The Cinder Lake Intrusive Complex, Knee Lake area, Central Manitoba: a Syenite- Carbonatite Association from a Neoarchean Continental Margin
The Cinder Lake intrusive complex is the only known occurrence of feldspathoid rocks in Manitoba. These rocks were initially mapped in the southeastern part of the Lake by Elbers (in Gilbert, 1985) and Lenton (1985), but have not been adequately studied. On the basis of new field, petrographic and geochemical evidence acquired in 2008, three discrete intrusive phases can be presently identified at Cinder Lake: fine-grained aegirine-nepheline syenite, fine-grained biotite-vishnevite syenite and syenitic pegmatite. There is also convincing mineralogical and geochemical evidence for the presence of unexposed clinopyroxenite and carbonatitic units genetically associated with the alkaline syenitic rocks. The evidence for the presence of unexposed carbonatite includes pervasive calcitization of the syenitic rocks, occurrence of rare-earth minerals (britholite, monazite and REE-rich apatite) in association with Sr-rich calcite in metasomatised pegmatite, and andradite veins crosscutting the syenites. The geochemistry of the Cinder Lake rocks is most consistent with the HFSE-depleted, potassic, high-Ba/La and high-Th/Nb signature of arc magmas (Edwards et al., 1994). In common with island-arc and continental-margin phonolites, the Cinder Lake syenites are potassic rocks with a chondritic Zr/Hf ratio, strong enrichment in Ba relative to La and Th relative to Nb. Uranium-lead dating of zircon crystals recovered from the biotite-vishnevite syenite yielded an age of 2705±2 Ma, interpreted as the timing of syenite emplacement. This value is close to the age of the incipient accretion of subprovinces in the northwestern Superior province at 2.70-2.71 Ga (Davis et al. 2005). Given this age relationship, the Cinder Lake complex is probably derived from magmas produced in a Neoarchean subduction zone underlying the North Caribou microcontinent. The regional geological setting of the complex (abundance of tonalite and granodiorite among the plutonic rocks and the predominance of subaerially deposited sediments and calc- alkaline volcanics in the local stratigraphy) is more consistent with an active continental margin, rather than an island-arc environment.The economic potential of the Cinder Lake complex remains undetermined. According to Müller et al. (2001), over 20% of the world's largest gold deposits are associated with alkaline volcanics in subduction-zone settings. The unexposed carbonatite unit may also be of economic interest as a source of rare earth elements, Ba and Sr. Carbonanites in collisional settings are known for their enrichment in these elements and have been exploited for REE in the United States and China (Xu et al., 2004). Further work is required to ascertain the extent and petrographic makeup of the Cinder Lake complex and to gain a better understanding of its resources. References: Davis, D.W., Amelin, Y., Nowell, G.M. and Parrish, R.R. 2005: Precambrian Research, v. 140, p. 132-156. Edwards, C.M.H., Menzies, M.A., Thirlwall, M.F., Morris, J.D., Leeman, W.P. and Harmon, R.S. 1994: Journal of Petrology, v. 35, p. 1557-1595. Gilbert, H.P. 1985: Manitoba Energy and Mines, Geological Services, Geological Report GR83-1B, 76 p. Lenton, P.G. 1985: Report of Field Activities, Manitoba Energy and Mines, Geological Services, Mines Branch, p. 203-208. Müller, D., Franz, L., Herzig, P.M. and Hunt, S. 2001: Lithos, v. 57, p. 163-186. Xu, C., Zhang, H., Huang, Z., Liu, C., Qi, L., Li, W. and Guan, T. 2004: Geochemical Journal, v. 38, p. 67-76.
