Mineral Resources Tasmania Department of Infrastructure, Energy and Resources 1:25 000 Scale Digital Geological Map Series — Explanatory Report 1 — Explanatory Report for the Maydena, Skeleton, Nevada, Weld and Picton geological map sheets by C. R. Calver, S. M. Forsyth and J. L. Everard July 2007 ISBN 0 7246 4018 5 Mineral Resources Tasmania PO Box 56 Rosny Park Tasmania 7018 Phone: (03) 6233 8377 l Fax: (03) 6233 8338 Email: [email protected] l Internet: www.mrt.tas.gov.au While every care has been taken in the preparation of this report, no warranty is given as to the correctness of the information and no liability is accepted for any statement or opinion or for any error or omission. No reader should act or fail to act on the basis of any material contained herein. Readers should consult professional advisers. As a result the Crown in Right of the State of Tasmania and its employees, contractors and agents expressly disclaim all and any liability (including all liability from or attributable to any negligent or wrongful act or omission) to any persons whatsoever in respect of anything done or omitted to be done by any such person in reliance whether in whole or in part upon any of the material in this report. 2 CONTENTS EXECUTIVE SUMMARY………………………………………………………………………… 5 GEOLOGICAL OVERVIEW …………………………………………………………………… 9 Previous work………………………………………………………………………………… 9 Proterozoic successions ……………………………………………………………………… 9 Cambrian successions ………………………………………………………………………… 11 Cambrian mafic-ultramafic complexes ………………………………………………………… 15 Cambrian deformation: Tyennan Orogeny ……………………………………………………… 16 Late Cambrian–Ordovician …………………………………………………………………… 16 Devonian deformation ………………………………………………………………………… 16 Late Carboniferous–Triassic …………………………………………………………………… 16 Jurassic ……………………………………………………………………………………… 18 Cretaceous–Cainozoic ………………………………………………………………………… 18 ECONOMIC GEOLOGY (CRC) ………………………………………………………………… 19 Base metals …………………………………………………………………………………… 19 Maydena area……………………………………………………………………………… 19 The Mt Mueller and Humboldt prospects …………………………………………………… 19 Modern exploration ……………………………………………………………………… 19 Weld River area …………………………………………………………………………… 19 Gold ……………………………………………………………………………………… 19 Maydena area……………………………………………………………………………… 19 Forster prospect …………………………………………………………………………… 21 History ………………………………………………………………………………… 21 Geology and genesis ……………………………………………………………………… 21 Industrial Minerals …………………………………………………………………………… 21 Silica ……………………………………………………………………………………… 21 Maydena area (Maydena map sheet) ………………………………………………………… 21 Glovers Bluff area (Weld map sheet) ………………………………………………………… 22 Limestone ………………………………………………………………………………… 22 Dolomite ………………………………………………………………………………… 22 Talc ……………………………………………………………………………………… 22 Fuels ……………………………………………………………………………………… 23 Construction materials ………………………………………………………………………… 23 Marble …………………………………………………………………………………… 23 Gravel and Sand …………………………………………………………………………… 23 Prospectivity …………………………………………………………………………………… 23 References ……………………………………………………………………………………… 24 TABLES 1. Chemical analyses of igneous rocks …………………………………………………………… 26 FIGURES 1. General location plan and major adjoining tectonostratigraphic elements ………………………… 6 2. Map sheets on geology base, with contours to granite derived from gravity modelling …………… 8 3. Schematic stratigraphic column and economic geology of project area …………………………… 10 4. Bedrock geology of the Glovers Bluff inlier ……………………………………………………… 12 5. Cross sections, Forster Prospect area …………………………………………………………… 13 6. Incompatible element spider diagrams, normalised to model primitive mantle …………………… 14 7. Rare-earth diagrams ………………………………………………………………………… 15 8. Correlation of Parmeener Supergroup lithological units, Maydena–Picton and Kingborough areas … 17 9. SW–NE cross sections of Glovers Bluff inlier, showing interpreted cone sheet geometry …………… 20 3 PLATES 1. Graded bed of small-pebble to coarse-sand grade, Harrisons Opening Formation ………………… 28 2. Thick crossbed set in Needles Quartzite correlate, Jubilee Range ………………………………… 28 3. Symmetrical ripples in Needles Quartzite correlate, Glovers Bluff quarry ………………………… 29 4. Desiccation cracks and probable halite molds in unit Pcdc near Jubilee Range……………………… 29 5. Stromatolites in unit Pcdos, South Styx River …………………………………………………… 30 6. Photomicrograph of oolitic dolograinstone, Weld River Group …………………………………… 30 7. Outcrop of diamictite on the Huon River ……………………………………………………… 31 8. Outcrop showing lonestone in laminated dolomitic siltstone, unit Pwcg, Huon River ……………… 31 9. Exposure of ?Cambrian pillow lava in quarry at Maynes Hill, west of Maydena …………………… 32 10. Outcrop of ?Cambrian volcaniclastic lithicwacke and siltstone on the Huon River ………………… 32 4 EXECUTIVE SUMMARY The contiguous