATIAL DISTRIBUTION:
Porphyry Cu-Mo deposits occur throughout the world in a series of
extensive, relatively narrow, linear metallogenic provinces. They are
dominantly associated with orogenic belts in western North and South
America and around the western margin of Pacific basin particularly
within the southeast Asia Archipelago. Major deposits also occur in
Central Asia and Eastern North America.
FIG 2: DISTRIBUTION OF POROHYRY Cu-Mo ORES
4. TEMPORAL DISRIBUTION:
Porphyry Cu Mo deposits range in age from Archean to Recent, although
peak period of porphyry deposits are Jurassic, Cretaceous, Eocene and
Miocene age. Although porphyry deposits of Precambrian age are not so
common, important examples include Malanjkhand, India.
5. TECTONIC CONTROL:
Porphyry deposits occur mainly in areas of thickened crust, where
uplift was associated with crustal thickening over a shallow
subduction zone.
1. Compression resist magma ascending through upper crust, thus
inhibits volcanism.
2. Resultant shallow magma chambers in this compression zone are
larger than those of beneath extensional zones.
3. Fractionation process in this chamber promoted by inability to
erupt, resulting in volatile saturation and generation of large
volume of magmatic-hydrothermal fluid.
4. Compression restrict the number of apophyses formation on roof,
due to lack of steep faults; providing fluid focusing into a single
stock rather than a cluster.
5. Rapid uplift and erosion promotes efficient extraction and
transportation of magmatic-hydrothermal fluids due to abrupt
decompression.
6. MORPHOLOGY OF ORE:
Ore minerals in these deposits are
scattered throughout the host rock
either as disseminated
mineralization, or they are largely
and wholly restricted to quartz
veinlets which form a ramifying
complex called a stockwork –
generally mined by bulk mining
methods.
FIG 4: QUARTZ-PYRITE-CHALCOPYRITE VEIN STOCKWORKS
IN FELDSPAR (Bell deposit, Babine district, British
Columbia)
7. GRADE AND TONNAGE:
In porphyry Cu deposit, Cu
grade range from 0.2% to
more than 1%.
In porphyry Mo deposit, Mo
grades are 0.1% to 0.24%.
The tonnage generally range
from 10 to over 10,000.
FIG 5 : GRADE TO TONNAGE PLOT OF MAJOR
WORLD PORPHYRY DEPOSITS
8. MINERALOGY:
Principal ore minerals are
chalcopyrite, bornite,
chalcocite, tennantite,
other Cu sulphides and
sulphosalts, molybdenite
and electrum; associated
minerals include pyrite,
magnetite, quartz, biotite,
K-feldspar, anhydrite,
muscovite, clay minerals,
epidote and chlorite.
FIG 6: HAND SAMPLE SHOWING DISSEMINATED ORE MINERAL
CHALCOPYRITE, PYRITE, MAGNETITE..ETC
9. ALTERATION ZONES:
Generally four hydrothermal
alteration zones are present,
which from centre to outward
are – Potassic zone, Phyllic
zone, Argillic zone and
Porphylitic zone.
FIG 7: HYDROTHERMAL ZONES IN LOWELL-GUILBERT MODEL OF
PORPHYRY COPPER DEPOSITS.
12. Phyllic zone evolved from leaching of Na,
Ca, Mg by aluminusilicate and wholesale
replacement in zone.
Fe form cuprious pyrite which may run
upto 10 vol% of the rock
15. Isotopic composition of the hydrothermal
fluids responsible for the successive
alteration assemblages
1. evolving magmatic fluid with phase
separation
2. fluid rock interaction.
3. mixing of two isotopically distinct fluids.
17. Stage 1 represent primitive hydrothermal
fluid
Primitive fluid exhibit D depleted and O
enriched compared to water dissolved in
felsic magma
D depleted in residual magmatic fluid is
effect of degassing
18. Stage 2 – most D depleted due to the
minerals and isotopic composition injection
of new magmatic water in a
compositionally evolved hydrothermal fluid
system.
A large isotopic fractionation in D occur
due to injection of HT(>650C) saline
magmatic fluid into a cooling hydrothermal
system – consequence of the mixing
magmatic fluid to vapor and hyper saline
brine
19. Fluid rock interaction
Shift in the isotopic composition of the fluid
due to the net mass balance effect.
