A study of the base-metal, gold and barite mineralization in the Silurian volcanosedimentary belts of the Canberra region

Abstract

The Silurian volcano sedimentary belts of the Canberra region contain numerous small deposits of base-metal sulphides, iron oxides, barite and gold. Many different styles of mineralization are present including: veins, stockworks, massive stratiform bodies, and stratabound veins and disseminations. Primary features in the deposits and the host rocks are commonly masked by the development of greenschist facies mineral assemblages and a metamorphic foliation. The foliation in some rocks is domainal so that some premetamorphic minerals and textures are preserved and it is possible to infer premetamorphic alteration assemblages and textures. The distribution of volcanic and sedimentary facies in the Canberra region suggests that the Late Silurian palaeogeography consisted of a meridionally trending. deep-marine trough bounded on its western side by a broad shallow-marine shelf with local subaerial areas. The trough was bounded on its eastern side initially by a shallow-marine shelf and later by a belt of subaerial volcanics. Numerous zones of mineralization, including the Woodlawn and Captains Flat massive sulphide deposits, occur in rocks deposited in the trough. The shallow-marine and terrestrial sequences along both sides of the trough contain only minor, small vein deposits. The Late Silurian sequence in the Bredbo-Bunyan area consists of basal shallow-marine sedimentary rocks overlain by subaerial volcanic rocks with rare sedimentary rocks followed by a transgressive sequence overlain by interbedded mudstones and volcanic rocks deposited in a deep-marine environment. A number of zones of mineralization, including the Cosgrove Hill, Billilingra, Barite, Driscolls Hill, Hamett and Stonehenge Prospects, form a mineralized horizon along the top of the subaerial volcanic sequence and in the transgressive sequence. Mineralization consists of disseminations and veins of pyrite, base-metal sulphides and gold-rich sulphidic barite associated with zones of sericitic, K-feldspar, advanced argillic and propylitic alteration. In parts of the volcanics below this horizon, including at the Picasso and Stonehenge Prospects, there are narrow zones of sericite, pyrite, ±kaolinite alteration surrounded by broad zones of propylitic alteration. In the basal sedimentary sequence at the Gillans Prospect sericite, pyrite alteration zones are surrounded by propylitic zones which contain pyrrhotite and pyrite. Above the mineralized horizon there are a few jasper zones. There is no obvious difference in the primary geochemical composition of the volcanic rocks above and below the mineralized horizon in the Bredbo-Bunyan area. The sericitic rocks in the mineralized horizon have gained K and Rb and lost Fe, Mg, Ca, Na and Sr whereas chloritic rocks have gained Fe, Mg, K, Rb, and V and lost Ca, Na, and Sr. The rare earth element (REE) patterns of the volcanic rocks were preserved during low-grade alteration and metamorphism except for zones of intense sericite alteration where the heavy REE were lost during the destruction of hornblende. The sulphur isotope composition of sulphides from below the mineralized horizon form a tight cluster around 𝛅³⁴S=+5%o whereas those from the mineralized horizon range from -14.9 to +3.5%o. Barites from the main horizon range from +20.5 to +26.0 which is lighter than Late Silurian seawater sulphate (+26%o). The results suggest that seawater cannot have been the only source of sulphur in the system and are consistent with the equilibrium oxidation of the sulphide in the hydrothermal fluid, possibly by mixing with seawater. The mineralization in the Bredbo-Bunyan area formed in an epithermal system operated in a mixed subaerial and submarine setting. The Peak View massive sulphide deposit occurs a small outlier of Silurian volcanic and sedimentary rocks 53 km south of Captains Flat. Footwall rocks include volcanic rocks (deposited at least in part as lavas), volcaniclastic rocks and biogenic limestones. There is a broad zone of propylitic alteration with sulphides in veins and disseminations in the footwall beneath the sulphide lens. The mineralized horizon consists of a stratiform massive sulphide lens, fine-grained volcaniclastic rocks and carbonate bodies. The sulphide lens is composed of massive sulphides (pyrite, sericite and chlorite with minor sphalerite, galena, chalcopyrite, quartz and rare tennantite-tetrahedrite and arsenopyrite) with minor pyritic chert and veins of quartz and chalcopyrite. The hangingwall rocks are volcanic rocks, deposited partly as lavas, which contain zones of chloritic, albitic (originally zeolitic?) and sericitic alteration. The hangingwall rocks have a different primary geochemical composition to the footwall volcanic rocks. The Peak View deposit is of the volcanic associated, massive sulphide style and may have formed by convective circulation of seawater below the seafloor.