HYPOGENE AL UNITE FROM THE EL SALVADOR DISTRICT, CHILE, INDICATES POTENTIAL FOR A BLIND PORPHYRY COPPER CENTER

Surface samples of hypogene alunite that cement late breccia bodies from the El Salvador porphyry copper district of Chile were recently dated. One alunite sample over the principal Turquoise Gulch porphyry deposit has a 40Ar/ 39Ar total gas age of 40.64 +/- 1.04 Ma, overlapping the age of a late latite intrusion. Two other samples associated with quartz-alunite replacement of rhyolite, similar to 750 m southwest of the collapse zone over the block cave of the porphyry copper deposit, are distinctly younger, at 38.12 +/- 0.66 and 38.04 +/- 0.22 Ma (averages of duplicate analyses, with +/- 2 sigma errors). Previously reported U/Pb ages of zircons from 15 Eocene-age diorite, granodiorite, and granite porphyry intrusions have weighted mean ages that range from about 44 to 41 Ma, with peak magmatic flux interpreted at 44 to 43 Ma. Porphyry copper ores in the El Salvador district formed at about the same time as porphyry intrusions, with intrusive centers that migrated in a south-southwest direction, from the small deposits at Cerro Pelado (similar to 44.2 Ma), to Old Camp (similar to 43.6 Ma) and M Gulch-Copper Hill (similar to 43.5-43.1 Ma), to the main ore deposit at Turquoise Gulch (similar to 42 Ma). The granodiorite porphyry intrusions at Turquoise Gulch are associated with similar to 80% of the known copper ore of the district; they record waning stages of magmatism at 42.5 to 42.0 Ma, followed by weakly altered latite dikes at 41.6 Ma. Molybdenite in quartz veins returned Re-Os ages of 41.8 to 41.2 Ma. The two alunite samples from our study with coincident dates of similar to 38 Ma provide evidence for magmatichydrothermal activity younger than any recognized to date, consistent with the alteration overprint of quartzalunite on older muscovite after erosion. This younger activity must have been associated with a blind intrusion, likely located south of the Turquoise Gulch deposit, based on the distribution of alteration minerals, and offset from the zoning associated with the Turquoise Gulch center. Stable isotope values (delta S-34, delta O-18, delta D) of the similar to 38 Ma alunite indicate a high-temperature hypogene origin, consistent with formation in a lithocap environment that typically is located at shallow levels over and on the shoulders of porphyry copper deposits. Both observations-alteration overprint and markedly younger age of alunite-indicate the potential for porphyry copper mineralization south of Granite Gulch, as much as 1,000 m below the level of the coeval outcropping quartzalunite replacement, perhaps near similar to 2,000-m elevation; this is hundreds of meters deeper than the known copper ore of Turquoise Gulch. Surface samples of hypogene alunite that cement late breccia bodies from the El Salvador porphyry copper district of Chile were recently dated. One alunite sample over the principal Turquoise Gulch porphyry deposit has a 40Ar/ 39Ar total gas age of 40.64 +/- 1.04 Ma, overlapping the age of a late latite intrusion. Two other samples associated with quartz-alunite replacement of rhyolite, similar to 750 m southwest of the collapse zone over the block cave of the porphyry copper deposit, are distinctly younger, at 38.12 +/- 0.66 and 38.04 +/- 0.22 Ma (averages of duplicate analyses, with +/- 2 sigma errors). Previously reported U/Pb ages of zircons from 15 Eocene-age diorite, granodiorite, and granite porphyry intrusions have weighted mean ages that range from about 44 to 41 Ma, with peak magmatic flux interpreted at 44 to 43 Ma. Porphyry copper ores in the El Salvador district formed at about the same time as porphyry intrusions, with intrusive centers that migrated in a south-southwest direction, from the small deposits at Cerro Pelado (similar to 44.2 Ma), to Old Camp (similar to 43.6 Ma) and M Gulch-Copper Hill (similar to 43.5-43.1 Ma), to the main ore deposit at Turquoise Gulch (similar to 42 Ma). The granodiorite porphyry intrusions at Turquoise Gulch are associated