A hydrogen and oxygen isotope study of the San Cristobal Mine, Peru; implications of the role of water to rock ratio for the genesis of wolframite deposits

Abstract

Oxygen and hydrogen isotope ratios were measured from vein minerals and extracted fluid inclusion waters from the San Cristobal wolframite-base metal deposit in Peru. Oxygen isotope values for quartz range from 9.8 to 14.4 per mil and for wolframite from 0.6 to 4.6 per mil. The calculated delta values at 300 degrees C for water in equilibrium with quartz and wolframite are delta 18 O = 2.8 to 7.3 per mil for quartz and 1.4 to 5.4 per mil for wolframite. Hydrogen isotope values show an overall range of delta D = -58 to -148 per mil, but the range for any given mineral is much smaller. These data are interpreted in light of a model which examines the effect of temperature and water to rock ratio on the isotopic compositions of meteoric waters exchanging with granite. In a rock-dominated system an isotopically exchanged water becomes depleted in delta 18 O and enriched in delta D as the temperature of exchange decreases. When the water to rock ratio is also allowed to vary, a wide variety of water compositions is generated. The isotopic composition of minerals from San Cristobal can be interpreted as the result of deposition from a meteoric water which has undergone isotopic exchange with a granite at 400 degrees C, with a water to rock ratio ranging from 0.01 to 0.003. Isotopic data taken from two other wolframite deposits, Pasto Bueno (Landis, 1972) and Panasquiera (Kelly and Rye, 1979) are reinterpreted using the model developed for San Cristobal. Both sets of data are consistent with the hypothesis that wolframite deposits are formed by meteoric waters which have experienced exchange at very low water to rock ratios (<0.05). The importance of the low water to rock ratio may be related to the way in which tungsten is leached from a crystallized pluton.