PORPHYRY Cu+/-Mo+/-Au L04 by Andre
Panteleyev British Columbia Geological Survey |
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Panteleyev, A. (1995): Porphyry
Cu+/-Mo+/-Au, in Selected British Columbia Mineral Deposit Profiles,
Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E.,
Editors, British Columbia Ministry of Energy of Employment and
Investment, Open File 1995-20, pages 87-92.
IDENTIFICATION
SYNONYM: Calcalkaline porphyry Cu,
Cu-Mo, Cu-Au.
COMMODITIES (BYPRODUCTS):
Cu, Mo and Au are generally present but quantities range from
insufficient for economic recovery to major ore constituents. Minor
Ag in most deposits; rare recovery of Re from Island Copper
mine.
EXAMPLES (British Columbia -
Canada/International):
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Volcanic type deposits (Cu + Au * Mo) - Fish
Lake (092O041), Kemess (094E021,094), Hushamu (EXPO, 092L240),
Red Dog (092L200), Poison Mountain (092O046), Bell (093M001),
Morrison (093M007), Island Copper (092L158); Dos Pobres
(USA); Far Southeast (Lepanto/Mankayan), Dizon, Guianaong,
Taysan and Santo Thomas II (Philippines), Frieda River and
Panguna (Papua New Guinea). |
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Classic deposits (Cu + Mo * Au) - Brenda
(092HNE047), Berg (093E046), Huckleberrry (093E037), Schaft
Creek (104G015); Casino (Yukon, Canada), Inspiration,
Morenci, Ray, Sierrita-Experanza, Twin Buttes, Kalamazoo and
Santa Rita (Arizona, USA), Bingham (Utah, USA),El Salvador,
(Chile), Bajo de la Alumbrera (Argentina). |
 |
Plutonic deposits (Cu * Mo) - Highland Valley
Copper (092ISE001,011,012,045), Gibraltar (093B012,007),
Catface (092F120); Chuquicamata, La Escondida and Quebrada
Blanca (Chile). |
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Stockworks of
quartz veinlets, quartz veins, closely spaced fractures and breccias
containing pyrite and chalcopyrite with lesser molybdenite, bornite
and magnetite occur in large zones of economically bulk-mineable
mineralization in or adjoining porphyritic intrusions and related
breccia bodies. Disseminated sulphide minerals are present,
generally in subordinate amounts. The mineralization is spatially,
temporally and genetically associated with hydrothermal alteration
of the hostrock intrusions and wallrocks.
TECTONIC SETTINGS: In orogenic
belts at convergent plate boundaries, commonly linked to
subduction-related magmatism. Also in association with emplacement
of high-level stocks during extensional tectonism related to
strike-slip faulting and back-arc spreading following continent
margin accretion.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: High-level (epizonal) stock emplacement levels in
volcano-plutonic arcs, commonly oceanic volcanic island and
continent-margin arcs. Virtually any type of country rock can be
mineralized, but commonly the high-level stocks and related dikes
intrude their coeval and cogenetic volcanic piles.
AGE OF MINERALIZATION: Two main
periods in the Canadian Cordillera: the Triassic/Jurassic (210-180
Ma) and Cretaceous/Tertiary (85-45 Ma). Elsewhere deposits are
mainly Tertiary, but range from Archean to Quaternary.
HOST/ASSOCIATED ROCK TYPES:
Intrusions range from coarse-grained phaneritic to porphyritic
stocks, batholiths and dike swarms; rarely pegmatitic. Compositions
range from calcalkaline quartz diorite to granodiorite and quartz
monzonite. Commonly there is multiple emplacement of successive
intrusive phases and a wide variety of breccias. Alkalic porphyry
Cu-Au deposits are associated with syenitic and other alkalic rocks
and are considered to be a a distinct deposit type (see model L03).
DEPOSIT FORM: Large zones of
hydrothermally altered rock contain quartz veins and stockworks,
sulphide-bearing veinlets; fractures and lesser disseminations in
areas up to 10 km2 in size, commonly coincident wholly or in part
with hydrothermal or intrusion breccias and dike swarms. Deposit
boundaries are determined by economic factors that outline ore zones
within larger areas of low-grade, concentrically zoned
mineralization. Cordilleran deposits are commonly subdivided
according to their morphology into three classes - classic, volcanic
and plutonic (see Sutherland Brown, 1976; McMillan and Panteleyev,
1988):
* Volcanic type deposits
(e.g. Island Copper) are associated with multiple intrusions in
subvolcanic settings of small stocks, sills, dikes and diverse
types of intrusive breccias. Reconstruction of volcanic landforms,
structures, vent-proximal extrusive deposits and subvolcanic
intrusive centres is possible in many cases, or can be inferred.
