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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. Source

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. Source

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. Source

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. Source

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. Source

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