Shallow and Deep-level Alkalic Porphyry deposits (NTS 93A, 93N, 104G)
- Dick Tosdal, Mineral Deposits Research Unit

Alkalic gold-(copper) deposits are of increasing economic significance and can be an attractive exploration target. They include some of the world's highest grade and largest porphyry-related gold resources (e.g., Ridgeway: 77 Mt @ 1.87 g/t Au, 0.63 % Cu -- 5.09 Moz Au; Galore Creek resource of 13.75 Moz Au, 156.4 Moz Ag, and 12.0 billion pounds Cu), as well as some of the largest gold accumulations in epithermal settings (Ladolam, ~48.6 Moz Au; Porgera, ~19.6 Moz; Cripple Creek, ~26.7 Moz; and Emperor ~9.5 Moz) (http://geopubs.wr.usgs.gov/open-file/of99-556/). Furthermore, alkalic porphyry Cu deposits contribute to the economy of British Columbia, and are driving significant new exploration in the province. The economic importance of the alkalic class of porphyry deposits to British Columbia is thus well established. However, exploration for them is difficult because of their small footprint and alteration assemblages that are different in significant details from those characteristic of porphyry Cu deposits associated with subalkalic igneous complexes. Hence, there is a need to better define the characteristics of the somewhat anomalous but potentially very metal-rich alkalic porphyry systems from the academic perspective of building a holistic model but also from a practical perspective of providing simple tools for exploration in British Columbia.

Alkalic Porphyry Deposits Study Sites

This multi-disciplinary research project will advance the understanding of the characteristics of individual alkalic porphyry systems and integrate that information into a holistic model for the deposit class. The project will address fundamental questions regarding deposit genesis that will provide practical criteria useful for exploration. The proposed project is part of a larger project investigating epithermal and porphyry environments associated with alkalic rocks.

More information on this project can be found on MDRU's website.

Posters and Presentations
2009 : Breccia Systems in the Alkalic Porphyry Environment; Examples from the Galore Creek District, Northwestern British Columbia
- Mineral Exploration Roundup Poster (pdf, 9.59MB)


2008 : Towards an integrated model for alkalic porphyry and epithermal deposits in British Columbia
- Mineral Exploration Roundup Poster (pdf, 2.66MB)


2007 : Towards an integrated model for alkalic porphyry and epithermal deposits in British Columbia
- Mineral Exploration Roundup Poster (pdf, 5.08MB)

Technical Articles
2010: "Coherent and clastic rocks in the Southwest Zone alkalic porphyry copper-gold system, Galore Creek, northwestern British Columbia (NTS 104G)"
- Summary of Activities 2009, Report 2010-1 p.87-104 (pdf, 7.8 MB)

2009: "Distribution of alteration in an alkalic porphyry copper-gold deposit at Mount Milligan, central British Columbia"
- Summary of Activities 2008, Report 2009-1 p.33-48 (pdf, 20MB)


2008: "Hydrothermal breccia in the Mount Polley alkalic prophyry copper-gold deposit, British Columbia (NTS 093A/12)"
- Summary of Activities 2007, Report 2008-1 p.105-114 (pdf, 4.70MB)


2007: "Toward an Integrated Model for Alkalic Porphyry Copper Deposits in British Columbia (NTS 093A, N; 104G)"
- Geological Fieldwork 2006, Paper 2007-1 p.259-274 (pdf, 4.82MB)

Final Deliverables
M.Sc. Thesis - K. Byrne
The southwest zone breccia-centered silica-undersaturated alkalic porphyry Cu-Au deposit, Galore Creek, B.C : magmatic-hydrothermal evolution and zonation, and a hydrothermal biotite perspective - University of British Columbia, 2009 - Available digitally through the University of British Columbia (pdf, 7.381MB)

