Page 282 - Handbook of Gold Exploration and Evaluation

248 Handbook of gold exploration and evaluation

intervals in the Quaternary then profoundly affected both local and global
weathering conditions.

Glacial till
Spoil deposited by glaciers is termed drift or till. Tills that are brought together
directly by the ice, i.e. without fluvial transport, are deposited in the form of
lateral moraines along the sides of the glacier and as terminal moraines during
glacial retreat. A terminal moraine appears at the furthest advance of the glacier
as it recedes and successive recessional moraines are deposited at intervals of
stillstand in its retreat. Medial moraines occur where lateral moraines of
intersecting glaciers join together centrally in the ice flow. Drumlins of clayey
till form groups of oval shaped hills tapering in the direction of the ice flow.
Long sinuous ridges of sands and gravels (eskers) of fluvio-glacial origin, mark
the sites of melt water streams flowing in crevasses and tunnels within or at the
base of the ice. Eskers, capable of transporting large volumes of englacial
sediments at high velocities are formed under considerable hydrostatic pressure.
Exposure of till to glacially induced flow at the base of a glacial system
creates `lodgement' till, which is to a greater or lesser extent both stratified and
sorted. Eyles and Kocsis (1989) note the common enrichment of the basal
portions of lodgement till as a result of the sluicing action of sub-glacial melt
waters. The presence of intraformational gravels within lodgement till sequences
record erosion and deposition by sub-glacial rivers. A glacially reworked
lodgement till is often covered by `ablation' till when the ice melts in a stagnant
marginal zone. Lodgement till and ablation till are termed `sub-glacial' tills and
are classified on the basis of the processes involved in their formation and
location. Other till classification applies to `sublimation' tills and `melt-out'
tills, as products of glacial reworking.

Sub-glacial placers

Glacier-related placers have been discussed widely in the literature but until
recently, gold-bearing glacial debris has been regarded as of little economic
importance except where it has been upgraded by post-glacial stream processes.
Wells (1969) quotes Blackwelder (1932): `Since it is the habit of a glacier to
scrape off loose debris and soil but not to sort it at all, ice is wholly ineffective as
an agency of metals concentration.' And ± `If a glacier advances down a valley
which already contains gold-bearing gravel, it is apt to gouge out the entire
mass, mix it with much other debris and deposit it later as useless till. Under
some circumstances however, it merely slides over the gravel and buries it
without distributing it.' On the other hand, Boyle (1979) recognised the import-
ance of `auriferous glacial outwash gravels' and `post-glacial stream gravels' in
placers of the Cariboo (Barker) Mining District, British Columbia, Canada.