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Geochemical constraints on the genesis of AG-PB and ZN deposits, Sandon, British Columbia

University of British Columbia

Deposits of Slocan mining camp are Ag-rich, Pb- and Zn-bearing veins localized around Sandon, southeastern British Columbia. These small to medium tonnage (to 10⁶ tonnes) deposits occupy a major lode/fault system which crosscuts the regional fold structure of the enclosing Upper Triassic Slocan Group. The Late Jurassic Nelson batholith intrudes this clastic sequence and contains fewer and mineralogically different deposits than the sedimentary unit. Ore paragenesis and deposit-scale metal zonation patterns for the Hallmac deposit reveal two stages of silver mineralization. An early stage is characterized by exsolution inclusions of silver-sulphantimonides in galena. A later stage postdates the bulk of galena deposition, and is characterized by antimony-poor/silyer-rich sulphosalts and acanthite. These two stages of Ag mineralization represent respectively the 'wet' sediment-hosted and 'dry' batholith-hosted ore classifications of Cairnes (1934). 'Dry' ores are late stage, spatially restricted to the batholith or related stocks and peripherally distributed about the central area of 'wet' ore deposits at Sandon. The late stage mineral assemblages thereby establish a relatively later period of mineralization for batholith-hosted structures than those within the sedimentary sequence. Lithogeochemistry of the Slocan Group and lower-most Rossland Group indicates a single unit within the Slocan, the Slate belt, to be enriched in Ag, Ba and S. Nowhere in the stratigraphic sequence are Pb and Zn values recognizably higher than average black shale values. The Slate belt contains syn/diagenetic trace metal sulphides of pyrite, sphalerite, chalcopyrite, millerite and galena. Fe, S and possibly trace metals have been released during contact/regional metamorphism of pyritic sediments, in part accompanying the conversion of pyrite to pyrrhotite. The Slate belt coincides with the highest spatial density distribution of vein deposits at Sandon; source rock volume calculations suggest it could have acted as a source bed for base and precious metals in the veins. Pb-isotope studies of layered sulphides in the clastic sequence have established initial isotope ratios and μ-value approximations for the Upper Slocan Group. An evolutionary path starting at 1890 Ma (departure from the curve of Stacey and Kramers, 1975) with μ = 11.1 could have generated the required ²⁰⁶Pb/²⁰⁴Pb ratios for the vein deposits at t₃ = 160 ± 6 Ma. This suggests that lead and perhaps silver and zinc in epigenetic veins was extracted from the clastic sedimentary sequence during emplacement of the Late Jurassic Nelson batholith.

Text

2010-06-21

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http://hdl.handle.net/2429/25914

Geological Sciences

University of British Columbia

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