Academic literature on the topic 'Geology Victoria Lachlan Fold Belt'

Seismic refraction data were obtained for the Bass and Gippsland Basins during the 1988 cruise of the BMR research vessell "Rig Seismic". Seismic recorders were deployed on land by BMR and Monash University to record long-offset wide-angle reflection and refraction data using the ship's air-guns as the energy source. Preliminary results have now been obtained from these data providing information on deep crustal structure related to the basin formation. Two crustal layers have been detected with velocities of 4.5 km/s increasing to 7.4 km/s (unreversed) at depths exceeding 20 km. Additional data have now been obtained over a traverse length of 170 km to provide constraints on the deep structure of Bass Strait and the Lachlan Fold Belt in Victoria and Tasmania.

The majority of economic gold deposits in NSW are associated with Ordovician-aged igneous rocks and are examples of the Cu-Au porphyry-skarn-epithermal association commonly developed in convergent margin to orogenic settings. They are among the oldest porphyry Cu-Au deposits in the Pacific Rim region. They are similar to younger deposits in terms of tectonic setting and structure, but the largest are chemically distinct, being associated with shoshonite magmas (Cadia, Ridgeway and Northparkes). The Lachlan Fold Belt (LFB) porphyries are subdivided into four sub-groups based mainly on their age relative to development of the Lachlan Transverse Zone (LTZ) structure. Two subgroups pre-date the LTZ, one group is syn�LTZ and one group post-dates the LTZ. No mineralisation has been found or reported among pre-I.TZ porphyries. but it is common in post- . l Z_ porphyries. Petrographic analysis and microprobe results establish a wide range of primary and secondary features within the Ordovician rocks examined in this study. Cale alkaline to shoshonitic affinities are supported by the variable abundance of primary K-feldspars. Primary mineral phases such as pyroxenes and igneous magnetite provide an indication of fractioning mineral assemblages responsible for igneous trends in magma chemistry. The hydrothermal mineral assemblages documented in these LFB study areas are characteristic of younger Cu-Au Porphyry style mineralisation. As expected, the most pervasive alteration is associated with highly mineralised shoshonitic Ordovician rocks at Ridgeway, and Cadia. the less strongly mineralised calc alkaline Ordovician rocks at Cargo. Copper Ilill and Fairholme. are correspondingly less strongly altered overall. although secondary mineral assemblages are locally abundant. Many varieties of oxides and carbonates are observed at the different study localities. Most of the studied samples conform to igneous chemical trends because they are weakly altered, although post magmatic processes, such as veining, are detectable in certain trends. The K2O enrichment of the studied samples is consistent with subductionmoditied mantle wedge sources. A few effects, such as the high Fe203 contents of some Ridgeway samples, probably reflect porphyry-style hydrothermal alteration processes. Host rocks at the Cadia and Ridgeway are entirely alkalic on the K2O versus SiO2 plot and shoshonitic on the Total Alkalies versus SiO2 plot. Igneous rocks at the other deposits display a range of compositions between low K tholeiites to shoshonites that in some cases reflects multiple igneous suites. The LREE and L1LE enrichments, and HFSE depletions (Nb, Ta and Ti) of the magmas associated with these deposits are characteristics of a subduction-related tectonic setting. They all fall in the volcanic-arc granite and syn-collisional granite field of the Nb-Y tectonic discrimination diagram. Several magma types are identified by differences in the HFSE and REE trends. Differences in the extent and style of magma fractionation are evident in the trace element data. The Ridgeway samples define a wider range of trace element concentrations than the Cadia samples that may indicate a greater extent of fractionation during emplacement of the Ridgeway magmas. Fairholme samples display a high Nh and /If trends that are distinct from the main fields on Zr variation diagrams. Compositional differences between larger Cu-Au deposits, Cadia-Ridgeway and smaller deposits, Copper Ifill, Cargo and Fairholme are evident in terms of Nb-Ta depletion and variation. The smaller deposits show constant Nb/Ta or negative Nb/Ta trends that extend to high Nb. The larger deposits display positive Nb/Ta trends that do not extend to high Nb. This distinction reflects a difference of preferential incorporation of Nb in a mineral phase (magnetite). Comparisons between Cadia-Ridgeway and other shoshonite (altered samples of Bajo de la Alumbrera, Argentina), calc alkaline magmas from New Zealand and rocks from other areas indicate that Nb/Ta is not directly correlated with the shoshonitic classification, K2O vs. SiO2, and that the Cadia-Ridgeway Nb and Ta variation is not the result of alteration. The fact that the weakly altered LFB Capertee shoshonites exhibit a narrow range of Nb and low Nb/Ta suggest the shoshonite trend for the LFB as a whole is a steep one on the Nb/Ta versus Nb plot. The results of this study could provide important information for exploration within the LFB. Only the Cadia and Ridgeway deposits display a wide range of Nb/Ta values and lack the near-horizontal trend seen for other localities associated with smaller deposits. The tectonic evolution of the LFB is a major factor contributing to occurrence of large porphyry Cu-Au deposits. The sequence of important events, however, commences with sub-crustal generation of oxidised magma and finishes with efficient Cu-Au accumulation by hydrothermal processes at favourable structural sites. The increase in Au-Cu deposit size from small (Copper Hill-Cargo) to world class (Cadia-Ridgeway) indicates the importance of magma composition during this process. The most obvious differences between the Cadia-Ridgeway and New Zealand rocks is that the latter are volcanic in origin and associated with an arc-back arc system. Therefore, they did not form in a tectonic regime suitable for the evolution of porphyries and the focussed movement of hydrothermal fluids during dilatant episodes. As a result, they are not linked to mineralisation despite having Nb-Ta and Nb/Ta variations that are typical of the high oxidation states in Au-prospective magmas of the LFB.

