Geology and Volcanic Facies Architecture of the Lower Ordovician Waterloo Massive Sufide Deposit, Australia

Abstract

The Waterloo massive sulfide deposit occurs in the Cambro-Ordovician Seventy Mile Range Group in the Charters Towers district in northern Queensland, one of the principal submarine successions in eastern Australia that contains abundant volcanic-hosted base metal deposits. The deposit consists of several small dismembered, stratiform, blanketlike, pyrite-chalcopyrite-sphalerite-galena massive sulfide lenses hosted by intensely altered volcanic rocks. The deposit and its host rocks have been deformed during regional deformation of the Middle to Late Ordovcian and were affected by two subsequent events of brittle deformation. The Waterloo deposit formed on top of a lava-dominated andesite succession comprising coherent volcanic units and related juvenile volcaniclastic rocks. The hydrothermal activity leading to the deposition of massive sulfides commenced during the waning stages or after the cessation of the andesite volcanism. The blanketlike mineralization is interpreted to have formed largely at or very close to the sea floor in a moderately deep to deep marine environment. Formation of the massive sulfides was accompanied by the mass-flow deposition of coarse quartz-feldspar crystal-rich volcanic debris that records broadly contemporaneous, probably explosive, rhyolitic volcanism occurring outside the immediate study area. This resedimented volcaniclastic unit retains a record of hanging-wall alteration, indicating that the Waterloo hydrothermal system was operative during and after burial by the coarse volcaniclastic material. The deposition of the coarse volcaniclastic rocks was followed by the intrusion of a feldspar-porphyritic dacite cryptodome that was also subject to hydrothermal alteration associated with the massive sulfide formation. Facies relationships suggest that the cryptodome locally breached the wet and unconsolidated coarse volcaniclastic material to emerge on the sea floor. The emplacement of the partially extrusive dacite cryptodome coincided with the waning of the Waterloo hydrothermal system. The subsequent period of relatively quiet sedimentation was occasionally interrupted by the emplacement of synsedimentary basalt and andesite sills as volcanism at Waterloo shifted back to more mafic compositions. This period was followed by the mass-flow deposition of coarse feldspar-quartz-vitric volcanic debris and the onset of another period of intense basalt- to andesite-dominated volcanism. Geochemical analyses of volcanic rocks from the host-rock succession of the Waterloo deposit and correlation with regional data suggest that the massive sulfide deposits in the Charters Towers district formed during a specific period of Cambro-Ordovician back-arc basin development dominated by mantle-derived volcanism. Comparison with the volcanic architecture of other known massive sulfide deposits in the district indicates that massive sulfide mineralization at Charters Towers is mainly associated with silicic shallow, intrusion-dominated volcanic centers and occurrences of this volcanic facies should be considered prospective for other deposits in the belt.