ALBERT

Description: The Xietongmen district is located 260 km west-southwest of Lhasa in the Tibet Autonomous Region, China. The district occurs within the Gangdese belt, which forms the eastern part of the Trans-Himalayan magmatic belt and is the product of complex magmatic activity that began during the Late Triassic or Early Jurassic and ended in the Eocene. The Xietongmen Cu-Au and Newtongmen Cu-Au-Mo deposits contain a total measured and indicated resource of approximately 610 million metric tons, with additional mineralization in the Langtongmen and Olitongmen Cu-Au prospects. Porphyry mineralization in the Xietongmen district formed during Middle Jurassic volcanic arc activity in the Lhasa terrane, prior to its accretion to the southern margin of Eurasia, and establishes that an economically important, but only recently recognized, metallogenic event is present in the region. Rock types in the Xietongmen district range from Early Jurassic to Eocene in age. Early Jurassic (~188-177 Ma) volcanic, volcaniclastic, and coeval intrusive rock types are crosscut by Middle Jurassic (176-171 Ma) hornblende diorite and quartz diorite porphyry dikes and stocks, including intrusions related to porphyry Cu-Au ± Mo mineralization. The Jurassic igneous assemblage was intruded by mafic dikes between the Late Jurassic and the Cretaceous, then by an Eocene (50-47 Ma) biotite granodiorite batholith and related dikes, and finally by, volumetrically minor lamprophyre dikes. The most important structures in the Xietongmen district are four E-striking, moderately N-dipping, sinistral-oblique thrust faults. Crosscutting and suturing relationships between the TSF-2 thrust fault, located in the south part of the district, and intrusions dated to between 174 and 180 Ma constrain the main stage of thrust fault activity to the Middle Jurassic. The Contact and Adit-1 thrust faults truncate the Xietongmen deposit and form the footwall and hanging wall to mineralization, respectively. Numerous zones of cataclasis deform the Xietongmen deposit between these bounding thrust surfaces. The strongly deformed Langtongmen Cu-Au prospect is located ~1.3 km west of the Xietongmen deposit and occurs in the immediate hanging wall of the Adit-1 thrust fault. The Newtongmen deposit and the Olitongmen Cu-Au prospect occur to the north in the hanging wall to the SBF thrust fault and are not strongly deformed. Mineralization and hydrothermal features in the Xietongmen district are fully compatible with porphyry Cu-Au ± Mo deposits. Alteration, vein types, and mineralization are zoned around quartz diorite porphyry intrusions. Early K silicate alteration and related veins occur within and proximal to the intrusions and contain the highest grade mineralization. In the Xietongmen deposit, the grade of mineralization decreases outward from a core of early biotite-rich K silicate alteration, through a transitional zone in which early K silicate alteration is partially overprinted by quartz-sericite-pyrite alteration, to a peripheral zone of poorly mineralized quartz-sericite-pyrite ± pyrrhotite alteration. Incipient sodic alteration occurs as albite alteration envelopes to quartz-sulfide veinlets in the deepest part of the deposit. Late polymetallic veins and veinlets contain sphalerite, galena, and other base metal sulfides and sulfosalts, occur throughout the Xietongmen deposit, and reflect telescoping during late-stage collapse of the hydrothermal system. Partially developed supergene mineralization forms less than 10% of the Xietongmen deposit. Underlying hypogene mineralization comprises ubiquitous pyrite, chalcopyrite, lesser and more erratically distributed pyrrhotite, and rare molybdenite. The characteristics of the Langtongmen prospect are identical to those found in the deeper parts of the Xietongmen deposit. Characteristics of the Newtongmen deposit are generally similar to those in the Xietongmen deposit but Newtongmen contains only minor supergene mineralization, is cut by very few late polymetallic veinlets, and contains zones of strong, weakly mineralized sodic alteration related to a relatively later stage of quartz diorite porphyry intrusion. The Olitongmen prospect has characteristics similar to the Newtongmen deposit. The Xietongmen deposit and the Olitongmen prospect were thermally recrystallized in a hornfels aureole to the Eocene biotite granodiorite batholith, whereas thermal effects are minor to absent at Langtongmen and Newtongmen. Copper and gold (Au/Cu; ppm / % ) are closely correlated within each of the two main deposits and ratios range from 1.5 to 1.2 in the Xietongmen deposit to between 0.8 and 0.6 in the Newtongmen deposit. Mineralization in the Xietongmen district formed in several coeval mineralized centers and the vein types, alteration and metal assemblages among these centers span a continuum in hydrothermal characteristics. The differences between the mineralized zones are interpreted to reflect exposure at different relative paleodepths as a result of displacement and deformation by posthydrothermal, sinistral-oblique movement on thrust faults. The Xietongmen deposit was transposed to the south by deformation on and between the bounding Adit-1 and Contact thrust faults. The Langtongmen deposit was separated from the deep hanging wall of the Xietongmen deposit and displaced approximately 600 m vertically and 1,300 m to the west by the Adit-1 thrust fault. The Newtongmen deposit and the Olitongmen prospect were uplifted relative to Xietongmen and Langtongmen in the hanging wall to the SBF thrust fault and were not significantly deformed. The genesis and relationship between porphyry deposits in the Xietongmen district can be reconciled by the combined effects of vertical and lateral displacement by thrust faults, preservation of the deposits at different relative paleodepths, and varying degrees of posthydrothermal mechanical and thermal recrystallization.

