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


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Bath, A 2005 , 'Petrography and geochemistry of the mesoproterozoic Gawler Range volcanics, South Australia', Honours thesis, University of Tasmania.

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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.

Item Type: Thesis - Honours
Authors/Creators:Bath, A
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Copyright 2005 the Author

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