Meta-evaporite sequence, exhalative chemical sediments and associated rocks in the Proterozoic Willyama Supergroup, South Australia: implications for metallogenesis

Abstract

The Willyama Supergroup comprises Proterozoic meta-sedimentary and meta-igneous rocks in western New South Wales and eastern South Australia, and contains the giant Broken Hill PbZnAg deposit, in addition to numerous small stratiform, stratabound and vein mineral deposits. New lithological, geochemical and fluid inclusion data from the Olary Block, west of Broken Hill, suggest the presence of former evaporite rocks now metamorphosed to a variety of chemically and mineralogically unusual lithologies. These evaporite rocks overlie relatively oxidised strata, and are in turn overlain by reduced, graphite-bearing pelites. Rocks of sodic composition abound, represented by albite-quartz-rich rocks (+calcite, actinolite, epidote, garnet, clinopyroxene), inferred to represent metamorphosed analcime-rich strata of felsic volcaniclastic provenance, with variable proportions of carbonates; for example calcite, dolomite, trona and shortite. These rocks are interspersed with potassic rocks, possibly also with an evaporitic origin, in addition to “normal” clastic meta-sediments. The presence of halogen-rich minerals in a distinctive mineral assemblage, hypersaline fluid inclusions, pseudomorphic voids and minerals after gypsum and carbonate, in addition to tourmalines with isotopically light boron isotopes is interpreted as evidence for a non-marine evaporite parent rock for portions of the Olary Block. Types of mineral occurrences in the Olary Block form a continuum between chemical sediments formed by the exhalations of hydrothermal fluids to intrasediment deposits, inferred to have been formed due to the chemical changes imposed on these and/or diagenetic fluids during their passage through the sedimentary pile. Disseminated stratabound pyrite (including cobaltian types) and chalcopyrite extend along strike for many kilometres and are interpreted to have formed by a diagenetic mechanism. The presence of massive sulphide-bearing strata, iron formations, barite, tourmalinite and manganiferous rocks are typical of chemical sediments derived from hot spring sources. The Olary Block meta-evaporites have similar whole-rock compositions to Tertiary analcime-rich rocks of the Green River Formation, Wyoming. These new data imply that portions of the adjacent Broken Hill Block may also comprise meta-evaporites, for example the quartz-albite-rich strata of the Thackaringa Group and the calcsilicates of the lower Broken Hill Group, and that the disseminated cobaltian pyrite mineralisation in the Thackaringa Group may not be of exhalative origin.