PATTERNS OF OROGENIC EROSION CONSTRAINED BY PORPHYRY COPPER DEPOSITS

Geologic controls on the emplacement and preservation of porphyry copper deposits have long been a focus of scientific and industry inquiry. Porphyry copper deposits are formed at an average depth of ~2 km below the earth’s surface and exposed through tectonic uplift and climate driven erosion processes. The time between formation of porphyry copper deposits and their arrival at or near the surface should therefore be related to the vigor of geomorphic processes on the surface, especially for geologically young deposits. To assess this prediction, we have compared data on global Cenozoic porphyry copper deposits with patterns of tectonic activity, mean annual precipitation, and topographic slope. To first order, we find a concentration of young (<10 Ma) deposits globally between 15^oS and 15^oN, where precipitation is generally high (> 1 m/yr) and tectonic rock-uplift is rapid (e.g. Papua New Guinea, Northern Andes, and the Philippines). To second order, steeper landscapes also tend to produce younger deposits. Regional complexities in factors such as rock erodibility complicate simple global quantitative relationships between deposit age (i.e. erosion) and precipitation rate or topographic slope. However, focusing the analysis on a single orogen, the Andes in South America, reveals a clear relationship between tectonics, climate, and porphyry copper age. The oldest deposits (~30-60 Ma) are found in the Atacama region where hyper-aridity and flat-slab subduction have stalled erosional processes. The youngest deposits (<10 Ma) are found in the tropical regions and along the eastern margin of the Sierra Pampeanas. Previous analysis has revealed that the vertical distribution of porphyry copper deposits over long timescales behaves diffusively. The results here imply that over shorter timescales (e.g. within the last 60 My), the spatial distribution of deposit ages is influenced by the geomorphic system. We conclude that the ages of ore deposits emplaced at constant average depths provide a new means of constraining long-term erosion rates in active convergent margins.