INSIGHTS INTO THE MIGRATION AND CONCENTRATION OF CU DURING WEATHERING FROM CU ISOTOPE FRACTIONATION

Cu isotopes are known to fractionate in low temperature aqueous environments. The fractionation can be observed in minerals that are the residues and products of oxidative chemical weathering. To show case how the Cu isotope composition of these minerals can be used to understand ancient fluid pathways, we compare and contrast Cu isotope data from 5 porphyry copper deposits. We examined deposits from three different locations (the Southwestern USA, Central Peru, and the Philippines).

The southwestern USA and Philippines porphyry copper deposits display a predicted pattern in Cu isotope compositions of minerals where the leach cap minerals are depleted inδ⁶⁵Cu in comparison to enrichment minerals at depth. The pattern demonstrates the removal of ⁶⁵Cu and precipitation at depth. A pattern in the plan view map pattern of the Cu isotope composition of chalcocite from the Philippines can be used to determine the lateral migration of Cu. In contrast, the deposit from central Peru lacks enriched δ⁶⁵Cu chalcocite beneath the leach cap. The results from Peru could indicate a hypogene source of chalcocite instead of a supergene source of chalcocite. This comparative analysis is used to summarize how the Cu isotope composition of minerals can be used to unravel complex supergene processes.