Abstract

The Woodlawn zinc-lead-copper-silver deposit, New South Wales, Australia, contains two styles of mineralization, polymetallic massive sulfides and footwall chalcopyrite mineralization (stockwork copper ore). The deposit was discovered following roadside reconnaissance soil geochemical sampling in the late 1960s. At the start of production in 1978, in situ ore reserves amenable to open-cut mining totaled 6.3 million metric tons of complex ore with an average grade of 1.7 percent Cu, 5.5 percent Pb, 14.4 percent Zn, and 89 ppm Ag, and 3.7 million metric tons of stockwork copper ore averaging 1.9 percent Cu.The deposit occurs in Middle to Upper Silurian pyroclastics, volcanogenic sediments, and carbonaceous shales intruded by diabase sills. On a regional scale the Silurian rocks occupy synclinorial zones over a basement of Upper Ordovician metasediments with Early Devonian granitoids in adjacent anticlinorial zones.In the main lens of massive sulfides, pyrite is the major mineral with variable sphalerite, galena, and chalcopyrite and minor arsenopyrite, tetrahedrite-tennantite, pyrrhotite, marcasite, and electrum. Silicate gangue minerals include talc, quartz, chlorite, and phlogopite.The stockwork ore is a network of fracture-filling assemblages of pyrite and chalcopyrite with subordinate to minor sphalerite, galena, pyrrhotite, quartz, calcite, chlorite, and barite. Stockwork ore is the center of an aureole of hydrothermal alteration around the deposit.Reconstruction of the structural-depositional setting indicates that sulfide deposition occurred on an unconformity within a submerged graben. Metalliferous fluid beneath the sea floor migrated from the south-bounding 760 fault updip in a northerly direction through permeable volcanic horizons. The north-bounding 790 fault was apparently active in a growth sense during the later stages of sulfide deposition. The ore horizon, hanging-wall sequence, and 790 fault were displaced in a right-lateral sense by the 750 fault during regional deformation.A preliminary interpretation of ore formation is developed from field observations and blast hole base metal zoning relationships. Above the sea floor, base metal zonation patterns outline mounds of chalcopyrite and pyrite capped by massive sphalerite-galena ore. Beneath the mounds copper concentration patterns (>0.5 wt %) in the footwall are interpreted as a network of fossil feeder channels, the confluence regime of the convection cell streamlines.The depositional history of the main Woodlawn lens and stockwork copper zone is divided into three stages: (I) precipitation and deposition ofpyrite-chalcopyrite from early, hot, silica-rich metalliferous solutions mixing with seawater to form sulfide mounds adjacent to vents; current dispersion of colloidal galena and sphalerite quench precipitated higher in the plume; (II) deposition of sphalerite and galena (in a basinal low north of the mounds) from cooler, fractionated, negatively buoyant ore solutions; and (III) onlapping sedimentation of sulfides over mounds choking off venting ore solutions followed by subsurface hydrothermal alteration and diffusion of solutions through unconsolidated sulfides to form Zn-Pb-rich chimney structures above and to the north of mounds.Woodlawn's detailed metal zoning architecture provides an interesting comparison between Paleozoic volcanogenic massive sulfide mineralization and modern sea-floor hydrothermal activity. Despite the different tectonic settings and metal contents, similarities can be drawn between off-axial sulfide structures on the East Pacific Rise and the Woodlawn Cu-rich mounds.

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