Maydena, Skeleton, Nevada, Weld and intercalated dolostone, diamictite, sandstone and Picton 1:25 000 scale map sheets cover 1000 km2 of mudstone (Weld River Group). Pebbly laminated central-south Tasmania (fig. 1, 2), and were mapped siltstone, and at least some of the diamictite, are between 1987 and 1999. The mapping was undertaken probably glacial in origin. This succession is middle to in order to better understand the industrial and metallic late Neoproterozoic in age, and is probably broadly mineral potential of this previously poorly-known correlative with the Togari Group of northwest area. The maps cover parts of two geological provinces: Tasmania, but does not contain mafic volcanic rock. 0 in the west, an older province known as the The Weld River Group occupies extensive areas in the Adamsfield–Jubilee region, composed of deformed, Weld valley, the upper Styx valley and the Huon valley relatively unmetamorphosed Proterozoic to lower around Blakes Opening. Very large resources of Palaeozoic rocks; industrial dolomite are present. Quartz veining and 0 silicification are widespread in the dolostone, and to the east, the Tasmania Basin, of flat-lying Upper much of the silicification is probably Mesozoic in age. Carboniferous to Triassic sedimentary rocks (Parmeener Supergroup) which unconformably The Forster Au-Zn-Ni prospect (Indicated Resource of overlie the older province, and Jurassic dolerite. one million tonnes Au @ 0.42 g/t, Summons, 1999) is The project built on earlier 1:50 000 scale mapping of hosted in weathered magnesian skarn and siliceous adjacent areas to the west (Huntley and Pedder map breccia derived from dolostone of the Weld River sheets). Mapping was also undertaken in The Needles Group, which forms part of the Glovers Bluff inlier in area (southeastern part of the Adamsfield 1:25 000 scale the eastern part of the Weld map sheet. The contact sheet); these results are included as Appendix 1 in metamorphism, metasomatism and mineralisation Calver et al. (2006). Areas of the Tasmania Basin, within the Glovers Bluff inlier are attributed to a thick Jurassic dolerite, and those areas incorporated within (600 m) intrusion of Jurassic dolerite that surrounds the World Heritage Area (WHA) in 1989, and and underlies the inlier (Bottrill et al., 1999). This is the unavailable for mineral exploration, were mapped at first recognised, significant mineralisation associated reconnaissance level only. The rest of the area was with the widespread Tasmanian Jurassic dolerite, and covered with 1:25 000 scale level of detail. Mapping this recognition significantly enhances the reliability information can be obtained from the prospectivity of the entire eastern Tasmanian region. responsibility diagrams on each of the maps. In faulted contact with the Proterozoic rocks is a unit of The main outcomes of the mapping are summarised lithicwacke, mudstone, chert and basalt, a correlate of below, and given in greater detail in Calver et al. (2006). the Ragged Basin Complex of presumed Early to The area contains large reserves of industrial minerals Middle Cambrian age. This crops out extensively west (silica, dolomite and limestone), and is prospective and of Maydena, and as a small, newly discovered inlier at under-explored for gold, base metals and platinum the eastern edge of the Weld map sheet. Chemically, the group elements. The geologically unusual Forster basaltic flows fall within the field of within-plate Au-Zn-Ni prospect lies in the south of the mapped area. tholeiite and basaltic andesite. Small serpentinite The work described here has led to a better bodies are juxtaposed with these rocks, and are the understanding of the Forster mineralisation, and of the source of small placer deposits of platinum-group geology and prospectivity of the exposed and elements. concealed basement rocks of southeastern Tasmania. A probably-Cambrian succession of turbiditic Three major Proterozoic successions were recognised lithicwacke and conglomerate is found west of (fig. 3). Probably the oldest of these is a succession of Maydena, in the middle and lower reaches of the Weld conglomeratic and sandy turbidite, passing up into River and in the Huon River east of Blakes Opening. distal sandy turbidite and black slate (Harrisons These rocks were derived from a source terrain of Opening Formation). The base and top are unknown. quartzose metasediments, sedimentary rocks Faulted against this formation is a sequence of (including dolostone similar to the Weld River Group), orthoquartzite (quartzarenite), dolostone, siltstone and mafic and felsic volcanic rocks, microgranite and mudstone, known as the Clark Group, which forms ultramafic rocks. Conglomerate in the Glovers Bluff large inliers at Jubilee Range, Glovers Bluff