The application of S isotopes and S/Se ratios in determining ore-forming processes of magmatic Ni–Cu–PGE sulfide deposits: a cautionary case study from the northern Bushveld Complex

The application of S/Se ratios and S isotopes in the study of magmatic Ni–Cu–PGE sulfide deposits has long been used to trace the source of S and to constrain the role of crustal contamination in triggering sulfide saturation. However, both S/Se ratios and S isotopes are subject to syn- and post-magmatic processes that may alter their initial signatures. We present in situ mineral δ34S signatures and S/Se ratios combined with bulk S/Se ratios to investigate and assess their utility in constraining ore-forming processes and the source of S within magmatic sulfide deposits. Magmatic Ni–Cu–PGE sulfide mineralization in the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex was used as a case study based on well-defined constraints of sulfide paragenesis and local S isotope signatures. A crustal δ34S component is evident in the most primary sulfide assemblage regardless of footwall lithology, and is inferred that the parental magma(s) of the GNPA member was crustally contaminated and sulfide saturated at the time of emplacement. However, S/Se ratios of both the primary and in particular secondary sulfide assemblages record values within or below the mantle range, rather than high crustal S/Se ratios. In addition, there is a wide range of S/Se ratio for each sulfide mineral within individual assemblages that is not necessarily consistent with the bulk ratio. The initial crustal S/Se ratio is interpreted to have been significantly modified by syn-magmatic lowering of S/Se ratio by sulfide dissolution, and post-magmatic lowering of the S/Se ratio from hydrothermal S-loss, which also increases the PGE tenor of the sulfides. Trace element signatures and variations in Th/Yb and Nb/Th ratios support both an early pre-emplacement contamination event as seen by the S isotopes and S/Se ratios, but also a second contamination event resulting from the interaction of the GNPA magma with the local footwall country rocks at the time of emplacement; though this did not add any additional S. We are able to present an integrated emplacement and contamination model for the northern limb of the Bushveld Complex. Although the multitude of processes that affect variations in the δ34S signature and in particular S/Se ratio may be problematic in interpreting ore genesis, they can reveal a wealth of additional detail on a number of processes involved in the genetic history of a Ni–Cu–PGE deposit in addition to crustal contamination. However, a prerequisite for being able to do this is to utilize other independent petrological and mineralogical techniques that provide constraints on both the timing and effect of various ore-forming and modifying processes. Utilizing both bulk and in situ methods in concert to determine the S/Se ratio allows for the assessment of multiple sulfide populations, the partitioning behaviour of Se during sulfide liquid fractionation and also the effects of low temperature fluid alteration. In comparison, S isotopes are relatively more robust and represent a more reliable indicator of the role of crustal S contamination. The addition of trace element data to the above makes for an incredibly powerful approach in assessing the role of crustal contamination in magmatic sulfide systems

How the Neoproterozoic S-isotope record illuminates the genesis of vein gold systems: an example from the Dalradian Supergroup in Scotland

The genesis of quartz vein-hosted gold mineralization in the Neoproterozoic–early Palaeozoic Dalradian Supergroup of Scotland remains controversial. An extensive new dataset of S-isotope analyses from the Tyndrum area, together with correlation of the global Neoproterozoic sedimentary S-isotope dataset to the Dalradian stratigraphy, demonstrates a mixed sedimentary and magmatic sulphur source for the mineralization. d34S values for early molybdenite- and later gold-bearing mineralization range from 22 to +12‰, but show distinct populations related to mineralization type. Modelling of the relative input of magmatic and sedimentary sulphur into gold-bearing quartz veins with d34S values of +12‰ indicates a maximum of 68% magmatic sulphur, and that S-rich, SEDEX-bearing, Easdale Subgroup metasedimentary rocks lying stratigraphically above the host rocks represent the only viable source of sedimentary sulphur in the Dalradian Supergroup. Consequently, the immediate host rocks were not a major source of sulphur to the mineralization, consistent with their low bulk sulphur and lack of metal enrichment. Recent structural models of the Tyndrum area suggest that Easdale Subgroup metasedimentary rocks, enriched in 34S, sulphur and metals, are repeated at depth owing to folding, and it is suggested that these are the most likely source of sedimentary sulphur, and possibly metals, for the ore fluids

The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals

publisher: Elsevier articletitle: The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals journaltitle: Minerals Engineering articlelink: http://dx.doi.org/10.1016/j.mineng.2015.09.026 content_type: article copyright: Copyright © 2015 The Authors. Published by Elsevier Ltd.© 2015 Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Evolution of the Munali Intrusive Complex: host to a carbonate-rich Ni-(Cu-PGE) sulfide deposit