Potential Greenstone Belt Continuity Undercover, Zimbabwe
The Zimbabwean craton is one of the most metal-endowed cratons on the Earth with abundant deposits of Au, Ni, Cr and PGE that are spatially and genetically associated with greenstone belts. This study aims at tracing the possible undercover continuity of these greenstone belts. A conjunctive interpretation of regional-scale geological, gravity and magnetic data was carried out, focusing on the Harare-Shamva region, NE Zimbabwe. The datasets used in the study included: (1) public geological maps and reports; (2) Zimbabwean Government aeromagnetic surveys (1983-1992) with line spacing and terrain clearance ranging 250-1000 m, and 120-305 m, respectively; and (3) the national gravity dataset of Zimbabwe (1996) with >12,000 gravity measurements. The Harare-Shamva gravity coverage corresponds to one station per ∼10 km2. A digital elevation model was generated from the NASA's Shuttle Radar Topography Mission data (90m resolution) to preprocess the gravity data. Standard gravity corrections, namely, the latitude, free-air, Bouguer- slab and terrain corrections, were applied to the observed gravity data. A distance of 167 km was used for the terrain correction, and an average density value of 2.67 g.cm3 (expected granite density) was used for the Bouguer-slab correction. In the Harare-Shamva area, long wavelength Bouguer anomalies were filtered out by applying a second-order polynomial and extracting the residual anomalies interpreted as representing the effect of density heterogeneities located below the surface down to a depth of ∼10 km. These residual gravity anomalies are used to characterize the geometry of the greenstone bodies in the area. Based on the composite Bouguer map, positive anomalies are associated with the geometry of the Harare, Shamva, Mutoko, Guruve, Filabusi, Masvingo, Fort Rixon, Shangani, Bulawayo and Gwanda greenstone belts. These greenstone belts comprise metamorphosed ultramafic-mafic suites that are intercalated with felsic volcanics, sedimentary units and banded iron formation (BIF). Based on the available geological information, magnetic positive anomalies are interpreted as BIF units and ultramafic-mafic suites, whereas the negative ones are interpreted as phyllitic and quartzitic units. On the residual Bouguer map, positive anomalies in the Harare-Shamva are associated with the Reliance F. within the previously mapped greenstone belt, which comprises up to 6-km thick succession of mafic- ultramafic rocks. On the contrary, the Zeederbergs F. (2-3 km thick sequence of sediments and tholeiitic basalts) and the Chinamora Complex (tonalites-granodiorites) are characterized by negative anomalies. Positive gravity anomalies in the Harare-Shamva area continue ∼20 km further to the N and probably ∼30 km to the E beyond the mapped greenstone belt, which suggests continuity of the mafic-ultramafic rocks under the cover sediments. Possible undercover continuity and thickening of the Harare-Shamva greenstone belts to the E is tested by applying petrophysical measurements, and direct 2D joint gravity and magnetic modeling using Geosoft-GM- SYS software by integrating the potential field data and available geological information. Several transverse cross-sections are developed through some key areas to verify the above interpretations. The 2.7 Ga greenstone belts with komatiite sequences in Zimbabwe are highly prospective for commodities such as Ni. This study contributes data for a potential re-evaluation of geological relationships in Zimbabwe, as well as delineation of new potential areas for targeting greenstone-hosted mineral deposits.
Petrogenesis of the Khabr-Marvast Tectonized Ophiolite in the middle part of Nain-Baft Ophiolite belt, NW Shahrebabak, Iran
The Late Cretaceous Khabr-Marvast tectonized Ophiolite, exposed in the middle part of Nain-Baft ophiolite belt,
at the South edge of the Central Iran micro-continent. Intrusive sequence of this association comprise by:
serpentinized harzburgite, isotropic gabbro, diabase sheeted dikes complex, which intruded by isolated
diabase dikes, pegmatite gabbro and trondjhemite. Extrusive sequence are included: pillow lava, sheetflow
and hyaloclastic breccias at passing upward. The relationship between the units of this association are
The primitive mantle-normalized multi-element plot (Sun and McDonough, 1989) indicate moderate enrichement in the LILE and show clear depletion for Nb in all samples, and depletion for Ta and Ti in most of samples. The chondrite-normalized multi-element plot indicate different enrichment of samples from LREE to HREE, and could be compare with subduction related magmatism.
The variable rare earth elements patterns of the Khabr-Marvast tectonized ophiolite may reflect presence of heterogeneous source to generation of these rocks.The high Ba/Th(209-1460) ratios, low Th/Ta(0.5-1.7) and Nb/Ta(2.02-4.04) ratios in sheeted dikes and pegmatite gabbros, indicate more participant of fluids with partial melting of depleted mantle wedge in a subduction zone (Pearce et al., 2005; König et al., 2008 ). In contrast, sheetflows and isolated diabase dikes and trondjhemites have lower Ba/Th(20-170) ratios and higher Th/Ta(7.9-16.1), Nb/Ta(12.12-24) ratios which indicative of partial melting of depleted mantle and lower fluids to participating in generation of these rocks.
The geological and geochemical studies suggest that the Khabr-Marvast tectonized Ophiolite had heterogeneous source and form in a supra-subduction zone system which is consistent with the model that assumes subduction of the Arabian plate under the Central Iran micro-continent.
Keywords: Tectonized Ophiolite, Central Iran micro-continent, heterogeneous source, supra-subduction system.
