Trace elements in native gold by solution ICP-MS and their use in mineral exploration: A British Columbia example

An elegantly simple, aqua regia-based, ICP-MS analytical procedure is used to compare the trace element composition of density-separated alluvial native Au from seven stream silt samples with three samples of geographically-associated Au from a prospective ore deposit in central British Columbia. Not all of the alluvial Au could have come from the ore deposit based on present drainage. The silt sample Au, averaging four alluvial grains and totaling 12–250 μg per sample, generally yielded measurable concentrations for V, Fe, Cu, As, Pd, Ag, Sb, Pt and Bi. The bedrock Au samples represent the three dominant rock types in the showing. Their Au trace element compositions largely bracket the alluvial Au. Multidimensional scaling (exploratory statistics) shows that trace elements in the native Au form lithophile, chalcophile and siderophile groupings. This indicates that a small set of geochemical processes formed all the Au in one geologic environment. Previous work shows that Au from individual deposits has distinct assemblages of detectable elements. Given these observations and that detectable elements are the same in both the deposit and alluvial Au, and that concentrations in the former bracket those of the latter, it is concluded that the source of the alluvial Au has probably been identified. Apparently neither mineral inclusions nor weathering impaired fingerprinting of the Au. The simplicity of the approach indicates that this is a useful exploration tool for determining the bedrock source of alluvial Au. The study also shows that silt sample exploration in glaciated terrains must recognize that paleo-ice movement and paleo-stream directions can yield geographic distributions of alluvial Au that cannot be explained by present-day drainage patterns. Thus this simple analytical/exploration technique is potentially very useful to the exploration industry.