Mineralization at depths of 1 km, or less, is mainly associated
with breccia development or as lithologically controlled
preferential replacement in hostrocks with high primary
permeability. Propylitic alteration is widespread and generally
flanks early, centrally located potassic alteration; the latter is
commonly well mineralized. Younger mineralized phyllic alteration
commonly overprints the early mineralization. Barren advanced
argillic alteration is rarely present as a late, high-level
hydrothermal carapace. * Classic deposits (e.g., Berg)
are stock related with multiple emplacements at shallow depth (1
to 2 km) of generally equant, cylindrical porphyritic intrusions.
Numerous dikes and breccias of pre, intra, and post-mineralization
age modify the stock geometry. Orebodies occur along margins and
adjacent to intrusions as annular ore shells. Lateral outward
zoning of alteration and sulphide minerals from a weakly
mineralized potassic/propylitic core is usual. Surrounding ore
zones with potassic (commonly biotite-rich) or phyllic alteration
contain molybdenite * chalcopyrite, then chalcopyrite and a
generally widespread propylitic, barren pyritic aureole or 'halo'.
* Plutonic deposits (e.g., the Highland Valley
deposits) are found in large plutonic to batholithic intrusions
immobilized at relatively deep levels, say 2 to 4 km. Related
dikes and intrusive breccia bodies can be emplaced at shallower
levels. Hostrocks are phaneritic coarse grained to porphyritic.
The intrusions can display internal compositional differences as a
result of differentiation with gradational to sharp boundaries
between the different phases of magma emplacement. Local swarms of
dikes, many with associated breccias, and fault zones are sites of
mineralization. Orebodies around silicified alteration zones tend
to occur as diffuse vein stockworks carrying chalcopyrite, bornite
and minor pyrite in intensely fractured rocks but, overall,
sulphide minerals are sparse. Much of the early potassic and
phyllic alteration in central parts of orebodies is restricted to
the margins of mineralized fractures as selvages. Later
phyllic-argillic alteration forms envelopes on the veins and
fractures and is more pervasive and widespread. Propylitic
alteration is widespread but unobtrusive and is indicated by the
presence of rare pyrite with chloritized mafic minerals,
saussuritized plagioclase and small amounts of
epidote.
TEXTURE/STRUCTURE: Quartz,
quartz-sulphide and sulphide veinlets and stockworks; sulphide
grains in fractures and fracture selvages. Minor disseminated
sulphides commonly replacing primary mafic minerals. Quartz
phenocrysts can be partially resorbed and overgrown by
silica.
ORE MINERALOGY (Principal and
subordinate): Pyrite is the predominant sulphide
mineral; in some deposits the Fe oxide minerals magnetite, and
rarely hematite, are abundant. Ore minerals are chalcopyrite;
molybdenite, lesser bornite and rare (primary) chalcocite.
Subordinate minerals are tetrahedrite/tennantite, enargite and
minor gold , electrum and arsenopyrite. In many deposits late
veins commonly contain galena and sphalerite in a gangue of quartz,
calcite and barite.
GANGUE MINERALOGY (Principal and
subordinate): Gangue minerals in mineralized veins are
mainly quartz with lesser biotite, sericite, K-feldspar,
magnetite, chlorite, calcite, epidote, anhydrite and
tourmaline. Many of these minerals are also pervasive
alteration products of primary igneous mineral grains.
ALTERATION MINERALOGY: Quartz,
sericite, biotite, K-feldspar, albite, anhydrite/gypsum, magnetite,
actinolite, chlorite, epidote, calcite, clay minerals, tourmaline.
Early formed alteration can be overprinted by younger assemblages.
Central and early formed potassic zones (K-feldspar and biotite)
commonly coincide with ore. This alteration can be flanked in
volcanic hostrocks by biotite-rich rocks that grade outward into
propylitic rocks. The biotite is a fine-grained, 'shreddy' looking
secondary mineral that is commonly referred to as an early developed
biotite (EDB) or a 'biotite hornfels'. These older alteration
assemblages in cupriferous zones can be partially to completely
overprinted by later biotite and K-feldspar and then phyllic
(quartz-sericite-pyrite) alteration, less commonly argillic, and
rarely, in the uppermost parts of some ore deposits, advanced
argillic alteration (kaolinite-pyrophyllite) .
