The mineralogical history of the Broken Hill Lode, NSW
Titel:
The mineralogical history of the Broken Hill Lode, NSW
Auteur:
Plimer, Ian R.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 31 (1984) nr. 4 pagina's 379-402
Jaar:
1984-12
Inhoud:
The Broken Hill deposit, western NSW, is an intensely deformed and metamorphosed submarine exhalative deposit, with associated hydrothermal alteration and exhalites, formed in a lineament-controlled rift. The sulphide and enclosing silicate rocks at Broken Hill have undergone the same complex deformational and metamorphic history. High-grade metamorphism and coeval deformation were accompanied by mass movement of sulphides with fold hinges, sulphide brecciation, development, of sulphide and silicate porphyroblasts, coarsening of grainsize, development of sulphide-bearing pegmatites, attainment of surface-tension equilibrium between coexisting minerals, and the formation high-grade metamorphic mineral assemblages comprising sulphides, pyroxenes, pyroxenoids, garnets, spinels and feldspars. At the waning stage of high-grade metamorphism, ore breccias were formed, sulphide minerals (e.g. galena, chalcopyrite, pyrrhotite) were mechanically injected into transgressive fractures, and further redistribution took place by secondary hydrothermal processes. Exsolution and replacement of earlier prograde silicate and sulphide minerals formed many new minerals. Secondary hydrothermal processes formed pegmatoid masses, transgressive veins (quartz, sulphide and silicate) and cavities. Retrogression of the sulphide rocks occurred in shear zones, which transgressed and displaced prograde ore, resulting in brecciation, plastic flow, sulphide injection and retexturing. Ag, As, Sb, Cu and Pb moved from prograde ore into shear zones, and prograde minerals underwent decomposition to other sulphides, exsolution and replacement. Secondary hydrothermal processes formed transgressive quartz, sulphide and carbonate veins. The orebodies are transgressed and displaced by minor fault zones. Sulphide rocks were fractured and retextured and new phases formed by decomposition, exsolution and replacement. Sequential precipitation of carbonates and other phases on fault planes and in crystal-lined cavities is characteristic. The oxidised zone of the Broken Hill orebody is characterized by telescoped mineral assemblages, collapse structures, solution features, overgrowths, boxworks after cerussite, greatly variable depth of oxidation, and the lack of a supergene zone—all indicative of a long history of multiple rejuvenation of the weathering of the Broken Hill deposit. The Willyama Supergroup has undergone Proterozoic erosion and it is possible that the secondary zone at Broken Hill is Palaeozoic in age.
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