The Tasmanian Geologist - Geological Society of Australia

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King Island Field Symposium 13 th -16 th March 2009 This is now fully booked, with arrangements made for 24 registrants and 2 reserves. Eight participants have chosen an add-on trip to visit possible Tasmanian correlates at Waratah Bay, Victoria on 16 th - 17 th March (organized by David Taylor, GSV). Further details are available on the GSA website: www.gsa.org.au For further inquiries contact the Secretary andrew.mcneill@utas.edu.au Organizing committee: Nick Direen, Clive Calver, Andrew McNeill. W. H. Twelvetrees Medal The Committee has proposed that a medal be struck in honour of the memory of the eminent Tasmanian government geologist, William Harper Twelvetrees (1848 – 1919). The suggestion is that the medal be awarded at the discretion of the Tasmanian Division of the Geological Society of Australia, usually annually, to recognize either: • important contributions to the Earth Sciences with Tasmania (including Macquarie Island); or • outstanding contributions to Earth Sciences while resident in Tasmania. In the spirit of W. H. Twelvetrees, contributions shall be considered to include meritorious feats of geological exploration in Tasmania or elsewhere; significant published works; or meritorious service to the Earth Sciences. Two or more members of the Society can nominate a member for the award by submitting to the Divisional Committee the name of the candidate, biographical data, and a citation in about 300 words describing the candidate’s work and its significance. Nominations will be valid for up to five years after the nomination date. The medal may be awarded posthumously if the candidate was alive at the time of nomination. Comments and suggestions from members are invited. The proposal will be put to a meeting in early 2009. “Geological Evolution of Tasmania” Volume The committee met with the editors on 22 nd January to review progress. The majority of contributions have been received, and letters have been sent to the remaining contributors. The following timetable will be implemented: 30 th June: final deadline for all submissions 15 th August: deadline for return of peer reviews 15 th September: deadline for author corrections 15 th October: deadline for completion of editorial content. Dr Clive Calver has been appointed joint editor, in addition to Dr Keith Corbett and Prof. Patrick Quilty. The committee has also resolved that the book will be produced in electronic, as well as printed form. Membership New applications Jafar Taheri Lindsay Clarke (student) J. Moye (student) Students remember- the Tasmanian Division will pay your $10 membership fee in full! Geological Society of Australia website: www.gsa.org.au 2009/2010 Divisional Committee Speakers wanted The Annual General Meeting is planned for Please April. contact All executive the Secretary and if you committee know of any positions visitors will or returning fall vacant. Tasmanians To assist willing in and planning, able to present expressions a talk at of a GSA interest meeting. from members willing and able to serve are sought by the current Secretary. Please consider if you are able to assist in your society! The role of diagenetic and metamorphic processes in the multi-stage concentration of The Tasmanian Geologist, 10th February 2008
gold, arsenic and vanadium in black shale and turbidite-hosted gold deposits. Ross R Large Organic-rich mudstones or shales, within thick packages of calcareous or siliciclastic turbidites, have long been recognised as important host rocks for major gold deposits (e.g. Victorian Goldfield, Carlin District, Lena Gold Province, Otago Schist belt). Many of the deposits in these districts are referred to as orogenic gold deposits, emphasising the tectonic and metamorphic processes involved in the concentration of gold (Groves et al., 2003; Goldfarb et al., 2005). The current genetic models propose that the organic-rich sedimentary host rocks are the trap-rock for gold precipitated during deformation-related fluid flow, with the gold being transported by deep-sourced fluids of mantle or lower crustal origin, or in some cases from felsic magmas. Recent research at CODES (Wood and Large, 2007; Large et al., 2007) suggests the possibility that organic-rich black shales are potential source rocks for gold, arsenic and vanadium, and that the gold was originally trapped on seafloor organic material by chemical process during sedimentation and concentrated in arsenian pyrite during diagenesis. Research on the giant black shale-hosted Sukhoi Log deposit in the Lena gold province of Siberia (Large et al. 2007, Scott et al, 2007) has revealed a multi-stage process of