Suprasubduction Origin for the Nari Ophiolite, Southwestern Belt of Fariman, Northeast Iran
In the west and southwest region of Fariman, northeast of Iran, two Late Cretaceous-Neotethys ophiolitic belts meet. The western belt of Fariman is the continuation of the eastern belt of the Sabzevar ophiolitic belt that runs parallel to the Miami fault and separates the Central Iran from the Alburz-Binalud structural zone. The continuation of the southwestern belt of Fariman, non-continuously and parallel to the Doruneh fault, joins to the Naein ophiolite. The study area, situated in the ophiolitic belt of southwest Fariman between 35o, 30'N and 35o 37'N latitudes, and 59o 37'E and 59o 45'E longitudes. The Nari Ophiolite exposes the tectonized mixture of ultramafic rocks, a few kilometers long mass of cumulate gabbro, sheeted dikes, pegmatite gabbro, individual diabase dikes and an extrusive sequence of pillow basalt and sheeted lava flow underlied by pelagic limestone and chert. The Eocene conglomerate is deposited on top of the sequence with a disconformity. Recent paleontology studies on the pelagic limestone give a Late Cretaceous, Coniasian-Santonian age for the limestone. Based on petrography studies, the ultramafic rocks are serpenticized and have harzburgite, clinopyroxene- bearing harzburgite, and lherzholite composition. Olivine porphyroclasts are affected by dense kink bands and the orthopyroxene porphyroclasts are stretched and suggest that the rocks are affected by mantle deformation. Massive gabbros have cumulated texture and display layering. The thickness varies from a few centimeters to one meter. Their composition is olivine gabbro and pyroxene gabbro. Pyroxenite is formed in thin non- continuous layers throughout the sequence however pegmatite gabbro only forms at the lower layers of the sequence. The microscopic texture of the majority of these masses is mesocumulate and heteradcumulate however pyroxenite shows an adcumulate texture. The presence of these textures suggests an open system for mineral crystallization. The layered gabbros in the Nari region are intersected with pegmatitic veins and dikes, individual diabase dikes, plagiogranite veins, and several epidote veins. The structural measurements for layers of gabbro show an E-W orientation while a N-S orientation is measured for dikes and veins, sheeted dikes, and individual diabase dikes. These findings suggest an opening of a basin while the ophiolitic belt of southwest Fariman was forming during the Late Cretaceous. Sheeted dikes are diabasic and the quartz contained in the majority of the dikes is the result of secondary processes. Pillow basalts and sheeted dikes have basaltic and basaltic andesitic composition. Even though in layered gabbros, the crystallization trend is olivine - plagioclase - clinopyroxene - orthopyroxene - amphibole, the study of the pattern of elements on primitive mantle-normalized spider diagrams show clear depletion of Nb in gabbros. The patterns of these elements in gabbros are less enriched than sheeted dikes and pillow basalts. However, sheeted dikes and pillow basalts also show clear negative anomalies. The tectonomagmatic diagrams also show that the Nari ophiolite separates from the MOR regions and they inclined more toward subduction tectonomagmatic regions. It seems that the N-S oriented Nari ophiolite of the southwest belt of Fariman is formed during the Late Cretaceous in a supra-subduction system. Our data is consistent with the model suggesting that the Arabian plate is continuously subducted beneath the margin of the Iranian plate and that the remnant of the Paleotethys ocean is subducting under northeastern Iran.
A Possible Northern Extension of the Reunion Hotspot Track; Based on Samples From the Bela Ophiolite, Pakistan.
This study deals with a selection of rocks thought to be related to the Reunion Hotspot. Sample rocks were collected from igneous intrusions that run through the Bela Ophiolite in Pakistan, which would make these samples part of the Pre-Deccan extension of the Reunion Hotspot track. Petrographic and geochemical analyses were performed to provide information about the formation of these rocks. The information should help to further understanding of the tectonics involved in the collision of the Eurasian and Indian plates. Petrographic analyses were performed on eleven of the samples. There are five dacites, two of which are ignimbrites, and one of them has been highly altered to clay minerals. The remaining samples consist of two pyroxene andesites, two basalts, one diorite, and one hornblende pyroxenite. Of these, only the hornblende pyroxenite has olivine present, with the pyroxene and amphiboles being leached and partially altered to prehnite. The other samples all show weak to moderate alteration of their plagioclase grains, which have been pitted from the center out and replaced by clay minerals. Plagioclase is present throughout all the samples except the pyroxenite, with compositions ranging up to An60, making it the dominant feldspar in these samples. The dacites all have glassy groundmasses which have undergone slight to moderate alteration to clay minerals and calcite. Geochemical analyses were performed for ten of the samples, with duplicate analyses run on two of those samples. These analyses show that the samples represent low K tholeiites, with continental signatures. Based on the degree of fractionation, and chemical affinities from discrimination diagrams (such as FeO-MgO- Al2O3) the samples have been separated into three groups: mafic (basalts and basaltic andesites), intermediate (dacites and andesites), and the pyroxenite. Chondrite normalized spider diagrams for the felsic samples show a divide between the two ignimbrites, which are less enriched in LIL elements, at 10- 20×chondrite, compared to the other felsic samples, 60-200×chondrite. The chondrite normalized mafic samples LIL element enrichment was 20-60×chondrite. Both the felsic and mafic chondrite normalized samples exhibit depletions of HFS elements in a linear fashion to levels of only 10- 30×chondrite. The pyroxenite had LIL and HFS element concentrations below the other samples, 7- 10×chondrite and 5-6×chondrite respectively. Primitive mantle normalized spider diagrams show depletions in P and Ti in all of the samples, with three of the felsic samples being depleted in Nb and the other two showing a slight enrichment in Nb. Based on the chemical analysis it appears that these samples may be related to two different sources. The pyroxenite does not appear to readily match with any of the other samples, and may in fact be unrelated to them.
