Re: Genetic Models and Exploration Metalguy Great stuff!
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HIGH-GRADE NI-CU-PT-PD-ZN-CR-AU-V-TI DISCOVERIES IN THE "RING OF FIRE"
NI 43-101 Update (September 2012): 11.1 Mt @ 1.68% Ni, 0.87% Cu, 0.89 gpt Pt and 3.09 gpt Pd and 0.18 gpt Au (Proven & Probable Reserves) / 8.9 Mt @ 1.10% Ni, 1.14% Cu, 1.16 gpt Pt and 3.49 gpt Pd and 0.30 gpt Au (Inferred Resource)
Message: Genetic Models and Exploration for Nickel Ores
| Genetic Models & Exploration for Nickel Ores | |||||||||
There are two principal types of nickel deposit which are of economic interest: magmatic sulphide deposits associated with mafic and ultramafic rocks, and lateritic deposits formed over olivine-rich protoliths as a result of tropical weathering. Further subdivision of the magmatic sulphide deposits can be made on the basis of the host-magma: extrusive and ultramafic versus intrusive and mafic. The former deposits occur in ancient komatiitic lava flow-fields. The latter are associated with layered mafic-ultramafic intrusions. The lateritic deposits can also be subdivided according to whether the nickel is contained predominantly within silicate phases in the saprolite zone ("garnieritic" deposits) or within oxides phases higher in the weathering profile ("limonitic" deposits). Clearly the criteria for area selection and technologies employed for exploration will vary according to the type of nickel deposit being sought. Research within CSIRO-EM over the last fifteen years has focused on genesis of magmatic sulfide ores, particularly those hosted by komatiites. The following discussion is a brief review of the current state of understanding of the origins of both major classes of magmatic sulphide ores.
Genesis of Magmatic Nickel-Sulphide Deposits The genesis of magmatic nickel-sulphide deposits depends on the formation of immiscible magmatic sulphide liquids (the natural analogue of blast furnace mattes) in magmatic plumbing systems, where large volumes of magnesium- and iron-rich magma penetrate the Earth's crust or erupt over it (Figures 1 and 2). The process of ore formation is analogous to that of separating matte from slag, and involves two processes: addition of sulphur to the system to generate the "matte", than physical segregation and accumulation of the metal-bearing sulphide liquid. The essential process is the same in both komatiit-hosted and gabbroic intrusion-hosted deposit types. Massive magmatic nickel-sulphide deposits occur where the following two essential elements are present.
Disseminated ores can form purely as a result of fractional crystallisation. As minerals crystallise from a mafic/ultramafic magma, the composition of the remaining magma can change is such a way that it becomes sulphur-saturated (can no longer hold all its contained sulphur in solution). Mixing of two sulphur-undersaturated magmas can also have similar results and may also produce disseminated sulphide mineralisation. The layered mafic-ultramafic intrusions which host nickel-copper sulphide ores crystallise from mafic magmas and commonly exhibit a complex history of crystallisation, with periods of quiescence and fractional crystallisation in the magma chamber being interrupted by the injection of fresh pulses of magma, followed by mixing and convective overturn. Ore-bearing intrusions can be found at any crustal level, provided the two requirements listed above are met (Figure 1). Ore bearing intrusions can be found in deeply eroded terrains, where mid-crustal magma chambers were intruded into metamorphosed sedimentary rocks containing sulphide-rich bands. Subsequent erosion reveals remnants of these now-solidified magma bodies at the surface. Voisey's Bay is an example of this (Naldrett 1997; Ryan 2000). Favourable conditions can also be found at higher crustal levels, where relatively little erosion has occurred since formation of the orebodies. The best known example of this is the Talnakh-Noril'sk province, where rich orebodies are hosted by small subvolcanic sills which fed a thick overlying sequence of flood-basalt lava flows (Naldrett, 1997). The Sudbury deposits are a unique and enigmatic exception to these general principals, in that they formed from relatively Mg- and Fe-poor magmas generated by wholesale melting of the crust during a large meteorite impact. The ores formed by segregation of an immiscible sulphide melt, but the source of the sulphur and mechanism of segregation remain incompletely understood.
