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Olson N.H.,Oregon State University | Dilles J.H.,Oregon State University | Kent A.J.R.,Oregon State University | Lang J.R.,Hunter Dickinson Inc.
American Mineralogist | Year: 2017

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 km2 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 (87Sr/86Sri = 0.70329-0.70424; ϵNdi = 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. © 2017 by Walter de Gruyter Berlin/Boston.

Chakhmouradian A.R.,University of Manitoba | Reguir E.P.,University of Manitoba | Kressall R.D.,Dalhousie University | Crozier J.,Hunter Dickinson Inc. | And 3 more authors.
Ore Geology Reviews | Year: 2015

The Aley Nb deposit in northern British Columbia, Canada, is hosted by metamorphosed calcite and dolomite carbonatites of anorogenic affinity emplaced in Lower Paleozoic sedimentary carbonate rocks in the Devonian. Primary Nb mineralization consists of pyrochlore (commonly comprising a U-Ta-rich and F-poor core) and ferrocolumbite developed as discrete crystals and replacement products after the pyrochlore. These phases and associated heavy minerals (apatite ± magnetite ± zircon ± baddeleyite) precipitated early in the magmatic history and probably formed laterally extensive cumulate layers up to at least 1.5 m in thickness. Fractionation of copious amounts of pyrochlore is reflected in the chemical composition of the carbonatites and their constituent minerals, which show large variations in Nb/Ta value, but a near-chondritic Zr/Hf ratio. Alkali-rich metasomatic rocks (in particular, fenites and glimmerites) associated with the carbonatites are barren; the bulk of Nb in these rocks is contained in rutile, phlogopite and, to a much lesser extent, amphibole. When the passive margin of North America became the zone of plate convergence in the Cretaceous, the host carbonatites were strongly deformed, which is manifested in structures and textures indicative of grain comminution, ductile flow, folding and, locally, brecciation. The structure and continuity of the cumulate units enriched in Nb minerals were profoundly affected by these processes. Interaction of the carbonatites with crustal fluids of complex chemistry resulted in extensive dolomitization, replacement of the pyrochlore and ferrocolumbite by fersmite, and development of hydrothermal parageneses consistent with the lower greenschist-facies conditions. At these late evolutionary stages, Nbwas mobilized only to a very limited extent and sequestered in a variety ofminerals (fersmite, euxenite,Mg-rich ferrocolumbite and Nb-bearing rutile) typically occurring as scarce minute crystals associated with hydrothermal dolomite, quartz and chlorite. Progressive enrichment of the deformed dolomite carbonatites in heavy C and O isotopes relative to primary calcite, coupled with changes in the trace-element composition of Nb phases, indicate that the fluids were equilibrated with the wall-rock sedimentary rocks hosting the Aley deposit and were capable of transporting F-, (PO4)3-, U, Th and rare-earth elements, but not Nb. © 2014 Elsevier B.V.

Simandl G.J.,British Geological Survey | Simandl G.J.,University of Victoria | Paradis S.,Geological Survey of Canada | Stone R.S.,British Geological Survey | And 5 more authors.
Geochemistry: Exploration, Environment, Analysis | Year: 2014

