Alteration of felsic volcanics hosting the Thalanga massive sulfide deposit (Northern Queensland, Australia) and geochemical proximity indicators to ore

Thalanga is an Early Ordovician, stratiform Zn-Pb-Cu-Ag-rich massive sulfid e deposit in northern Queensland (Australia) with a total resource of 6.6 M t of ore. The host-rack succession consists of variably altered rhyolite in the footwall and a dacite-dominated volcano-sedimentary sequence in the ha nging wall. The sulfide deposit and the enclosing volcanic sequence were me tamorphosed under upper greenschist conditions. A laterally continuous footwall alteration zone extends beneath the entire deposit (similar to3,000 m) and to a stratigraphic depth of at least 300 in below the ore lenses. The bulk of this zone is occupied by feldspar-destru ctive, muscovite-biotite-chlorite-rich, mottled alteration facies with diss eminated pyrite, representing the metamorphic equivalent of phyllosilicate- dominated hydrothermal alteration. Discordant zones of intense quartz-pyrit e -alteration represent the principal fluid pathways during mineralizing hy drothermal activity, Locally, quartz-K feldspar alteration facies exist on the fringes of the system, and calcareous alteration and chlorite-pyrite al teration facies exist in the upper part of footwall rhyolite, proximal to s ulfide lenses. Quartz-feldspar-porphyritic rhyolite laterally surrounding t he footwall alteration zone and feldspar-porphyritic dacite in the hanging wall are unaltered or weakly altered. The compositional diversity of altered footwall rhyolite implies that hydro thermal alteration at Thalanga was a complex processes. This paper presents a model for the evolution of the footwall alteration zone, Calcareous -alt eration probably represents the initial phase of hydrothermal activity This was followed by diffuse upwelling of acidic, seawater-dominated fluids cau sing destruction of primary feldspar, precipitation of pyrite, and formatio n of hydrothermal sericite, chlorite, and clay minerals. Subsequent, intens e quartz-pyrite alteration was directly associated with mineralization. The Thalanga footwall alteration zone has several geochemical characteristi cs that show systematic changes with increasing proximity to ore. These inc lude Na depletion and elevated Mg, S, alteration index (AI), chlorite-carbo nate-pyrite index (CCPI), Mo, Bi, and As. Furthermore, the X-Mg of chlorite and biotite increases systematically from values of 0.45 to 0.5 in least a ltered rhyolite to values >0.9 immediately below ore. These geochemical fea tures can be used in exploration for massive sulfide deposits as vectors to ore, at the prospect scale, and in discrimination of prospective hydrother mal from unprospective diagenetic alteration systems, at the regional scale .