BALLANTYNE, JUDITH M., XIONN Geoscience, Park City, UT

ABSTRACT: Hydrocarbon-Associated, Sediment-Hosted Gold Deposits: A Genetic Model

The common association between disseminated, sediment-hosted gold deposits and hydrocarbons or carbonaceous material indicates a genetic link. Deposition of gold in sufficient quantities to produce an ore deposit requires sustained flow of hydrothermal fluid through a narrow zone that coincides with a stable physico-chemical interface. Hydrocarbon-enriched rocks provide such an interface, at which thermal cracking of hydrocarbons can generate an oxidizing environment by depleting hydrogen. Gold complexes are destabilized, depositing gold on available pyrite and other mineral surfaces. Pyrite may later be oxidized, releasing free gold. The sequence of reactions occurring as hydrogen depletion increases can explain the mineralogy and paragenesis observed at many hydrocarbon-associated g ld deposits. The model also explains the absence of silicification in many ore zones and the existence of barren jasperoids distal to such ore zones.

Contrasting gold-depositing interfaces include the pressure-temperature decline near the earth's surface, and mixing zones between hydrothermal fluids and cold groundwater. In the complex environment of a hydrothermal ore deposit, with its distinct set of structural, lithologic, and hydrologic characteristics, recognizing the type or types of interfaces involved can enhance exploration success.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.