the spatial associations between geology, structure, magmatism and known ...... mineralisation in northeastern Tasmania; implications for the tectonic and ...
Using Geomodelling and Geophysical Inversion to Evaluate the Geological Controls on LowSulphidation Epithermal Au-Ag mineralisation in the Drummond and Bowen Basins, Australia Leonardo Feltrin'', Timothy Baker"*, Nick Ohver"*, Margaretha Scott'', Kate Wilkinson'' Melanie Fitzell'', Owen Dixon'' and Martina Bertelli'' "School of Earth and Environmental Sciences, James Cook University, 4811, Townsville Geological Survey of Queensland, Department of Mines and Energy, Indooroopilly, 4068, Brisbane Abstract. We present a 3D geological model that integrates different datasets and incorporates geophysical inversion of airborne gravimetric and magnetic surveys of the northern part of the Drummond and Bowen basins. These basins are known for their endowment of low-sulphidation, epithermal Au-Ag mineralisation. The objective of this computer based reconstruction is to empirically evaluate the key controlling variables that contributed to the spatial localization of 147 Palaeozoic and Mesozoic shallow hydrothermal gold systems, found predominantly in veins and breccia hosted in basal volcanics and volcaniclastic intervals developed in a back-arc rift environment. The model provides a three-dimensional, regional scale (100,000 km^) perspective on the spatial associations between geology, structure, magmatism and known mineral occurrences, representing a 3D framework for precious-metals exploration. Results of 3D visualisation of geological and geophysical data suggest that magmatic intrusions, and correlative volcanic centres localized most of the major deposits and also controlled the arrangement of clusters of uneconomic Au-Ag occurrences. The empirical modelling supports a model for the genesis of low-sulphidation epithermal Au-Ag mineralisation that favours a strong spatial association with shallow felsic to intermediate magmatic intrusions, similarly to that proposed for high-sulphidation systems. Geophysical inversion estimates for the depth of magmatic bodies suggest also that they may have controlled the efficiency of the hydrothermal, mineralising systems - shallow intrusions tend to be associated with greater tonnages of Au-Ag and are spatially associated with the larger clusters of occurrences. However, the composition of magmatic intrusions (mafic, intermediate, felsic) may have been important in regulating the amount of available bisulphide in the volatile phase, exerting a control on Au grade/tonnage independent of the depth of emplacement of intrusions. Keywords: 3D Modelling, WORMS, Geophysical Inversion, Low-Sulphidation Epithermal Gold PACS: 01.30.Cc; 91.10.Jf, 91.10.Kg, 91.25.Rt, 91.45.Wa, 91.50.Ac, 95.75.Tv, 95.75.Mn
CP1009, GIS in Geology and Earth Sciences, edited by K. Oleschko, S. Cherkasov, J. L. Palacio Prieto, V. S. Torres Argiielles, C. I. Gaona Salado, A. G. Castaiieda Miranda, and S. A. Zamora Castro O 2008 American Institute of Pliysics 978-0-7354-0528-8/08/$23.00
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INTRODUCTION Eastern Queensland, Australia, hosts major examples of vein-style gold deposits. In the northern Drummond and Bowen Basins epithermal deposits represent the most significant gold deposits of this type in Australia, with Pajingo mine as the major past Au-Ag producer (Table 1). Despite these significant known resources, the Drummond and Bowen basins remain relatively unexplored because of their extensive Cenozoic and Tertiary cover. Scarce available outcrops hamper the identification of alteration systems, so that more geophysical exploration predominated recently.