The Trace Element Diversity of Anorthitic Plagioclase and Melt Inclusions in MOR Basalts
This investigation centered on the spatial and compositional relationship between anorthitic feldspars and included melts. Such phenocrysts are characteristic of primitive lavas in suites of high Al MORB most often associated with slow to intermediate spreading ridges close to fracture zones. This study uses the relationship between the melt inclusions and host feldspar to evaluate the degree to which the melt inclusions represent original trapped liquid, versus the degree to which melt inclusion compositions are the product of entrapment or post-entrapment processes, and to evaluate their relationship to MORB magmas. Feldspar phenocryst from several samples from the Southeast Indian and Gorda Ridges were analyzed for major and minor components. Feldspar and Melt inclusion major element compositions show little variation with An content from 86 to 92, with individual phenocrysts rarely ranging more than +/- 2 An%. One of the striking characteristics of plagioclase hosted inclusions in MORB is the wide range of Ti contents from single phenocrysts [1, 2, 3]. The origin of this chemical signal has been attributed to a number of processes, including post entrapment diffusive re-equilibration , diffusive process at the time of entrapment , local disequilibrium events , and reactions in melt channels . Measurement of the trace element content of the feldspar host allowed us to evaluate some of those models. Our results show that the trace element content of plagioclase exhibits an even greater range of composition than that exhibited in the melt inclusions. Concentration in the feldspar changes extremely rapidly as one traverses the crystal, mirroring the variations seen in the melt inclusions. Each feldspar crystal appears to have a distinct history, and each sample contains a complex crystal cargo of phenocrysts. Partition coefficient "proxies" were calculated for Mg, Ti, Fe, Sr, Ba, Y, Zr, La, Ce, Pr, Eu, and Pb by analyzing plagioclase spots directly adjacent to melt inclusions. These calculated partition coefficients do not vary systematically with changes in concentration in the melt or with MI size. Our partitioning data is generally consistent with the predicted partitioning behavior for high An plagioclase based on the limited available experimental data. The trace element patterns are consistent with some degree of diffusive re-equilibration of some of the more mobile/compatible elements (Sr, Eu) and to a lesser degree for other less mobile elements (LREE). The degree to which diffusive re-equilibration has gone to completion versus the degree to this signal represents an original trapped signal appears to vary from crystal to crystal depending on residence time. The primary conclusion we draw from our data set is that plagioclase hosted melt inclusion are broadly representative of the magmas from which anorthitic phenocrysts form. However, they do not represent an unmodified array of primary mantle melts - rather they are produced by a complex set of processes that include mantle melting, post entrapment re-equilibration, and reaction with the gabbroic crust. Sources 1 -Nielsen et al. (1995) Contrib. Mineral. Petrol. 122, 34. 50., 2 - Sours-Page (1999) Contrib. Mineral. Petrol. 134, 342-363., 3- Michael et al.. (2002) Chemical Geology, 183, 43-64., 4 - Cottrell et al. (2002). Geochem. Geophys. Geosyst., 3, 1525-2027, 5- Danyushevsky, et al (2003) Contrib. Mineral. Petrol, 144, 619- 637, 6- Spiegelman and Kelemen (2002) Geochem. Geophys. Geosyst., 4
Toward an Integrated Model for the Composition, Structure, and Physical Properties of the Crust in Icelandic Rift Zones
The rift zones that extend across Iceland roughly southwest to northeast are the only portion of the mid-Atlantic Ridge that is exposed above sea level. This reflects anomalously high melt productivity in the mantle leading to anomalously thick oceanic crust. There are 30 active volcanic centers in the rift zones. Petrologic studies of the 30 volcanic centers in the active rift zones show that, magmas pond at a mid-crustal level as well as at the base of the crust prior to eruption. The depth of magma chambers at the base of the crust provides an estimate of crustal thickness of (20 ± 2.5 km) in these zones. Melts erupting to the surface directly from chambers at the base of the crust provide one constraint on the composition of the crust because any compositional variations within the crust must be the result of differentiation of these melts. However, the glass compositions indicate that relatively evolved magmas erupted from the deep chambers, suggesting that crystallization of compositionally more primitive magmas also occurred at the base of the crust. Knowledge of crustal thickness, the temperature of melts at the base of the crust, and the compositions of these melts allows development of comprehensive models of the composition, structure and properties of crust within the rift zones. We have developed two end member models: one with variation of mineralogy with depth in the crust due to metamorphism, and one with variation of crustal composition with depth due to fractionation processes. We have also considered models that are plausible combinations of these two end member models. We have calculated well constrained geothermal gradients and used these to predict variations in density, seismic velocity, and bulk modulus with depth. These models which include petrologic and geochemical data are consistent with published geophysical data, therefore provide important constraints on interpretation of geophysical data. In particular, results of this work provide well constrained values of crustal composition, mineralogy, and physical properties that constitute input data that are essential for processing seismic and gravity data for the Icelandic crust.
