Abstract

The stratiform gold zones and massive sulfide (Zn-Pb-Cu-Au-Ag) deposit at Montauban, Quebec, occur as part of a series of Helikian supracrustal rocks metamorphosed to the upper amphibolite facies. The mine sequence at the north gold zone consists of nodular sillimanite gneiss, cordierite-anthophyllite (zincian staurolite-kyanite) gneiss, and manganiferous garnet-gahnite quartzitic gneiss. Porphyroblastic gahnite is present only in the gold-bearing unit but finer grained gahnite was observed in the remainder of the sequence for up to 20 m on either side. Gahnite was formed mainly by desulfurization of sphalerite, breakdown of zincian staurolite, and breakdown of Zn-bearing biotite. Peak metamorphic conditions are estimated at T = 650 degrees C, P = 6 kb, f (sub O 2 ) = 10 (super -17) bars, and f (sub S 2 ) = 10 (super -2) bars.Mn ranges up to 25 wt percent in garnet and Zn up to 8 wt percent in staurolite. Zn ratios in staurolite correlate closely with those of gahnite (r = 0.85) and bulk rock (r = 0.72). Zn/(Cu + Zn + Pb) bulk ratios increase symmetrically away from the ore zone.The mine sequence is interpreted as a metamorphosed exhalative system consisting of a siliceous exhalite containing gold and sulfide mineralization and anomalous Mn, Ba, and B. The other units were initially chlorite-sericite-quartz hydrothermal alteration assemblages.

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