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Numerous skarn-type Sn deposits have been identified in the Nanling Range (South China), of which the Shizhuyuan W-Sn-Bi-Mo, Xianghualing Sn, Jinchuantang Sn-Bi, and Hehuaping Sn deposits are the largest. The Xianghualing deposit, which is the focus of this study, hosts a resource of 0.17 Mt Sn grading 0.93–1.39 wt% SnO2. Whether the distal skarn-type mineralization and the cassiteritesulfide vein-type orebody in the Xianghualing district are genetically related to the Laiziling granitic pluton, which produced the proximal skarn-type Sn mineralization, however, is still unknown. The Xianghualing Sn mineralization occurs exclusively as cassiterite and has been subdivided into four ore-types: (1) lenticular proximal skarn ore (Cst I) containing the mineral assemblage cassiteritepyrrhotite-chalcopyrite-actinolite-wollastonite; (2) layered distal skarn ore (Cst II) containing the mineral assemblage cassiterite-pyrrhotite-chalcopyrite-actinolite; (3) vein cassiterite-sulfide ore (Cst III) distal from the skarn and associated granite containing the mineral assemblage cassiterite-arsenopyrite-pyrrhotite-muscovite-fluorite; and (4) veinlet Sn-Pb-Zn ore (Cst IV) distal from the skarn and associated granite containing the mineral assemblage cassiterite-galena-sphalerite-topaz-quartz. Here, we report the results of in situ laser ablation inductively coupled plasma mass spectrometric (LA-ICP-MS) U-Pb age determinations for garnet from the Xianghualing skarn and the above four types of cassiterite. Our age determinations indicate that there were two independent magmatic-hydrothermal events at ~160 and 156~150 Ma, both of which led to Sn mineralization. The first Sn mineralization event at ~160 Ma (Cst IV U-Pb ages of 159.6 ± 1.4 to 158.5 ± 0.8 Ma) is interpreted to have been associated with a speculative unexposed granitic pluton, which is coeval with the nearby Jianfengling granite intrusion. The second Sn mineralization event at 156~150 Ma (Cst I to Cst III U-Pb ages of 155.9 ± 0.7 to 152.3 ± 1.1 Ma and garnet U-Pb ages of 153.6 ± 7.6 to 151.5 ± 3.5 Ma) is genetically related to the adjacent Laiziling granitic intrusion (152.8 ± 1.2 Ma, zircon U-Pb age). This event was responsible for the bulk of the Sn resource (>95%). Our age determinations provide convincing evidence for superimposed Jurassic Sn mineralizing systems at Xianghualing. They also show the value of combining garnet and cassiterite U-Pb age determinations to constrain the timing of skarn and Sn mineralization and distinguish discrete Sn mineralizing events in a protracted metallogenic history. [ABSTRACT FROM AUTHOR]

Earth is the 'blue planet,' with more than 70% of its surface covered in water and the equivalent of up to four oceans of water in its interior. This abundance of water has a profound impact on the processes that shape our planet as well as the development of the organisms that inhabit it. To understand this impact, it is necessary to know the properties and compositions of this fluid. At present, this information is largely unavailable, because direct samples of fluid are rare, especially for early Earth and Earth's interior, and other estimators are semiquantitative, at best. Here we propose a different approach in which the composition of the fluid is reconstructed from that of minerals, based on the characteristic trace element partitioning between minerals and aqueous fluids. We show experimentally that this partitioning is systematic and obeys lattice-strain theory. It depends strongly on element complexation in the fluid, but this dependence is predictable and can be accommodated. Unlike fluids, minerals with preserved compositions are readily available in the geological record, and this approach therefore provides a powerful and widely applicable tool to reconstruct a quantitative record of fluid composition for the full range of Earth environments and for its earliest history. doi: 10.1130/G31112.1

