Detrital Gold as a Deposit-specific Indicator Mineral by LA-IPS-MS Analysis

Gold derived from alkalic porphyry systems in BC has previously been shown to exhibit a generic Hg-Pd signature revealed in both alloy composition and the suite of minerals present as inclusions within gold particles. Development of an indicator mineral methodology based on this result has been hindered by the number of gold detrital gold grains required to confidently establish a signature, and the associated implications for the design of field exploration campaigns. \n\nTrace element analysis of detrital gold grains has been undertaken using Laser-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) in an attempt to identify more elemental discriminants than has been possible by analysis using electron microprobe (EMP), and consequently reduce the sample size necessary for accurate characterization. Two LA-ICP-MS instruments have been used which collect data in different ways. The quadrupole system has been applied to all grains studied. The mass spectrometer sequentially scans the plasma stream to generate an analysis of material liberated by continual ablation. In contrast, the Time of Flight (ToF) instrument makes a single ablation, but measures all elements simultaneously. This approach has been used with only a few samples, but has generated new understanding of the heterogeneity of natural gold. \n\nConsideration of the large element suite generated by LA-ICP-MS confirmed the elemental associations previously identified, and has facilitated characterization of gold populations in terms of the binary Au-Ag alloy and occurrence of minor alloying metals. Orogenic gold grains generate analyses which lie along a Au-Ag binary mixing line, but many gold grains from alkalic porphyry systems show clear deviation from this trend. Natural gold has been shown to be highly heterogeneous at the trace element level. Comparison of results from a LA quadrupole and a ToF system revealed that only a small number of elements are homogenously distributed in Au alloy. Most are present either as local concentrations or as inclusions of minerals too small to observe by SEM. This heterogeneity has profound implications for the interpretation of LA-ICP-MS data of natural gold generated by quadrupole systems, as the analyses obtained may be dependent upon the site of ablation within any particular gold grain. \n\nThe application of LA-ICP-MS to detrital gold grain characterization during exploration does not currently offer an advantage to the established workflow of microchemical characterization which measures alloy compositions using EMP and identifies the associated inclusions using SEM. Both microchemical characterization and LA-ICP-MS approaches currently require approximately equivalent numbers of detrital gold particles to generate useful information. However, the results of this study have raised the possibility that extreme partitioning of trace elements to mineral inclusions within the gold could form the basis of a methodology by which individual gold grains could be confidently ascribed to a specific source style.