Facies architecture of the felsic lava-dominated host sequence to the Thalanga massive sulfide deposit, Lower Ordovician, northern Queensland

The Thalanga volcanic-hosted massive sulfide deposit occurs in the Cambro-O rdovician Mt Windsor Subprovince in northern Queensland. The orebody compri ses steeply dipping, stratiform, sheet-like. polymetallic massive sulfide l enses. Overall, the volcanic facies architecture at Thalanga is dominated b y quartz- and/or feldspar-phyric lavas and synvolcanic intrusions that comp rise coherent facies and in situ and resedimented autoclastic facies. Syste matic phenocryst logging (mineralogy, abundance, size) has been used to dis criminate separate emplacement units of rhyolite in the footwall and dacite in the hangingwall. Some of the petrographically different rhyolite and da cite types can also be distinguished using immobile-element geochemistry. R hyolitic lavas and intrusions in the footwall are weakly to strongly altere d. Apparent clastic textures resulting from hydrothermal alteration and met amorphism are widely developed in the coherent facies. Genuine elastic text ures are characterised by clasts with randomly oriented internal laminar or banded fabric (e.g. rotated, flow-laminated clasts), marked and consistent differences in quartz phenocryst abundance and/or size range between clast s and matrix, and normal grading. Mass-flow-emplaced, rhyolitic breccia uni ts delineate palaeo-sea-floor positions in the footwall that are potentiall y prospective for exhalative massive sulfide mineralisation, a comparison o f the distribution of elastic and coherent facies with the geometry of stro ngly altered zones in the footwall indicates that intense hydrothermal flui d flow was independent of the facies arrangement. The massive sulfide lense s conformably overly altered footwall rhyolite and occur in a distinctive f acies association which includes coarse quartz-phenocryst-rich rhyolitic si lls with peperitic contacts and crystal-rich polymictic breccia. The hangin gwall to the orebody consists of largely unaltered dacitic lavas and synvol canic intrusions and minor dacitic pumice breccia, dacitic breccia and poly mictic volcanic breccia. The facies architecture shows that the Thalanga ma ssive sulfide deposit formed in a below-storm-wave-base depositional enviro nment on top of an elevated, lava-dominated, rhyolitic volcanic centre. A m odern analogue for the setting or the Thalanga massive sulfide is the PACMA NUS hydrothermal field on the crest of the dacite lava-dominated Pual Ridge in the eastern Menus backarc basin (Papua New Guinea).