The Munali Intrusive Complex is hosted within supracrustal metasedimentary rocks located along a major structural lineament within the Zambezi Belt in southern Zambia. The complex comprises unmineralised gabbro surrounded by a marginal heterogeneous mafic–ultramafic breccia unit that is host to Ni-Fe sulfide. This marginal unit comprises a range of variably evolved brecciated mafic–ultramafic rocks that include gabbro, olivine-gabbro and dolerite, alongside younger, pegmatitic, apatite-magnetite-bearing clinopyroxenite, wehrlite and dunite. The magmatic evolution is most consistent with a model whereby early mafic rocks interact with hot, MgO- and volatile-rich melts along gabbro contacts, causing localised metasomatism of gabbro and pyroxenites, and progressively replacing pyroxene-rich rocks with olivine, forming pegmatitic ‘replacive dunites’. Sulfide mineralisation is characterised by a carbonate-rich apatite-magnetite-bearing assemblage predominately present as lenses of semi-massive to massive sulfide ore. The complex is enveloped almost entirely within a unit of marble, yet C and O isotope signatures of carbonate at Munali have revealed a clear mantle signature for some of the carbonate associated with sulfide, alongside a more dominant, crustally derived component. The carbonate occurring alongside sulfide displays micro to macro textures signifying the presence of carbonate melts formed from anatectic melting of the country rocks. The presence of fracture sets that define coarse breccia clasts (>1 m) indicate that the host rock was significantly crystallised and brittly deformed prior to carbonate and sulfide melt infiltration. Both carbonate and sulfide melts appear to have independently utilised these pre-existing weaknesses producing a pseudobreccia, and accounting for the seemingly chaotic nature of the orebody. The indication of sulfide being a significantly later phase suggests that the sulfide did not form in situ and was mobilised from elsewhere to be subsequently emplaced late within the Munali system

Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth’s spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a ‘Goldilocks’ temperature zone at ~1100–1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits

Extreme enrichment of Se, Te, PGE and Au in Cu sulfide microdroplets: evidence from LA-ICP-MS analysis of sulfides in the Skaergaard Intrusion, east Greenland

The Platinova Reef, in the Skaergaard Intrusion, east Greenland, is an example of a magmatic Cu–PGE–Au sulfide deposit formed in the latter stages of magmatic differentiation. As is characteristic with such deposits, it contains a low volume of sulfide, displays peak metal offsets and is Cu rich but Ni poor. However, even for such deposits, the Platinova Reef contains extremely low volumes of sulfide and the highest Pd and Au tenor sulfides of any magmatic ore deposit. Here, we present the first LA-ICP-MS analyses of sulfide microdroplets from the Platinova Reef, which show that they have the highest Se concentrations (up to 1200 ppm) and lowest S/Se ratios (190–700) of any known magmatic sulfide deposit and have significant Te enrichment. In addition, where sulfide volume increases, there is a change from high Pd-tenor microdroplets trapped in situ to larger, low tenor sulfides. The transition between these two sulfide regimes is marked by sharp peaks in Au, and then Te concentration, followed by a wider peak in Se, which gradually decreases with height. Mineralogical evidence implies that there is no significant post-magmatic hydrothermal S loss and that the metal profiles are essentially a function of magmatic processes. We propose that to generate these extreme precious and semimetal contents, the sulfides must have formed from an anomalously metal-rich package of magma, possibly formed via the dissolution of a previously PGE-enriched sulfide. Other processes such as kinetic diffusion may have also occurred alongside this to produce the ultra-high tenors. The characteristic metal offset pattern observed is largely controlled by partitioning effects, producing offset peaks in the order Pt+Pd>Au>Te>Se>Cu that are entirely consistent with published D values. This study confirms that extreme enrichment in sulfide droplets can occur in closed-system layered intrusions in situ, but this will characteristically form ore deposits that are so low in sulfide that they do not conform to conventional deposit models for Cu–Ni–PGE sulfides which require very high R factors, and settling of sulfide liquids

Assessing the potential involvement of an early magma staging chamber in the generation of the Platreef Ni–Cu–PGE deposit in the northern limb of the Bushveld Complex: a pilot study of the Lower Zone Complex at Zwartfontein

