A depositional model for the upper Mount Isa group and implications for ore formation

Abstract

This sedimentological study provides new information about the Middle Proterozoic sediments of the Upper Mount Isa Group (Northwest Queensland, Australia) which host the Mount Isa Pb-Zn and Cu ores. The sedimentary sequence consists predominantly of clastic rocks which accumulated in a fault-bounded basin following the deposition of thick sequences of shallow-water sediments and continental volcanics. The stromatolites, sabkha-type evaporite nodules and halite casts occurring in the immediate vicinity of the mine area and found during the course of this study indicate shallow-water and saline conditions in the general depositional system including the environment of deposition of the ore-bearing sediments. The clastic sediments are of silt-size and generally consist of varying proportions of ferroan dolomite, quartz, K-feldspar and mica, with minor calcite, siderite, albite and chlorite and with accessory rutile, zircon, tourmaline and carbonaceous matter. Throughout most of the sequence thin beds of laminated to massive coarse siltstones are interbedded with non-laminated mudstones (fine siltstones). In many instances the vertical order of sedimentary structures indicates deposition of sandy silts from waning currents followed by settling of fine silts from the water column. Locally, chemical sediments are important additions to the sequence, especially in the cyclic deposits stratigraphically below the ore deposit where clastic fining-upward sequences are overlain by carbonate crusts and/or displaced by carbonate nodules, Zones of strong mineral authigenesis are present in various parts of the Upper Mount Isa Group. In the upper formations the clastic sediments are highly albitic. In the Pb-Zn orebodies clastic sediments rich in authigenic cherty K-feldspar and chert are common. The sedimentary sequence is interpreted as the depositional record of a saline-lake complex. Most of the sequence above and below the ore deposit formed in flood-plain and subaerial to subaqueous saline mudflat environments. The main part of the Pb-Zn deposit is hosted by slope to basin facies sediments which formed in a perennial saline lake. The immediate source of this sediment was terrigenous and intrabasinal fine clastic material which was deposited during flooding stages of the saline-lake complex. The composition of the intrabasinal sediment source evolved in time from carbonate, to carbonate-sulphate, to carbonate-sulphate-Na-evaporites. The ore-hosting sediments formed during the main sulphate precipitating stage of the depositional system. Clastic and mainly current-deposited sulphate-evaporite-rich sediments provided the sulphur for the formation of diagenetic pyrite and epigenetic galena, sphalerite and pyrrhotite. Pyrite formed by microbial sulphate-reducing reactions during diagenetic dissolution of sulphate minerals. Galena, sphalerite and pyrrhotite formed later when brines rich in chloride-complexed metals invaded the sequence and reacted inorganically with sulphate-rich sediments.