This project involves a detailed investigation of oxygen isotope variations in volcanic rocks of the Comstock Lode. A remarkable and perhaps unique opportunity exists to combine oxygen data on surface samples with data on samples from mine workings and drill cores in order to construct the first detailed 3-dimensional image of a hydrothermal system in an important mining district. This work will include examination of 18 variations down several 1-km-deep vertical mine shafts as well as the preparation of an isotopic cross-section of the famous Con Virginia bonanza. The research will disclose critical relationships between circulation patterns and geologic factors that will advance understanding of these systems and may prove to be invaluable guides to ore deposits.