Sulphide and supergene nonsulphide Zn-Pb deposits of the southern Kootenay Arc, British Columbia, Canada
The Kootenay Arc (KA) hosts a large number of carbonate-hosted base metal (Zn-Pb) deposits. These deposits occur mainly within the dolomitized limestone of the Lower Cambrian Badshot Formation (or its equivalent the Reeves Member of the Laib Formation), and the Upper Cambrian to Lower Ordovician Nelway Formation. They range in size from 6-10 million tonnes with average grades of 3-4% Zn, 1-2% Pb, 0.4% Cd and traces of Ag. The deposits, their dolomitic envelopes, and the limestone hostrock lie within secondary isoclinal folds along the limbs of regional anticlinal structures. Most of the deposits are stratabound lenticular concentrations of sphalerite, galena, pyrite, local pyrrhotite and rare arsenopyrite in isoclinally folded dolomitized or silicified carbonate layers. Brecciated zones are common within the more massive sulphide mineralization. Several deposits are past producers (e.g., Reeves MacDonald, Jersey, and HB) and others are advanced prospects. They are interpreted as metamorphosed MVT- or Irish-type Pb-Zn deposits. The main concentrations of these deposits define the Salmo and Duncan camps. The near-surface portions of these carbonate-hosted sulphide deposits are weathered and strongly oxidized (supergene environment). They consist of extensive Zn- and Pb-bearing iron oxide gossans and base metal-bearing nonsulphide mineralization. The most common nonsulphide minerals are goethite, hematite, hemimorphite, smithsonite, cerussite, anglesite, and hydrozincite. The Reeves MacDonald, Jersey-Emerald, Lomond, and Oxide group of deposits are the best examples of carbonate-hosted nonsulphide base metal (CHNSBM) deposits in the KA. The shape, mineralogy and paragenesis of the known CHNSBM deposits are indicative of direct-replacement of sulphides by nonsulphide base metal-bearing minerals. Zn-rich (low Pb) CHNSBM deposits commonly form by interaction of Zn-rich fluids with carbonate wallrock (replacement process). Such deposits (consisting of "white ore") may have been overlooked in the past and represent new and exciting exploration targets within the KA.
Genesis of Halloysite-Alunite Deposits in the Balıkesir Area, Turkey
The field studies revealed that all halloysite deposits in the Biga Peninsula were formed along the major fault zones between Miocene calc-alkaline tuffs and Permian limestone blocks. Hypogene halloysite mineralization controlled by the intersection of minor faults in the vicinity of clay deposits. Halloysite is found in both hydrated and dehydrated states and assumes a tubular morphology. Observations by transmission and scanning electron microscopy are consistent with a model of halloysite dehydration, where the shapes transform from an open-hole tubular morphology to a closed-hole unfurled morphology. The thickness of clay deposits are about 20 m on the underlying limestone blocks, which provide excellent drainage system for the discharging geothermal waters. This results in the differential mobility of Ca, Na, Fe, Si and Mg type mobile elements and, consequently, in the enrichment of the Al, which led to the formation of halloysite and alunite-halloysite deposits. The occurrence of alunite requires additional K+ and SO4= supplies from hypogene origin and often occurring with a minor Ca-phosphate phase. Pyrite, realgar, cinnabar, orpiment, azurite, chalcopyrite, psilomelane, hematite, kaolinite, gibbsite and alunite are associated with halloysite mineralization, pyrite is mostly concentrated within the fault clays. Hematite and psilomelane enrichments from a few cm to 20 cm bands are commonly found as surrounding of the carbonate blocks due to changes in pH of geothermal solutions. There are little evidence of halloysitization in overlying highly altered andesitic rocks and they are generally rich in smectite due to either multi-stage acidic solutions were used up before reaching to overlying country rocks or original volcanic rocks might had been deposited in different episodes and consequently, have different chemical compositions. In addition, permeability is reduced in overlying highly altered andesitic rocks and the flowing of geothermal waters became stagnant or sluggish in fine pores where the differential mobility of elements significantly reduced, which increased salinity and alkalinity, favoring the formation of smectite rather than kaolinite or halloysite.
