Similarities in mineralization and alteration style, host rocks, fluid compositions, and alteration-related 40 Ar/ 39 Ar ages from several recently discovered gold prospects support the existence of a significant Mesoproterozoic gold province spanning an arcuate region at least 300 km in length in the central Gawler craton, South Australia. At the recently discovered prospects of Tunkillia, Nuckulla Hill, Barns, and Weednanna, as well as at the historically mined Au deposit at Tarcoola, gold occurs as disseminated and veinlet-hosted mineralization focused in brittle to brittle-ductile faults and shear zones. Host rocks are principally ~1715 to ~1680 Ma granites but also include Hutchison Group (2000-1860 Ma) and Tarcoola Formation (~1650 Ma) metasedimentary rocks. Hydrothermal alteration is characteristically zoned around gold mineralization, with intense sericitepyrite alteration and quartz veining proximal to gold mineralization and chlorite ± epidote ± hematite alteration distal from mineralization. Alteration was either synchronous with or, in some cases, continued after deformation. Gold is associated with pyrite and minor to trace galena, sphalerite, and chalcopyrite. Iron oxides are low in abundance in mineralized zones, which correspond to demagnetized zones. Fluid inclusion analyses suggest that multiple fluids were present in some systems, but one key fluid type occurs in each of the gold prospects: a low-to moderate-salinity (up to 10 wt % NaCl equiv) fluid with homogenization temperatures mostly in the range of ~150°to 300°C and commonly containing CO2 or associated with CO2-rich inclusions. Several features of the prospects are similar to those of orogenic-and intrusion-related gold deposits, but neither the prospect-scale geology nor the regional geologic setting in the central Gawler craton is sufficiently well understood to establish a genetic model or to confidently classify the mineralizing system. Hydrothermal white micas from several of the central Gawler craton gold prospects yield reproducible 40 Ar/ 39 Ar plateau ages of between 1567 and 1583 Ma. These ages, together with U-Pb zircon ages of granitic host rocks, constrain the timing of mineralization to the interval ~1690 to 1570 Ma, although we interpret the 40 Ar/ 39 Ar results to indicate a narrower mineralizing interval at ~1580 ± 10 Ma. This age range overlaps with, and is indistinguishable from, the range of U-Pb zircon ages reported from regional Hiltaba Suite granites and Gawler Range Volcanics, although igneous rocks of this age have not been identified locally at each of the prospects.

Abstract. 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 c~-fl inversion temperature (~ 573 ~ C) massive emission of sound occurs if the quartz is twinned according to the Dauphin6 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.

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Quartz, as the most ubiquitously occurring mineral, was chosen for investigations of the acoustic emission upon heating. Various samples from a wide range of conditions of formation were analyzed in the frequency range of 400–10 000 Hz with a high-resolution decrepitometer. The acoustic emission patterns showed a variation of 5000–80 000 total signals over the entire temperature range of 90 °C-610 °C (1-cm3 sample, grainsize fraction 250–500 μm). Microthermometric investigations of the sound-emitting processes and correlation with microcrack patterns determined by scanning electron microscopy (SEM) showed that various sources contribute to the sometimes vehement noise production. Stress built up because of thermal anisotropy causes the formation of microcracks along inter- and intragranular grain boundaries over most of the observed temperature range. Accumulation of signals from the decrepitation of fluid inclusions reveals information about the conditions of formation and subsequent alterations of the investigated sample. Acoustic emission caused by phase transitions in the crystal structure (α〈 − 〉β-transition at 573 °C) is related to changes in the twinning state of the sample. Measurement of the complex patterns of acoustic emission produces a “fingerprint” of the conditions of formation and subsequent alterations the sample underwent in its geological history. They can be used to determine the extension of mineralizations and alterations or detection of geological structures even when covered by soil. Real-time frequency analysis of the signals shall reveal further information about the origin of single acoustic signals. [Work supported by the DFG.]