21. Highest D associate with lowest
temperature
Interpreted in terms of mixing between
magmatic composition and the expected
isotopic composition of paleomagmatic
water
22. GENESIS OF PORPHYRY Cu-Mo DEPOSIT
The most striking characteristics of porphyry Cu deposits when
compared with other hydrothermal ore bodies is their enormous
dimension- the size and shape of these deposits imply that the
hydrothermal solutions had passed through large volume of rock,
including country rock as well as parent pluton. The Genesis of
Porphyry copper deposit can be explained by the following stages
FIG : PORPHYRY COPPER MINE IN CHILI
24. STAGE 2
The outer part of the pluton coming in contact
with shallow crustal rock (cold), crystallizes
quickly (equigranular crystallization)
25. STAGE 3
Now, the centre of the pluton cools slowly,
crystallizing anhydrous minerals- magma
becoming richer in volatiles- which increases
vapor pressure
26. STAGE 4
At one time vapor pressure exceeds confining
pressure and retrograde boiling will appear –
and a rapidly boiling liquid escapes (increasing
the volume).
27. STAGE 5
The vapor pressure exceeds the confining
pressure and tensile strength of the rapid
brecciation occur
28. STAGE 6
Crystallization of solid phase is exothermic but bubble
formation is endothermic which results in decrease of
temperature rapidly, causing high amount of nucleation
and subsequently forms fine grain ground mass
surrounding large crystals - that is porphyritic texture
29. STAGE 7
The boiling liquid that gets separated from the magma
are enriched in chloride and bisulifde, chloride acts as a
transporting mechanism for base metals which also
fractionate strongly. Sulfur concentration depends on
fugasity of oxygen and thus gets fractionated strongly in
case of “I” type than “S” type
30. STAGE 8
When brecciation occurs a large amount of pressure
gets released and thus the fractionated liquid gets
separated into a low density gas phase (carrying H2O,
SO2, HCl, Br2O3, CO2) and a dense Cu bearing brine.
The low density gas flushed through the fractures first
followed by copper rich brine.
31. STAGE 9
The low density, low salinity, acidic, oxidized
liquid (gas) is believed to mix with the meteoric
water present in the surrounding and forms a
hybrid liquid
32. STAGE 10
Subsequent upwelling of magmatic hydrothermal brine
resulted in sulfide deposition. High grade copper deposit
is interpreted to have occurred in response to mixing of
hybrid liquid and metal rich brine. The second phase of
void infill was the main stage of mineralization
developing chalcopyrite, pyrite, bornite etc.
33. LOWELL – GUILBERT MODEL
Two hydrothermal system coexist in Porphyry Cu ore formation
First hydrothermal system constitute the magmatic water derived from
pluton, which causes potassium silicate alteration
As the pluton heats up the surrounding country rock, the thermal gradient
sets up a convective circulation of water in country rock
The relatively rapid gradients in pH, temperature and salinity across the
interface between these two hydrothermal system accounts for
concentration of copper around the boundary zone between the potassium
silicate and phyllic zones
34. ENRICHMENT OF COPPER
Copper has an affinity for octahedral sites
Number of octahedral sites is directly proportional to
(alumina)/(alkali) ratio
Thus copper rich melts tend to follow Calk-alkaline trend
35. 34S STUDY TO ACCOUNT FOR EVIDENCE
OF FLUID MIXING
The general observation is that whereever 34S value
is low it also signifies highest copper grade
36. IMPLICATION
The transition occurs over a vertical range of 100
mts. from a value of +13.4%o at depth to about – 4.1
at the top
This variation can be produced by precipitation from
a cooling reduced fluid
Such a large decrease over a short range would
require 150 degrees of cooling over a very narrow
vertical interval
Such enormous temperature gradients are rare in
hydrothermal systems, expect where mixing of fluids
have occurred
37. REFRENCE
Croo D R, Hollings P, 2005, Giant Porphyry Deposits: characteristic,
distribution and tectonic control , Economic Geology, vol 100, PP
801 to 818.
Evans A.M, An Introduction to Ore Geology
Frikken P.H, Crook D.R,2005, Mineralogical and isotopic zonation in
the sur – sur Tourmaline Braccia, Rio Blanco- Ios Brancos Cu-Mo
Deposit, Chile, Implication of Ore Genesis, Economic Geology, vol
100 PP 935-961.
Harris A.C, Golding S.D, White N.C,2005, Bajo dela Alumbrera
Copper Gold deposit: Stable isotope evidence for a porphyry related
hydrothermal system dominated by magmatic aqueous fluids,
Economic Geology, vol 100, pp 863 – 886.
Mookherjee A, Ore Genesis : A Holistic Approach, Allied Publishers.
Sindair W.D, Porphyry Deposits, Mineral Deposits of Canada, edited
by Wayne D Goodfellow.
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