Unraveling the Effects of Melt–Mantle Interactions on the Gold Fertility of Magmas

We thank Tony Lanzirotti and Matt Newville for assistance with μ-XANES analyses at the GeoSoilEnviroCars (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory.The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/feart.2020.00029/full#supplementary-materialThe oxidation state of the Earth’s mantle and its partial melting products exert a key control on the behavior and distribution of sulfur and chalcophile and siderophile elements between the mantle and crust, underpinning models of ore deposit formation. Whether the oxidized nature of magmas is inherited from the asthenospheric mantle source or acquired during ascent and differentiation is vigorously debated, limiting our understanding of the mechanisms of extraction of sulfur and metals from the mantle. Here, we focused on the redox-sensitive behavior of sulfur in apatite crystallized from quenched alkaline basaltic melts preserved within a peridotite xenolith from the El Deseado Massif auriferous province in southern Patagonia. We took advantage of this unique setting to elucidate the redox evolution of melts during their ascent through the subcontinental lithospheric mantle (SCLM) and grasp the inner workings of the Earth’s mantle during gold metallogenesis. Our data reveal that an initially reduced silicate melt (ΔFMQ −2.2 to −1.2) was oxidized to ΔFMQ between 0 and 1.2 during percolation and interaction with the surrounding peridotite wall-rock (ΔFMQ 0 to +0.8). This process triggered changes in sulfur speciation and solubility in the silicate melt, boosting the potential of the melt to scavenge ore metals such as gold. We suggest that large redox gradients resulting from the interaction between ascending melts and the surrounding mantle can potentially modify the oxidation state of primitive melts and enhance their metallogenic fertility. Among other factors including an enriched metal source and favorable geodynamic conditions, redox gradients in the mantle may exert a first-order control on the global-scale localization of crustal provinces endowed with gold deposits.This study was funded by the Iniciativa Científica Milenio through Millennium Nucleus for Metal Tracing along Subduction Grant NC130065. Additional funding for analytical work was provided by the FONDAP project 15090013 “Centro de Excelencia en Geotermia de Los Andes, CEGA.” ST acknowledges CONICYT for support through a Ph.D. scholarship #21170857. Grants RTI2018-099157-A-100 and RYC-2015-1796 provided funding for sample preparation and EPMA analyses. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The DID-UACh project #S-201505 financed the fieldwork

NEW ELEMENTS OF THE PALEOHERPETOFAUNA OF THE PLEISTOCENE FROM MAR CHIQUITA COUNTY (BUENOS AIRES, ARGENTINA)

Fossils of amphibians and squamates are relatively common in Pliocene–Pleistocene sediments in the southeastern Atlantic coast of Buenos Aires Province (Argentina), particularly in the southern slope of the Tandilia mountain system. These taxa are less known in the northern slope, which in the county of Mar Chiquita are solely represented by one anuran record. Here, new herpetofaunal material is described from Mar Chiquita county in the northern slope of the Tandilia mountain system. These specimens, while few, present an interesting taxonomical and morphological diversity. From the Ensenadan outcrops, amphibians are represented by cf. Rhinella. On the other hand, Stenocercus, an indeterminate tropidurid, Colubroides, and an indeterminate snake account for the Ensenadan squamates. In the Bonaerian outcrops, squamates are represented by the viperid Bothrops and an indeterminate Pleurodont. These latter taxa are also present in the Lujanian with an indeterminate snake. These are the first fossil records of squamates from Mar Chiquita and together with the new and previous amphibian records constitute a herpetological assemblage (Ceratophrys ornata, cf. Rhinella, Stenocercus, and Bothrops) that suggests that the paleoclimatic conditions during the Pleistocene of the Pampean region would have been similar to those of the present day. This inference should be considered with caution, since it contradicts those suggested by the mammalian record, which is richer and presents taxonomic identifications that are more precise. Mammalian fossil records indicate a cold and dry glacial Pleistocene interspersed with short, warmer, and more humid periods

Osmium isotopes fingerprint mantle controls on the genesis of an epithermal gold province

