AN INTEGRATED FLUID MINERAL STABLE-ISOTOPE STUDY OF THE GRANITE-HOSTED MINERAL DEPOSITS OF THE NEW ROSS AREA, SOUTH MOUNTAIN BATHOLITH, NOVA SCOTIA, CANADA: EVIDENCE FOR MULTIPLE RESERVOIRS

Abstract

Granitic rocks in the New Ross area, Nova Scotia, host polymetallic (Cu, Mo, Mn, Sn, U, W), aplite‐pegmatite, greisen, and vein-type mineral deposits. The host granitic rocks belong to the Late Devonian (380 Ma) peraluminous South Mountain Batholith (SMB), which consists of several coalesced plutons. Among these, the New Ross Pluton, composed of monzogranite, leucomonzogranite, and leucogranite, is one of the youngest. White mica separates from aplite‐pegmatite, greisen, and veins have 18 O values between +4.0 and +10.0‰, and D values between ‐42 and ‐108‰. Fluid-inclusion extracts from quartz in samples from the area of the same deposits have D values between ‐42 and ‐97‰. The isotopic composition of fluids in equilibrium with the white mica and isotopic data for the fluid-inclusion extracts record the transition from an early, orthomagmatic stage, in which magmatic fluid dominated (i.e., aplites, pegmatites), to a subsequent hydrothermal stage (i.e., greisens, veins) where an increasing amount of another fluid, inferred to be meteoric water, infiltrated the systems and mixed with the magmatic fluid. In addition, the low D values for fluid-inclusion extracts compared with the Dfluid values calculated for coexisting white mica samples are considered to record infiltration of a second meteoric water much later in the evolution of the SMB. Integration of the new stable isotope data with our previous results on fluid inclusions suggests the following history of fluid evolution within the New Ross area: (1) exsolution of a magmatic fluid at ~600°C, (2) incursion of a low- 18 O, high-D meteoric fluid at ca. 380 Ma through faults, and subsequent mixing with magmatic and metamorphic fluids through circulation by convection before cooling to ~400‐500°C, and (3) incursion at a much later time of a low-D meteoric fluid, mostly affecting the composition of fluid-inclusion extracts.