The Platreef of the northern limb of the Bushveld Complex is one of the world's most significant deposits of platinum-group elements (PGE) with associated Ni and Cu. The origin of the Platreef is controversial. Some workers suggest that it is a northern facies of the Merensky Reef or part of the Upper Critical Zone, while others have suggested that the Platreef formed by processes entirely contained within the northern limb, unrelated to mineralisation events elsewhere in the complex. The northern limb is separated from the rest of the complex by the Thabazimbi–Murchison Lineament (TML) and the effect that this structure had on the intrusion of Bushveld magmas is debated. The presence of chilled rocks and cross-cutting relationships between the Platreef and its hangingwall gabbronorites would seem to preclude the magma that formed the hangingwall also acting as a source of PGE to the Platreef. The base metal sulphides in the Platreef carry very high PGE tenors (comparable with the Merensky Reef) indicating that the PGE must have been concentrated from a large volume of magma, but the source of that magma has not been established. In order to solve this PGE mass balance paradox, McDonald and Holwell have suggested that the magmas that formed the (pre-Platreef) Lower Zone may have been the source of PGE. At present, other models do not involve any significant role for the Lower Zone magmas in forming the Platreef. The data presented in this pilot study of the Lower Zone intrusion at Zwartfontein test some of the predictions arising from the McDonald and Holwell model. They highlight some important first order differences between Lower Zone intrusions in the northern limb compared with the rest of the Bushveld Complex. The enrichment in Th and LREE that characterizes the Lower Zone rocks in the eastern and western Bushveld appears to be missing in the northern limb. The lithophile element signatures of the different types of Lower Zone are suggested to result from mafic magmas intruding north and south of the TML and being contaminated by these different types of crust. Most significantly, the study has also revealed strong depletion of chalcophile elements (Ni, Cu and PGE) in the Lower Zone intrusion at Zwartfontein. The results are consistent with the depleted products expected from the processing of pre-Platreef magma(s) by interactions with sulphides at a deeper level within the magmatic plumbing system. The results provide a positive first test of one of the predictions arising from the McDonald and Holwell Platreef model. The existence of a system capable of removing PGE and producing a large volume of depleted ultramafic cumulates, in close proximity to the most highly mineralised sector of the Platreef, is suggested to be highly significant

Precious and base metal geochemistry and mineralogy of the Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member, northern Bushveld Complex, South Africa : implications for a multistage emplacement

The Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a mineralized, layered package of mafic cumulates developed to the south of the town of Mokopane, at a similar stratigraphic position to the Platreef. The concentration of platinum-group elements (PGE) in base metal sulfides (BMS) has been determined by laser ablation inductively coupled plasma-mass spectrometry. These data, coupled with whole-rock PGE concentrations and a detailed account of the platinum-group mineralogy (PGM), provide an insight into the distribution of PGE and chalcophile elements within the GNPA member, during both primary magmatic and secondary hydrothermal alteration processes. Within the most unaltered sulfides (containing pyrrhotite, pentlandite, and chalcopyrite only), the majority of IPGE, Rh, and some Pd occur in solid solution within pyrrhotite and pentlandite, with an associated Pt-As and Pd-Bi-Te dominated PGM assemblage. These observations in conjunction with the presence of good correlations between all bulk PGE and base metals throughout the GNPA member indicate the presence and subsequent fractionation of a single PGE-rich sulfide liquid, which has not been significantly altered. In places, the primary sulfides have been replaced to varying degrees by a low-temperature assemblage of pyrite, millerite, and chalcopyrite. These sulfides are associated with a PGM assemblage characterized by the presence of Pd antimonides and Pd arsenides, which are indicative of hydrothermal assemblages. The presence of appreciable quantities of IPGE, Pd and Rh within pyrite, and, to a lesser, extent millerite suggests these phases directly inherited PGE contents from the pyrrhotite and pentlandite that they replaced. The replacement of both the sulfides and PGM occurred in situ, thus preserving the originally strong spatial association between PGM and BMS, but altering the mineralogy. Precious metal geochemistry indicates that fluid redistribution of PGE is minimal with only Pd, Au, and Cu being partially remobilized and decoupled from BMS. This is also indicated by the lower concentrations of Pd evident in both pyrite and millerite compared with the pentlandite being replaced. The observations that the GNPA member was mineralized prior to intrusion of the Main Zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal S. We therefore believe that our observations are more compatible with a multistage emplacement model, where preformed PGE-rich sulfides were emplaced into the GNPA member. Such a model explains the development and distribution of a single sulfide liquid throughout the entire 400-800 m thick succession. It is therefore envisaged that the GNPA member formed in a similar manner to its nearest analogue the Platreef. Notable differences however in PGE tenors indicate that the ore-forming process may have differed slightly within the staging chambers that supplied the Platreef and GNPA member