gold concentration and release that occurred throughout the 50 million year period of sedimentation, diagenesis and metamorphism in the Bodaibo trough. We have used the rapidly developing laser ablation ICP-MS technology to study the siting and partitioning of gold, arsenic, vanadium and other metals during diagenesis and metamorphism of the black shales. During periods of extreme euxinic conditions on the seafloor, gold and a range of other trace elements (As, Zn, Ag, V, Ni, Mo, Pb, Se, Te, Cu, Cr, U) are trapped on organic-surfaces forming strong organometallic ligands, leading to a concentration of metals in the most organic-rich facies of the sediments (e.g. Algeo and Maynard, 2004; Rimmer, 2004). During diagenesis, pyrite framboids, and clusters of pyrite micro-crystals of various textural forms, grow within the organic-rich sediments, commonly mediated by the biological reduction of marine sulfate to H 2 S. A portion of the gold and arsenic, plus many other trace elements (particularly Ni, Mo, Se, Pb, Ag, Te, Cu) are partitioned into the diagenetic pyrite. Other trace elements remain in the organics or form insoluble oxy-hydroxides (particularly V, U, Cr, Algeo and Maynard, 2004). The arsenic content of the diagenetic pyrite is a critical control on the amount of gold that is trapped within the sediment (Reich et al., 2005). The more arsenic-rich the black shale package, the more gold that is likely to be trapped during early diagenesis, thus contributing to form a more suitable gold source rock. With the onset of late diagenesis, the early forms of micro-crystalline gold-bearing arsenian pyrite, recrystallise to larger crystals ( commonly < 0.2 mm) of subhedral to euhedral pyrite, or in some cases pyrite nodules, developed parallel to bedding. Some gold and selected other trace elements are released during this process forming a gold-bearing diagenetic fluid, that may migrate and deposit free gold in late diagenetic structures. As the sediment passes through the oil window, mobile organics migrate along structures and may transport further gold, dissolved within the organic-rich fluid (William-Jones and Migdisov, 2007). These processes have also been observed in the Carlin district (Emsbo and Koenig, 2005). Deformation and metamorphism is accompanied by further recrystallisation and growth of pyrite in the black shales. The syn-deformation pyrite is commonly coarser grained (0.2 to 20 mm), with less sediment inclusions, and a significantly lower dissolved trace element content (Large et al., 2007). Gold originally dissolved in the diagenetic pyrite is released during metamorphism to form free gold, which concentrates in preferred structural sites (anticlinal zones or shears) associated with either metamorphic pyrite or syn-deformation quartz veins. Not only is the gold released from the diagenetic pyrite during metamorphism, but other trace elements, in particular, Cu, Zn, Pb, Ag, Bi, Te, are also released to form minor discrete sulfides (sphalerite, chalcopyrite, galena and various goldsilver-tellurides) which commonly form inclusions within the metamorphic pyrites. Certain elements, that are strongly held within the structure of pyrite (e.g. As, Ni, Co, Se), remain to become concentrated in the metamorphic pyrite. Vanadium which was originally concentrated in organic matter in the sediments, by reduction of V 5+ to V 4+ (Wood, 1996), is subsequently incorporated into gold-bearing diagenetic pyrite as micro-inclusions of V-bearing illite, and finally occurs as roscolite, intergrown with pyrite and free gold, in metamorphic assemblages. At higher metamorphic grades, above middle greenschist facies, much of the early diagenetic pyrite is converted to pyrrhotite. This is accompanied by the complete release of gold to the metamorphic fluid, as pyrrhotite, unlike pyrite, has a low trace element content and does not accommodate significant gold or arsenic within its structure (Buryak, 1982; Large et al, in press). For this reason pyrrhotite-rich black shales are unlikely to be good host-rocks for gold. In conclusion, organic rich black shales, especially those enriched in As, Ni, Mo, Pb, Zn and V are ideal source rocks for gold. Diagenetic and metamorphic processes are critical to releasing the gold, originally bound in organic-matter and dissolved within early arsenian pyrite, to become concentrated in later stages of syn-deformation pyrite and associated quartz veins. Full references are available from the editor or author on request. Abstracts from IAVCEI meeting The GSA (Tasmanian Division) gave financial support to assist two students attending the IAVCEI ( International The Tasmanian Geologist, 10th February 2008
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