NORTHERN TERRITORY GEOLOGICAL SURVEY Geological Survey Record 2001-003 - electronic pre-release Igneous rocks of the Tanami Region Alison A. Dean 1 NORTHERN TERRITORY DEPARTMENT OF MINES AND ENERGY MINISTER: Hon Daryl Manzie, MLA SECRETARY: Peter Blake NORTHERN TERRITORY GEOLOGICAL SURVEY DIRECTOR: Dr R Dennis Gee Bibliographic reference: Dean AA 2001. Igneous rocks of the Tanami Region - Northern Territory Geological Survey Record GS 2001-0003 ISSN 1443-1149; Geological Survey Record GS 2001-0003 ISBN 0 7245 7008 X Keywords: Tanami 1:250 000 SE 52-15; The Granites 1:250 000 SF52-03; Archaean; Palaeoproterozoic; igneous petrology; mafic; felsic; dolerite; granite; intrusives; extrusives; petrogenesis; geophysics; geochemistry This report may be obtained from: Department of Mines and Energy (DME) NT Geological Survey 3rd Floor, Centrepoint Building Smith Street Mall DARWIN NT 0800 or from the NTGS web site:http://www.dme.nt.gov.au/ntgs Cover photograph Granite Outcrop, Frederick Suite, MACFARLANE, AMG 504632-778659 ACKNOWLEDGEMENTS The author thanks Chris Field, Max Heckenberg and Liam McNally for their assistance and company during fieldwork. We thank the staff of Normandy NFM, Otter Gold, Anglo Gold, Tanami Gold and Glengarry Resources for their on going support of the Tanami Project. Authorship © Copyright in this information is vested in the Crown. The Government of the Northern Territory retains ownership of the intellectual property rights. No reproduction of this information, or any part of the information is permitted in any form except as the user may be licensed to produce by the Minister for Mines and Energy for the Northern Territory. All rights are reserved to the Crown. 2 Preface This publication represents the preliminary interpretations of petrographical and geochemical data from the Tanami Region by the Northern Territory Geological Survey (NTGS). The intention is to disseminate new information and ideas as quickly as possible to help stimulate the generation of new exploration models. Readers are advised that interpretations may change as new information becomes available. It is intended that after further editing and interpretation, the material contained in this Record will be incorporated, along with companion Records on structure and srtatigraphy, into a comprehensive bulletin on the geology and mineralisation of the Tanami Region. Disclaimer This information is provided on the understanding that the user agrees to release and indemnify the Northern Territory, the Commonwealth of Australia, companies who supplied and acquired the data, and their employees, agents and contractors, in respect of all liability for actions, claims, costs, expenses, loss, damage or injury, which may be suffered by them, or any other persons, arising from the users use of the data, or as a consequence of any unlawful or negligent act or omission of the user. 3 ABSTRACT Grantoids comprise a large percentage (~60%) of the Tanami Region. They are predominantly biotite (± hornblende) monzogranite and granodiorite, with minor syenogranite, quartz syenite, monzonite, tonalite and quartz diorite. Grantoids are distinguished on macro, meso and microscopic features and are subdivisible into the following: Coomarie Supersuite, comprising three suites: • Coomarie Suite, non-magnetic, metaluminous, oxidized and reduced, biotite monzogranite. • Inningarra Suite, moderately magnetic, foliated, metaluminous or weakly peraluminous, oxidized and reduced biotite monzogranite and granodiorite. • The Granites Suite, non to moderately magnetic, metaluminous, oxidized and reduced, biotite monzogranite and granodiorite. • numerous unassigned minor felsic stocks and dykes. Defined also are: • Frederick Suite, non-magnetic to highly magnetic, metaluminous, reduced to weakly oxidized, biotite hornblende monzogranite and granodiorite. • Winnecke Suite, non-magnetic to moderately magnetic, metaluminous, reduced to highly oxidized, hornblende biotite monzogranite, granodiorite and dacite. • Borefield Road Suite, moderately to highly magnetic, metaluminous, hornblende - biotite monzogranite to gabbro. Suites display predominantly calc-alkaline character and are metaluminous to weakly peraluminous (>1.1ASI). Peraluminous and peralkaline analyses are variably differentiated or metasomatized. Two genetic groups are discerned on trace element abundances: • Sr, Ti, Nb, and P depleted, Y undepleted or slightly undepleted – infer residual plagioclase, ilmenite, rutile, and apatite, with variable hornblende content in the source area. • Sr undepleted, Ti, Nb, P and Y depleted - infer residual hornblende, garnet, ilmenite, rutile, and apatite in the source area. This implies partial melting of protoliths at increasing depth with time. Three overlapping events of granitoid magmatism are recognised: an early 1830-1820 Ma event (Winnecke Suite), an 1820-1810 Ma event (Coomarie Supersuite, Inningarra Suite), and a later 1810- 1790 Ma period (The Granites Suite, Frederick Suite). Granite chemistry is equivocal with regard to tectonic setting. Distinctive chemical signatures defining suites suggest variable depths for partial melting and variable protolith compositions. Intermittent felsic volcanism concurrent with sedimentation is indicated by interbedded rhyolitic lava and ash-fall tuff. Mafic plutonism and volcanism was syn and post kinematic, and prior to or synchronous with granite intrusion. Tanami Region mafic rocks can be classified as subalkaline tholeiitic and exhibit a broad range in Mg # (26-71). Two groups are defined from stratigraphic petrographic and geochemical features: • Dead Bullock Soak Dolerite - intruded into Tanami Group, continental flood basalt chemistry, MgO wt% 6-7, LREE enriched, weak Eu anomaly. • Mount Charles Formation Basalt - extruded within Mount Charles Formation, distinctive low-K chemistry, similar to continental rift tholeiite. In the absence of evidence to the contrary and based on regional stratigraphy, this study supports the findings of previous authors that Tanami Group dolerite and associated mafic rocks intruded into developing basins during initial extension, and Mount Charles basalt and metamorphosed equivalents were extruded during a subsequent rifting event. 4 CONTENTS ABSTRACT INTRODUCTION 9 REGIONAL SETTING ARCHAEAN .............................................................................................................................................. 8 PALAEOPROTEROZOIC.............................................................................................................................. 9 FELSIC ROCKS PETROGRAPHIC FEATURES ...........................................................................................................10 GENERAL GEOCHEMICAL FEATURES......................................................................................... 14 GENERAL FEATURES OF GRANITOIDS........................................................................................ 21 COOMARIE SUITE................................................................................................................................... 24 Coomarie Dome Intrusives............................................................................................................... 25 Frankenia Monzogranite.................................................................................................................. 26 Talbot South Monzogranite.............................................................................................................. 26 MacFarlane Granodiorite................................................................................................................ 27 Browns Range Dome Intrusives ....................................................................................................... 27 INNINGARRA SUITE................................................................................................................................29 Muriel Range Intrusives................................................................................................................... 30 Grimwade Ridge Intrusives.............................................................................................................. 31 Murdoch Cliffs Intrusives................................................................................................................. 31 Officer Hill Intrusives....................................................................................................................... 32 Bunkers Tonalite .............................................................................................................................. 33 Watertower Tonalite......................................................................................................................... 33 THE GRANITES SUITE ............................................................................................................................ 34 The Granites Monzogranite.............................................................................................................. 35 Ptilotus Granodiorite ....................................................................................................................... 35 Twin Bonanza Porphyry................................................................................................................... 36 UNASSIGNED FELSIC INTRUSIVES .......................................................................................................... 36 Hurricane Dacite Dyke