Reduced orthomagmatic C-O-H-N-NaCl fluids in the Strange Lake rare-metal granitic complex, Quebec/Labrador, Canada

The Strange Lake complex is a small, Late-Proterozoic pluton containing F-enriched hypersolvus and subsolvus peralkaline granites with unusually high concentrations of Y, Zr, REE, Nb, Be. The subsolvus granite hosts abundant pegmatites and displays extensive evidence of hydrothermal alteration. Quartz in the pegmatites contains primary inclusions concentrated along growth zones, and pseudosecondary inclusions in the cores of crystals. Four types of fluid inclusions have been recognized: aqueous, liquid-rich (type I); CH 4 , vapour-only (type II); aqueous-CH 4 (type III) and higher hydrocarbon-bearing (fluoresce under UV light; type IV). Coexisting type-III inclusions show highly variable aqueous/carbonic phase ratios, suggesting that the system comprised separate aqueous and carbonic fluids. Type-I inclusions are accurately modeled by the system H 2 O-NaCl, and mainly have salinities close to the eutectic. A small proportion of type-I inclusions have lower salinities. The aqueous phase in most type-III and type-IV inclusions also has a salinity close to the H 2 O-NaCl eutectic. The salinities of type-III inclusions that appear to condense a hydrocarbon liquid which does not fluoresce under UV light are similar to those of the lower-salinity type-I inclusions. Both type-I and type-III inclusions homogenize mainly at temperatures above 300 o C. The CH 4 phase displays critical homogenization. Raman analyses failed to detect CO 2 in the CH 4 -bearing inclusions. Significant N 2 is, however, present, with X CH4 as low as 0.2 in N 2 -rich inclusions. The latter belong to the low-salinity type-III inclusion group. The trapping temperature of the inclusions was estimated to be approximately 340 o C, i.e., the modal homogenization temperature of the aqueous inclusions; the trapping pressure was estimated to be 70 MPa, from the intersection of this isotherm with the critical isochore of CH 4 . Both aqueous and hydrocarbon fluids are interpreted to have originated from the magma, possibly in two stages of aqueous-carbonic fluid exsolution marked by changes in the salinity and N 2 contents of the exsolved fluids. The presence of CH 4 in the absence of CO 2 is unusual, and to our knowledge, has not been previously reported from siliva-saturated igneous rocks, but is consistent with the low fO 2 estimated for the subsolvus granite