Abstract Trace-element zoning in igneous phenocrysts and cumulus phases is an informative record of magmatic evolution. The advent of microbeam X-ray fluorescence (XRF) mapping has allowed rapid chemical imaging of samples at thin section to decimeter scale, revealing such zoning patterns. Mapping with synchrotron radiation using multidetector arrays has proved especially effective, allowing entire thin sections to be imaged at micrometer-scale resolution in a matter of hours. The resolution of subtle minor element zoning, particularly in first-row transition metals, is greatly enhanced in synchrotron X-ray fluorescence microscopy (XFM) images by scanning with input beam energy below the FeKα line. In the examples shown here, from a phenocryst rich trachybasalt from Mt Etna (Italy) and from a Ni-Cu-PGE ore-bearing intrusion at Norilsk (Siberia), the zoning patterns revealed in this way record aspects of the crystallization history that are not readily evident from XFM images collected using higher incident energies and that cannot be obtained at comparable spatial resolutions by any other methods within reasonable scan times. This approach has considerable potential as a geochemical tool for investigating magmatic processes and is also likely to be applicable in a wide variety of other fields.
Small intrusions dominated by olivine- and pyroxene-rich cumulates are well known to be favourable hosts to magmatic Ni-Cu-(Platinum Group Element - PGE) sulfide mineralization. Such intrusions are common in a variety of settings around the world, but only a very small proportion contain economically exploitable sulfides; these tend to be of conduit or chonolith style. If prospectivity could be discriminated from sparse sampling at early exploration stages, then the discovery rate for deposits of this type could be improved. To this end, a number of pyroxene-bearing samples from small intrusions containing magmatic sulphide deposits have been investigated including the Noril'sk-Talnakh camp in Siberia, the Kotalahti nickel belt in Finland, Ntaka Hill in Tanzania, Nova-Bollinger in the Albany-Fraser Orogen of Australia, Savannah in the Halls Creek Orogen of Australia, Jinchuan in central China, Xiarihamu in Tibet and Huangshanxi in the east Tianshan Ni province of NW China. To compare, samples from unmineralised intrusions in four of these regions were also investigated along with four mafic intrusions from other localities that are not associated with any known economic sulfide mineralisation. Using fine-scale (<5 μm/pixel) chemical imaging on the Australian Synchrotron, complex zoning in chromium was found in cumulate and poikilitic pyroxenes within the strongly mineralised intrusions. The zoning patterns can be separated into three distinct types: 1) abrupt zoning: a single change in trace element concentration with a sharp boundary; 2) sector zoning: hourglass style zonation; and 3) oscillatory zoning: small scale oscillations that are usually cyclic. Zoning of all three types can be present in a single grain. The presence of cumulus orthopyroxene with a combination of abrupt zoning, sector zoning and resorbed olivine inclusions has so far only been detected in mineralised intrusions. This combination of zoning patterns is postulated to be an indication of high magma flux and fluctuating cooling rates that accompany wall rock assimilation in dynamic conduits where sulphide liquid forms and accumulates. The distinctive zoning patterns reported here can, in many cases, be easily imaged using desktop microbeam XRF mapping techniques and may provide a useful fertility indicator for the exploration of new magmatic Ni-Cu-(PGE) deposits.
Abstract The Norilsk-Talnakh orebodies in Siberia are some of the largest examples on Earth of magmatic Ni–Cu–platinum group element (PGE) deposits, formed by segregation of immiscible sulfide melts from silicate magmas. They show distinctive features attributable to degassing of a magmatic vapor phase during ore formation, including: vesiculation of the host intrusions, widespread intrusion breccias, and extensive hydrofracturing, skarns, and metasomatic replacement in the country rocks. Much of the magmatic sulfide was generated by assimilation of anhydrite and carbonaceous material, leading to injection of a suspension of fine sulfide droplets attached to gas bubbles into propagating tube-like host sills (“chonoliths”). Catastrophic vapor phase exsolution associated with a drop in magma overpressure at the transition from vertical to horizontal magma flow enabled explosive propagation of chonoliths, rapid “harvesting” and gravity deposition of the characteristic coarse sulfide globules that form much of the ore, and extensive magmatic fluid interaction with country rocks.
