Beschreibung: Magmatic sulphides are a widespread component in mafic and ultramafic rocks and contain variable concentrations of nickel, copper and platinum-group elements. Previous literature has been concerned with the whole-rock geochemistry of magmatic sulphide ores and their host-rocks and relatively little attention has been paid to the physical nature of magmatic sulphide transport and accumulation. Our high-resolution X-ray computed tomography study quantifies for the first time the 2D and 3D size, shape and textural relationships, and distribution of disseminated magmatic sulphides and olivine in adcumulates from komatiites. These new data are combined with analysis of trace-element concentrations within sulphides to provide important information about the mechanisms of transport, deposition and post-accumulation migration of sulphide liquid in dynamic magmatic systems. Olivine shows evidence of textural maturation, with larger crystals growing at the expense of small ones to different degrees depending on the sulphide content of the rock. The olivine texture and the presence of poikilitic chromite provide evidence of in situ nucleation of olivine and chromite at the interface between a flowing magma and a basal pile of crystals. Disseminated to strongly interconnected base-metal sulphides are located at contacts between olivine crystals or in some cases can be entirely or partially enclosed within chromite. Based on their 3D morphologies, their size distribution and their Pd concentrations, the sulphides are divided into four main categories: finely disseminated sulphides; disseminated to slightly interconnected sulphides; disseminated to globular sulphides; disseminated to strongly interconnected sulphides. All samples contain a population of sub-spherical sulphide blebs (〈1000 µm equivalent sphere diameter; ESD), which are observed in the olivine–sulphide cotectic proportion and which contain the lowest Pd concentrations. These small droplets are interpreted to have formed by segregation of immiscible sulphide liquid upon cooling of a komatiitic magma flowing in a magma conduit or channel. These newly formed droplets were trapped in situ by the crystallizing framework of olivine and/or chromite. Larger sulphide blebs (up to 10 mm ESD) are present where the sulphide abundance is 〉3 wt % and the sulphide bleb size population is multi-modal. The Pd content of the sulphide blebs is variable and positively correlated with the sulphide bleb size. The overall sulphide abundance, sulphide bleb size and Pd concentrations indicate that these sulphides have been transported in a flowing sulphur-saturated magma over some distance and accumulated at their present site by mechanical processes. Strongly interconnected network to matrix sulphides are observed in samples containing more than 5 wt % sulphide with small variability in Pd concentrations within and between blebs. These sulphides are interpreted to reflect the accumulation and coalescence (by film drainage) of small sulphide blebs. Overall our results show that komatiite-hosted disseminated sulphides form by a mechanical accumulation process that takes place against a background of steady-state in situ nucleation of small blebs along the olivine–sulphide liquid cotectic.

Beschreibung: Copper-rich massive sulfides are an important source of Pt and Pd in magmatic Ni-Cu-platinum group element ore deposits. At the McCreedy East deposit, Sudbury, they constitute a classic magmatic assemblage of chalcopyrite, cubanite ± pentlandite, located in sharp-walled footwall veins. These Cu-rich ores represent the subsolidus (〈600°C) exsolution products of intermediate solid solution (ISS) that crystallized (950°–800°C) from a highly fractionated sulfide liquid rich in Pt, Pd, Ag, As, Bi, Cd, Pb, Se, Sn, Te, and Zn. Laser ablation-inductively coupled plasma-mass spectrometry and scanning electron microscope analyses have revealed the distribution of platinum group elements (PGE) and trace elements in these ores, which is important for a better understanding of the petrogenesis of Cu-rich sulfide deposits and to improve PGE extraction. Chalcopyrite and cubanite are the dominant hosts of Se and Sn, with Co in pentlandite. Lead is hosted by galena and Zn and Cd by sphalerite, with only a small proportion of these elements present at trace level in ISS (now equally distributed between chalcopyrite and cubanite). Platinum, Pd, Au, As, Bi, and Te, however, are not concentrated in the base metal sulfides and are accounted for almost entirely by the platinum group mineral assemblage, which is dominated by Pt-Pd-Bi-Te phases, such as michenerite [(Pt,Pd)BiTe] and froodite [PdBi 2 ], with minor sperrylite [PtAs 2 ] and Sn-bearing platinum group minerals (PGM), such as niggliite [PtSn], paolovite [Pd 2 Sn], and an unnamed Pt-Sn-Te phase. The PGM form complex, composite grains hosted at the grain boundaries of the base metal sulfides. They typically comprise a core of Sn- or As-bearing PGM (stable at higher temperatures) hosted in Bi-Te-PGM (lower temperatures), which are commonly surrounded by accessory tellurides (Ag, Bi, Pb bearing) and sulfides (galena, sphalerite, and stannite [Cu 2 FeSnS 4 ]). Primary chloride minerals such as cottunite [PbCl 2 ] and ferropyrosmalite [(Fe,Mn) 8 Si 6 O 15 (OH,Cl) 10 ] also form composite grains with hessite [Ag 2 Te] and galena. In contrast to much of the previous work at Sudbury, which has invoked the role of late-magmatic and/or hydrothermal fluids in the collection of precious metals, we show that, in this case, PGM in Cu-rich ore have a magmatic origin. Due to the incompatibility of Pt, Pd, Bi, and Te during the crystallization of ISS, these elements became concentrated in a small volume of late-stage S-bearing melt trapped between intermediate solid solutions. A sequence of PGM, followed by accessory tellurides and sulfides, crystallized from this late-stage melt and formed composite grains. Toward the end of crystallization, the small amount of Cl that was soluble in the sulfide liquid crystallized as primary chloride minerals at low temperatures (〈500°C), either from the late-stage melt or from an exsolved Cl-rich late-magmatic fluid. Some of the primary PGM have been partially altered by Cl-rich fluids (late magmatic and/or hydrothermal) that leached Bi in preference to Pd to form an unnamed Pd-Bi-O-Cl phase. Many PGM also show scalloped edges and truncated grain boundaries, indicating partial corrosion and dissolution, most likely by late-magmatic/hydrothermal fluids that probably remobilized and deposited PGE in the footwall surrounding the veins.

