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The activity of volatile-rich fluids may be important in the evolution of basaltic magmatic systems and associated precious metal ore formation. There is evidence for Cl-rich fluids within the Stillwater Complex (Montana, U.S.A.), which have been linked to platinum and palladium mineralization in the economically important Johns-Manville (J-M) Reef ore body. We present the first data set for heavy halogens (Cl, Br, and I) and natural noble gas isotopes in bulk rock and mineral separates from the Peridotite Zone and the Olivine-Bearing Zone I of the Stillwater Complex, including samples from the J-M Reef and G Chromitite bodies. Our data reveal concentrations of 4 to 13 500 ppm for Cl, 26 ppb to 360 ppm for Br, and <1 ppb to 9 ppm I over the whole sample set. Cl, Br, and I correlate well with each other implying a shared process and/or distribution in mineral species. Br/Cl and I/Cl ratios span a range from 0.3 to 35 × 10−3 and 5 to 900 × 10−6 by weight, respectively, encompassing MORB-like to more enriched compositions, particularly for Br/Cl. High-Br/Cl ratios compared to MORB in some Stillwater samples suggest fractionation of halogens during the exsolution of a volatile-rich fluid to explain the most Br-enriched samples. More generally, the presence of minerals such as scapolite, hornblende, and apatite in the most halogen-enriched samples suggests that the halogen-bearing fluids were derived from the cooling of the intrusion rather than late-stage (low-temperature) metamorphism. The combined halogen abundance and noble gas isotope data set imply that crustal contamination may have played a limited role in the crystallization of pegmatoids and the G Chromitite but is not required to account for the halogen budget of the J-M Reef. High-halogen contents in the sulfide-bearing J-M Reef and associated lithologies are consistent with the influence of fluid-related activity during platinum-group element (PGE)-Reef formation, lending weight to the hydromagmatic model for mineralization in the Stillwater intrusion. Our new data also imply chalcophile tendencies of Br and I over Cl in sulfides in natural systems, hinting at the importance of sulfide liquid interaction with halogen-rich fluids in the formation of sulfide-hosted precious metal ore deposits. [ABSTRACT FROM AUTHOR]

Hydrothermal alteration of oceanic crust represents a globally significant geochemical exchange between the crust and oceans, introducing volatile elements into the oceanic lithosphere and removing latent heat from crystallization. The extent to which hydrothermal systems penetrate the lower oceanic crust is debated and is significant for competing models of lower oceanic crust formation. In this study, we apply halogen and noble gas geochemistry in order to better understand the fluid sources and conditions during a hydrothermal alteration in a suite of gabbros sampled from drill cores in the mid‐ (hole GT3A) and lower‐ (hole GT1A) crust of the Oman Ophiolite. Low Br/Cl in mid‐crustal samples is controlled by amphibole formed at relatively high fluid/rock ratios. Lower crustal samples, meanwhile, have Br/Cl ranging from seawater to much higher values (<5 × 10−3). These require alteration at highly variable fluid/rock ratios from ∼1 to <0.01 in the lower crust. Noble gas isotopes also suggest alteration at relatively high and low fluid/rock ratios in the mid‐ and lower‐crust, respectively. Indicators of alteration and proxies for fluid/rock ratios (loss on ignition, Pb/Ce, Br/Cl, 40Ar/36Ar96Ma) vary systematically with distance from major fault zones in the lower crust with the strongest correlation in the least altered minerals. Together with age constraints derived from K‐Ar decay, these data indicate faults were major conduits, supplying seawater to the lower crust roughly contemporaneous with the formation of the ophiolite, and commencing at the magmatic stage. These results suggest that hydrothermal alteration of the lower crust is primarily controlled by major fault structures. Plain Language Summary: After a new oceanic crust is formed by volcanic activity at mid‐ocean ridges, it cools and reacts with circulating seawater. However, there is controversy over whether water penetrates to the lower oceanic crust, or whether this is cooled by conduction alone. This is important for theories of how the lower oceanic crust is formed. Investigating the lower crust is difficult since few samples have been recovered by deep seafloor drilling. However, ophiolites–sections of oceanic crust, uplifted onto continental margins during tectonic collisions–offer a potential window into processes in the deep oceanic crust. In this study, we use halogens and noble gases as chemical fingerprints for the origin of water in rocks from the Oman Ophiolite. We show that rocks from the mid‐oceanic crust and from the lower‐oceanic crust interacted with water under markedly different conditions and that the supply of water to the lower crust was very variable. Rocks close to large fractures cutting through the lower crust were altered by seawater ∼96 Myr ago. Further from these fractures, compositions are increasingly different from seawater and water was scarce. This provides important evidence that large fractures delivered water to the lower crust and may be important for cooling new oceanic crust globally. Key Points: Gabbros from the Oman Ophiolite record hydrothermal circulation in both the upper and lower oceanic crustHalogen and noble gas systematics suggest alteration under high and low fluid/rock ratiosFluid supply to the lower crust is variable and controlled by major fault zones commencing at the magmatic stage [ABSTRACT FROM AUTHOR]

