The CanTung E Zone scheelite skarn orebody, Tungsten, Northwest Territories; oxygen, hydrogen, and carbon isotope studies

Abstract

Contact skarn formation at the CanTung scheelite deposit (Tungsten, Northwest Territories, Canada) involved replacement of calcitic marbles by garnet-pyroxene, pyroxene-pyrrhotite, amphibole-pyrrhotite, and bidtite-pyrrhotite skarns, adjacent to a peraluminous biotite mon-zogranite pluton (delta 18 O (sub whole rock) = 10.9ppm). Replacement of marble (delta 18 O = 19.7-20.5ppm; delta 13 C = -0.5 to +1.7ppm) by skarns resulted in significant 18 O and 13 C depletion. Fluid inclusion studies presented in Mathieson and Clark (1984) and oxygen isotope thermometry data presented here are consistent with the initial development, essentially simultaneously, of a zoned array of both anhydrous and hydrous skarns at 450 degrees to 500 degrees C, as proposed by Dick and Hodgson (1982). Development of hydrous skarn continued to lower temperatures, as indicated by the fluid inclusion studies of Mathieson and Clark (1984) and suggested by the calcite-pyroxene isotopic data presented here.The hydrogen and oxygen isotope data at CanTung suggest that early skarn-forming fluids were comprised mainly of fluid in equilibrium with the intrusion (> or = 75 mole % intrusive fluid; or = 90 mole % intrusive fluid; < or = 10 mole % meteoric water). These portions of meteoric water would be lower for any hydrous skarns formed at T < 475 degrees C. Because the calculated delta D values of the hydrous skarn fluid would increase with decreasing temperature, any retrograde development of hydrous skarn was not accompanied by a progressive influx of meteoric water. Such increases could be the result of an influx of D-enriched formation water or of progressive formation of CH 4 in the skarn fluids at lower temperatures (<350 degrees C). The hydrogen isotope data indicate that meteoric water was not a dominant component of the skarn fluids throughout most of the period of skarn development. However, the isotopic data alone cannot rule out the possibility that the skarn fluids were derived from formation waters which were isotopically similar to magmatic water.Mass balance calculations of 18 O and 13 C depletions resulting from equilibrium decarbonation at 475 degrees C indicate that the observed depletions of 18 O and 13 C in skarn calcites (delta 18 O = 12.9ppm, delta 13 C = -6.2ppm) could not have arisen simply from decarbonation of the marble host rocks. Mass balance calculations also indicate that the depletions could not have arisen from simple isothermal isotope exchange between the skarn-forming fluid and the marble wall rock at 475 degrees C. However, the oxygen isotope compositions of the skarn calcites can be interpreted to have formed over a range of falling temperature ( approximately 400 degrees -270 degrees C) from a fluid of nearly constant oxygen and carbon isotope composition, which was isotopically equivalent to that responsible for formation of the higher temperature ( approximately 475 degrees C) skarn silicates.The hydrogen and oxygen isotope data indicate that there was only a limited and nearly constant influx of meteoric water into the skarn system. The hydrothermal fluids associated with each of the skarn facies have nearly identical ranges of salinities (Mathieson and Clark, 1984) and nearly identical oxygen isotope compositions. With one exception, the calculated delta D values of the skarn fluid for both amphibole and biotite skarns are similar and display relatively little variation (7ppm). These characteristics contrast with the wide variations in scheelite and pyrrhotite contents in the skarns (Mathieson, 1982; Mathieson and Clark, 1984) and suggest that variation in the influx of meteoric water, with accompanying changes in salinity, hydrogen isotope composition, or temperature, was not the controlling factor governing scheelite deposition at CanTung. The isotope data also do not support a model of skarn petrogenesis for CanTung which invokes an early high-temperature, exclusively anhydrous skarn stage formed dominantly from magmatic water, followed by a later, distinctly lower temperature (retrograde) hydrous stage formed from fluids dominated by meteoric water.