New Mexico Geological Society Annual Spring Meeting — Abstracts

Copper oxides are presently a topic of great intrest in the mining and metallurgical industry because of the ease with which they can be leached and the resulting low cost of copper extraction. Copper oxide minerals occur in the oxidized zone of porphyry copper deposits as a result of the oxidation of hypogene copper sulfides, and of former enrichment zones as a result of in situ oxidation of "enrichment blankets". The formation of copper oxide minerals can occur in any climate, but extremely arid conditions engender the formation of exceptionally deep oxidized zones as a result of sparse flushing with surface water and very deep phreatic zones. Oxide zones of laterally continuous copper oxide development in extremely arid environments have been noted as deep as 500 meters below the present topographic surface.

The depth of the local phreatic zone, in general, controls the depth of oxidation. Oxidizing conditions persist into the phreatic zone although the oxygenated water at depth is very reduced compared to the near surface water. Reducing conditions persist into the oxidized volume as well, resulting from capillary action moving reduced water along fractures. As a result of the channelized fracture flow and capillary action a vadose zone is formed in which a combination of sulfide and oxide minerals are stable.

Acid conditions are created as oxygenated ground water migrates, especially along fractures and oxidizes sulfide minerals. The result of this is the formation of oxide and sulfate minerals that are dissolved and transported by periodic flushing in to reduced rock volumes where supergene sulfide minerals form. If the local phreatic zone is lowered and if the sulfide minerals experience in situ oxidation, the result will be a geochemically thick copper oxide zone.

The copper oxide minerals that form are a function of local pH conditions within the deposit. Chalcanthite and krohnkite, along with a series of iron oxides, occur in very acid conditions: these minerals are very water-soluble and tend to be redissolved, surrendering copper to copper oxide minerals such as antlerite and brochantite. In moderate pH conditions minerals such as neotocite, copper pitch and cuprite are stable. Conditions favorable for malachite and azurite exist when there is a sufficient amount of reactive carbonate material in the wall rock or sufficient carbonate in solution. Chrysocolla will form if there is adequate silica in solution.