Page 221 - Handbook of Gold Exploration and Evaluation

192 Handbook of gold exploration and evaluation

Cementation may be due in part to residual concentration and surface wash
during surface erosion and partly to mobility in solution, or as colloids
complexed by humic acids produced by rapid degradation of organic matter in
soil. Some gold may also have been contributed directly to the soil after intake
by plants. Reduction of the complexes by oxidation of ferrous iron results in the
incorporation of fine-grained gold with low silver content in iron oxides in the
ferruginous and mottled zones. Lateritic profiles are commonly modified by
cementation in response to change from a humid to an arid climate. In the
Yilgarn Block of Western Australia, the first protracted period of deep chemical
weathering took place in the humid, warm to sub-tropical climates of the
Cretacious to mid-Miocene, probably similar to the wetter savanna climate of
the present (Butt, 1997). High water tables and generally acid conditions during
this regime gave rise to extensive deep lateritic weathering and to profiles
containing great variations in mineralogical and chemical composition.
A post-Miocene arid to semi-arid climatic regime coupled with minor uplift
on the continental margin resulted in a general lowering of water tables and
changes to alkaline chemical weathering reactions (Butt, 1997). Partial trunca-
tion of the regolith occurred due to climatic change (e.g., warm tropic to arid
climatic conditions and/or drainage rejuvenation). The failure of weathering
rates to keep pace with increased erosion rates and erosion of the upper horizon
led to exposure of the lower horizon including bedrock, further reducing the
already low relief.

Dispersion of gold and associated elements
Lateritic deposits with their mixture of high-fineness and low-fineness gold form
widespread blankets over relatively narrowly weathered mineralised zones.
Generally flat-lying, lateritic supergene deposits are developed contiguously with
the ferruginous horizon and underlying mottled layers of the weathering profile.
These deposits contain very fine-grained, low-fineness (>2% Ag) primary gold
with occasional nuggets (Wilson, 1984) and Ag-poor (<0.5% Ag) secondary gold
as fine-grained particles with some euhedral crystals typically associated with
iron oxides.
The behaviour of gold under lateritic conditions is uncertain due to the
overprint of remobilisation under subsequent arid conditions. Butt (1997)
suggests that some indications can be gained by analogy with studies (Freyssinet
et al., 1989; Zeegers and Lecomte, 1992) of regolith geochemistry and the
dispersion of gold and associated minerals in tropical and sub-tropical terrains.
These studies propose that in humid lateritic environments gold is essentially
immobile throughout the saprolite but is physically dispersed in the lateritic
horizons and chemically associated with Fe oxides, to give widespread surficial
haloes. Hence, although the Au-Fe oxide association is dominant in humid tropic
areas, there may be a strong Au-carbonate association in arid areas due to