Application of microchemical characterization of placer gold grains to exploration for epithermal gold mineralization in regions of poor exposure

The style of gold mineralization (e.g., mesothermal, porphyry, intrusion related, high-sulphidation epithermal, low sulphidation epithermal) is normally established at an early stage in the gold exploration process from the study of the mineralogical and lithological examination of outcrop, but this approach is problematic in areas of poor exposure. However, placer gold is frequently present in local drainages, and may be characterized according to the alloy composition and the suite of mineral inclusions. In the temperate areas which are the subject of this study, this microchemical signature of the cores of placer gold grains reflects that of gold from the hypogene source. Thus, informed speculation on the nature of the source bedrock mineralization is possible through comparison of the microchemical signature of placer grains with the generic characteristics of gold from different styles of mineralization.A holistic approach involving consideration of the local geology and microchemical signatures of populations of placer gold grains can be used to infer the presence of epithermal gold mineralization. Epithermal gold deposits are characterized by a wide range of Ag values in the electrum grains and a corresponding complex mineralogy of the opaque inclusion suite. Having established that the placer grains are likely to be derived from epithermal mineralization, the sulphidation style may be inferred through comparison of the sulphide mineral inclusion suite with the compositional fields which define the sulphidation state of the mineralization. In addition, suites of telluride minerals can be used to constrain the possible sulphidation state of the mineralization using log fTe2/log fS2 diagrams. Further useful information is available from the study of non-opaque inclusions which can be used to indicate sulphidation state and the type of wall rock alteration associated with the mineralization.Two case studies have been undertaken to illustrate the value of this approach. The Eureka Creek locality in Yukon Territory, Canada lies in an unglaciated region where bedrock is covered by mainly residual overburden derived from local bedrock sources. At Borland Glen in Scotland there have been several periods of glaciation which have influenced geomorphology in particular by forming deep thicknesses of till and rare bedrock exposures. In neither case has outcropping mineralization been identified. This study describes how consideration of compositional and morphological data of placer gold together with an understanding of the geological setting has indicated that both placers are derived from low-sulphidation epithermal mineralization.The Eureka Creek locality forms part of a goldfield with a recorded production of roughly 150,000 oz. The microchemical signature of the placer gold is consistent with the signature of the low-sulphidation epithermal gold and consequently distinct from the major gold mineralization found to the north in the Klondike District. Interpretation of the opaque mineral inclusion suite using log fS2–temperature diagrams indicates a temperature of formation near 200 °C.A similar approach has shown gold from a highly anomalous placer occurrence in Borland Glen, Ochill Hills, Scotland to be of local origin and most probably derived from an episode of low-sulphidation epithermal mineralization unique to the region. Three phases of mineralization corresponding to different mineral inclusion/gold alloy compositions have been identified, only one of which has the potential to be of economic importance. The sequence of deposition of the three phases has been established. The mineralization from which the placers were derived is hosted at least in part by propylitically altered wall rock. The areal extent of the surface expression of the most significant gold type has been investigated through identification of gold grains which exhibit a diagnostic internal texture of different phases of alloy compositions each associated with specific inclusion assemblages. This approach has proved an efficient tool in demarking a new target area within a region where placer gold derived from sources of low economic potential is also moderately abundant.This study has highlighted the potential value of characterizing the mineral inclusion suites of placer gold grains as part of a wider array of analytical approaches including determining the alloy compositions and morphological studies.