Hermit Park, Australia
Hermit Park, Australia

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Hill E.J.,CSIRO | Oliver N.H.S.,Holcombe Coughlin Oliver | Cleverley J.S.,CSIRO | Nugus M.J.,AngloGold Ashanti | And 2 more authors.
Journal of Structural Geology | Year: 2014

In high-nugget gold ore bodies, samples taken from drill core for gold assay are typically too small to allow for the extreme spatial variability in grade and are a poor representation of the underlying distribution of mineralisation. The GQ North lode at the Sunrise Dam Gold Mine in Western Australia is a good example of an orebody which has a very strong nugget effect (coefficient of variation >20) and that has proved very problematic to model. Gold is hosted in vein stockworks and shear zones and although there is a clear spatial relationship between mineralisation and alteration, high vein density and well-developed foliations, the relationship is best defined statistically because the association between high gold grades and various combinations of these features is non-trivial. We present a method for automating the inclusion of geological data (proxies for gold mineralisation) into the prediction of mineralised rocks, using conditional probability. The method uses the gold assays and the logged geological data to calculate the probability that rocks with particular geological features will be mineralised. The ore body can then be modelled automatically using interpolation software with isosurfaces indicating the regions with highest probability of gold mineralisation. A good understanding of the geological features associated with mineralisation and consistent geological logging are important prerequisites for successful conditional probability modelling of drill hole data. © 2013 Published by Elsevier Ltd. All rights reserved.

Hill E.J.,CSIRO | Oliver N.H.S.,Holcombe Coughlin Oliver | Oliver N.H.S.,James Cook University | Fisher L.,CSIRO | And 2 more authors.
Journal of Geochemical Exploration | Year: 2014

Gold distribution in vein-hosted hydrothermal ore deposits is commonly nuggety (i.e. occurs as very localised concentrations of gold). In these cases samples for gold assay from diamond drill core may be too small to model the underlying heterogeneity of gold distribution and result in poorly constrained ore body models and underestimated gold resources. Hence, it is common practice to use more spatially continuous proxies for mineralisation to help define the boundaries of mineralised regions. We present a method for automating the use of geochemical proxies for nuggety gold ore bodies. Sunrise Dam Gold Mine, in Western Australia, is a world-class gold deposit with a very high nugget effect. Multi-element geochemical data has been collected at this site in order to improve prediction of mineralised regions. Suitable proxy elements have been selected from this data set, in particular, those that are spatially related to gold mineralisation but do not display nuggety distribution, such as Sb, Rb and Cr. We applied a probabilistic approach to the problem of quantifying the relationship between gold assay values and geochemical elements. It is shown that a kernel density estimator and Bayes conditional probability can provide an effective method for calculating the probability of a sample having elevated gold content and that this measure will be more spatially continuous than gold assay values if the appropriate geochemical proxies are selected. Using conditional probability and suitable cut-off values, we reclassified approximately 27% of samples as mineralised which returned low Au assay results. When plotted on drill holes conditional probability values provided a much more spatially continuous guide to mineralised regions than Au assay values alone. © 2014 Elsevier B.V.

Oliver N.H.S.,James Cook University | Thomson B.,Kinross Brazil Mineracao S A | Freitas-Silva F.H.,University of Brasilia | Holcombe R.J.,Holcombe Coughlin Oliver | And 8 more authors.
Economic Geology | Year: 2015

Multiscale structural and geochemical studies have been applied to understand the genesis of the Paracatu deposit in Brazil, a shallow-dipping, bulk-tonnage, vein-style low-grade Au(-Ag-Pb-Zn) orebody hosted in ∼1000 Ma black phyllites of the Paracatu Formation, associated with intense shearing accompanying thrusting during the ∼680 Ma Brasiliano orogeny. Massive to laminated quartz-sulfide-carbonate veins and associated alteration and Au mineralization formed early in the deformation history, and these veins were boudinaged during subsequent progressive increase in shear strain. Geochemical profiles across the ore allow recognition of a relatively homogeneous protolith at 100-m scales with respect to Ti, Al, Zr, V, and rare earth elements, including a footwall with protolith attributes similar to those of the ore. Oxygen, hydrogen, and sulfur isotopes do not reveal a distinctive external fluid source; rather, they reflect fluid-rock equilibration with the host phyllites during greenschist facies regional metamorphism. Detailed geochemical sampling around a prominent population of smaller boudinaged veins shows that Si, Ca, and Sr were removed and Al, K, and Ti were residually concentrated during formation of synboudinage dark selvages. This process of mass transfer occurred at scales more local than the processes responsible for ore genesis. Despite the local processes of chemical equilibration and redistribution, many of the main ore components appear externally derived at scales broader than the orebody. Gold, As, Ag, Sb, Bi and, to some extent, Pb and Zn show a strong spatial and statistical correlation with boudinaged quartz-sulfide ± carbonate veins, and these are surrounded by a broader (10-50-m vertical scale) halo of enriched K, Ba, and volatiles (loss on ignition), with depleted Na and Sr. The majority of gold is present as inclusions in pyrite and arsenopyrite within and immediately surrounding the veins, and gold-bearing pyrite shows mineral chemistry consistent with a hydrothermal origin. The main population of large veins locally preserve preboudinage, complex internal vein textures, including sulfide-rich wall-rock laminae similar to laminated fault-fill veins in many orogenic-style vein deposits. These larger veins contain most of the gold in the Paracatu resource. Metals were most likely sourced distal to the Paracatu Formation and were precipitated in sheeted shear or tensile veins during the early stages of the Brasiliano thrusting. The oblate strain recorded by the boudinaged, mineralized veins is anomalous and distinctive to Paracatu at km scales, even though the thrusting is regional. Although the orebody may have been physically dislocated by thrusting from the place where the early veins formed, the anomalous strains recorded suggest a perturbation in the footwall to the regional thrust or, perhaps, a spatially restricted competency contrast within the stratigraphy, which contributed to the localization of the present mineralization during the thrusting. Copyright © 2015 by Economic Geology.

