SAMPLE Ag Co Cu Fe Ni Pb Zn
DESCRIPTION ppm ppm ppm % ppm ppm ppm
QFP 1 1.1 370 2170 21.9 132 45 986
.217%Cu + 370 ppm Cobalt Copper equivalent 0.71%
SAMPLE Ag Co Cu Fe Ni Pb Zn
DESCRIPTION ppm ppm ppm % ppm ppm ppm
QFP 2 2.7 824 2880 38.8 152 11 120
.28%Cu + 820 ppm Cobalt Copper equivalent 1.38%
Saturday, April 30, 2005
Wrangellia's Drift
Starting from a position below the paleoequator in the the ancient Pacific Ocean in the Carboniferous, the terrane Wrangellia drifted northward a distance of about 10,000 km and eventually collided with western Laurentia in the Cretaceous. Wrangellia is the only terrane shown on these sketch maps. The ancient Pacific was crowded by other isolated terranes. Some of these terranes eventually collided with Laurentia to form parts of the Cordillera, others formed parts of eastern Asia. The maps also depict the break-up of the supercontinent Pangaea (Ma = million years).
Wednesday, April 27, 2005
Mineral Deposit Profile L04
PORPHYRY Cu+/-Mo+/-Au
L04
by Andre Panteleyev
British Columbia Geological Survey
MDP Logo
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):
bullet 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).
bullet 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).
bullet 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:
bullet 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.
bullet 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.
bullet 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] [Published Profile Index]
[Deposit Profiles]
Last Updated June 13, 2003
L04
by Andre Panteleyev
British Columbia Geological Survey
MDP Logo
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):
bullet 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).
bullet 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).
bullet 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:
bullet 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.
bullet 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.
bullet 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] [Published Profile Index]
[Deposit Profiles]
Last Updated June 13, 2003
HPH Alkaline Porphyry Petrographic Report Photo
FELSIC (ALKALIC) VOLCANIC OR HIGH-LEVEL INTRUSIVE WITH AMYGDULES(?) OF ?ZEOLITE/NEPHELINE?, GARNET, SODIC CLINOPYROXENE IN MATRIX OF SERICITIZED KSPAR-GARNET-CLINOPYROXENE-SPHENE/RUTILE-EPIDOTE
One sample submitted for petrographic description and identification of a mineral circled by black marker (unfortunately, this marking was cut off during section preparation, so not visible to the petrographer). The hand specimen is grey-buff in colour, fine-grained, with an altered porphyritic or amygdular volcanic texture; the amygdules are filled with a mixture of minerals that are partly oxidized to orange-brown limonite. The rock is not magnetic, and shows no reaction to cold dilute HCl, but the matrix largely stains bright yellow for K-feldspar in the etched offcut. Modal mineralogy in polished thin section is approximately:
K-feldspar (primary?) 45%
Garnet 20%
Clinopyroxene (aegirine-augite?) 15%
Unidentified (zeolite? or nepheline?) 10%
Sericite (after feldspar) 5%
Chlorite 2-3%
Sphene/rutile 1-2%
Epidote <1%
Apatite <1%
Chalcopyrite (partly replaced by limonite) <1%
Limonite <1%
This sample consists mainly of about 15-20% amygdules (or possibly relict mafic phenocrysts) filled with or replaced by a colourless mineral containing abundant poikilitic inclusions of clinopyroxene and locally garnet plus rare apatite, in a matrix composed mainly of K-feldspar partly to largely replaced by fine-grained sericite, plus garnet, clinopyroxene(?) and minor sphene/rutile and epidote.
The amygdules have subrounded to subangular outlines up to 2-9 mm long and are primarily filled with coarse subhedral crystals of a colourless mineral up to 3 mm in diameter, with the following optical properties: low (first-order grey to white) birefringence, moderate negative relief compared to epoxy (n<1.55), possibly biaxial negative (no well oriented sections available for interference figures). Its identity is not clear, but it could be a zeolite or nepheline(?), although lack of parallel extinction argues against the latter. The included pyroxene, forming small sub- to euhedral crystals mostly <0.35 mm long, has pale green pleochroism and moderate extinction angle up to about 35 degrees, suggesting it may be a sodic clinopyroxene such as aegirine-augite (?). The pyroxene is locally partly altered to a very fine-grained mixture of ?sericite and ?chlorite or "hydrobiotite (Fe-rich chlorite). In places, the amygdules are rimmed by, or locally almost completely composed of, garnet. The garnet forms mainly euhedral to subhedral, pale yellow-brown crystals up to about 0.7 mm in diameter, in masses up to as much as 3.5 mm across. The garnet is locally unusually anisotropic, showing zoning parallel to the crystal margins. Locally, apatite forms stubby euhedral prisms up to 0.15 mm long, or part of the amygdule is filled with chlorite (sheafs of small subhedral flakes mostly <0.1 mm in diameter), or relics of strongly sericitized K-feldspar forming laths mostly <0.5 mm long).