Abstract: Situated in northwest B.C Canada, the Southwest Zone Cu-Au breccia-centered deposit is one of twelve mineralized centers in the Galore Creek alkalic porphyry district. Formed in an island arc-setting outboard of ancestral North America in the lateTriassic, deposits in the Galore Creek district have a combined measured and indicated resource of 785.7 Mt at 0.52 per cent Cu, 0.29 g/t Au and 4.87g/t Ag. Mineralisation in the Southwest Zone is centered in narrow hydrothermally cemented breccias. Composite dikes of megacrystic orthoclase-phyric syenite and megacrystic orthoclase and plagioclase-phyric monzonite are cut by polylithic, poorly sorted pebblecobble clast size matrix-rich breccias. Hydrothermally cemented breccias, characterised by the cement (infill) assemblage phlogopite ± K-feldspar ± magnetite ± anhydrite ± diopside ± suiphide, overprint the western contact between matrix-bearing breccias and megacrystic porphyries. Coeval with cemented breccia formation, intrusions of biotite phyric monzodiorite occur at the matrix-bearing breccia-wall rock contact and in the matrix-bearing breccias. Biotite-phyric monzodiorite and the principal cemented breccia domains are co-spatial and syn-Cu-Au. Drilling has outlined a zone of >0.3% hypogene Cu approximately 20-100m thick, 500m wide and 400m in length that strikes 100, dips 45-60°S, and has a semiellipsoidal morphology. This mineralisation is coincident with potassic (stage D) alteration and infill. Cu-poor, diopside-dominated (calc-[potassic]) alteration formed contemporaneously with, and locally flanks, potassic-D infill. Sulphide minerals are zoned from a core of chalcopyrite-bomite, to chalcopyrite>pyrite, to pyrite>chalcopyrite out to pyrite only. Garnet-bearing peripheral propylitic alteration overlaps with a pyrite and Au-halo and locally overprints potassic and calc-Q,otassic) assemblages. Based on electron microprobe analysis, systematic spatial variations in Ti-content and Feł⁺/Fe˛⁺ of infill biotite are evident. Increases in Ti-contents and Feł⁺/Fe˛⁺ overlap with positive gradients in Cu concentration, taken with interpreted alteration reactions, this suggest Cu-deposition is caused by decreasing fO₂ coupled with an increase in pH at 420-475°C. Low log(fH₂/fHF)₂, determined from infill biotite, distinguish potassic fluids in the Southwest Zone, and other alkalic porphyry deposits, from fluids in calcalkalic systems and reflects the contrasting magmatic composition.

M.Sc. Thesis - M.L. Jackson
Evolution of the Northeast zone breccia body, Mount Polley mine, British Columbia - University of British Columbia, 2008 - Available digitally through University of British Columbia (pdf, 14.2MB)

Abstract: The Mount Polley deposit in north-central British Columbia is a Cu-Au porphyry system related to alkaline magmatism. The deposit is composed of numerous discrete zones of Cu-Au mineralization related to bodies of hydrothermal breccia, including the Northeast zone which hosted a pre-mining proven and probable reserve of approximately 11 Mt of ore averaging 0.88% Cu and 0.28 g/t Au. The polylithic breccia body at the Northeast zone is irregular in shape and intruded by multiple generations of post-mineral dykes. Post-mineral faults cut the breccia, making the original breccia geometry unclear. 11,000 meters of core logging along two vertical sections perpendicular to the long axis of the orebody show that a significant percentage (locally >50%) of the breccia is composed of K-feldspar-phyric monzonite porphyry clasts. Many of these have globular shapes, implying that this material was ductile at the time of brecciation. Cu-sulfides are most abundant in and immediately above zones containing the greatest concentration of K feldspar-phyric monzonite porphyry clasts. This relationship suggests that the fluids responsible for both mineralization and brecciation originated during this phase of intrusion. Most of the Cu-Au-bearing sulfide minerals form hydrothermal chemical infill in the breccia. Where fine-grained clastic material is abundant or where movement of clasts was minimal, sulfides are generally less abundant or absent. The entire breccia body of the Northeast Zone at Mount Polley appears to be the result of a single brecciation event. Variation in breccia character within the body is attributable to variations in fluid flux, pre-existing rock character, and fluidization processes. Furthermore, the permeability structure established during the brecciation event exerted the fundamental control over ore distribution within the breccia body.

M.Sc. Thesis - C.P. Jago
Metal- and alteration-zoning, and hydrothermal flow paths at the moderately-tilted, silica-saturated Mt. Milligan copper-gold alkalic porphyry deposit - University of British Columbia, 2008 - Available digitally through University of British Columbia (pdf, 125MB)

