Dating of supergene alunite-group mineral separates from various ore facies and elevations by laser 40Ar/39Ar incremental heating reveals a supergene history extending over at least 20 m.y. Fifteen multistep age spectra for alunite, natroalunite, and jarosite yielded acceptable plateau dates, ranging from the earliest Oligocene, 35.26 +/- 0.68 Ma, to the middle Miocene, 14.59 +/- 2.46 Ma; five samples of alunite-natroalunite mixtures, however, produced staircase spectra with no direct age significance. Following hypogene mineralization in the middle Eocene, the deposit was probably unroofed at ca. 42 Ma, at the initiation of the Incaic orogeny in northern Chile. An age of 35.26 +/- 0.68 (2sigma) Ma for alunite associated with hematite in a clast in gravel overlying the Leached cap demonstrates that both formation and oxidation of a stage I chalcocite blanket had occurred by earliest Oligocene. During stage II (>35.26 +/- 0.68 Ma to <22.42 +/- 1.6 Ma), continued uplift and pedimentation caused leaching and thickening of the blanket, forming the surviving hematitic Leached cap and chal-cocite-dominated Upper Supergene ore. Stage III (>21.49 +/- 0.49 Ma to 19.25 +/- 0.43 Ma) was initiated by the major Pehuenchean (Aymara) tectonic event, when rapid regional uplift and a drastic fall in the water table renewed intense leaching to form the Lower Leached zone. At this time the chalcocite blanket was thickened below the Lower Leached zone, generating the Lower Supergene ore, and oxidized above it to form the Cu oxide-dominated Upper Supergene ore. At 19.25 +/- 0.43 Ma, an ignimbrite flow, up to 95 in thick, covered much of the deposit, temporarily interrupting supergene activity but imposing lateral ground-water flow through the lower 20 m. Oxidation was then reactivated, generating chrysocolla veins along northwest-trending fractures, which are inferred to be genetically linked to exotic mineralization in gravels below the ignimbrite 1.5 km north of the deposit. Further uplift and climatic desiccation in the middle Miocene, during stage V (<14.59 +/- 2.46 Ma), terminated significant supergene processes and preserved the existing supergene orebody from lateral dispersion of metal. Dating of supergene alunite-group mineral separates from various ore facies and elevations by laser 40Ar/39Ar incremental heating reveals a supergene history extending over at least 20 m.y. Fifteen multistep age spectra for alunite, natroalunite, and jarosite yielded acceptable plateau dates, ranging from the earliest Oligocene, 35.26 +/- 0.68 Ma, to the middle Miocene, 14.59 +/- 2.46 Ma; five samples of alunite-natroalunite mixtures, however, produced staircase spectra with no direct age significance. Following hypogene mineralization in the middle Eocene, the deposit was probably unroofed at ca. 42 Ma, at the initiation of the Incaic orogeny in northern Chile. An age of 35.26 +/- 0.68 (2sigma) Ma for alunite associated with hematite in a clast in gravel overlying the Leached cap demonstrates that both formation and oxidation of a stage I chalcocite blanket had occurred by earliest Oligocene. During stage II (>35.26 +/- 0.68 Ma to <22.42 +/- 1.6 Ma), continued uplift and pedimentation caused leaching and thickening of the blanket, forming the surviving hematitic Leached cap and chal-cocite-dominated Upper Supergene ore. Stage III (>21.49 +/- 0.49 Ma to 19.25 +/- 0.43 Ma) was initiated by the major Pehuenchean (Aymara) tectonic event, when rapid regional uplift and a drastic fall in the water table renewed intense leaching to form the Lower Leached zone. At this time the chalcocite blanket was thickened below the Lower Leached zone, generating the Lower Supergene ore, and oxidized above it to form the Cu oxide-dominated Upper Supergene ore. At 19.25 +/- 0.43 Ma, an ignimbrite flow, up to 95 in thick, covered much of the deposit, temporarily interrupting supergene activity but imposing lateral ground-water flow through the lower 20 m. Oxidation was then reactivated, generating chrysocolla veins along northwest-trending fractures, which are inferred to be genetically linked to exotic mineralization in gravels below the ignimbrite 1.5 km north of the deposit. Further uplift and climatic desiccation in the middle Miocene, during stage V (<14.59 +/- 2.46 Ma), terminated significant supergene processes and preserved the existing supergene orebody from lateral dispersion of metal. Dating of supergene alunite-group mineral separates from various ore facies and elevations by laser 40Ar/39Ar incremental heating reveals a supergene history extending over at least 20 m.y. Fifteen multistep age spectra for alunite, natroalunite, and jarosite yielded acceptable plateau dates, ranging from the earliest Oligocene, 35.26 +/- 0.68 Ma, to the middle Miocene, 14.59 +/- 2.46 Ma; five samples of alunite-natroalunite mixtures, however, produced staircase spectra with no direct age significance. Following hypogene mineralization in the middle Eocene, the deposit was probably unroofed at ca. 42 Ma, at the initiation of the Incaic orogeny in northern Chile. An age of 35.26 +/- 0.68 (2sigma) Ma for alunite associated with hematite in a clast in gravel overlying the Leached cap demonstrates that both formation and oxidation of a stage I chalcocite blanket had occurred by earliest Oligocene. During stage II (>35.26 +/- 0.68 Ma to <22.42 +/- 1.6 Ma), continued uplift and pedimentation caused leaching and thickening of the blanket, forming the surviving hematitic Leached cap and chal-cocite-dominated Upper Supergene ore. Stage III (>21.49 +/- 0.49 Ma to 19.25 +/- 0.43 Ma) was initiated by the major Pehuenchean (Aymara) tectonic event, when rapid regional uplift and a drastic fall in the water table renewed intense leaching to form the Lower Leached zone. At this time the chalcocite blanket was thickened below the Lower Leached zone, generating the Lower Supergene ore, and oxidized above it to form the Cu oxide-dominated Upper Supergene ore. At 19.25 +/- 0.43 Ma, an ignimbrite flow, up to 95 in thick, covered much of the deposit, temporarily interrupting supergene activity but imposing lateral ground-water flow through the lower 20 m. Oxidation was then reactivated, generating chrysocolla veins along northwest-trending fractures, which are inferred to be genetically linked to exotic mineralization in gravels below the ignimbrite 1.5 km north of the deposit. Further uplift and climatic desiccation in the middle Miocene, during stage V (<14.59 +/- 2.46 Ma), terminated significant supergene processes and preserved the existing supergene orebody from lateral dispersion of metal.
