The Rustenburg Layered Suite of the Bushveld Complex, South Africa, contains the world’s largest resource of chrome and platinum group elements (PGEs). Both Cr and PGEs are found in chromitite layers within an approximately 1,000-m-thick section of ultramafic to mafic rocks known as the Critical zone. Neither the process of how the chromitite layers form nor the role that chromite plays in collecting the PGEs is clear. Major and trace element contents of chromite and silicate minerals from each of the 13 chromitite layers, and from chromite in the adjacent peridotites and norites, have been determined. The concentrations of PGEs in both chromite and silicates are less than detection levels (10–20 ppb). Thus, neither are the host of the PGEs in these rocks. The Cr# and Fe# of the chromites from chromitite layers are similar to those found in experiments carried out to model the crystallization of the initial magma (B1) of the Bushveld, with the same decrease in Cr# with increase in Fe#. The fO2 of the experiments Δ 0 FMQ (where FMQ = fayalite-magnetite-quartz buffer) and those of the chromitite chromite calculated from the Fe3+/FeTotal ratios and the V contents of the chromite are similar. Variations in trace element contents of the chromitite chromite can also be modeled using the B1 composition and allowing for ~40% crystal fractionation across the stratigraphy.

The behavior of platinum-group elements (PGE) in sulfide-undersaturated primitive magmas may be controlled by crystallization of either Cr-spinel, platinum-group minerals (PGM) or olivine. Evaluation of the PGM-hosted PGE portion is challenging because PGM in volcanic rocks are small and rare. We report on the sizes and compositions of PGM associated with Cr-spinel, the PGE content of Cr-spinel and olivine, and the bulk rock PGE content from sulfide undersaturated arc volcanics of the Tumrok range (Eastern Kamchatka). Platinum-iron and Ir-Os alloys (< 30 to > 3000 nm in size) form inclusions in Cr-spinel. Their presence and composition are mostly independent of Cr-spinel chemistry. Bulk rock PGE contents are erratic and PGE concentrations in Cr-spinel are exceptionally variable (up to 2 orders of magnitude). Whereas PGM inclusions in Cr-spinel and PGE in solid solution in Cr-spinel significantly contribute to the bulk rock PGE budget, a considerable portion of the PGE must be present associated with minerals other than Cr-spinel. The variable PGE content of the rocks is attributed to a combination of: (a) disequilibrium kinetic effects at the Cr-spinel-melt boundary layer, strongly affecting PGE partitioning into Cr-spinel and crystallization of PGM in the layer; (b) the presence of some sufficiently large PGM to cause nugget effects; and (c) melt degassing.

The Merensky Reef of the Bushveld Complex represents a magmatic unconformity that some researchers attribute to chamber replenishment by relatively primitive magma. It is propounded that cumulate rocks in this chamber reacted with replenishing melt, as part of the process that ultimately produced chromitite stringers and reef-style platinum-group element mineralization. This study investigates as to whether chamber replenishment contributed to the formation of the Merensky Reef and its underlying anorthosite at the Rustenburg Platinum Mine in the western lobe of the Bushveld Complex. At this location, the Merensky Reef is a coarse-grained pyroxenite bracketed by millimeter-scale chromitite stringers. This sequence is underlain by a centimeter-scale anorthosite which in turn is underlain by leuconorite. The leuconorite comprises normally zoned cumulus orthopyroxene with poikilitic rims (Mg80-79) and cumulus plagioclase (An80-58), where the latter defines a magmatic fabric indicative of gravitational settling of tabular crystals in a quiescent melt. The contact between leuconorite and anorthosite is marked by an increased abundance of late-stage accessory minerals, and the composition of poikilitic orthopyroxene at this horizon is consistent with trapped liquid shift. Plagioclase crystals in the anorthosite are variably zoned (An79-64) and record a magmatic fabric that strengthens with proximity to the reef. This unit is traversed by sinuous networks of sulfides, pyroxenes, quartz, and very fine-grained chromite that terminate at the contact with the leuconorite. The lower chromitite hosts both amoeboidal and blocky chromite crystals that are enclosed by complexly zoned plagioclase oikocrysts in the lower two-thirds and by orthopyroxene oikocrysts in the upper third. The upper chromitite hosts only blocky crystals, similar to those in the upper portion of the lower chromitite. Microtextural characteristics of the amoeboidal crystals coupled with their propensity to host polymineralic inclusions, suggests that these were initially skeletal crystals that subsequently underwent dissolution-reprecipitation. There is no discernible chemical difference between amoeboidal and blocky crystals; however, accessory mineralogy and chromite chemistry imply that the upper portion of the lower chromitite and the upper chromitite experienced post-cumulus re-equilibration with evolved intercumulus silicate melt. Our observations are consistent with the anorthosite being a restite of partially molten leuconoritic cumulates. This theory is supported by thermodynamic modelling that demonstrates that under certain conditions, replenishing melts can reconstitute noritic cumulates to anorthosite, troctolite, or feldspathic orthopyroxenite restites. The porosity generated during this process was exploited by downward percolating sulfide melt that displaced a proportionate amount of intercumulus silicate melt upward to the level of the nascent reef. Initially, these partial melts were likely relatively volatile-rich, triggering Cr-supersaturation at the cumulate-melt interface, and later became Cr-bearing with the consumption of poikilitic orthopyroxene and very fine-grained chromite.