This study evaluates the suitability of portable (handheld) X-Ray fluorescence spectrometry (pXRF) in the exploration for Aley-type 'hard-rock' (primary) carbonatite-hosted Nb deposits. The assessment consists of comparisons between: (1) results of pXRF analyses on selected pulp samples and results of analyses of the same pulps using traditional laboratory methods; (2) results of averaged, multiple pXRF spot field analyses performed directly on 10 to 15 cm long pieces of core (before pulverization) compared with those of traditional laboratory analyses of the same pieces of core after pulverization; and (3) results of a manual core scanning method compared with the results of conventional analytical methods of the pulps of the corresponding scanned sections. A strong correlation exists between pXRF measurements on pulps and laboratory methods for most specialty metals, such as Nb (r2 = 0.99), La (r2 = 0.97), Ce (r2 = 0.67), Y (r2 = 0.93), and P (r2 = 0.89); however, the values of r2 for Pr and Nd are 0.19 and 0.38, respectively. As expected, textural heterogeneities within sample intervals reduced the quality of pXRF results when multiple spot readings were taken directly on the core. Nevertheless, the data can still be used to identify carbonatite-related Nb (± other specialty metal mineralization) and delimitate potentially economically significant zones within it. The core scanning reduced the degree of variation associated with spot analyses. Scanning is useful during the early exploration stages, but provides data limited by the inability of the operator to maintain constant scanning speed. The scanning results correlate with laboratory methods for Nb (r2 = 0.88), Th (r2 = 0.80), Fe (r2 = 0.84), Sr (r2 = 0.74), Ba (r2 = 0.73), Y (r2 = 0.59), and Zn (r2 = 0.75). The values of r2 for La, Ce, Pr, and Nd were only 0.31, 0.26, 0.01 and 0.03, respectively, suggesting that concentrations of these elements were too low, and/or that the light rare earth elements (LREEs) were present not only in the crystal structure of fersmite, pyrochlore and apatite, but also in minor or accessory minerals such as REE-bearing fluorocarbonates or zircon erratically distributed throughout the core. Portable XRF is a robust tool facilitating exploration-related decision-making in the field, assuming that elements of interest such as Nb are present in concentrations within the analytical range of the instrument. The pXRF core scanning reduces the need for sample preparation (no pulps) and can be done directly on the drill-site, but the precision and accuracy of the data are reduced relative to laboratory and pXRF pulp analyses. The multiple spot analyses (no pulps) approach is good for instant verification of unknown, potentially ore-bearing minerals and for analysing discrete homogeneous features, layers, veins, etc; however, under normal circumstances this method is inferior to pulp analyses in precision and accuracy, and to scanning for determining average grade of core intervals. © 2014 AAG/The Geological Society of London.

Gregory M.J.,Pebble Ltd | Gregory M.J.,University of British Columbia | Lang J.R.,Hunter Dickinson Inc | Gilbert S.,University of Tasmania | And 2 more authors.
Economic Geology | Year: 2013

The Pebble Cu-Au-Mo deposit in southwest Alaska is one of the world's largest porphyry deposits. The deposit contains over 80 billion pounds (Blbs) Cu, 107 million ounces (Moz) Au, and 5.6 Blbs Mo in all resource categories. Copper and gold grades vary across diverse hydrothermal alteration assemblages with higher grades associated with sericite- and pyrophyllite-rich assemblages in an advanced argillic alteration zone. Moderate grades are associated with the potassic and sodic-potassic alteration assemblages that dominate the deposit and low grades are found in quartz-illite-pyrite assemblages. These variations reflect fluctuations in both the temperature and composition of the magmatic-hydrothermal fluids in time and space. QEMSCAN scanning electron microscope-based mineral mapping and in situ laser ablation inductively coupled plasmamass spectrometry methods designed to document the spatial and temporal variations in metal deportment across alteration zones show that gold occurs as inclusions of electrum and high-fineness gold in chalcopyrite and pyrite and as free grains hosted by silicate minerals, with the proportion of each related to the principal host alteration assemblage. Gold associated with the earliest high temperature sodic-potassic and potassic assemblages occurs as electrum inclusions in chalcopyrite and to a lesser extent pyrite, and pyrite trace element signatures have high gold, silver, and copper concentrations (pyrite-1). During cooling of the hydrothermal system lower temperature illite and illite-kaolinite alteration overprinted the earlier assemblages with varying degrees of recrystallization and precipitation of more pyrite-rich assemblages with pyrite having a gold-poor trace element composition (pyrite-2). Gold released during recrystallization formed high-fineness gold inclusions in newly formed pyrite and narrow gold-rich pyrite rims (pyrite-3). A second magmatic-hydrothermal event resulted in a sericite- and pyrophyllite-rich advanced argillic overprint, which introduced more copper and gold to the system and recrystallized preexisting sulfides into both high and low sulfidation assemblages with high-fineness gold inclusions in pyrite and chalcopyrite and solid solution gold in bornite. A second generation of pyrite-1 with elevated palladium concentrations is associated with this hydrothermal fluid pulse and also possibly high arsenic pyrite (pyrite-4). Understanding the controls on gold deportment provides genetic constraints on ore deposit genesis. Alteration and sulfide assemblages and gold compositions provide information on the hydrothermal fluid compositions, pH, and temperature evolution of the magmatic-hydrothermal system. The presence of electrum inclusions in chalcopyrite and pyrite in potassic and sodic-potassic alteration is consistent with these zones forming from early high-temperature magmatic fluids because both gold and silver are transported together in such fluids. High-fineness gold inclusions in younger pyrite related to low-temperature clay alteration are evidence that the gold in early electrum was partially remobilized and reprecipitated. The low-temperature clay alteration most likely formed from a mixture of H2S-rich vapor with meteoric waters, conditions where gold is transported as a bisulfide complex and silver is not, resulting in the separation of the two metals. A second, structurally controlled pulse of magmatic fluids formed a well-mineralized advanced argillic alteration assemblage containing high-fineness gold inclusions in chalcopyrite and pyrite, the only part of the deposit where high-fineness gold is hosted by chalcopyrite. In the pyrophyllite stability field acidic fluid compositions transport gold and silver as different complexes and gold solubility is significantly lower than silver leading to the precipitation of highfineness gold inclusions. Palladium is transported by the same complex as gold under these conditions, consistent with the elevated palladium content of pyrite in the advanced argillic alteration assemblage. The temporal and spatial studies of variations in gold deportment across the Pebble deposit provide critical inputs to optimization of mineral processing design. The greatest influence on metallurgical gold recovery at Pebble is the proportion of gold that is hosted by pyrite. Pyrite-hosted gold may require different mineral processing methodologies compared with gold hosted by chalcopyrite. Therefore, defining domains of consistent hydrothermal alteration, and sulfide mineralogy and gold deportment is the key to the geometallurgical characterization of the deposit. © 2013 Society of Economic Geologists, Inc.

