School of Earth Sciences - Theses

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    Geochemistry and mineralisation of primary and secondary platinum-group elements in the ultramafic "Alaskan-type" Owendale complex and laterites in the Fifield Region, New South Wales, Australia
    Shi, Bielin ( 1995)
    The Owendale Complex belongs to a family of ultramafic-mafic intrusions that is characterised by a zonal, nonstratiform arrangement of the principal ultramafic units. The ultramafic rocks of the Owendale Complex are virtually identical to many of the Alaskan-type intrusions, however the associated gabbroic rocks (wehrlites) are K-rich and Si-undersaturated, in contrast to the tholeiitic gabbroic rocks of the Alaskan examples. The intrusion history of the Owendale Complex is thought to have involved emplacement of a gabbroic intrusion that was invaded by an ultrabasic magma, possibly while the former was still only partly solidified. Emplacement of both magmas probably occurred during Late Devonian tectonism and deformation synchronous with emplacement and crystallisation is necessary to explain the present non-stratiform arrangement of the rock units. The most obvious linkage factor between the two proposed parent magmas (gabbroic and ultrabasic) of the Owendale suites is their mutual affinity with tholeiitic basalt magmas and the similarities of their products with intrusions of alkalic basalt derivation. This suggests the possibility that the Owendale Complex rocks and those of other tholeiitic intrusions of the regions are comagmatic products of an ancestral magma that may have also produced the widespread assemblage of complexes. Viewed from this perspective, the ultramafic rocks of Owendale Complex would thus represent a very minor product of a period of regional magmatic activity. Most alloys, erlichmanite, cooperite and some grains with exclusion texture of Pt-Os-Ir-Pd-Rh are considered to represent a primary high-temperature paragenesis. Concentration of PGE in pegmatoidal units of dunite-wehrlite is explained by the accumulation of platinum-rich alloys that segregated directly from the melt at an early stage in the evolution of the complex. The high-temperature PGM segregate directly from a silicate melt and were not generated by exsolution from spinels or magmatic sulphides. These suggest that fS2 was generally low (subordinate sulphide formation) and, after some influence at the beginning, has given way to rising fO2 (chromite, olivine and Pt-Fe-Cu-Ni alloys formation). After lithification, the ultramafic rocks become subject to "reducing" conditions, i.e., conditions of lower O2 and S2 activities. Ni-Fe alloys, native Fe and Bi formed in cracks which filled the serpentine matrixes. The former PGM (erlichmanite, cooperite and Pt-Fe alloys) were exposed to the reducing conditions via cracks were desulphurated to form porous cooperite with Pt-Fe alloys and multiphase textural Os-Ir-Ni, Pt-Ir aggregates. It is plausible that the veinlets and aggregates of unnamed Rh-Sb-S, (Pt, Ir)2(Fc, Cu)3(S, Sb, AS)3 in the dunites may also have been formed by reduction of Ni-rich sulphides and erlichmanite, Pt-Fe alloys or cooperite. Late PGM are dominated by sperrylite-geversite solid solution resulting from the reaction of early PGM with a fluid phase. A hydrothermal origin is also indicated for native Fe, native Bi and awaruite (NiFe) and the base-metal sulphides (pentlandite, chalcopyrite, sphalerite, arsenopyrite, pyrite, pyrrhotite, and some Ni-Co-Fe sulfide). The cause of the reducing conditions may have been related to H2 production accompanying hydrous alteration of the dunites and clinopyroxenites. The laterites overlying the ultramafic complexes in the Fifield region are exceptionally well-developed and well-preserved weathering profiles. Field, textural and geochemical data all support a chemical weathering origin for the profiles and compatible with meteoric and ground water origins. Meteoric water with intermediate Eh and pH and negligible dissolved species sinks into the laterite where these parameters are modified. The Eh rises and pH decreases to the conditions typical of lateritic soils and the concentration of dissolved species increases. In this state the water is able to take PGE and Au into solution from a finely disseminated form in the bedrock as a part of the process of lateritisation. When the soil solution transports the PGE and Au towards a transitional interface must exist between the ferruginous and saprolite zones with lower Eh, neutral pH and lower concentration of dissolved salts. At this transitional region, deposition of the PGE and Au occurred. The presence of magnetic Pt-Fe-Cu-Ni alloys suggests that hydrothermal solutions play a later role in the Fifield region, and the alloys have grown in situ in a lateritic soil by a process involving laterite water solution in the high Eh, low pH conditions prevalent in such soil, followed by deposition when the conditions become less extreme. Some examples of the Pt-Fe alloys from such an environment become frequently strongly magnetic with larger size. It is assumed that the temperature of the hydrothermal solution is in the range of 300° - 500° C (Bowles, 1990). PGE mineralisation in the primary rocks and laterite in this region has demonstrated a good example of multi-stage process mineralisation including primary high temperature magmatic formation; low temperature postmagmatic hydrothermal alteration and residual lateritic enrichment.