ABSTRACT The spatial association between Pt minerals, magmatic sulfides, and chromite has been investigated using microbeam X-ray fluorescence (XRF) element mapping and the Maia Mapper. This lab-based instrument combines the Maia parallel energy dispersive (ESD) detector array technology with a focused X-ray beam generated from a liquid metal source. It proves to be a powerful technique for imaging Pt distribution at low-ppm levels on minimally prepared cut rock surfaces over areas of tens to hundreds of square centimeters, an ideal scale for investigating these relationships. Images of a selection of samples from the Bushveld Complex and from the Norilsk-Talnakh ore deposits (Siberia) show strikingly close association of Pt hotspots, equated with the presence of Pt-rich mineral grains, with magmatic sulfide blebs in all cases, except for a taxitic low-S ore sample from Norilsk. In all of the Bushveld samples, at least 75% of Pt hotspots (by number) occur at or within a few hundred microns of the outer edges of sulfide blebs. In samples from the leader seams of the UG2 chromitite, sulfides and platinum hotspots are also very closely associated with the chromite seams and are almost completely absent from the intervening pyroxenite. In the Merensky Reef, the area ratio of Pt hotspots to sulfides is markedly higher in the chromite stringers than in the silicate-dominated lithologies over a few centimeters either side. We take these observations as confirmation that sulfide liquid is indeed the prime collector for Pt and, by inference, for the other platinum group elements (PGEs) in all these settings. We further propose a mechanism for the sulfide-PGE-chromite association in terms of in situ heterogeneous nucleation of all these phases coupled with transient sulfide saturation during chromite growth and subsequent sulfide loss by partial re-dissolution. In the case of the amygdular Norilsk taxite, the textural relationship and high PGE/S ratio is explained by extensive loss of S to an escaping aqueous vapor phase.
Low-sulfide ores in the upper marginal series of the Norilsk intrusions are distinct from the main massive and disseminated sulfide orebodies in a high platinum-group element (PGE) tenor up to 1,300–1,700 ppm PGE + Au in 100 % sulfide, lower Cu/Pd of ∼100–300 and increased Pt/Pd of ∼0.2–2. These features have previously been considered either as a result of collection by sulfide liquid at extremely high R-factor (silicate to sulfide melt mass ratio) or postmagmatic sulfide removal, although the decoupling between PGE grade and PGE tenor may suggest an additional process of PGE concentration in addition to partitioning into immiscible sulfide liquid. In this study, the distribution of diverse platinum-group minerals (PGM) and their spatial associations with sulfide-silicate assemblages were investigated by automated mineralogy techniques to recognize the mechanism of the PGE concentration. Mineral assemblages were studied from 2D surfaces of polished sections and mineral separate blocks using the Mineral Liberation Analysis and TESCAN Integrated Mineral Analysis in combination with X-ray microtomography and lab- and synchrotron-based microbeam X-ray fluorescence mapping collected over slabs and thick sections, representative of the 3D sub-surficial layer. Palladium minerals (mostly stibnides, bismuthides, tellurides and arsenides) prevail over Pt minerals in all three intrusions (Norilsk 1, Talnakh and Kharaelakh) based on ∼6,600 identified PGM grain statistics from both thin sections and heavy mineral separates. Among Pt minerals, sperrylite and Pt-Fe alloys occur in an equal proportion in Norilsk 1 whereas the alloys are exceptionally rare in Talnakh-Kharaelakh low-sulfide ores. The PGM grains are predominantly locked within secondary silicates with a percentage of their mutual boundaries of ∼50 rel. % in consistency with a high abundance (35–90 area %) of secondary silicates in mineralized rocks. No correlation between the proportions of secondary silicates and PGMs as well as with the degree of their spatial affinity was found. The preferred confinement of PGM grains to the disseminated chromite schlieren is observed in the studied samples, although, among PGMs, only