Page 281 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 247

· Discontinuous valley margin paystreaks were produced by a radial dispersion
of primary gold paystreaks away from centre valley positions.
· Some paystreaks were dispersed and reconcentrated at the base of the glacial
system while others were telescoped to lower levels.
· Glaciers with outlets into the ocean deposited englacial material at the
shoreline, to be reconcentrated by shallow marine processes on beaches and
deltas.
· Further entrenchment then occurred as a result of interglacial warming and
renewed sedimentation in the valleys.
This situation is demonstrated in a schematic cross-section of the Upper Turon
River, NSW (Fig. 4.29). The youngest alluvials lying topographically below the
older gravels following the reworking of alluvials during three periods of uplift.
Stratigraphically the third cycle is the present stream and the cycle runs late
Pliocene, Pleistocene, and recent. Volcanic action accompanying orogenic
upheavals protected sections of existing placers from further erosion under a
cover of ash and lava to depths of up to 500 m and more. Renewed uplift and
tilting in late Pliocene-early Pleistocene times caused streams to cut deeply into
the volcanic rocks. Successive episodes of glacial and humid interglacial

4.29 Superposition of drainage in the Upper Turon River, NSW.