Harraden C.L.,Pebble Ltd | Harraden C.L.,Hecla Mining Company | McNulty B.A.,Pebble Ltd | McNulty B.A.,University of British Columbia | And 3 more authors.
Economic Geology | Year: 2013

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, quartzsericite- 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. © 2013 Society of Economic Geologists, Inc.

Lang J.R.,Hunter Dickinson Inc. | Gregory M.J.,Pebble Ltd | Gregory M.J.,University of British Columbia | Rebagliati C.M.,Hunter Dickinson Inc. | And 3 more authors.
Economic Geology | Year: 2013

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. © 2013 Society of Economic Geologists, Inc.

Mathur R.,Juniata College | Munk L.,University of Alaska Anchorage | Nguyen M.,Juniata College | Gregory M.,Pebble Ltd | And 3 more authors.
Economic Geology | Year: 2013

Copper isotope ratios measured in minerals and shallow groundwater and surface waters provide insight into high-temperature mineralization and active weathering processes at the Pebble porphyry Cu-Au-Mo deposit, Alaska. The West zone of the deposit contains hypogene mineralization with a supergene overprint and a thin oxide leached capping, whereas the contiguous East zone contains only hypogene mineralization. Sulfide- rich rock powders and mineral separates have δ65Cu values that range from 0.78 to 2.28% (hypogene West), 0.02 to 1.55% (hypogene East), -3.49 to 1.88% (oxide West), and -5.04 to 1.27% (supergene West). The results from hypogene samples show that there is a systematic increase in δ65Cu values from deeper to shallower portions of the deposit. Furthermore, the δ65Cu values correlate with silicate alteration assemblages; mostly positive values correspond to quartz-illite-pyrite, sericite and quartz-pyrophyllite alteration zones which formed at relatively lower temperatures, whereas negative values characterize the higher temperature potassic and sodic-potassic domains. This empirical evidence could indicate that fractionation of Cu isotopes during hypogene alteration is controlled by pH and/or temperature variations. Shallow surface waters proximal to the deposit, and which likely interacted with underlying concealed mineralization, have heavy δ65Cu values which contrast with lighter values in waters distal from the deposit. Patterns measured in the copper isotope ratios of both solids and surface waters demonstrate the potential use of copper isotope distribution as a vectoring tool in mineral exploration and aid in understanding the sources of copper in the surface and near-surface environments. © 2013 Society of Economic Geologists, Inc.