Thesis (B. Sc.(Hons.))--University of Adelaide, Dept. of Geology and Geophysics, 2000?
Australian National Grid Reference 1:250000 SJ 54-3 and SJ 54-7. Includes bibliographical references (6 leaves ).

Southeastern Australia, part of the Phanerozoic Tasmanides, is a unique region where large amounts of granite (~30% of the surface rocks) with a very wide range of compositions (S-, I- and A-types) were intruded in the Ordovician to Triassic. The distinctive Carboniferous granites in the Lachlan Fold Belt (LFB) are transitional in time and space between the major magmatic episodes of the LFB and New England Orogen (NEO). There was a contemporaneous continental-arc developed in the NEO, products of which became the dominant source for the NEO Early Permian S-type granites. The Carboniferous granites in the LFB have characteristic compositions (K, Sr and LILE-enrichment and Y-depletion) similar to the Late Permian I-type granites (and to the S-type granites in part) in the NEO. The in situ microanalyses of zircon (both melt-precipitated and inherited; over 1280 grains analysed from 31 samples) from those granites, using SIMS and LA-MC-ICPMS, show that the Carboniferous magmatism in the LFB was closely related to the tectonic movement of the NEO, and that the granites with similar compositions which transect the two contrasting orogens had similar source compositions. From the isotopic compositions of zircon from the Carboniferous granites in the LFB, it is evident that the granites are distributed in zones of different ages and Hf and O isotopic compositions. The zones are approximately parallel to the NEO boundary. It is likely that tectonic activity related to the NEO triggered the production of the Carboniferous magmas and their source rocks. The new data demonstrate, however, that the Carboniferous granites were not directly related to the contemporaneous arc volcanism in the NEO. The source rocks of the Carboniferous granites in each zone consisted of different mixtures between a mantle-like underplate and relatively mature pre-existing lower crust during the Devonian. The distinctive O isotopic compositions of inherited zircon from the NEO S-type granites demonstrate that the source rock of the granites was a Carboniferous arc-related volcanogenic sedimentary pile which included increased amounts of Devonian volcanogenic sediments with time. The age difference between the inherited and melt-precipitated zircon indicates rapid crustal recycling to produce the peraluminous magmas within 15 Ma of source rock deposition. The I-type Moonbi Supersuite granites were generated from the underplate with a minor crustal contribution. As all the studied NEO granites have remarkably similar initial Hf isotopic compositions, the components comprising most of the NEO, including sediments, are similarly radiogenic, probably due to the geologic processes operating in the young accretionary orogen. Combining all the results of U-Pb dating and Hf and O isotopic analyses of zircon from the Carboniferous granites in the LFB and the Permian I- and S-type granites in the NEO, it is also inferred that the source rocks of the studied granites had similar isotopic compositions. This suggests a potential petrogenetic link between the granites across the two orogens. From a source composition similar to that of the most primitive LCG, three igneous components have evolved in different ways depending on the nature of the crustal materials incorporated into each component.