Lithological Associations
Generation of gold-rich porphyry copper deposits is believed to be
independent of the composition and thickness of underlying crust, which
ranges from primitive oceanic to evolved cratonic. A relationship between
gold-rich porphyry deposits and anomalously high gold contents in
underlying crust (e.g., Titley, 1990) fails to account for isolated gold-rich
deposits (e.g., Marte, Lobo, Dos Pobres) in otherwise gold-poor porphyry
copper provinces; neither can such a relationship account for closely spaced
deposits with markedly different gold contents (e.g., Saindak, Pakistan:
Sillitoe and Khan, 1977). However, relatively reduced crustal units may
buffer the redox state of magmas that ascend through them, and magmas with
relatively low oxidation states are considered by some investigators (e.g.,
Keith and Swan, 1987; Leveille et al., 1988) as especially favourable for
generation of gold-rich deposits. The commonly accepted correlations
between molybdenum-rich deposits and continental settings and between
gold-rich deposits and oceanic settings (e.g., Hollister, 1975) are discredited
further by the recent discovery at Marte and Lobo of porphyry gold deposits
that are underlain by the thick Andean crust of northern Chile (Vila and
Sillitoe, 1991).
Gold-rich porphyry copper deposits are commonly associated with stocks
emplaced at shallow (1-2 km) crustal levels (Cox and Singer, 1988); this
association is illustrated by widespread preservation of coeval volcanic
sequences. Indeed, 25 deposits (86%) listed in Table I cut coeval volcanic
rocks. The volcanic rocks are commonly broadly andesitic or tracbyan-desitic
and in most districts constitute erosional remnants of stratovolcanoes. In
some districts, however, any coeval volcanic sequences have been eroded
entirely, and stocks are hosted by rocks as disparate as serpentinites at
Mamut (Kosaka and Wakita, 1978), gneiss at Skouries (Kalogeropoulos,
1986), and limestone at 0k Tedi (Bamford, 1972). At Ok Tedi, however, the
former presence of a stratovolcano is supported by nearby volcano-clastic
sedimentary rocks of the Birim Formation (Rush and Seegers, 1990).