The matrix is composed of relict laths of K-feldspar, mostly with euhedral outlines <0.5 mm long, with interstitial garnet (locally as aggregates or skeletal crystals, sieved by inclusions, up to 1 mm across), plus abundant fine semi-opaque crystals of sphene mostly <25 microns in diameter with minute (micron-sized) inclusions of rutile, and minor epidote (subhedra to 0.1 mm). There also appears to be relict pale green clinopyroxene as ragged subhedra <0.1 mm long (mainly replaced by fine-grained sericite and chlorite).The K-feldspar is commonly partly to locally completely replaced by fine-grained sericite; the garnet is the same as described above for the amygdules.
Traces of chalcopyrite, forming subhedral crystals mostly <0.25 mm in diameter, are found in both the amygdules, associated with the pyroxene, and (to a lesser degree) the matrix. The chalcopyrite is generally rimmed by or partly to completely replaced by limonite.
In summary, this appears to be a strongly altered, volcanic or high-level intrusive rock probably of alkalic composition to judge by the abundant, possibly primary, K-feldspar and possibly sodic clinopyroxene. It is likely that the grey to white, porcellaneous mineral you circled is either the garnet, or possibly the ?zeolite/?nepheline, both of which (with the clinopyroxene) fill the amygdules. Trace sulfides appear to be mainly chalcopyrite, partly oxidized to limonite.
One sample submitted for petrographic description and identification of a mineral circled by black marker (unfortunately, this marking was cut off during section preparation, so not visible to the petrographer). The hand specimen is grey-buff in colour, fine-grained, with an altered porphyritic or amygdular volcanic texture; the amygdules are filled with a mixture of minerals that are partly oxidized to orange-brown limonite. The rock is not magnetic, and shows no reaction to cold dilute HCl, but the matrix largely stains bright yellow for K-feldspar in the etched offcut. Modal mineralogy in polished thin section is approximately:
K-feldspar (primary?) 45%
Garnet 20%
Clinopyroxene (aegirine-augite?) 15%
Unidentified (zeolite? or nepheline?) 10%
Sericite (after feldspar) 5%
Chlorite 2-3%
Sphene/rutile 1-2%
Epidote <1%
Apatite <1%
Chalcopyrite (partly replaced by limonite) <1%
Limonite <1%
This sample consists mainly of about 15-20% amygdules (or possibly relict mafic phenocrysts) filled with or replaced by a colourless mineral containing abundant poikilitic inclusions of clinopyroxene and locally garnet plus rare apatite, in a matrix composed mainly of K-feldspar partly to largely replaced by fine-grained sericite, plus garnet, clinopyroxene(?) and minor sphene/rutile and epidote.
The amygdules have subrounded to subangular outlines up to 2-9 mm long and are primarily filled with coarse subhedral crystals of a colourless mineral up to 3 mm in diameter, with the following optical properties: low (first-order grey to white) birefringence, moderate negative relief compared to epoxy (n<1.55), possibly biaxial negative (no well oriented sections available for interference figures). Its identity is not clear, but it could be a zeolite or nepheline(?), although lack of parallel extinction argues against the latter. The included pyroxene, forming small sub- to euhedral crystals mostly <0.35 mm long, has pale green pleochroism and moderate extinction angle up to about 35 degrees, suggesting it may be a sodic clinopyroxene such as aegirine-augite (?). The pyroxene is locally partly altered to a very fine-grained mixture of ?sericite and ?chlorite or "hydrobiotite (Fe-rich chlorite). In places, the amygdules are rimmed by, or locally almost completely composed of, garnet. The garnet forms mainly euhedral to subhedral, pale yellow-brown crystals up to about 0.7 mm in diameter, in masses up to as much as 3.5 mm across. The garnet is locally unusually anisotropic, showing zoning parallel to the crystal margins. Locally, apatite forms stubby euhedral prisms up to 0.15 mm long, or part of the amygdule is filled with chlorite (sheafs of small subhedral flakes mostly <0.1 mm in diameter), or relics of strongly sericitized K-feldspar forming laths mostly <0.5 mm long).
The matrix is composed of relict laths of K-feldspar, mostly with euhedral outlines <0.5 mm long, with interstitial garnet (locally as aggregates or skeletal crystals, sieved by inclusions, up to 1 mm across), plus abundant fine semi-opaque crystals of sphene mostly <25 microns in diameter with minute (micron-sized) inclusions of rutile, and minor epidote (subhedra to 0.1 mm). There also appears to be relict pale green clinopyroxene as ragged subhedra <0.1 mm long (mainly replaced by fine-grained sericite and chlorite).The K-feldspar is commonly partly to locally completely replaced by fine-grained sericite; the garnet is the same as described above for the amygdules.