Description: The Pebble deposit is located ~320 km southwest of Anchorage, Alaska. It is one of the largest porphyry deposits known with a total resource of 10.78 billion tons (Bt). It comprises the East and West zones, which are approximately equal in size, with slightly lower grade mineralization in the center of the deposit where the peripheries of the two zones merge. The West zone was discovered by Cominco America in 1989 and the East zone was discovered by Northern Dynasty Minerals in 2005. The oldest rock in the Pebble district is the Jurassic-Cretaceous Kahiltna flysch unit, which contains basinal turbidites, interbedded basalt flows, and associated gabbro intrusions. These rocks were intruded between 99 and 96 Ma by coeval granodiorite and diorite sills, followed shortly thereafter by alkalic monzonite intrusions and related breccias. Subalkalic hornblende granodiorite porphyry plutons of the Kaskanak batholith were emplaced at ~90 Ma. Similar, smaller granodiorite plutons were emplaced around the margins of the batholith and are related to Cu-Au-Mo mineralization. Re-Os dates on molybdenite are between 89.7 and 90.4 Ma. A Late Cretaceous volcanic and sedimentary "cover sequence" completely conceals the East zone, whereas the West zone is overlain only by glacial sediments and is exposed in one small outcrop. Eocene volcanic rocks and subvolcanic intrusions occur east and southeast of the Pebble deposit and unconsolidated glacial sediments are widespread. The East and West zones represent two coeval hydrothermal centers within a single system. The West zone extends from surface to ~500-m depth and is centered on four small granodiorite plutons emplaced into flysch, diorite, and granodiorite sills, and alkalic intrusions and breccias. The much higher grade East zone extends to at least 1,700-m depth and is hosted by a larger granodiorite pluton and adjacent granodiorite sills and flysch. The granodiorite plutons merge with depth. On the east side of the deposit, high-grade mineralization has been dropped 600 to 900 m into the NE-trending East graben, where the deposit remains undelineated to the east and to depth. Variations in hypogene grade and metal ratios reflect multiple stages of metal introduction and redistribution. Hornfels related to the Kaskanak batholith formed prior to hydrothermal activity at Pebble and is most intensely developed in flysch. Disseminated and vein-hosted Cu-Au-Mo mineralization, dominated by chalcopyrite and locally accompanied by bornite, formed with potassic alteration in the shallow part of the East zone and with approximately coeval sodic-potassic alteration in the West zone and at depth in the East zone. Illite ± kaolinite alteration overprinted potassic and sodic-potassic alteration throughout the deposit and variably redistributed copper and gold. High-grade copper-gold mineralization is related to advanced argillic alteration controlled by a synhydrothermal brittle-ductile fault zone which overprinted potassic, sodic-potassic, and illite ± kaolinite alteration in the East zone. Advanced argillic alteration comprises a core of pyrophyllite alteration associated with chalcopyrite, bounded to the west by an upward-flaring zone of sericite alteration which contains hypogene bornite, digenite, covellite, and trace enargite and tennantite. Late quartz veins introduced additional molybdenum in several parts of the deposit. Grade-destructive quartz-sericite-pyrite alteration forms a halo to the entire deposit and yields outward to propylitic alteration. A quartz-illite-pyrite cap is preserved in the weakly mineralized center of the deposit. Mineralization at Pebble is predominately hypogene. A thin, incompletely developed zone of supergene mineralization occurs in the West zone and is overlain by a thin leached capping. There is no evidence for paleosupergene mineralization or leaching below the cover sequence in the East zone. Molybdenite contains high concentrations of rhenium throughout the deposit. Elevated palladium concentrations are associated with pyrophyllite alteration in the East zone. The Pebble deposit occurs in one of a number of large, deep-seated magnetic anomalies which are located at the intersection of crustal-scale structures both parallel and at high angles to a mid-Cretaceous magmatic arc. This setting is similar to fertile porphyry environments in northern Chile and suggests that southwestern Alaska is highly prospective for porphyry exploration. The large size and high hypogene grades of the Pebble deposit may reflect a combination of multiple stages of metal introduction with vertically restricted, lateral fluid flow induced by hornfels aquitards in flysch.

