Hydrothermal maturation of indigenous organic matter at the Alligator Ridge gold deposits, Nevada

Abstract

The organic matter of the Devonian-Mississippian Pilot Shale, which hosts the disseminated gold mineralization at Alligator Ridge, Nevada, has been investigated in an attempt to determine its source and maturation processes and to find an alteration parameter which reflects precious metal distribution more accurately than mineralogical alteration patterns. The Alligator Ridge deposits are similar to other disseminated gold deposits in the Great Basin of the western United States. Visible alteration features, consisting of quartz replacement of carbonates and oxidation of sulfides and organic matter, do not provide significantly useful guides for exploration.In this study, the type, abundance, and maturity of organic matter in the Pilot Shale were determined using techniques standard to petroleum exploration. The type and abundance of organic matter in background and mineralized samples are similar. Transmitted light examination of kerogen concentrates shows that most of the organic matter consists of fine amorphous (algal) aggregates with a minor amount of vitrinite. This corresponds with type I or II organic matter of Tissot and Welte (1978) which is derived from a marine source. Total organic carbon makes up 1 to 3 percent of most carbonaceous rocks at Alligator Ridge. No evidence was found for hydrothermal introduction or remobilization of organic matter.The maturity of organic matter, as determined by pyrolysis and vitrinite reflectance, is considerably higher in mineralized samples than in samples unaffected by the processes of mineralization. Hydrogen index values for mineralized samples are generally <40 (H/C approximately 0.5) compared with a background of 230 to 315 (H/C approximately 1). Changes in vitrinite reflectance show a similar but less dramatic change. Mineralized samples have reflectance values of 1 to 1.3 percent; background reflectance is 0.4 to 0.5 percent. All reflectance values, however, are anomalously low due to the extremely small grain size of the vitrinite and therefore cannot be used to determine paleotemperatures.Spatially, hydrogen index contours increase concentrically away from gold mineralization, except near the lower Pilot Shale contact. Here hydrogen index isopleths are nearly concordant and parallel other alteration features. The spatial pattern indicates that hydrothermal flow was dominantly along this contact and away from mineralization.This study shows that the organic matter in the Pilot Shale is of a marine sedimentary origin. The total organic carbon of mineralized and background material is similar, indicating that the organic matter was not added to the Pilot Shale hydrothermally. However, hydrothermal activity has increased the organic matter maturity, suggesting that maturation studies can be useful to exploration programs.