Tectonic controls, timing and geochemistry of supergene enrichment of the Tyrone porphyry copper deposit, Grant County, New Mexico

The Tyrone porphyry copper deposit is located in the Big Burro Mountains 16 km (10 miles) southwest of Silver City, New Mexico, and is part of the Laramide porphyry copper province in the southwestern US and northern Mexico. The deposit is associated with the Tyrone stock, a composite stock of granodiorite, monzonite porphyry, and quartz monzonite porphyry that intrudes Proterozoic granite. The Tyrone stock has been dated by Phelps Dodge (unpublished) at 55.8 Ma to 52.1 Ma by 40Ar/39Ar on magmatic biotite and orthoclase. Hypogene copper grades average 0.09% Cu in chalcopyrite; supergene ore currently averages 0.3% for run-of-mine leach material, but chalcocite ore averaging 2-3% Cu was mined historically.;The research reported here addresses formation of the Tyrone supergene system and had five main objectives: (1) determine the number and timing of uplift, unroofing and supergene enrichment events; (2) assess the lateral variations in hypogene sulfide content and alteration assemblage across the district and the controls they exercised on formation of the laterally-zoned supergene system; (3) map the distribution of limonite minerals that reflect the supergene ore types; (4) determine the amount of erosion of the hypogene copper-bearing system and the extent of northeast copper flux in the supergene environment; and (5) assess the groundwater conditions during supergene enrichment.;Tyrone underwent extensive supergene sulfide enrichment during the Tertiary. Detailed 40Ar/39Ar dating for this study has resolved three stages of supergene enrichment, which record the progressive unroofing of the deposit.;40Ar/39Ar age determinations on alunite indicate that main-stage enrichment occurred in the Miocene between 22.2 and 8.7 Ma. The Miocene enrichment event was related to initiation of Basin and Range normal faulting that probably began as early as 26 Ma.;A third, minor supergene enrichment event occurred in the Pliocene-Pleistocene with uplift of the Little Burro Mountains on the Mangas fault, a northwest-striking Basin and Range normal fault that separates the Big Burro and Little Burro mountains. A sample of supergene alunite from the southern Little Burro Mountains yielded an age of 4.6 Ma. Supergene enrichment diminished in the Pliocene with the onset of arid conditions, and the chalcocite blanket underwent widespread in-situ oxidation as pyrite was consumed in the oxidizing column.;Drill hole logs indicate that the hypogene system in the northeastern part of the district is characterized by a thick, nonreactive phyllic alteration assemblage, pyrite:chalcopyrite averaging ∼11:1, and hypogene sulfide content of 2-5 vol. %. The leached cap in the northeastern part of the district is dominated by hematite and overlies a thick, well-developed chalcocite blanket with a poorly-developed, mostly late-stage copper oxide zone. The hypogene system in the southwestern part of the deposit is characterized by a reactive potassic alteration assemblage, pyrite:chalcopyrite averaging ∼7:1, and low total hypogene sulfide content of 0.5-2.5 vol. %. The leached cap is dominated by goethite and overlies a well-developed copper oxide zone and a thin, poorly-developed chalcocite blanket.;The unreactive phyllic alteration assemblage, high pyrite:chalcopyrite, and high total hypogene sulfide content in the northeastern part of the district resulted in strongly acidic conditions during supergene enrichment, thorough copper leaching, and strong enrichment. In contrast, the reactive potassic alteration assemblage and lower content of acid-producing hypogene sulfides in the southwestern part of the district resulted in weak leaching and poor enrichment. The copper oxide zone in the southwestern part of the district formed in part during active enrichment. The northeast to southwest lateral zoning of the hypogene system is due in large part to approximately 20° of northeast tilt of the deposit that elevated the potassic zone in the southwestern part of the district while depressing and preserving the phyllic zone in the northeastern part of the district.;Tyrone is one of few deposits in the Laramide porphyry copper province with clear evidence for both Eocene and Miocene supergene enrichment events that have been dated by 40Ar/39Ar. The Tyrone data indicate that evidence for Eocene supergene enrichment in these deposits is probably often destroyed by Miocene uplift, leaching, and erosion. (Abstract shortened by UMI.)...