Why are copper minerals near the top of porphyry deposits often oxides?

Copper minerals near the top of porphyry deposits are typically oxides because they have been subjected to weathering at the surface.12

When sulfide minerals—which are the primary ore minerals found in the deeper levels of these deposits—are exposed to surface conditions, they undergo oxidation. This process produces sulfuric acid, which facilitates the leaching of copper from the upper portion of the deposit. The acidified rainwater carries the copper downward as copper sulfate toward the water table.12

Consequently, the upper, weathered zone of the deposit becomes depleted of its original sulfide minerals and is left with copper oxides, while the copper that was leached away is often redeposited at or just below the water table in a process known as supergene sulfide enrichment.31