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Organic maturity is a key property for assessment of hydrocarbon potential and generation of black shales. Early Paleozoic strata are usually void of terrestrial organic matter such as vitrinite, thus evading conventional, reflectance based maturity analysis. To overcome this, graptolites in samples from the basal hot shales of the Silurian Qusaiba Formation, with known, reflectance based vitrinite equivalent maturities from 0.54 to 1.76 VRE%, were analyzed with Raman spectroscopy to establish a spectroscopic maturity scale. All samples provided spectra showing the D isorder (D) peak and the G raphite (G) peak typical for organic carbons. Systematic variation of these peaks with sample maturity is used to quantify the relationship of reflectance with the graptolites molecular condition. Several spectral properties were tested for use as maturity indicators. The most reliable maturity tracers are the variation of position of the G peak between 1570 and 1610 cm − 1 and the widening band distance (Δ (G-D) ) between the G peak and D peak position at 1320 to 1370 cm − 1 . Comparison of the mean and modal maxima of peak positions showed that the mean of G peak position and Δ (G-D) provide the best fit of Raman and optical reflectance determinations with R 2 of 0.884 and 0.935, respectively. Algorithms for the calculation of vitrinite equivalent reflectance from the Raman spectral properties were developed based on best fit exponential regressions, with a linear cross correlation of the measured and calculated values at R 2 = 0.9178 for the mean of Δ (G-D) . Raman based maturity measurements on graptolites provide a welcome alternative method for on particles as small as 3 μm diameter.

The Lower Silurian Qusaiba Hot Shales (QHS) are proven source rocks for oil within the Paleozoic, and possibly some of the Mesozoic, reservoirs in Saudi Arabia. Moreover, these shales have oil shale potential where they are immature and shallow enough for mining, as well as unconventional shale oil and shale gas potential, in areas where they are within oil-maturity and gas-maturity levels, respectively. The QHS were deposited in relatively shallow marine environments under anoxic water conditions, resulting in accumulation of amorphous kerogen, organic-walled graptolites and acritarchs. Across the Arabian Basin, organic matter quality does not show much variation for the QHS, but the maturity varies greatly as a result of varying burial history. Thermal maturity assessment of shale source rocks that lack vitrinite, such as the Qusaiba Hot shales, continues to be challenging, especially where conflicting measurements are obtained from different sources. This paper presents an integrated assessment of QHS thermal maturity parameters, based on cores from 13 carefully selected boreholes that – based on regional basin models – are believed to cover a wide maturity range (ca. 0.5 to 2.0% Ro). We conducted some analyses on kerogen (maceral petrography, graptolite reflectance, UV-fluorescence, whole rock pyrolysis, Raman spectroscopy of graptolite) and other analyses on bitumen extracts (GC and GC–MS of saturate and aromatic fractions, and FTIR spectroscopy of the asphaltene fraction). We show that using many thermal maturity parameters reduces uncertainty significantly, and we therefore recommend that graptolite reflectance analyses should be conducted in support of other maturity indicators. Proper reflectivity measurements of the graptolites set the reference for comparison of other maturity parameters obtained from petrographic, geochemical, and spectroscopic techniques. Our work provides a template by which Qusaiba thermal maturity can be more accurately estimated when only a limited set of maturity parameters is available.

Calcrete (pedogenic Ca carbonate) is an important sampling medium for geochemical gold (Au) exploration in semi-arid and arid regions of Australia, because it is widespread, easy to sample and calcium (Ca) shows a strong positive correlation with Au, but not with base metals, in calcrete overlying buried Au mineralization. In this study we show that the formation of Au-anomalous calcrete can be biomediated through the activity of resident microorganisms, and may not simply be the result of passive nucleation on inactive cells or evapotransporative processes. Calcified microfossils are highly abundant in calcrete from the Barns Au-prospect in South Australia. These microfossils are morphological analogues of calcified cells and biofilms formed in laboratory experiments conducted with active bacterial cultures enriched from Au-anomalous calcareous sand from the Barns prospect. Calcium carbonates precipitated by these cultures consisted mostly of calcite, which is the main carbonate mineral in calcrete. Synchrotron micro-X-ray fluorescence (S-μXRF) mapping was used to assess the distribution of Au, Zn, Ca and other metals in Ca carbonates precipitated by active bacterial cultures. On a μm-scale the distribution of Au was heterogeneous in these Ca carbonates and differed from base metal distribution, thus mimicking the spatial separation of these metals observed in calcrete. The speciation of Au in Ca carbonates precipitated by active bacteria was measured using micro-X-ray absorption near edge structure spectroscopy (μ-XANES) and resembled that observed in Au-anomalous calcrete closely. While metallic Au was observed in Au ‘hotpots’, ionic Au was detected in the halo surrounding the ‘hotspot’. In contrast, the precipitates produced in the presence of dead bacterial cells or by raising solution pH or pCO 2 , i.e., hydroxylapatite, portlandite and vaterite, respectively, did not reflect the mineralogy of calcrete. Gold distribution and speciation in vaterite, formed by raising pCO 2 , were homogenous and did not reproduce the variation observed in calcrete and Ca carbonates precipitated by active cells. Increasing the supersaturation with respect to Ca in solution by incremental drying of the medium produced only X-ray amorphous precipitates, or hydroxylapatite in the presence heat-killed cells. In conclusion, this study shows that active microbial processes that combine biogenic Ca carbonatogenesis with Au precipitation are likely to drive the formation of Au-anomalous calcrete.

