High Sulfur in Primitive Arc Magmas, Its Origin and Implications

Sulfur contents in 98.5% of melt inclusions (MI) from calc-alkaline subduction basalts do not exceed 4000 ppm, whereas experimentally established limits of sulfur solubility in basaltic melts with high fO2 (characteristic of subduction zones, e.g., QFM + 2) surpass 14,000 ppm. Here we show that primitive (Mg# 62-64) subduction melts may contain high sulfur, approaching the experimental limit of sulfur solubility. Up to 11,700 ppm S was measured in olivine-hosted MI from primitive arc basalt from the 1941 eruption of the Tolbachik volcano, Kamchatka. These MI often contain magmatic sulfide globules (occasionally enriched in Cu, Ni, and platinum-group elements) and anhydrite enclosed within a brown, oxidized glass. We conclude that the ubiquitous low sulfur contents in MI may originate either from insufficient availability of sulfur in the magma generation zone or early magma degassing prior to inclusion entrapment. Our findings extend the measured range of sulfur concentrations in primitive calc-alkaline basaltic melts and demonstrate that no fundamental limit of 4000 ppm S exists for relatively oxidized subduction basalts, where the maximum sulfur content may approach the solubility limit determined by crystallization of magmatic anhydrite.

The effects of post-cumulus alteration on the distribution of chalcophile elements in magmatic sulfide deposits and implications for the formation of low-S-high-PGE zones: The Luanga deposit, Carajás Mineral Province, Brazil

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.

Genesis of sulfide vein mineralization at the Sakatti Ni-Cu-PGE deposit, Finland

ABSTRACT The Sakatti Ni-Cu-platinum-group element deposit is situated in northern Finland and comprises massive, disseminated, and vein sulfide mineralization. A stockwork is formed by chalcopyrite-rich sulfide veins, which contain exceptionally high platinum-group elements and Au grades. The mineralogy and geochemistry of this stockwork zone ore is documented in this investigation. The results are used to develop the first robust genetic concept and its relationship to massive and disseminated mineralization of the Sakatti deposit. This model is similar to that proposed for many Cu-rich magmatic sulfide ores, most importantly the Cu-rich footwall veins described from the Sudbury Complex in Canada and the Cu-rich ore at Noril'sk-Talnakh in Russia. Detailed petrographic studies using a sample suite from exploration drill core intersecting vein-style mineralization revealed a classic magmatic sulfide assemblage of chalcopyrite ± pyrrhotite, pentlandite, and pyrite. More than 1000 platinum-group mineral grains belonging almost exclusively to the moncheite (PtTe2) – merenskyite (PdTe2) – melonite (NiTe2) solid solution series were identified in the studied samples. Notably, almost two thirds of the platinum-group element-bearing minerals consist of melonite. Some of the platinum-group minerals contain inclusions of Ag-rich gold (AgAu2) and muthmannite (AuAgTe2). Most of the platinum-group minerals occur as inclusions in chalcopyrite, although a few grains are located at base-metal sulfide grain boundaries and in fractures in base-metal sulfides. The whole-rock compositions of the stockwork veins are Cu-rich and are interpreted to represent a fractionated Cu-rich sulfide liquid enriched in Pt, Pd, Au, Ag, As, Bi, Pb, Se, Te, Zn, which separated from a monosulfide solid solution (mss). An intermediate solid solution (iss) solidified from the Cu-rich sulfide liquid, recrystallizing chalcopyrite at <550 °C. Simultaneously, small volumes of intercumulus residual melt contained mainly the precious metals, Bi, and Te due to their incompatibility in iss. Solitary and composite platinum-group minerals as well as Au-minerals crystallized first from the residual melt (<600 °C), followed by a succession of various Bi-, Ag-, and Pb-tellurides (∼540 °C), and finally sphalerite and galena. Melonite crystallized as mostly large, solitary grains exsolved directly from Ni-bearing intermediate solid solution (∼600 °C), shortly after the formation of moncheite and merenskyite from the residual melt. Finally, remobilization of the platinum-group minerals occurred at temperatures of <300 °C, as suggested by the presence of minor amounts of Cl-bearing minerals and ragged grain shapes.

