Porphyry-epithermal and orogenic gold are two of the most important styles of gold-bearing mineralization within orogenic belts. Populations of detrital gold resulting from bulk erosion of such regions may exhibit a compositional continuum wherein Ag, Cu, and Hg in the gold alloy may vary across the full range exhibited by natural gold. This paper describes a new methodology whereby orogenic and porphyry-epithermal gold may be distinguished according to the mineralogy of microscopic inclusions observed within detrital gold particles. A total of 1459 gold grains from hypogene, eluvial, and placer environments around calc-alkaline porphyry deposits in Yukon (Nucleus-Revenue, Casino, Sonora Gulch, and Cyprus-Klaza) have been characterized in terms of their alloy compositions (Au, Ag, Cu, and Hg) and their inclusion mineralogy. Despite differences in the evolution of the different magmatic hydrothermal systems, the gold exhibits a clear Bi-Pb-Te-S mineralogy in the inclusion suite, a signature which is either extremely weak or (most commonly) absent in both Yukon orogenic gold and gold from orogenic settings worldwide. Generic systematic compositional changes in ore mineralogy previously identified across the porphyry-epithermal transition have been identified in the corresponding inclusion suites observed in samples from Yukon. However, the Bi-Te association repeatedly observed in gold from the porphyry mineralization persists into the epithermal environment. Ranges of P-T-X conditions are replicated in the geological environments which define generic styles of mineralization. These parameters influence both gold alloy composition and ore mineralogy, of which inclusion suites are a manifestation. Consequently, we propose that this methodology approach can underpin a widely applicable indicator methodology based on detrital gold.
A study of both in situ and detrital gold from different deposit types in British Columbia was undertaken to establish deposit-specific compositional characteristics in terms of alloy composition and suites of mineral inclusions. The study is based on 11,840 particles from 160 localities in which nine gold deposit types are represented, although there is a strong bias towards gold of orogenic, low-sulphidation epithermal, and alkalic porphyry origin. In general, Ag values in gold alloys are not a powerful discriminator for deposit type, but minor metals may prove useful where detectable, e.g., Cu in gold from ultramafic associations and Pd and Hg in gold from alkalic porphyry systems. The characterization of inclusion suites is far more illuminating, as they correlate strongly with the mineralogy of auriferous ores from different deposit types. This outcome has confirmed the validity of designing an indicator methodology based on inclusion suites and has permitted the prediction of inclusion suites for gold from other deposit types where data are more scarce. The compositional templates generated in the study were applied to identify the source deposit type(s) of gold from 41 localities (a total of 2916 detrital gold particles) where gold genesis was previously unknown.
AbstractThe Late Pliocene (3.6–2.6 Ma) was a period of significant global warmth, considered a potential analogue for future anthropogenic climate change. Newly discovered fine-grained sediments from between the gold-bearing lower and upper White Channel Gravels show the presence of a wetland or lake within Bonanza Creek, Dawson Mining District, Yukon. This environment was surrounded by a diverse Pinaceae-dominated boreal forest with significant stands of angiosperms in favourable sites. Quantitative climate reconstructions derived from pollen and spores reveal a mean annual temperature at least 6 °C warmer than today with warm summers and relatively mild winters. Finally, the new pollen assemblage is used to discuss the age of the White Channel Gravels.Keywords: Late Pliocenepollen and sporesYukon, Canadaboreal forestwarmer climateKlondike Mining region AcknowledgementsSpecial thanks is given to the placer miners of the Dawson area, namely John Alton, the Christie family, Ivan Daunt, Daryl Fry, Doug Jackson, Mike McDougall, Eric Rauguth, Don Ruman and Chuck Sigurdson, without whom this research would not have been possible. Thanks are also given to the Natural Environment Research Council, Yukon Geological Survey, Canadian Northern Development Agency, Society of Economic Geologists, and International Association of Sedimentologists, for providing support for the field component of this project. We gratefully thank A. Dolan and H. Dowsett for their reviews that have greatly improved the manuscript.Additional informationNotes on contributorsMatthew J. PoundMATTHEW J. POUND is a research fellow at Northumbria University in Newcastle, United Kingdom. He is involved in several projects including