Volcanology and sedimentology of the host succession to the Hellyer and Que River volcanic-hosted massive sulfide deposits, northwestern Tasmania

Abstract

The Hellyer and Que River high-grade polymetallic volcanic-hosted massive sulfide deposits of western Tasmania are hosted by a sequence of late Middle Cambrian subaqueous mafic to felsic volcanics and volcaniclastics known as the Que-Hellyer Volcanics. These volcanics occur toward the top of a thicker sequence of mainly felsic volcanics, also hosting mineralization, known as the Mount Read Volcanics.The base of the footwall sequence within the Que-Hellyer Volcanics is marked by massive to pillowed basaltic lavas and volcaniclastics of variable thickness. These are conformably overlain by a thick sequence of andesitic, basaltic, and minor dacitic lavas, and associated volcaniclastics. Eruption of these footwall sequences was rapid, with breaks in volcanism represented only by thin discontinuous epiclastic horizons.A hiatus in volcanism followed the emplacement of the footwall sequences. This hiatus is represented by the deposition of locally derived, coarse-grained mass flows. The distribution and geometry of these epiclastics suggest that they accumulated in local syndepositional depressions. Sedimentation was also accompanied by syngenetic Zn-Pb-Cu-Ag-Au-rich mineralization in the central or axial area of these depressions. Rapid burial by further mass flows assisted in the preservation of mineralization. Shallow intrusive to extrusive dacitic lava flows and domes in the host horizon were emplaced during and immediately after mineralization. Autoelastic material from these dacitic domes was then resedimented and incorporated in the epiclastic horizons overlying the orebodies.Eruption of massive to pillowed basaltic, and minor andesitic, lavas of the Hellyer basalt occurred soon after mineralization and was the final phase of volcanism in the Que-Hellyer Volcanics. The basalts were emplaced as extrusions and as shallow intrusions into wet, fine-grained sediments which began to accumulate above the coarse-grained epiclastic horizons. These basalts buried the remaining parts of the sulfide mound not covered by epiclastics. Immediately after the cessation of volcanic activity in the area, a 100-m-thick sequence of black shales and minor sandstones, known as the Que River Shale, was deposited.The general lack of products of explosive volcanism within the Que-Hellyer Volcanics is most likely a function of the composition of the lavas combined with their emplacement in deep water under high confining pressures. Although the absence of such products does not rule out the possibility of emplacement in shallow-water depths, the absence of abundant large-scale tractional sedimentary structures, the presence of the overlying black, anoxic Que River Shale, and the mass-flow characteristics of elastic units favor a relatively deep water setting for mineralization in the Que-Hellyer Volcanics.