The compositions of fluid inclusions trapped in garnet, pyroxene, and sphalerite, measured with laser ablation--inductively coupled plasma--mass spectrometry, from the distal, dominantly limestone-hosted, El Mochito Zn-Pb-Ag skarn, Honduras, are very similar to those of proximal magmatic fluids in granitoid-related mineral deposits. However, the El Mochito mineralizing fluids are chemically distinct from basinal brines, demonstrating that in distal skarns, magmatic fluids dominate, and that most element concentrations and ratios are unaffected by water-rock interaction. High fluid temperatures (~325-425[degrees]C) throughout the paragenesis are also consistent with this hypothesis. A variety of elements and element ratios (e.g., Rb, Mn, K/Na, Mn/Na, Rb/Na, Zn/Na) clearly discriminates between magmatic fluids and basinal brines. Elevated Ca most likely resulted from carbonate dissolution along the flow path. Zinc and Pb contents indicate that the ore metals were precipitated in the deposit in proportions that approached their ratios in the fluid.

A single-pass flow-through experiment was conducted to induce serpentinization, and to relate serpentine mineralogy to reaction progress during open-system alteration of San Carlos olivine by air-saturated water. The fluid flowed at 300[degrees]C and 300 bars for 1368 hours through crushed olivine contained in a tubular titanium reaction cell. Chemical analysis of the fluids indicated a steady-state composition with respect to Mg and pH after 550 hours, which was interpreted to reflect saturation with brucite and one or more serpentine-group minerals. The alteration products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Nucleation and growth characteristics of serpentine phases on the fracture surfaces of olivine were studied by TEM using platinum/carbon replica techniques. After reaction, olivine fragments displayed clear signs of dissolution, and growth of neoformed phases. Brucite and magnetite were distributed uniformly through the reaction cell, although magnetite was more abundant. There is, however, a distinct zonal distribution of the serpentine phases from the inlet to the outlet of the reaction cell. Lizardite is the dominant serpentine phase at the inlet, whereas fibrous serpentines are predominant near the outlet. Lizardite crystals are more numerous (but smaller) in the middle of the cell, and are topotactically aligned along a single preferential crystallographic orientation of the olivine. Cylindrical chrysotile formed predominantly on the surfaces of lizardite, conical or polygonal serpentine. The morphology and sorting of the serpentine phases through the reaction cell suggest that crystallization of lizardite was controlled by heterogeneous nucleation on olivine under supersaturated conditions, and crystallization of chrysotile by nucleation on lizardite at higher levels of supersaturation.

The 2176Ma peralkaline and agpaitic Nechalacho Layered Suite (NLS) is the youngest and most evolved member of the Paleoproterozoic Blatchford Lake Igneous Complex, which comprises a series of gabbro, granite, quartz syenite and nepheline syenite intrusive rocks that were emplaced within the Slave Craton of Canada. The nepheline-sodalite syenites of the NLS form a laccolithic intrusive body >4km2 in plan and >1.1km deep that crops out at Thor Lake, Northwest Territories, in the centre of the complex. Igneous layering is defined by variable proportions of nepheline, sodalite, end-member aegirine and annite, K-feldspar, albite and eudialyte. Two layers in the upper part of the intrusion, the Basal Zone and the Upper Zone, host one of the world's largest rare earth element (REE) and niobium resources, the Nechalacho Rare Metal Deposit. The principal magmatic REE-Nb-mineral is kentbrooksite (a eudialyte group mineral); fluornatropyrochlore, bastn€asite-(Ce) and fluorbritholite-(Ce) occur in minor proportions. Based on whole-rock and mineral chemistry and phase equilibria for the NLS and cogenetic granitoids, the units of the Blatchford Lake Igneous Complex are considered to have formed by batch melt extraction from a deep crustal magma chamber in which a mantle-derived, heterogeneous mafic parental melt underwent fractional crystallization. The magma that crystallized to form the NLS represents the final and most evolved batch. It was essentially magnesium-free and highly enriched in Zr, REE, Nb, carbon species, fluorine and chlorine. Based on its mineralogy, the NLS crystallized from ∼800 to<500 °C at a pressure of ∼4 kbar and oxygen fugacity initially below the annite-aegirine equilibrium (QFM + 0.2 to QFM + 1.3 at 500-882 °C, where QFM is the quartz-fayalite-magnetite buffer); fO2 later increased in response to auto-oxidation of the residual melt by annite crystallization. In situ fractional crystallization of the intrusion proceeded both upwards and downwards. Early crystallization of REE-Nb-Zr minerals was inhibited owing to the low melting temperature of eudialyte. The REE, Nb, Zr and volatile species (fluorine, carbon) were enriched in an iron-rich residual melt until the magma became saturated with respect to eudialyte, forming cumulates and layers containing interstitial eudialyte in the upper part of the intrusion. Mineral fractionation during internal crystallization led to large variations in light/heavy REE, Zr/Hf and Nb/Ta, with the Basal Zone recording the strongest heavy REE enrichment. [ABSTRACT FROM AUTHOR]

