The genesis of the Grey River tungsten prospect: a fluid inclusion, geochemical, and isotopic study

Higgins, Neville Charles (1980) The genesis of the Grey River tungsten prospect: a fluid inclusion, geochemical, and isotopic study. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Mineralisation in the Grey River Tungsten Prospect, Newfoundland, is contained within a swarm of quartz veins and fractures which traverse the contact between a sheared Devonian (405 ± 10 Ma) K-feldspar megacrystic granite, and metamorphic rocks which are typical of the gneissic terrain of the Gander Zone. The mineralisation is genetically related to a suite of post-tectonic, highly differentiated, alkali-rich leucogranite dykes. -- The tungsten-bearing hydrothermal veins, dated at 330 Ma, range from tensional fractures and veinlets to lodes containing several injections of quartz. The mineralisation is divided into four stages; the Early Stage, characterised by quartz-feldspar-molybdenite veins; the Composite Stage, consisting of five vein types (in paragenetic order), quartz-bismuthinite, quartz-wolframite (Fe-rich), greisen, quartz-sulphide, and quartz-wolframite (Mn-rich) veins; the Sulphide Stage, characterised by silver-bearing quartz-galenasphalerite veins; and the Late Stage, composed of zoned fluorite-calcite-barite veins. A spatial mineral zonation from south to north matches the temporal sequence outlined above, with the exception of the Late Stage veins which crosscut Composite Stage veins. -- Fluid inclusion data indicate a complex evolutionary history for the hydrothermal fluid. Initially a homogeneous supercritical fluid (XCO₂ = 0.4) with a density of 0.65 g/cc deposited quartz-feldspar-molybdenite veins at temperatures of 450-500°C and fluid pressures of 1200-1400 bars (120-140 MPa). The simultaneous trapping of CO₂-rich and H₂O-rich fluid inclusions, as well as solid inclusions of calcite in quartz of the quartz-bismuthinite vein type is evidence for the existence of an immiscible heterogeneous fluid during this stage of mineralisatiion. Fluid phase equilibria indicate immiscibility occurred at temperatures of 390 - 430°C and fluid pressures of 1000-1200 bars (100-120 MPa). Oxygen isotope data indicate that greisen alteration halos of the composite lodes were formed at temperatures between 350-400°C. -- In these lodes quartz-sulphide and quartz-wolframite veins were deposited in open spaces created by normal faulting. These movements prompted rapid decreases in fluid pressure and temperature and caused retrograde boiling of the hydro-thermal fluid (at 350 bars and 300-390°C) during deposition of quartz-sulphide veins. Deposition of wolframite in quartz-wolframite veins occurred at 270-330°C and fluid pressures less than 350 bars, from an aqueous fluid of low salinity (<0.5 wt% NaCl) and CO₂ content (< 10 bars PCO₂), and after separation of a CO₂ vapour phase by retrograde boiling. -- Calculated δ¹⁸OH₂O values indicate a progressive depletion in ¹⁸O isotopic composition of the hydrothermal fluid with time (7.3 °/oo to 0.5 °/oo). The depletion is due to the loss of 40 mol% CO₂ from the hydrothermal fluid by immiscibility and retrograde boiling, and subsequent fractionation effects. The oxygen isotope data suggest that the hydrothermal fluid was dominantly of magmatic origin. -- Heavy REE enrichment and light REE depletion during greisenisation indicate extensive REE mobility, and imply both that CO₃²⁻ was the dominant anionic species in the hydrothermal fluid and that mobilisation of REE occurred by REE carbonate complexing. A positive correlation between heavy REE enrichment and high tungsten concentration suggests that REE mobility is related to tungsten transport. Together with the fluid inclusion evidence these data suggest that the transport of tungsten in hydrothermal fluids might be due to carbonate/bicarbonate complexing. -- The fluid inclusion and isotopic data from the Grey River Tungsten Prospect serve to illustrate the previously unrecognised role of CO₂ in the transport and deposition of tungsten in the hydrothermal environment.

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