EXPERIMENTAL INVESTIGATION OF THE THERMOCHEMICAL REDUCTION OF ARSENITE AND SULFATE: LOW TEMPERATURE HYDROTHERMAL COPPER, NICKEL, AND COBALT ARSENIDE AND SULFIDE ORE FORMATION

Author

Nicholas Allin, Montana Tech

Date of Award

Spring 2019

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geosciences

Committee Chair

Christopher Gammons

First Advisor

Glenn Shaw

Second Advisor

Douglas Cameron

Third Advisor

Gary Wyss

Abstract

Experiments were conducted to determine the relative rates of reduction of aqueous sulfate and aqueous arsenite (As(OH)₃,aq) using foils of copper, nickel, or cobalt as the reductant, at temperatures of 150ºC to 300ºC. At the highest temperature of 300°C, very limited sulfate reduction was observed with cobalt foil, but sulfate was reduced to sulfide by copper foil (precipitation of Cu₂S (chalcocite)) and partly reduced by nickel foil (precipitation of NiS₂ (vaesite) + NiSO₄·xH₂O). In the 300ºC arsenite reduction experiments, Cu₃As (domeykite), Ni₅As₂, or CoAs (langisite) formed. In experiments where both sulfate and arsenite were present, some produced minerals were sulfarsenides, which contained both sulfide and arsenide, i.e. cobaltite (CoAsS). These experiments also produced large (~10 µm along longest axis) euhedral crystals of metal-sulfide that were either imbedded or grown upon a matrix of fine-grained metal-arsenides, or, in the case of cobalt, metal-sulfarsenide. Some experimental results did not show clear mineral formation, but instead demonstrated metal-arsenic alloying at the foil edges. Below 250ºC in the mixed experiments, reduction of sulfate was not observed, but reduction of arsenite by copper to form domeykite was prominent at temperatures down to at least 150ºC, and reduction of arsenite by nickel to form an un-named mineral, Ni₃As, as a crystalline crust occurred as low as 150°C. The implication is that a low temperature fluid carrying both aqueous sulfate and arsenite will quickly precipitate metal-arsenide minerals at a reducing interface, whereas sulfate reduction is much slower, especially at temperatures below 250ºC. This helps to explain the abundance of metal-arsenide minerals and relative lack of metal-sulfide minerals in certain ore deposit types, including unconformity-type U-Ni and “5-element suite” vein deposits.

Comments

A thesis submitted in partial fulfillment of the requirements for the degree of Masters in Geoscience: Geology Option

Recommended Citation

Allin, Nicholas, "EXPERIMENTAL INVESTIGATION OF THE THERMOCHEMICAL REDUCTION OF ARSENITE AND SULFATE: LOW TEMPERATURE HYDROTHERMAL COPPER, NICKEL, AND COBALT ARSENIDE AND SULFIDE ORE FORMATION" (2019). Graduate Theses & Non-Theses. 213.
https://digitalcommons.mtech.edu/grad_rsch/213