Comparative elemental and oxygen isotope geochemistry of jasperoid in the northern Great Basin; evidence for distinctive fluid evolution in gold-producing hydrothermal systems

Abstract

This comparative geochemical study of jasperoid in the northern Great Basin is based on 65 samples from 10 Carlin-type gold deposits and 22 similar but apparently barren hydrothermal systems. Multielement geochemistry coupled with oxygen isotope data indicate that hydrothermal fluids in barren and mineralized systems evolved in different ways, and that there are fundamental geochemical differences among the various gold-producing deposits of the area.Much of the variation in the jasperoid geochemical data can be explained in terms of seven abstract end-member components obtained through factor analysis. Three of these components (factors) dominate the results and are related to common products of alteration and mineralization in epithermal systems of the northern Great Basin. Element associations for these factors are: factor 1: TiO 2 , Al 2 O 3 , La, K 2 O, Sr, Fe 2 O 3 , Th; factor 2: Au, Ag, Sb, SiO 2 , As, Pb; and factor 3: W, B, V, Zn, Co, Au, CaO, Ni, Mn, Cu.Samples from barren systems are predominantly associated with high factor 1 loadings, whereas samples from mineralized systems are generally characterized by high loadings of factor 2 and/or factor 3. Factor 1 is related to residual, argillically altered, noncarbonate constituents in original host rocks. Factor 2 is related to hydrothermal silica (jasperoid) and is characterized by many of the so-called pathfinder elements used in Great Basin exploration. Factor 3, although characterized by a transition metal assemblage, is related to syngenetic inclusions of hydrothermal calcite within jasperoidal silica.High factor 2 loadings are associated with samples from the Windfall and Northumberland deposits but not with samples from other gold-producing systems in the study. Factor 3, on the other hand, is the dominant component in most samples from the Carlin, Gold Quarry, Maggie Creek, Pinson, and Preble deposits. It therefore appears that there are at least two geochemically discernible types of gold deposits in the northern Great Basin. These types are represented in this study by the Windfall deposit (factor 2) at one extreme and by the Gold Quarry deposit (factor 3) at the other. Other deposits, including Alligator Ridge and Tonkin Springs, are intermediate between these extremes. In this study, the hydrothermal geochemistry of the Jerritt Canyon samples is obscured by a chert-related compositional factor.Jasperoid samples from gold-producing ore deposits have delta 18 O values ranging from 3 to 20 per mil, whereas jasperoid samples from analogous barren systems have delta 18 O values ranging from 3 to 11 per mil. This indicates that at least some fluids in ore-related hydrothermal systems were relatively 18 O rich compared to those in barren systems. Jasperoid samples having high delta 18 O values invariably have high factor 3 loadings.The elemental and isotopic geochemistry of jasperoid samples indicates that hydrothermal fluids in individual gold-producing systems of the northern Great Basin evolved in different ways. It appears that high CO 2 contents were a critical distinguishing feature of deeply circulating fluids in the hydrothermal systems associated with high factor 3 loadings. Within the scope of this study, it therefore appears that the systems located on the Carlin and Getchell trends (mineral belts) must have been relatively CO 2 rich. Fluids in these systems would have been relatively reactive and perhaps more efficient at extracting gold and other metals from rocks in fluid exchange reservoirs. Moreover, the generally high 18 O character of ore-stage fluids in these systems, as inferred from jasperoid compositions, is indicative of extensive water-rock exchange.The largest gold deposits in the jasperoid study area appear to have been high CO 2 , high 18 O systems. The primary source of CO 2 in these systems is uncertain. It does appear, however, that deep crustal structure, rather than the lithologic character of fluid exchange reservoirs, was probably a dominant factor governing the distribution of high CO 2 systems in the northern Great Basin.