Page 36 - Handbook of Gold Exploration and Evaluation

18 Handbook of gold exploration and evaluation

Table 1.8 Composition of placer gold from Osa Peninsula (after Berrange, 1987)

Gold (%) Silver (%)
Analytical Ave. Max. Min. Ave. Max. Min. Other trace
technique elements identified (%)
Banco Central 93.8 94.5 92.4 5.4 6.8 4.6 Fe, P, Te, Pb, Zn, Ni, Mg
Microprobe (UL) 93.8 3.96 Cu=0.13 Zn=0.09
Microprobe (L) 97.1 0.46 Cu=0.06 Zn=0.15
XRD 95.96 3.68 Fe=1.045 PGE=0.244
(sd=2.08) (sd=1.92) Pb=0.163 Te=0.127
Hg=0.118 Zn=0.044
Cu=0.039 Bi=0.036
As=0.026 As=0.003?
UL = unleached area.
L = Ag-leached rim.

corroded in supergene or soil environments are protected against mechanical
wear. Prior to achieving physical equilibrium the patina may seal the grain
against both internal corrosion and physical decrepitation. Before this happens
however, anodic effects in the depths of cracks may be intense because of the
dissimilar nature of pure gold patinas and native gold. In a physically active
environment such as an abrasive fluvial setting, development of paths of access
may also involve significant and largely destructive processes entailing the
formation of impact craters when small particles are pressed or impacted into
grain surfaces. Cracks and striations may result from high-energy fluvial
deformation or high-pressure glacial action. In some cases, the end result is
fracturing of the grain along the line of corrosion and the liberation of small
equant particles of supergene gold.
Microprobe analyses of a selection of coarse-sized, low-fineness gold from
two placer deposits in Gold Creek, Granite County, Montana (Loen, 1994)
produced data relating to the evolution of nuggets and effects of supergene
processes on both silver depletion and gold enrichment. An ARL SEMQ elec-
tron microprobe was used for quantitative analysis. Semi-quantitative spectro-
graphic data combined with the microprobe data suggested a trace element
signature that could be related back to the type of lode source. Polished sections
of placer gold grains showed areas of colour and reflectance that corresponded
to differences in composition. Many of the grains are rimmed by 0.020 to
0.100 mm thick high-fineness gold whereas the core gold is of lower reflectance
and higher silver content.
Figure 1.3 illustrates a tracing of polished sections of gold grains from the
Master Mine showing location of rims (stippled) and gold fineness patterns
determined by electron microprobes (dots representing analysis points). Grains