U-Pb Zircon LA-ICPMS Ages of Granitoids From the Dahomeyide Segment of the Pan- African Trans Saharan Belt
The Dahomeyide orogen in southeastern Ghana and adjoining parts of Togo and Benin records the suture of West African craton (WAC) into northwest Gondwana. The suture zone is characterized by distinctive high- pressure mafic rocks that mark the collision of WAC with exotic blocks to the east. Granitoids to the east of the suture zone are postulated to be juvenile crust representing the arc terrane that formed during subduction and oceanic closure. We have compiled a map of granitoid gneisses in Ghana, Togo and Benin which allows us to distinguish between two main rock types: (1). migmatitic gneisses with biotite as the dominant mafic mineral and (2). dioritic gneiss characterized by hornblende as the main mafic mineral and with occasional garnet. These rocks types are separated by a shear zone inferred to be a splay of Kandi shear zone, a segment of the Trans-Saharan shear zone that extends for >2500 km from the Sahara to the Gulf of Guinea. The migmatitic gneiss unit with a zone of highly strained granitoids (straight gneiss) forms a sliver just east of the suture zone and yielded U-Pb zircon ages 615-589 +/- 40 Ma which are similar to the metamorphic crystallization age of the suture zone rocks. The zircons analyzed display concentric zoning (in Cl images) and are inferred to be magmatic. On the other hand, zircon separates from the dioritic gneiss, with grains also showing igneous zoning in CL images, yielded U-Pb ages ranging from 2.19 to 2.14 Ga. These are the first U-Pb zircon ages to document Paleoproterozoic (Birimian) rocks in the postulated juvenile terrane west of the Kandi fault zone and indicate the existence of widespread involvement of older crust in the Pan-African collision zone, similar to available geochronologic data from the correlative Medio Coreau domain of the northwest Borborema province in northeast Brazil.
Mineralogic, Petrographic, Mechanical And Physical Properties Of Incesu Ignimbrite (Central Anatolia - Turkey) Used Building Stone In Historical Buildings
The Central Anatolian Volcanic Province (CAVP) is located in the Central Turkey and one of the four major volcanic provinces in Turkey. It extends 300 km along a NE-SW direction, over a large area (32500 km2). The CAVP includes various volcanic, pyroclastic rocks, which are related to collision volcanism between the Arabian and Eurasian plates, occurred in Neogene, and Quaternary times There are numerous natural stone pits in the CAVP. They have been commonly used in historical building which had been built in the Anatolian Seljuk and Ottoman empires times. The subject of this study is to figure out petrographic, geochemical, some mechanical and physical properties which include Schmidt hardness, density, porosity, water absorption by weight, point load index and failure load.Incesu ignimbrite is subdivided into three levels as lower, middle and upper according to color, welding degree, crystal contents and lithic components. All of the natural stone pits is the upper part which is characterized by grey pinkish in color, poorly welded, high porosity ratios, high amount of lithic fragments and almost 2 m thickness. In terms of the mineralogical composition, it is composed of plagioclase (oligoclase, andesine) + pyroxene (augite, clinoenstatite) + opaque minerals and low amount of amphibole, biotite and quartz. Vitrofiric texture is dominant in upper level. Al2O3 content of the upper level from 12.75-13.96wt%, SiO2 66.70-68.10wt%, MgO 0.73-1.40wt%, Fe2O3 3.77-4.04wt%, TiO2 0.46-0.51wt%, CaO 1.97-2.91wt%, Na2O 3.41-4.29wt%, K2O 3.21-4.20wt%, P2O5 0.13-0.18wt% and LOI 3.65-4.52wt%. Geochemical analyze results reveal that Incesu ignimbrite has rhyolite, rhyodacite-dacite composition, medium to high-K calc-alkaline and peraluminous nature.Building stones can be classified according to their mineralogy, mechanical and physical properties and processing types. Mechanical and physical properties are more significant depending on the stones practice aim. The mean schmidt hardness value of the upper level of Incesu ignimbrite is 35, bulk density ranges between 2.42 - 2.66 g/cm3, mean water absorption by weight is 90%, mean point load strength is 38.2 MPa and failure load is 1890 kgf/cm based on the mechanical and physical test results.