ABSTRACT An idealized stratigraphic section of the five main chromitite-bearing cyclic units of the Upper Critical Zone of the Bushveld Complex typically consists of basal chromitite overlain by pyroxenite, norite, and anorthosite. Such units can be explained by classical fractional crystallization from a Mg-rich basalt followed by injection of new magma. However, in some cases both the underlying and overlying rocks of the chromitite are anorthosite. Furthermore, most of the interstitial material in the chromitites is not pyroxene, which would be the mineral expected to crystallize with chromite, but rather it is oikocrystic plagioclase. Finally, recent age dating suggests that the cyclic units may not have been emplaced in stratigraphic order, with the upper group chromitites being older than the underlying middle group chromitites. In order to reconcile some of these data we compiled field and drill core observations from the Upper Critical Zone. Of particular interest is the MG2–MG3 interval, showing pronounced layering, abundant autoliths of various shapes and compositions, as well as flame structures and boudinage. Many of these features are typical of sedimentary mass flows, suggesting that the MG2–MG3 interval formed via granular flow of intruding chromite-orthopyroxene crystal slurries that deposited chromitite and orthopyroxenite. The heat of the new magma influx caused partial melting of the host leuconorite to produce restitic anorthosite and buoyant noritic melt. The latter infiltrated the slurry, resulting in a disequilibrium assemblage of cumulus chromite and oikocrysts of plagioclase.

The compositions of chromites from volcanic rocks is of interest for two reasons. First, most chromite-rich rocks from intrusions and the mantle are enriched in Os, Ir, Ru (Ir-platinum-group elements, IPGE), and Rh relative to volcanic rocks and in some cases are also enriched in Pd and Pt. There is a small data base showing that chromites from volcanic rocks contain IPGE and Rh, but that chromites from intrusions generally do not contain any of the PGE. Thus, the role that chromite plays in enriching chromite-rich rocks in PGE is not clear. Secondly, the major ore deposits are found within three layered intrusions and the compositions of the magmas which formed the intrusions and conditions under which the deposits formed are a subject of debate. All three intrusions contain massive chromite layers and because chromite is one of the fi rst minerals to crystallize it is possible that chromite compositions could indicate the compositions of the magmas. However, the composition of chromite in an intrusion may change after crystallization due to a number of processes, including re-equilibration during slow cooling or injection of new magma and alteration during metamorphism. In order to evaluate what the composition of chromite can tell us a baseline of the initial chromite composition is required. A comparison of the variations in the compositions of chromites from three komatiite fl ows shows that the chromite compositions reflect the composition of the evolving liquids. Furthermore, the Cr# records higher equilibration temperatures than the Fe#. The 3+ and 4+ ions (Al, Ti, Sc, Ga, V, and Zr) correlate with Cr#, whereas 2+ ions (Mn, Co, Ni, and Zn) correlate with Fe#, indicating that 3+ and 4+ ions are more reliable indicators of initial magma composition. Compared with compositions of volcanic chromites from the literature, the 2+ ions concentrations are not distinctive. In contrast, the 3+ and 4+ ions combined with Cr# distinguish chromites from MORB, boninite, arc picrites, high-Ti picrites, and komatiites. The Alexo komatiite chromites contain IPGE and Rh, but only Ru is controlled by chromite. Another mineral, either olivine or an Ir-Rh platinum-group mineral, controls Ir and Rh. Compared with chromites from volcanic rocks of island arcs, the komatiite chromites and chromites from large igneous provinces are richer in Ru and poorer in Rh. This difference is attributed to the higher f O 2 of arc magmas, a point reinforced by the lower V content and higher Fe3+/FeT ratios of chromites from volcanic rocks of island arcs.

