Page 49 - Handbook of Gold Exploration and Evaluation

30 Handbook of gold exploration and evaluation

Table 1.10 Estimated earth movement and gold recovered in NW Spain during
Roman times

Deposits Earth Gold Average
movement recovery gold grade
3
3
(m ) (kg) (mg/m )
Alluvial fan (Miocene) 203,000,000 10,200 50
Fluvial deposits (Pliocene) 20,000,000 1800 90
Fluvial deposits (Quaternary) 73,000,000 7300 100
Moraines and residual placers 12,000,000 1200 100
(Quaternary)
Total placers (Neogene-Quaternary) 308,000,000 20,000 67
Quartz veins (late Hercynian) 290,000,000 170,000 600
Total gold 190,000

· the diversity of conditions affecting the liberation and modification of gold
grains in zones of weathering (Chapter 2)
· base level changes induced by isostatic adjustments (Chapter 4)
· the effects of climatic cyclicity and extremes on fluvial transport and sorting
(Chapter 5).
Classifications in Western countries generally follow schemes based upon
geological location and tectonic uplift or depression, as proposed by Lindgren
and summarised in Table 1.10 by Boyle (1979). Lindgren (1911, 1933) taught
that gold placers do not occur haphazardly or by chance, but as a result of
particular sets of geomorphic processes in specific locations. In Sierra Nevada,
he linked the effects of long periods of deep tropical weathering during the
Eocene with the large-scale production of gold-bearing sediments and their
deposition in valleys of the time. By tracing the subsequent history of volcanism
and tectonic uplift, Lindgren provided cogent reasons for the restriction of the
larger deposits to pre-volcanic valley systems and elucidated the genesis of deep
leads. He demonstrated, moreover, that geomorphic processes leave their
imprint upon the geological record, so that an original imperfect understanding
of the evolution of a particular deposit may become clearer when fundamental
principles are used to interpret observed facts.
Boyle (1979) stressed the great complexity of the geological history of
many productive placers noting in particular the wide range of sources from
which auriferous placers derive and the involvement of intermediate collectors
of the gold such as quartzites and conglomerates. Geologists in Asian countries
have generally followed upon Russian experience. Groupings by Smirnov
(1962) consider aspects of both the mechanism of formation of placers and
their geological environment at the time of formation. Considerable attention is
paid to the geomorphic analysis of placer formation and preservation in the
Urals.