Distal Limits and Composition of a Late Ordovician (Mohawkian) Biotite-Bearing Volcanic ash, Foreland Carbonate Platform (Verulam Formation), Ottawa Embayment: Helping to Define Magmatic Change in Volcanism Following Later Platform Foundering
Two thin (~3 cm) biotite-bearing clay beds that are separated by a 1-cm thick crinoidal-brachiopod floatstone can be traced in two cores through the Upper Ordovician Verulam Formation (Ottawa Group) along a 25-km transect in the western portion of the Ottawa Embayment, eastern Ontario. Only a further 3 km to the northwest, neither the beds nor related mineralogy are recognized in either core or outcrop,. The beds contain abraded fragments (5-10 %) of partially chlorite- and illite-altered biotite that floats within a soft, friable clay-mineral matrix of kaolinite, illite, chlorite, and minor smectite. Accessory mineralogy includes phenocrystic apatite, feldspar (orthoclase and albite), amphibole, and magnetite. In the lower bed, biotite grains decrease in both abundance (<15 % to trace amount) and grain size (300-400 microns to 50 to 75 microns) to the northwest over the transect distance. The upper bed shows a similar decrease in biotite abundance and grain size but with a significant increase of admixed normal-marine carbonate particles and mud matrix. Mineralogy and phenocrystic texture of biotite and rare apatite, along with the stratigraphic continuity of twin thin beds over 25 km, are interpreted to document deposition of now altered volcanic ash from two related eruptive events; the second with much less volume or lateral extent such that its presence is much reduced along the transect. The ash was deposited rapidly in a normal marine shallow-shelf environment, carbonate facies developed between and following ash deposition suggest that there was no adverse influence from volcanic input. The Verulam altered ash deposits represent the distal margin of a double volcanic eruption, the source mostly likely within the developing Taconic volcanic arc that once lay along the Late Ordovician foreland basin of eastern Laurentia. Geochemical proxies (Zr/TiO2 and Nb/Y) identify a trachyandesitic to rhyodacitic parent magma. Chlorite alteration may indicate minimum burial temperatures of 130oC, in keeping with burial temperature estimates according to conodont alteration indices (CAI = 3) for the host carbonate platform succession. The MgO and FeO % of biotites from these beds plot within the same field as the slightly older Millbrig and Deike bentonites that also represent ash deposition within shallow-water carbonate platform environments from widespread Late Ordovician eruptions that occurred along the foreland basin margin. Collectively, these compositions are higher in FeO and lower in MgO % values compared those associated with a bentonite within the overlying Taconic foreland shale succession of the Ottawa Embayment, and bentonites in Lower Silurian successions of western Europe. This contrast strengthens a previous hypothesis (Sharma et al., 2005) that there was significant change in magmatic composition along the Taconic arc following or coincident with foundering of the foreland carbonate platform.
Evidence for In-situ Cretaceous Volcanism From La Conception Quarry in the Noumea Basin, New Caledonia
The Noumea Basin in New Caledonia, forms a narrow band, about 10-30km wide that extends from the city of Noumea north-westward towards La Tontouta for about 60km. The Noumea Basin represents a sequence of late Cretaceous basalts, rhyolites and marginal marine sedimentary typical of continental margin volcanic arcs. Directly north of the Tina Peninsula, in the Bay of La Conception, there is an old basalt quarry known as the La Conception Quarry. This project involved mapping La Conception quarry, which may be an extinct volcano. This is of particular importance as it is one of the only in-situ volcanic centers that exist in the South Pacific during the late Cretaceous. Through petrophraphic analyses of the basalts we determined that they have experienced minimal low-grade metamorphism, and that the temperatures and pressures never increased enough to re-orientate the phenocrysts. The quarry is roughly oval in shape, and is oriented north-south. The area of the exposed rock is roughly 0.5km2 and the quarry lake itself encompasses about ¾ of the exposed area. We mapped this site by using orientation of phenocrysts and vesicles to determine the direction of flow. We took measurements on the exposed rock every meter by setting up a square meter grid around the entire quarry. This was achieved by establishing a starting point, with a known GPS position, in the quarry and then having two groups of two people move directly north and south from that point. Compasses were used to orientate the lines of the grid; spray paint was used to mark the grid. According to the data we compiled, there are three different basaltic flows in La Conception quarry. These flows were discernable only on the west side of the quarry lake, and are stacked one on top of the other, forming a steep sided mound. On the east side of the quarry all orientations were random and there were no recognizable flows. Of the three flows we identified, the upper flow has random orientation of phenocrysts, the middle flow has a westward orientation, and the bottom is again random. Further work includes: electron microprobe and laser ablation mass spectrometry analyses to determine if there are chemical differences between the three flow units, and possibly then determine the chemical evolution of the magma chamber. If the La Conception quarry is an in-situ eruptive center, its existence supports the hypothesis that there was extensive subduction beneath the eastern Gondwana margin during the late Cretaceous and helps us to model the processes active during this phase of volcanism.