The formation of crustal blocks enriched with gold (Au) deposits above subduction zones is intimately bound to the genesis and evolution of magmatic-hydrothermal systems. A longstanding question, however, is whether the metal fertility of these systems stems from distinct sources that are anomalously enriched in Au or from subsequent processes occurring during crustal magma emplacement and hydrothermal activity. The Deseado Massif auriferous province in southern Patagonia (Argentina) is a unique place to test these contrasting hypotheses because Au-bearing mantle xenoliths indicate the presence of an underlying Au-rich lithospheric mantle reservoir. However, direct geochemical links between the Au-rich mantle source and the formation of the Deseado Massif auriferous province in the overlying crust remain to be established. To address this prominent gap in knowledge, we used sulfide Re-Os geochronology to identify the source of Au at Cerro Vanguardia, the largest low-sulfidation epithermal Au-Ag deposit in the Deseado Massif. Pyrite from high-grade Au quartz veins yielded an isochron age of 147.4 ± 2.9 Ma (mean square of weighted deviates = 1.04, n = 8) and an initial 187Os/188Os ratio of 0.26 ± 0.01, fingerprinting a dominant mantle control for the source of Os and, by inference, the source of Au. Our data provide a unique geochemical linkage between an Au-rich subcontinental lithospheric mantle source and the genesis of epithermal Au deposits, supporting the hypothesis that pre-enriched mantle domains may be a critical factor underpinning the global-scale localization of Au provinces.Fil: Tassara, Santiago. University of Yale; Estados UnidosFil: Rooney, Alan D.. University of Yale; Estados UnidosFil: Ague, Jay. J.. University of Yale; Estados UnidosFil: Guido, Diego Martin. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Recursos Minerales. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto de Recursos Minerales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Reich, Martin. Universidad de Chile; ChileFil: Barra, Fernando. Universidad de Chile; ChileFil: Navarrete Granzotto, César Rodrigo. Universidad Nacional de la Patagonia "San Juan Bosco"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Slab-derived devolatilization fluids oxidized by subducted metasedimentary rocks

Metamorphic devolatilization of subducted slabs generates aqueous fluids that ascend into the mantle wedge, driving the partial melting that produces arc magmas. These magmas have oxygen fugacities some 10–1,000 times higher than magmas generated at mid-ocean ridges. Whether this oxidized magmatic character is imparted by slab fluids or is acquired during ascent and interaction with the surrounding mantle or crust is debated. Here we study the petrology of metasedimentary rocks from two Tertiary Aegean subduction complexes in combination with reactive transport modelling to investigate the oxidative potential of the sedimentary rocks that cover slabs. We find that the metasedimentary rocks preserve evidence for fluid-mediated redox reactions and could be highly oxidized. Furthermore, the modelling demonstrates that layers of these oxidized rocks less than about 200 m thick have the capacity to oxidize the ascending slab dehydration flux via redox reactions that remove H2, CH4 and/or H2S from the fluids. These fluids can then oxidize the overlying mantle wedge at rates comparable to arc magma generation rates, primarily via reactions involving sulfur species. Oxidized metasedimentary rocks need not generate large amounts of fluid themselves but could instead oxidize slab dehydration fluids ascending through them. Proposed Phanerozoic increases in arc magma oxygen fugacity may reflect the recycling of oxidative weathering products following Neoproterozoic–Palaeozoic marine and atmospheric oxygenation

Plume-subduction interaction forms large auriferous provinces

Gold enrichment at the crustal or mantle source has been proposed as a key ingredient in the production of giant gold deposits and districts. However, the lithospheric-scale processes controlling gold endowment in a given metallogenic province remain unclear. Here we provide the first direct evidence of native gold in the mantle beneath the Deseado Massif in Patagonia that links an enriched mantle source to the occurrence of a large auriferous province in the overlying crust. A precursor stage of mantle refertilisation by plume-derived melts generated a gold-rich mantle source during the Early Jurassic. The interplay of this enriched mantle domain and subduction-related fluids released during the Middle-Late Jurassic resulted in optimal conditions to produce the ore-forming magmas that generated the gold deposits. Our study highlights that refertilisation of the subcontinental lithospheric mantle is a key factor in forming large metallogenic provinces in the Earth’s crust, thus providing an alternative view to current crust-related enrichment models.Funding for this study has been provided by Millennium Science Initiative through Millennium Nucleus for Metal Tracing along Subduction Grant NC130065. Additional support was provided by FONDAP project 15090013 ‘Centro de Excelencia en Geotermia de Los Andes, CEGA’, FONDECYT projects #11140005 and #1140780, and DID-UACh project #S-2015-52. Additional funding for LA-ICP-MS analyses of sulfides at Macquarie University was provided by RYC-2015-17596. Minerals Targeting International Pty Ltd is acknowledged for providing directions regarding suitable sample sites for gold-bearing xenoliths. S.T. thanks CONICYT 21170857 for providing support through a PhD scholarship

The role of the subcontinental lithospheric mantle in metallogenesis

Tesis para optar al grado de Doctor en Ciencias, Mención GeologíaNúcleo Milenio Trazadores de Metales en Zonas de Subducción, el Centro de Excelencia en Geotermia de Los Andes, CEGA (FONDAP-CONICYT # 15090013), y la beca doctoral CONICYT # 2117085722/05/202

New Record of the Vampire Desmodus draculae (Chiroptera) from the Late Pleistocene of Argentina

The presence of Desmodus draculae, an extinct giant vampire bat, from the Pleistocene of Buenos Aires is confirmedFil: Brizuela, Santiago. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Biología; ArgentinaFil: Tassara, Daniel Adrian. Museo Municipal de Ciencias Naturales Pachamama; Argentin