The ~3033 Ma Stella layered intrusion is hosted by supracrustal rocks of the Kraaipan–Madibe greenstone terrane, South Africa. The studied portion of the intrusion consists mainly of magnetite leucogabbro and magnetite anorthosite, as well as several massive magnetite layers. The intrusion hosts a laterally continuous, ~60-m-thick, PGE mineralized interval, with total resources amounting to 108t Pt + Pd + Au, constituting one of the oldest known PGE reef-style mineralizations on Earth. The richest reef, with a grade of 4.4 ppm Pt+Pd over a width of 5–8 m, occurs in semi-massive magnetitite. It is suggested that the mineralized oxide and silicate layers formed through a combination of primary magmatic, late magmatic, and hydrothermal processes, including granular flow and phase sorting of a magnetite- and sulfide-bearing gabbroic crystal mush that crystallized from a tholeiitic basalt, as well as remobilization of S and metals by late magmatic and hydrothermal fluids that led to crystallization of platinum-group minerals.

The Musgrave Province of central Australia was the focus of long-lived mantle upwelling that produced large volumes of magnesian basaltic to tholeiitic magma and their felsic derivatives. The Musgrave Province contains one of the greatest concentrations of mafic-ultramafic layered intrusions globally, grouped as the Giles intrusions. In the present paper, we study the magmatic ore potential of the Kalka, Gosse Pile and Ewarara layered intrusions located in South Australia. Ewarara and Gosse Pile appear to have relatively low potential for platnium-group element (PGE) reefs and magmatic Ni-Cu, based on lack of evident metal enrichment and the absence of a mafic-ultramafic transition zone that hosts most PGE reefs globally. However, mafic-ultramafic pipes within the intrusions that could have higher ore potential have not been studied by us. At Kalka, the mafic-ultramafic transition interval is exposed, rendering this intrusion potentially more prospective for PGE reefs. However, based on the available data, this zone appears to be barren. Instead, there are signs of PGE enrichment and metal ratio variation in the magnetite-bearing upper portion of the intrusion suggestive of undiscovered PGE reefs. This interpretation is consistent with subtle Cu-Pd enrichment of soils adjacent to the upper portion of the intrusion.

Python script.LeapFrog software (SEEQUENT), a visual software commonly used in geological mining industry, was used to produce 3D images of the ablated pit and MI (Fig. 1).Measured major and trace element concentration in the melt inclusion are in good agreement with expected concentration from the literature [3] (Fig. 2).

By examining the minor and trace element contents of chromites from three intrusions—the Bushveld Complex (South Africa), the Stillwater Complex (USA), and the Great Dyke (Zimbabwe)—and comparing these chromite compositions with those of magmas from which they could have formed (komatiites and picrites) we conclude that ( i) the variations in Ti, V, Sc, and Ga contents across stratigraphy and across individual layers do not support the model of magma mixing leading to chromite-only crystallization, ( ii) the chromites from the lowest levels of the intrusions could have crystallized from komatiite liquids that were contaminated with continental crust, ( iii) the Great Dyke chromites have the highest Cr# and lowest incompatible element contents and formed from a liquid closest to komatiite, ( iv) all of the chromites, except those of the Dunite Succession of the Great Dyke have equilibrated with a liquid that also had crystallized pyroxene, ( v) the Great Dyke and Stillwater chromites show a narrower range in composition than the Bushveld chromites, and ( vi) Chromites from the western limb of the Bushveld Complex contain much higher V contents than all the other chromites. This requires either, that the oxygen fugacity ( fO2) was lower in the western Bushveld or that the chromites equilibrated with a V-rich magma. We favor a model where chromite and silicate minerals crystallized in cotectic proportions (∼2:98). The chromite, silicates, and transporting liquid are emplaced into the magma chamber. During emplacement the chromite and silicate separated due to viscous particle flow to form a massive chromite layer overlain by silicates.