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    Genesis of volcanogenic epithermal gold-silver mineralization, Budawang Rift, New South Wales, Australia
    Glaser, Lawrence Martin ( 1988)
    The genesis of four volcanogenic epithermal Au-Ag deposits located within the Budawang Rift of South Coastal New South Wales, Australia have been investigated. Co-genetic pyrophyllite deposits have also been studied. Mineralization is hosted within peraluminous rhyolites which comprise approximately 50% of the bi-modal (rhyolites and tholeiitic basalts) Budawang Volcanic Complex. All mineralization occurs within the confines of the Budawang Rift of early Late Devonian age, with which mineralization is temporally related. Based upon geochemical, isotopic, structural, and lithologic investigations, the former division of the intra-rift volcanic rocks into three units (Boyd, Comerong, and Yalwal Volcanics) has been abandoned, with the adoption of a new name to include all three co-magmatic rocks; the Budawang Volcanic Complex. The name Eden-Comerong-Yalwal Rift has also been abandoned, and the new name Budawang Rift applied. The Pambula, Wolumla, Grassy Gully, and Yalwal deposits are each located along the margins of separate rhyolite flow domes, located within cumulo flow dome complexes. All four deposits plus co-genetic pyrophyllite deposits occur along N-S trending faults of similar orientation and are probably genetically related to rift graben faults. The Pambula and Wolumla deposits, plus at least two proximally located pyrophyllite deposits are situated adjacent to E-W trending cross graben block faults which predate rifting but which were re-activated by that event. Fluid inclusion studies yield temperatures of mineralization of between 320° C and 380 ° C for the Au-Ag deposits, and 290 ° C for the pyrophyllite deposits. System fluids were highly saline, ranging from 12 to 17 wt. % NaCI equiv. for the Au-Ag deposits, to 9% for the pyrophyllite deposits. Salt species are NaCI dominant with variable amounts of CaC!. No carbon dioxide was documented in fluid inclusions. Depth calculations for mineralization using the salinity corrected critical path of boiling fluids (most of the intra-rift deposits display evidence of phase separation) yield depths of: Pyrophyllite deposits 800-1,000 m, Pambula 1,200 m, Yalwal 1,300 m, Grassy Gully 1,450 m, and Wolumla > 1,600 m. Ore mineralogy is dominated by electrum which displays a distinct Au:Ag compositional ratio for each deposit, and which conforms to a temperature-depth profile for the suite of deposits, with increasing Ag in the higher temperature deposits. At Wolumla, other Ag minerals identified include native Ag, acanthite, stephantite, antimonpearceite, arsenpolybasite, pearceite, and proustite. cerargyrite, and the very rare selenide minerals naumannite and aguilarite. At Grassy Gully, trace amounts of the telluride minerals hessite and petzite were also identified. Ore associate mineral assemblages include chalcopyrite, galena, sphalerite, chalcocite, chalcostibite, tetrahedrite, tennantite, and arsenopyrite. The bulk of these minerals are co-depositional to electrum. Multiple episodes of pyrite have been identified in all goldfields, and always occur post-brecciation and shearing, but pre-electrum and pre-electrum associate mineral deposition. Some pyrites display As as well as optical zonation patterns. The chemical composition and paragenetic sequence of all mineral species identified have been documented. The levels of Se substitution of S in the Ag sulphosalts, and composi tion of naumannite and aguilarite are also discussed in detail. Ore geochemistry yields a vertical metal zonation pattern among the deposits, with the deepest and highest temperature deposit containing higher concentrations of base metals, Se, and Ag. In the shallower deposits, correlation coefficients and metal ratios indicate a decoupling of base and precious metals, and a previously un-recognized behavioural aspect of Sand Se in boiling epithermal systems.......