Sulfur isotopes and hydrothermal mineral deposits

Abstract

In the past, hopeful attempts have been made to determine the type of mineral deposit (i. e., pegmatitic, magmatic, hydrothermal, and so forth) by means of S isotope ratios. The recognition of the overlap and similarity of S 32 /S 34 ratio values determined from specimens from such deposits is a clear indication that these ratios provide no simple panacea to the classification of mineral deposits. On the other hand, however, S ratios of specimens from some hydrothermal deposits, although they do not exhibit identical values, do show a comparatively narrow spread, usually < + or - 0.5%, in ratio values which is a striking contrast when compared to other hydrothermal deposits that contain sulfides that vary in ratio by as much as 4% and more. The recognition that these rather similar hydrothermal deposits are dissimilar in degree of S isotopic homogeneity is quite striking when considered in relation to the known geology of the deposits. In those deposits that are intimately associated with the intrusive body from which it is believed the ore solutions were derived, the spread in ratio values is very narrow, but those deposits that are not associated with an apparent magmatic source exhibit a broad spread in ratio values. As a result, it is suggested that S isotope ratios may provide an additional aid or tool whereby the economic geologist may be better equipped to learn more about the different sources of hydrothermal solutions and obtain a better understanding about the processes by which hydrothermal mineral deposits have been formed. It is suggested, furthermore, as an initial step, that hydrothermal deposits might be subdivided according to different ore fluid sources, or at least according to what may be believed to be the sources. Apparently, S isotope ratios do aid in providing some evidence for subdividing many hydrothermal deposits as Magmatic Hydrothermal deposits, Metamorphic Hydrothermal deposits, and Ground Water Hydrothermal deposits. It is evident, therefore, that the use of the term hydrothermal in this paper includes those minerals ormed by heated waters that are not at all necessarily magmatic in origin. It is believed that most students of ore genesis would not restrict the term to this sole source.