Discussion
Which one of the following mineral associations is diagnostic of the highly-oxidized zone of a weathered primary base metal vein?
Answer: A
The association diagnostic of the highly oxidized zone is smithsonite, malachite, anglesite.
Above the water table, oxygenated water breaks down the primary sulphides of a base metal vein, and zinc, copper and lead are re-deposited as oxidized minerals: smithsonite (ZnCO3), malachite (Cu2CO3(OH)2) and anglesite (PbSO4). All three are secondary oxidation products of the very metals a base metal vein carries.
Calcite, fluorite and pyrite are primary vein gangue and sulphide, unaltered. Chalcocite belongs to the supergene enrichment zone below the water table, which is reducing, and realgar is a sulphide. Goethite is an oxidation product, but orpiment is an arsenic sulphide and boehmite is an aluminium hydroxide of bauxites, not of base metal veins.
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