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Effects of oxidation and weathering on pyrite chemistry, and implications for evaluating regional gold fertility of the Selwyn Basin, Yukon

conference contribution
posted on 2023-05-24, 13:47 authored by Sack, PJ, Leonid Danyushevsky, Gilbert, S, Ross LargeRoss Large, Gregory, D
Pyrite from surface outcrops and diamond drill core near sediment-hosted gold occurrences in the Selwyn basin have been analyzed using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) . Samples from the orogenic gold 3Ace occurrence, in southeast Yukon, are interbedded fine-grained sandstones and silty black phyllites from the Neoproterozoic Hyland Group . Samples from the Carlin-type Conrad occurrence, in central-east Yukon, are dark grey to black shales from the Neoprotero-zoic Windermere Supergroup . Surface outcrop samples are representative of regional lithologies and are distal (>5 km) from known mineral occurrences . Diamond drill holes are distal, in a deposit sense, and show minor alteration and mineralization; samples are from barren intervals of shale and phyllite .
LA-ICPMS spot analyses were all conducted on fresh pyrites, including unoxidized pyrites from weathered surface samples . All data are from microcrystalline pyrite nodules and framboids interpreted to have grown early in diagenesis . Forty-two major and trace element data from 118 spots are used in this comparison: from the 3Ace orogenic gold occurrence, 38 spots from four samples in 3A11-24A are compared against 11 spots from one surface sample; from the Conrad Carlin-type occurrence, 57 spots from six samples in OS11-70 are compared against 12 spots from one surface sample .
Similar results between surface and core samples are defined as having approximately 25%, or more, overlap between the first and third quartiles of each element in the data set . From both occurrences, S, Ti, V, Cr, Co, Cu, Cd, Sn, Te, W, Pt, Au, Pb, and U are similar; at the Conrad occurrence, Mn, Ni, Zn, As, Se, Mo, Ag, Sb, and Bi are also similar; Tl is higher in surface than core data at the 3Ace occurrence and lower at the Conrad . At the 3Ace occurrence, Mn, Ni, Zn, and Sb are higher in core compared to surface data, whereas As, Se, Mo, and Bi are lower . With the exception of Tl, data between core and surface samples are similar . Thallium is a low temperature hydrothermal fluid indicator and the regional surface samples may not have been influenced by local hydrothermal conditions . There are more dissimilar elements between core and surface pyrite at the 3Ace oc-currence than the Conrad, but without systematic enrichment or depletion; this is interpreted as relating to differing original compositions of the pyrite, not weathering effects.
Our preliminary conclusion is that careful selection of unoxidized pyrite in weathered surface samples yields LA-ICPMS results similar to those of pyrite collected from the same lithology in unweathered diamond drill core and, thus, surface sampling is a viable technique for measuring pyrite chemistry in shales . Recent work suggests that gold content, and possibly associated trace elements, of diagenetic pyrite may be used as a first order proxy for evaluating the regional gold fertility of sedimentary basins . We pro-pose to test this model using surface samples from the Selwyn basin in Yukon, which is known to have Carlin-type and Orogenic gold occurrences, as well as significant intrusion-related gold deposits and base metal deposits .


Publication title

Whistler 2013: Geoscience for Discovery




School of Natural Sciences

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Whistler 2013: Geoscience for Discovery

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Whistler, Canada

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Other mineral resources (excl. energy resources) not elsewhere classified

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