GU Report 24 - Gold Undercover - summary of geological findings: Exploring for buried gold in northern Victoria
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Product description:Executive Summary The Gold Undercover initiative was a three-year scientific program (2006 to 2009) to provide new data and to improve exploration techniques in gold-bearing Palaeozoic basement rocks that are buried beneath Cainozoic sediments of the Murray Basin in the Stawell, Bendigo and Melbourne structural zones of northern Victoria. The Gold Undercover initiative included a number scientific studies and literature reviews resulting in twenty-four Gold Undercover reports and two papers published in scientific journals. The world class central Victorian gold province has produced 2500 t Au (~80 Moz Au). Most deposits are classified as orogenic gold deposits that formed during regional deformation and metamorphism in the Late Ordovician to Silurian or in the Late Devonian. For mineral endowment studies under the Gold Undercover initiative, a 3-part assessment was used to estimate the undiscovered gold endowment of the northern parts of the Stawell, Bendigo and Melbourne zones. These studies showed that the likely undiscovered gold endowment is about 75 Moz. Geochemical studies were undertaken to characterise primary and secondary alteration haloes surrounding typical orogenic gold deposits and new hyperspectral data were obtained for primary alteration and secondary dispersion samples. The primary alteration study confirmed that three alteration zones surround many Victorian gold deposits. An inner zone generally is defined by wall-rock sulphidation with elevated S, Au, As and Sb and disseminated arsenopyrite and pyrite. A poorly defined zone of phyllic alteration either coincides with, or lies within, the sulphidic alteration halo. An outer and laterally extensive ferroan carbonate halo is present up to 200 m away from mineralised structures. A study of secondary dispersion of primary alteration in the Stawell area resulted in the definition of new threshold values for various elements that indicate the presence of anomalous Au, As and Sb concentrations and anomalous Cu/Fe and As/Fe ratios. Recommended lower threshold values, which separate background from anomalous samples, are generally 10 ppb for Au, between 20 and 40 ppm for As, and generally 3 ppm for Sb in sandstone. Upper threshold values, which distinguish anomalous from strongly anomalous samples are typically between 60 and 100 ppb for Au and between 50 and 100 ppm for As. A soil regolith and groundwater study in the Lockington area, northeast of Bendigo, confirmed that soil samples can reveal the presence of geochemically anomalous basement at depths of 40 to 60 m beneath transported Cainozoic cover. Groundwater analysis thus provides a potential reconnaissance exploration method in the Murray Basin where drilling has penetrated basement, or from cover above basement highs not blanketed by organic-rich sediments. Geophysical studies provided a crustal-scale transect of the Victoria gold province and data to help identify buried structures under the Murray Basin sedimentary cover. The interpretation of four seismic reflection lines totalling 398 km revealed the presence of greatly thickened mafic volcanic rocks beneath the Stawell and Bendigo zones and a set of crustal-scale listric faults that extend from the surface to the lower crust. The mafic volcanic rocks were identified as possible source rocks for orogenic gold. The major faults may have provided important fluid pathways for mineralisation. The survey also identified a horizontally layered basement in the Melbourne Zone that is consistent with the Selwyn Block model for central Victoria. Magnetotelluric survey lines across the northern Bendigo Zone revealed a layered crust consistent with the seismic survey interpretation. A Versatile Time Domain Electromagnetic (VTEM) survey was conducted along the four seismic survey lines. Detailed infill gravity data were collected over the northern Bendigo Zone to reduce the existing station spacing from 1.5 to 0.5 km. Interpretation of the new data revealed buried faults and basement highs under the Murray Basin sedimentary cover. Specialist AutoMag™ software was used to determine the depth to magnetic basement under the Murray Basin sedimentary cover. The magnetic model was constrained by 993 borehole basement intersections. New rock density measurements increased the existing GeoScience Victoria rock density database from 41 to 1235 density entries for 54 rock types. A geochronology study that reviewed previous work added new age determinations from central Victorian gold deposits and showed that all gold mineralisation occurred between about 440 and 370 Ma with a major mineralisation event at 440 Ma. Some gold was metamorphosed or remobilised during the emplacement of intrusions at approximately 410–400 Ma and 380–370 Ma. Fluid inclusion noble gas and halogen geochemistry was reviewed and new data were obtained from some major Victorian gold deposits. The review showed that gold mineralisation is associated with low salinity fluids with variable CO2 content. The fluid composition varied by the process of phase separation. Current chemical and isotopic data indicate that the so-called intrusion-related gold deposits had a similar fluid origin as those of orogenic gold deposits in Victoria. Consequently, the source of gold is most likely the metamorphosed basement sedimentary and volcanic rocks. Download The downloadable version of this report is supplied in PDF format. Bibliographic reference Willman, C.E., 2010. Gold Undercover - summary of geological findings: Exploring for buried gold in northern Victoria. GeoScience Victoria Gold Undercover Report 24. Department of Primary Industries. Related products:
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