Chemical and Mineralogical Signatures of Gold Formed in Oxidizing Chloride Hydrothermal Systems and their Significance within Populations of Placer Gold Grains Collected during Reconnaissance

Abstract

The close association of Pd with Au has been previously reported in highly oxidized chloride hydrothermal systems from several regions, including Australia, Brazil, Poland, and the United Kingdom. The distinctive mineralization precipitated usually as a consequence of contact with reducing environments is characterized by Au alloys containing low Ag ± Pd ± Hg ± Cu, associated with selenides ± tellurides ± antimonides ± arsenides. Placer gold grains derived from this style of mineralization inherit these chemical and physical characteristics which are shown to be distinct from those placer grains derived from other styles of mineralization. The discovery of placer gold grains of this type in areas remote from known in situ mineralization initiated the present project and provided a methodology for the study of a relatively large number of occurrences. Data describing alloy chemistry and mineralogy of polished sections of 957 placer grains from 19 localities in the United Kingdom and 1 locality in the Czech Republic have augmented previously published data for 303 gold grains from 10 localities in the United Kingdom. Six generic gold alloy types have been identified based on the relative proportions of Au, Ag, Pd, and Hg. Mineralogical descriptions of similar gold alloys from localities worldwide usually conform to this classification, but slight deviations are noted where P-T conditions differ from those previously identified in Devon. Alloy types may occur either as discrete grains or as phases within heterogeneous grains. Type 1 gold (defined as >99 mass % Au), type 2 gold (Au-Pd), and type 3 gold (Au-Ag) alloys (with selenide inclusions) are the most common compositions and occur at most localities, although in different proportions. Type 4 gold is also an (Au-Ag) alloy, but contains sulfide and sulfarsenide inclusions. Gold alloys or intermetallics containing Pd-Hg and/or Pd-Sb alloys (type 5), and Cu (type 6) are found only at some localities, as is a variety of type 3 gold (± selenide inclusions), which contains Hg. The spatial arrangement of the main alloy types within heterogeneous gold grains shows a consistent precipitation order of type 1, type 2, and type 3, independent of locality. Interpretation of the chemical controls on alloy precipitation indicates that ƒO2 controls whether type 1 or type 2 gold precipitates initially. The formation of type 4 gold (Au-Ag alloys with sulfide inclusions) is ascribed to a fall in ƒO2 to around the hematite-magnetite buffer where HS− (aq) replaces Cl−(aq) as the aqueous gold complex. Types 3 and 5 gold commonly mantle earlier alloys, and show evidence for aggressive replacement of the core. This texture is interpreted as evidence for influx of a more oxidizing fluid which also accounts for the abundance of selenide and telluride mineral inclusions within this gold type. This approach provides a plausible explanation of the complex heterogeneity within and between gold grains from the localities and accounts for the common sequence of alloy precipitation, without invoking multiple episodes of hydrothermal activity to account for different alloys. However, the absence of thermodynamic data describing the Au-Cu, Au-Ag-Cu, and Au-Ag-Pd-Hg systems prevents interpretation of the significance of some alloy types. This study has demonstrated that gold mineralization associated with oxidizing strata is more widespread than currently appreciated. The methodology described here not only permits identification of gold from this style of mineralization but allows initial evaluation of the economic potential of the source through the study of the alloy types. This information may be gained at an early stage in the exploration process from gold grains collected during routine reconnaissance.