Phanerozoic orogenic gold mineralization at craton margins is related to the metasomatism of the lithospheric mantle by crustal material. Slab subduction transfers Au from the crust to the metasomatized mantle and oxidizes the latter to facilitate the mobilization of Au into mantle melts. The role of volatiles in the mobilization of Au in the mantle is unclear because of the absence of direct geochemical evidence relating the mantle source of Au to Au mineralization in the overlying crust. This study uses lithium isotopes from a large suite of lamprophyres to characterize the mantle beneath the eastern North China Craton, which hosts giant Mesozoic gold deposits. Our results indicate a strong genetic link between carbonate metasomatism in the mantle and Au mineralization in the overlying crust. Although pre-enrichment of Au in the mantle is critical for forming giant Au provinces, the oxidation of the lithospheric mantle controls the mobilization of Au. [ABSTRACT FROM AUTHOR]

Most tungsten (W) and tin (Sn) deposits are associated with highly evolved granites derived from the anatexis of metasedimentary rocks. They are commonly separated in both space and time, and in the rare cases where the W and Sn mineralization are part of a single deposit, the two metals are temporally separate. The factors controlling this behavior, however, are not well understood. Our compilation of whole-rock geochemical data for W- and Sn-related granites in major W-Sn metallogenic belts shows that the Sn-related granites are generally the products of higher-temperature partial melting (~800 °C) than the W-related granites (~750 °C). Thermodynamic modeling of partial melting and metal partitioning shows that W is incorporated into the magma formed during low-temperature muscovite-dehydration melting, whereas most of the Sn is released into the magma at a higher temperature during biotite-dehydration melting; the Sn of the magma may be increased significantly if melt is extracted prior to biotite melting. At the same degree of partial melting, the concentrations of the two metals in the partial melt are controlled by their concentration in the protolith. Thus, the nature of the protolith and the melting temperature and subsequent evolution of the magma all influence the metallogenic potential of a magma and, in combination, helped control the spatial and temporal segregation of W and Sn deposits in all major W-Sn metallogenic belts. [ABSTRACT FROM AUTHOR]

Porphyry and epithermal ore deposits, which are the products of magmatic hydrothermal fluids, are intimately associated with volcanoes in continental and island arcs above subduction zones, but the exact nature of this relationship has remained enigmatic. Although metal deposition is usually thought to occur during the waning stages of volcanism, numerous ore deposits have been demonstrated to be synvolcanic. Here we show how the formation of these deposits is tied to volcanic cycles. We relate the chemical variations in vapors from Merapi volcano, Indonesia, to different stages of its eruptive cycle. The chemical compositions of volcanic vapors from subduction zone volcanoes are then compared globally to those of fluid inclusions from porphyry-epithermal deposits. These data show that adiabatic decompression is the principal control on mineralization. The data also suggest that volcanic and subvolcanic magmatic-hydrothermal systems are under lithostatic pressure during quiescence but decompress rapidly during injections of mafic magma and explosive eruptions. During quiescence, the magma evolves through fractional crystallization and devolatilization, gradually becoming oxidized and enriched in gold and other incompatible metals. Upon the injection of sulfur-rich mafic magmas, subvolcanic intrusions brecciate the overlying rocks, the systems are depressurized, the volcanoes erupt explosively, supercritical fluids unmix into vapor and brine, and base metal sulfides precipitate. [ABSTRACT FROM AUTHOR]