Description: The Pebble Cu-Au-Mo porphyry deposit is located approximately 320 km southwest of Anchorage, Alaska. Shortwave infrared (SWIR) spectroscopy on drill core from the deposit has been used to document the distribution of alteration assemblages characterized by subtle variations in phyllosilicate minerals that cannot be confidently distinguished by visual criteria alone. At Pebble, these phyllosilicate alteration types have different histories of metal introduction and/or redistribution. Delineation of the distribution of these assemblages is critical to the geologic and genetic interpretations of the deposit. Spectral absorption features between 1,300 and 2,500 nm (in particular, small shifts in the position of the absorption wavelength related to AlOH bonds around 2,200 nm) allows distinction among illite-, sericite-, kaolinite-, and pyrophyllite-bearing alteration assemblages. Electron microprobe and X-ray diffraction analyses were used to validate the chemical composition and crystallinity of the phyllosilicate minerals identified using spectral data. The results confirm the use of SWIR spectroscopy to confidently identify and spatially delineate phyllosilicate alteration assemblages at Pebble. These alteration types include potassic, illite ± kaolinite, quartz-illite-pyrite, sericite, pyrophyllite, quartz-sericite-pyrite, and sodic-potassic assemblages. The highest gold and copper concentrations within the deposit are in the eastern pluton and are coincident with low AlOH values associated with pyrophyllite and sericite alteration. An additional zone of low AlOH values, not associated with high metal grades, occurs in the northeast along the margins of the deposit, coincident with quartz-sericite-pyrite alteration. The approach described here has significantly improved three-dimensional alteration mapping and shows that short wave infrared spectroscopy may successfully distinguish variations in phyllosiclicate species. This has implications for exploration because clay speciation is genetically related to the distribution of metals in the Pebble deposit. The recognition and utilization of these relationships has produced a robust three-dimensional alteration model, which can be applied to optimizing mine planning, comminution, and mineral process design.

Description: The key magmatic processes that lead to the formation of large magmatic-hydrothermal porphyry copper mineral deposits remain uncertain, and a particular question is why a few of these deposits, such as the Pebble porphyry Cu-Au-Mo deposit, are strongly enriched in both gold and molybdenum. This study investigated the igneous rocks of the Pebble district and obtained major and trace element compositions, Sr and Nd isotopic compositions, and zircon age and trace element data to model the origin of the ore-forming magmas. The Pebble porphyry Cu-Au-Mo deposit, one of the world’s largest Cu-Au resources, formed during the final stages of regional Late Cretaceous arc magmatism (101–88 Ma) in the Southwest Alaska Range. Local pre-mineral intrusions (99–95 Ma) are dominated by alkaline compositions including monzodiorite stocks, shoshonite dikes, and monzonite porphyries, but also include lesser volumes of high-K calc-alkaline diorite and granodiorite sills. The occurrence of early alkaline magmas has been noted at other gold-rich porphyry systems, including Bingham and Kerr-Sulfurets-Mitchell. Mineralization at Pebble is associated with granodiorite to granite porphyry dikes related to the 〉165 km 2 high-K calc-alkaline Kaskanak granodiorite batholith (91–89 Ma). Over a period of 10 m.y., Late Cretaceous melts evolved from high temperatures (930–730 °C) and modestly hydrous and oxidized conditions to relatively low temperatures (760–680 °C) and very hydrous and oxidized conditions. Collectively, all Late Cretaceous igneous rocks at Pebble contain magnetite and little or no ilmenite, are metaluminous to weakly peraluminous, and have typical arc trace element enrichments and depletions. They have moderate Sr/Y ratios (20–55) and gently sloped REE profiles (La/Yb = 5–20) that are not adakitic, which supports a source area lacking garnet that is consistent with a thin crust in southwest Alaska. Radiogenic isotopes for Late Cretaceous intrusions at Pebble have a restricted range of primitive Sr and Nd isotopic compositions ( 87 Sr/ 86 Sr i = 0.70329–0.70424; Nd i = 4.9–6.1), which overlap with volcanic and plutonic basement rocks of the Jurassic Talkeetna Arc along the Alaska Peninsula. The Kaskanak batholith intrudes the Late Jurassic–Early Cretaceous Kahiltna flysch, and mixing models using Sr and Nd isotopes indicate that the Kaskanak batholith assimilated ≤10 wt% Kahiltna flysch in amounts that did not likely affect magma fertility. Xenocrystic zircon samples are abundant in Cretaceous pre-mineral intrusions and have U-Pb ages similar to detrital zircon samples in the Kahiltna flysch. These data support some assimilation of upper crustal Kahiltna flysch, but the dominance of Devonian–Mississippian xenocrystic zircon populations in some intrusions suggests derivation from unexposed older basement. The extraordinary endowment of Cu and Au at Pebble is inferred to result from primitive calc-alkaline and alkaline arc magmas and the hydrous and strongly oxidized conditions that suppressed the formation and fractionation of Cu- and Au-enriched sulfide melts. Furthermore, differentiation to silicic compositions was a product of extensive crystal fractionation of parental melts accompanied by minor crustal assimilation. The trace element content of the intermediate composition intrusions indicates that both hornblende and titanite fractionation processes in the mid- to shallow-crust were both required to produce the more evolved granodiorite and granite porphyry compositions. Despite the apparent lack of Mo-enriched continental crust in the region, primitive hydrous melts were produced by protracted arc magmatism and were modified by minor crustal assimilation including early alkaline magmatism, periodic recharge of mafic hydrous basalts and hybrid andesites, and fractional crystallization, which was apparently sufficient to enrich Mo in late stage felsic melts.