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Abstract

Compared to stratigraphically equivalent Lower Ordovician slates and graywackes some 300 to 1,000 m from mineralization, the turbidite host rocks over an interval (80 m true width) that includes auriferous quartz veins at Clunes display more intense cleavage, are bleached in appearance, and possess conspicuous porphyroblasts and veinlets of ankerite and siderite. Apart from enrichments in CO2 and As, however, there is little geochemical contrast between remote and proximal wall rocks. Minor mobility of lithophile elements during cleavage development prior to mineralization and wall-rock alteration masks a slight potassium enrichment within tens of centimeters from lode veins. This potassium enrichment only partly explains the bleaching, a more general cause of which appears to be enhanced microporosity, probably related to cleavage development rather than wall-rock alteration, causing anomalously low densities within and adjacent to the carbonate-bearing zone. The lack of a conspicuous geochemical halo defined by lithophile elements is attributed to near equilibrium between mineralizing fluids and wall rocks in this metamorphogenic lode environment. Although the practical outer limits have not been defined, a halo of penetrative arsenic dispersion, creating enrichments from 10 to 30 times background within unveined wall rocks, has potential exploration significance.

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