Granite-hosted mineral deposits of the New Ross area, South Mountain Batholith, Nova Scotia, Canada.

摘要

The most evolved rocks of the South Mountain Batholith host polymetallic (Sn-W-U-Mo-Cu-Mn) mineral deposits in the New Ross area (NRA). This project presents new data (fluid inclusions, stable isotopes, mineral chemistry, and 40Ar/39Ar and 187Re/187Os dating) to define the hydrothermal fluids related to the mineralisation of the NRA, and to create an integrated model for the origin of the mineral deposits. Single-grain Ar/Ar laserprobe ages for white micas from unmineralised and mineralised samples range from 382 to 320 Ma. Molybdenite Re/Os ages on two pegmatite samples are 377 +/- 3 Ma and 371 +/- 3 Ma. The main phases of hydrothermal activity leading to polymetallic mineralisation occurred in close temporal relationship to granite crystallisation at ∼380 Ma, and episodic reheating events occurred at ∼371 Ma and ∼320 Ma, partially resetting the Ar/Ar system. Three distinct fluids in the mineral deposits of the NRA are: (i) a Na/K-rich magmatic fluid; (ii) a Ca-rich metamorphic fluid; and (iii) a meteoric fluid. Mixing of these fluids occurred, implying that the three fluids were contemporaneous. The isotopic compositions (delta 18O, deltaD) of white mica from unmineralised and mineralised samples of the NRA, and deltaD values for fluid inclusion extracts from coexisting quartz, record a transition from a magma-dominated, low water/rock ratio system to a fluid-dominated, higher water/rock ratio system, into which meteoric water infiltrated at the time of SMB crystallisation and greisen/vein formation. Fluctuating pH conditions of hydrothermal fluids at the contact with host granitoid rocks and variations in redox conditions, triggered by incursion of increasing amounts of meteoric fluid along deep-penetrating faults as the system evolved from magmatic to hydrothermal, caused metal deposition. Over ∼10 My, the concurrence of highly evolved granitic melts rich in incompatible elements in the late-stage of granite emplacement with intense fluid circulation, particularly those of meteoric origin, formed the NRA mineral deposits.