Petrography and geochemistry of the mesoproterozoic Gawler Range volcanics, South Australia

The Gawler Range Volcanics (GRV), located in the central region of South Australia, represents the eruption of enormous volumes of felsic lava (25,000km3 ) at the beginning of the Mesoproterozoic (~1.59Ga) and is closely associated with the emplacement of the widespread Hiltaba Suite (HS) granites both spatially and temporally. Since the discovery of the Giant Olympic Dam Cu-U-Au-Ag deposit in 1975 (which is intimately related to the emplacement of the HS), both the HS and GRV have attracted vast amounts of attention from research, government and exploration groups. Efforts to understand the HS has been largely hampered by the thick regolith cover which blankets much of the central region of South Australia. The GRV in contrast, is well-exposed and detailed volcanology (Morrow, 1998; Gamer and McPhie, 1999) and geochemical studies (Stewart, 1994) suggest that it represents lava flows, which were produced from the fractionation of a mafic source. One problem associated with the GRV is the wide extent of its homogenous rhyolitic and dacitic lava flows. Creaser and White (1991) reported that the upper GRV, which represents a single emplaced unit >8000 km2 , is characterized by dry (<1 wt% H20), high temperature magma (900-1010°C), which was derived from a single homogeneous source. Gamer and McPhie (1999) advocated that the GRV are lava flows as opposed to ignimbrites (e.g. Stewart, 1994). The notion that these widespread homogenous volcanics are lava flows is at odds with the high viscosity associated with rhyolitic and dacitic lavas. Geochemical studies undertaken by myself however show that rhyolites from the GRV contain 1400-1800ppm ofF and glass inclusions also from the GRV (which are thought to represent melt trapped at depth) contains up to 12,500ppm ofF, which is remarkably high for degassed lava and trapped melt respectively. I propose that F, known as an effective depolymeriser, may have significantly reduced the viscosity of the lava and thus allowed it to flow over an extensive area. Melt inclusions which are hosted within quartz phenocrysts from the upper units of the Eucarro Rhyolite (GRV) were collected from seven samples belonging to three different rock types, which included; 1) plagioclase rhyolite, 2) vesicular rhyolite, 3) quartz rhyolite (also see Morrow et a!., 2000). Each rock type contains excellent examples of large glass inclusions along with magmatic fluid inclusions which were found within the same trapping planes. Samples from the plagioclase rhyolite and the vesicular rhyolite also contain quartz with high-density C02 inclusions, glass inclusions and non-silicate melt inclusions. From the three rock types over fifty homogenized glass inclusions were analyzed for major elements (SX-100 Cameca microprobe) and over eighty for trace elements (laser ablation/IC-PMS). Comparisons were than made between the groundmass and melt inclusions. Each sample showed remarkable similarities in REE (except Eu, which is enriched within the groundmass ). In contrast though, elements such as F, Mo (x1-6), W (xl0-20), U (x1-2.5) and Pb (x3-9) were significantly enriched within melt inclusions. A similar REE concentration, between the groundmass and melt inclusions, suggests that both were derived from the same source, whereas enrichments ofF (x6-7), Mo (xl-6), W (x1 0-20), U (x1-2.5) and Pb (x3-9) within melt inclusions may reflect the concentration of these elements within the melt at depth and therefore concentrations prior to eruption and subsequent degassing. The presence of high-density C02 inclusions, magmatic fluid inclusions and nonsilicate melt inclusions suggests that a significant amount of degassing of the magma may have occurred at depth. This suggests that a significant amount ofF, Mo, W, U and Pb may have been lost as volatiles from the melt and therefore have likely either ended up as mineral deposits within the country rock, or were released into the atmosphere during eruption. If the former was significant, than the location the volcanic vent(s) which link the GRV and HS may be of enormous economic value.