A tightly folded, gold-rich, massive sulfide deposit; Que River Mine, Tasmania

Abstract

The Que River deposit in western Tasmania is a high-grade, gold-rich, Cambrian volcanogenic sulfide deposit, comprising two major subvertical ore lenses hosted by andesitic lavas and volcaniclastics of the Mount Read Volcanics. Contrary to previous published interpretations, the major orebodies (PQ and P north lenses) are considered to lie at the same stratigraphic level and be folded into a tight asymmetric syncline which has been sheared along its western limb. Massive sulfides are thickened in the hinge of the syncline. Extensive footwall stringer pyrite-galena-sphalerite mineralization within strongly altered andesitic volcaniclastics occurs on both limbs of the fold. A copper-bearing pyrite-rich stringer zone occurs immediately below the thickened central section of the PQ lens in the keel of the fold and is interpreted as marking the principal focus of ore fluid exhalation. Copper, lead, and zinc distribution within the lenses provides evidence of a stratigraphic younging direction around the fold structure. Gold grades of 5 to 30 ppm are concentrated toward the interpreted stratigraphic top of the folded lens. In the hinge zone of the major fold, pyrite bands within the ore are folded, with galena remobilized into the axial plane of the folds. The galena defines a banding throughout the mine which is subparallel to the cleavage measured in the adjacent alteration zone. The galena banding varies from a spaced cleavage in the moderately deformed zones to a mylonitic foliation in the sheared western limb of the fold. A conspicuous fuchsite-bearing horizon of altered coarse-grained polymict volcaniclastic immediately overlies the major ore lenses, and it is in turn overlain by flow-banded dacite lavas and lava breccias which lack significant hydrothermal alteration. A Cu-rich, massive sulfide body, the S lens, occurs stratigraphically below and on the eastern side of the PQ lens. This lower lens consists of massive pyrite and banded Pb-Zn sulfide overprinted by stringer Cu and stringer Pb-Zn.