Firth E.A.,University of Leicester | Holwell D.A.,University of Leicester | Oliver N.H.S.,Holcombe Coughlin Oliver | Mortensen J.K.,University of British Columbia | And 2 more authors.
Mineralium Deposita | Year: 2015

Mineral de Talca is a rare occurrence of Mesozoic, gold-bearing quartz vein mineralisation situated within the Coastal Range of northern Chile. Quartz veins controlled by NNW–SSE-trending faults are hosted by Devonian-Carboniferous metasediments of greenschist facies and younger, undeformed granitoid and gabbro intrusions. The principal structural control in the area is the easterly dipping, NNW–SSE-trending El Teniente Fault, which most likely developed as an extensional normal fault in the Triassic but was later reactivated as a strike-slip fault during subsequent compression. A dilational zone in the El Teniente Fault appears to have focussed fluid flow, and an array of NW–SE-trending veins is present as splays off the El Teniente Fault. Mineralised quartz veins typically up to a metre thick occur in three main orientations: (1) parallel to and within NNW–SSE-trending, E-dipping faults throughout the area; (2) along NW–SE-trending, NE-dipping structures which may also host andesite dykes; and (3) rarer E–W-trending, subvertical veins. All mineralised quartz veins show evidence of multiple fluid events with anastomosing and crosscutting veins and veinlets, some of which contain up to 3.5 vol.% base metal sulphides. Mineralogically, Au is present in three textural occurrences, identified by 3D CT scanning: (1) with arsenopyrite and pyrite in altered wall rock and along the margins of some of the veins; (2) with Cu-Pb-Zn sulphides within quartz veins; and (3) as nuggets and clusters of native Au within quartz. Fluid inclusion work indicates the presence of CO2–CH4-bearing fluids with homogenisation temperatures of ∼350 °C and aqueous fluids with low-moderate salinities (0.4–15.5 wt% NaCl eq.) with homogenisation temperatures in the range of 161–321 °C. The presence of Au with arsenopyrite and pyrite in structurally controlled quartz veins and in greenschist facies rocks with evidence of CO2-bearing fluids is consistent with an orogenic style classification for the mineralisation. However, the significant amounts of base metals and the moderate salinity of some of the fluids and the proximity to felsic granitoid intrusions have raised the possibility of an intrusion-related origin for the mineralisation. Vein sulphides display S isotope signatures (δ34S +2.1 to +4.3 ‰) that are intermediate between the host rock metasediments (δ34S +5.3 to +7.5 ‰) and the local granitoids (δ34S +1.3 to +1.4 ‰), indicating a distinct crustal source of some of the S in the veins and possibly a mixed magmatic-crustal S source. The local granite and granodiorite give U-Pb zircon ages of 219.6 ± 1 and 221.3 ± 2.8 Ma, respectively. Lead isotopic compositions of galena in the veins are consistent, suggesting derivation from a homogeneous source. Differences, however, between the isotopic signatures of the veins and igneous feldspars from nearby intrusions imply that these bodies were not the source of the metals though an igneous source from depth cannot be discounted. The Triassic age of the granitoids is consistent with emplacement during regional crustal extension, with the El Teniente Fault formed as an easterly dipping normal fault. The change to a compressional regime in the mid-Jurassic caused reactivation of the El Teniente Fault as a strike-slip fault and provided a structural setting suitable for orogenic style mineralisation. The intrusions may, however, have provided a structural competency contrast that focused the mineralising fluids in a dilational jog along the El Teniente Fault to form WNW-trending veins. As such, the mineralisation is classified as orogenic style, and the identification of the key mineralogical, isotopic and structural features has implications for exploration and the development of similar deposits along the Coastal Range. © 2014, Springer-Verlag Berlin Heidelberg.

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