Figure 1. Schematic diagram illustrating the genesis of sulphide ores in mafic/ultramafic intrusions. (click to view larger image) The other principal type of sulphidic nickel deposit occurs within komatiite sequences. Komatiite volcanism was largely restricted to the Archaean (>2.5billion years ago). These lavas were highly magnesian (>18wt % MgO) and were erupted at extremely high temperatures (in excess of 1500oC as compared with the typical 1100-1200oC eruption temperatures of basalts), which allowed them to thermo-mechanically erode their substrate during eruption (thereby picking up sulphur, where the substrate was sulphidic). As they cooled, they crystallised large quantities of olivine. The textural variation of olivine in komatiite lavas gives important clues as to the local cooling rate at the time of crystallisation, which in turn allows geologists to reconstruct the geometry of large komatiite eruptions. Nickel deposits such as those at Kambalda, Mt Keith, Thompson and Raglan are formed in long-lived lava pathways within large compound komatiite flows, and these pathways are typified by olivine-rich cumulate rocks.
Figure 2. Schematic diagram illustrating the genesis of sulphide ores in komatiite lava-flows. (click to view larger image) More on komatiites -including lava flow emplacement mechanisms
Exploration for Magmatic Nickel-Sulphide DepositsThis involves application of a variety of strategies. Historically, most of the major known nickel sulphide camps have been discovered during regional prospecting, as a result of sampling of outcropping gossans (weathered ores). This is true of most of the major nickel camps: Sudbury (discovered during excavation of a railway cutting), Noril'sk/Talnakh, Kambalda, Perseverance, Thompson, Raglan and, most recently, Voisey's Bay in Labrador. Current exploration techniques are aimed at discovering new deposits under cover of extensive weathering or transported sediments, both in recognised mining camps and in greenfields terrains. A variety of geological, geophysical and geochemical methods are in common use, the common purpose being to detect physical or chemical haloes which are much larger then than the orebodies themselves. Geological understanding of genetic mechanisms is critical from regional target selection through to detailed resource evaluation. Geologists apply understanding of ore forming processes and subsequent structural modification to predict prospectivity, geometry of orebodies and other properties. Geophysical techniques rely on detecting physical properties of ores and host rocks. Aeromagnetic techniques are used to locate potential host rocks, which in many but not all cases are strongly magnetic, but magnetic signatures only rarely detect orebodies directly. Electromagnetic prospecting techniques rely on detecting large interconnected bodies of electrically conductive Fe-Ni sulphides, but are hampered by the proximity of other conductors such as barren sulphide-bearing sediments or saline groundwaters. This is a rapidly advancing area of technological development, particularly in the area of measuring electromagnetic responses using instruments located down bore holes, thus avoiding problems of conductive overburden and increasing the depth of penetration. Another geophysical prospecting technique based on electrical properties of sulphides, Induced Polarisation (IP), has been widely applied in the past but with relatively limited success. Geochemical techniques involve recognising patters of dispersion of nickel and pathfinder elements, such as platinum and iridium, during weathering of ores. Common methods typically involve analysing low-level concentrations of the elements of interest in a variety of surficial sample media, such as soils, stream sediments, glacial till and lateritic lags, but these strategies are only effective where target orebodies intersect the weathering profile. Lithogeochemical methods, involving chemical analysis of fresh unweathered rock, rely on detecting primary magmatic signatures of ore formation in potential host rocks. They have some potential in regional target selection, limited by the highly dynamic magmatic environment in which these ores form, but have the potential to find blind (i.e. non-outcropping) deposits. Great advances have been made in geochemical prospecting techniques for gold in the last two decades, through greater understanding of sample media and improved sensitivity of analytical techniques. These procedures have yet to be refined specifically for nickel prospecting. Text by Caroline Perring and Steve Barnes, July 2002; diagrams by Steve Barnes and Rob Hill. ReferencesNaldrett, A. J. (1997). Key Factors In The Genesis Of Norilsk, Sudbury, Jinchuan, Voiseys Bay And Other World-Class Ni-Cu-PGE Deposits - Implications For Exploration. Australian Journal of Earth Sciences 44(3): 283-315. Reid, J.G. (1996). Laterite ores - nickel and cobalt resources for the future. In: "Nickel '96 Mineral to Market", Kalgoorlie, November 1996, Aus I.M.M. Publication Series 6/96: 11-16. Ryan, B. (2000). The Nain-Churchill boundary and the Nain plutonic suite: A regional perspective on the geologic setting of the Voisey's Bay Ni-Cu-Co deposit. Economic Geology and the Bulletin of the Society of Economic Geologists 95: 703- 724. Publications of CSIRO Nickel Group to 2002.
Read more about Nickel [Nickel Commodity Overview][Nickel Market][Nickel Production][Australian Nickel Mines and Discoveries] [Genetic models and exploration methods for nickel sulfide deposits][Komatiites and Associated Ni-Cu-PGM Deposits][Nickel Industry Organisations]
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