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

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

NOEL C. WHITE 1 and JEFFREY W. HEDENQUIST 2

I. BHP-Utah Minerals International, P.O. Box 619, Hawthorn, Vic. 3122, Australia

2. Mineral Resources Department, Geological Survey of Japan, 1-1-3 H igashi,

Tsukuba 305, Japan

(Received June 12, 1989; accepted for publication August 22, 1989 )

ABSTRACT

White, N.C. and Hedenquist, J.W., 1990. Epithermal environments and styles of mineralization:

variations and their causes, and guidelines for exploration, II. In: J.W. Hedenquist, N.C. White

and G. Siddeley (Editors), Epithermal Gold Mineralization of the Circum-Pacific: Geology,

Geochemistry, Origin and Exploration. J. Geochem. Explor., 36: 445-474.

Epithermal precious- and base-metal deposits are diverse, reflecting the different tectonic, igneous

and structural settings in which they occur, the complexities of their local setting, and the

many processes involved in their formation. Most epithermal deposits form at shallow crustal

levels where abrupt changes in physical and chemical conditions result in metal deposition and

attendant hydrothermal alteration. The principal factors that influence the conditions prevailing

in the epithermal environment, and which ultimately determine the sites and character of mineralization,

include: geology (structure, stratigraphy, intrusions and rock type, which affect the style

and degree of permeability and the reactivity of the host); pressure and temperature (which in

the epithermal environment are related on the boiling point with depth curve); hydrology (the

relationship between permeability and topography which governs fluid flow, and discharge/recharge

characteristics, as well as access of steam-heated waters); chemistry of the mineralizing

fluid (which determines the metal-carrying capacity, as well as the associated vein and alteration

assemblage); and syn-hydrothermal development of permeability and/or changes in hydraulic

gradients.

Many attempts have been made to classify epithermal deposits based on mineralogy and alteration,

the host rocks, deposit form, genetic models, and standard deposits. All have their strengths

and weaknesses. We prefer a simple approach using the fundamental fluid chemistry (high or low

sulfidation, reflecting relatively oxidized or reduced conditions, respectively) as readily inferred

from vein and alteration mineralogy and zoning, together with the form of the deposit, and using

comparative examples to clarify the character of the deposit.

Guidelines for exploration vary according to the scale at which work is conducted, and are

commonly constrained by a variety of local conditions. On a regional scale the tectonic, igneous

and structural settings can be used, together with assessment of the depth of erosion, to select

areas for project area scale exploration. At project area scale, direct (i.e. geochemical ) or indirect

guidelines may be used. Indirect methods involve locating and interpreting hydrothermal alteration

as a guide to ore, with the topographic and hydrologic reconstruction of the system being of high priority. These pursuits may involve mineralogic, structural, geophysical or remote sensing

methods. On a prospect scale, both direct and indirect methods may be used; however, they can

only be effective in the framework of a sound conceptual understanding of the processes that occur

in the epithermal environment, and the signatures they leave.