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The lithology and lithogeochemistry of the San Antonio Gold Mine, Bisset, Manitoba Whiting, Bernard Henry


The San Antonio Gold Mine, at Bissett, Manitoba, was a major precious metals producer. Over the history of the mine more than 45 thousand kilograms (1.35 million ounces) of gold and 6 thousand kilograms (194 thousand ounces) of silver were produced. The mine is in the Archean Rice Lake Greenstone Belt of the Superior Province. Ore zones are structurally controlled within extensional stockwork veins and shear veins, which are primarily restricted to one basaltic unit. The purpose of this study is to describe the mine site lithologies, particularly the auriferous host rock, and to examine lithogeochemical patterns within the mine. Key objectives of the research were; i) to establish hydrothermal alteration patterns, ii) to detail the effects of igneous differentiation on the chemistry of the gold hosting SAM Unit, and iii) to define a suite of elements that can be useful as gold pathfinders in future exploration. The SAM Unit, host to most of the productive veins on the property, represents a sequence of basalt flows with thin interflow sediments. The lower part of the SAM Unit is a single flow member that has undergone differentiation by crystal settling of olivine, plagioclase and pyroxene from the melt. Around the large gold bearing extensional stockwork veins in the SAM Unit, both visible and cryptic alteration can be detected. An examination of XRF whole rock chemical analyses, using Gresens (1967) mass balance methods, indicate that a 5 percent volume increase and multi-element enrichment occurs in a halo (envelope) about gold rich zones, as compared to unaltered host rock. Visible (megascopic) alteration zones underwent a 10 percent volume increase. Albitization of wallrock is the dominant alteration type. Multi-element ICP analysis patterns were studied, revealing discrete populations and elemental groupings associated with ore zones, halos and host rocks. Silver, arsenic and tungsten are the most sensitive pathfinder elements for gold. The halo zones are marked by an enrichment of the base metals, copper, lead and zinc, whereas both halos and ore zones exhibit elevated values for sodium, potassium, and barium. The significance of the alteration element suites and presence of halos is in enhancing the ability to recognize mineralized structures where gold values are sporadic. In the orientation study case, the halos effectively doubled the width of the exploration target. Potential for gold exploration has also been identified for the North Basalt Unit, the Gabrielle Unit and Unit 'A' in the minesite area. These are competent volcanic units that reacted in a similar manner to the SAM Unit to regional tectonism. An ancillary objective of the research was to examine the variation of gold with depth in the San Antonio Gold Mine. It is the author's finding that gold values in the lowest levels of the mine (Domain III), where the bulk of the current reserves are situated, should yield grades as rich as the historical recovery from the upper levels (Domain I).

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