Abstract

Most of the mineral deposits in the Mount Read Volcanics are of Cambrian age and are either stratiform exhalative bodies or epigenetic feeder systems (e.g., Rosebery, Mount Lyell, Que River). Others were formed at depth during the same or other phases of Cambrian seawater circulation (Lake Selina, Prince Darwin) and a few consist of probably remobilized veins in late Cambrian volcanics (Lake Dora, Jukes Proprietary). A number were formed from solutions circulating during Devonian deformation and metamorphism or derived directly from granitoid plutons.Sulfur in the stratiform deposits and feeder zones (-10 to + 19ppm but mostly 7-15ppm) and in the deep circulation systems exposed near granite at Murchison, Lake Selina, and Darwin (8-17ppm) is probably a mix of sulfur derived from the host volcanics and granites, and sulfate from reduced Cambrian seawater. A rock sulfur content declining with time is a more likely explanation for the upward increase of delta 34 S values in the Rosebery ore (8-17ppm; Green et al., 1981) than is an increasing water/rock ratio. No comparable zoning is apparent at Mount Lyell or Que River where the systems appears to be dominated by rock sulfur (6-10ppm). Late stratiform barite seen in the major deposits develops from systems containing only partially reduced Cambrian seawater sulfate (delta 34 S values [raquo] 30ppm), the process of sulfate removal during circulation having decreased in efficiency owing to destruction of reductants.Devonian fluid activity in the north of the region was dominated by magmatic solutions derived from shallow granite plutons. These reduced, sulfur-poor solutions commonly increased their sulfur content by dissolution of Cambrian sulfides having relatively high delta 34 S values (Rosebery pyrrhotite, North Mount Farrell). In the southern part of the region the Devonian fluids were relatively oxidized, resulting in precipitation of barite and barite + sulfide veins (19-31ppm), and at Mount Lyell, high-grade bornite-chalcopyrite-chalcocite assemblages (-10.5 to +3.5ppm).

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