Economic Geology News
Orogenic gold deposits forming under our feet (11 December 2014)
In the approaching festive season, people like to give presents. There must be prospectors and geologists out there exploring for gold? Yes? What if I gave you a tiny orogenic gold system that promises to grow into a sizeable deposit?
Of course, I cannot. But I can amuse you with the reverse application of James Hutton’s (1726-1797) uniformitarian principle, that agents now at work on and within the Earth have operated with general uniformity through immensely long periods of time.
In New Zealand, metasediments of the Torlesse Terrane (Australian Plate) are advected underneath the transpressional orogen of the Southern Alps (Pacific Plate), establishing an ongoing conveyor belt of metamorphism, fluid release and rise, and precipitation of numerous auriferous veins (Pitcairn et al. 2014). Ore-forming elements (Au, As, Sb, Hg, etc.) are measurably depleted in metamorphic rocks relative to unmetamorphosed protolith samples; the same elements appear in the mineralised veins. Amphibolite facies rocks commonly contain between 50 and 80% less gold than their unmetamorphosed protolith equivalents. Concentration of Au, As, Hg and Sb in the metamorphic fluids is estimated at 0.08, 711, 3, and 40 mg/kg, respectively. Processing of a 10 km wide × 5 km deep section of Torlesse rock through the orogen at tectonic rates (0.01 m/yr) to be metamorphosed up to amphibolite facies is estimated to mobilise over 1100 t Au, 10 Mt As, 47,000 t Hg, 560,000 t Sb and 14,000 Mt H2O in one million years (Pitcairn et al. 2014). With favourable conditions of flow paths and a trap, 1000 t of gold make a large, though not a giant deposit.
Orogenic gold deposits forming now under our feet, is this not wonderful?
Pitcairn, I.K., Craw, D. & Teagle, D.A.H. (2014) The gold conveyor belt: large-scale gold mobility in an active orogen. Ore Geol. Rev. 62, 129–142.
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Recently, I collected the Wardell Armstrong Prize, from the Institute of Materials, Minerals and Mining (IOM3) in London (30 October 2014)
This award is given for the best paper printed in Applied Earth Science (refer to “Publications”, “News” entry 30 July 2014 or the Gallery in my websites). The award dinner at the premises of the society was a very formal event, but in some ways, rather exhilarating... Just look at the photograph at http://www.walter-pohl.com/gallery2/index.html
When I wrote this paper, the targeted audience was exploration staff of junior, but also established mining companies that consider to enter, or already operate in the Great Lakes region in Africa. I am convinced that regarding the use of modern concepts and technologies, this region is drastically underexplored. There is a considerable potential for the occurrence of Sn–Ta–Be–Li pegmatites and hydrothermal single vein, sheeted vein, saddle vein or stockwork tin, tungsten and gold deposits. Therefore, my aim was to provide a coherent framework of metallogeny to be filled in with detail if needed, which can easily be found in the references cited. My greatest reward would be to learn of new deposits discovered.
The paper combines elements from my surface and underground mine mapping in the region, several exploration programs, structural studies, geochemical and fluid inclusions research, a distillation of work published by fellow scientists, and some audacious new insights.
Read some of my concluding recommendations:
“Because of the generally dense vegetation and thick soil cover, the contribution of remote sensing to mapping and exploration is limited. Geological field work and geophysical methods will play a leading role in improved mapping and discovery. Modern geophysical data, such as airborne gravity, magnetic and radiometric surveys in Rwanda, are only available for limited parts of the whole region. Systematic petrophysical characterization of mafic sills (metadolerite) and G4 granites is needed for airborne surveys to map their distribution, to locate outcrops and to model buried granite cupolas and elongated ridges. Iron-rich hydrothermal systems (Fig. 8) formed by reaction with the metabasalts are prospective for gold. The morphology of the contact surface between granites and metasediments should be mapped to about 2000 metres below ground in order to locate favourable structures such as intrusive protuberances. Graphitic schists may mark high temperature contact metamorphism and hydrothermal alteration bodies hosted in black shale. Sulphides can indicate the potential of associated Au or Sn mineralisation.”
In this context, note a recent paper by Chicharro et al. 2014: Hidden cupolas of tin granites can by found by geophysical methods at greenfield scales. Detailed gravimetric surveys at brownfields reveal exploration targets by subsurface modelling of roots, steep flanks or cupolas, that may control magmatic-hydrothermal ore bodies. You can ask Eva Chicharro in ResearchGate for a PDF of this paper:
Chicharro, E., Martin-Crespo, T., Gomez-Ortiz, D. et al. (2014) Geology and gravity modeling of the Logrosan Sn- (W) ore deposits (Central Iberian Zone, Spain). Ore Geology Reviews (2014), doi: 10.1016/j.oregeorev.2014.10.005
Sincere acknowledgements are due to my co-authors Bernd Lehmann (TU Clausthal) and Michael Biryabarema (Department of Geology and Mines, Kigali), and thanks go to IOM3 for this recognition of my work.