The Karelian craton contains abundant-2.45 Ga mafic dykes that are compositionally, temporally and, in some cases, spatially related to PGE-mineralised mafic-ultramafic layered intrusions. The dykes can be sub-divided into four groups, namely siliceous high-magnesian basalts (SHMB), gabbronorites (GBNO), low-Ti tholeiites and Fe -rich tholeiites. In this study, we group the SHMB and GBNO dykes to one group as SHMB group, and the two tholeiitic group dykes as tholeiite group, based on their similar geochemical and mineralogical features. In the SHMB group dykes, plagioclase has initial 87Sr/86Sr ratios of 0.7028 to 0.7036 and initial bulk-rock & epsilon;Nd values vary from-2.5 to-1.0, indicating moderate degrees of contamination with Archaean basement. Tholeiitic dykes show a less-radiogenic Sr isotope composition with an average initial 87Sr/86Sr ratio of 0.7024 and higher initial & epsilon;Nd values ranging from +0.3 to +1.7. Thermodynamic and geochemical modelling suggests that the SHMB group dykes could have formed by crustal contamination of a komatiitic magma at deeper crust followed by fractional crystallisation at shallower depth, whereas the tholeiitic group mainly experienced fractional crys-tallisation with less crustal contamination. Alternatively, the SHMB dykes may have been derived from a SCLM mantle whereas tholeiitic dykes from a plume mantle, or the two types of dykes derived from different part of a mantle plume with different melting degrees, though these latter two models are not favoured in this study. Based on trace element and isotope characteristics, the SHMB dykes are suitable candidates for the parental magmas to some of the Finnish PGE-mineralised intrusions (e.g., Penikat and Portimo), whereas the tholeiitic dykes may represent the parental magma of the Tsipringa layered intrusion in Russia. Both the SHMB and tholeiitic dyke types are fertile with regard to PGE, with up to 10-20 ppb Pt and Pd and mantle-like Cu/Pd ratios in their least evolved members, suggesting that the magmas remained sulphide undersaturated during mantle melting and en route to the upper crust. This interpretation is consistent with the fact that most of the-2.45 Ga Fennoscandian layered intrusions contain PGE mineralisation. Sulphide melt saturation in the dykes and layered intrusions was mostly attained after their final emplacement, likely due to crystal fractionation.

Although in situ analysis by LA‐ICP‐MS is considered a rapid technique with minimal sample preparation and data reduction, mapping areas of millimetres in size using a small beam (< 15 μm) can be time consuming (several hours) when a quadrupole ICP‐MS is used. In addition, fully quantitative imaging using internal standardisation by LA‐ICP‐MS is challenging in samples with more than one mineral phase present due to varying ablation rates. A new protocol for the quantification of multiple coexisting phases, mapped at a rate of about 12 mm 2 h ‐1 and a resolution of 12 μm × 12 μm per pixel, is presented. The protocol allows mapping of most atomic masses, ranging from 23 Na to 238 U, using a time‐of‐flight mass spectrometer (ICP‐ToF‐MS, TOFWERK) connected to a 193 nm excimer laser. A fast‐funnel device was successfully used to increase the aerosol transport speed, reducing the time usually required for mapping by a factor of about ten compared with a quadrupole ICP‐MS. The lower limits of detection for mid and heavy masses are in the range 0.1–10 μg g ‐1 , allowing determination of trace to ultra‐trace elements. The presented protocol is intended to be a routine analytical tool that can provide greater access to the spatial distribution of major and trace elements in geological materials.

Аннотация. На примере палеопротерозойского Мончегорского расслоенного комплекса предлагает

