Shooting the touchers / Documenting human capital

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Mantle-derived magmas and magmatic Ni-Cu-(PGE) deposits

Magmatic Fe-Ni-Cu ± platinum-group element (PGE) sulfide deposits form when mantle-derived mafic and ultramafic magmas become saturated in sulfide and segregate immiscible sulfide liquid, commonly following interaction with crustal rocks. Although the metal contents of primary magmas influence ore compositions, they do not control ore genesis because the metals partition strongly into the sulfide liquid and because most magmas capable of segregating sulfide liquid contain sufficient abundances of ore metals. More important controls are the temperature, viscosity, volatile content, and mode of emplacement of the magma, which control the dynamics of magma emplacement and the degree of interaction with crust. By this measure, high-temperature, low-viscosity komatiites and tholeiitic picrites are most capable of forming Ni-Cu-(PGE) deposits, whereas lower-temperature, volatile-rich alkali picrites and basalts have less potential. In most deposits, ore formation is linked directly to incorporation of S-rich country rocks and only indirectly to contamination by granitic crust. However, the geochemical signature of contamination is easily recognized and is a useful exploration guide because it identifies magmas that had the capacity to incorporate crustal material. Several aspects of the ore-forming process remain poorly understood, including the control of mantle melting processes on the PGE contents of mafic-ultramafic magmas, the mechanisms by which sulfur is transferred from wall rocks to ores (bulk assimilation, incongruent melting, and/or devolatilization), the distances and processes by which dense sulfide melts are transported from where they form to where they become concentrated (as finely-dispersed droplets, as segregated layers, or by deformation-driven injection of massive sulfide accumulations), and the dynamic processes that increase the metal contents of the ores

Magmas Erupted during the Main Pulse of Siberian Traps Volcanism were Volatile-poor

The eruption of the Siberian Traps Large Igneous Province (SLIP) at the Permo-Triassic boundary was synchronous with environmental degradation and the largest known mass extinction in the geological record. The volatile emissions associated with these eruptions have been linked to the environmental change yet we understand little of their source and magnitude and how they varied with time. There are a number of possible sources for the volatiles that were emitted during the eruptions: the mantle (including metasomatized lithosphere), volatile-rich sediments (through metamorphism or direct assimilation) and the crustal basement. To assess the relative importance of these sources (with the exception of the metamorphic outgassing source), we have conducted a geochemical study of melt inclusions hosted by clinopyroxene in Siberian Traps low-Ti tholeiitic lavas and sills of the Khakanchansky, Ayansky and Khonnamakitsky Formations. The magmas were not emplaced into or erupted onto evaporite deposits, in contrast to samples studied previously. The trace element compositions of the melt inclusions are highly variable compared with the uniform whole-rocks, exhibiting a wide range of La/Yb ratios from 0·7 to 9·5. The melt geochemistry is consistent with relatively large degrees of partial melting of a dominantly peridotite mantle source. A negative Nb anomaly indicates a degree of crustal contamination, but there is no evidence for contamination by volatile-rich evaporites. Enrichment of some of the melts in large ion lithophile elements (Ba, Sr) indicates their interaction with a fluid. We suggest that, consistent with the observed depletion in other incompatible trace elements in the melt inclusions, the volatile concentrations in the melts were relatively low, and that subsequently the melts underwent variable degrees of degassing in the crust. Overall, the melts are more volatile-poor than those reported previously from the SLIP and were erupted after the first “pulse” of more volatile-rich magmas described by Sobolev et al. (2015). These volatile-poor magmas may have been widespread across the region during the Siberian Traps eruptions once a pyroxenite component in the mantle source had been exhausted

The Aguablanca Ni–(Cu) sulfide deposit, SW Spain: geologic and geochemical controls and the relationship with a midcrustal layered mafic complex

