This study comprises the first systematic classification of native gold geochemistry within alkalic porphyry Cu-Au systems and the placer expression of such systems. The geochemistry and mineral associations of gold from four alkalic porphyry deposits in British Columbia, Canada (Afton, Mount Milligan, Mount Polley and Copper Mountain) have been compared to comment on the likely detrital gold expression of similar systems globally. Populations of gold grains collected from in situ hypogene mineralization as well as fluvial deposits downstream of these deposits have been characterized in terms of their alloy composition (Au, Ag, Cu, Hg, and Pd) and associated mineral inclusions. These data are combined to generate a ‘microchemical signature’. Gold compositions vary according to the alteration zone within a porphyry system. Previous compositional studies of gold in porphyry systems have focused on the most economically important ore associated with potassic alteration, wherein native gold hosted either by bornite or chalcopyrite comprises tiny blebs (typically 5-20 μm), containing detectable Cu in the gold alloy to a maximum of around 5% but quite variable (2-30 wt.%) Ag. The presence of such grains have been confirmed in hypogene ore from the four systems studied, but they have been shown to be compositionally distinct from detrital gold collected from nearby fluvial placers which exhibit a strong Pd and Hg signature, both in the alloy and as mineral inclusions. Several workers have described late stage veins associated with alkalic porphyries which contain distinctive Pd-Hg bearing minerals in association with other sulphides and sulphosalts. This unusual mineralogy has been observed in the mineral inclusion suites of populations of detrital gold grains collected in the environs of the porphyry systems. We conclude that whilst the micron-scale Cu-rich gold grains formed in potassically altered rocks are in general too small to be recovered during routine sediment sampling, those formed in later stage hydrothermal systems are larger, and exhibit a distinctive microchemical signature which may be differentiated from those of gold formed in other mineralizing systems. Consequently, compositional studies of detrital gold could underpin a mineral indicator methodology in the exploration for alkalic porphyry Cu-Au deposits.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)