BREACHING OF VOLATILE-RICH FELSIC MAGMAS: A KEY STAGE IN THE FORMATION OF MAGMATIC-HYDROTHERMAL ORE SYSTEMS (Invited Presentation)

Magmatic-hydrothermal ore systems reflect focusing of metal-rich fluids sourced from volatile-rich magmas. As such enrichment is P dependent, this also implies depth is a critical factor in volatile saturation. Importantly, P also governs the nature of structures which control mineralization. Herein three varied Sn deposit settings are used to explore and argue that fluid egress in them, and similar deposits in general, was due to the breaching of volatile- and metal-rich causative felsic magmas, the requisite final stage for fluid release, and relates to the active tole of structures at the time of ore-deposit formation.

The East Kemptville (EK) Sn-Zn-Cu-Ag(-In) deposit (Nova Scotia, Canada) is hosted by the large (7300 km²), multiphase, ca. 375 Ma meta- to peraluminous batholith. The batholith’s outline was controlled by NE- and NW-trending basement structures, including NE striking shear and fault zones that localized more evolved intrusions, such as the mineralized EK topaz leucogranite. The Main Zone ore body contains NE-trending, sheeted quartz-topaz-cassiterite-sulfide greisens centred on a subvertical fault zone locally coated with quartz-sulfide fibre veins. A later, subvertical, discordant quartz-topaz-sulfide greisen and breccia body hosts the adjacent Baby Zone ore body.

The San Rafael (SE Peru) Sn(-Cu) deposit is dominated by a single, NW striking and 40-75°NE dipping vein (Veta San Rafael) traversing a small (<1 km²), high-level (<2 km) ca. 25 Ma biotite cordierite monzogranite, itself cut by chemically evolved tourmaline leucogranite dikes. That mineralization post-dates the host granite by ca. 1 Ma reflects fluid ingress from a sustained, more evolved magmatic system at depth. The main vein, a poly-genetic reverse fault-controlled breccia body, relates to regional-scale compressional tectonics active during mineralization.

The Minas Pirquitas (N Argentina) Ag-Zn-Sn-Pb deposit is centred on sheeted veins and poly-genetic breccia bodies cutting Ordovician metaturbidite rocks exposed in a horst structure surrounded by Miocene age felsic volcanic and sedimentary rocks and small felsic stocks. The subparallel vein systems formed perpendicular to the N-striking anticlinal closures in the basement rocks they cut. The heterolithic mineralized breccias, likely sourced from a buried felsic cupola, contain fragments of altered basement rock, disaggregated granite, and hydrothermal- and mineralized quartz veins.