The geology, geochronology, and geochemistry of the Kaskanak Batholith, and other late Cretaceous to Eocene magmatism at the Pebble porphyry Cu-Au-Mo deposit, SW Alaska Public Deposited

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  • The Pebble porphyry Cu-Au-Mo deposit in Alaska is one of the world's largest Cu-Au mineral resources. Late Cretaceous magmatic evolution in the Pebble district culminated with the intrusion of the Kaskanak Batholith and associated porphyry copper-gold-molybdenum mineralization. The Kaskanak Batholith is a multiphase granodiorite intrusion with an estimated footprint of ≥150 km². The batholith is exposed at surface west of the deposit and lies at >600 m depth in the East Zone. The geology, geochemistry, and geochronology of the Pebble district intrusions were investigated to better understand the magmatic processes and their relationship to the formation of this giant ore deposit. The principal zones of mineralization in the Pebble district are the West Zone and East Zone, with prospects containing mineralized porphyry intrusions located at the 38 Zone, 308 Zone, and 65 Zone. Together, they constitute an estimated resource of 1.08 Bt of ore containing 36.5 Mt Cu, 2.54 Mt Mo, and 3340 M gm Au. The main and equigranular granodiorite phase of the Kaskanak Batholith forms a series of porphyritic cupolas along its roof. These cupolas are cross-cut by three distinct porphyry dike sets associated with mineralization in the district. These are: 1) a voluminous granodiorite porphyry plug in the East zone, 2) quartz granite porphyry dikes in the West Zone, East Zone, and 38 Zone, and 3) narrow leucocratic quartz granite porphyry dikes in the 38 Zone and 65 Zone. New SHRIMP-RG and LA-ICP-MS U-Pb analyses of zircon from 25 samples, together with previously published ages establish a period of 9 m.y. for Late Cretaceous magmatism in the district from 98 to 89 Ma. Pre-ore diorite, alkalic monzodiorite and monzonite porphyry intrusions, and granodiorite sills were emplaced between ~98-95 Ma; the main equigranular granodiorite was emplaced at ~91 Ma; and the younger mineralized porphyries were emplaced at ~90-89 Ma (e.g., 90.3±1.0 Ma, 89.4±1.4 Ma; 89.2±1.2 Ma for these porphyries, respectively). New recognition of pre-mineral and post-mineral andesite porphyry dikes are observed in the 1 Gold Zone, the 308 Zone, and in the East Zone. New U-Pb ages on the pre-mineral andesite porphyry dikes in the 308 Zone are 90.6±1.5 Ma and 91.1±1.2 Ma. Following the cessation of hydrothermal activity, the area was magmatically quiescent until the end of the Cretaceous. The age of renewed magmatism is established by new U-Pb age determinations of dacite and andesite dikes of 63.9±1.0 Ma and 65.9±1.4 Ma, respectively. These dikes cross-cut mineralization in the East Zone, but are eroded by a Paleocene erosional unconformity. The maximum age of the erosional unconformity is constrained by the 11 youngest detrital zircons from a sample of basal conglomerates of the Copper Lake Formation that immediately overlies the unconformity (e.g., 61.2±0.8 Ma). Based on petrographic and geochemical evidence, the Kaskanak Batholith was apparently more oxidized and hydrous than earlier local intrusions that predate mineralization. The Kaskanak Batholith is characterized by high ratios of whole rock V/Sc (up to 160), zircon Eu/Eu[subscript CN]* (>0.4) and zircon Ce/Nd[subscript CN] (>40). Zircon trace element compositions of the Kaskanak Batholith are distinct from pre-ore or post-ore Paleocene-Eocene intrusions. Ti-in-zircon geothermometry indicates pre-ore intrusions were hotter (~750-940° C) than Kaskanak Batholith phases (~685-760° C). Zircon Ce/Ce[subscript CN]* and Eu/Eu[subscript CN]* values are elevated in all phases of the Kaskanak Batholith as well as in pre-ore granodiorite sills, and within some zircons