Abstract

Copper ore grades of the Cuajone, Quellaveco, and Toquepala porphyry Cu(-Mo) deposits, situated at 3,000 to 4,000 m a.s.l. on the Pacific slope of the Cordillera Occidental of southernmost Peru (lats 17 degrees 02'-17 degrees 15' S), have been markedly increased by supergene sulfide enrichment. Following the emplacement of the hypogene mineralization in the early Eocene (52-57 Ma) as the terminal stage in the development of the Toquepala Group continental volcano-plutonic terrane, this Andean transect was gradually reduced by erosion in a semiarid climate to a low altitude topography; unroofing of the deposits had taken place by the mid-Oligocene. The initiation of cordilleran uplift at ca. 25 to 26 Ma, accompanied by the episodic eruption of felsic ash-flow tuffs at the oceanward front of the volcanic arc, led to more rapid erosion and the conversion of the mid-Tertiary landscape into the subplanar Altos de Camilaca surface. This regionally extensive pediplain constitutes the major landform component of the present precordillera surrounding the porphyry centers; its final configuration was attained at ca. 18 to 19 Ma. Ash-flow tuff eruption was widespread and frequent at this time.Supergene enrichment began in the late Oligocene during the progressive lowering of topography and continued through the more abrupt water-table depression resulting from the latest Oligocene to early Miocene uplift. However, the distribution of chalcocite, sensu lato, in the three porphyry deposits and the local postmineralization landforms and volcanic histories differed significantly during the latter interval. At Toquepala and, to a lesser extent, Quellaveco, the landform regimes were dominated by open valleys and ignimbrite blanketing was short-lived, whereas the exposed Cuajone deposit was both the site of aggressive early Miocene valley incision and the deposition of an unusually thick, in part welded, ash-flow tuff at 22.8 + or - 0.7 Ma. As a result, the enrichment blanket at Cuajone remained relatively thin and was even partially eroded in the early Miocene, whereas the blankets at Quellaveco and Toquepala were thickened.Continued strong uplift in the mid-Miocene (ca. 8-15 Ma) generated the array of apron and terrace pediments of the Multiple Pediment stage which dominates the lower cordilleran slopes and, in the mineralized areas, led to the deepening of existing valleys and the development of new fluvial channels. Ignimbrite eruption persisted throughout the Miocene. Again, local regimes of erosion and volcanism proved inimical to supergene activity at Cuajone, but enrichment continued at Quellaveco and, particularly, Toquepala, where there is no record of later Miocene ash-flow accumulation. Uplift in this period was apparently also most extensive in the vicinity of the Toquepala deposit, which experienced the formation of a deep chalcocite blanket, while the extant enrichment zone at Quellaveco was thickened.The development of enriched assemblages containing "massive" chalcocite had clearly terminated by 13.1 + or - 0.4 Ma at Cuajone and by 9.5 + or - 0.5 Ma at Quellaveco; the timing of later enrichment at Toquepala is less constrained, but a mid-Miocene age is probable. Supergene upgrading of the three Peruvian deposits was contemporaneous with that of copper mineralization in northernmost and northern Chile, as at Chuquicamata and La Escondida, and in the Copiapo mining district, in all of which areas a late Oligocene to mid-Miocene age has been inferred for supergene alteration. This major and regionally developed metallogenic episode took place during and between two major periods of cordilleran uplift, under semiarid climatic conditions. The termination of intense enrichment along this 2,000-km stretch of the Cordillera Occidental in the late Miocene was a direct result of marked climatic desiccation, which not only reduced the overall supergene activity but focused fluvial erosion, leading to the incision of steep-walled canyons less favorable for supergene alteration than the earlier subplanar landforms.

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