The Lithologic, Stratigraphic, and Structural Setting of the Giant Antamina Copper-Zinc Skarn Deposit, Ancash, Peru

摘要

Antamina, located at latitude 9 deg 32 ft S and longitude 77 deg 03 ft W in the Ancash Department of north-central Peru, is the largest known Cu-Zn skarn ore deposit. It incorporates a mineral reserve of 561 Mt, which has an average grade of 1.24 percent Cu, 1.03 percent Zn, 13.71 g/t Ag and 0.029 percent Mo, calculated at a 0.7 percent Cu equiv cutoff grade. The grandite-dominated calcic skarn formed in and around an upper Miocene porphyritic monzogranite stock emplaced into Upper Cretaceous carbonate strata that had experienced thin-skinned, northeast-verging thrusting and folding in the late Eocene Incaic orogeny. The exoskarn Cu-Zn ore is discordant to the strata of the Jumasha and overlying Celendin Formations, which comprise, respectively, massive to thick-bedded, relatively pure limestones and thin-bedded, predominantly marly limestones. The Jumasha Formation, the upper contact of which is locally defined as the top of the uppermost thick-bedded limestone or marble unit, hosts approximately three-quarters of the known exoskarn. Approximately the same fraction of the contiguous endoskarn Cu ore occurs adjacent to this formation. The overlying Celendin Formation is less extensively mineralized but, because it is widely metamorphosed to hornfels and locally converted to diopsidic skarnoid, may have inhibited the upward and outward migration of hydrothermal fluids, thereby promoting the development of the unusually large endoskarn ore zone. Ore also occurs in late hydrothermal breccias emplaced during the formation of mineralized endoskarn. The preskarn thermal metamorphic aureole around the ore deposit is expressed differently in the two host formations. Jumasha Formation limestone is coarsened and bleached to banded gray marble and locally to white marble peripheral to the intrusion and skarn. Minor scapolite occurs in dark gray bands in marble, concentrated in a discontinuous halo tens of meters wide and commonly separated from the skarn by tens of meters. Three facies of calc-hornfels are recognized in the marl beds of the Celendin Formation adjacent to the intrusion extending hundreds of meters beyond sulfide-bearing skarn: a peripheral, very fine grained, light brown phlogopitic fades; an intermediate, fine-grained, gray tremolitic facies; and a proximal, medium-grained, light green diopsidic facies. At an X_((CO)_2) of 0.1 to 0.9 and P = 100 MPa, these zones reflect temperatures increasing to circa 495 deg C adjacent to the intrusion. In addition, in nodular beds of the Celendin Formation that have been metamorphosed to hornfels, diagenetic calcite nodules are selectively replaced by diopside for distances of tens of meters beyond the skarn front. Such calc-silicate formation through both metamorphism and metasomatism, together with a 9 km~2 cluster of Pb-Zn-Ag vein deposits, provides district-scale vectors to ore.