Abstract

High resolution 40 Ar/ 39 Ar dating of porphyry stocks and related hydrothermal minerals from porphyry copper and epithermal precious metal deposits in the Potrerillos district of Chile demonstrates the very short duration of hydrothermal activity in these ore-forming systems. Ages of primary magmatic phenocrysts and hydrothermal alteration minerals in seven distinct intrusive centers show that individual porphyry stocks and their hydrothermal systems formed in very brief episodes, within a period of igneous activity lasting at least 8 m.y. during late Eocene through early Oligocene time. During this period of igneous activity, porphyry stocks that produced classic porphyry copper ore and porphyry-related epithermal gold ore were emplaced intermittently with barren porphyry stocks.At the Cobre porphyry copper deposit, 40 Ar/ 39 Ar ages of fresh hornblende phenocrysts (35.87 + or - 0.21 Ma), primary and secondary biotite (35.65 + or - 0.03 Ma), and sericite from late-stage vein envelopes (35.64 + or - 0.03 Ma) suggest that this hydrothermal system cooled from hornblende argon closure temperature (ca. 570 degrees C) to biotite closure (ca. 300 degrees C) in about 230,000 yr or less. At the smaller Norte porphyry copper deposit, ages of hornblende phenocrysts (36.76 + or - 0.20 Ma) and plagioclase phenocrysts (36.64 + or - 0.08 Ma) suggest that this porphyry stock and its hydrothermal system cooled below the plagioclase argon closure temperature (ca. 250 degrees C) in 120,000 yr or less.Magmatic biotite (40.82 + or - 0.19 Ma) in a porphyry stock that is spatially associated with the El Hueso epithermal gold deposit and hydrothermal sericite (40.25 + or - 0.05 Ma) from gold-bearing quartz veinlets in porphyry at El Hueso are 5 m.y. older than the Cobre porphyry copper deposit. This result contradicts previous claims of a genetic link between the El Hueso and Cobre deposits but supports a genetic link between epithermal- and porphyry-type deposits in the district. The age of coarse-grained alunite (36.23 + or - 0.08 Ma) associated with diaspore, pyrophyllite, dickite, and zunyite demonstrates that advanced argillic alteration adjacent to the epithermal deposit significantly postdated Au mineralization there, attesting to the superimposition of a second hydrothermal system at El Hueso. The heat source that drove this younger hydrothermal system has not been identified.A barren porphyry stock (Bochinche, 37.91 + or - 0.06 Ma) is the only porphyry in the district that has known cogenetic volcanic deposits. This barren stock was emplaced after the Gonzalez copper porphyry (38.85 + or - 0.06 Ma) and before the Norte copper porphyry. Like the mineralized porphyries, dating shows that the Bochinche porphyry cooled very rapidly (within 90,000 yr) from hornblende argon closure temperature to below the argon closure temperatures of biotite and plagioclase (250 degrees -300 degrees C). Two other barren porphyry stocks, Cerro El Hueso (33.13 + or - 0.06 Ma) and Cuesta (32.56 + or - 1.80 Ma), have no known related volcanic rocks.Numerical modeling of conductive heat flow in a shallowly emplaced, 500-m-wide porphyry stock shows that it would cool below the argon closure temperature of all datable minerals in less than 40,000 yr; hydrothermal circulation would shorten this time span. This result suggests that the duration of many ore-forming, porphyry-related hydrothermal systems is not resolvable by the best available geochronologic methods unless hydrothermal activity has been prolonged by multiple, spatially coincident intrusive events.

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