Geology and genesis of the Lewis Ponds carbonate and volcanic-hosted massive sulfide deposits, New South Wales, Australia

The Lewis Ponds carbonate and volcanic-hosted Zn-Pb-Cu-Ag-Au-rich massive sulfide deposits are located near the western margin of the Hill End Trough, in the eastern Lachlan Fold Belt of New South Wales. Two stratabound massive sulfide zones, Main and Toms occur in a tightly folded Late Silurian marine succession of volcaniclastic sandstone, polymictic breccia, limestone-clast breccia, siltstone and mudstone. They have a combined indicated resource of 5.7 Mt, grading 3.5% Zn, 2.0% Pb, 0.19% Cu, 97 g/t Ag and 1.9 g/t Au. Main zone occurs within a thick unit of poorly-sorted mixed provenance breccia, limestone-clast breccia and pebbly-granular sandstone, whereas Toms zone is hosted by siltstone. The sedimentary rocks unconformably overlie a thick succession of quartz-plagioclase phyric dacite and strongly foliated, chlorite-sericite-altered dacite. Variably recrystallised fossiliferous limestone occurs throughout the Lewis Ponds host sequence in thick, tabular units of poorly-sorted breccia and fault-bound lenses of megabreccia. Limestone clasts vary in size from small pebbles to 90 m boulders. The mixed provenance breccia, limestone-clast breccia and sandstone were deposited in a moderately deep water, below wave-base slope environment, around the margins of a high-level intrusive dacite centre. Detrital volcanic and sedimentary components were derived from multiple source areas within the basin and in the adjacent hinterland. The massive sulfide lenses occur along the eastern limb of a regional-scale D I anticline. The adjacent syncline has been partly truncated by a 200-250 m wide, NNW-trending high strain zone termed the Lewis Ponds fault. Syn-tectonic quartz ± sulfide veins and steeply dipping, anastomosing shear zones surround the Toms massive sulfide lens. The variably folded and boudinaged quartz veins resulted from periodic brittle shear failure and extension during and after the D I deformation. Pinch-and-swell structures, boudins, catalcastic breccia and kink folds occur in the massive sulfide. Main zone, located west of the fault is significantly less deformed than Toms zone. However, reversals in stratigraphic facing and vergence indicate that tight parasitic folds occur in the Main zone host sequence. Mineralisation at Lewis Ponds probably pre-dated shearing along the Lewis Ponds fault. An asymmetric, semiconformable Mg-Fe-Ca-Ba-rich hydrothermal alteration envelope surrounds the massive sulfide lenses. Mg-chlorite occurs at the top the footwall volcanic succession south of Main zone and grades outwards into a weak pervasive sericite-quartz ± Fe-Mg-chlorite assemblage. The compositions of recrystallised phyllosilicates vary systematically with whole rock geochemistry, alteration intensity and proximity to the Toms massive sulfide lens. Hydrothermal alteration of dacite in the Toms zone footwall involved MgO, Fe203, K2 0 and Ba enrichment and Na2O, CaO and Sr depletion. The addition or removal of Si02 contributed to net gains of 0-75 g/100g in dacite C and net losses of 0-50 g/100g in dacite A, except where MgO and Fe 203 gains offset the loss of Si02. In contrast, weak sericite-chlorite-calcite alteration of coherent dacite in the Main zone footwall led to net losses of 10-40 g/100g. Conformable, texturally destructive alteration assemblages associated with the two mineralised zones include dolomite-chlorite-talc, chlorite-pyrite, quartz-dolomite-chlorite and quartz-sericite ± hyalophane. Dolomite, Mg-chlorite, talc, phlogopite, calcite, quartz and sulfides have overprinted the clasts and matrix in the breccia and sandstone units in Main zone. Relict crinoid fossils are preserved in even the most intensely altered rocks, where dolomite, chlorite and talc have replaced the original calcite. Irregular honeycomb vuggy and botryoidal sulfide-dolomite textures in the breccia and sandstone units indicate dissolution and precipitation of dolomite during mineralisation. Dolomite associated with the massive sulfide lenses is characterised by low [delta to the power 18]vsmow (6 to 16%o) and [delta to the power I3Cvpdb (-II to 0%o) values relative to the regional fossiliferous limestone. Fluid inclusion and stable