During the Palaeoproterozoic, a large number of granitic intrusions were
emplaced in the various parts of the Pine Creek Geosyncline (PCG). In
the central region, the granitic magmatism is represented either by the
coalesced mass, eg the Cullen Batholith, or individual plutons such as the
Mount Bundey Granite which together with the Mount Goyder Syenite
intrude the metasedimentary strata, causing wide spread contact aureoles.
Some of these granitic intrusions are temporally and genetically related to
gold (and less commonly base-metals) mineralisation present in the
adjacent metasediments. These plutons are characterised by the presence of biotite and homblende
which appear late in the paragenetic sequence. Electron microprobe
analyses of biotite and homblende show that these minerals are poorly to
moderately zoned. Biotites are characterised by a decrease in Fe/(Fe +
Mg) ratios when plotted against AIVI. Similarly a modest decrease in Ti
from centre to the margins of the biotite crystals has also been observed.
Furthermore, an increase in Mg/(Mg + EFe) ratios from centre to the
margins as well as in mafic to felsic rocks is also apparent. On the other
hand, homblendes from the granitic bodies display less well marked
trends and are poorly zoned. However, they show systematic increases in
Mg/(Mg + EFe) ratios, and decreases in Fe, Al"' and Fe/(Fe + Mg) ratios,
when plotted against Si. Magnetic susceptibility measurements on powdered samples of the
plutons give results characterised by high values ranging from 0.8 x 10-4
to 2.0 x 10-3 emu/g which place the granites in magnetite- series field.
This indicates that the oxidation state of the magma was high during
crystallisation. The range of chemical compositions of the mafic phases indicates a
rise in the oxidation state of the magma as it progressively became
saturated in H20-volatiles and other metals, not partitioned into the
earlier crystallised phases in the final stages of magma consolidation.
These variations in chemical compositions also indicate that initially fOz
was high and became progressively higher during fractional
crystallisation of magmas; this subsequently led to the emanation of
hydrothermal fluids responsible for the mineralisation in the adjacent
metasediments.