Background concentrations of gold in different rock types

Abstract

The abundance of gold in crustal rocks is an important constraint on the formation of gold deposits. Gold concentrations in unmineralised igneous, sedimentary and metamorphic rocks range from 0?05 to 20 ppb with average concentrations commonly between 0?5 and 5 ppb. Analytical methods with ultra-low detection limits are required to observe the full range of concentrations. Gold concentrations in igneous rocks are strongly controlled by the behaviour of sulphur. Higher gold concentrations occur in sulphur-undersaturated rocks compared to sulphur saturated igneous rocks. Mid ocean ridge basalt has lower gold concentration than ocean-island and volcanic-arc basalt, due mainly to lower oxygen fugacity at MOR settings that causes sulphur saturation. Gold concentrations in sedimentary rocks increase with increasing abundance of diagenetic sulphide minerals and organic matter. Gold concentrations in metamorphic rocks decrease systematically with increasing metamorphic grade. Amphibolite facies rocks commonly contain between 50 and 80% less gold than their unmetamorphosed protolith rocks.

Mobility of gold during metamorphism of the Dalradian in Scotland

“…12B,13B,and 14B). Previous studies have shown that Au, As, Sb and other trace metals can be concentrated in sulphide rich carbonaceous mudstones as these elements are preferentially incorporated into diagenetic sulphides (Large et al, 2007(Large et al, , 2011Pitcairn, 2011;Tomkins, 2010Tomkins, , 2013. Sulphur concentrations are marginally higher in pelitic compared to psammitic rocks (Fig.…”

“…The range in concentrations of each element is large causing a large standard deviation which in the case of As is larger than the mean value. This is a natural product of the heterogeneous distribution of these elements in metasedimentary rocks (Pitcairn, 2011;Pitcairn et al, 2006aPitcairn et al, , 2010aPitcairn et al, , 2014a. The concentrations of these elements decrease significantly with increasing metamorphic grade (Figs.…”

Hydrothermal alteration in the Witwatersrand goldfields

“…3.5 g/t Au). The elevated gold in these basaltic rocks of 10 3 times background cannot be explained by any reasonable magmatic process (Pitcairn, 2011), and given the extreme alteration of many of these basaltic rocks (see Appendix A8), the introduction of gold during their alteration appears to be the most reasonable explanation. An origin of the unusual basaltic rocks through early weathering has already been dismissed (Palmer et al, 1989; the alteration is on the bottom as well as the top of the basalt units).…”

“…A context for this is the background levels of gold in a variety of common igneous, sedimentary and metamorphic rocks determined by Pitcairn (2011). He shows that shales commonly have 1-2 ppb Au with somewhat higher values where pyrite or hydrocarbons are present.…”

“…There is~6 t of gold in a cubic km of rock if one assumes an average background concentration of gold of 2 parts-per-billion and density near 3 t per cubic meter (Pitcairn, 2011). This gold does not occur as 5 mm to 100 μm grains because repeated analyses of rocks in such sequences indicate that the 2 ppb of gold must be evenly spread throughout the rock mass, presumably associated with minor sulfide minerals (Pitcairn et al, 2006).…”