Beschreibung: More than 390 chromite grains from komatiites and komatiitic basalts from the Yilgarn craton of Western Australia and the Finnish part of the Fennoscandian Shield were analyzed using in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to identify ruthenium (Ru) signatures in chromite associated with nickel sulfide-bearing rocks. Results indicate a potential method to discriminate mineralized and barren komatiite and komatiitic basalt units based on Ru concentrations in chromite and indicate potential for chromite to be used as a resistate indicator mineral in exploration for komatiite-associated nickel sulfide deposits. Chromites from barren komatiites and komatiitic basalts display Ru concentrations mostly between ~150 and 600 ppb. Chromites from mineralized units have distinctly lower Ru contents (〈150 ppb). These results can be interpreted in terms of the much higher partition coefficient for Ru into sulfide liquid compared to that of Ru into chromite, resulting in much lower Ru concentrations in chromite where both chromite and sulfide liquid are present and competing for Ru. As a result, the Ru content of chromite can be used to determine if a komatiite melt equilibrated with a sulfide liquid during crystallization, and therefore, if a system and/or sequence is prospective for nickel sulfide mineralization. The strength of this method compared to previous whole-rock exploration techniques derives from combining (1) the geochemical properties of a chalcophile element that records an ore-forming process while being strongly immobile during postmagmatic processes, with (2) the in situ analysis of a mineral that is generally preserved even in highly altered and mildly weathered komatiites and that is a common constituent of detrital heavy mineral samples. Chromite Ru content has potential as a prospectivity indicator, applicable to a wide range of media including bedrock, laterites, and detrital resistates heavy mineral samples.

Beschreibung: Sulfide droplets from fresh Mid-Ocean-Ridge Basalt (MORB) glasses show different textures. Some are fine-grained droplets consist of Monosulfide Solid Solution (Mss) and Intermediate Solid Solution (Iss) micrometric intergrowths with pentlandite at the Mss-Iss interface and disseminated Fe-oxide grains; other droplets display a characteristic "zoned" texture consisting of segregated massive grains of Mss and Iss, with euhedral Fe-oxides and pentlandite occuring as equant grains and as flame-shaped domains in the Mss formed by exsolutions. The difference in the textures implies a difference in the crystallization history of the sulfide droplets. These different textures are observed in droplets that are only millimeters apart in the same sample, and thus had an identical cooling history. Therefore, some other factors controlled the textural development. There is relationship between the size and the texture of the droplets. The larger sulfide droplets tend to have zoned textures and the smaller ones fine-grained textures. We propose that the latter have experienced greater undercooling before crystallization. The reason for the delay in crystallization could be that, in the small sulfide droplets, large stable grains with low surface to volume ratio cannot form, which results in higher effective solubility of the Mss. Due to the high degree of undercooling in the small droplets, there were numerous nucleation sites and the diffusion rates of the crystal components in the liquid were lower, leading to fine-grained Mss-Iss intergrowths. In contrast, larger droplets with lower effective solubility of Mss began to crystallize at higher temperature, and thus had fewer nucleation sites, higher diffusion rates, and more time for sulfide differentiation.

Beschreibung: The generation and evolution of Earth’s continental crust has played a fundamental role in the development of the planet. Its formation modified the composition of the mantle, contributed to the establishment of the atmosphere, and led to the creation of ecological niches important for early life. Here we show that...

Beschreibung: The Monts de Cristal Complex of Gabon consists of several igneous bodies interpreted to be remnants of a tectonically dismembered, 〉100 km long and 1–3 km wide, ultramafic–mafic intrusion emplaced at 2765–2775 Ma. It is the most significant mafic–ultramafic layered complex yet identified on the Congo Craton. The complex consists largely of orthopyroxenite cumulates, with less abundant olivine-orthopyroxenite and norite, and rare harzburgite and dunite. Mineral compositions (Fo ol 84, Mg# Opx 85, An plag 60–68, Cr/Fe chromite 1–1·45) and whole-rock data suggest that the parent magma was a low-Ti basalt containing approximately 10% MgO and 0·5% TiO 2 . Trace element and Rb–Sr and Sm–Nd isotope data indicate the presence of an enriched component, possibly derived from crustal contamination of a magma generated in the sub-lithospheric mantle. Most rocks show a highly unusual pattern of strong Pt enrichment (10–150 ppb) at low concentrations of Pd (1–15 ppb), Au (1–2 ppb), Cu (1–20 ppm), and S (〈500 ppm), suggesting that unlike in most other PGE-rich intrusions globally, platinum in the Monts de Cristal Complex is not hosted in magmatic sulfides. Synchrotron X-ray fluorescence mapping has revealed the location of buried small Pt particles, most of which are associated with As. We propose that this constitutes some of the strongest evidence yet in support of magmatic crystallization of a Pt–As phase from S-undersaturated magma.