Tafti R.,University of British Columbia | Tafti R.,British Columbia Institute of Technology | Lang J.R.,Hunter Dickinson Inc. | Mortensen J.K.,University of British Columbia | And 2 more authors.
Economic Geology | Year: 2014

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. ©2014 Society of Economic Geologists, Inc.

Liu L.,University of Alberta | Richards J.P.,University of Alberta | Dufrane S.A.,University of Alberta | Rebagliati M.,Hunter Dickinson Inc
Canadian Journal of Earth Sciences | Year: 2015

Newton is an intermediate-sulfidation epithermal gold deposit related to Late Cretaceous continental-arc magmatism in south-central British Columbia. Disseminated gold mineralization occurs in quartz-sericite-altered Late Cretaceous felsic volcanic rocks, and feldspar-quartz-hornblende porphyry and quartz-feldspar porphyry intrusions. The mineralization can be divided into three stages: (1) disseminated pyrite with microscopic gold inclusions, and sparse quartz-pyrite ± molybdenite veins; (2) disseminated marcasite with microscopic gold inclusions and minor base-metal sulfides; and (3) polymetallic veins of pyrite-chalcopyrite-sphalerite-arsenopyrite. Re-Os dating of molybdenite from a stage 1 vein yielded an age of 72.1 ± 0.3 Ma (published by McClenaghan in 2013). The age of the host rocks has been constrained by U-Pb dating of zircon: Late Cretaceous felsic volcanic rocks, 72.1 ± 0.6 Ma (Amarc Resources Ltd., unpublished data, reported by McClenaghan in 2013); feldspar-quartz-hornblende porphyry, 72.1 ± 0.5 Ma; quartz-feldspar porphyry, 70.9 ± 0.5Ma(Amarc Resources Ltd., unpublished data, reported by McClenaghan in 2013). The mineralized rocks are intruded by a barren diorite, with an age of 69.3 ± 0.4 Ma. Fluid inclusions in quartz-pyrite ± molybdenite ± gold veins yielded an average homogenization temperature of 313 ± 51 °C (number of samples, n = 82) and salinity of 4.8 ± 0.9 wt.% NaCl equiv. (n = 46), suggesting that a relatively hot and saline fluid likely of magmatic origin was responsible for the first stage of mineralization. Some evidence for boiling was also observed in the veins. However, the bulk of the gold mineralization occurs as disseminations in the wall rocks, suggesting that wall-rock reactions were the main control on ore deposition. © 2016, National Research Council of Canada. All right reserved.

McNulty B.,Heatherdale Resources Ltd. | Roberts K.,Hunter Dickinson Inc.
2012 SME Annual Meeting and Exhibit 2012, SME 2012, Meeting Preprints | Year: 2012

The Niblack polymetallic volcanogenic massive sulfide deposits are located on Prince of Wales Island in southeast Alaska, approximately 50 km southwest of Ketchikan. The property hosts a number of polymetallic volcanogenic sulfide occurrences within the folded Lookout rhyolite volcanic succession. The Lookout deposit is the primary focus of current resource delineation and contains an indicated resource of 4,136,000 tonnes at 1.13% Cu, 2.32 g/t Au, 2.27% Zn, 38.70 g/t Ag and an inferred resource of 1,736,000 tonnes at 1.09% Cu, 1.77 g/t Au, 2.02% Zn, and 25.52 g/t Ag as of March 2011. Sulfide mineralization is interpreted to have formed sub-seafloor, within permeable felsic fragmental volcanic rocks. Sulfide minerals of economic interest include chalcopyrite and Fe-poor, Zn-rich sphalerite. Of considerable importance, the Lookout deposit contains elevated gold that occurs as inclusions within the base metal sulfides. Copyright © 2012 by SME.

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