Dyke Swarms in Southeastern British Columbia: Mineralogical and Geochemical Evidence for Emplacement of Multiple Magma Types During Orogenic Collapse
Eocene dyke swarms in southeastern British Columbia provide an important record of the tectonic and magmatic history of the Cordillera following orogenic collapse. New field mapping, petrographic, and geochemical data is presented for a swarm of more than thirty dykes located near the mining town of Trail, B.C. Detailed field mapping revealed that individual dykes are highly diverse, both in composition and morphology. As a group, the dykes trend northwest (average strike of 338 degrees) and dip steeply to the southwest. Their average thickness is approximately 1.5m, with a range from 4.5m to less than 1cm. Three sub-parallel dykes were mapped for a length of 2km, and exhibit irregularities in their form such as branching and offshoots that follow fractures in the country rock. Thin-section analysis shows a wide variety of rock types within the swarm, including: micro-quartz syenite, micro-syenite, micro-monzonite, latite, basalt, basaltic andesite, and lamprophyre. Texturally, these samples are consistently porphyritic and partially altered to chlorite and sericite. This alteration commonly occurs in concentric rims around phenocrysts. The samples are typically intergranular, although some display trachytic texture. Whole-rock geochemistry shows that the dykes have a wide range in composition, with SiO2 between 76.45 wt.% and 45.15 wt.% and MgO between 0.13 wt.% and 13.16 wt.%. The results also revealed that one dyke has very high values of Ni (430 ppm), Cr (1420 ppm), and Co (50 ppm), giving it a fairly primitive composition. Harker diagrams and trace element plots show three distinct groups: mafic calc-alkaline dykes, felsic calc- alkaline dykes, and minette lamprophyres. The felsic dykes are characterized by negative Eu and Sr anomalies suggesting fractionation of plagioclase feldspar, as well as pronounced negative P and Ti anomalies. The minettes are enriched in LILE and depleted in HSFE relative to the mafic dykes. The three groups do not appear to be related through magma differentiation processes, and it is suggested that they originate from separate magma sources. This has important implications for magma generation and heat flow in an extensional stress regime in an orogenic belt, and the potential interaction of different reservoirs in the crust. The findings from this study suggest extension in an ENE-WSW oriented pull-apart direction during Eocene time in southeastern B.C., and the concurrent emplacement of several different magma types perpendicular to extension.
Petrogenesis of Silicic Magmas in the Afro-Arabian Flood Volcanic Province in Yemen: A Melt Inclusion Study
The Oligocene (∼26-30 Ma) Afro-Arabian Flood Volcanic Province in Yemen and Ethiopia contains significant silicic material primarily deposited as large volume (>1,000 km 3 ) ignimbrites. These deposits have been correlated to tephra layers in ODP cores ∼2700 km SE of Yemen in the Indian Ocean. We are using melt inclusions (MI) in 4 ignimbrite units to model the petrogenesis and pre-eruptive volatile contents of the ignimbrites. Although quartz is a better MI host in silicic systems, these units only contain plagioclase that has the potential for degassing and leakage in cleavage planes. Through a series of experiments to investigate the rehomogenization temperature of the MI, we found inclusions were glassy at 1075 ° C after 24 hours. We also have compositional data from shorter duration runs (∼20 min.) at 1050 ° C. Reconnaissance MI data show a wide compositional variation compared to whole rock samples of the ignimbrites, and they provide a better estimate of true magmatic compositions compared to the heterogeneous whole rock samples. The inclusions are generally lower in Si (∼59-85 wt.%), yet significantly higher in Na and Al, which both decrease with increasing Si. We are currently modeling these variations to determine the petrogenesis of these silicic magmas. In terms of pre-eruptive volatiles, sulfur in the long duration experiments appears degassed (<5 ppm) yet is retained in the shorter duration runs (up to 500 ppm). Cl appears robust in the MI at ∼340 ppm and is consistent with modeled high halogen contents in such peralkaline melts. We will present estimates for the total S and Cl released during these large ignimbrite eruptions.
