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

Development and Microstructural Improvement of Spin Cast High-Speed Steel Rolls

A detailed microstructural analysis was conducted on a series of radial shell samples extracted from commercially produced centrifugally spin casted high-speed steel (HSS) work rolls for finishing hot strip mills (HSM). The systematic microstructural analysis was coupled with a numerical and experimental investigation to improve the life of HSS rolls. An integrated computational-experimental approach was developed to optimize the response of the HSS roll material that permitted the enhancement of the microstructure and properties of the HSS roll shell layer. Local continuous microstructural transformations through the thickness of the shell: carbide formation, precipitation, dissolution sequence and phase changes, were studied in great details. The analyses were conducted with the aid of advanced metallographic and experimental methods, finite-element (FE) analysis, and using commercial software systems to conduct thermodynamic-kinetics predictions. In order to analyze a response of the HSS roll to the hardening heat treatment (HT) and to control stress-strain evolution, a 3-D FE model was developed of the composite structure of the roll. The multilayered model considers nonlinear material properties of each individual layer as a function of temperature, based on measured chemical composition gradients through the HSS shell. Transient coupled thermal-stress analysis was performed, using actual measured surface temperatures as boundary conditions (BC) for the FE model. The allowable thermal stress-strain levels were established and compared with a) thermodynamically predicted high temperature mechanical properties and b) room temperature test results of the shear strengths for the shell, bonding and core. In addition, sub-structuring and image-based processing techniques were implemented to aid in the development of a meso-scale FE model to simulate the local response of a given microstructural constituents and matrix under particular thermal conditions. The fundamental interpretation of multilayered structure and multi-scale approach help to understand the kinetics phenomena associated with continuous local microstructural transformations due to nonlinear heat transfer. The results from the microstructural observations were in good agreement with the numerical predictions. The major impact of this work clearly indicated that a refined as-cast structure prior to the heat treatment promoted an increased precipitation of carbides during final hardening, which greatly improved strength and performance. A non-conventional HT was defined and implemented in order to provide an additional degree of microstructural pre-conditioning, which homogenized the matrix throughout the HSS shell. The new HT defined the austenitization temperatures and times to modify the morphology of brittle interdendritic eutectic carbide networks and, hence, facilitating the kinetics of dissolution of these carbides. This behavior caused an increase in the solute content of the matrix. As a result, the matrix hardness and strength were increased during subsequent hardening HT in comparison to the conventional HT routes used for as-cast HSS rolls. Reports about rolls with the new material that have been placed in service indicate that the rolls last 50-70% longer

Managing uncertainty in exploration targeting

Mineral exploration is one of the best examples of a business run by judgement under conditions of extreme uncertainty. A study of manual targeting exercises over several groups, on several continents has revealed that targets derived by human-data interaction are fraught with systemic uncertainties dominated by the mineralisation model used (reflecting the preferences/experience of the explorer), how this is translated to a targeting model, and the inability to systematically apply the targeting model over geoscience datasets. Targets generated tend to show some clustering between groups, usually towards areas of outcrop or known mineralisation (a problem when the best opportunities are likely under cover), but very different spreads in ranking. The stochastic uncertainties of the data are important but secondary to these systemic uncertainties. \ud \ud Automated prospectivity analysis methods applied in GIS, although affected by systemic uncertainties in the selection of predictor maps, can partially mitigate the biases of human data interaction but in turn are severely affected by the stochastic uncertainties.\ud \ud Examples from several terranes are used to illustrate that a combination of manual and automated approaches can best manage these uncertainties and enhance the confidence of mineral exploration targets. Keys to applying this approach are: (1) creation of appropriate derived datasets and predictor maps to overcome stochastic uncertainty in areas of cover or poor quality data, (2) a mineral systems approach in generating targeting models, (3) application of manual targeting, followed by automated knowledge- and data-driven approaches in GIS, and (4) final refinement of manual targeting for final target decisions

Global constraints on exhumation rates during porphyry copper formation and supergene enrichment: applications to exploration as illustrated from the Central Andes

Currently, 60% of the world’s copper production comes from porphyry copper deposits, often significantly enriched by surface weathering. This paper uses new global datasets and previous work to review the critical processes required for porphyry copper formation and supergene enrichment. Porphyry copper formation requires a subducting arc to create a source magma which traverses a thickened crust subject to high exhumation rates during formation, ranging from 100’s to 1,000’s m/m.y. High exhumation rates potentially trigger magma decompression, causing fluid release, opening fluid pathways along faults and lineaments and/or facilitating telescoping, whereby early porphyry-style mineralization is overprinted and enriched by high-sulfidation mineralization at shallower crustal levels. Later supergene enrichment of the deposit requires precipitation rates \u3e 120 mm/yr and exhumation rates ranging from 10’s to 100’s m/m.y. This allows copper sulfide sources to be continually refreshed for weathering but restricts the amount of erosion. Using the Central Andes, one of the world’s most critical porphyry copper provinces, the understanding gained from analyzing these global databases can explain the temporal and spatial pattern of known deposits. These constraints were used to inform mappable target criteria and data required for mineral exploration at a range of different scales, from orogen (\u3e 100,000 km²), to terrane (100,000–1,000 km²) to arc (1,000–100 km²). The results can be used to help illustrate and inform global exploration strategies for supergene-enriched porphyry copper deposits