Chromite is among the first minerals to crystallize from mantle derived magmas and one of the last to be consumed during partial melting of the mantle. Chromite is also an important mineral in major ore deposits of Cr and Pt. The composition of chromite could, therefore be of use in interpreting the petrogenetic conditions during partial melting of the mantle, crystallization of primitive magmas and formation of Pt and Cr ore deposits. However, most mafic rocks contain very little chromite and post-crystallization processes such as re-equilibration during cooling, metamorphism and weathering could change the composition. The composition of chromites from high- and low-Ti picrites from the Emeishan large igneous province have been determined to assess the degree to which the chromite compositions reflect the melt compositions. Aluminium, Sc, Ti, Ga, Nb, Sn, Hf and Ta concentrations in the chromites do appear to reflect the melt compositions in that they correlate with the whole rock compositions and have empirical partition coefficients similar to those determined in experiments. The V contents of both types of picrite are similar, but concentrations of chromites from high-Ti picrites are higher than those of low-Ti picrites. This can be explained if, in the high-Ti picrite more of the V was in the V3+ state (which can more readily substitute into chromite than V4+ or V5+) than in the low-Ti picrite. This implies that fO2 was lower for high-Ti picrites than low-Ti picrites. Concentrations of elements with a 2+ charge, Mg, Mn, Fe, Co, Ni and Zn are different in chromites included in olivine and chromites in the matrix and appear to have re-equilbrated. The compositions of the UG-2 chromites from the World's largest Pt deposit show some similarities with the chromites from the high-Ti picrite, but are depleted in Sc and Ti and enriched in Ga and Al.

It is proposed to distinguish two genetic types of primary magmatic contact-style PGE-Cu-Ni mineralization using the example of the Monchegorsk Complex: 1) "gravitational" and 2) "intrusive". The presentation discusses features of the formation of these two types of the mineralization with an emphasis on their potential economic significance.

In magmatic nickel-copper-platinum-group element (PGE) deposits, the PGEs are found both in solid solu-tion in base metal sulfides and as platinum group minerals (PGMs). Apart from S, the most common elements that the PGEs combine with to form PGMs are Te, As, Bi, Sb, and Sn (TABS). Whether the TABS play a role in collecting the PGEs or simply partition into the sulfide liquid along with the PGEs and later combine with PGEs when the sulfide phase becomes saturated with PGMs is not currently clear. This is in part because the concentrations of TABS in the magmas (picrites and basalts from large igneous province and komatiites) that form these types of deposits are not well established, and hence it is not evident whether the magmas contain sufficient TABS to control PGEs. In order to establish the concentrations of Te, As, Bi, Sb, and Se (TABS+) in these rock types and to document the processes affecting these concentrations, we have determined TABS+ concentrations in komatiites, in mid-ocean ridge basalt, and in picrites and basalts from large igneous provinces. Using TABS+ mantle-normalized diagrams, the affects of different mantle sources, crystal fractionation, crustal assimilation, degassing, and alteration are considered. We estimate the concentrations of TABS+ in komatiites to be approximately twice primitive mantle values. In picrites the concentrations vary: approximately 10 times primitive mantle values for As and Sb and decreasing through Bi to Te from 7 to 2 times primitive mantle. Assimilation of S-bearing sedimentary rocks is thought to be important in triggering sulfide saturation lead-ing to the formation of Ni-Cu-PGE deposits. Assimilation of such sediments would enrich the magma in Th over Nb and in As, Sb, and Bi. Evidence of assimilation in the form of TABS and Th enrichment is clear in the PGE reef deposits of the Bushveld and Stillwater Complexes, but the deposits do not contain sufficient TABS to control the PGEs. This is also true in the Norilsk-Talnakh Ni-Cu-PGE deposits. However, at Norilsk degas-sing of the magma has resulted in the loss of TABS, which results in negative As, Bi, Se, and Te anomalies on primitive mantle-normalized plots.

Three models are considered for the origin of normal Meresky reef: a) PGE are collected by a sulphide liquid; b) PGE are collected by clusters of PGE ions; c) PGE are collected by volatiles. No model can explain all the aspects of the reef. A new model involving sulphide collection of the PGE followed by re-equilibration of the sulphides with chromite resulting in PGM crystallization from the sulphide liquid is proposed.