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High-Tech Exploration Without Economic Geologists? (1st October 2014)
Several weeks ago I attended a Technical Meeting on Advances in Exploration Techniques for Uranium Deposits at the International Energy Agency (IEA). Reports from countries that are less known as producers of U were interspersed with papers that highlighted the state of the art in Australia and Canada. The two are the world’s largest U-producers after Kazakhstan.
I don’t know about Kazakhstan, but the results of novel exploration methods and technologies employed in Canada and Australia are amazing. In some cases, skilful use of big data and processing in Geographic Information Systems (GIS) enables pinpointing sites of hidden mineralisation, down to about 2000 m below surface.
Naturally, I searched for papers that describe the new methods and found out that a Special Issue of Ore Geology Reviews (2010) contains 13 papers on the subject. Unfortunately, OGR is not one of the cheapest journals, so I condense here some information from the introductory paper (Porwal & Kreuzer 2010). Other papers in the issue describe application examples, such as orogenic gold (Victoria), volcanic massive sulfides (Sweden) and platinum (Finland).
Three families of methods are used in exploration:
(1) Traditional exploration targeting as used by the majority of the mining industry relies on economic geology expert knowledge and interpretation of available or newly acquired geological, geochemical and geophysical data. Conceptual genetic models of the targeted deposit type and past exploration results are used to assemble characteristics that may be guides or vectors to mineralisation.
(2) Statistical techniques of mineral prospectivity analysis aim to reduce the human bias. Computational techniques employed include weights of evidence, fuzzy logic, and artificial neural networks. Latest developments propose hybrids of knowledge-driven and data-driven fuzzy-weights-of-evidence statistics.
(3) Modern mineral prospectivity modelling relying on a process-based, or “mineral systems” approach using Geographic Information Systems (GIS) was developed in Australia during the last 25 years. This method considerably improves predictive capabilities. Mineral systems models of targets are based on the underlying mineralization processes and their mappable features. The latter are considered to be more reliable compared with type deposit models because no two deposits of one type are fully equal. Critical mineralization processes acting together to form a hydrothermal mineral deposit, for example, include (a) the establishment of an energy gradient to drive the system; (b) the generation of hydrothermal fluids; (c) the extraction of metals and chemical ligands for metal complexation from suitable sources; (d) the transport of fluids and metals from source regions to traps (i.e. effective flow channels); (e) deposition of metals, triggered by chemical and physical processes that affect fluids migrating through traps; and (f) the preservation of mineral deposits through time. Although the processes cannot be directly observed or mapped, proxies such as wide alteration haloes or syngenetic faults may be recognizable. Other key elements are modern systematic grid-sampled regional geophysical data (magnetics, gravity, electromagnetic, seismic and soon, deep-sounding magnetotelluric tomographic surveys) and remote sensing. Output maps are decision-support tools for delineating, ranking, and prioritizing exploration targets.
Until now, however, reports of successful applications are rare apart from the publication of mineral potential maps. A very instructive example is the detailed report on the comparison between mineral prospectivity modelling and the results of past traditional exploration covering the important porphyry copper-gold province around Cadia and Northparkes mines in eastern Australia (Kreuzer et al. 2014).
At the first view, (3) looks like a machine spilling out maps with the targets, including size, depth and probability. But we all know better – improved mineral deposit models including ever better understanding of genetic processes provided by research will continue to raise the success rate. Knowledgeable professionals will feed the software with data. All components of acquiring and processing the data will be continuously updated. In fact, specialised IT+GIS economic geologists will be needed, but also those that work nearer to the rocks in the field, from mapping and drilling the prospects to servicing producing mines.
I do trust that economic geologists will not be replaced by roboters. There are clear signs, however, that the role of data will continue to increase. Read my blog “Gold Exploration – Contrasting Recent Strategies” (News Archive 1 August 2012).
Kreuzer, O.P., Miller, A.V.M., Peters, K.J. et al. (2014) Comparing prospectivity modelling results and past exploration data: A case study of porphyry copper-gold mineral systems in the Macquarie arc, Lachlan fold belt, New South Wales. Ore Geol. Rev.
Porwal, A.K. & Kreuzer, O.P. (2010) Introduction to the special issue: Mineral prospectivity analysis and quantitative resource estimation. Ore Geol. Rev. 38, 121-127.
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The Origin of Bayan Obo, the Giant Rare Earths Deposit in N-China, Finally Unravelled? (23 September 2014)
Recently in LinkedIn, Linda Campbell announced the publication of her paper on zircon SHRIMP data of ore from this giant mine. As creative commons, it is freely downloadable from the Springer website.