Distribution of noble metals in magmatic sulfide occurrences in the Montagnais Sill Complex, Labrador Trough, Canada

ABSTRACT We have characterized the distribution of noble metals among six styles of magmatic sulfide mineralization in the Montagnais Sill Complex of the Labrador Trough in northern Québec using optical and electron microscopy combined with laser ablation-inductively coupled plasma-mass spectrometry trace element analysis of sulfides. The principal sulfide minerals include pyrrhotite, chalcopyrite, and pentlandite with accessory sphalerite and sulfarsenides. In addition, cubanite, troilite, and mackinawite are present in ultramafic-hosted assemblages. The precious metal mineral assemblages are dominated by tellurides, Ag-rich gold, and sperrylite which generally occur at the margins of sulfides. Few iridium-group platinum group element- and Rh-bearing grains were identified and mass-balance calculations show that these elements are generally hosted in pyrrhotite and pentlandite. Virtually all Pt and Au are hosted in precious metal grains, whereas Pd is distributed between precious metal grains and pentlandite. Where present, sulfarsenides are a key host of iridium-group platinum group element, Rh, Pd, Te, and Au. The presence of troilite, cubanite, and mackinawite and the absence of pentlandite exsolution lamellae in the ultramafic-hosted sulfides indicates an initial sulfide melt with a high metal/S ratio. Sulfarsenides present among globular sulfide assemblages derive from an immiscible As-rich melt that exsolved from the sulfide melt in response to the assimilation of the As-bearing floor rocks. In this study, the composition of sulfides is consistent with those derived from Ni-Cu-dominated deposits and not platinum group element-dominated deposits.

Spatial Association Between Platinum Minerals and Magmatic Sulfides Imaged with the Maia Mapper and Implications for the Origin of the Chromite-Sulfide-PGE Association

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.

The Ni-Cu-PGE-(Au-Te) potential of the Permo-Triassic boundary between Laurasia and Gondwana

<p>The long-lived geodynamic evolution of the Permo-Triassic boundary between <span>Laurasia</span> and Gondwana may have created the ideal conditions for the genesis of a trans-continental Ni-Cu-PGE-(Au-Te) mineralised belt in Europe. This working hypothesis stems from the recent understanding that orogenic processes play a fundamental role in the onset of chemical and physical triggers for the transport of metals from the metasomatised mantle through to various crustal levels. An insight into our renewed framework for the polyphased genetic evolution of magmatic sulfide mineral systems is provided by a series of mineralised occurrences in the Ivrea Zone of NW Italy, which formed at multiple stages over a > 80 Ma time interval. Between 290-250 Ma, a series of hydrated and carbonated ultramafic alkaline pipes containing Ni-Cu-PGE-(Te-Au) mineralisation was emplaced in the lower continental crust. At ~200 Ma, a subsequent mineralising event occurred in association with the emplacement of the La Balma-Monte Capio (LBMC) intrusion. Modelling of the LBMC parental magma shows derivation from ~30% partial melting of an anhydrous juvenile mantle at moderate pressure (< 7 GPa). The inferred composition of the parental melt is consistent with magmatism associated with the Central Atlantic Magmatic Province (CAMP). However, its tellurium-enriched composition together with the S-C-O isotope signature of the associated magmatic sulfide mineralisation cannot be reconciled with the CAMP source. It is argued that the geochemical and isotopic signature of the LBMC intrusion reflects interaction and mixing of a primitive magma sourced from a juvenile source with localised domains enriched in carbonate and metal-rich sulfides located in the lower crust, consistent with the composition of the Permo-Triassic pipes. Evidence of this magmatic interaction informs on the first-order processes that control enhanced metallogenic fertility along the margins of lithospheric blocks. The scenario depicted here is consistent with reactivation and enrichment of a Gondwana margin Ni-Cu-PGE-(Te-Au) mineral system during the breakup of Pangea. The lessons learnt in the Ivrea Zone natural laboratory may inform on the genesis of other Permo-Triassic magmatic mineral systems in continental Europe, such as the deposits in north-west Czech Republic and southern Spain, which display significant analogies with their counterparts in the Ivrea Zone. We suggest that these systems may have a common DNA related to a metallogenic belt forming at different stages during the complex evolution and multi-phase activation of the margin between <span>Laurasia</span> and Gondwana. The nature and localisation of the magmatic sulfide mineral systems along this belt indicate that enhanced potential for ore formation at lithospheric margins may be due not only to favourable architecture, but also to localised enhanced metal and volatile fertility. Importantly, this hypothesis may explain why ore deposits along the margins of lithospheric blocks are not distributed homogeneously along their entire extension but generally form clusters. As mineral exploration is essentially a search space reduction exercise, this new understanding may prove to be important in predictive exploration targeting for new mineralised camps in Europe and elsewhere globally, as it provides a way to prioritise segments with enhanced fertility along extensive lithospheric block margins.</p>