The stability fields of binary selenide minerals calculated thermodynamically relative to native elements, sulfide, telluride, and oxide minerals as a function of temperature have been used to characterize the genesis of selenide-bearing deposits. Selenide-bearing hydrothermal deposits are restricted mainly to four major types: "telethermal" selenide vein-type deposits, unconformity-related deposits, sandstone-hosted uranium deposits, and Au-Ag epithermal volcanic-hosted deposits. The selenide minerals in telethermal selenide vein-type and selenide-bearing unconformity-related uranium deposits formed from hydrothermal fluids at conditions within the stability field of hematite, between 5.8 and 7 log units in the f (sub O2(g)) above the hematite-magnetite (HM) buffer (at 100 degrees C), and with an f (sub Se2(g)) /f (sub S2(g)) ratio higher than unity. These high f (sub O2(g)) conditions enhance the separation of selenium from sulfur, leading to a high f (sub Se2(g)) /f (sub s2(g)) ratio and deposition of a diverse group of selenides. Selenide deposition was not significantly influenced by wall-rock buffers, but late reequilibration and dissolution of some earlier formed selenides, such as krutaite (CuSe 2 ), probably occurred in some selenide-bearing unconformity-related uranium deposits. Mineral parageneses in sandstone-hosted uranium deposits suggest that they formed at higher f (sub Se2(g)) and lower f (sub O2(g)) (0-5.8 log units above the hematite-magnetite buffer at 100 degrees C) than did unconformity-related uranium deposits and selenide vein-type deposits. The selenide-bearing Au-Ag epithermal, volcanic-hosted deposits were formed from hydrothermal fluids with f (sub O2(g)) below or near the hematite-magnetite buffer and at an f (sub Se2(g)) /f (sub S2(g)) ratio lower than unity. Low f (sub Se2(g)) /f (sub S2(g)) ratios and the presence of sulfide assemblages that buffer f (sub Se2(g)) and f (sub S2(g)) prevent the enrichment of selenium in the hydrothermal fluid and deposition of selenide minerals except for silver selenides. Therefore, a selenium-rich, relatively reduced (e.g., below the hematite-magnetite buffer) hydrothermal fluid can deposit only silver selenides; no other selenide minerals can deposit from a fluid of this type. An oxidizing environment (e.g., close to the anglesite-galena buffer) is essential to form most selenide minerals. The actual mineralogy and the amount of selenide minerals that form from such an oxidizing fluid is a function of f (sub Se2(g)) and ultimately reflects the concentration of selenium in the fluid. A selenium-rich fluid alone can account for the presence of silver selenides in epithermal Au-Ag deposits, but both an oxidizing and a selenium-rich fluid is required to form the selenide mineral assemblages observed in telethermal selenide vein, unconformity-related, and sandstone-hosted uranium deposits.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.