Page 148 - Handbook of Gold Exploration and Evaluation

126 Handbook of gold exploration and evaluation

Alunite KAl 3 (SO 4 ) 2 (OH) 6 and clays such as kaolinite, picilite and pyro-
phyllite are stable under acidic conditions and are common, but minor, con-
stituents of high-sulphidation gold systems. Quartz is the principal gangue
mineral and propylitic alteration, controlled by rock composition, occurs outside
the conduit zones in both low and high-sulphidisation styles. Albite, calcite,
epidote and pyrite are typical minerals in regions of low water±rock ratios.
Alteration varies both vertically and laterally and high-sulphidation orebodies in
zones of most acidic alteration are surrounded by mineral assemblages indicat-
ing less acid conditions as the acidic water is neutralised by reaction with the
host rock away from the fluid conduit.

2.5 Provenance

The term `provenance' is derived from the French `provenir' and the Latin
`provenere' meaning to originate or come forth. In a residual gold setting,
provenance refers to both the source and country rocks from which gold-bearing
placers and lateritic regoliths are derived. In order to link economic concen-
trations of detrital gold to broad groups of gold-bearing rocks in the immediate
hinterland:

· There must have been an adequate catchment of source rocks.
· The rocks must have been sufficiently weathered to release the gold.
· The regime must have induced efficient methods of transport.
· Concentration must have taken place under lateritic conditions of weathering,
or in alluvial deposits by some form of fluvial or aeolean transportation.
· Detailed petrology may be needed to trace detrital gold that has been recycled
through sediments such as beach concentrations back to their original source.

2.5.1 Significance of provenance
Residual gold deposit types have unique features and locally, the weathering of
source rocks provides for the movement of chemically altered and partly
fragmented gold-bearing material into the drainage system. The detritus is
diagnostic of the source and associated country rocks from which it is derived.
Fragments of unstable rock varieties close to the source, have sharp edges and
are only partly weathered. A lesser amount of fresh rock is incorporated in the
next alluvial cycle and so on until ultimately little or no rock pebbles remain in
the stream sediment. At this stage, although the altered country rocks could
contain distinctive assemblages of secondary or hydrothermally altered minerals
as haloes around the parent ore bodies, the initial source may be rendered
difficult or impossible to trace by conventional panning methods.
The free gold content of the sediment is determined by the extent to which the
gold grains have been released and/or modified during the weathering processes.