Page 283 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 249

Boulton (1982) and Drewry (1986) were amongst the first to recognise the
effects on placer formation of wet-based ice flow smearing of englacial debris
over the underlying substrate. Working in the same general area of British
Columbia, they all drew attention to the highly dynamic nature of sub-glacial
flow in both the high-pressure transport of fine-grained sediment at the base of
the ice and the channelled fluvial transport of coarse and fine-grained sediment.
The process by which englacial till is released from the base of the ice as the
glacier moves over the underlying bed involves frictional resistance and
pressure melting. Debris within the basal layer of the glacier is lodged against
the substrate when ice velocities are less than 50 m/y; at higher velocities, the
bed is swept clear and erosion becomes dominant. Rates of lodgement till
deposition in modern sub-glacial settings are reported to be around 2 cm/y.
Eyles and Eyles (1992) illustrate the widely varying response of conditions at
the base of large ice sheets to different ice temperatures and velocities in Fig.
4.30 (a), (b), (c) and (d).
Eyles and Kocsis (1989) describe the genesis and overall characteristics of
economic glacial placers within lodgement till complexes of the Cariboo Mining
District. The principal gold pay zones associated with these complexes are
shown in Fig. 4.31. The authors in Fig. 4.32 present a schematic representation
of Pleistocene stratigraphy and associated placer mines in north central British
Columbia. The very coarse, nuggety character of the Cariboo gold placers is
thought to have resulted from the incorporation of pre-glacial Tertiary
paystreaks in basal tills along low parts of the valley floors.

Glacial outwash fans
Outwash fans are built up by melt-water erosion of glacial debris on gravel
plains. Over time, some of them may extend up to several kilometres in length
and hundreds of metres in width at their terminal end. Many carry economic
gold and are quite productive both for themselves and as secondary provenances
for ongoing fluvial processes. As early as the Inca period, glacial outwash fan
deposits were worked on both the southwestern (altiplano) and Cordilleran
slopes in the Andes of Peru and Bolivia (Fornari et al., 1982). In Papua New
Guinea, auriferous conglomerates deposited under periglacial conditions in the
Lakekamu Embayment, extend for about 40 km from their source. Small-scale
mining ventures in the outwash fan have recovered more than 70,000 oz. of gold
from shallow alluvial operations. Preliminary testing of channels at lower
stratigraphic levels has indicated a much greater potential for the area as a
whole.
Geomorphological control of gold evolution and distribution in glacial and
fluvio-glacial placers of the Ancocala-Ananea Basin, Southeastern Andes of
Peru has been studied by Herail et al. (1989). The moraines of the two glacia-
tions Ancocala and Chaquiminas (middle and upper Pleistocene) provide