Giant porphyry deposits: Characteristics, distribution, and tectonic controls

More than half of the 25 largest known polphyry copper deposits, defined in terms of contained copper metal. formed during three time periods: the Paleocene to Eocene, Eocene to Oligocene, and middle Miocene to Pliocene. These giant deposits are c;istered within three provinces , central Chile, northern Chile, and southwest Arizona-northern Mexico. Other giant deposits occur in Montana, Utah, Panama, Peru, Argentina, Irian Java Mongolia and Iran. Compressive tectonic em,viroments, thickened continental crust,are active up-lift and erosion were associated with the Formation of many y oof these deposits. Cale-alkalic magmas are most favorable for the formation of giant porphyry, copper deposits, although several of the largest systems are associated with high K calc-alkalic intrusions. The 25 largest gold-rich porphyry, deposits are concentrated in the southwest Pacific and South America, other occurrences in Eurasia, British Columbia, Alaska, and New South Wales. Many of the deposits formed ill the last 1.3 The largest of the deposits are associated with high K calc-alkalic intrusions. Many calc-alkalic porphyritic intrusions have also produced giant gold-rich porphyries. In the last 20 m.y., the formation of giant porphyry copper-molybdenum and copper-gold deposits in the circum-Pacific reion has been closely associated with subduction of aseismic ridges, seamount chains, and oceanic plate ILIS beneath oceanic islalld and continental arcs. In several examples, these tectonic perturbations have promoted flat-shab subduction, crustal thickening, uplift and erosion, and adakitic magmatism coeval with the formation of well-endowed polphyry and/or epithermal mineral provinces. Similar tectonic features are inferred to be associated with the aiant porphyry copper-molybdemim provinces of northern Chile (Eocene-Oligocence) and sonthwest United States (Cretaceous-Paleocenc). Topographic and therinal anomalies oil the downgoing slab appear to act as tectonic triggers for porphyry ore formation. Other factors, such as slitures ill tile overridilig plate, permeability architecture of the upper crust, efficient Processes of ore transport and deposition, alld, ill some cases, formation and preservation of'stipergene einichment blankets are also vital for the development of higli-grade giant one deposits. A low-grade o aeochemical anomaly may be the final product of mineralization, if ore-forming processes do not operate efficiently evcn in the most hAvorable gcodynanlic settings.