AbstractsEarth & Environmental Science

Trace element analysis provides an efficient method of fingerprinting placer gold populations in order to characterize bedrock-source hypogene mineralization style and effects of supergene gold mobilization in surficial settings. LA-ICP-MS analysis, using the AuRM2 gold reference material as an external standard, measured concentrations of Mg, Al, Ti, V, Mn, Fe, Ni, Cu, Zn, As, Se, Rh, Pd, Sn, Sb, Te, Pt, Hg, Pb, Bi, and U in placer gold samples from: the Prophet Mine Australia, Nus River Colombia, Piaba Brazil, and four locations in British Columbia. Additional reference materials NIST610 and FAU7 were used to quantify the micro-homogeneity of AuRM2, monitor precision, and confirm accuracy. The AuRM2 reference material had not previously been utilized for micro-analysis, but is shown to be sufficiently homogenous at a micro-scale (64-108 µm). Precision of analyses of AuRM2 range between ±10-15% and accuracy is better than ±10%. Semi-quantitative concentrations for V, Hg, and U (0.3, 3.7, and 0.1 ppm respectively) were determined in AuRM2 along with Hg in NIST610 (0.6 ppm). Quantitative concentrations were determined for Se, Rh, Sb, and Te (24.9, 0.1, 0.3, and 2 ppm respectively) in FAU7. Placer gold in the central Okanagan from Mission Creek and the Winfield paleoplacer share the same hypogene trace element signature indicating that modern-day placer gold in Mission Creek is reworked from Miocene paleoplacers similar to the Winfield occurrence. Samples from Lambly Creek, also in the central Okanagan, exhibit two discrete trace element populations of native gold; one has elevated Cu indicative of greenstone-hosted orogenic gold, the other has a low Cu signature similar to intrusive-hosted mineralization. Both groups contain primary mineral inclusions and limited supergene rims indicating two proximal hypogene gold sources within the catchment of Lambly Creek. The samples from the Prophet Mine contain biologically precipitated supergene gold. Gold-rich rims are up to 100 µm thick indicating significant supergene gold accumulation in this placer occurrence. Analyses of regions of gold grains containing primary hypogene inclusions retain a distinct hypogene signature rich in base metals (Ni, Zn, Pb, and V) whereas others had an indistinct signature of supergene gold richer in Sb and Bi.