If you wish to have the results compressed in one phrase - Bayan Obo is a carbonatite-related REE concentration, although with a rather complex postmagmatic history that is not fully understood.
Somewhat longer is the updated description of Bayan Obo in my “Economic Geology” book (2011, page 259) as below.
Or read Linda’s full paper, which is a very good example of excellent science in methods and contents.
The Economic Geology text:
Magmatic-hydrothermal Bayan Obo (Baiyenabo) in Inner Mongolia, China, is both economically outstanding and genetically complex. Total reserves comprise 1500 Mt at 35% of haematite and magnetite, 135 Mt with 6% LREE2O3 (monazite, parisite and bastnaesite), and 1 Mt with 0.13% Nb2O5. Country rocks are Mesoproterozoic slates, sandstones and quartzites. Complex “dolomite marble” hosting the ore bodies is the product of multiple alkaline-carbonatitic activity that includes fenitization affecting sedimentary carbonates (Smith et al. 2014). Ore is thinly banded by magnetite, apatite, carbonates, alkali silicates and fluorite. Fluid inclusions in ore and gangue minerals have elevated salinity (7-10 wt.% NaCl equiv.) and CO2 contents. At formation pressures of 1 kbar and 300 - 400 degree C episodes of phase separation (boiling) are indicated. The fluids have characteristics that can be related to carbonatites and alkali complexes (Smith & Henderson 2000).
Yet, aspects of the origin of this giant deposit remain disputed (Smith et al. 2014, Yang et al. 2008, Fan et al. 2004). It displays features of both (magmatic) carbonatite and (magmatic-hydrothermal) oxide-copper-gold (IOCG) deposits (Wu 2008). The ore is enclosed in, and coeval with a magmatic, intrusive, carbonatitic protolith suite that crystallized at 1325 ± 60 Ma as determined on zircon cores resembling those generally found in carbonatites (Campbell et al. 2014). This magmatism may have been related to a break-up rifting phase occurring throughout supercontinent Columbia. Ages of zircon rims at 455 ± 28 Ma (Late Ordovician) date a profound tectonothermal overprint, disturbance, foliation and recrystallisation of Fe-REE ore by alkaline hydrothermal fluids and by Caledonian deformation and metamorphism (Smith et al. 2014).
http://link.springer.com/article/10.1007%2Fs00410-014-1041-3 (Open Access full text)
Campbell, L.S., Compston, W., Sircombe, K.N. & Wilkinson, C.C. (2014) Zircon from the East Orebody of the Bayan Obo Fe-Nb-REE deposit, China, and SHRIMP ages for carbonatite-related magmatism and REE mineralization events. Contributions Mineralogy & Petrology 168, published 8 august 2014.
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New Papers: Reconciling Coltan (Sn, Ta) Mining with People, Nature and the Environment in the Great Lakes Region, Africa (20 August 2014)
I reported on related matter in several previous blogs. Here are two newly published papers that together provide a baseline for responsible mining in the region. Take note, if you should be involved in environmental impact studies for rare-metal mining projects in the region or in supervising/controlling current mining.
The Coltan Environmental Management Project (full name: Sustainable Restitution/Recultivation of Artisanal Tantalum Mining Wasteland in Central Africa) is a research initiative by academics from four countries in the Central Africa Region (Burundi, DR Congo, Rwanda and Uganda, all sharing part of the Kibaran tantalum-tin-tungsten-gold metallogenetic province) and Germany, and is supported by VW-Foundation (Hannover). Recognizing that the prosperity of the region is advanced by exploitation of the rare metals, we aim to reconcile mining with people, nature and the environment, by searching best methods of rehabilitating abandoned mines and of environmental and socio-economic management of current mining. Geology, geomorphology, biology, agriculture, and economic, soil and social sciences are involved. A science-based toolbox of mitigation technologies, and guidelines for socially and environmentally responsible tantalum mining and mine closure are being developed.
Lehmann, B., Halder, St., Ruzindana Munana, J., Ngizimana, J. de la Paix, Biryabarema, M. (2014) The geochemical signature of rare-metal pegmatites in Central Africa: Magmatic rocks in the Gatumba tin–tantalum mining district, Rwanda. J. Geochem. Exploration (Special Volume on Mining and the Environment in Africa) 144, 528-538.
