Abstract

Base metal and gold ores in thin calc-silicate and cordierite gneiss units at Montauban-Les-Mines have historically been described as pyrometasomatic deposits related to granitic intrusions. They are stratigraphically overlain by quartzo–feldspathic gneiss and amphibolite, the uppermost amphibolite unit being a pillowed metabasalt.Chemical analysis shows all the amphibolites to be derived from basic igneous rocks, probably basaltic flows or shallow intrusives. Some analyses of quartzo–feldspathic gneisses follow igneous trends on variation diagrams and plot closely with those of indisputable volcanic rocks associated with massive sulfide deposits from the Kuroko District, Japan, and Noranda, Quebec. They appear to be metamorphosed intermediate to acidic volcanic tuffs and associated sediments, and are thus termed 'leptites'.The volcanic environment of the ore deposits, their general conformability to stratification, and other distinguishing features, strongly suggest they may be exhalite deposits formed in the overlapping carbonate–sulfide facies.

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