The Aguablanca Ni–(Cu) sulfide deposit is hosted by a breccia pipe within a gabbro–diorite pluton. The deposit probably formed due to the disruption of a partially crystallized layered mafic complex at about 12– 19 km depth and the subsequent emplacement of melts and breccias at shallow levels (<2 km). The ore-hosting breccias are interpreted as fragments of an ultramafic cumulate, which were transported to the near surface along with a molten sulfide melt. Phlogopite Ar–Ar ages are 341– 332 Ma in the breccia pipe, and 338–334 Ma in the layered mafic complex, and are similar to recently reported U–Pb ages of the host Aguablanca Stock and other nearby calcalkaline metaluminous intrusions (ca. 350–330 Ma). Ore deposition resulted from the combination of two critical factors, the emplacement of a layered mafic complex deep in the continental crust and the development of small dilational structures along transcrustal strike-slip faults that triggered the forceful intrusion of magmas to shallow levels. The emplacement of basaltic magmas in the lower middle crust was accompanied by major interaction with the host rocks, immiscibility of a sulfide melt, and the formation of a magma chamber with ultramafic cumulates and sulfide melt at the bottom and a vertically zoned mafic to intermediate magmas above. Dismembered bodies of mafic/ultramafic rocks thought to be parts of the complex crop out about 50 km southwest of the deposit in a tectonically uplifted block (Cortegana Igneous Complex, Aracena Massif). Reactivation of Variscan structures that merged at the depth of the mafic complex led to sequential extraction of melts, cumulates, and sulfide magma. Lithogeochemistry and Sr and Nd isotope data of the Aguablanca Stock reflect the mixing from two distinct reservoirs, i.e., an evolved siliciclastic middle-upper continental crust and a primitive tholeiitic melt. Crustal contamination in the deep magma chamber was so intense that orthopyroxene replaced olivine as the main mineral phase controlling the early fractional crystallization of the melt. Geochemical evidence includes enrichment in SiO2 and incompatible elements, and Sr and Nd isotope compositions (87Sr/86Sri 0.708–0.710; 143Nd/144Ndi 0.512–0.513). However, rocks of the Cortegana Igneous Complex have low initial 87Sr/86Sr and high initial 143Nd/144Nd values suggesting contamination by lower crustal rocks. Comparison of the geochemical and geological features of igneous rocks in the Aguablanca deposit and the Cortegana Igneous Complex indicates that, although probably part of the same magmatic system, they are rather different and the rocks of the Cortegana Igneous Complex were not the direct source of the Aguablanca deposit. Crust–magma interaction was a complex process, and the generation of orebodies was controlled by local but highly variable factors. The model for the formation of the Aguablanca deposit presented in this study implies that dense sulfide melts can effectively travel long distances through the continental crust and that dilational zones within compressional belts can effectively focus such melt transport into shallow environments

Hercynian late-post-tectonic granitic rocks from the Fornos de Algodres area (Northern Central Portugal)

The Fornos de Algodres Complex (FAC) comprises several intrusions of late-post-tectonic Hercynian granitic rocks ranging in composition from hornblende granodiorites and quartz monzodiorites, through coarse porphyritic biotite granites and two-mica granites (coarse-, medium- and fine-grained), to muscovite-rich leucogranites. Field and regional constraints show that the emplacement of this large, composite, batholithic complex post-dates the main Variscan regional deformation phases (D1 + D2 + D3) and associated metamorphic events. Field, petrographic and geochemical data suggest a strong genetic relationship between most of the members of the FAC. However, their Rb-Sr and Sm-Nd isotopic signatures appear to rule out any genetic process involving a single homogeneous source and/or closed-system fractional crystallization of the same parental magma. A model involving hybridization of mantle-derived basaltic liquids with crustal anatectic melts followed by further contamination and fractional crystallization is proposed to explain the isotopic and geochemical variation trends defined by the FAC granitic rocks

Composition, crystallization conditions and genesis of sulfide-saturated parental melts of olivine-phyric rocks from Kamchatsky Mys (Kamchatka, Russia)

Highlights • Parental melts of sulfide-bearing KM rocks have near primary MORB-like composition. • Crystallization of these S-saturated melts occurred in near-surface conditions. • Extensive fractionation and crustal assimilation are not the causes of S-saturation. • S content in melts can be restored by accounting for daughter sulfide globules. Abstract Sulfide liquids that immiscibly separate from silicate melts in different magmatic processes accumulate chalcophile metals and may represent important sources of the metals in Earth's crust for the formation of ore deposits. Sulfide phases commonly found in some primitive mid-ocean ridge basalts (MORB) may support the occurrence of sulfide immiscibility in the crust without requiring magma contamination and/or extensive fractionation. However, the records of incipient sulfide melts in equilibrium with primitive high-Mg olivine and Cr-spinel are scarce. Sulfide globules in olivine phenocrysts in picritic rocks of MORB-affinity at Kamchatsky Mys (Eastern Kamchatka, Russia) represent a well-documented example of natural immiscibility in primitive oceanic magmas. Our study examines the conditions of silicate-sulfide immiscibility in these magmas by reporting high precision data on the compositions of Cr-spinel and silicate melt inclusions, hosted in Mg-rich olivine (86.9–90 mol% Fo), which also contain globules of magmatic sulfide melt. Major and trace element contents of reconstructed parental silicate melts, redox conditions (ΔQFM = +0.1 ± 0.16 (1σ) log. units) and crystallization temperature (1200–1285 °C), as well as mantle potential temperatures (~1350 °C), correspond to typical MORB values. We show that nearly 50% of sulfur could be captured in daughter sulfide globules even in reheated melt inclusions, which could lead to a significant underestimation of sulfur content in reconstructed silicate melts. The saturation of these melts in sulfur appears to be unrelated to the effects of melt crystallization and crustal assimilation, so we discuss the reasons for the S variations in reconstructed melts and the influence of pressure and other parameters on the SCSS (Sulfur Content at Sulfide Saturation)