Nieder, R., Weber, T.K.D., Paulmann, I., Muwanga, A., Owor, M., Naramabuye, F.-X., Gakwerere, F., Biryabarema, M., Biester, H. & Pohl, W.L. (2014) The geochemical signature of rare-metal pegmatites in the Central Africa Region: Soils, plants, water and stream sediments in the Gatumba tin-tantalum mining district, Rwanda. J. Geochem. Exploration (Special Volume on Mining and the Environment in Africa) 144, 539-551. doi: 10.1016/j.gexplo.2014.01.025
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We Have Been Awarded with the Wardell Armstrong Prize 2014 (30 July 2014)
Who is we? - Walter L Pohl, Michael Biryabarema and Bernd Lehmann
This award is given for the best paper published in Applied Earth Science (AES). AES is published by Maney on behalf of the Institute of Materials, Minerals and Mining and the Australasian Institute of Mining and Metallurgy. The winning article in 2014 is:
Early Neoproterozoic rare metal (Sn, Ta, W) and gold metallogeny of the Central Africa Region: a review
Maney Publishing is generously sharing free downloads of our paper and of other winners of the 2014 Institute of Materials, Mining and Minerals (IOM3) awards for published work.
The award is named for Wardell Armstrong, an international, Britain-based, environmental and engineering consultancy (http://www.wardell-armstrong.com/).
If you wish for more information on the article, look up my blog dated 10 January 2014.
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For Rare Metal Exploration, go to Supercontinents? (16 July 2014)
Peak amalgamation of supercontinents appears to control the timing of numerous important Ta deposits (as shown for Africa by U/Pb dating of CGM: Melcher et al. in press), and globally, of many provinces of the “granitophile” rare element deposits (tin, tungsten, Ta, Be, Li etc.).
Yet, I am not convinced that there is a difference in metallogeny between amalgamation of all continents into one (forming a true supercontinent such as Pangaea), or of only a few continents (no true supercontinent, e.g. Gondwana; or a simple continental collision such as the Caledonian one between Laurentia and Baltica). If, to continue with the example of the granitophile elements, significant deposits occur likewise in Rodinia, Gondwana and Pangaea, the common factor is wide-spread flooding of the crust by granites. And that may be caused by a variety of plate tectonic processes (to be specified), and not the 5th or 6th continent being amalgamated. I tend to doubt that there is a magic “metallogenic supercontinent process” and I suggest that the tool box of plate tectonics or better, of the Earth’s dynamics, is the rational approach to scientific analysis and to building exploration models. To answer the question stated in the title – by all means, go to the supercontinents for rare metal exploration. But do not trust the label alone!
Correlation is no positive proof of causation ... or is it?
If you wish to read more on this subject – look at my blogs dated 16 June, and 20 February, 2014.
Melcher F., Graupner T., Gäbler H.-E. et al. (in press) Tantalum–(niobium–tin) mineralisation in African pegmatites and rare metal granites: Constraints from Ta–Nb oxide mineralogy, geochemistry and U–Pb geochronology. Ore Geology Reviews http://dx.doi.org/10.1016/j.oregeorev.2013.09.003
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Metallogenesis Related to Supercontinents – Still a Hot Topic! (16 June 2014)
In my earlier News blog “Are Supercontinents an Exploration Guide? (20 February 2014)” I doubted that the sole existence of a supercontinent would provide valuable hints for exploration. Meanwhile, I continued to follow this topic closely. Many current papers mention some relation between supercontinents and ore deposit formation, but omit to provide details. There seem to be authors who attribute any mineralisation, if dated before, during and after the actual assembly of a supercontinent as relevant in this respect. Others maintain that a supencontinent is only in existance when it is fully amalgamated. Before assembly, during welding and breakup, supercontinents display a variety of plate tectonic processes that we have learnt to understand and to use for exploration and scientific studies. And what about the peak of welding and the following period of supercontinent stasis before new-born plates start to drift away? Are all deposits that originate during this period causally related to the supercontinent?
If you are interested in this topic, do come to Leoben/Austria on 24 June 2014 (16,00 h). I’ll talk on the subject at Mining University, Chair of Economic Geology, Peter Tunnerstrasse 5, Seminarraum B.
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Are we Running out of Gold? If you Believe this Statement, look at the Australian Success Story (23 May 2014)
There is a deluge of warnings in public and scientific media that we are running out of mineral resources of substances such as oil, phosphate and, yes, gold too. While I do recognize the need for forecasts I am skeptical of many predictions of doom.
Concerning gold, I would like to support my views by reporting on a short paper by Neil Phillips (2013) who is a leader in the Australian gold exploration industry. Look at some facts:
Australia's gold production has been 265 +/- 50 tonnes per year for the past 23 years. For 2012, the figure of 248 t is only 20 per cent below the historical peak for Australia of 314 t in 1997 and second only to China (403 t, USGS 2014). From 1980 onward, annual production rose steeply from <50 t to present levels.
