Gold Abundance in Igneous Rocks; bearing on Gold Mineralization

Abstract

HydrothermM deposits of gold and other metals are commonly associated with igneous rocks that are thought by many to be the source of the ore metals. However, hypotheses linking gold mineralization and gold abundance in the associated igneous rock have been ambiguous and nonspecific. For example, abundance levels higher as well as lower than those considered as background or normal have been invoked as evidence to support theories that the gold was derived from the igneous rock.A review of the quantitative data now available indicates that unaltered igneous rocks have a rather restricted range in gold content, rarely exceeding 10 ppb (parts per billion) and generally well below 5 ppb. Mafic rocks tend to have more gold than felsic and intermediate rocks in both plutonic and volcanic suites. Furthermore, gold also tends to be higher in the early crystallizing minerals (mafic silicates, Fe-Ti-oxides, etc.) than in the later crystallizing quartz and feldspar. These observations suggest that gold becomes depleted in the residual silicate melt in differentiating calc-alkalic magmas.Comparison of fresh and altered rocks from the Butte and Marysville districts, Montana, shows that higher than background gold values invariably can be ascribed to secondary introduction rather than to initial high abundances in the magma. Evaluation of available abundance data for greenstone-granite gold belts demonstrates no appreciable differences in gold content between the granitic rocks and the metavolcanic country rock;hence, the gold data neither support or refute the currently popular hypothesis that the gold in the deposits had its ultimate source in the country rock.Geochemical abundance data do not provide reliable guides to areas favorable (or unfavorable) for gold mineralization, nor do they help to identify source rocks or to clarify processes that bring about economic concentration of gold. Precipitation of oregrade concentrations of gold by thermal waters that are extremely low in gold (0.05 ppb or less) and that have circulated through rock also low in gold (1 ppb or less) shows convincingly that low concentration of gold in a sufficiently long-lived hydrothermal system poses no deterrent to gold mineralization (e.g., as in the Yellowstone area and the Taupo Volcanic Zone, New Zealand). Whether or not gold mineralization occurs is largely determined by geologic, geochemical, and geophysical factors other than the concentration of gold available at various places in the mineralizing system. Available data suggest that regions of gold mineralization do not necessarily coincide with segments of the Earth's crust intrinsically higher (or lower) in gold abundance.

Petrochemical evolution of subvolcanic granitoid intrusions within the Late Devonian Mount Pleasant Caldera, southwestern New Brunswick, Canada: comparison of Au versus Sn-W-Mo-polymetallic mineralization systems

“…These data suggest that background Au abundance in the granitoids is not an essential parameter controlling associated gold mineralization. This observation is in accord with the conclusion by Tilling et al (1973) that it is impossible to relate gold abundance in igneous rocks and gold mineralization. The geochemical behaviour of gold during the magmatic-hydro-thermal evolution of granitoid systems may be a key factor responsible for gold mineralization, which is controlled by the physiochemical conditions and composition of the magmas.…”

Source of lead in the gold-bearing quartz-fuchsite vein at the Dome mine, Timmins area, Ontario, Canada

“…If gold and lead came from a common source, then, according to the lead isotope signature of the galenas, either the bulk of the gold was the product of the degassing of a typical greenstone belt sequence, or gold was derived from local quartzfeldspar porphyries with a possible small input from local chemical sediments and volcanic rocks. However, unaltered igneous rocks ranging in composition from ultramafic to felsic commonly have gold contents between 2 and 5 ppb (Tilling et al 1973;Kwong and Crocker 1978;Saager et al 1982;Crocker and MacRae 1986), whereas in the Dome mine, the altered quartz-feldsplar porphyries and the mafic to ultramafic volcanic rocks have gold abundances which are 102 to 104 times higher Moritz 1988). Consequently gold must have been introduced into these rocks rather than leached from them during mineralization and alteration.…”

Geological, petrochemical and metallogenic characteristics of the Costa Rican gold belt: Contribution to new explorations

“…TAYLOR 1973). Recent geochemical studies (TILLING et al 1973) point out that gold is mainly associated to mafic rocks and minerals. However, magmatic processes related to the emplacement of granitoid ptutons in island-arc terranes may mobilize and transport gold producing specific mineral deposits.…”