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Magmatic and hydrothermal stratigraphy of Paleocene and Eocene porphyry Cu-Mo deposits in southern Peru Simmons, Adam Thomas


A geological investigation of the of the Paleocene-Eocene Cu-Mo porphyry belt of southern Peru was undertaken at the Quellaveco Cu-Mo porphyry system with complimentary work completed at the Cuajone and Toquepala mines. This work adds to the understanding of the formation and evolution of igneous systems associated with Cu-Mo porphyry formation in an understudied but economically significant copper source globally. The Quellaveco Igneous Complex is composed of at least five phases of porphyritic intrusions which were emplaced into a slightly older equigranular granodiorite batholith and Late Cretaceous Toquepala Group volcanic rocks. These intrusions were emplaced from approximately 58Ma to 53Ma. The intrusions with the closest temporal and spatial association to copper may have been emplaced in as short a timespan as approximately 1.5m.y. These time periods and time spans are almost identical to those for the emplacement of the igneous systems at Cuajone and Toquepala. At Quellaveco, zircon mineral chemistry indicates that the igneous systems associated with porphyry copper formation are different to those not associated with porphyry copper formation. Geochemical disruptions, flat to cup shaped Eu/Eu* anomalies and rapid changes in temperature of formation of the zircons with their growth are indicative of zircons from the porphyry intrusions from Quellaveco. It is postulated that these geochemical signatures may indicate that intrusions associated with the formation of the hydrothermal systems are derived from fractionating and cooling magmatic systems which remained relatively oxidized and were subject to periodic geochemical disturbance. The geochemical disturbances may reflect a dynamic deeper magmatic system where magma mixing, replenishment and scouring of wall rock took place. At Quellaveco, each of the porphyritic intrusions are temporally related with a hydrothermal system. Each hydrothermal system is an evolution of mineral assemblages in veins from biotite-magnetite-chalcopyrite rich veins to quartz-sulphide rich veins to quartz vein with quartz-sericite selvages. This sequence of vein evolution repeats itself at Quellaveco at least four times. Each hydrothermal system occurs immediately after the emplacement of at least five porphyry intrusion suites.

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