The Mt. Mulga barite Cu–Au deposit in the central part of the Olary Domain is hosted by the Palaeoproterozoic Willyama Supergroup of the Curnamona Craton. The lenticular, stratiform ore bodies are hosted by the Ethiudna Subgroup at and near the contact to the underlying Wiperaminga Subgroup. The ore bodies consist of massive, coarse crystalline barite up to 6 m thick interspersed with granular ( A pronounced positive Eu-anomaly of the mineralisation hosting unit indicates a reducing character of the mineralising fluids and is characteristic for exhalative sediments. 87 Sr/ 86 Sr-isotope ratios of barite fall into a narrow range of 0.708 to 0.713 suggesting an origin of the mineralising fluids from the underlying Lower Willyama sequence. The δ 34 S values of barite from the main ore body range from + 13 and + 16‰, while the barites of the other ore bodies range from + 6 and + 11‰. Chalcopyrite revealed δ 34 S values of − 1.3 to + 4.5‰. Barite is suggested to be formed by mixing of a hydrothermal, reduced, hydrothermal Ba-rich fluid with oxidised, sulphate-rich water of a non-marine basin. Although an epigenetic formation of the Cu–Au mineralisation during the Olarian Orogeny is possible, we suggest a cogenetic formation with the barite–iron-oxide mineralisation.

The formation of pedogenic carbonate (calcrete) in terrestrial environments is commonly mediated by microorganisms. In Australia, Au-anomalous calcrete is an important sampling medium for geochemical exploration, but current models describing its formation do not include a confirmed microbial component. This study demonstrates that bacterial communities in calcareous sands from dunes overlying the Barns Gold Deposit in semi-arid South Australia, are capable of mediating the biomineralisation of Au-anomalous carbonates. Bacterial enrichment cultures obtained from calcareous sands at three depths (0.1, 0.64 and 2.1 m, plus abiotic control) were incubated in urea and Ca 2+ -containing growth media (pH 8), unamended and amended with Au (100 parts-per-billion, ppb) as Au–aspartic-acid complex. During the incubation of the enrichment cultures urea was turned over to NH 4 + within 96 h to 220 h. The solution pH increased concurrently by approximately 1.2 units, and Au-anomalous Ca-carbonate crystallites were precipitated on cells, which functioned as nucleation sites; no carbonate precipitation was observed in abiotic controls. Compared to the medium, Au was strongly enriched in these carbonates and appeared to be uniformly dispersed in the individual crystallites, as shown using LA-ICP-MS; a similar distribution is present in naturally occurring Au-anomalous calcrete. Phylogenetic 16S rRNA PCR DGGE analyses, shotgun cloning and functional microbial analyses (BioLog, ureC quantitative PCR) demonstrated that naturally occurring and culture-enriched bacterial communities were dominated by alkaliphylic, halotolerant Bacillus spp. The indigenous bacterial communities were capable of utilising amino acids (including l -aspartic acid) and urea, which appears to lead to the destabilisation of the Au–amino acid complexes and concomitant co-precipitation of Au in the Ca-carbonates. In conclusion, a model combining geomicrobial– with evapotranspiration– and plant-based components is likely to best describe the formation of (Au-anomalous) calcrete in semi-arid and arid zones.

