Page 131 - Handbook of Gold Exploration and Evaluation

110 Handbook of gold exploration and evaluation

ships, if studied in conjunction with other exploration data, give useful informa-
tion about the type of ore-forming fluid (pH, Eh, etc.) as a guide to identifying
lateral and vertical zoning around individual veins. The intensity of the hydro-
thermal alteration varies from minimal in ores hosted by arenaceous wackes, to
intense in some intermediate to mafic volcanics and ultramafics (Nesbitt and
Muchlenbachs, 1989).
Ore-associated alteration is produced at decreasing temperatures both
towards the surface and at increasing distances from the fluid conduits. This
alteration mineralogy can be observed and the temperature measured in active
systems to indicate the thermal stability of temperature alteration minerals
(Henley and Ellis, 1983; Reyes, 1990). Information can then be deduced from
the distribution of the alteration minerals to determine the locus of palaeo-flow
and level of erosion. This is significant because most epithermal gold ores occur
in conduit zones over the temperature range 180±280 ëC, equivalent to depths
below the palaeo-water table of about 100 m to 800±1,500 m (Hedenquist and
Lowenstern, 1994).
The nature of the alteration mineralogy changes with depth, thus suggesting
that current levels of erosion can be predicted from the exposed rock. In high
sulfidation systems, the ore is associated with a zone of intense acid alteration
and is surrounded by mineral assemblages indicating less acid conditions. The
gold in low sulfidation systems is typically transported as a chloride complex
with different controls on precipitation, such as dilution and/or cooling
(Giggenbach, 1991; Hedenquist, 1992). The ore is associated with the least-
acid alteration and the gold is likely to be transported in solution as a bisulfide
complex. Hence, epithermal orebody exposures with alteration assemblages
indicating low palaeotemperatures are encouraging for primary gold exploration.
Orebody exposures indicating alteration palaeotemperatures approaching 280 ëC
are indicative of a long period of erosion with the possibility of significant
placer formation.
Gold-rich porphyry deposits are generated in volcanic-plutonic arcs in both
island arc and continental arc settings. They are associated with stocks and
volcanic rocks ranging in composition from low potassium calc-alkalic, through
high potassium calc-alkalic to potassium alkalic (Sillitoe, 1993). Introduction of
the copper and gold took place mainly during potassium silicate alteration (
biotite, magnetite, amphibole and calcic minerals). High fineness gold is present
in veins, stockworks and disseminations, mainly in zones of potassium silicate
alteration but in some cases, with advanced argillic alteration as in the Phillipines
at Santo Thomas 11 and Guinaoang. Hydrothermal magnetite is abundant in most
gold-rich potassium silicate zones. Figure 2.19 illustrates the generalised intrusive
and alteration relationships in and around gold-rich porphyry copper deposits.
The term `propylitic' was originally coined by Becker in 1882 to describe the
alteration of diorite and andesite beside the Comstock gold-silver lode, Nevada,
USA where epidote, chlorite and albite are the main alteration products.