SEG 45 Final_qx4 - Society of Economic Geologists

More documents

Recommendations

Info

SEG NEWS 20 SEG NEWSLETTER No 63 • OCTOBER 2005 Editor’s note: Part I of the report on the gold forum, covering Carlin-type deposits, was written by Greg Arehart and appeared in the July Newsletter. This second part, on the Witwatersrand, was contributed by Laurence Robb. A large audience of several hundred attended the much-anticipated debate on the origin of the Witwatersrand gold and uranium deposits in South Africa…testimony to the fascination that this mother of all gold fields still holds for the economic geology community at large. Ironically, it was a hydrothermal quartz vein in the West Rand Group near Roodepoort from which the very first gold was extracted in the Wits Basin….this was by the Streuben brothers a few years prior to the discovery of conglomerate-hosted gold, on the Ridge of White Waters, by George Harrison and George Walker in 1886. Right from the very beginnings of mining, it was clear that an understanding of the occurrence of gold, and its origin, was no simple matter. As long ago as 1917, Robert Burns Young, in his book The Banket of South African Gold Fields, described the authigenic character of gold within what was clearly a fossilized, heavy mineral-bearing gravel placer. And so it is, still in the year 2005, that geologists continue to argue about the nature of the gold and its origin, either as detrital particles sourced from a hinterland that must itself have been fabulously rich in primary gold deposits, or hydrothermal precipitates from a fluid—it, too, either very voluminous or extraordinarily enriched in the precious metal. The Reno debate started off with a talk by Hartwig Frimmel, who presented the case for the modified placer point of view. This notion is based on the existence in the basin of flat, rounded and “peened” grains that are interpreted as detrital gold toroids. This gold was initially concentrated by hydrodynamic processes during sedimentation and then substantially dissolved and remobilized by later fluids flowing through the sediments. It was shown that there must have been at least 2 episodes of fluid flow, resulting in the re-precipitation of gold and pyrite as authigenic particles within the conglomerates. The argument in favor of a purely SEG Forum on Gold Deposits: Part II May 14, 2005 • Reno, Nevada Laurence J. Robb (SEG 1985 F) Greg B. Arehart (SEG 1988 F) Report on the Witwatersrand Debate hydrothermal introduction of gold was presented by Neil Phillips, whose model invokes early circulation of diagenetic/meteoric fluids with hydrocarbon generation, followed by a pre-Platberg (the Platberg volcanics form part of the flood basaltic succession that immediately overlies the Witwatersrand sediments at 2709 Ma) introduction of metamorphogenic fluid that sulfidized the “black sand” heavy mineral suite that had accumulated during sedimentation and also precipitated gold. The essential differences between the 2 viewpoints are that the modified placer model requires initial accumulation of detrital gold grains, with later modification, whereas the hydrothermal model does not recognize the presence of allogenic gold and mineralization is essentially by precipitation from a fluid phase. What followed these two presentations was a combination of audience questions and comments, interspersed with brief presentations by each of five panel members, themselves experts on the Witwatersrand Basin. David Groves initiated the discussion by questioning the discrepancy between the very large scale alteration halos around putative fluid conduits and the mineralization that occurs at a more limited scale and is restricted to the thin conglomerate horizons. He also posed the question as to what exactly was the hydrothermal model, pointing out that its proponents did not seem to agree on the timing of the principal ore fluid circulation event. John Chesley also spoke in favor of a detrital origin of gold by summarizing the recent Re-Os isotope dating and characterization of gold and pyrite in the basin. This data, although itself controversial—as pointed out by Jonathan Law—is interpreted to indicate an unusual mantle source for the gold and its introduction as detrital particles at a time that preceded the end of sedimentation in the basin. Proponents of the hydrothermal view, such as Andy Barnicoat, re-emphasized the ubiquity of alteration assemblages and pointed to evidence showing that the latter are intimately associated with both gold and uraninite, often in fractures within the conglomerate packages. Vic Wall provided evidence for a fluid circulation and mineralizing event in post- Transvaal times (i.e., later than ca. 2550 Ma) and drew interesting analogies between the Au-U-C-PGM association in the Witwatersrand and a similar association in Proterozoic unconformity-related uranium deposits. Audience participation was lively and the four hours allocated to the debate passed by in a flash. No vote was taken at the end of proceedings and it is difficult to say which way the body of opinion swayed. Is there more consensus now about the origin of this most enigmatic of deposits than there was in the past? The answer is probably yes, in that more is known about the processes involved and also because proponents of the two models do agree on a wide range of issues. A fundamental division, nevertheless, still remains— namely, that of whether a fertile hinterland existed from which pyrite, gold, and uraninite detritus was sourced. The morphology of grains is not sufficient, in the view of some, to provide a conclusive answer to this question. Hundreds of zircons have now been individually dated and provide unequivocal indications of the age and location of the source of clastic sediment in the depository. The eventual answers to the Witwatersrand debate may also have to come from well-constrained and more accurate ages of key minerals such as pyrite, uraninite, and gold. The writer for one is fervent in the hope that we will have such answers well before the last bucket of ore is extracted from this phenomenal set of deposits. 1
