Carbon and oxygen isotopic covariations in hydrothermal calcites

Abstract

Isotopic covariations of carbon and oxygen in hydrothermal calcites are quantitatively modeled in terms of the following three mixing processes: (1) mixing between two different fluids which leads to the precipitation of calcite; (2) mixing between fluid and rock: (a) calcite precipitation due to fluid/rock interaction, (b) secondary alteration of primary calcite by interaction with a subsequent fluid. The models are derived from mass balance equations. A distinction among the three mixing processes can be made on a δ ¹³C vs δ ¹⁸O diagram, which places important constraints on the genesis of hydrothermal mineralization. The variables which control the ultimate isotopic composition of hydrothermal calcites include the composition of the initial fluid and the wallrock, temperature, and dissolved carbon species. Owing to significant temperature-dependent fractionation effects during equilibrium precipitation of calcite from a hydrothermal fluid, the mixing processes may be distinguished by telltale patterns of isotopic data in δ ¹³C vs δ ¹⁸O space. In particular, caution must be exercised in postulating the fluid mixing as the cause for mineral deposition. This is demonstrated for hydrothermal Pb-Zn deposits in the western Harz Mountains, Germany. A positive correlation between δ ¹³C and δ ¹⁸O values is observed for calcites from the Bad Grund deposit in the Upper Harz. Two sample profiles through calcite veins show similar correlations with the lowest δ-values at the center of the veins and the highest δ-values at the vein margins. Because the correlation array has a greater slope than for calcite precipitation at equilibrium in a closed system and because fluid mixing may not proceed perpendicular to the vein strike, it is assumed that a fluid/rock interaction is responsible for the observed correlation and thus for the precipitation of calcite. A deep-seated fluid is inferred with a δ ¹³C value of — 7% and a δ ¹⁸O value of +10%., as well as H₂CO₃ as the dominant dissolved carbon species; precipitation temperatures of the calcites are estimated to be about 280 ∼ 170°C. Quite different isotopic distributions are observed for calcites from the St. Andreasberg deposit in the Middle Harz. An alteration model is suggested based mainly on the isotopic distribution through a calcite vein. In addition to a primary fluid which has the same isotopic composition as that in the Bad Grund deposit and thus seems to be responsible for the precipitation of calcite associated with sulfides, an evolved, HCO ^-₃ -dominant subsurface fluid with δ ¹³C about -20 ∼ — 15% and δ ¹⁸O ≤ 0% is deduced to alter the primary calcite at low temperatures of 70 ∼40°C.