Anomalously high Au concentrations (2.5 to 50 ppb) in regolith carbonate accumulations, such as calcrete and calcareous sands in aeolian sand dunes overlying Au mineralisation of the Gawler Craton, South Australia, show a marked covariance of Au with K, Mg and most notably Ca. This relationship appears to be linked to the authigenic formation of smectites and carbonates within the aeolian dunes in the region. However, little is known about the processes involved in the formation of carbonates under semi-arid and arid conditions. In this study the geochemical properties of aeolian dunes along several depth profiles of 2 to 4 m are investigated in order to assess the relationship between Au mobility and calcrete formation. In the profiles a strongly systematic relationship between Au and the increasing Ca–Mg contents at depth highlights the close association between the enrichment of Au in the calcrete and the underlying hydrothermal mineralisation. Intense calcrete formation and concurrent Au enrichment also occurs in the vicinity of roots penetrating the dune. Thin section petrography and cathodoluminescence show that most of the calcrete in the regolith profiles is micritic; some sparic crystallites have also been identified. To demonstrate the presence of microbial processes that may mediate the formation of calcrete, samples from a depth profile in the dune were taken under sterile conditions. After amendment with urea and incubation of up to 24 h, up to 18 mg/l of NH 4 + were detected in near surface samples. At depth of 2.3 m 1 mg/l NH 4 + were detected compared to a control that contained below 0.05 mg/l NH 4 + . These results suggest that the genesis of calcrete and pedogenic carbonate in the area may be partly biologically mediated via processes such as the metabolic breakdown of urea by resident microbiota which generates a pH and pCO 2 environment conducive to the precipitation of carbonate. In the process of urea breakdown organic Au complexes such as Au-amino acid complexes may become destabilised in solution and Au may be co-precipitated, resulting in the fine, non-particulate distribution of Au throughout the micritic calcrete carbonate. In conclusion, this study suggests a coupled mechanism of biologically mediated and inorganic mechanisms that lead to the formation of Au-in-calcrete anomalies.

Samples of diagenetic and hydrothermal minerals from drillcores along the DEKORP 9601 seismic traverse in the North German Basin were investigated for their fluid inclusion and sulphur isotope composition to reconstruct the thermal and chemical evolution of fluid systems during basin evolution. The stages of basin subsidence and subsequent inversion are marked by fluid systems of distinct composition. The basinwide Zechstein evaporite units formed an aquiclude, disconnecting convecting fluids in the underlying Rotliegend volcanics from the overlying units. δ 34 S ratios of sulphides suggest a sedimentary sulphur source. δ 34 S ratios of diagenetic and hydrothermal sulphates indicate that sulphur was derived partly from Rotliegend volcanic units and partly from the evaporitic Zechstein.

Uranium-bearing hematite (containing up to several wt.% U), also containing Al, Mo, W and radiogenic Pb, is described from Olympic Dam, South Australia. These elements are present in grains that display both oscillatory zonation and porous structures. Laser-ablation inductively coupled mass spectrometry (LA-ICP-MS) element mapping confirms oscillatory and sectorial zoned domains in hematite grains are enriched in 238U- and 206Pb, and are distinct from those of W and Mo. The crystal structure and absence of inclusions within zoned hematite was assessed by transmission electron microscopy on foils obtained by in situ slicing across zonation patterns using the scanning electron microscope-focused ion beam technique. Satellite reflections on the electron diffraction patterns obtained from banded zones in hematite are attributable to long-range superstructure ordering, that is inferred to assist metal incorporation via the substitution 2Fe3+ ↔ Me6+ + vacancy, where Me = U, W, Mo. The suitability of U-bearing hematite for Pb–Pb geochronology as a first pass was tested on both zoned and porous hematite grains via LA-ICP-MS, using the GJ-1 zircon as the primary external standard. Only Pb–Pb ages were considered and resulted in 207Pb–206Pb ages of 1590 ± 8 Ma and 1577 ± 5 Ma for oscillatory and sector zoned hematite from two samples. Although reconnaissance in nature, these ages potentially support the supposition that mineralization is coeval with emplacement of the Gawler Range Volcanics and associated Hiltaba Intrusive Suite. The geochronological application utilizing an abundant refractory mineral represents a new tool for dating iron-bearing ores.

Acoustic emission (AE) from quartz during heating was measured with a high resolution decrepitometer on more than 350 samples from various conditions of formation and geological settings. The emitted acoustic signals can accumulate in very sharp peaks or extend over a wide temperature range. Different types and conditions of quartz formation can be distinguished from total counts and individual AE-patterns. Correlation of the determined AE peaks with microthermometric investigations, optical and scanning electron microscopical (SEM) studies of polished and etched thin sections showed that the sound, generated during heating, is caused by several distinguishable mechanisms. The main cause is the thermal expansion mismatch and thermal anisotropy of the quartz. In all cases the sound is emitted from opening and propagating microcracks. At temperatures below 380° C the main sources of AE are the rupture of grain boundaries, transgranular fracturing and decrepitation of large fluid inclusions. In the temperature range of 350–550° C, intragranular fracturing, sometimes enhanced by Brasil twinning, and reopening of healed fissures, often decorated with large numbers of small secondary fluid inclusions, cause distinct peaks of acoustic emission. At the α-β inversion temperature (≈ 573° C) massive emission of sound occurs if the quartz is twinned according to the Dauphine law. Measurement of AE can help to determine critical temperatures of material failure and reveal information on the residual strain in rocks. Different generations of quartz veins can be distinguished, even if covered by soil and the extent of alteration zones can be determined by the varying AE patterns.