Magmatic sulfide ore deposits

Magmatic sulfide ore deposits are products of natural smelting: concentration of elements from silicate magmas (slags) by immiscible sulfide liquid (matte). Deposits occupy a spectrum from accumulated pools of matte within small igneous intrusions or lava flows, forming orebodies mined primarily for Ni and Cu, to stratiform layers of weakly disseminated sulfides, mined for platinum group elements, within large mafic-ultramafic intrusions. One of the world’s most valuable deposits, the Platreef in the Bushveld Complex in South Africa, has aspects of both of these end members. Natural matte compositions vary widely between and within deposits, controlled largely by the relative volumes of matte and slag that interact with one another

Geochemistry and mineralogy of platinum group element mineralization in the river valley intrusion, Ontario, Canada : A model for early-stage sulfur saturation and multistage emplacement and the implications for "contact-type" Ni-Cu-PGE sulfide mineralization

The River Valley Intrusion (RVI) within the ~2.48 Ga East Bull Lake Intrusive Suite in Ontario, Canada, is an example of a mafic igneous intrusion with 'contact-type' Ni-Cu- PGE sulfide mineralization along its base. Whereas many 'contact-type' deposits are thought to form from in situ contamination of the magma by the addition of crustal S during emplacement, there are some intrusions, including the RVI, which appear to have a much more complex history where the timing of S saturation, and thus the critical ore genesis processes, may have occurred much earlier, prior to emplacement. The RVI is made up of a basal ~100 m of unlayered, inclusion-bearing units, overlain by layered cumulates. The basal units contain autoliths of gabbroic rocks and inclusions of footwall gneiss and amphibolites, all within a gabbroic matrix. Platinumgroup element-rich magmatic sulfide mineralization occurs throughout both the inclusions and the matrix as blebby and disseminated sulfides. The matrix and inclusions can be separated into two distinct textural types: hydrothermally altered greenschist assemblages and unaltered metamorphic amphibolite assemblages. The platinum-group mineral (PGM) assemblages vary only between textural types, and not between inclusions and matrix, being dominated tellurides in all rock types. The hydrothermally altered rocks, however, have fewer tellurides, and an increased amount of Sb- and As-bearing PGM, indicative of minor fluid interaction, although the PGM have not been mobilised significantly away from the base metal sulfides. Precious and base metal geochemistry shows all rock types to have an excellent correlation between all the PGE, indicating the presence of a single, well homogenised, PGE-rich sulfide liquid. However, Au and Cu appear to be decoupled from the PGE at low concentrations, although correlate well with each other, which is interpreted to be due to minor fluid redistribution and alteration of sulfide bleb margins. The overlying Layered Units above the mineralized units are not PGE depleted. Trace element data, including (Th/Yb)PM and (Nb/Th)PM ratios, demonstrate that all River Valley rocks were formed from crustally contaminated magmas following interaction with local country rocks in a deeper subchamber; although some samples have S/Se ratiosindicative of crustal S, most have S/Se ratios lower than the mantle range, indicative of S loss. We propose a multi-stage model for the formation of the mineralization in the RVI with a major contamination event at depth with the addition of S from local crustal rocks, inducing sulfide saturation. Sulfide droplets were then enriched in PGE within a conduit system with possible further upgrading of sulfide metal tenors (and reduction of S/Se ratios) via partial dissolution of sulfide. The PGE-enriched sulfide liquid then settled in a staging chamber and partially crystallised before a major pulse of magma entrained sulfide liquid, eroded blocks of pre-crystallised and mineralized gabbro and footwall rocks and emplaced an inclusion-bearing package as the lower 100m or so of the RVI. Later emplacement of main RV magma was from a S-undersaturated, PGE-fertile magma. The RVI is thus an example whereby 'contact-type' mineralization is purely a function of the earliest magma intruded containing pre-formed sulfide mineralization, rather than contamination triggering sulfide saturation in situ. In such cases, processes at depth determine the generation and subsequent tenor of the mineralization. In particular, dissolution of the sulfide can upgrade metal tenor, but subsequently will reduce S/Se ratios, masking the signature of crustal contamination. In addition, a multi stage emplacement such as this will not necessarily preserve the characteristic increase in Cu/Pd ratios in the overlying cumulates that is often used in exploration for PGE deposits in mafic intrusions. Thus, a full understanding of all the field, geochemical and mineralogical factors is required to construct genetic models for such deposits, and especially in the interpretation of S/Se and Cu/Pd ratios as an indicator of crustal contamination and the presence of PGE mineralisation