Solubility of Ore Minerals in Magmatic-Hydrothermal Deposits: Constraints from Natural and Synthetic Fluid Inclusions

Abstract

Magmatic-hydrothermal ore deposits are among the most important resources of many elements that are crucial for our technologically oriented civilization. Fluids expelled from crystallizing and cooling plutons are the subject of this study, as they are responsible for the transport and selective concentration of many economically important elements, such as, e.g., Cu, Mo, Sn, W and Au. To understand the potential of such magmatic-hydrothermal fluids to transport elements of interest, the trace element concentrations in fluid inclusions in minerals need to be analyzed quantitatively, and in addition synthetic inclusions need to be prepared and analyzed at relevant geological conditions. To reach these goals a new analytical LA-ICP-MS setup was developed based on the combination of a femtosecond-laser with a fast scanning sector field ICP-MS. An experimental protocol for the formation of synthetic fluid inclusions, trapped from fluids at high temperatures and pressures, was tested and refined to ensure the achievement of equilibrium during syntheses. Concentrations of Mo, W and Au in synthetic fluid inclusions in equilibrium with various ore minerals were determined at different T, P, salinity, fO2 and fS2 conditions and compared to existing natural and experimental data as well as results from thermodynamic modelling. With W being of tremendous economic importance, a particular attention was given to the determination of the solubility of the two most abundant W minerals, i.e., wolframite and scheelite. According to our experiments, T is the most important parameter controlling W transport and deposition, followed by the salinity of the fluid, fO2 and fS2. Pressure has no significant effect in the investigated range of 100-300 MPa. The temperature dependence of wolframite and scheelite solubility is different, which may explain natural observations indicating a decreasing wolframite/scheelite ratio with depth in some W-deposits, or the dominance of ferberite over scheelite in many W-deposits. In addition, geochemical interaction with country-rocks plays an important role in the formation of magmatic-hydrothermal W-deposits, which is supported by our study of natural fluid inclusions from the Panasqueira W-Sn-(Cu-)deposit in Portugal. Fluids from the Main Sulfide Stage at Panasqueira further show a sharp decrease of Cu and As concentrations concomitant to the precipitation of chalcopyrite and arsenopyrite and were likely derived from a single pulse of a rather homogenous magmatic-hydrothermal fluid. The combination of our experimental data with measured W concentrations in natural fluid inclusions from Panasqueira indicate that W concentrations during the mineralization stages (~ 1-70 ppm W) are ca. one to two orders of magnitude higher than previously estimated (~ 0.2 ppm W). This results in a reduction of the required amount of hydrothermal fluid to ca. 10-100 km3 to explain the amount of W precipitated in the Main Oxide Silicate Stage. This finding diminishes, but does not preclude, the necessity of the involvement of meteoric fluids in the formation of the Panasqueira W-Sn-(Cu-)deposit.