Description: Magmatic-hydrothermal systems associated with upper crustal plutons strongly influence volcanic and geothermal processes and form important mineral deposits. Fluids released from plutons are commonly saline and undergo phase separation into high-salinity brines and low-salinity vapors upon ascent. While brine-vapor immiscibility has been extensively studied, precipitation of solid salt during phase separation in magmatic-hydrothermal systems has generally been considered a rare phenomenon. Here we show that most porphyry deposits exhibit fluid inclusion evidence best interpreted by solid salt precipitation from ore-forming solutions. This interpretation naturally links thermodynamics, numerical simulations, and independent estimates of porphyry ore formation depths. Salt precipitation imposes major changes on the permeability of the system. Moreover, salt precipitation has implications for ore formation along the liquid-vapor-halite curve. The recognition of salt-saturated systems is challenging, but very relevant for understanding the evolution of magmatic-hydrothermal systems.

Description: Tungsten mineralization is typically associated with reduced granitic magmas of crustal origin. While this type of magmatism is widespread, economic tungsten deposits are highly localized, with ~90% produced from only three countries worldwide. Therefore, the occurrence of reduced magmatism, while necessary for tungsten enrichment, seems to be insufficient to form such rare deposits. Here we explore the mechanisms that lead to wolframite precipitation and evaluate whether they may exert a decisive control on tungsten global distribution. Tungsten differs from other rare metals enriched in magmatic-hydrothermal ore deposits because it is transported as an anionic species. Precipitation of the main tungstate minerals scheelite, CaWO 4 , and wolframite, (Fe, Mn)WO 4 , thus depends on the availability of calcium, iron, or manganese. We demonstrate quantitatively that magmatic fluids at Panasqueira, Portugal, provide tungsten in solution, whereas the host rock contributes the iron required to precipitate wolframite. The combination of special source conditions with specific reactive host rocks explains why major wolframite deposits are rare and confined to a few ore provinces globally.

Description: Panasqueira is a world-class tungsten-vein deposit. Several paragenetic stages have been proposed (Polya et al., 2000) including two pre-ore stages (crack-seal quartz-seam, and muscovite selvages) and four ore stages (main oxide-silicate stage, main sulfide stage, pyrrhotite alteration stage, and late carbonate stage). In this study, compositions of the mineralizing fluids at Panasqueira have been determined by a combination of detailed petrography, microthermometric measurements and LA-ICPMS analyses. We have characterized the fluids related to several mineralizing stages and determined the information they provide about the fluid sources in this system. Three fluid generations recorded in pseudosecondary to secondary fluid inclusions have been identified at Panasqueira. The first fluid generation identified consists of CO2-bearing fluid inclusions with homogenization temperatures ranging between 260 and 320 °C and salinities between 5 and 8 eq wt % NaCl. Petrographic constraints indicate that this first generation (1) is paragenetically related to the main oxide-silicate stage. Two lower-temperature CO2-absent fluid generations (2a and 2b) have been identified and are represented by secondary fluid inclusions postdating the main oxide-silicate stage. This stage was likely trapped under high pressures and lithostatic conditions (Jacques and Pascal, 2017). Generation (2a) consists of high-salinity (20-30 eq wt % NaCl) fluids with homogenization temperatures ranging between 180°C and 250°C. Generation (2b) consists of low-salinity (〈2 wt %) low homogenization temperature (100-150°C) fluid inclusions. Conclusive petrographic evidence of the relationship between these two late-stage fluid generations and specific late mineral stages are scarce. However, fluid compositions suggests that generation (2a) is related to the main sulfide stage and generation (2b) is related to the late carbonate stage. The PTX evolution of fluids at Panasqueira indicate a transition from magmatic dominated fluids to a likely influx of non-magmatic fluids at least in the latest stages of mineralization (main sulfide stage and late carbonate stage) which is in good agreement with recent results from isotopic studies (Codeço et al., 2017).

Description: Magmatic-hydrothermal systems associated with upper crustal plutons strongly influence volcanic and geothermal processes and form important mineral deposits. Fluids released from plutons are commonly saline and undergo phase separation into high-salinity brines and low-salinity vapors upon ascent. While brine-vapor immiscibility has been extensively studied, precipitation of solid salt during phase separation in magmatic-hydrothermal systems has generally been considered a rare phenomenon. Here we show that most porphyry deposits exhibit fluid inclusion evidence best interpreted by solid salt precipitation from ore-forming solutions. This interpretation naturally links thermodynamics, numerical simulations, and independent estimates of porphyry ore formation depths. Salt precipitation imposes major changes on the permeability of the system. Moreover, salt precipitation has implications for ore formation along the liquid-vapor-halite curve. The recognition of salt-saturated systems is challenging, but very relevant for understanding the evolution of magmatic-hydrothermal systems.

Description: More than 50 % of the world’s total reserves of tungsten are in China, and most tungsten deposits are located in the Nanling range (Cathaysia block, southeast China). This study explores the genetic relationship between W-specific magmatic events and shallow (W)-Ag-Pb-Zn deposits in the Nanling range based on data from the Wutong deposit, Guangxi Province. Because the temperatures obtained from mineral thermometry and homogenization temperatures of fluid inclusions are the same (~300°C), the fluid inclusion homogenization temperatures are interpreted to be approximately equal to the trapping temperatures, indicating formation at low pressures, i.e., slightly above the liquid-vapor curve. This, in turn,indicates that the deposit formed at relatively shallow levels. The C-O-S-Sr-Pb isotopic composition of minerals indicate that one single fluid was responsible for mineralization. The chemical composition of fluid inclusions indicates that the fluid evolved from a fractionated magma. Therefore, the Wutong deposit is likely to represent the shallow expression of a magmatic-hydrothermal system. The Sr-Pb isotopic data indicate that the magmatic fluids are associated with melting of crustal rocks of the Cathaysia block. The age of the Wutong mineralization, obtained from hübnerite dating, is Late Yanshanian (Cretaceous). Most Ag-Pb-Zn deposits in this region are typically attributed to Late Jurassic mineralization. In contrast, Wutong is a Cretaceous system formed during the Cretaceous mineralization peak in south China. As this relatively shallow hydrothermal system is related to deeper magmatism, Cretaceous mineralization is not limitted to the western limit of Cathaysia but may extend eastward at least to the Nanling range. Furthermore, regional grouping of the age of granite-related magmatism indicates that the same kind of crustal source has been melted repeatedly, possibly in different tectonic settings.