Page 147 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 125

zone extending into the basement. Such deposits are commonly distributed in a
roughly symmetrically pattern due to mushrooming of the ascending hydro-
thermal fluids. In high relief areas (andesitic stratovolcanoes) a large degree of
lateral flow takes place, which results in strongly asymmetrically altered zones,
relative to the upward flow.
The ores are texturally diverse in banded, crustiform quartz and chalcedony
veins, and in druse-lined cavities and multiple episodic vein breccias (Berger
and Eimon, 1983). They are associated with the least acidic alteration minerals,
e.g., calcite and adularia although calcite, formed as a result of boiling, may be
replaced by quartz as the system cools. White and Hedenquist (1995b) stress the
importance of determining the origin of alteration minerals that indicate acidic
conditions. These conditions include hypogene activity due to magmatic HCl
and SO 2 ; steam heated acid-sulphate waters formed near the surface; and post-
hydrothermal weathering of sulphide minerals.

High-sulphidation ores
A well-documented genetic association exists between magmas and epithermal
gold deposits where the deposits are formed by high sulphidation (acidic and
oxidised) fluid, typical of acidic springs near volcanoes. Isotopic studies have
shown that reactive components in the high sulphidation environment are
derived from a relatively oxidised magmatic source, with little wall-rock
interaction at depth as they rise to the surface. According to Rye (1993), SO 2 and
HCl vapour absorbed by ground water causes SO 2 to disproportionate to H 2 SO 4
and H 2 S followed by dissociation of the H 2 SO 4 and HCl. This results in hot
(200±300 ëC) highly acidic (pH 0±2) oxidised solutions which react with and
leach wall rocks at shallow depths; as distinct from low-sulphidation fluids,
which rise from great depths and react extensively with deep seated rocks (Gray,
1997b).
High sulphidation ore deposits are distinguished from their low-sulphidation
counterparts by features, which relate to differences in physico-chemical condi-
tions of formation and zoning of their alteration products. Both deposit styles are
associated with economically important orebodies. But whereas low-sulphidation
deposits usually comprise veins, breccias and stockworks of veins in which the
filled cavities have sharp edges, high-sulphidation deposits are typically
disseminated ore bodies, usually in leached zones of most acidic alteration
extending outwards in the surrounding country rock from the fluid conduit. The
gold is transported mainly as a chloride complex, with dilution and/or cooling as
controls on precipitation. Suitable conditions for the deposition of high grade ore
bodies are sometimes provided by localised veining or brecciation, but the
dominant texture overall, is massive vuggy quartz caused by leaching at pH 2
(Stoffregen, 1987). Such vuggy quartz bodies may be cut by massive to banded
sulphide veins consisting of pyrite and enargite (White and Hedenquist, 1995b).