Australia's national gold resources are expressed as economic demonstrated resources (EDR) annually published by Geoscience Australia (2013). EDR is essentially a sum of reserves and most of the measured and indicated resources (these are JORC terms; see my News blog dated (8 April 2013).
At the end of 2013, Australia's gold resources (EDR) were approximately 9900 t Au. You do recognize that obviously, when resources increase, more ounces must have been found than were produced. This is true for most years since Australian gold production started to rise in 1980. Today, Australia owns the largest gold resources on the Earth.
Explorations costs in Australia are reported on a national scale. During the past nine years, EDR has risen by 4518 t and Australia has produced 2241 t Au, implying added resources of 6759 t in that period. The total expenditure on gold exploration over the same period was $4950 million (2012 AUD) that equates to $22.8 per ounce.
Neil concludes that adding 6000 t gold to Australia’s EDR in ten years is an extraordinary achievement. The EDR resource/production ratio is currently >30 years. The outlook for gold mining in Australia is bright. “Risks are complacency, hubris and high costs of extraction, rather than running out of resources.”
Would you agree with me that three hearty cheers are due to our Australian mates in gold exploration?
There is one point that I would like to add to this story:
Australia is not unique in its gold endowment. In my opinion, this success is due to a number of factors, of which outstanding people in geological research and its application, and general support of mining by society and government may be the most important. Other countries might emulate this path to prosperity.
A few weeks ago I spoke to students at Vienna University on “Geological energy resources – How much longer will they last?” I presented the current global reserve/resource and production figures and explained that forecasts based on these figures necessarily reflect present and past technologies. Revolutionary jumps in technology (remember shale gas and oil) cannot be predicted. Humans are extremely inventive and ours is a time of precipitate advance. So we should not stare at statistical predictions of “the end of gold, oil etc.” but get to work inventing new methods.
Phillips, G. N. (2013) Australian gold exploration – a quick audit (or how do we measure success?). AusIMM Bulletin no. 4 August 2013, 22-25.
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Metals can be Recycled, but Coal and Hydrocarbons are Burnt. Is Uranium Dissolved in the Global Oceans the Long Term Solution to Humanity’s Energy Thirst? The Novel Technology of Molecular Recognition may be the Key (14 April 2014)
Large potential U resources assure future supplies, but new mines and technologies will be needed, especially for eventual exploitation of the giant uranium mass dissolved in seawater (estimated at 4.5 x 10 9 tonnes U). This is one of the goals persued by applying molecular recognition principles to the design of new separation agents selective for individual metal ions (Izatt et al. 2014). In nuclear waste cleanup, hydrometallurgical applications of the technology are already now well established. This is only one highlight in an excellent review paper that provides many insights, apart from explaining and illustrating the principle of molecular recognition. Current commercial applications in the mining industry include the large-scale purification of metals such as cobalt (deleterious Cd), gold (Cu) and copper (Bi).
Izatt, R.M., Izatt, St.R., Bruening, R.L. et al. (2014) Challenges to achievement of metal sustainability in our high-tech society – a tutorial review. Chem. Soc. Rev. 43, 2451-2475.
Abstract: Achievement of sustainability in metal life cycles from mining of virgin ore to consumer and industrial devices to end-of-life products requires greatly increased recycling rates and improved processing of metals using conventional and green chemistry technologies. Electronic and other high-tech products containing precious, toxic, and specialty metals usually have short lifetimes and low recycling rates. Products containing these metals generally are incinerated, discarded as waste in landfills, or dismantled in informal recycling using crude and environmentally irresponsible procedures. Low recycling rates of metals coupled with increasing demand for high-tech products containing them necessitate increased mining with attendant environmental, health, energy, water, and carbon-footprint consequences. In this tutorial review, challenges to achieving metal sustainability, including projected use of urban mining, in present high-tech society are presented; health, environmental, and economic incentives for various government, industry, and public stakeholders to improve metal sustainability are discussed; a case for technical improvements, including use of molecular recognition, in selective metal separation technology, especially for metal recovery from dilute feed stocks is given; and global consequences of continuing on the present path are examined.
Key learning points (Izat et al. 2014)
(1) Since Earth’s metal supply is finite, burgeoning consumption of metals by consumers and industry is unsustainable without efficient recycling.
(2) Due to the unique chemical and physical properties of metals, scientific discoveries have enabled the creation of a myriad of high-tech products that have transformed 21st century society.
(3) Vastly improved value recovery from end-of-life metal products is needed both by increasing the rate of metal recycling and by improving the performance of collection and recycling technologies, some of which rely on informal methods that are disastrous from environmental and health standpoints but essential to the economies of many nations.
(4) Improved selective, environmentally friendly, and commercial processes for separation and recovery of metals (precious, specialty including rare earth, toxic, and radioactive) in pure form with minimum carbon footprint are essential to sustainability of metal life cycles from mining to end-of-life to recycling.
