Groundwater geochemistry, hydrogeology and potash mineral potential of the Lake Woods region, Northern Territory, Australia

We collected 38 groundwater and two surface-water samples in the semi-arid Lake Woods region of the Northern Territory to better understand the hydrogeochemistry of this system, which straddles the Wiso, Tennant Creek and Georgina geological regions. Lake Woods is presently a losing waterbody feeding the underlying groundwater system. The main aquifers comprise mainly carbonate (limestone and dolostone), siliciclastic (sandstone and siltstone) and evaporitic units. The water composition was determined in terms of bulk properties (pH, electrical conductivity, temperature, dissolved oxygen, redox potential), 40 major, minor and trace elements, and six isotopes (δ18Owater, δ2Hwater, δ13CDIC, δ34SSO42–, δ18OSO42–, 87Sr/86Sr). The groundwater is recharged through infiltration in the catchment from monsoonal rainfall (annual average rainfall ∼600 mm) and runoff. It evolves geochemically mainly through evapotranspiration and water–mineral interaction (dissolution of carbonates, silicates and to a lesser extent sulfates). The two surface waters (one from the main creek feeding the lake, the other from the lake itself) are extraordinarily enriched in 18O and 2H isotopes (δ18O of +10.9 and +16.4‰ VSMOW, and δ2H of +41 and +93‰ VSMOW, respectively), which is interpreted to reflect evaporation during the dry season (annual average evaporation ∼3000 mm) under low humidity conditions (annual average relative humidity ∼40%). This interpretation is supported by modelling results. The potassium (K) relative enrichment (K/Cl– mass ratio over 50 times that of sea water) is similar to that observed in salt-lake systems worldwide that are prospective for potash resources. Potassium enrichment is believed to derive partly from dust during atmospheric transport/deposition, but mostly from weathering of K-silicates in the aquifer materials (and possibly underlying formations). Further studies of Australian salt-lake systems are required to reach evidence-based conclusions on their mineral potential for potash, lithium, boron and other low-temperature mineral system commodities such as uranium.This project was undertaken as part of the salt-lake mineral prospectivity project at Geoscience Australia during 2012–2013, which was supported by appropriation funding from the Commonwealth of Australi

The Relevance of Fluid Inclusions to Mineral Systems and Ore Deposit Exploration

Fluid inclusions provide the only direct samples of palaeofluids that may be related to mineralisation processes. In order to apply fluid inclusion data to study fluid flow on a larger scale, we have used a mineral systems approach, which regards a mineral deposit as part of a much larger system and considers all the processes that are involved in mobilising ore components from a source, transporting and accumulating them in a more concentrated form and then preserving them throughout the subsequent geological history. This not only enables a better understanding of fluid flow processes but also enables fluid inclusions to provide an exploration target that is much larger than the ore deposit itself. As an example, a study of fluid inclusions associated with gold mineralisation in the Tanami Region of Northern Australia was used to determine the temperatures and compositions of the ore fluids. Once the parameters of the mineralising fluids were established, the study was then expanded to a region of central Australia covering almost 100,000 km(2). It was concluded that a high temperature (320-360 degrees C), low salinity fluid containing CO2 and other gases was circulating in the northern part of this region at around 1720 Ma. This suggests the circulation of an orogenic gold style fluid and indicates that this region has potential for other orogenic gold deposits. In the southern part of this region, a lower temperature (120 to 190 degrees C), high salinity fluid with no detectable gases was present and appears to represent circulation of a basinal brine. In the second example, fluid inclusion data from Cu-U-Au- Ag-REE prospects in the Olympic Copper-Gold Province in South Australia were used to constrain geochemical modelling of the mineralisation processes. By combining the inversion-generated, 3-D geophysical maps with geochemical and magnetic susceptibility values derived from the modelling, it has been possible to divide the range of observed magnetic susceptibilities into divisions that represent the various alteration assemblages within this region. This approach allows us to use geochemical modelling to relate alteration assemblages, and hence, the predicted sites of mineralisation, to the geophysical expressions of the mineral deposit

Pine Creek, Northern Territory : explanatory notes

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Intrusion related gold deposits in the Tanami and Kurundi-Kurinelli goldfields, Northern Territory, Australia: Constraints from LA-ICPMS analysis of fluid inclusions