Traces of chalcopyrite, forming subhedral crystals mostly <0.25 mm in diameter, are found in both the amygdules, associated with the pyroxene, and (to a lesser degree) the matrix. The chalcopyrite is generally rimmed by or partly to completely replaced by limonite.
In summary, this appears to be a strongly altered, volcanic or high-level intrusive rock probably of alkalic composition to judge by the abundant, possibly primary, K-feldspar and possibly sodic clinopyroxene. It is likely that the grey to white, porcellaneous mineral you circled is either the garnet, or possibly the ?zeolite/?nepheline, both of which (with the clinopyroxene) fill the amygdules. Trace sulfides appear to be mainly chalcopyrite, partly oxidized to limonite.
HPH Alkaline Porphyry Petrographic Report Photo
FELSIC (ALKALIC) VOLCANIC OR HIGH-LEVEL INTRUSIVE WITH AMYGDULES(?) OF ?ZEOLITE/NEPHELINE?, GARNET, SODIC CLINOPYROXENE IN MATRIX OF SERICITIZED KSPAR-GARNET-CLINOPYROXENE-SPHENE/RUTILE-EPIDOTE
One sample submitted for petrographic description and identification of a mineral circled by black marker (unfortunately, this marking was cut off during section preparation, so not visible to the petrographer). The hand specimen is grey-buff in colour, fine-grained, with an altered porphyritic or amygdular volcanic texture; the amygdules are filled with a mixture of minerals that are partly oxidized to orange-brown limonite. The rock is not magnetic, and shows no reaction to cold dilute HCl, but the matrix largely stains bright yellow for K-feldspar in the etched offcut. Modal mineralogy in polished thin section is approximately:
K-feldspar (primary?) 45%
Garnet 20%
Clinopyroxene (aegirine-augite?) 15%
Unidentified (zeolite? or nepheline?) 10%
Sericite (after feldspar) 5%
Chlorite 2-3%
Sphene/rutile 1-2%
Epidote <1%
Apatite <1%
Chalcopyrite (partly replaced by limonite) <1%
Limonite <1%
This sample consists mainly of about 15-20% amygdules (or possibly relict mafic phenocrysts) filled with or replaced by a colourless mineral containing abundant poikilitic inclusions of clinopyroxene and locally garnet plus rare apatite, in a matrix composed mainly of K-feldspar partly to largely replaced by fine-grained sericite, plus garnet, clinopyroxene(?) and minor sphene/rutile and epidote.
The amygdules have subrounded to subangular outlines up to 2-9 mm long and are primarily filled with coarse subhedral crystals of a colourless mineral up to 3 mm in diameter, with the following optical properties: low (first-order grey to white) birefringence, moderate negative relief compared to epoxy (n<1.55), possibly biaxial negative (no well oriented sections available for interference figures). Its identity is not clear, but it could be a zeolite or nepheline(?), although lack of parallel extinction argues against the latter. The included pyroxene, forming small sub- to euhedral crystals mostly <0.35 mm long, has pale green pleochroism and moderate extinction angle up to about 35 degrees, suggesting it may be a sodic clinopyroxene such as aegirine-augite (?). The pyroxene is locally partly altered to a very fine-grained mixture of ?sericite and ?chlorite or "hydrobiotite (Fe-rich chlorite). In places, the amygdules are rimmed by, or locally almost completely composed of, garnet. The garnet forms mainly euhedral to subhedral, pale yellow-brown crystals up to about 0.7 mm in diameter, in masses up to as much as 3.5 mm across. The garnet is locally unusually anisotropic, showing zoning parallel to the crystal margins. Locally, apatite forms stubby euhedral prisms up to 0.15 mm long, or part of the amygdule is filled with chlorite (sheafs of small subhedral flakes mostly <0.1 mm in diameter), or relics of strongly sericitized K-feldspar forming laths mostly <0.5 mm long).
The matrix is composed of relict laths of K-feldspar, mostly with euhedral outlines <0.5 mm long, with interstitial garnet (locally as aggregates or skeletal crystals, sieved by inclusions, up to 1 mm across), plus abundant fine semi-opaque crystals of sphene mostly <25 microns in diameter with minute (micron-sized) inclusions of rutile, and minor epidote (subhedra to 0.1 mm). There also appears to be relict pale green clinopyroxene as ragged subhedra <0.1 mm long (mainly replaced by fine-grained sericite and chlorite).The K-feldspar is commonly partly to locally completely replaced by fine-grained sericite; the garnet is the same as described above for the amygdules.
