Mineralogy and geochemistry of the Simberi epithermal Au deposit, Papua New Guinea

Abstract

The Simberi epithermal Au deposit is located on Simberi Island, part of the Tabar Group in the Tabar-Lihir-Tanga-Feni Island Chain in Papua New Guinea. It is located ~80km from the world-class Ladolam deposit on Lihir, and is associated with silica-undersaturated alkaline igneous rocks, which formed following cessation of subduction of the nearby Manus- Kilinailau Trench. The host rock of this deposit is mostly brecciated andesite and tuff, and the mineralisation is structurally controlled and preferentially associated with the andesite and its contacts. Whole rock geochemistry, petrography and XRD analyses identify two alteration styles: initial potassium metasomatism characterised by an adularia + illite + pyrite (± quartz ± chlorite) mineral association; and later carbonate alteration, characterised by a calcite + pyrite (± quartz ± chlorite) association, where calcite is a replacement mineral, and is not bladed. Mineralisation is almost invariably disseminated, which can be attributed to intensive fluid-rock interactions and the permeable nature of the host rock. Pyrite is the main sulphide mineral, and occurs concurrently with minor pyrrhotite, sphalerite, and galena; and prior to minor chalcopyrite and arsenopyrite. Minor tetrahedrite and anatase have uncertain timing. Much of the pyrite contains As in variable concentrations, and LA-ICP-MS analyses suggest that while there is some association between Au and As in pyrite, deposition of arsenian pyrite did not always have associated Au deposition. Variations in the mineral chemistry suggest that the chemistry of the depositional fluid was highly complex and variable both spatially and temporally. QEMSCAN and SEM identify gold and silver occurring within telluride minerals, mostly calaverite and hessite, and which associate mainly with silicates, sulphides and other ore-bearing tellurides. Gold and silver tellurides form both inclusions and Nano-inclusions within generally more porous pyrite, and textural relationships suggest an approximately synchronous deposition. The alteration and ore mineralogy is consistent with models of alkali-type epithermal systems. The close relationships between alkalic rocks and the mineralising fluid, along with telluride mineralisation, suggest that this fluid is magmatic in origin, which is particularly unusual considering the classic adularia-sericite epithermal alteration mineralogy of the deposit, which is commonly formed from near-neutral fluids of mostly meteoric origin.