Multistage Mineralization of the Inata Gold Deposit, Burkina Faso: Insights from Sulphide and Fluid Inclusion Geochemistry

International audienceThe Inata gold deposit formed during the Eburnean orogenesis and is hosted in meta-volcano-sedimentary rocks. Gold mineralization is directly linked to disseminated arsenopyrite and pyrite assemblages which are associated to a quartz-ankerite-albite-tourmaline vein system that formed inside a N-NNE-striking, steeply dipping, D2 deformation zone. LA-ICP-MS analyses reveal that early sulphides, which formed parallel to the metamorphic D1 foliations, have low Au/Ag ratios of 40) in contrast to late D2 sulphide overgrowths which show more or less equal Au/Ag ratios. These assemblages are locally crosscut by narrow D3 fault corridors, where visible gold is observed within fractured sulphides and is interpreted being the result of late stage remobilization during D3. Extraction of fluids indicates that veins associated to D1 and D3 are characterized by high CO2 content whereas especially early D2 veins are dominated by H2O-rich fluids. δD values of fluid inclusions versus δ 18 O isotopic compositions of hosting quartz veins indicate two distinct sources, one of metamorphic/magmatic origin, rich in H2O, and the other one being the result of extensive fluid-rock interactions with C-rich volcano-sediments

Paleogene post-collisional lamprophyres in western Yunnan, western Yangtze Craton: Mantle source and tectonic implications

A suite of lamprophyres, spatially associated with mafic lavas and potassic felsic intrusive rocks, was emplaced between 36.5±0.2 and 33.7±0.5Ma (based on phlogopite 40Ar/39Ar dating) on the eastern side of the Ailao Shan-Red River shear zone in the western Yangtze Craton. These shoshonitic and ultrapotassic intrusive rocks post-date the ~60-55Ma collisional event between the Indian and the Asian continents. They are characterized by: (1) enrichment in large-ion lithophile elements and light rare-earth elements with (La/Sm)n =3.15-7.15; (2) strong positive Pb spikes; (3) depletion in high-field-strength elements (e.g. Nb/La=0.08-0.98); (4) high initial 87Sr/86Sr (0.706-0.709) with negative eNd(t) values of -10.5 to -0.9; (5) old Nd model ages of 1542-945Ma; and (6) radiogenic (207Pb/204Pb)i of 15.57-15.70 and (208Pb/204Pb)i (38.70-39.06). These features suggest that the mantle source was metasomatized by Proterozoic subduction beneath the Yangtze Craton. The lamprophyres have similar trace element patterns, and Sr-Nd-Pb isotope compositions, as coeval mafic lava, indicating a common source of metasomatized veined continental lithospheric mantle (CLM). Lower degree partial melting of metasomatic veins likely generated the lamprophyres, whereas the coeval mafic lava was likely derived from melting of phlogopite harzburgite. The lamprophyres and mafic lava have similar Sr-Nd isotope systematics as CLM-derived Neoproterozoic mafic rocks and Late Permian Emeishan low-Ti basalt in the region, indicating that they share the same Proterozoic source. We envisage that mantle plumes thermally eroded the Proterozoic metasomatized CLM beneath the western part of the Yangtze Craton during 825-750Ma and 260-250Ma, although residual metasomatized domains remained before being tapped by delamination after the India-Asia continental collision during the Paleogene period

Generation of Early Indosinian enriched mantle-derived granitoid pluton in the Sanjiang Orogen (SW China) in response to closure of the Paleo-Tethys

