Epithermal environments and styles of mineralization: variations and their causes, and guidelines for exploration

EPITHERMAL GOLD DEPOSITS:

STYLES, CHARACTERISTICS AND EXPLORATION

Noel C. White1 and Jeffrey W. Hedenquist2

1BHP Minerals International Exploration, 229 Shepherds Bush Road, London W6 7AN, U.K.

2Mineral Resources Dept., Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan

Epithermal ore deposits form at shallow depth. This conclusion was initially based on geologic reconstructions, ore mineralogy and related textures (Lindgren, 1933). It has subsequently been refined withfluid inclusion data to indicate that epithermal ores form over the temperature range of <150oC to ~300oC, from the surface to as deep as 1 to 2 km.

Here we highlight the general characteristics of the two principal styles of epithermal mineralization in which gold is the dominant economic metal. We base our generalizations on observations of many deposits and prospects in the circum-Pacific region. Distinguishing between the two styles is crucial for effective exploration. Although they show similar alteration mineralogies, the distribution of the alteration zones is different, and the economic mineralization is associated with different parts of the system. The alteration zoning can be used as a pointer towards the most prospective part of the system, but only when the style has been correctly recognized. In addition, the two styles of mineralization have differences in their geochemical associations.