Page 291 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 257

4.34 Course change due to obstruction in the original Olipai River, Lakekamu
Embayment, Papua New Guinea.

of weakness as shown in (c). Finally the former channel deposits remain as high-
level gravels protected by a capping of basalt high above present stream levels
(d). Initially the outpouring of Tertiary basalts preserved a fluvial gold placer
over pre-Tertiary alluvial landforms. Ultimately the Tertiary basalt was almost
completely removed by the erosive forces leaving remnants of fluvial placers
such as the one depicted in the above illustration on the tops of hills protected by
the basaltic capping.

Fluvio-Aeolean placers

Deserts cover about 30% of the continental surface and vary from small areas
covered by bare rock undergoing erosion, to vast areas covered by dunes that are
in constant motion. The great tropical deserts of the world occur along the tropic
of Cancer at latitudes 15±35 ëN and the tropic of Capricorn at latitudes 15±35ë S.
These deserts lie under virtually stationary sub-tropical cells of high pressure
characterised by a subsiding air mass that is adiabatically cooled and dried as it
sinks. Precipitation is largely convectional and unreliable in tropical deserts,
typically less than 25 cm annually and sometimes less than 5 cm. The principal
tropical desert regions are the Kalahari and Sahara (Africa) and the Thar Parka
(India and Pakistan).
Major desert regions in the middle latitudes, 35±50ë N and 35±50ë S, occur in
central Asia, Australia and the Great Basin and Mojave Desert areas of the