The formation age of ores from the Pebble Cu-Au-Mo giant deposit (Alaska, United States)

Abstract

In the past decade, several copper-porphyry deposits, to which significant resources of associated gold are related, have been explored [1]. The Pebble copper-molybdenum-porphyry deposit in southwest Alaska is of this kind as well (Fig. 1a). As of January 31, 2009, the explored copper resources of this deposit are 36.6 mil. t (with the cutoff grade of conditional copper being 0.3 vol. %), gold resources are 3042 t (average gold content is 0.30 g/t), and the molybdenum resources are 2529 t (0.02 vol. %) [2].The ore body is an ENE extended stockwork of 4.0 × 2.4 km in area; the depth of ore mineralization reaches 300-400 m in the west and 1200-1650 m in the east (Fig. 1b). The principal ore minerals are pyrite, chalcopyrite, and molybdenite; minor miner als, such as bornite, covellite, digenite, and enargite, occur mainly in quartz sulfide veinlets in the form of impregnation in altered host rocks.Based on the geological ratios, the following stages of metasomatic changes in porphyry bosses and host rocks are identified (with the corresponding types of ore mineralization) [2]:(1) Early, quartz-K spar-biotite with magnetite (pyrite-chalcopyrite mineralization).(2) Late, quartz-K spar-biotite (molybdenite mineralization with low grade gold).(3) Illite metasomatism, which is not accompanied by specific sulfide mineralization and superimposed products of the first two stages.(4) Intensive argillization (copper redeposition, release of free gold, substitution of bornite-covellitedigenite-(enargite) association for the chalcopyritepyrite one). Argillizated rocks and accompanying mineralization are superimposed as a continuous cover both on the porphyry intrusives and on the host rocks, including metasomatic ones and ores of all the preceding stages, forming a united ore body of the deposit. In the ore body, the general vertical Mo → Cu → Au zoning is clearly manifested from bottom to top (Fig. 1c).(5) Quartz-sericite-pyrite alteration in the exter nal zone of the ore field: pyritization of the early stages with copper and molybdenum removal and gold pres ervation (up to 100-150 mg/g).Summarizing, we can conclude that the Pebble Cu-Au-Mo deposit formed as a result of spatial com bination of the early post igneous copper-molybde num mineralization, which was related to crystalliza tion of porphyry intrusives, and the late epithermal gold-sulfide mineralization caused by circulation of hydrothermal solutions, which were related to another source. This process led both to redistribution of pri mary ores and to their hypogenic secondary enrich ment caused by mobilization of copper and gold from the scattered mineralization zones beyond the contour of the primary ore body [3] and from the eroded upper part of the deposit. Abstract-Zircons from the porphyry like quartz-diorite boss of the Pebble Cu-Au-Mo deposit (southwest Alaska) have been examined. By their appearance and internal structure (cathode luminescence and electron probing), the zircons have been subdivided into four genetic groups: (1) xenogenic detrital (mainly ro...