Traces of chalcopyrite, forming subhedral crystals mostly <0.25 mm in diameter, are found in both the amygdules, associated with the pyroxene, and (to a lesser degree) the matrix. The chalcopyrite is generally rimmed by or partly to completely replaced by limonite.
In summary, this appears to be a strongly altered, volcanic or high-level intrusive rock probably of alkalic composition to judge by the abundant, possibly primary, K-feldspar and possibly sodic clinopyroxene. It is likely that the grey to white, porcellaneous mineral you circled is either the garnet, or possibly the ?zeolite/?nepheline, both of which (with the clinopyroxene) fill the amygdules. Trace sulfides appear to be mainly chalcopyrite, partly oxidized to limonite.
One sample submitted for petrographic description and identification of a mineral circled by black marker (unfortunately, this marking was cut off during section preparation, so not visible to the petrographer). The hand specimen is grey-buff in colour, fine-grained, with an altered porphyritic or amygdular volcanic texture; the amygdules are filled with a mixture of minerals that are partly oxidized to orange-brown limonite. The rock is not magnetic, and shows no reaction to cold dilute HCl, but the matrix largely stains bright yellow for K-feldspar in the etched offcut. Modal mineralogy in polished thin section is approximately:
K-feldspar (primary?) 45%
Garnet 20%
Clinopyroxene (aegirine-augite?) 15%
Unidentified (zeolite? or nepheline?) 10%
Sericite (after feldspar) 5%
Chlorite 2-3%
Sphene/rutile 1-2%
Epidote <1%
Apatite <1%
Chalcopyrite (partly replaced by limonite) <1%
Limonite <1%
This sample consists mainly of about 15-20% amygdules (or possibly relict mafic phenocrysts) filled with or replaced by a colourless mineral containing abundant poikilitic inclusions of clinopyroxene and locally garnet plus rare apatite, in a matrix composed mainly of K-feldspar partly to largely replaced by fine-grained sericite, plus garnet, clinopyroxene(?) and minor sphene/rutile and epidote.
The amygdules have subrounded to subangular outlines up to 2-9 mm long and are primarily filled with coarse subhedral crystals of a colourless mineral up to 3 mm in diameter, with the following optical properties: low (first-order grey to white) birefringence, moderate negative relief compared to epoxy (n<1.55), possibly biaxial negative (no well oriented sections available for interference figures). Its identity is not clear, but it could be a zeolite or nepheline(?), although lack of parallel extinction argues against the latter. The included pyroxene, forming small sub- to euhedral crystals mostly <0.35 mm long, has pale green pleochroism and moderate extinction angle up to about 35 degrees, suggesting it may be a sodic clinopyroxene such as aegirine-augite (?). The pyroxene is locally partly altered to a very fine-grained mixture of ?sericite and ?chlorite or "hydrobiotite (Fe-rich chlorite). In places, the amygdules are rimmed by, or locally almost completely composed of, garnet. The garnet forms mainly euhedral to subhedral, pale yellow-brown crystals up to about 0.7 mm in diameter, in masses up to as much as 3.5 mm across. The garnet is locally unusually anisotropic, showing zoning parallel to the crystal margins. Locally, apatite forms stubby euhedral prisms up to 0.15 mm long, or part of the amygdule is filled with chlorite (sheafs of small subhedral flakes mostly <0.1 mm in diameter), or relics of strongly sericitized K-feldspar forming laths mostly <0.5 mm long).
The matrix is composed of relict laths of K-feldspar, mostly with euhedral outlines <0.5 mm long, with interstitial garnet (locally as aggregates or skeletal crystals, sieved by inclusions, up to 1 mm across), plus abundant fine semi-opaque crystals of sphene mostly <25 microns in diameter with minute (micron-sized) inclusions of rutile, and minor epidote (subhedra to 0.1 mm). There also appears to be relict pale green clinopyroxene as ragged subhedra <0.1 mm long (mainly replaced by fine-grained sericite and chlorite).The K-feldspar is commonly partly to locally completely replaced by fine-grained sericite; the garnet is the same as described above for the amygdules.
Traces of chalcopyrite, forming subhedral crystals mostly <0.25 mm in diameter, are found in both the amygdules, associated with the pyroxene, and (to a lesser degree) the matrix. The chalcopyrite is generally rimmed by or partly to completely replaced by limonite.
In summary, this appears to be a strongly altered, volcanic or high-level intrusive rock probably of alkalic composition to judge by the abundant, possibly primary, K-feldspar and possibly sodic clinopyroxene. It is likely that the grey to white, porcellaneous mineral you circled is either the garnet, or possibly the ?zeolite/?nepheline, both of which (with the clinopyroxene) fill the amygdules. Trace sulfides appear to be mainly chalcopyrite, partly oxidized to limonite.
Subscribe to:
Posts (Atom)