Halogens are primarily located within surface reservoirs of the Earth; as such they have proven to be effective tracers for the identification of subducted volatiles within the mantle. Subducting lithologies exhibit a wide variety of halogen compositions, yet the mantle maintains a fairly uniform signature, suggesting halogens may be homogenized during subduction to the mantle or during eruption. Here we present halogen (Cl, Br, and I), K, noble gas, and major and trace element data on olivines from three seamounts along the Hawaiian‐Emperor seamount chain to determine if the deep mantle source has retained evidence of halogen heterogeneities introduced through subduction. High Ni contents indicate that the Hawaiian‐Emperor mantle source contains a recycled oceanic crust component in the form of pyroxenite, which increases from the 46% in the oldest (Detroit) to 70% in the younger seamount (Koko). Detroit seamount retains mid‐ocean ridge basalts (MORB)‐like Br/Cl and I/Cl, while the Br/Cl and I/Cl of Suiko and Koko seamounts are higher than MORB and similar to altered oceanic crust and dehydrated serpentinite. Helium isotopes show a similar evolution, from MORB‐like values at Detroit seamount toward higher values at Suiko and Koko seamounts. The correlation between pyroxenite contributions, Br/Cl, I/Cl, and 3He/4He indicates that subducted material has been incorporated into the primordial undegassed Hawaiian mantle plume source. The identification of recycled oceanic crustal signatures in both the trace elements and halogens indicates that subduction and dehydration of altered oceanic crust may exert control on the cycling of volatile elements to the deep mantle. Key Points: Trace elements in Emperor seamount olivines indicate magmatism sampled an increasing proportion of pyroxenite through timeHigher than mantle Br/Cl and I/Cl in the younger seamounts indicate that the pyroxenite contains a subducted volatile componentHelium isotope variations indicate that Hawaiian plume magmatism also sampled primitive material derived from the deep mantle [ABSTRACT FROM AUTHOR]

Halite crystals from the Zag H3-6 chondrite contain essentially pure (monoisotopic) xenon-129 ([sup.129]Xe) produced in the early history of the solar system by the decay of short-lived iodine-129 ([sup.129]I) (half-life = 15.7 million years). Correlated release of [sup.129]Xe and [sup.128]Xe, produced artificially from [sup.127]I by neutron irradiation, corresponds to an initial ([sup.129]I/[sup.127]I) ratio of (1.35 ± 0.05) x [10.sup.-4], close to the most primitive early solar system value. If the [sup.129]Xe was produced by in situ decay, then the halite formed from an aqueous fluid within 2 million years of the oldest known solar system minerals., The occurrence of halite has recently been reported in two primitive chondritic meteorites, Monahans (1) and Zag (2, 3). Both meteorites are H group chondrites, indicating an origin on the [...]

The Batouri gold mining area in southeastern Cameroon is part of the Adamawa–Yadé Domain of the Central African Fold Belt (Pan-African). It is underlain by a variety of granitic rocks, including alkali-feldspar granite, syeno-monzogranite, granodiorite, and tonalite. Geochemical data suggest that these rocks formed by differentiation of I-type tonalitic magma under oxidizing conditions in a continental volcanic arc setting. U–Pb dating of zircons from gold-associated monzogranite-granodiorite at Kambélé gave concordant ages of 619 ± 2 and 624 ± 2 Ma, while Ar–Ar dating of alkali-feldspar granite yielded a non-plateau maximum age of 640–620 Ma. These ages imply that the Batouri granitoids were emplaced during the collision of the West African Craton and the Congo Craton. The geochemical characteristics of the Batouri granitoids as well as their oxidized state (magnetite series) are typical of gold-associated felsic rocks in subduction settings elsewhere. The similarities in age, composition, and geochemical affinities of these granitoids with those reported from other localities in the Adamawa–Yadé Domain reinforce the earlier assumption that the granitic rocks of this domain represent parts of a regional-scale batholith, with commonly small-scale, high-grade auriferous quartz veins in structurally favourable sites. The spatial and temporal association of gold mineralization and the Batouri granitoids may suggest potential for regional-scale, high-tonnage, granite-related gold ore. [ABSTRACT FROM AUTHOR]

A 40Ar/39Ar age of 75 ± 6 Ma (2σ) was obtained for aggregates of Mn-Ba oxide intergrown with palladiferous gold and Pd-Pt minerals, representative of near-surface bonanza Au-Pd-Pt mineralization that triggered the gold rush to Serra Pelada in northern Brazil. The mineralization occurs in goethite-rich tectonic breccia and records recurrent fault displacement in the still active Cinzento strike-slip fault system. The goethite-rich tectonic breccia is truncated by the bonanza-style Au-Pd-Pt-Mn-Ba enrichment, with which tabular hematite is spatially associated on a micrometric scale. The tabular hematite is coarsely crystalline, >5 µm in grain size, and lines cavity walls in the goethite-rich breccia. The presence of coarsely crystalline tabular hematite constrains the metalliferous overprint to temperatures in excess of about 100°C. We suggest that the bonanza mineralization is a Late Cretaceous, epithermal-like, fault-breccia system that overprinted Paleoproterozoie intrusion-related Au-Pd-Pt mineralization preserved at depth. [ABSTRACT FROM AUTHOR]