Page 125 - Handbook of Gold Exploration and Evaluation

104 Handbook of gold exploration and evaluation

shallow formed epithermal and base metal deposits as shown in Table 2.2 are
representative of hydrothermal ore deposits associated with subduction related
magmatism (Hedenquist and Lowenstern, 1994).
Formed by gradual igneous replacement of country rock at great depths or by
stoping during a mountain building period, batholitic intrusions provide
important long-term sources of heat energy for the large-scale circulation of
hydrothermal fluids. There is, however, some doubt that such large parent bodies
can themselves host significant gold-bearing vein systems. Batholiths located
along the gold-bearing cordillera of North and South America do not show any
direct relationship with gold deposition. More suitable conditions for folding and
faulting and the supply of heat for hydrothermal deposition are provided by
smaller diapiric intrusions injected into relatively cold country rock. Metallo-
genic gold provinces usually contain clusters of similar deposits or areas of host
rock associations in which mineralisation has been active at one or more
intervals of geological time. The most productive areas of the Californian
goldfields appear to be closely related to multiple smaller intrusions and/or
apopheses of the larger parent granitic bodies.

2.3.2 Fluid phase
Hydrothermal fluids are of several different types. Magmatically related fluids
emanate from felsic to intermediate igneous intrusives during late stages of
cooling and crystallisation. Connate fluids comprise water trapped in the pores
of sedimentary rocks or in the joints of igneous rocks. Metamorphic water,
incorporated in silica structures, is released from rocks undergoing metamorphic
change. Meteoric waters include rainwater, river water, lacustrine water and
ground water. The metal complexing capacity of brines leads to a much greater
exchange of chemicals between seawater and mantle basalts than occurs
between meteoric waters and basaltic magmas.
According to recent studies, magmatic fluids are present in most active
hydrothermal systems but are not always recognised because of masking by the
large volume (>95%) of meteoric fluids convected during cooling of an intrusion
at shallow depths in the crust (Cathies, 1991). Temperature, salinity and redox
conditions are critical factors for a range of deposits. Tables 2.3 and 2.4 show
temperature-salinity and temperature redox conditions respectively for a range
of hydrothermal gold ore types. Meteoric and fluid inputs can usually be traced
from oxygen and seawater hydrogen isotopes.
Despite the apparent dominance of magmatic water in the formation of many
hydrothermal ore deposits, meteoric fluids are widespread throughout the crust
and under suitable conditions gold and other soluble minerals may be leached
from the rocks through which they pass. Rocks with high clay content and low in
carbonates, carbon and pyrite are good sources of gold, the content generally
ranging from about 0.005 to 0.01 ppm Au with some carbonaceous shale having