Pt sulfides demonstrate significant spatial affinity (15–34 rel. % mutual boundary) intergrowing with chromite. The spatial association between PGM and base-metal sulfides (∼20 rel. %) is notably higher than the proportion of sulfides in rocks (1.8–2.8 area %) that is interpreted as a quantitative proof of the cogenetic link between PGM and sulfides. However, the significant mm-scale detachment of the PGM grains from the sulfide blebs is persistent throughout the mineralized sections. Element mapping techniques reveal widespread textures of early magmatic degassing (such as amygdules, segregation vesicles, fluid caps and halos), the common tight intergrowths of PGM with volatile-rich minerals and the presence of PGM grains nucleating on the former gas bubble walls. These features are all distinct from the textures of postmagmatic hydrothermal-metasomatic alteration. We propose that an initial composition of sulfide liquid was modified due to S partitioning into a fluid phase during early magmatic degassing. Therefore, the high PGE tenor is not solely a result of sulfide-silicate exchange but was likely enhanced by early magmatic S loss during degassing and PGE fluid transport.
Abstract The stratiform and vein-hosted Kapunda Cu deposit in South Australia contains a saprolitized hydrothermal vein with 12.37 wt.% total rare earth oxide (TREO). The vein was analyzed by X-ray diffraction, scanning electron microscopy, synchrotron-based X-ray fluorescence microscopy and electron backscatter diffraction to understand the controls that govern high-grade REE accumulation during periods of intense weathering. Petrological assessments indicate the transformation of an apatite-calcite-aluminosilicate-bearing protolith to a supergene assemblage of Fe-oxides, kaolinite and REE-phosphate minerals that include rhabdophane-(Ce), monazite-(Ce) and florencite-(Ce). This transformation was facilitated by progressive acidification of the weathering fluid, which is indicated by: 1) the increasing crystallinity of authigenic Fe-oxides and kaolinite, which led to REE desorption; 2) the textural evolution and increase in grain size of authigenic REE-phosphates from nanoscopic crystallites, to acicular needles, to micro-scale hexagonal prisms; 3) the late dissolution of REE-phosphates; and 4) the replacement of goethite by jarosite, whose sulfate component originated from the oxidation and weathering of proximal sulfide minerals. Alongside the depletion of pH-buffering carbonate minerals that are indicated by the preservation of calcite menisci, this sulfide dissolution also facilitated acid generation. Results illustrate how highly acidic weathering fluids might facilitate either REE mobilization or REE accumulation in regolith. High-grade REE accumulation under acidic supergene conditions is prioritized when the host-rock contains a significant source of depositional ligands (i.e., phosphate in the form of apatite) that can be readily leached during intense weathering. Exploration companies should therefore assay routinely for REEs in any heavily weathered phosphatic rock, due to the observed efficiency of phosphate minerals as geochemical traps for REE accumulation.
SummaryWhilst detailed geochemical analyses have been conducted to mitigate the complications regolith cover poses to modern exploration efforts, less attention has been directed to the mineralogical associations that exist between lateritic residuum and potentially underlying intrusive-hosted Li-deposits. The characterisation of mineral hosts for Li in the lateritic duricrust would assist exploration efforts within poorly outcropping regions of Australia, by providing early indication and characterisation of potentially concealed deposits. Pisolitic laterite and lateritic duricrust from Greenbushes, Western Australia, was analysed as an example, using a suite of laband field mineralogical analytics, to understand the regolith mineralogical signature of buried Li ore.The major and minor mineral phases, including the Li host minerals, in the regolith and underlying pegmatites are identified and compared. This provides insight into the distribution and mobility of Li in this setting, and the evolution of the Greenbushes ore deposit.