Reptiles escamosos (Squamata) del Pleistoceno Medio del Norte de la ciudad de Mar del Plata (provincia de Buenos Aires, Argentina)

In this study, we describe two fossil remains of squamate reptiles found in Middle Pleistocene outcrops at the northern marine cliffs of the city of Mar del Plata (Buenos Aires province). The specimens were found forming a taphocenosis with remains of other microvertebrates (amphibians, mammals and birds). The reptiles recognized in the association are represented by remains of an undetermined colubrid, and the anguid Ophiodes sp. This latter finding represents the first fossil record for the family Anguidae exhumed in Argentina

Fe-Ni-bearing serpentines from the saprolite horizon of Caribbean Ni-laterite deposits: new insights from thermodynamic calculations

Fe-Ni-bearing serpentine from the saprolite horizon is the main Ni ores in hydrous silicate-type Ni laterites and formed by chemical weathering of partially serpentinized ultramafic rocks under tropical conditions. During lateritization, Mg, Si, and Ni are leached from the surface and transported downwards. Fe2+ is oxidized to Fe3+ and fixed as insoluble Fe-oxyhydroxides (mostly goethite) that incorporate Ni. This Ni is later leached from goethite and incorporated in secondary serpentine and garnierite. As a result, a serpentine-dominated saprolite horizon forms over the ultramafic protolith, overlapped by a Fe-oxyhydroxide-dominated limonite horizon. The serpentine from the protolith (serpentine I) is of hydrothermal origin and yields similar Ni (0.10-0.62 wt.% NiO) and lower Fe (mostly 1.37-5.81 wt.% FeO) concentrations than the primary olivine. In contrast, Fe-Ni-bearing serpentine from the saprolite (serpentine II) shows significantly higher and variable Fe and Ni contents, typically ranging from 2.23 to 15.59 wt.% Fe2O3 and from 1.30 to 7.67 wt.% NiO, suggesting that serpentine get enriched in Fe and Ni under supergene conditions. This study presents detailed mineralogical, textural, and chemical data on this serpentine II, as well as new insights by thermodynamic calculations assuming ideal solution between Fe-, Ni- and Mg-pure serpentines. The aim is to assess if at atmospheric pressure and temperature Fe-Ni-bearing serpentine can be formed by precipitation. Results indicate that the formation of serpentine II under atmospheric pressure and temperature is thermodynamically supported, and pH, Eh, and the equilibrium constant of the reaction are the parameters that affect the results more significantly.FEDER Funds Spanish projects CGL2009-10924 CGL2012-36263 Catalan project 2014-SGR-1661 Ministerio de Educacion (Spain

Refertilization of the Subcontinental Lithospheric Mantle and its link to the formation of Metallogenic Provinces

Refertilisation of the subcontinental lithospheric mantle is a key process controlling the noble metal budget of the mantle, and recent views point to anomalously enriched mantle sources as a critical factor in the formation of noble metal (e.g., Au) provinces at a lithospheric scale. Here we test this hypothesis by studying peridotite xenoliths from the mantle beneath the Deseado Massif auriferous province in southern Patagonia, Argentina. Extensive Neogene back-arc plateau magmatism composed of alkaline basalts (~3.5 Ma) has brought to the surface deep-seated mantle xenoliths from beneath the crust that host the Au mineralization. In the studied xenolith samples we found gold particles enclosed within primary olivine and pyroxene, and embedded in a highly alkaline interstitial glass or sulphides. Detailed inspection of the sulphide hosts using FESEM reveals abundant native Au nanoparticles, which are consistent with the high Au (up to 6 ppm) obtained by LA-ICP-MS analysis of these sulphides. It is relevant to note that these sulphides also contain significant amounts of Ag (up to 163 ppm). Different generations of sulphides were identified on the basis of their chondrite-normalized PGE patterns, and they can be systematically associated with different events of melt depletion and metasomatism in the mantle. Noticeably, Cu-Pd-Pt-Au rich sulfides are associated with clinopyroxene showing typical carbonatite markers (i.e., large LREE/HREE, Zr and Hf negative anomalies) and accessory minerals such as carbonates and apatite. Still, clinopyroxene commonly has high Ti contents suggesting that a “basaltic” component was also present during the metasomatism. These results suggest that overprinting of events of melt depletion and metasomatism lead to the formation of several generations of sulfides. We propose that the Cu-Pd-Pt-Au rich sulfides may be associated with carbonated silicate melts in the mantle. Our results point to 1) a link between an enriched source of gold (and silver) in the mantle and the formation of the Deseado Massif auriferous province; and 2) carbonated silicate melt metasomatism as an important factor in the PPGE + Au refertilisation of the mantle