PRELIMINARY STUDY OF BISMUTH PARTITIONING IN MINERAL-MELT SYSTEMS

Understanding the deposition of ore metals in magmatic-hydrothermal deposits begins with an examination of the partitioning of metals in magmatic systems. Little research has been done on bismuth (Bi) in these systems. In hydrothermal ores, Bi behaves, in part, as a chalcophile element. Bi has also been reported to substitute in Ca-rich minerals, due to the similar ionic radius of Bi³⁺ and Ca²⁺ (Greenland, et al., 1973, GCA, p 283). The purpose of this study is to examine the partitioning of Bi into mineral phases including sulfides, oxides and Ca-bearing phases. These early-forming phases may be left behind as crystalline residue in deeper levels in the earth's crust, and may become unavailable to any aqueous ore-forming fluid at the crustal level of ore formation.

Experiments were performed to evaluate the partitioning of Bi among the melt and various crystalline phases at magmatic P & T. Approximately 40 mg of Bishop Tuff, 40 mg magnetite (Mt) or pyrrhotite (Po), 100 ml of either a 1:1 KCl:NaCl or a 1:1:1 KCl:NaCl:HCl solution, along with Bi (metal; <1 mg) were combined in Pt capsules. Capsules were loaded into Rene 41 cold-seal pressure vessels and run at 800°C and 100 MPa for 1-3 days. Run products were analyzed using a combination of EPMA and LA-ICP-MS.

Preliminary data for the partitioning of Bi between Mt/melt and Po/melt were obtained. The average Nernst-type partition coefficient (mineral in ppm/melt in ppm) for Mt/melt was ~2.8 ± 1 (all errors reported to 2σ). The average Nernst-type partition coefficient for Po/melt was ~1.8 ± 1. These values indicate that Bi is only slightly compatible with Mt and Po. It should also be noted that while the current experiments were only run for 1 to 3 days, longer experiments are planned.

At this time, few data have been collected regarding the concentration of Bi in Ca-rich Pyx and alkali feldspar, due to analytical difficulties. However, with concentrations of Bi as high as 43.5 and 28.5 ppm, respectively, these phases should be considered as other important phases in segregating Bi in magmatic systems.