A diverse suite of intermediate to felsic rocks from the Baimaxueshan pluton within the Sanjiang Orogen, SW China, documents the tectonomagmatic history of closure of the eastern Paleo-Tethys and associated arc accretion onto the margin of Asia. The pluton consists mainly of diorite, tonalite and granodiorite, and contains mafic magmatic enclaves (MMEs). SHRIMP U–Pb analyses on zircons from the MMEs and host granitoids yield crystallization ages of 253–248 Ma. Considering analytical errors, the age of MMEs and their host granitoids are indistinguishable with a weighted mean of 249 ± 1 Ma (N = 71, MSWD = 1.09), indicating Early Triassic emplacement. They display similar chemical and isotopic characteristics with a medium- to high-K, calc-alkaline, metaluminous I-type character, and similar whole rock Sr–Nd isotopic ratios, zircon εHf (t) (− 10 ± 1) and identical zircon δ18O (> 8‰ VSMOW). These observations, in combination with variably elevated Mg# (0.53–0.65), MgO content and Cr and Ni concentrations for these rocks suggest an origin from hydrous partial melting of subduction-modified subcontinental lithospheric mantle (SCLM).The MMEs may represent a mantle-derived mafic component that replenished the magma chamber and which survived mixing with the evolved host magma. The isotopically evolved signatures and arc-like pattern of incompatible elements on a primitive-mantle normalized spidergram suggest considerable input of supracrustal materials to the SCLM magma source (20–30% estimated by mixing calculation based on Sr–Nd isotopes). The striking resemblance of the zircon Hf–O isotopic systematics in both MMEs and host rocks also favors a crustal recycling paradigm dominated by source mixing rather than assimilation en route, and suggests that zircon crystallization commenced after the ingestion of supracrustal materials. Fractional crystallization operated during magma ascent and emplacement, and produced the wide spectrum of rock types seen in the pluton. Our geochronological and geochemical results also demonstrate that the Early Triassic Baimaxueshan pluton constitutes a part of the volcano-plutonic arc created by westward-directed Jinshajiang subduction system, and cannot be treated as the northern continuation of the Lincang batholith in the southern Lancangjiang zone

Generation of Early Indosinian enriched mantle-derived granitoid pluton in the Sanjiang Orogen (SW China) in response to closure of the Paleo-Tethys

A diverse suite of intermediate to felsic rocks from the Baimaxueshan pluton within the Sanjiang Orogen, SW China, documents the tectonomagmatic history of closure of the eastern Paleo-Tethys and associated arc accretion onto the margin of Asia. The pluton consists mainly of diorite, tonalite and granodiorite, and contains mafic magmatic enclaves (MMEs). SHRIMP U-Pb analyses on zircons from the MMEs and host granitoids yield crystallization ages of 253-248 Ma. Considering analytical errors, the age of MMEs and their host granitoids are indistinguishable with a weighted mean of 249 +/- 1 Ma (N = 71, MSWD = 1.09), indicating Early Triassic emplacement. They display similar chemical and isotopic characteristics with a medium- to high-K, calc-alkaline, metaluminous I-type character, and similar whole rock Sr-Nd isotopic ratios, zircon epsilon(Hf) (t) (-10 +/- 1) and identical zircon delta O-18 (&gt;8 parts per thousand VSMOW). These observations, in combination with variably elevated Mg# (0.53-0.65), MgO content and Cr and Ni concentrations for these rocks suggest an origin from hydrous partial melting of subduction-modified subcontinental lithospheric mantle (SCLM). The MMEs may represent a mantle-derived mafic component that replenished the magma chamber and which survived mixing with the evolved host magma. The isotopically evolved signatures and arc-like pattern of incompatible elements on a primitive-mantle normalized spidergram suggest considerable input of supracrustal materials to the SCLM magma source (20-30% estimated by mixing calculation based on Sr-Nd isotopes). The striking resemblance of the zircon Hf-O isotopic systematics in both MMEs and host rocks also favors a crustal recycling paradigm dominated by source mixing rather than assimilation en route, and suggests that zircon crystallization commenced after the ingestion of supracrustal materials. Fractional crystallization operated during magma ascent and emplacement, and produced the wide spectrum of rock types seen in the pluton. Our geochronological and geochemical results also demonstrate that the Early Triassic Baimaxueshan pluton constitutes a part of the volcano-plutonic arc created by westward-directed Jinshajiang subduction system, and cannot be treated as the northern continuation of the Lincang batholith in the southern Lancangjiang zone. (C) 2012 Elsevier B.V. All rights reserved.</p

Age and origin of the Bulangshan and Mengsong granitoids and their significance for post-collisional tectonics in the Changning–Menglian Paleo-Tethys Orogen

The subduction and collision tectonics in the Sanjiang region provide important insights into the evolution of the Paleo-Tethyan Orogen. Here we present LA-ICP-MS zircon U–Pb ages, Sm–Nd and Lu–Hf isotopic data, whole-rock major and trace element compositions of the Late Triassic magmatic rocks from the Mengsong and Bulangshan granitoids in this region. Zircon U–Pb dating of the Bulangshan and Mengsong granitoids yields crystallization age of 216–228 Ma, with zircon εHf(t) values ranging from −15.2 to −1.5. These granitoids are high-K calc-alkaline and muscovite-bearing. The rocks show peraluminous nature, and are enriched in large ion lithophile elements (Rb, K, U, and Th) and depleted in high field strength elements (Nb and Ti). They also display enrichment in light rare earth elements with strong negative Eu anomalies (δEu = 0.16). The geochemical and isotopic features suggest that the primary magma of the Bulangshan and Mengsong granitoids most likely originated from partial melting of the underlying Proterozoic metasedimentary rocks. The granitoid magmatism is correlated with post-collisional tectonics, and provides important constraints on the timing of closure of the Paleo-Tethyan Ocean