isotope data indicate that the dolomite precipitated from a low temperature (166-232°C for 1 000 m water depth), weakly saline (1.4 to 7.7 eqiv wt % NaCl) fluid, possibly depleted in [omicron] and C isotopes ([delta to the power 18 omicron] = -2.5 to 0.3%o, [delta to the power 13]C = -14 to -4%o). The dolomite probably formed during diagenesis and hydrothermal alteration, by fluid-rock interactions between evolved seawater at 150-250°C and the limestone-bearing host sediment, and/or mixing between evolved seawater at 350°C and a seawater-dominant pore fluid at 100°C. The massive sulfide lenses consist of pyrite, sphalerite and galena, with subordinate tetrahedrite, chalcopyrite, arsenopyrite, pyrrhotite, stannite, pyrargyrite and electrum. Paragenetically early framboidal, dendritic, reticulate, botryoidal and spongy Fe-sulfide aggregates and bladed pyrrhotite pseudomorphs of sulfate occur throughout the breccia and sandstone beds that host Main zone, but are rarely preserved in the coarse grained, annealed massive sulfide in Toms zone. Pre-tectonic carbonate-chalcopyrite-pyrite veins in the footwall volcanic succession, immediately south of Toms zone may represent a stringer zone displaced from the Toms massive sulfide lens. The stringer veins also contain native bismuth, sphalerite and Se-Bi-Ag-rich galena. Sulfur isotope values in the massive sulfide ([delta to the power 34]S = 1.7-5.0%0) and footwall stringer veins (6345= 3.9-7.4%0), indicate that the hydrothermal fluid contained a homogenous mixture of magmatic S, derived from the host volcanics rocks and reduced seawater sulfate. The lower average [delta to the power 34]34S values in the massive sulfide lenses may have resulted from a component of partially reduced seawater sulfate or biogenic S, leached out of the host sediment. Textural, geochemical and isotopic data indicate that the Main zone massive sulfide lenses formed by lateral fluid flow and sub-sea floor replacement of the poorly-sorted, carbonate-bearing breccia and sandstone beds. Low temperature dolomitisation of the carbonate-bearing sediment during diagenesis created secondary pore spaces and provided a reactive host for fluid-rock interactions. Base metal sulfides, chlorite, dolomite, calcite, quartz and talc filled pore spaces throughout the carbonate-altered breccia and sandstone units. Toms zone probably formed in fine-grained sediment at or near the sea floor, above a zone of focused up-flowing hydrothermal fluids. Pyrite, pyrrhotite, sphalerite, galena, tetrahedrite and electrum precipitated from a relatively low temperature 150-250°C, reduced hydrothermal fluid. Paragenetically early dendritic, reticulate and spongy Fe-sulfide aggregates in the Main zone host sequence formed by rapid mixing between the hydrothermal fluid and cooler pore fluids in the matrix of the breccia and sandstone. Base metal sulfide deposition in Main zone probably resulted from fluid mixing, dissolution of dolomite and increased fluid pH. Galena, sphalerite and chalcopyrite overprinted the primitive Fe-sulfide textures. As the hydrothermal system intensified, a high temperature >280°C, strongly reduced fluid carrying Sn, Cu, Se and Bi was sourced from deep within the footwall volcanic succession. Carbonate-chalcopyrite-pyrite stringer veins and dolomite-chalcopyrite-pyrite-stannite veins formed in the Footwall Copper zone and Toms zone Central lens respectively. Chalcopyrite partly replaced the Zn-Pb-rich massive sulfide in Main zone. During the DI deformation, fracture-controlled fluids remobilised sulfides into syn-tectonic, quartz and carbonate veins within the Lewis Ponds fault zone and adjacent footwall volcanic succession, resulting in extensive Cu, Au and Zn anomalies. Massive sulfide remobilisation may have occurred over tens to hundreds of metres. Talc, quartz-sericite, chlorite and Fe-Mg-Mn-carbonate assemblages overprinted the dolomite, chlorite and sericite-altered rocks in the Toms zone host sequence. Lewis Ponds is an unusual stratabound, carbonate and volcanic-hosted massive sulfide deposit. The intimate spatial association between fossiliferous limestone, hydrothermal carbonate and base metal sulfides at Lewis Ponds provides a basis for new exploration targets in Siluro-Devonian marine successions elsewhere in New South Wales.