WEATHERING: Secondary (supergene)
zones carry chalcocite, covellite and other Cu*2S minerals
(digenite, djurleite, etc.), chrysocolla, native copper and copper
oxide, carbonate and sulphate minerals. Oxidized and leached zones
at surface are marked by ferruginous 'cappings' with supergene clay
minerals, limonite (goethite, hematite and jarosite) and residual
quartz.
ORE CONTROLS: Igneous contacts,
both internal between intrusive phases and external with wallrocks;
cupolas and the uppermost, bifurcating parts of stocks, dike swarms.
Breccias, mainly early formed intrusive and hydrothermal types.
Zones of most intensely developed fracturing give rise to ore-grade
vein stockworks, notably where there are coincident or intersecting
multiple mineralized fracture sets.
ASSOCIATED DEPOSIT TYPES: Skarn Cu
(K01),
porphyry Au (K02),
epithermal Au-Ag in low sulphidation type (H05)
or epithermal Cu-Au-Ag as high-sulphidation type enargite-bearing
veins (L01),
replacements and stockworks; auriferous and polymetallic base metal
quartz and quartz-carbonate veins (I01,
I05),
Au-Ag and base metal sulphide mantos and replacements in carbonate
and non- carbonate rocks (M01, M04), placer Au (C01,
C02).
COMMENTS: Subdivision of porphyry
copper deposits can be made on the basis of metal content, mainly
ratios between Cu, Mo and Au. This is a purely arbitrary,
economically based criterion, an artifact of mainly metal prices and
metallurgy. There are few differences in the style of mineralization
between deposits although the morphology of calcalkaline deposits
does provide a basis for subdivision into three distinct subtypes -
the 'volcanic, classic, and plutonic' types. A fundamental contrast
can be made on the compositional differences between calcalkaline
quartz-bearing porphyry copper deposits and the alkalic (silica
undersaturated) class. The alkalic porphyry copper deposits are
described in a separate model - L03.
EXPLORATION
GUIDES
GEOCHEMICAL SIGNATURE: Calcalkalic
systems can be zoned with a cupriferous (* Mo) ore zone having a
‘barren’, low-grade pyritic core and surrounded by a pyritic halo
with peripheral base and precious metal-bearing veins. Central zones
with Cu commonly have coincident Mo, Au and Ag with possibly Bi, W,
B and Sr. Peripheral enrichment in Pb, Zn, Mn, V, Sb, As, Se, Te,
Co, Ba, Rb and possibly Hg is documented. Overall the deposits are
large-scale repositories of sulphur, mainly in the form of metal
sulphides, chiefly pyrite.
GEOPHYSICAL SIGNATURE: Ore zones,
particularly those with higher Au content, can be associated with
magnetite-rich rocks and are indicated by magnetic surveys.
Alternatively the more intensely hydrothermally altered rocks,
particularly those with quartz-pyrite-sericite (phyllic) alteration
produce magnetic and resistivity lows. Pyritic haloes surrounding
cupriferous rocks respond well to induced polarization (I.P.)
surveys but in sulphide-poor systems the ore itself provides the
only significant IP response.
OTHER EXPLORATION GUIDES: Porphyry
deposits are marked by large-scale, zoned metal and alteration
assemblages. Ore zones can form within certain intrusive phases and
breccias or are present as vertical 'shells' or mineralized cupolas
around particular intrusive bodies. Weathering can produce a
pronounced vertical zonation with an oxidized, limonitic leached
zone at surface (leached capping), an underlying zone with copper
enrichment (supergene zone with secondary copper minerals) and at
depth a zone of primary mineralization (the hypogene
zone).