of late monzonite porphyry dikes, which reflects an increase in ƒO₂ and H₂O content from early pre-ore intrusions to emplacement of the Kaskanak Batholith. Investigation of apatite SO₃ and halogen concentrations suggest that the Kaskanak Granodiorite melt initially contained 0.1-0.3 wt. % SO₃ and had initial Cl/H₂O ratios of 0.3-0.6. The presence of SO₃-rich apatites hosted in primary biotite, occasionally in magnetite, and commonly within interstitial quartz and K-feldspar was observed. These sulfur-rich apatites may have crystallized from hot andesitic melts that subsequently mixed with the Kaskanak Granodiorite, or by breakdown and release of magmatic anhydrite upon volatile exsolution, or a combination of both. Mafic enclaves have been observed locally within the Kaskanak Granodiorite, but observed andesitic melts in the district make up much less than a fraction of a percent of the volume of the Kaskanak Batholith. On the basis of whole rock major and trace element compositions, the Kaskanak Batholith likely differentiated from hydrous and oxidized calc-alkaline andesitic melts. Compositions of andesitic porphyries from the 1 Gold Zone are inferred to represent parental melt compositions. Raleigh fractionation modeling suggests the Kaskanak Granodiorite can be produced by 10-12 wt. % crystal fractionation of amphibole, biotite, magnetite, apatite, and zircon, and the evolved porphyry dikes could have been produced by an additional 10-14 wt. % fractionation of amphibole, titanite, apatite, and zircon. Titanite fractionation at relatively low temperatures was apparently important for sharply depleting evolved porphyry melts of REEs, Nb, and Ta. Jurassic to Eocene age Pebble district intrusions of basaltic to granitic compositions, all have non-radiogenic initial isotopic signatures (⁸⁷Sr/⁸⁶Sr[subscript i] = 0.70329 - 0.70424 and ¹⁴³Nd/¹⁴⁴Nd[subscript i] = 0.51278 - 0.51284 (ƐNdi = +4.9 - +6.1); t = 180, 90, & 65 Ma) reflecting the age and bulk composition of the crustal section. These intrusions are interpreted to have been derived from homogenous shoshonitic and calc-alkaline andesites parental melts generated by melting of the mantle wedge and overlying lower crust yielding similar Sr and Nd isotopic compositions to those of the Peninsular Terrane in the Talkeetna Mountains. Xenocrystic zircon from Pebble district intrusions were derived primarily from the Kahiltna Basin sediments upon emplacement, but a greater component of Paleozoic and Proterozoic grains have also been observed that are unlikely derived in whole from the Kahiltna Basin sediments, consistent with xenocrystic zircons found in some Talkeetna Arc volcanics along the Alaska Peninsula that pre-date the formation of the Kahiltna Basin sediments. These zircons may have been derived from late Triassic - early Jurassic metamorphosed sediments and volcanics that predate Talkeetna Arc magmatism. During the lifespan of the Kaskanak Batholith, it is estimated that ~2 km of cover rocks were unroofed which produced telescoping advanced argillic ledges on top of potassic alteration in the East Zone. By latest Late Cretaceous – Paleocene time (~67-58 Ma), an additional ~2.5 km were rapidly eroded and subsequently buried by volcanoclastic rocks and tuff deposits of the Copper Lake Formation, which may have been initiated by the subduction of the Kula-Farallon mid-ocean ridge. The bulk of the displacement of the ZG Fault that down-dropped high-grade copper ore in the East Graben likely occurred at this time, and much of the mineralized advanced argillic alteration and epithermal-style mineralization overlying the Pebble deposit had been removed. Subsequent eastward Eocene - Quaternary tilting (~20°) has exposed the Kaskanak Batholith to the bedrock surface at Kaskanak Mountain and in the West Zone.
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