Facies and palaeoenvironment analysis of the Archaean volcanic-sedimentary succession hosting the Golden Grove Cu-Zn massive sulphide deposits, Western Australia

Abstract

The Archaean geology of the Golden Grove district was examined in the context of recent advances in the understanding of Archaean granite-greenstone terrains and utilising modem facies analysis techniques. It was determined that the geology of the Golden Grove district can be considered as a coherent tectonostratigraphic domain, but more geochronological data is required to confidently define its relationship to four other discrete domains of the Warriedar Fold Belt. Within the defined Golden Grove tectonostratigraphic domain a detailed stratigraphic subdivision was achieved, with three groups, the Gossan Hill, Thundelarra and Min jar Groups being defined. The stratigraphy of the Gossan Hill Group is further refined, with the definition of five formations and the subdivision of the central three formations into fourteen members. The establishment of a detailed stratigraphy for the Gossan Hill Group required the careful examination of the lithofacies association of the succession. The documentation and interpretation of the epiclastic and chemical sediment facies of the succession established that deposition occurred in a consistently deep subaqueous setting. In addition, the style of sedimentation, facies geometry and sediment provenance indicates that the basin was steep sided, extension related subsidence continued during deposition and the regional setting of the basin included a major continental crust influence. The characteristics of the volcanic facies present indicate that proximal volcanism was effusive in style and the eruption centres for the volcanism were felsic dominated, submarine dome and lava complexes. Three coherent-autoclastic volcanic facies associations are defined and their characteristics used to infer controls on the style of volcanism. The most important controls on the style of volcanism are: ( 1) high ambient hydrostatic pressure constraints in the eruptive environment; (2) relatively low magma eruptive viscosity; and (3) some insulation of the flows by stable film-boiling processes. Additional variables that influenced facies association development are: (1) proximity to the vent; (2)vent; (2) magma effusion rates and volume; (3) erupted volume relative to syn-depositional topography; and (4) the nature of the flow emplacement substrate. Magma composition, within the range of calc-alkaline andesite to rhyolite, did not strongly influence the style of volcanism. In contrast to previous interpretations of the succession tuffaceous volcaniclastic facies were found to be a minor component. The lithological characteristics of these facies indicates their derivation from contemporaneous silicic hydrovolcanic and magmatic eruptive centres in shallow marine to subaerial settings distal to the depositional site of the debris. Other facies characteristics, including the absence of evidence for welding, favours their emplacement by water supported mass-flows, precluding their interpretation as primary pyroclastic facies. Controls on the occurrence of VMS deposits within the Gossan Hill Group are consistent with generalised models for this deposit style, including a deep marine depositional setting, relatively low rates of sedimentation, the occurrence of syndepositional extensional faulting and volcanic facies with geochemical characteristics suggesting the presence of high crustal level magma chambers. The footwall hydrothermal alteration style indicates the Golden Grove VMS deposits are 'Mattabi-type', within the two -fold division of Archaean VMS deposits. These VMS deposits are also distinguished by the occurrence of active hydrothermal discharge preceding the major pulse of felsic volcanism within the host succession. In the absence of a coherent volcanic dominated footwall succesion the presence of juvenile tuffaceous volcaniclastic facies in the clastic dominated footwall stratigraphy may be critical to the development of concentrated hydrothermal discharge.