Page 290 - Handbook of Gold Exploration and Evaluation

256 Handbook of gold exploration and evaluation

end of the Pliocene repeated the process with some gold being passed into the
Pleistocene gravels still further below and flanking the Turon River some 3±5 m
above river levels. Erosion of these Pleistocene gravel beds fed gold into the
present active river which, itself, has two levels; the slightly older one through
which the presently active river flows is only active at flood time (refer back to
Fig. 4.29).

The Lakekamu model
Elevated terraces with lateritised clays and outwash boulder beds or fanglom-
erates make higher ground above the flood plain occur along the Olipai River
and elsewhere in the Lakekamu Embayment of Papua New Guinea. The
terraces, possibly of epi-Pliocene age, rest with low-angle unconformity on
bedded mudstone, conglomerates and fine tuffs of Pliocene age, which form the
basement for both the Olipai River and its palaeochannel and of the terraced
fanglomerates. Placer gold occurs firstly in the fanglomerates or palaeochannels
therein and secondly in the flood plain with concentrations in palaeochannels.
Examination of the gold of these two environments suggests that there are more
points of similarity than there are differences. It is probable that the two had a
common primary origin in the late Mesozoic Owen Stanley Metamorphic series
though a somewhat different geomorphic history.
The Olipai palaeochannel is stepped or terraced with four levels, including its
base, indicating pulsatory periods of minor uplift followed by subsidence which
has buried the channel and covered the flood plain with silty, sandy and gravelly
sediment. A discontinuous obstruction to the flow had the effect of accumulating
a barrier of gravels and cobbles that abruptly changed the course of the main
350 m wide palaeochannel into a narrow (less than 50 m wide), scoured channel.
This corresponds roughly with the course of the present Olipai River, which is
diverted for about 500 m to the west before once more opening out and flowing
again in its original southerly direction (Fig. 4.34). The original cause of the
obstruction is not known, but flow in the narrowly confined sector was almost
certainly super-critical and chaotic; only traces of gold were identified in
bedrock drill samples.

Tertiary deep leads
High level sub-basaltic placers are common in Victoria and New South Wales,
Australia and in California, USA and are distinguished from ordinary deep leads
by having a basaltic lava covering. Figure 4.35 (a), (b), (c), (d) illustrates how
remnants of such earlier fluvial deposits may occur as deep leads. A pre-Tertiary
surface (a) with a fluvial placer occupies the lowest portion of the valley. An
outpouring of basalt has mantled the area in (b) and stress fractures develop due
to stretching along the higher portions. This initiates new erosion along the lines