New geochemical exploration tools to aid discovery of porphyry deposits - a case study from Resolution, Arizona

摘要

Porphyry deposits are a signature ore deposit type of magmatic arcs, and numerous examples have formed around the Pacific Rim. Porphyry deposits have distinctive geochemical and geophysical footprints that are centred on shallow crustal mineralising intrusive complexes. The footprints vary as a function of various parameters, including depth of erosion, protolith compositions and size of the hydrothermal system. When exploring for porphyry deposits, at shallow crustal levels, explorers must contend with lithocaps, domains of intense, texturally destructive, silicic, advanced argillic and argillic alteration that contain abundant pyrite. Lithocaps can form stratabound domain that extend for more than 10 km2 and may be more than 1 km thick. At deeper erosion levels in volcanic terrains, porphyry mineralisation can be found centred on hydrous, oxidized I-type intrusive complexes that were emplaced into the shallow crust. Stockwork quartz-sulfide veins characterise the core of the deposits and are typically associated with magnetite-bearing potassic alteration. The potassic assemblages may be intensely overprinted by phyllic and/or (in cases of extreme uplift and exhumation) advanced argillic alteration. The flanks of volcanic-hosted porphyry deposits are typically altered to propylitic alteration assemblages. This distal 'green rock' environment is a domain of weak to moderate hydrothermal alteration and provides significant challenges to explorers due to the low intensity of alteration, which produces subdued geochemical and geophysical anomalies that may extend 1-2 km laterally from the porphyry centre. At deeper erosion levels, explorers are generally searching for porphyry deposits within an extensive domain of green rock alteration.