OCTOBER 2005 • No 63 SEG NEWSLETTER 21 AFRICA Regional Correspondent: Judith Kinnaird (SEG 2002) SEG Regional Vice President Africa School of Geosciences University of the Witwatersrand, South Africa Tel: +27 11 7176583 Fax +27 11 7176579 Email: kinnairdj@geosciences.wits.ac.za With contributions from Dr. Stephen Frindt Geological Survey, Namibia COTE D’IVOIRE Cluff Gold bought the exploration data covering the Mt. Yaoure licence. This includes drill assay results from drilling completed around the Angovia mine, from the previous licence-holder Comincor. Internal company reports by Comincor indicate a resource of 90,000 t at a grade of 3.7 g/t, based on drilling completed in 2002. GHANA Golden Star Resources announced that the plant at Wassa gold mine is now operating at its design capacity of 10,000 tpd. The standby powerhouse was recently upgraded to supply sufficient power while connections to the national grid are being completed. MADAGASCAR Rio Tinto announced the approval of a US$585M mineral sands operation and port. First production in the Fort Dauphin region is expected in late 2008 and initial capacity will be 750,000 tpa of ilmenite. The development is the largest project in Madagascar’s history and will be the catalyst for broader economic development in the region. With a grade of 60% titanium dioxide, the Madagascar orebody is the largest known undeveloped high-grade mine, according to Rio Tinto. MALI Randgold Resources, the Malian gold producer, approved a US$100M underground project at its open-cast Loulo mine. EXPLORATION REVIEWS NAMIBIA Navachab gold mine (AngoGold Ashanti Ltd) situated in central-western Namibia, was developed in the late 1980s and was interpreted as a skarn deposit with a life of mine until 2003. However, with the discovery of the significance of associated sheeted auriferous quartz veins and the reinterpretation of the deposit as a mesothermal gold deposit, exploration located additional resources both in the hanging wall and the footwall. This increased life of mine to 2013 and, pending on the economic conditions, there is a chance of extending it even further. Namibia Stone Processing (NSP) plant, situated in the town of Omaruru, is one of the biggest dimension stone processing facilities in southern Africa. NSP is a black economic empowerment initiative, which was inaugurated by the Namibian President Hifikepunye Pohamba on July 20, 2005. The plant, built at a cost of N$50M, will process granite and marble tiles derived from local quarries as well as darker varieties of dimensions stone from Angola, Zimbabwe, and South Africa. The plant will employ between 70 and 100 people, with staff to be trained by Italian experts in stone processing, especially for marble tiles. The plant houses 2 granite gangue saws, 2 granite block cutters, 2 marble block cutters and 1 marble gangue saw. At full capacity the plant will produce some 3,000 m 2 of polished tiles per machine per month. The market is currently aimed at local and SADC countries, in future the company plans to expand their market to include European, American and Middle and Far Eastern countries. Vancouver-based junior, Helio Resource Corp, through its 100% owned Namibian subsidiary BAFEX Exploration, is currently exploring for copper and gold in northern Namibia. Helio has just completed a first-pass RC and diamond drilling program on the Otjitombo project, where significant intersections including 39.82 m @ 1.03% Cu and 0.12 g/t Au, 32 m @ 0.97% Cu & 0.11 g/t Au and 22 m @ 1.06% Cu, and 0.26 g/t Au have been reported, with numerous assays still pending. Elsewhere Helio has entered into a significant option agreement with Teck Cominco whereby Teck can earn up to a 75% interest on delivery of a feasibility study on the Vredelus gold prospect. Helio is also active exploring a number of IOCG prospects in the north of the country, two of these are in conjunction with fellow Vancouver Juniors Boulder Mining Corp and Yale Resources. Helio is also active in diamond exploration in Botswana, where it has optioned the Lokgwabe prospect to Indicator Minerals. Historic data at Lokgwabe indicates the presence of G10 indicator garnets. Rössing, the large open-pit uranium mine producer, which is situated 65 km inland from the coastal town of Swakopmund, started operations in 1976. Rössing is one of the largest open-pit uranium mines in the world and with solid reserves will continue to serve the world nuclear energy industry throughout the world, especially in Central Europe, North America, and Southeast Asia. The mine currently produces about 7.7% of the world’s uranium. In 2004, the life-of-mine plan forecast closure in 2009. However, they aim to develop and expand their capacity to deliver value to shareholders and local stakeholders for the long term and they are working on plans to justify extending the mine life to 2017. On the operations side, production will be increased to 3,800 t of uranium oxide for 2005 to counteract the negative economic impacts of the “weak” US dollar, in which Rössing’s uranium is sold. The work of the Rössing Foundation in the mine’s neighboring town, Arandis, was expanded as the Rössing Foundation strives to ensure an economically independent and selfsustaining town. Avdale Namibia (Pty) Ltd is a wholly owned subsidiary of African Rainbow Minerals (ARM). Thirteen EPL’s were awarded during 1997, covering 1,060,000 hectares over an area from Tsumeb in the north to Otjiwarongo in the south, Outjo in the west to Grootfontein in the east. This has now been reduced to four EPL’s covering 308,000 hectares. Copper, zinc, and gold were initially the main exploration commodities in the northern Carbonate platform, northern rift-margin and Northern Zone of the Pan- African Damara orogen. The terrain is extensively covered by a thin calcrete to page 22 ... EXPLORATION REVIEWS
Page 1 and 2: Celebrating a Century of Economic G
Page 3 and 4: OCTOBER 2005 • No 63 SEG NEWSLETT
Page 19: OCTOBER 2005 • No 63 SEG NEWSLETT
Page 27 and 28: DAY 1 - Saturday, May 13, 2006 SEG
Page 51 and 52: NEW IN 2005 The Economic Geology 10

SEG 45 Final_qx4 - Society of Economic Geologists

Create successful ePaper yourself

Delete template?

Save as template?