Textural and P-T-X characteristics of the hydrocarbon-bearing stages of the paragenesis at Carlin, Nevada

Early stages of the paragenesis at the Carlin, Nevada sediment-hosted gold deposit are represented by veinlets and/or cleats filled with calcite, quartz and organic matter that has been thermally altered to non-graphitizing mesophase. The pyrobitumen has a mean reflectance of 3-4.5%, and consist of a fine mosaic of micron-sized anisotropic domains. Bitumen in the host rocks has migrated to low-pressure zones. In veinlets and cleats the organic matter was introduced as an organic phases; more rarely it has been introduced as micelles. Multiple generations of pyrobitumen+/-calcite and quartz occur, often within a single veinlet. Alteration of some of the pyrobitumen, shown by changes in the mean reflectance and relative size of anisotropic domains, is speculatively interpreted as oxidation. Fluid inclusions in the adjacent quartz and calcite appear to contain either 1 or 2 fluid phases in various proportions at room temperature. Combined crushing, heating/freezing and laser Raman microprobe studies have revealed 3 general types of inclusions in this stage of the paragenesis. Type I inclusions homogenize between 140-220/sup 0/C, with 160+/-20/sup 0/C being most common. Type II inclusions homogenize below 31/sup 0/C, and melt very near -56.6/sup 0/C. Type III inclusions homogenize between -114 and -85/sup 0/C and have meltingmore » events at approximately -182/sup 0/C and a second at T/sub m/ < T/sub h/. Temperatures, compositions and geobarometric constraints provided by type I and III inclusions, as well as textural characteristics of the pyrobitumen are consistent with the metagenesis of indigenous organic matter under near lithostatic conditions.« less