COMBINING FLUID INCLUSION AND STABLE ISOTOPE ANALYSES WITH NUMERICAL MODELLING TO RECONSTRUCT MAGMATIC-HYDROTHERMAL ORE-FORMING SYSTEMS

Magmatic-hydrothermal systems form a great wealth of economic mineral resources, such as Cu-Mo-Au porphyry, Au-Ag epithermal, and Sn-W greisen and vein-type deposits. With exploration targets moving to deeper crustal levels there is a growing need to understand the geological and hydrological controls on metal enrichments on the scale of entire ore-forming systems from the evolution of the driving magmatic system to precipitation mechanisms at the deposit itself. We present new advances combining fluid inclusion and stable isotope analyses with numerical modelling to reconstruct magmatic-hydrothermal ore-forming processes. The results elucidate how ore formation critically depends on the balance between the expulsion of magmatic fluids from incrementally recharged magma chambers and convection of ambient hydrothermal fluids, which can be affected by near-surface hydrology. A new body of evidence by combining fluid inclusion and oxygen isotope data from hydrothermal ore-stage quartz supports previous numerical results indicating that ore precipitation in porphyry Cu deposits happens during cooling at the onset of fluid mixing at the transition between ductile and brittle rock behavior. Cooling and fluid mixing can also be demonstrated to cause ore precipitation in a number of Sn-W deposits. However, recent results from the lode-type Panasqueira W-Sn-Cu deposit (Portugal), the greisen and vein-type Zinnwald Sn-W-Li deposit (Germany/Czech Republic) and the skarn-hosted Hämmerlein Sn-Zn-In deposit (Germany) suggest a variety of triggers for ore formation, including fluid-rock interaction and phase separation of magmatic fluids in absence of meteoric fluids. These results also show that key information may not be captured by quartz-hosted fluid inclusions but can be revealed with the use of geochemical thermometers, such as the Ti-in-quartz method or the newly described boron-isotope mica-tourmaline thermometry, or the analyses of fluid inclusions in the ore minerals themselves using infra-red microscopy. The remaining key challenges for these reconstructions are the geological complexity of the deposits and the accurate numerical representation of the magmatic-hydrothermal transition, chemical fluid-rock reactions, structural controls and dynamic permeability variations.