Granite-hosted mineral deposits of the New Ross area, South Mountain Batholith, Nova Scotia, Canada: P, T and X constraints of fluids using fluid inclusion thermometry and decrepitate analysis

Abstract

The 370 Ma peraluminous South Mountain Batholith (SMB) intrudes Meguma Supergroup metasedimentary rocks in Nova Scotia. The New Ross area of the SMB contains polymetallic mineralisation (Sn, W, U, Mo, Cu and Mn) in pegmatite, greisen and vein directly or indirectly associated with highly evolved fractions of the SMB. Eight mineral deposits from this area have several fluid inclusion types hosted by quartz: (1) monophase liquid (L); (2) monophase vapour (V); (3) aqueous, L-V (4) aqueous, L-rich + solids; (5) aqueous, L-rich + halite. Inclusions have irregular to equant shapes and are pseudo-secondary or secondary. The irregularity and variability of L:V ratios within fluid inclusion populations suggest post-entrapment modifications of inclusions (i.e. necking).Thermometric data indicate three distinct fluids in terms of salinity: (1) 19-25 wt. % equiv. NaCl (rarely 14-25 wt. % NaCl equiv.), (2) 29-43 wt. % equiv. NaCl, and (3) 0-9 wt. % equiv. NaCl. Temperatures of first melting and ice/hydrohalife melting indicate CaCl2 in solution. Proximity of the deposits to Meguma Supergroup metasedimentary rocks suggests that this Ca component may be externally derived. The majority of the low-salinity fluid population has the composition of meteoric water. Electron microprobe analyses of artificially decrepitated mounds identify Na, Ca and K as major solutes, with a continuum in terms of compositions. Other solute components in the mounds are Fe and Ba, and a variety of metals of unknown speciation also occur (Cu, Zn, Fe, Ni). Homogenisation temperatures (Th) range from c. 80°C to 370°C, but for inclusion assemblages the range is 10°C to 20°C. Given the 3 kbar depth of emplacement of the SMB, estimated entrapment temperatures are c. 200°C to 550°C. The fluid inclusion data appear to reflect: (1) trapping of mixed Na-K-Ca brines during isobaric cooling in pegmatite and greisen deposits as indicated by large ranges in Th; (2) formation of deposits at different ambient pressures (i.e. depth); and (3) mixing of fluids of different reservoirs (i.e. magmatic, metamorphic, meteoric).