Mineral Chemistry of Silicate Phases From the Summit Creek Stock, Southeastern British Columbia: Evidence for Associated "Xenolith" Origin and Dyke Emplacement
The Summit Creek stock is a Mid-Cretaceous aged granitic intrusion that is located within the Cordilleran Omineca belt in the southeastern corner of British Columbia. Included within the stock are numerous xenoliths of intermediate composition that range in size from 5cm to 2m, as well as a prominent 1m-wide mafic dyke. According to the QAP diagram, the stock is classified as a muscovite-biotite monzogranite, with a typical sample containing 35% quartz, 25% plagioclase, 30% K-feldspar, 7% biotite, 2% muscovite, and 1% accessory phases. Minor accessory phases identified in this study include pyrite, molybdenite, apatite, magnetite, ilmenite, monazite, and zircon. Samples collected from the stock proper are predominantly equigranular; however the xenoliths are porphyritic and contain phenocrysts of both quartz and plagioclase, as well as glomerophyric aggregates of biotite. New investigation into the mineral chemistry of the stock, xenoliths, and dyke indicates that the xenoliths have a strong geochemical similarity to the main body of the stock. Plagioclase feldspar compositions from the xenolith overlap with those from the stock, and REE abundances in the stock and the xenoliths are indistinguishable (LREE 100x chondrite; HREE 10x chondrite). In thin section, the boundary between the xenoliths and the stock is very irregular, and protruding grains of plagioclase and biotite can be found piercing into both the granitic host, as well as into the xenolith itself. This evidence suggests that these inclusions are better termed autoliths rather than xenoliths, as it appears that they are derivatives of the same parent magma as the main stock. The mafic dyke differs in both mineralogy and geochemistry from the stock, and is characterized by low SiO2 content (48.98 wt.%), high REE abundances (30-40x chondrite), presence of rare euhedral hornblende, and greater variation in mineral chemistry. Samples from the dyke contain plagioclase with cores ranging from bytownite to relatively Ca-rich andesine, and rims varying from Ca-rich andesine to more Na-rich andesine. Biotite from the dyke differs in Mg/Fe ratio from that of the autoliths and monzogranite stock. In addition, the dyke has an alkaline affinity with relatively high Na2O + K2O values, while the stock and autoliths are both calc- alkaline. Field relations suggest that the dyke may represent an immiscible liquid that was present during the time that the stock was cooling. Further work using trace element modeling and geochronology will help to constrain the relation of the dyke to the stock.
Orientation Dependent Polarized Micro-XAS Study of U, Th and Sr in Single Crystal Apatites
In order to evaluate apatite as a potential solid nuclear waste form and a contaminant sequestration agent, the complimentary use of single crystal X-ray diffraction and X-ray absorption spectroscopy (XAS) is applied to the study of U, Th, and Sr doped apatite single crystals to investigate the site preference, oxidation state, and structural distortions created by these substituents. Single crystal X-ray diffraction provides average information regarding the site occupancy of U and Th in apatites. Extended X-ray absorption fine-structure (EXAFS) yields quantitative information of the local structure of these substituents, which includes near-neighbor distances, coordination numbers and variations in bond distances; while X-ray absorption near edge structure (XANES) is used to determine the oxidation states of U. Restricted by the typical small size (20-100 μm) and volume of our synthetic samples, Micro-XAS is required. Different from studies which take full advantage of the polarization of synchrotron radiation, our Micro- XAS study on single crystal apatites was hampered by the polarization effects. In order to extract precise information of valence state and structural variation from XAS, it is necessary to know the crystallographic orientation of the sample with respect to the polarization direction of the incident X-ray beam during data collection. To do this we have designed and built a portable goniometer that duplicates the geometry of our laboratory standard Bruker Apex diffractometer goniometer. Crystal orientation is determined by X-ray diffraction at our home institution. The portable goniometer is then set up on the experimental table at synchrotron facilities and the crystal can be set in any specific known orientation. The lattice orientation determined by X-ray diffraction is applied to XAS data analysis, specifically calculation of scattering amplitudes and phase shifts, to account for polarization effects of synchrotron radiation. The goniometer also allows evaluation of possible orientation dependence on the U absorption edge position in our samples. In this study, we found orientation has strong effects on U, Th, and Sr EXAFS we measured in all of our apatite single crystals. After taking account of polarization effects, results from EXAFS suggest that U and Th substitute dominantly into Ca1 site in fluorapatite, whereas U and Th are essentially equally distributed between Ca1 and Ca2 site in chlorapatite. We also tested the possibility of orientation dependence on the U absorption edge position in our sample and found no observable X-ray absorption pleochroism.