(5) Along with improvements in technology, increased awareness of the urgency of global metal sustainability and resolve-to-action on the part of public, government, press, scientific, and industry stakeholders is needed to ensure a workable solution to sustainability of metal life cycles for future generations.
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First Industrial-Scale Application of Gold Thiosulfate Leaching (19 March 2014)
In an article “Taking cyanide out of the gold equation” CSIRO (Australia) reports that later this year, Barrick Gold’s thiosulfate leaching process will attain full-scale production at the company’s Nevada Goldstrike mine.
Goldstrike is one of the largest Carlin-type gold deposits. Exploitation started in 1986 with oxidized ore. Later, gold was extracted by autoclaving or roasting and cyanide leaching. Starting in Q4/2014 in the autoclave circuit designed to process refractory sulfide ore, thiosulfate (S2O32-) lixiviant is to replace cyanide. CSIRO supported Barrick Gold by developing and applying an improved resin elution process at a demonstration plant at Goldstrike.
You are probably aware, that Carlin-type ore is different from typical gold ore; therefore, the thiosulfate leaching process is not applicable and economic for all gold deposits. Main features and genetic interpretations of Carlin gold are summarised in my “Economic Geology” (page 212); alternatively start a search with Muntean et al. (2011).
Muntean, J.L., Cline, J.S., Simon, A.C. & Longo, A.A. (2011) Magmatic–hydrothermal origin of Nevada's Carlin-type gold deposits. Nature Geoscience 4, 122 – 127.
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Are Supercontinents an Exploration Guide? (20 February 2014)
To state the question in a less shortened way – do geological data about the amalgamation, stasis and disaggregation of supercontinents (SC) serve strategic exploration? We might ask also (1) are there mineralisation types that occur in all or in most supercontinents and (2) which deposits may be prospective exploration targets? (3) is there any type of mineralisation that is restricted to SC and therefore, can be a pathfinder to supercontinents? And finally, (4) is there any practical advantage for exploration in the study of supercontinents?
Xin-Fu Zhao et al. (2013), for example, write that “continental blocks that once were part of Columbia, notably where rift basins developed in such blocks, may have potential to host sedimentary rock-hosted stratiform Cu deposits (SSC)". Should explorationists pack their gear and search all fragments of former Nuna/Columbia supercontinent for SSC?
Supercontinents are amalgamations of several or all continental plates of the Earth, formed by long-lived plate convergence punctuated by distensive events before peak assembly. After a geologically short period of stasis (the SC stage), tensional tectonics initiate fragmentation and the growth of new oceans, thus terminating one “supercontinent cycle”. Of course, ore formation is potentially associated with all contributing individual process systems, including subduction and delamination, arc and anorogenic magmatism, oceanic and continental rifting, basin formation, orogenesis, diagenesis, metamorphism and post-collisional collapse.
Let us first list the major supercontinents that are reasonably well known and widely accepted:
- Nuna also called Columbia (1900-1500 Ma; peak assembly at ca. 1800 Ma: Evans & Mitchell 2011)
- Rodinia (peak assembly at ca. 1000 Ma: Pohl et al. 2013)
- Gondwana (peak assembly at ca. 550 Ma: Abu-Alam et al. 2013)
- Pangaea (peak assembly at ca. 300 Ma: Wegener 1924)
Comprehensive studies on the metallogenetic characteristics and evolution of the individual supercontinents are not available. Generally, continental deposit types such as sedimentary rock-hosted stratiform Cu deposits or sedimentary Fe ore deposits are cited as examples of the link between supercontinents and ore formation.
Always being short of time, for this blog I can only scan easily available information. One useful source is Chapter 6 “Ore deposits in a global tectonic context” (p 311-343) in Robb (2005). Laurence Robb does not, however, describe metallogeny in the respective supercontinent cycle but in time periods.
Nuna in the Paleoproterozoic, seems to lack remarkable mineralisation apart from the giant Pb-Zn-Ag deposits of eastern Australia (Mt. Isa etc.) and the unique IOCG deposit at Olympic Dam in Southern Australia, all of which seem to be related to the postpeak-assembly fragmentation of Nuna.
Rodinia in the Neoproterozoic, amalgamated by pan-Rodinian (or “Grenvillean”) orogenic events in the Late Mesoproterozoic. In North America, prepeak-assembly anorogenic magmatism and rifting are related to deposits of Fe-Ti, Pb-Zn and Cu-Ag. In central Africa, a flare-up of Sn-W-Au fertile granites marks the time of peak assembly, possibly caused by collisional indentation followed by delamination (Pohl et al. 2013).
