Significant tellurium enrichment occurs in many orogenic gold deposits but the factors causing this are little understood; some authors suggest this demands a magmatic input whereas others suggest it need not. Fractionation of Te from Se and S could offer insight into source/pathway processes of auriferous fluids. The metasedimentary-hosted Cononish vein gold deposit, Scotland, is unusually Te-rich compared to many orogenic gold deposits with Te/Au ≈ 2.4 whereas most orogenic deposits have Te/Au < 1. Here, Ag in Au-Ag alloy increases from ∼10 to 90 wt% through the paragenesis, correlating with decreasing hessite (Ag2Te) abundance. This suggests the Au-Ag alloy composition was controlled by the fluid Te activity, and that this decreased through time. This is coupled to an increase in pyrite δ34S from −2.0‰ to +11.4‰ through the paragenesis. Thus, the deposit formed from a primary fluid with a low-δ34S and high Te + Au + Ag that evolved to a high δ34S-low Te, Pb + Cu bearing fluid. The high δ34S of the later fluid suggests it can only be sourced from specific nearby metamorphosed SEDEX horizons. The early fluid that deposited most of the gold could be sourced from other metasedimentary units in the stratigraphy or be magmatic in origin. We argue that two observations taken together suggest it is most likely that this fluid was magmatic; the age of the mineralisation is identical to the last stage of crystallization of nearby granite batholiths, and the fluid has a S-isotope signature consistent with a magmatic source. Gold deposits in orogenic belts are almost certainly polygenetic and this study demonstrates evidence for Te-rich "orogenic" deposits having a significant magmatic component.
Gold compositional studies have been advocated to resolve genetic relationships between alluvial and in situ occurrences based on the assumption that the P-T-X conditions at the sites of mineral deposition are reflected in common compositional signatures of gold. Here, we explore two refinements to a simple ‘same or different’ approach, namely (i) in situ gold sources at different localities may correspond to multiple stages of mineralization and, therefore, different gold grade, and (ii) any duplication of gold signatures between localities requires fluid conduits compatible with the prevailing structural framework. The high-grade gold paragenetic stage at the Cononish Mine, Scotland, is characterized by relatively low Ag alloy associated with Ag-Au and Ag tellurides. This signature is replicated in the inclusion signature of alluvial gold from the adjacent drainage and is also present in two other drainages for which there are no known in situ sources. There is a strong correlation between the spatial extent of this signature and the fault linkage zone, but outside this zone, gold exhibits other compositional signatures. The study shows how structural considerations provide an independent and robust framework to evaluate genetic relationships suggested by compositional studies of alluvial gold in areas where the source location and economic potential are unknown.
Abstract Studies of populations of gold particles are becoming increasingly common; however, interpretation of compositional data may not be straightforward. Natural gold is rarely homogenous. Alloy heterogeneity is present as microfabrics formed either during primary mineralization or by modification of pre-existing alloys by chemical and physical drivers during subsequent residence in either hypogene or surficial environments. In electron-probe-microanalysis (EPMA)-based studies, the combination of Cu, Hg, and Pd values and mineral inclusion suites may be diagnostic for source style of mineralization, but Ag alone is rarely sufficient. Gold characterization studies by laser-ablation-ICP mass spectrometry linked to both quadrupole and Time-of-Flight (ToF-MS) systems show that only Ag, Cu, and Hg form homogenous alloys with Au sufficiently often to act as generic discriminants. Where present, other elements are commonly distributed highly heterogeneously at the micron or submicron scale, either as mineral inclusions or in highly localized, but low concentrations. Drawing upon our own data derived from individual inspection and analyses of approximately 40,000 gold particles from 526 placer and in situ localities worldwide, we show that adequate characterization of gold from a specific locality normally requires study of a minimum of 150 particles via a two-stage approach comprising spatial characterization of compositional heterogeneity, plus crystallographic orientation mapping, that informs subsequent targeted acquisition of quantitative compositional data by EPMA and/or laser-ablation ICP-MS methods. Such data provide the platform to review current understanding of the genesis of gold particle characteristics, elevating future compositional studies from empirical descriptions to process-focused interpretations.
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