Abstract

Previously published and newly determined K-Ar dates of biotite, hornblende, and augite in the Bingham area are reinterpreted according to their relative sequence of geological events. All K-Ar dates from altered and unaltered igneous rocks in and around the Bingham mine fall within a time span of 3.2 m.y., between 39.8 + or - 0.4 m.y. and 36.6 + or - 0.3 m.y., and suggest a close genetic relationship between igneous and hydrothermal activity.The oldest igneous rocks in the area are represented by the equigranular monzonite of the Last Chance stock and the equigranular monzonite phase of the Bingham stock. The monzonites form part of one intrusive phase, dated at 39.8 + or - 0.4 m.y. Younger ages occur in the same rock type and are probably the result of argon degassing close to the younger quartz monzonite porphyry intrusion. This porphyry was the hydrothermal heat source around which the different alteration and sulfide assemblages are zoned. The time of emplacement of the quartz monzonite porphyry cannot be ascertained because no unaltered quartz monzonite porphyry exists at the surface nor in drill holes.Plugs of quartz latite porphyry in the central pit have few fractures and are weakly altered and sparsely mineralized, suggesting that they intrude mainly after sulfide mineralization and alteration. The plugs cut strongly mineralized, altered, and fractured monzonite and quartz monzonite porphyry; they appear to be the youngest intrusive rocks in the mine. The average age of four K-Ar determinations of biotites from the plugs is 38.8 + or - 0.3 m.y.Porphyry copper mineralization most likely took place during several pulses of hydrothermal activity that started shortly after the intrusion of equigranular monzonite (39.8 m.y.) and probably culminated prior to the emplacement of quartz latite porphyry plugs in the central pit (38.8 m.y.). The youngest K-Ar age obtained from the mine area was determined from a selected sample of biotites with a large proportion of hydrothermally altered. Mg-rich biotite and probably represents one of the last pulses (36.6 + or - 0.3 m.y.).

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