Gold deposits in the Tanami and Kurundi-Kurinelli goldfields of the Northern Territory, Australia, have geological and geochemical characteristics that distinguish them from the typical vein-hosted, orogenic gold deposits that occur in other parts of Australia. This study used cathodoluminescence (CL) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analyses of fluid inclusions to investigate the source of mineralising fluids from both the highly mineralised Tanami goldfield and weakly mineralised Kurundi-Kurinelli goldfields in northern Australia. Cathodoluminescence showed three generations of quartz in fractured and mineralised veins from the Groundrush deposit in the Tanami goldfield but the remaining deposits had uniform dark CL images. The LA-ICPMS trace element data indicate an evolution from B-As–enriched, magmatic fluid to a highly Sr-Baenriched fluid which results from interaction with sedimentary rocks. The fluids from both regions have chlorinity normalised Zn and Pb concentrations at the magmatic end of the metamorphic – magmatic mixing trend for these elements. This is in agreement with previous studies showing that gold mineralisation is coeval with magmatism in the Tanami region. The relatively high salinity of the fluids in both the Tanami and Kurundi – Kurinelli goldfields, and their trace element contents indicates that the deposits in both regions are intrusionrelated gold deposits. The higher gold endowment of the Tanami region may result from the multiple goldmineralisation events in this region whereas fluid flow associated with mineralisation in the Kurundi – Kurinelli goldfields appears to be of limited extent.This work was financially supported by Chinese Academy of Sciences President’s International Fellowship Initiative (Grant No. 2017VCB0018) to TPM, and by the National Natural Science Foundation of China (41773058), the Science and Technology Foundation of Guizhou Province ([2013]3083), the Opening Foundation of State Key Laboratory of Ore Deposit Geochemistry, and the Institute of Geochemistry, Chinese Academy of Sciences (201504)

Uranium mobility and deposition over 1.3 Ga in the Westmoreland area (McArthur Basin, Australia)

The Westmoreland area is located in the southern part of the McArthur Basin (Australia) and hosts several uranium (U) deposits and prospects. Previous studies proposed that U mineralisation formed under conditions similar to the unconformity-related U deposits of the Alligator Rivers Uranium Field (ARUF), located north of the McArthur Basin. Detailed mineralogical, geochemical and geochronological studies on mineralised intervals from the Redtree, Junnagunna, and Huarabagoo deposits, and integration with previous studies, identified at least six generations of uranium oxides. These formed between ca. 1680 and 350 Ma, highlighting protracted mobility of U over 1.3 Ga. Each generation of uranium oxide has a specific chemical composition, indicating variable physicochemical conditions for their formation throughout this period. Although the 1680-Ma mineralising event formed at the same time and with similar mineralising fluids as the unconformity-related U deposits from the ARUF, the physicochemical conditions differed between the two areas. Deposits of the Westmoreland area lack the typical Mg– and Bmetasomatism of the unconformity-related U deposits in the ARUF: in the Westmoreland area, the chlorite is Mg-poor and no B minerals are observed, whereas the ARUF is marked by Mg-rich chlorite, between clinochlore and Mg–amesite, Mg–foitite, and alumino–phosphate–sulfate minerals. The chondrite-normalised REE patterns of uranium oxides are enriched in light rare earth elements (LREEs) or flat, compared with the typical bell shape of the unconformity-related U deposits. Chlorite thermometry indicates significantly higher temperature conditions (> 300 °C) than in the ARUF for the early U stages. Based on these results, mineralisation in the Westmoreland area is thus significantly different from the unconformity-related U deposits in the ARUF and in other sedimentary basins. Collectively, the data point towards atypical ore-forming processes for basin-related U deposits in the Westmoreland area, but show some similarities with those known in the U deposits of the Otish Basin (Québec, Canada).This research was funded by (i) a French Ministry of Higher Education and Research PhD salary Grant to J.G., (ii) a Région LorraineFEDER grant to J.M: « Rôle des phases fluides dans la distribution spatiale des ressources métalliques dans les bassins sédimentaires paléoproterozoiques australiens », (iii) OSU OTELo grants to J.M.: « Conditions de transport des métaux dans un mégabassin protérozoïque » and A.R.: « Transferts de fluides et métaux dans le bassin de McArthur (Australie) » and (iv) a CNRS-INSU-CESSUR grant to J.M. « Transferts des fluides et métaux dans les méga-bassins paléoprotérozoïques ». The French National Agency through the national programme « Investissements d’avenir » of the Labex Ressources 21 (reference ANR-10-LABX-21-RESSOURCES21) partly funded the last developments of the SIMS lab

Combination of Rough and Fuzzy Sets Based on α-Level Sets

A fuzzy set can be represented by a family of crisp sets using its α-level sets, whereas a rough set can be represented by three crisp sets. Based on such representations, this paper examines some fundamental issues involved in the combination of rough-set and fuzzy-set models. The rough-fuzzy-set and fuzzy-rough-set models are analyzed, with emphasis on their structures in terms of crisp sets. A rough fuzzy set is a pair of fuzzy sets resulting from the approximation of a fuzzy set in a crisp approximation space, and a fuzzy rough set is a pair of fuzzy sets resulting from the approximation of a crisp set in a fuzzy approximation space. The approximation of a fuzzy set in a fuzzy approximation space leads to a more general framework. The results may be interpreted in three different ways