Abstract

Mapping of the Permian stratigraphy at the Mount Chalmers massive sulfide + footwall stockwork Cu-Au orebody has revealed shallow-water body fossils, trace fossils, and non-welded to partly welded pyroclastic units within the flat-lying host sequence. The massive sulfide-bearing Lower Chalmers sediment and the Upper Chalmers sediment are each 20 to 40 m thick and comprise fossiliferous tuffaceous mudstone and wacke. They are separated by the North Star pumice breccia. The breccia is 50 m thick at the mine and comprises water-transformed, pyroclastic-flow deposits, but 2 km to the south and southwest it is 120 m thick and includes probable nonwelded ignimbrite. The Upper Chalmers sediment is overlain by a dacitic tuff which has a 5-m-thick, blocky to columnar-jointed interval near the base termed the Chalmers welded tuff. The tuff contains strongly spherulitic pumice lenses within a weakly spherulitic matrix with relict fine banding; it extends at least 450 m north and 300 m west from the mine.The Upper Chalmers sediment has yielded an assemblage of brachiopods, gastropods, bi-valves, and bryozoans from five localities 1 to 2 km north and south of the mine. These body fossils show limited reworking and are typical of a shallow-marine shelf setting, near or below wave base. Although seawater depths of 50 to 300 m are possible, a range of 100- to 200-m depth is considered most likely. The Upper and Lower Chalmers sediments contain trace fossils typical of the Cruziana ichnofacies below and above the ore position, also suggesting shelf sedimentation below the effective wave base. These consistent estimates suggest that seawater depths of 200 to 300 m prevailed during deposition of the massive sulfide, contrary to the 1,000-m seawater depth usually regarded as necessary to form VHMS ore deposits.The shallow depth would have allowed the ascending ore-forming fluids to boil prior to reaching the sea floor, unless prevented by channel constrictions. Boiling may account for the strong, Cu-rich, footwall stringer zone and elevated fluid inclusion salinities at Mount Chalmers (Large and Both, 1980); the duplication of these features at other deposits suggests that Mount Chalmers is not an isolated case. The relatively shallow-water depths have also permitted deposition of two pyroclastic-flow units. Sequences containing primary ignimbrites (at least those which are nonwelded) should therefore not be automatically excluded from exploration for VHMS deposits.

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