Title page for ETD etd-02072013-040058
|Type of Document
||Groen, John Corwyn
||Gold-enriched rims on placer gold grains:an evaluation of formational processes
||Master of Science
|Craig, James R.
|Rimstidt, james Donald
|Ribbe, Paul H.
|Date of Defense
Placer gold is frequently reported to assay at overall higher values of tineness than
the gold in the rock from which it was liberated. A related phenomenon is the historical
discovery of many extremely large gold nuggets (up to 28 lbs) in the southeastern United
States that have no apparent source rock. Placer gold grains from the southeastem United
States have been examined and found to frequently exhibit the development of nearly pure
to pure gold rims around their borders. These gold rims are suggested as a possible cause
of the high frneness placer deposits. Formation of very thick rims may also be the cause
of the large nuggets. Formation of these gold-enriched rims by the often attributed
mechanism of simple silver leaching is disputed on the basis of ineffective mechanisms
for the removal of silver from the alloy. Diffusion of silver through the gold at low
temperatures proceeds far too slowly to produce the chemical gradients observed in the
placer gold grains. Comparison of the complexation capacities of 41 ligands with
subsequent modelling of expected complex concentrations in natural stream and stream
sediment waters indicates CN- and S2- as the most likely functional ligands for the
transport and redeposition of supergene gold. Electrolytic refming of placer Au—Ag grains
is also a process for forming gold-enriched rims that can operate together with secondary
enrichment to produce the observed phenomena.
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