Wolframite deposition in a hydrothermal vein system; the Grey River tungsten prospect, Newfoundland, Canada

Abstract

Mineralization in the Grey River tungsten prospect, Newfoundland, is contained within a sheared Devonian K-feldspar megacrystic granite and metamorphic rocks which are typical of the gneissic terrain of the Gander zone. The mineralization is genetically related to a suite of post-tectonic, highly differentiated, alkali-rich leucogranite dikes.The tungsten-bearing hydrothermal veins range from tensional veinlets to lodes containing several injections of quartz. Major composite lodes have greisen alteration envelopes which are characterized by enrichment in volatiles, K 2 O, Al 2 O 3 , Rb, Li, F, heavy rare earth elements, and W. The heavy rare earth elements enrichment appears to be related to deposition of carbonate and fluorite in the greisen. The mineralization is divided into four stages: stage I, characterized by quartz + feldspar + molybdenite veins; stage II, composite lode formation, consisting of five vein types (in paragenetic order), quartz + bismuthinite (IIa), quartz + Fe-rich wolframite (IIb), greisen (IIc), quartz + sulfide (IId), and quartz + Fe-poor wolframite veins (IIe); stage III, characterized by silver-bearing quartz + galena + sphalerite veins; and stage IV, composed of fluorite + calcite + barite veins. A spatial mineral zonation from south to north matches the temporal sequence outlined above, with the exception of the stage IV veins which crosscut stage II veins.Fluid inclusion data indicate a complex evolutionary history for the hydrothermal fluid. The simultaneous trapping of CO 2 -rich and H 2 O-rich fluid inclusions, as well as solid inclusions of calcite, in quartz of the quartz+feldspar+ molybdenite and quartz + bismuthinite vein types, is evidence for the existence of a heterogeneous fluid during stage I and early stage II mineralization. Fluid phase equilibria indicate that the inclusions were trapped at temperatures of 330 degrees to 360 degrees C and fluid pressures of less than 500 bars.In composite lodes, quartz + sulfide and quartz + Fe-poor wolframite veins were deposited in open spaces created by normal faulting. These movements prompted rapid decrease in fluid pressure and temperature and caused retrograde boiling of the hydrothermal fluid (at 200 bars and 300 degrees to 330 degrees C) during deposition of quartz + sulfide veins. Deposition of wolframite in quartz + Fe-poor wolframite veins occurred at 270 degrees to 300 degrees C from an aqueous fluid of low salinity (<0.5 wt % NaCl) and low CO 2 content (<1.0 wt % CO 2 ) and after separation of a CO 2 vapor phase by retrograde boiling. Isotopic and fluid inclusion data suggest that deposition of Fe-rich wolframite may have occurred at a slightly higher temperature (330 degrees C) and from a fluid of higher CO 2 content than the bulk of the wolframite which has an Fe-poor composition.Progressive CO 2 loss from the hydrothermal fluid by immiscibility and retrograde boiling had a marked effect on the solution pH which shifted from 5 weakly acid) to 6 (weakly alkaline at 300 degrees to 350 degrees C) during tungsten deposition. Thermodynamic data on the solubility of wolframite suggest that a pH increase would provide an effective mechanism for tungsten deposition and explain the variation in wolframite composition, from Fe rich to Fe poor, with time.The association of hematite, Fe-rich wolframite, and muscovite in the greisen suggests high f (sub O 2 ) (10 (super -20) -10 (super -30) ) conditions and is supported by Eu behavior during alteration. Sulfides formed after tungsten deposition at temperatures less than 300 degrees C and under f (sub S 2 ) conditions of less than 10 (super -11) .CO 2 -rich fluids are commonly associated with tungsten deposits from a variety of environments and the evidence from the Grey River tungsten prospect shows that CO 2 plays an important role in the transport and deposition of tungsten in the hydrothermal environment.