CALC-ALKALINE PORPHYRY COPPER-MOLYBDENUM-GOLD-TUNGSTEN
CONCISE DESCRIPTION: Fracture-controlled quartz-sulphide veinlets and veins, and sulphide disseminations in fractures hosted by, or proximal to, high-level, calc-alkaline, intermediate to felsic, porphyritic intrusions.
GEOLOGICAL ENVIRONMENT
Host Rock Types: Spatial and genetic relationship to high-level (epizonal), calc-alkaline, intermediate to felsic stocks, dykes, sills, and breccia pipes, with porphyritic phases, that are intrusive into volcanic and sedimentary rocks. These commonly occur as subvolcanic intrusions to volcanic complexes. The porphyritic intrusions and/or the surrounding country rocks may host the mineralization. Multiple intrusive phases and brecciation are common. Typical general associations are: quartz monzonite to alkali feldspar granite: Mo-W; granodiorite to quartz monzonite: Cu-Mo; and diorite-quartz diorite-tonalite: Cu-Au-(Mo).
Rock Textures: Intrusions are medium- to coarse-grained phaneritic, with prominent phases that are feldspar, quartz-feldspar, and/or quartz porphyritic, typically with an aplitic groundmass. Various types and styles of intrusive and hydrothermal breccia are very common.
Ages of the Host Rocks and Mineralization: On a world scale they range from Archean to Tertiary. However, most of the economic deposits are Mesozoic to Tertiary. This is probably related to erosion, in that these deposits are generally formed within 5 km of the surface in orogenic belts and the older deposits are more likely to have been removed. In Saskatchewan, the known deposits are Paleoproterozoic.
Depositional Environment: High-level (< 5 km) porphyritic intrusion emplacement at convergent plate boundaries and in large-scale continental rifts. Often occur as subvolcanic intrusions in volcano-plutonic complexes. Magma transport and emplacement was facilitated by deep-seated faults. Volatile exsolution under very high pressures resulted in extensive, multiple-phase fracturing and breccia development in the
host rocks. High-temperature, magma-generated, hydrothermal fluids transported the metals towards the surface, where precipitation in the open fractures and breccias resulted, mainly in response to a rapid drop in temperature. Both Saskatchewan examples are interpreted to have formed in volcanic island arcs.
Tectonic Setting: Convergent plate boundaries represented by island arc and continental margin arc environments; and in large-scale continental rifts. Associated deep-seated faults, especially strike-slip, pull-apart systems that formed low pressure zones in the upper crust, and facilitated magma transport and emplacement and associated hydrothermal fluids. Local zones of intense fracturing and brecciation, formed by hydrostatic overpressure, are necessary as the metal deposition sites.
Associated Deposit Types: Epithermal Au-(Ag-base metal) deposits, especially the high-sulphidation subtype, are very commonly associated with the overlying volcanic complex. Also base metal skarn, replacement, and polymetallic vein deposit types are common peripherally. The Laurel Lake epithermal Au-Ag deposit is contained by the Missi Island Vent Complex that overlies the Amisk Lake Cu-Mo-Au porphyry deposit.
DEPOSIT DESCRIPTION
Mineralogy: Pyrite is the principal sulphide mineral. Ore minerals include chalcopyrite, with subordinate molybdenite and bornite, and minor to rare scheelite, chalcocite, electrum, gold, tetrahedrite, tennantite, enargite, tellurides, wolframite, cassiterite, bismuthinite, and arsenopyrite. Magnetite is common, while hematite is rare. Supergene zones may contain chalcocite, covellite, digenite, native Cu, and copper oxide minerals. Late veins may contain sphalerite and galena. Common gangue minerals include quartz, with subordinate biotite, K-feldspar, sericite, epidote, chlorite, calcite, anhydrite, and tourmaline, which may also be alteration minerals.
The classic mineral zonation is developed concentrically outward from the core from magnetite to molybdenite-(scheelite) to chalcocite + bornite to chalcopyrite + pyrite to pyrite to magnetite to peripheral gold + silver and sphalerite + galena. The pattern is usually much more complex and irregular, including missing relationships, in any given deposit. In gold-rich Cu porphyries, the Au tends to be centrally located with the Cu, with an increase in the Ag/Au ratio outward.
Textures and Styles of Mineralization: Veinlets, veins, and stockworks containing quartz, quartz- sulphide, and sulphide in fractures and breccias; and disseminated sulphide grains in fractures and replacing mafic minerals. Ore bodies have variable shapes that may include irregular, oval, cylindrical, and inverted cup. Deposits are typically very large with dimensions in plan of 100s to 1000s of m2 and a vertical extent of 100s to 1000s of metres.
Alteration: Hydrothermal alteration is extensive, pervasive, and typically zoned, both on a deposit scale and around individual veins and veinlets. Typical alteration assemblages are of four main types: 1) potassic: K-feldspar and/or biotite + quartz +\- amphibole, anhydrite, magnetite, sericite, and albite; 2) phyllic: quartz, sericite, and pyrite; 3) argillic: intermediate assemblage – quartz, illite, and pyrite +\- kaolinite, montmorillonite, smectite, and calcite; advanced assemblage – kaolinite-dickite, pyrophyllite, alunite, diaspore, and quartz; and 4) propylitic: quartz, chlorite, epidote, and calcite +\- albite and pyrite. Typical zonation consists of an inner potassic zone and an outer propylitic zone. Phyllic and intermediate argillic alteration are younger and are commonly superimposed between the potassic and propylitic zones, or as irregular to tabular zones superimposed on the older alteration and mineralization. Potassic alteration is typically coincident with the ore zone. In practice the alteration types and patterns may vary greatly between deposits. Advanced argillic alteration assemblages are often developed in the upper portion of the hydrothermal system and are commonly associated with high-sulphidation epithermal mineralization.