the incorporation of geological data into palaeoclimate models, Cenozoic palynology and onshore Miocene successions of the United Kingdom. Display full size Robert I. LowtherROBERT I. LOWTHER is a PhD researcher at the University of Leeds, United Kingdom. His research focuses on placer deposits, combining both field studies and physical modelling to investigate the processes involved in their formation and evolution. Display full size Jeff PeakallJEFF PEAKALL is Professor of Process Sedimentology at the University of Leeds, United Kingdom. His work focuses on sedimentation processes and deposits across a wide range of terrestrial and deep marine environments. Display full size Robert J. ChapmanROBERT J. CHAPMAN is a Senior Lecturer in the School of Earth and Environment at the University of Leeds and Principal Investigator of the Placer Minerals Group. His research interests centre on the relationships between placer and lode gold, and their application to mineral exploration. Display full size Ulrich SalzmannULRICH SALZMANN is a palynologist and Professor of Palaeoecology at Northumbria University in Newcastle, United Kingdom. His research focuses on global palaeoecology and climatology and the reconstruction of past environments using a combined proxy data and modelling approach. Display full size
Funding was provided by the Minerals Management Service under contract Number 14-12-0001-30262; Sea Grant under contract Number NA86AA-D-FG090; and the Office of Naval Research Young Investigator Program under contract Number N00014-86-K-0579.
"An investigation into the relationship of orogenic quartz veins and disseminated mineralisation in the Klondike." Applied Earth Science, 126(2), pp. 59–60
Abstract Compositional studies of natural gold usually have a geological focus, but are also important in archaeological provenancing. Both methodologies rely on compositional comparison of two sets of samples, one of which is geographically constrained. Here we describe how experiences in gold characterization resulting from geological studies are relevant to archaeology. Microchemical characterization of polished sections of natural gold identifies alloy compositions, alloy heterogeneity and mineral inclusions. Gold from all deposit types shows Cu and Sn values much lower than those recorded during numerous studies of artefacts. Inclusions in artefact gold include various Cu- and Sn-bearing compounds which indicate specific high temperature reactions that could ultimately illuminate the conditions of (s)melting. The use of LA-ICP-MS to generate a wide range of elemental discriminants for provenance studies may be compromised by alloy adulteration and/or unrepresentative analysis of natural/artefact alloys, which are commonly highly heterogeneous at the micron scale. Geological studies normally characterize only the earliest-formed (hypogene) alloy, whereas archaeology-focused studies should entail analyses of bulk alloy compositions and impurities that may be incorporated during (s)melting. Isotopic-based provenancing alleviates many of these problems but, to date, generates regional rather than locality-specific targets. A dual isotopic–compositional approach is recommended.
Compositional features of a total of 1887 gold alluvial particles from six localities to the south of Loch Tay in central Scotland were interpreted to establish different types of source mineralization. Populations of gold particles from each locality were grouped according to alloy and inclusion signatures. Inclusion suites provided the primary discriminant with gold from Group 1 localities showing a narrow range of simple sulphide and sulphoarsenide inclusion species, whereas a wide range of minerals including molybdenite, bornite and various Bi and Te- bearing species were identified in gold from Group 2 localities. Whilst the range of Ag in alloys in all populations was similar, higher incidences of measurable Hg and Cu were detected in Group 1 and Group 2 gold samples respectively. The application of compositional templates of gold from other localities worldwide indicated that Group 1 gold is orogenic and Group 2 gold is a mixture of porphyry and epithermal origin; a result that is sympathetic to the spatial relationships of sample localities with local lithologies. This approach both provides an enhanced level of understanding of regional gold metallogeny where in situ sources remain undiscovered, and permits clearer targeting of deposit types during future exploration.
This is a repository copy of Characterization of gold mineralization in the northern Cariboo Gold District, British Columbia, Canada, through integration of compositional studies of lode and detrital gold with historical placer production: a template for evaluation of orogenic gold districts.
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