ABSTRACT Most of the World's platinum-group element ore deposits occur as thin stratiform layers within layered intrusions. These layers generally contain disseminated base-metal sulfides or chromite. However, cryptic platinum-group element deposits also occur without chromite or base-metal sulfides in what are known as low-S-high platinum-group element deposits. The origin of these deposits is not clearly understood. The Luanga Complex hosts the largest platinum-group elements resource in South America (i.e., 142 Mt at 1.24 ppm Pt + Pd + Au and 0.11% Ni) and hosts both a platinum-group element deposit containing disseminated base-metal sulfides (style 1) and a low-S-high platinum-group element deposit (style 2). It therefore offers the opportunity to compare the two deposit types in the same overall geological setting and consider how the low-S-high platinum-group element deposit could have formed. The first deposit style is termed the Sulfide zone and consists of a 10–50 meter-thick interval with disseminated base metal sulfides, whereas the second style is named low-S-high-Pt-Pd zone and consists of 2–10 meter-thick discontinuous lenses of 1–5 meter-thick sulfide- and oxide-free harzburgite and orthopyroxenite with discrete platinum-group minerals. Secondary assemblages commonly replace primary igneous minerals to a variable extent throughout the deposit, and thus allow for investigating the effects of post-cumulus alteration on the distribution of a wide range of chalcophile elements in a magmatic sulfide deposit at both whole-rock and mineral scale. This study presents the whole-rock distribution of S, platinum-group elements, and Te, As, Bi, Sb, and Se in both mineralization styles and the concentration of trace elements in base-metal sulfides from the Sulfide zone. The Sulfide zone has Pt/Pd ratios around 0.5 and high concentrations of Te, As, Bi, Sb, and Se, whereas the low-S-high-platinum-group element zone has Pt/Pd ratios greater than 1 and much lower Se, Te, and Bi concentrations, but comparable As and Sb contents. This is reflected in the platinum-group element assemblage, comprising bismuthotellurides in the Sulfide zone and mostly arsenides and antimonides in the low-S, high platinum-group elements zone. Moreover, the base-metal sulfides from the Sulfide zone have anomalously high As contents (50–500 ppm), which suggest that the sulfide liquid segregated from a very As-rich silicate magma, possibly illustrated by an average komatiitic basalt that assimilated a mixture of upper continental crust and black shales. We interpret the low-S-high platinum-group elements zone as a product of S loss from magmatic sulfides during post-cumulus alteration of the Luanga Complex. Selenium, Te, Bi, and Pd were also lost together with S, whereas As and Sb were expelled from base-metal sulfide structures and combined with platinum-group elements to form platinum-group minerals, suggesting they may play a role fixating platinum-group elements during alteration. The remobilization of chalcophile elements from magmatic sulfide deposits located in the Carajás Mineral Province may represent a potential source for hydrothermal deposits found in the region.

Abstract Some arc magmas lead to the formation of porphyry deposits in the relatively shallow upper crust (<5 km). Porphyry deposits are major sources of Cu and an important Au source but lack significant amounts of platinum group elements (PGE). Sulfide phases control the behavior of chalcophile elements and affect the potential to form ore deposits either by remaining in the mantle residue or by fractionating from arc magmas at lower crustal levels, although in detail the role of sulfide saturation in the lower crust remains poorly understood. Lower crustal cumulate rocks from the 85 Ma Chilas Complex of the Kohistan arc, Pakistan, provide insight into processes that occur at depth in arcs. Here we provide Cu, Ni, Au, and PGE concentrations and Os isotope ratios of the Chilas Complex in order to constrain the extent of sulfide saturation in the lower crust and the effect of sulfide saturation on the metal budget of evolved melts that ascend to the upper crust. The Chilas rock suite contains less than 0.17 wt % sulfides and low PGE concentrations. In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) measurements of the sulfide inclusions in silicate minerals show enrichment in several chalcophile elements (up to 34 wt % Cu, 23 ppm Au, 245 ppm Pd, and 20 ppm Pt), whereas iridium group PGE (IPGE- Os, Ir, Ru) are mainly below detection limits. The metal content of the parental melt was modeled based on the elemental concentrations of the sulfides. The modeled parental arc magmas contain 70 to 140 ppm Cu, 0.2 to 1.5 ppb Au, and 1.2 to 8 ppb Pd, but low concentrations of IPGE, suggesting that IPGE were likely retained in the mantle source. Mass balance calculations show that segregation of a sulfide melt in the lower crust could further deplete the melt by more than 95% in Pd and Pt, 33 to 85% in Au, and 13 to 60% in Cu. Thus, magmas that ascend to the upper crust would contain very low concentrations of Au (< 0.2 ppb) and Pd (< 0.04 ppb), but they would retain sufficient concentration of Cu (~45–57 ppm) to form porphyry Cu deposits upon emplacement in the upper crust, as is commonly observed in arc settings.