Abstract: The Mt. Milligan deposit is a tilted (~45°) Cu-Au alkalic porphyry located 155 km northwest of Prince George, B.C., Canada. It is the youngest of the BC alkalic porphyry deposits, all of which formed between 210 to 180 Ma in an extensive belt of K-enriched rocks related to the accretion of the Quesnellia-Stikinia superterrane to ancestral North America. Mt. Milligan has a measured and indicated resource of 205.9 million tonnes at 0.60 g/t Au and 0.25% Cu containing 3.7 million oz. gold, and 1.12 billion lb. copper. Shoshonitic volcanic and volcaniclastic andesites host mineralization. These have been intruded by a composite monzonitic stock (MBX stock), and associated sill (Rainbow Dike). Early disseminated chalcopyrite-magnetite and accessory quartz veins are associated with K-feldspar alteration in the MBX stock. A halo of biotite alteration with less extensive magnetite replaces host rocks within a ~150 m zone surrounding the stock, while K-feldpsar alteration extends along the Rainbow Dike and permeable epiclastic horizons. Peripheral albite-actinolite-epidote assemblages surround the K-silicate zone. Albite-actinolite occurs at depth, and epidote dominates laterally. Copper and Au grade are maximal where the albite-actinolite assemblage overprints biotite alteration. Gold grade is moderate in association with epidote, whereas Cu is depleted. The post-mineral Rainbow Fault separates the core Cu-rich zone from a downthrown Au-rich zone. A similar zonation of metals occurs in the hanging-wall (66 zone), where a Cu-bearing, potassically-altered trachytic horizon transitions to a funnel-shaped zone of pyrite-dolomite-sericite-chlorite alteration with elevated gold. Sulfide S-isotope compositions range from -4.79 δ34S in the central Cu-Au orebody to near-zero values at the system periphery, typical of alkalic porphyries. Sulfur isotope contours reflect the magmatic-hydrothermal fluid evolution, and indicate late-stage ingress of peripheral fluids into the Cu-Au zone. Carbonate C- and O-isotope compositions corroborate the magmatic fluid path from the Cu-Au rich zone to Au-rich zone with decreasing depth. Strontium isotopic compositions of peripheral alteration minerals indicate a laterally increasing meteoric fluid component. Changes in major- and trace element composition of epidote and pyrite across the deposit are also systematic. These provide additional vectors to ore, and confirm the kinematics of the Rainbow Fault.

Ph.D. Thesis - J. Micko
The geology and genesis of the Central Zone alkalic copper-gold porphyry deposit, Galore Creek district, northwestern British Columbia, Canada - University of British Columbia, 2010 - Available digitally through the University of British Columbia (pdf, 12.98 MB)

Abstract: Located in the Late Triassic Galore Creek alkalic Cu-Au porphyry district in northwestern British Columbia, the Central Zone deposit represents the end-member of the silica-undersaturated class of alkalic porphyry systems. The deposit is hosted by volcano-sedimentary rocks of the Middle to Upper Triassic Stuhini Group that were intruded by a syenite-monzonite complex and hydrothermal breccias. Post-mineral tilt (45 to 60° W-SW) provides an opportunity to examine a vertically extensive depth range of the system, and the impact of host rocks and a redox control on the precipitation of sulfide and silicate alteration minerals. Early mineralization associated with potassic alteration is dominated by gold-bearing chalcopyrite + bornite (Cu:Au ~ 2:1). A second gold-poor mineralization event is associated with calc-potassic alteration and dramatically changes the Cu:Au ratio (5:1) in the core of the Central Zone. In general, greatest Cu-Au concentrations overlap lithological contacts characterized by contrasting ferromagnesian mineral content, thus forming redox gradients. Sulfur isotopic compositions emphasize the importance of fO₂ conditions in ore deposition. Sulfides in highly mineralized centers are characterized by moderately negative δ₃₄Ssulfide values (-10.66‰ to -7.84‰), whereas sulfides deposited distally show highly negative δ₃₄Ssulfide values (-17.13‰ to -4.03‰). These data suggest that the interaction of sulfate-rich (SO₄˛-(aq)) fluids with varying amounts of Fe˛⁺-bearing minerals in host rocks increased H₂S/SO₄˛- leading to formation of reduced S, and precipitation of sulfide minerals. Trace elements such as V and As in host rocks and Eu˛⁺ in hydrothermal garnet reflect the same redox influence. Vanadium and As are soluble under highly oxidizing conditions. The shift in oxidation state facilitates their incorporation in alteration minerals. Thus, highest V (>700ppm) and As (>40ppm) concentrations form halos distally to the redox gradients and ore bodies. Hydrothermal garnets near lithologic contacts contain excess Eu˛⁺. In contrast to V and As, Eu˛⁺ is soluble in reduced fO₂ conditions and precipitates close to the redox gradient. This study demonstrates that redox is the dominant control on ore deposition in the Central Zone. Recognizing redox changes may provide a valuable guide for future exploration in the Galore Creek district and perhaps other alkalic Cu-Au porphyry systems worldwide.

Ph.D. Thesis - H.E. Pass
Breccia-hosted chemical and mineralogical zonation patterns of the Northeast Zone, Mt. Polley Cu-Ag-Au alkalic porphyry deposit, British Columbia, Canada - University of Tasmania, 2010 - Available digitally through THE University of Tasmania

 



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