Gondwana was assembled by long-lived plate convergence in the Late Neoproterozoic; it includes the spectacular Cu-Co resources of the Copper Belt in South-Central Africa that are most likely of peak-assembly diagenetic-hydrothermal origin.
Pangea welded by Late Paleozoic Variscan (or Hercynian or Appalachian) orogenic convergence, displays a multitude of economically significant deposits types, including prepeak-assembly VMS and sedex deposits such as the giant massive sulphide ore province of the Iberian Pyrite Belt with pyrite, copper, tin, lead, zinc and gold, and in the US, MVT Pb-Zn. Peak-assembly granite-related deposits comprise Sn, Cu, W, Bi, U, Au-As-Sb, Li and F. One of the most metal-endowed sutures worldwide is the 3000 km-long Palaeozoic accretionary orogenic collage of the Altaids in Central Asia, which marks the collision zone between Gondwana and Eurasia (Xiao et al. 2009). Postpeak-assembly giants include the Noril’sk Ni-Cu-PGE ore bodies, coking coal across much of the northern hemisphere, the diagenetic-hydrothermal European Copper Shale, and evaporites that include K-salt deposits, in Canada and Germany.
Conclusions? I feel that within parts of one supercontinent, for example Nuna/Columbia, consideration of metallogenic characteristics of one fragment may assist exploration in another fragment of similar geology (e.g. Xin-Fu Zhao et al. 2013). This principle was already recognized by Petrascheck (1965). The introductory questions might best be answered by (1) there are no mineralisation types that occur in all or in most supercontinents; (2) therefore, there are no specific supercontinent-related exploration targets; (3) there is no mineralisation that is restricted to SC; (4) there appears to be little practical advantage for exploration in the study of supercontinents. Overall, the toolbox of plate tectonics (Wilson cycles etc.) at the present level of refinement seems to suffice for building exploration models.
Would you like to contribute your experiences and opinion to this discussion?
Abu-Alam, T.S., Santosh, M., Brown, M. & Stüwe, K. (2013) Gondwana collision. Miner. Petrol. 107, 631-634.
Evans, D.A.D. & Mitchell, R.N. (2011) Assembly and breakup of the core of the Palaeoproterozoic-Mesoproterozoic supercontinent Nuna. Geology 39, 443-446.
Petrascheck, W.E. (1965) Typical features of metallogenic provinces. Economic Geol. 60, 1620-1634.
Pohl, W.L., Biryabarema, M. & Lehmann, B. (2013) Early Neoproterozoic rare metal (Sn, Ta, W) and gold metallogeny of the Central Africa Region: a review. Applied Earth Science 122, 66-82.
Robb, L.J. (2005) Introduction to Ore-Forming Processes. 373 pp. Blackwell.
Wegener, A. (transl. J.G.A. Skerl) (1924) Origin of continents and oceans. Methuen, London.
Xiao Wenjiao, Kröner, A. & Windley, B. (2009) Geodynamic evolution of Central Asia in the Paleozoic and Mesozoic. Int. J. Earth Sci. 98, 1185-1188.
Xin-Fu Zhao, Mei-Fu Zhou, Jian-Wei Li & Liang Qi (2013) Late Proterozoic sedimentary rock-hosted stratiform copper deposits in South China: their possible link to the supercontinent cycle. Miner Deposita 48, 129-136.
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Sustainable mining - sodium carbonate extraction from Lake Magadi, Kenya (25 January 2014)
You may remember, that the term “sustainable” was first coined about 300 years ago for the management of forests, demanding “that the amount of wood cut should not exceed the growth rate” (Carlowitz 1713). According to this narrow definition, very few mineral deposits are renewable, but the Magadi soda mine may be one of the class:
In the recent past, I had the occasion to visit the Lake Magadi area in the East African Rift (EAR) Valley, Kenya, near the border to Tanzania in the South. Lake Magadi in Kenya and Lake Natron in Tanzania are parts of a large saline lake system that may have originated in the early Pleistocene. This part of the rift is marked by the giant active carbonatite volcano Oldoinyo Lengai, the peak of which towers about 2600 metres above the lakes. Both lakes are saline, with a chemical composition of waters characterised by high concentrations of sodium and carbonate. They are typical “soda lakes” of the EAR.
Holocene trona beds are 7 to 40 m thick, sourced from surface runoff, groundwater, geothermal springs and precipitated by evaporation (Eugster 1970). Fluorine contents in this trona ore are significant, occurring in the form of 22 wt. % villiaumite (NaF) and ~6% fluorite. This seems to imply a genetic relation to the unique fluorine-rich natrocarbonatite lavas of Oldoinyo Lengai, probably by supergene leaching.
