Geology of the zoned gold skarn system at Junction Reefs, New South Wales

Abstract

The Sheahan-Grants, Frenchmans, and Cornishmens orebodies at Junction Reefs are typical gold skarns. Gold ore is restricted to a pyrrhotite-pyrite-arsenopyrite-bearing distal skarn zone in which prograde pyroxene has been altered to a retrograde chlorite-quartz-carbonate assemblage. The skarns are associated with a Late Ordovician (440 Ma) shoshonitic intrusive complex which comprises a large, central monzodiorite stock and numerous small, peripheral stocks. The skarn orebodies are located within a mineralogically zoned metasomatic alteration halo centered on one of the satellite stocks, the Junction Reefs Monzodiorite. The ore is within skarn formed from a 39-m-thick sequence of Early Ordovician limestone, siltstone, and chert, known locally, as the ore bed sequence. The ore beds were deposited between thick accumulations of massive and graded tuffaceous sandstone and siltstone in the hanging wall, and laminated chert, shale, siltstone, and sandstone in the footwall. The ore beds are thought to have been tilted, faulted, and intruded by at least one suite of Early to Middle Ordovician (465-480 Ma) latite dikes and sills prior to gold mineralization in the Late Ordovician. The orebodies are variously bounded by sharp skarn or marble fronts, preore dikes, and gradational mining cutoffs. Gold occurs mainly as 2- to 10-mu m particles on sulfide, calcite, and chlorite grain boundaries. The skarn is sequentially zoned from a grandite garnet-rich inner assemblage, near the Junction Reefs Monzodiorite stock, through clinopyroxene-dominated and amphibole-bearing assemblages, to an outer calcite-chlorite-quartz retrograde assemblage that includes the orebodies. Proximity to the skarn front has provided a major control on ore localization and gold grades reach a peak in a zone about 20 m behind the front. Preore dikes also appear to have controlled the distribution of ore in places by acting as barriers to...