with similar to 80% of the known copper ore of the district; they record waning stages of magmatism at 42.5 to 42.0 Ma, followed by weakly altered latite dikes at 41.6 Ma. Molybdenite in quartz veins returned Re-Os ages of 41.8 to 41.2 Ma. The two alunite samples from our study with coincident dates of similar to 38 Ma provide evidence for magmatichydrothermal activity younger than any recognized to date, consistent with the alteration overprint of quartzalunite on older muscovite after erosion. This younger activity must have been associated with a blind intrusion, likely located south of the Turquoise Gulch deposit, based on the distribution of alteration minerals, and offset from the zoning associated with the Turquoise Gulch center. Stable isotope values (delta S-34, delta O-18, delta D) of the similar to 38 Ma alunite indicate a high-temperature hypogene origin, consistent with formation in a lithocap environment that typically is located at shallow levels over and on the shoulders of porphyry copper deposits. Both observations-alteration overprint and markedly younger age of alunite-indicate the potential for porphyry copper mineralization south of Granite Gulch, as much as 1,000 m below the level of the coeval outcropping quartzalunite replacement, perhaps near similar to 2,000-m elevation; this is hundreds of meters deeper than the known copper ore of Turquoise Gulch. Surface samples of hypogene alunite that cement late breccia bodies from the El Salvador porphyry copper district of Chile were recently dated. One alunite sample over the principal Turquoise Gulch porphyry deposit has a 40Ar/ 39Ar total gas age of 40.64 +/- 1.04 Ma, overlapping the age of a late latite intrusion. Two other samples associated with quartz-alunite replacement of rhyolite, similar to 750 m southwest of the collapse zone over the block cave of the porphyry copper deposit, are distinctly younger, at 38.12 +/- 0.66 and 38.04 +/- 0.22 Ma (averages of duplicate analyses, with +/- 2 sigma errors). Previously reported U/Pb ages of zircons from 15 Eocene-age diorite, granodiorite, and granite porphyry intrusions have weighted mean ages that range from about 44 to 41 Ma, with peak magmatic flux interpreted at 44 to 43 Ma. Porphyry copper ores in the El Salvador district formed at about the same time as porphyry intrusions, with intrusive centers that migrated in a south-southwest direction, from the small deposits at Cerro Pelado (similar to 44.2 Ma), to Old Camp (similar to 43.6 Ma) and M Gulch-Copper Hill (similar to 43.5-43.1 Ma), to the main ore deposit at Turquoise Gulch (similar to 42 Ma). The granodiorite porphyry intrusions at Turquoise Gulch are associated with similar to 80% of the known copper ore of the district; they record waning stages of magmatism at 42.5 to 42.0 Ma, followed by weakly altered latite dikes at 41.6 Ma. Molybdenite in quartz veins returned Re-Os ages of 41.8 to 41.2 Ma. The two alunite samples from our study with coincident dates of similar to 38 Ma provide evidence for magmatichydrothermal activity younger than any recognized to date, consistent with the alteration overprint of quartzalunite on older muscovite after erosion. This younger activity must have been associated with a blind intrusion, likely located south of the Turquoise Gulch deposit, based on the distribution of alteration minerals, and offset from the zoning associated with the Turquoise Gulch center. Stable isotope values (delta S-34, delta O-18, delta D) of the similar to 38 Ma alunite indicate a high-temperature hypogene origin, consistent with formation in a lithocap environment that typically is located at shallow levels over and on the shoulders of porphyry copper deposits. Both observations-alteration overprint and markedly younger age of alunite-indicate the potential for porphyry copper mineralization south of Granite Gulch, as much as 1,000 m below the level of the coeval outcropping quartzalunite replacement, perhaps near similar to 2,000-m elevation; this is hundreds of meters deeper than the known copper ore of Turquoise Gulch.