At Magadi, trona is extracted by dredging and lake brines are also used as a source of soda and halite. During my studies, heavy rains had raised the lake level and diluted the brines. Evaporation will, however, soon re-establish saturation levels. The rate of replenishement of the deposit is reported to exceed production making Magadi one of the rare examples of literally sustainable mining. In neighbouring Tanzania, a project to exploit soda from a bay of Lake Natron is disputed because of possible negative consequences for the lake’s famous flamingo population. At Magadi, flamingos seem to be perfectly oblivious of the industrial activity.
Use of soda ash (Na2CO3) is mainly in glass manufacturing, followed by chemicals, soap and detergents, pulp and paper production, water treatment, bentonite activation, animal feed and flue gas desulfurization, dechlorination and defluorination. The annual production of soda ash (natural and synthetic) is ~52 Mt/a but only 13 Mt are primary mining products, almost wholly from the Green River Basin, USA. This region dominates world mine output, whereas Turkey, Kenya and Botswana are minor producers. In Turkey, two newly discovered and very large trona deposits are presently prepared for production: Kazan with resources of 600 Mt at 31% trona and nearby Beypazari with 230 Mt at 70% trona. China leads in the production of synthetic soda.
Papers for further reading:
Eugster, H.P. (1970) Chemistry and origin of the brines of Lake Magadi, Kenya. SEPC Publ. Geol. Soc. 3, 215-235.
Jones, B.F., Eugster, H.P. & Rettig, S.L. (1977) Hydrochemistry of the Lake Magadi basin, Kenya. Geochimica et Cosmochimica Acta 41, 53-72.
Moor, J.M. de, Fischer, T.P., King, P.L. et al. (2013) Volatile-rich silicate melts from Oldoinyo Lengai volcano (Tanzania): Implications for carbonatite genesis and eruptive behavior. Earth and Planetary Science Letters 361, 379-390.
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Kibaran (Meso- to Early Neoproterozoic) Metallogeny in the Central Africa Region Updated (10 January 2014)
Early Neoproterozoic rare metal (Sn, Ta, W) and gold metallogeny of the Central Africa Region: a review. Pohl, Walter L., Biryabarema, Michael & Lehmann, Bernd (2013) Applied Earth Science (Transactions of the Institute of Mining and Metallurgy London, and the Australasian Institute of Mining and Metallurgy Melbourne) 122, 66-82.Abstract The four metals of economic significance in the Central Africa or Great Lakes region, i.e. gold, tin, tantalum and tungsten, are part of one composite metallogenic system that operated at about 980 ± 20 Ma. The main driving agent was peraluminous ilmenite-series granite magmatism, synchronous with intracratonic compression and associated with the final amalgamation of the supercontinent Rodinia. The granitic melts were emplaced at intrusive levels of ≥2 kbar (≥8 km); the intrusions display a variable and often advanced degree of fractionation, including abundant Sn-Ta-Li-Be-Rb-Cs pegmatites, and are associated with hydrothermal systems enriched in tin, tungsten and/or gold. Based on cumulative past production and present metal prices, gold in hydrothermal quartz veins is the major commodity, followed by tin either in rare metal pegmatites or in sheeted, hydrothermal quartz veins. Many deposits in the province occur in siliciclastic metasedimentary, or metabasaltic roof rocks above parental granites; mainly in its western part, the zone of mineralisation retracts into the granite roof. Typically in the first case, antiformal sites acted as fluid escape zones, with carbonaceous or metabasaltic rocks as chemical traps for tungsten and gold. Examples of pegmatitic and magmatic-hydrothermal deposits are presented in some detail in order to illustrate characteristics and genetic controls, and to support the metallogenic hypothesis here advanced. Impeding strategic exploration, published elements of understanding the evolution and mineralisation of the Kibara belt are contradictory and essential links are missing, foremost an understanding of the 1 Ga “flare up” of fertile granites. Towards solving this conundrum we suggest that the key is delamination of the mantle lithosphere and dense mafic lower crust, residual after extraction of voluminous 1.38 Ga granitic melts. During pan-Rodinian orogenic events, the Tanganyika spur of the Tanzania craton acted as an indentor whose impact caused foundering of the early Kibaran lithosphere. Consequent influx of asthenospheric heat triggered large-area crustal melting that resulted in the tin granites. The stress state was largely compressive but possibly punctuated by short or local extensional events. The correlation of geological evolution and mineralisation substantiates the formal recognition of a Kibara Metallogenic Domain, that is composed of two units: (1) The Mesozoic (1.4 Ga) Kabanga-Musongati nickel (±copper, cobalt, platinum) province; and (2) the early Neoproterozoic (1Ga) Kibara rare metal and gold province that is the main subject of this paper. The present understanding of the operating metallogenic systems remains limited. Regarding the application of modern concepts and technologies, this province is drastically underexplored.
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