A FLUID INCLUSION AND STABLE ISOTOPE (dD AND d18O) STUDY OF GRANITE-HOSTED MINERAL DEPOSITS OF THE NEW ROSS AREA, SOUTH MOUNTAIN BATHOLITH, NOVA SCOTIA, CANADA

The New Ross area in Nova Scotia hosts several polymetallic (Cu, Mo, Mn, Sn, U, W) mineral deposits. The hosting granitoid rocks belong to the Late Devonian (372±3 Ma) peraluminous South Mountain Batholith (SMB) consisting of a number of coalesced plutons. The New Ross Pluton is one of the late-stage intrusions composed of monzogranite, leucomonzogranite, and leucogranite. The mineralized areas occur in aplite/pegmatite, greisen, and veins in close spatial association with late-stage leucogranites.

A detailed fluid inclusion study provides the necessary data to characterise the fluids involved with metals acquisition, transportation, and deposition (fluid compositions and origin(s), P-T conditions during fluid entrapment). The data reveal the presence of fluids of inferred magmatic, metamorphic, and meteoric origin. For the greisen and pegmatitic mineral deposits, the range of entrapment temperatures suggests that fluids were trapped at constant pressure (the pressure of the SMB at 372 Ma), under conditions of decreasing temperature (500° to 200°C). For the vein type deposits, the homogenisation temperatures are too high for meteoric water, implying a lower pressure than that at 372 Ma. These data suggest that the vein deposits were either connected to the surface through fractures, or that they did not form at the same depth as greisen and pegmatite deposits, implying both general uplift or exhumation of the area and a temporal break in hydrothermal activity (compared to the 372 Ma magmatic age of the SMB).

Stable isotope data (dD and d¹⁸O) from mineral separates and fluid inclusion extracts confirm the existence of three different reservoirs (magmatic, meteoric, and metamorphic) for the fluids present in the New Ross area. Large ranges for dD and d¹⁸O data exist both within and among deposits and cannot be explained by a temperature range solely. Therefore, a model of fluid evolution involving mixing of fluids with different isotopic signature is proposed.