Regional metamorphism and the alteration response of selected Silurian to Devonian mineral systems in the Nymagee area, Central Lachlan Orogen, New South Wales: a HyLogger™ case study

<p>The southern Cobar Superbasin includes the Mount Hope, Cobar and Rast troughs and adjacent Winduck, Mouramba and Kopyje shelves. These stratotectonic units host a range of mineral systems within deformed upper Silurian to Lower Devonian rocks. Despite the metallogenic importance of the terrane the regional metamorphic- and alteration-related mineral assemblages have not been systematically described until now. Here, we present a study using the HyLogger™ spectral scanner to systematically map changes in both background and alteration-related mineralogy for 14 mineral systems across the Nymagee area. The study found that, with the exception of the late diagenetic zone assemblages associated with the Winduck Shelf, the mineral assemblages for zones away from mineralisation are consistent with uniform sub- to lowest-greenschist facies metamorphic conditions, with no evidence for a regional, thermal field gradient across the basin. By contrast, proximal to mineralisation, there are significant changes in alteration-related mineral abundances and species reflecting elevated fluid temperatures. For several zones, including Browns Reef, Great Central, Hera, May Day, Nymagee and Shuttleton, there is a systematic change in chlorite composition from Fe- and/or Fe–Mg-chlorites to more Mg-rich varieties towards mineralisation. In addition, talc was noted for May Day, Mineral Hill and Shuttleton. The change in chlorite composition suggests that these ore-forming fluids were undersaturated with respect to iron. Furthermore, although present in discrete zones, carbonate alteration is not a dominant alteration type. However, at Manuka and Blue Mountain, the mineralisation is associated with carbonate-rich (calcite, ankerite, dolomite) units that have been dolomitised (Mg alteration) but not all dolomite-rich zones are mineralised. For Mineral Hill, there are systematic changes to mineral species/chemistry with observed data consistent with Parkers Hill being proximal to the centre of a mineralising system. The G and H Lode area (Southern Ore Zone) is slightly more distal whereas the Pearse deposit may represent a zone more distal again.</p

Basin provenance and its control on mineralisation within the Early Devonian Cobar Basin, western Lachlan Orogen, eastern Australia

The Cobar Basin, in central New South Wales, is an Early Devonian extensional basin that formed in the western Lachlan Orogen. The basin was filled with shallow- to deep-water sequences of the Cobar Supergroup and hosts small to large polymetallic deposits. Detrital zircon geochronology and whole-rock geochemical data collected from the Amphitheatre Group of the Cobar Supergroup provide constraints on the history of basin fill and illustrate the dynamic interplay between basin provenance and mineralisation, corresponding to the evolving tectonic regime. Data reveal provenance dissimilarity between the southern and northern parts of the basin. In the south, units of the Amphitheatre Group received abundant detritus from ca 430–410 Ma magmatic rocks situated to the southwest and southeast of the basin. By contrast, the northern successions were predominately sourced from recycled Ordovician basement found to the northwest, north, northeast of the basin, along with contributions associated with the Macquarie Arc. This spatial provenance variation, however, is less significant in the younger formations: the northern and southern sequences both exhibit an increase in older recycled detritus upwards with time. This reflects a progressive modification of basin paleogeography, during the transition from rift phase to sag phase. The rift-phase basin geography is characterised by fault-restricted deposition with predominant sediments derived from local proximal sources. The subsequent sag-phase subsidence exhibits a uniform depositional system with more homogenised basin input. This provenance variation is coeval with the stratigraphically controlled mineralisation features within the Amphitheatre Group successions, implying a provenance influence on mineralisation. Data suggest the different sediment source regions have produced distinct detrital mineral compositions between the major mineral-hosting and mineral-barren formations. The enrichment of some detrital minerals in the mineral-hosting units, such as feldspar, muscovite, Ti-minerals (and carbonate), is suggested to be an important factor for mineralisation in the basin. Detrital zircon geochronology and whole-rock geochemistry illustrate the history of basin fill for the Cobar Basin. Spatial and temporal variation of basin provenance reflects a modification of basin geography, corresponding to the evolving tectonic regimes. The change in basin source regions is one of key controls on the mineralisation within the Cobar Basin.</p