The deeply weathered Yamarna Terrane is the easternmost, least explored Terrane of the Yilgarn Craton in Western Australia. Investigation of the landscape evolution, mineralogy and geochemistry of the transported cover showed that the near surface ferruginous regolith can provide a significant tool for exploration under cover. Three types of ferruginous materials were identified and examined in three gold prospects: Smokebush, Toppin Hill and Santana. The ferruginous materials include: 1) lateritic residuum and its detrital ferruginous clasts of underlying or nearby bedrock; 2) authigenic pisoliths and nodules of Permian glacial sediments (PPS); and 3) authigenic pisoliths of aeolian sand (PAS) overlying the other two. Our findings show that As is the main pathfinder element for Au mineralization. Arsenic and Au are elevated and form near surface anomalies in lateritic residuum, detrital ferruginous clasts, PAS, <75 μm and <2 μm soil size fractions at Smokebush. At Toppin Hill, Au migrated directly from the mineralization through the Permian cover and became enriched at the surface in the PPS above mineralization. Laser ablation ICP-MS mapping shows Au in the PPS as nano-grains, microcrystalline aggregates and veinlet fillings. This is supported by partial extraction analyses, where Au was extracted mainly by K-iodide and K-cyanide, indicating that it is dispersed in particulate and soluble forms. The absence of an As anomaly in transported cover over the Toppin Hill prospect compared to Smokebush is related to the absence of arsenopyrite in the Toppin Hill Au mineralization. Arsenic, which was extracted mainly by 0.1 M tetra‑sodium pyrophosphates, is associated with organic compounds and has a different fractionation pathway from Au near surface. This is also shown by an As anomaly in the Eucalyptus foliage over the Smokebush Au mineralization. At the Santana prospect, there are no Au and/or As anomalies in the PPS over the mineralization. The PPS at Santana are reworked from barren Permian cover and could be deposited with ferruginous clasts from different sources, as shown by textural and mineralogical variations between the grains and matrix. Furthermore, there is no evidence of hydromorphic dispersion of Au and As from the underlying mineralization. Thus, the PPS at Santana cannot be used to vector towards the mineralization. However, sampling at the unconformity (interface sampling) is more effective in locating the mineralization at Santana. This study shows that ferruginous pisoliths and clasts in the cover are potential sampling media for Au exploration in the Yamarna Terrane. The exploration methods used in this study can be also applied in similar covered terrains not only in Australia but also in areas with complex weathering histories in the tropics and sub-tropics.
Abstract A large proportion of the disseminated sulfide ores of the Norilsk-Talnakh camp are hosted within olivine-rich, ultramafic cumulate layers called picro-gabbrodolerite units. In this study we quantitatively analyze the chemistry and textures of the silicate and oxide minerals within olivine-bearing cumulates of the Kharaelakh, Norilsk 1, and Talnakh intrusions to determine how these intrusions compare to each other and to establish the liquidus phase assemblage and crystallization sequence and how the liquid component evolved during solidification. Crystal size distributions indicate that much of the olivine and clinopyroxene oikocrysts grew together in situ as the first of the cumulus phases at contrasting growth rates. These large clinopyroxene oikocrysts record a significant drop in Cr in the system by a significant decrease in Cr content of the outer rims compared to the cores. The chadacrysts of olivine and spinel within the clinopyroxene record the chemistry of the first stages of crystallization, while the minerals in the framework of the cumulate show a relative reduction in Cr and enrichment in incompatible elements such as Ti, Zn, Y, and the rare earth elements, indicative of the enrichment through reactions with the trapped liquid during postcumulate growth. Due to the entrapment of the olivine and spinel in rapidly growing clinopyroxene, these minerals record a history of the changing chemistry during cumulate and postcumulate growth, giving us an insight into the changing conditions during the solidification of intrusions.
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