ECONOMIC
FACTORS
TYPICAL GRADE AND TONNAGE:
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Worldwide according Cox and Singer (1988) based on
their subdivision of 55 deposits into subtypes according to
metal ratios, typical porphyry Cu deposits contain (median
values): Porphyry Cu-Au: 160 Mt with 0.55 % Cu, 0.003 % Mo,
0.38 g/t Au and 1.7 g/t Ag. Porphyry Cu-Au-Mo: 390 Mt with
0.48 % Cu, 0.015 % Mo, 0.15 g/t Au and 1.6 g/t Ag. Porphyry
Cu-Mo: 500 Mt with 0.41 % Cu, 0.016 % Mo, 0.012 g/t Au and
1.22 g/t
Ag. |
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A
similar subdivision by Cox (1986) using a larger data base
results in: Porphyry Cu: 140 Mt with 0.54 %Cu, <0.002 % Mo,
<0.02g/t Au and <1 g/t Ag. Porphyry Cu-Au: 100 Mt with
0.5 %Cu, <0.002 % Mo, 0.38g/t Au and 1g/t Ag. (This
includes deposits from the British Columbia alkalic porphyry
class, B.C. model L03.) Porphyry Cu-Mo: 500 Mt with 0.42 % Cu,
0.016 % Mo, 0.012 g/t Au and 1.2 g/t
Ag. |
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British
Columbia porphyry Cu * Mo ± Au deposits range from <50 to
>900 Mt with commonly 0.2 to 0.5 % Cu, <0.1 to 0.6 g/t
Au, and 1 to 3 g/t Ag. Mo contents are variable from
negligible to 0.04 % Mo. Median values for 40 B.C. deposits
with reported reserves are: 115 Mt with 0.37 % Cu, *0.01 % Mo,
0.3g /t Au and 1.3 g/t Ag. |
ECONOMIC LIMITATIONS:
Mine production in British Columbia is from primary
(hypogene) ores. Rare exceptions are Afton mine where native copper
was recovered from an oxide zone, and Gibraltar and Bell mines where
incipient supergene enrichment has provided some economic
benefits.
END USES: Porphyry copper deposits
produce Cu and Mo concentrates, mainly for international export.
IMPORTANCE: Porphyry deposits
contain the largest reserves of Cu, significant Mo resources and
close to 50 % of Au reserves in British Columbia.
REFERENCES
Beane, R.E. and Titley, S.R.
(1981): Porphyry Copper Deposits Part II. Hydrothermal
Alteration and Mineralization; in 75th Anniversary Volume,
Economic Geology, pages 235-269.
Cox, D.P. (1986): Descriptive
Model of Porphyry Cu, also Porphyry Cu-Au and Porphyry Cu-Mo; in
Mineral Deposit Models; United States Geological Survey, Bulletin
1693, pages 76-81, also pages 110-114 and 115-119.
Cox, D.P. and Singer, D.A. (1988):
Distribution of Gold in Porphyry Copper Deposits; U.S. Geological
Survey, Open File Report 88-46, 23 pages.
Gustafson, L.B. and Hunt, J.P. (1975
): The Porphyry Copper Deposit at El Salvador, Chile;
Economic Geology, Volume 70, pages 857-912.
Lowell, J.D. and Guilbert, J.M.
(1970): Lateral and Vertical Alteration- Mineralization Zoning
in Porphyry Ore Deposits; Economic Geology, Volume 65, pages
373-408.
McMillan, W.J. (1991): Porphyry
Deposits in the Canadian Cordillera; in Ore Deposits, Tectonics and
Metallogeny in the Canadian Cordillera, B. C. Ministry of Energy,
Mines and Petroleum Resources, Paper 1991-4, pages 253-276.
McMillan, W.J. and Panteleyev, A.
(1988): Porphyry Copper Deposits; in Ore Deposit Models,
Roberts, R.G. and Sheahan, P.A., Editors, Geoscience Canada Reprint
Series 3, Geological Association of Canada, pages 45-58; also
in Geoscience Canada, Volume 7, Number 2, pages 52-63.
Schroeter, T. G., Editor (1995):
Porphyry Copper Deposits of the Northwestern Cordillera of North
America; Canadian Institute of Mining, Metallurgy and
Petroleum, Special Volume 46, in preparation.
Sutherland Brown, A., Editor,
(1976): Porphyry Deposits of the Canadian Cordillera;
Canadian Institute of Mining and Metallurgy, Special Volume
15, 510 pages.
Titley, S.R. (1982): Advances in
Geology of the Porphyry Copper Deposits, Southwestern North America;
The University of Arizona Press, Tucson, 560 pages.
Titley, S.R. and Beane, R.E.
(1981): Porphyry Copper Deposits Part I. Geologic Settings,
Petrology, and Tectogenesis, in 75th Anniversary Volume, Economic
Geology, pages 214-234.
February 5,
1995
[L01]
[L03]
[L04] [L05]
[L06]
[L07]
[L08]
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Last Updated June
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