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The Drummond and Bowen basin study area is approximately 100000 km and is bounded by the Lolworth-Ravenswood block (Silurian-Ordovician) on its northern side, the Connors Arc (Carboniferous) and Yarrol terrane to the east, and unconformably
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overlain by the Permo-Triassic Galilee and Jurassic-Cretaceous Surat basins, on the western and southern parts respectively [1] (see Fig. 1). In this paper, we discuss the multiscale control exerted by basement/cover interactions on the localisation of low-sulphidation epithermal Au-Ag deposits. 3D-GIS is used to explore the various geological aspects considered to be important in the genesis of these occurrences. To locate blind deposits (undercover) Jaques et al. [2] suggest an approach that relies on new geophysical imaging tools and a better understanding of the processes controlling ore genesis. Here we further develop this perspective incorporating in our 3D model advanced processing of geophysical datasets. GEOLOGICAL SETTING Subduction Related Basin Evolution The extent and degree of superposition of the Late Devonian - Early Carboniferous Drummond basin, the Permo-Triassic Bowen basin, and other younger basins are interpreted as reflecting the evolution of an eastward directed volcanic arc (AuburnConnors arc) [4]. Henderson and Davis [5] remark that the basins were coeval with terranes representing a consuming plate margin as part of the New England Fold Belt. The presence of a subduction related arc was proposed as a possible explanation for the thick piles of volcaniclastic sediments found for example in the Yarrol terrane (Fig. 1), which has variously been interpreted as a forearc basin [3, 5-7], back-arc basin (Bryan et al. [8]) and as having both forearc and back-arc phases [9]. Henderson et al. [7] and Murray et al. [10] suggested that the Wandilla and Shoalwater terranes, further east, represent a subduction complex coeval with the Drummond sequence. Basin Comparison The connection between basin evolution and arc related magmatism is further evidenced by similarities observed between the Drummond and Bowen basins [5]. These include the spatial positioning according to the evolution of the continental margin, and their initiation in response to detachment faulting that promoted the development of an asymmetric profile of basin infill (Fig. 2). Basement units in the deepest sections of the basins have matching gravity highs that may be considered to be the result of mafic underplating [11, 12] further supporting an association with the active margin of the New England Fold Belt. The Drummond and Bowen basins also preserve comparable sedimentary records comprising three main depositional cycles: Cycle 1 is represented by an extensive basal unit of felsic to intermediate volcanics (e.g.. Silver Hill and Lizzie Creek volcanics) typical of rift dominated settings [1, 5]. Cycle 2 is characterised by sediments of cratonic derivation (e.g. Scartwater, Back Creek Group) reflecting an interval of paucity in the volcaniclastic input. The latter, however is rejuvenated in Cycle
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3 (e.g.. Star of Hope Formation, Blackwater and Rewan groups) [1]. Both basins experienced extensional and compressional phases throughout their history [e.g., 1, 3, 7, 13]. Early rifting of the Drummond sequence developed during the Mid-Late Devonian with NE-SW directed extension (375-350 Ma) that involved reactivation of N to NW trending faults (Fig. 3). In the younger Bowen basin. Early Permian (306-268 Ma), NWSE directed extensional deformation caused dextral transtension favouring the development of N striking, dip-slip faults. Two episodes of E-W directed shortening, the Early Carboniferous Kanimblan Orogeny (350-325 Ma) and the Permo-Triassic HunterBowen Orogeny (255-230 Ma Late Permian-Triassic) caused significant uplift of topographic highs (Anakie metamorphics and Connors arc). Consequent basinal deformation was accommodated by large monoclines, open folds with N-S axial planes and some complex folds with limbs dipping 30-60° (e.g., Scartwater Salient). Reverse faults also appear to be associated with these major folds, in some cases representing inverted syn-rift structures that in the Bowen basin accommodated deformation by rotating to lower angles [13], in response to the Hunter-Bowen orogeny. Thin-skinned thrusts of crustal blocks lying east of the central Bowen Basin is apparent [13]. This style of deformation may be also recognised in the northern Bowen basin [14]. However, synrift faults remain preserved in both basins and represent the main boundaries of an inherited architecture of sub-basins that accommodated sedimentation by half-graben tilting [6]. NORTHERN DRUMMOND AND BOWEN BASINS SfartiArar^r Salieni CIr
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Suites of magmatic intrusions and possibly co-magmatic extrusives occur in both basins [15, 16] (Fig. 3). These are represented by the Late Devonian Retreat batholith (385-366 Ma) [15], composed of mainly granodioritic suites, intruding the Ukalunda beds, and outcropping mostly in the southern end of the Anakie inlier. To the east, the majority of the Connors arc comprises the Urannah suite, mainly Carboniferous (320-290 Ma)
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granodiorite and early Upper Permian dikes intruding the Lizzie Creek volcanics (280-234 Ma). Basal boulders of magmatic intrusions however, suggest that some of the plutonic rocks predate the volcanics, their lower limit is also about the age of the Bulgonuima Volcanics [16]. A younger component of plutons and dikes of Early Cretaceous age (125 Ma) is also present. Epithermal Deposits and Occurrences I'aiJngo
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