Reverse Osmosis contributing to metal zoning in porphyry type deposits: A case study

• Metal zoning at Infiernillo deposit differs from a symmetric-expected cooling model.
• Reverse Osmosis favors ore concentration during the phyllic stage at Infiernillo.
• Potential-energy gradients induced phyllic halo to behave as semi-permeable membrane.
• Different rejection/passage values for ions induced osmotic differentiation of metals.

Representing the main source of copper in the world, porphyry copper deposits have been widely studied. Different models have tried to explain the observed metal zoning, but they did not completely explain it. A Permian Cu–Mo porphyry deposit in the San Rafael Massif in Argentina shows a similar metal zoning pattern to those described elsewhere. However, some particular features depart from the conventional cooling model. Based on data from this deposit and on theoretical background, we present and discuss Reverse Osmosis as a complementary process which could have contributed to porphyry metal zoning during the phyllic stage. The existence of potential-energy gradients and the different relative rejection values for distinct ions make Reverse Osmosis a natural mineral-concentration process. At temperatures corresponding to those of porphyry phyllic halo formation and in presence of a phyllic (clay) membrane, Cu, Ag, Zn and Pb would show a strong osmotic differentiation that could have led to the observed metal zoning.

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