The Murchison Gorge, Tasmania; a possible cross section through a Cambrian massive sulfide system

Univ. Manchester, Dep. Geol., Manchester, United Kingdom
Bur. Mines. Resour., Australia
Univ. Ariz., United States
Aust. Natl. Univ., Australia

The Murchison Gorge, northeast of Rosebery in Tasmania, is believed to expose a section through the main elements of an inferred volcanogenic massive sulfide-forming system. The top of the steeply dipping section is dominated by shallow marine siltstones of the Farrell Slates which host several sulfide bodies. The Farrell Slates conformably overlie an altered succession of rhyodacitic and dacitic tuffs and lavas, the Murchison Volcanics. A sill-like monzonitic-quartz monzonitic body, the Murchison Granite, is emplaced in the Murchison Volcanics about 2 to 3 km stratigraphically below the Farrell Slates.The sulfide deposits in the Farrell Slates are of Devonian age, but sulfur isotope studies and comparisons with the Cambrian massive sulfide horizons at Red Hills and Rosebery suggest that the slates are a local Cambrian mineralized horizon. The Murchison Granite and the Murchison Volcanics appear to be comagmatic and coeval and are similar to calc-alkaline intrusions and volcanics of orogenic continental margins.Both the volcanics and the granite are extensively altered. The granite has been subject to potassic alteration, chloritization, and late calcite-epidote alteration. Minor greisenization was also noted.There is a vertical zonation of alteration mineralogy in the volcanics between the Farrell Slates and the top of the Granite as follows: top: sericite zone--(a), sericite, quartz, (b), sericite, quartz, calcite, K-feldspar, chlorite, hematite, chalcopyrite, pyrite, (c), sericite, quartz, chlorite, calcite, hematite, (K-feldspar, albite); chlorite zone--chlorite, sericite, albite, calcite, (hematite, magnetite); epidote zone--epidote, chlorite, calcite, magnetite, (allanite); potassic zone--chlorite, epidote, calcite, K-feldspar, pyrite, magnetite.The increasing predominance of hematite over magnetite toward the top of the section is consistent with an increase in fluid f (sub O ₂ ) away from the granite. Chlorites from the four zones lie in fairly clear fields on an MgO vs. FeO plot. Trends of decreasing MgO and increasing FeO up the section are consistent with decreasing temperature.Sulfide delta ³⁴ S values increase from about 9 per mil within the granite through 10 to 14 per mil in the overlying volcanics to 17 per mil in the Farrell Slates. The high values suggest Cambrian seawater sulfate (30-35ppm) as the dominant sulfur source in the system. The up-section variation is ascribed to varying degrees of mixing between Cambrian seawater-derived sulfur and lighter magmatic sulfur.

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