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Updated:
07 September 2005
Abstract of Tennant Creek/Davenport SynthesisRecord 2001/12Compiled by Lesley Wyborn
Download the complete chapterDownload the complete Tennant Creek/Davenport Synthesis report in pdf format [PDF_4.5MB]. Executive summary - geologyDue to its high economic endowment and potential, the geology of the Tennant Creek Province has been the focus of many detailed studies over the years. The most recent summary of the whole province was by Le Messurier et al. (1990). The Tennant Creek Province is currently being remapped at 1:100 000 scale by the Northern Territory Geological Survey (Donnellan et al. 1994, 1995, in prep.). The Davenport Province was remapped in the early 1980’s as part of a major 1:100 000-scale geological mapping program by the BMR (Blake et al. 1987). In this report, the Tennant Creek and Davenport Provinces are treated as the one entity. The oldest unit in the Tennant Creek and Davenport Provinces is the Warramunga Formation, a polydeformed succession consisting of lithic and sublithic, arenite, wacke and siltstone, terrigenous mudstone and hematitic shale. The sediments contain immature volcanic detritus and are regarded as medium-grained turbidites of proximal to distal fan facies apparently derived as part of a prograding (coarsening upwards) succession. Magnetite is ubiquitous and locally forms distinct laminae; it is a major component in the hematite shales (Donnellan 1994; Donnellan et al. 1994; Le Messurier et al. 1990). Carbonaceous shales are not common (N. Donnellan, pers comm); one occurrence of hematitic shale with 0.2% C was reported by Reveleigh (1997). The Warramunga Formation was deformed and metamorphosed to greenschist facies, and ironstone hosts to Au mineralisation appear to have formed early in the deformation history (Ding and Giles 1993). Felsic volcanics and volcaniclastic and clastic sediments of the Flynn Subgroup and Ooradidgee Subgroup were deposited on deformed Warramunga Formation. The Tennant Creek Supersuite was emplaced throughout this part of the geological history as volcanics within the Warramunga Formation (~1870 Ma, mainly in the Davenport Province), as granites (1940-1858 Ma) which are pre-, syn- and post- deformation, and as bimodal volcanics in the lower Flynn and Ooradidgee Subgroups (~1840 Ma; Hussey et al. 1994). Sedimentation continued through to the Tomkinson Creek, Wauchope and Hanlon Subgroups, under predominantly orogenic terrigenous and stable shelf conditions (Donnellan et al., in prep.). A compositionally different suite of bimodal volcanics and shallow-level intrusions, the Treasure Suite was emplaced at ~1820 Ma. Sedimentation continued after this suite, but all known mineral deposits are hosted by rocks equivalent in age or older than this suite. After sedimentation, a final magmatic episode at about 1700 Ma resulted in the intrusion of shoshonitic lamphrophyres and a syenite body of the Devils Suite. Synthesis: Sediments of the Tennant Creek and Davenport Provinces are dominated by clastics, with felsic and mafic volcanics. The clastic sequences are relatively oxidised, containing magnetite and hematite. Carbonaceous sediments are extremely rare, and carbonate rocks are rare. The paucity of carbonate rocks within the sequences mean that skarn mineralisation will be limited (Hoatson and Cruickshank 1985), and the lack of carbonaceous sediments implies that reduction of oxidised magmatic fluids by interaction with methane-bearing connate brines is unlikely. The best hosts are the secondary ironstone bodies, although fluid mixing may be a viable alternative. Executive summary - metallogenicThis compilation has assessed the potential of each granite suite based on the criteria set out in the Project Proposal. All three suites have been identified as having potential for granite-related mineralisation, but only the Treasure Suite is considered to be highly significant. The Tennant Creek Supersuite was emplaced at around 1850 Ma. It is a restite-rich I- (granodiorite) type which shows limited fractionation in the most felsic end members due to restite separation. The supersuite is associated with very minor W mineralisation. Although similar in age to the proposed time of formation of the ironstones, the supersuite is not believed to have played any role in their formation other than to act as a possible heat source to enhance circulation of basinal brines. Also, the supersuite has no relationship to the Au mineralisation. The Treasure Suite at ~1820 Ma is mainly volcanics, intrusive granophyre and porphyry in the Davenport Province, and diorite to monzodiorite in the Tennant Creek Province. The suite is I-(granodiorite) fractionated, with the more mafic end members preserved in the northwest Tennant Creek area, and the more felsic fractionated members in the southeast. The suite is believed to be associated with the Au, Bi, Cu mineralisation at Tennant Creek and with W, Cu, Bi, Mo mineralisation in the Hatches Creek area. The more significant Au mineralisation associated with this suite is predominantly hosted by ironstones, but there is potential for quartz-vein Au mineralisation independent of the ironstone bodies. The Devils Suite at 1700 Ma is an extremely fractionated I-(granodiorite) type which is associated with minor vein W-mineralisation. The granite is oxidised and is associated with extensive alteration of the country rocks. However, the granite crystallised over a very narrow and high SiO2 range and seems to contain high F. Its potential is therefore limited to vein W deposits. Although abundant cassiterite has been recorded in stream sediments from the area, the oxidised nature of the granite means that vein Sn is unlikely to be well developed. Future workThere are two aspects worthy of follow up in the Tennant Creek Province:
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