Geological Ore Controls: 1) convergent tectonic settings represented by volcanic island arcs and continental margin arcs; and large-scale continental rifts; 2) associated large-scale, deep-seated, extensional faults; 3) high-level, calc-alkaline, intermediate to felsic, porphyritic intrusions; commonly subvolcanic; 4) formation of extensive and multiphase fracturing and brecciation; 5) magma-generated hydrothermal fluid transport of metals with deposition, due mainly to a sharp decrease in temperature, in open fractures and breccias; 6) associated extensive, pervasive, and zoned alteration; and 7) ore zones are generally associated with areas of high fracture densities and potassic alteration.
Geochemical Signature: 1) zoned alteration (see Alteration); 2) metal zonation (see Mineralogy); 3) characterized by a sulphur-rich system; and 4) surficial geochemistry programs for associated metals.
Geophysical Signature: 1) airborne and ground magnetic surveys to detect magnetite-rich zones and as an aid to mapping; 2) induced polarization/resistivity surveys to outline disseminated sulphides; 3) resistivity surveys to help map alteration zones; 4) airborne and ground radiometric surveys to help delineate K-rich alteration zones; 5) audio-frequency magnetotelluric surveys to define the limits of the porphyry systems; and 6) short-wave infrared spectroscopy for clay alteration identification in the field.
EXAMPLES (with grades and tonnages)
World examples include Butte, Montana; Bingham Canyon, Utah; Highland Valley, B.C.; and Chuquicamata, Chile.
The two recognized examples in Saskatchewan are the: 1) Phantom Lake – Boot Lake Intrusive Complex near Flin Flon with a Au-W-Cu-Mo metal association; and 2) a suite of porphyritic dykes that underlie the Missi Island Vent Complex at Amisk Lake with a Cu-Mo-Au metal association. Neither has a defined
resource.
At the Phantom Lake – Boot Lake Complex, pyrite with associated chalcopyrite, and variable molybdenite, scheelite, arsenopyrite, sphalerite, and gold, together with quartz, occur in veins, stockworks, and as sulphide disseminations in fractures. Alteration is of two types: a) hematite-quartz-carbonate-epidote (propylitic); and b) sericite-quartz-carbonate (phyllic). Alteration is pervasive and restricted to the intrusion.
At Amisk Lake, pyrite, chalcopyrite, minor molybdenite, and local sphalerite and pyrrhotite occur within quartz-filled veinlets and as disseminations in fractures within the porphyritic dykes and associated volcanic rocks. Pervasive silicification and sericitization are associated. Propylitic alteration (chlorite, epidote, hematite, carbonate, and pyrite) has also been noted.
It has been suggested (SGS, 1991; p.47) that some of the gold deposits in the La Ronge Domain, particularly in the Waddy Lake area, may have some similarities to the porphyry deposit type.
The 1.76 Ga Nueltin Granite Suite, that contains phases of fluoritic, porphyritic granite and monzogranite, locally intrudes the Wollaston Domain and the eastern portion of the Mudjatik Domain. These intrusions may have some potential to contain porphyry Mo-W mineralization. Saskatchewan Mineral Deposit Index location 0705 is a drill-tested Mo occurrence associated with these granites.
SELECTED REFERENCES
Panteleyev, A. (1995): Porphyry Cu +\- Mo +\- Au, model L04; British Columbia Geological Survey,
Mineral Deposit Profiles, www.empr.gov.bc.ca/Mining/Geoscience/MineralDepositProfiles.
Pearson, J.G. (1987): Gold mineralization in the Flin Flon – Amisk Lake area, Saskatchewan; in Economic
Minerals of Saskatchewan, Saskatchewan Geological Society, Special Publication No. 8, p.37-43.
Rogers, M.C. and Fyon, J.A. (1995): Calc-alkalic porphyry Cu-Mo-(Au-W); in Descriptive Mineral
Deposit Models of Metallic and Industrial Deposit Types and Related Mineral Potential Assessment
Criteria, Ontario Geological Survey, Open File Report 5916, p.82-88.
Saskatchewan Geological Survey (1991): Base metals in Saskatchewan; Saskatchewan Energy and Mines,
prepared by Coombe Geoconsultants Ltd., Open File Report 91-1, p.38-40, 47, 73-74, 76-78.
Seedorff, E., Dilles, J.H., Proffett, J.M., Einaudi, M.T., Zurcher, L., Stavast, W.J.A., Johnson, D.A., and
Barton, M.D. (2005): Porphyry deposits: characteristics and origin of hypogene features; in Economic
Geology 100th Anniversary Volume, Society of Economic Geologists, p.251-298.
Sinclair, W.D. (2007): Porphyry deposits; in Mineral Deposits of Canada, A Synthesis of Major Deposit
Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods; Geological
Association of Canada, Mineral Deposits Division, Special Publication No. 5, p.223-244.
Reviewed By:
November, 2009
