Pilar Lecumberri-Sanchez

Salt precipitation in magmatic-hydrothermal systems associated with upper crustal plutons

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.

Fluid-rock interaction is decisive for the formation of tungsten deposits

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.

Halogen Geochemistry of Ore Deposits: Contributions Towards Understanding Sources and Processes

Most hydrothermal ore-forming fluids are dominantly aqueous chloride solutions. Due to the incompatible nature of halogens in most mineral phases, geofluid reservoirs commonly have distinct halogen geochemistry signatures. Comparison of the halogen geochemistry of ore-related fluids with that of geofluid reservoirs provides insights into sources of halogens and, by extension, of the fluids associated with ore formation. The halogen content of fluids has also direct effects in transport and deposition of metals. Addition of halide salts to H2O modifies the Pressure-Volume-Temperature-Composition (PVTX) properties of hydrothermal fluids and controls the region of the Earth’s crust in which metal partitioning and depositional mechanisms such as fluid immiscibility (boiling) may occur. Furthermore, addition of chloride salts to H2O provides negatively-charged ligands that form complexes with the positively-charged metal ions, significantly increasing metal solubilities compared to solubilities in pure H2O and enhancing the metal carrying capability of ore forming fluids. Halogens also influence the partitioning behavior of metals between different fluid phases, including liquid, vapor and melt phases and therefore determine the spatial distribution of metals in different ore deposit types and the metallogenic potential of the different fluid phases. Due to the close relationship between halogen geochemistry and the physical and chemical properties of fluids (aqueous and silicate), halogens play a significant role in ore deposit formation. As a result, some types of ore deposits show a systematic variation in time and space of halogen abundances and geochemistry, which may be used in exploration for these deposits.