Clay and Zeolite Diagenesis of Two Miocene Felsic Tuffs from the Santa Rosalia Basin, Baja California Sur, Mexico
Miocene-age felsic volcanism associated with the early development of the Santa Rosalia basin, an incipient rift basin of the proto-Gulf of California, resulted in the deposition of the El Morro tuff (EMT) and Cinta Colorada (CC) pyroclastic units. The EMT was deposited between 10.5 and 7.9 Ma and is intercalated with rift-related basaltic andesites, which are unconformably overlain by the basal chemical and clastic sequences of the Boleo Formation. The CC is positioned approximately at the stratigraphic midpoint of the clastic sequence of the Boleo Formation and is hosted within tuffaceous claystones to sandstone of the third (of five) fan-delta cycle. Both units are limited in terms of their lateral distribution. The EMT is laterally discontinuous, but occurs sporadically throughout the Santa Rosalia basin. The CC is restricted to the central part of the basin, but is laterally continuous. The dominant lithology of the EMT is an altered vitric-crystal tuff containing phenocrysts of plagioclase, K-feldspar, glass shards and quartz, with trace amounts of amphibole, pyroxene and magnetite, in a matrix altered to montmorillonite, clinoptilolite, cristobalite and tridymite. In one location the EMT appears to be of ignimbritic origin, though the mineralogy is similar to vitric-rich tuff varieties. Mineralogically, the CC is similar to the EMT with the following exceptions: i) barren of glass shards; ii) heulandite is the dominant zeolite; iii) pervasively hematite altered; and, iv) intermixed with Boleo Formation detrital sediments due to reworking. The identification of diagenetic products has been confirmed by XRD following a variety of heat treatments (for zeolite and clay) and acid treatments (for silica varieties). Both EMT and CC vitric tuffs were diagenetically altered through the sequence: (1) devitrication of glass to smectite, cristobalite and tridymite; (2) transformation of cristobalite and tridymite to quartz; (3) clinoptilolite (EMT) or heulandite (CC); and, (4) supergene hematization. Diagenetic alteration of the EMT is the reflects a sequential process of interaction of volcanic glass with: (1) magmatic volatiles dervived from contemporaneous basaltic andesites; (2) downwelling hypersaline waters associate with evaporite deposition of the Boleo chemical sequence; and (3) ambient marine waters of the San Rosalia basin. Alteration of the CC is due to reaction between normal marine waters and volcanic glass, followed by supergene hematization by oxidizing ground water. Work in progress includes: i) SEM-EDS analysis of phenocrysts and diagenetic products; ii) bulk-rock XRF to quantify detrital products; and, iii) SEM-EDS and LA-ICP-MS of volcanic glass for petrogenetic modeling.
Changes in Aerosol Chemistry in the Plume of Kilauea Volcano Caused by the 2008 Summit Eruption
In March 2008 an eruption began in Halema'uma'u summit crater of Kilauea volcano; this was the first summit eruption since 1982. Prior to the new active phase, degassing in the crater was predominantly from several small fumaroles emitting a weak translucent plume. The 2003-2007 average SO2 emission rate was 140 tonnes per day and increased drastically to over 2000 tonnes per day in March 2008. The plume emitted from the crater during the eruption was concentrated and opaque, containing both ash and aerosol particles. Aerosol particles were sampled in the plume from Halema'uma'u before the start of the new eruptive phase (August 2007) and during it (May 2008). Particles emitted from Pu'u'O'o crater were collected at the rim and 8- 10km downwind. Sampling was done with a cascade impactor which collects and segregates PM10 (particle matter <10 μm) into 14 size fractions. There is a significant increase in PM sulphate concentration during the eruptive phase, or from 0.11 up to 6.3 μg per m3 of sampled air. Cl- concentration increased from 0.097 to 0.338 μgm-3, while F- was not detected either before or during the eruption. The SO42-/Cl- ratio increased from 0.15 to 18.8. The concentration peak of SO42- shifts to a coarser PM size fraction during the active phase, or from 0.18-0.32 to 0.32-0.56 μm. It is possible that higher water vapour content during the eruption favours more rapid particle growth. PM collected at Pu'u'O'o rim shows a noteworthy bimodal SO42- concentration distribution with a finer peak between 0.32-0.56 μm and a coarser peak between 1.0-1.4 μm. The coarser PM is efficiently removed from the plume and is not detected when sampled 8km downwind of the source. Near-vent nitrate was not detected in pre-eruptive samples but was found in concentrations between 0.17-0.58 μgm-3 in syn-eruptive PM; these are much lower than the concentrations seen at Pu'u'O'o (up to 3.0 μgm-3). Work in progress is analysis of metal content in the pre- and syn-eruptive PM which will be correlated with the size-resolved chemistry of anions. Further field sampling will be made in April 2009 now that the eruptive activity is significantly diminished and potentially coming to an end.
The Degassing Behavior of Volatile Heavy Metals in Natural and Synthetic Silicate Melts
Volcanic plumes transport metals, often in particulate or gaseous forms complexed with sulfur or halogens, from magma to the bio- and hydrosphere. The plumes are assumed to be the cooled, air-diluted equivalents of the high-temperature vapor phases originally in contact with degassing melt (Hinkley et al, 1994 Geochimica). Trace metal concentrations in plumes can be significantly different than the melt that liberated them, sometimes orders of magnitude greater. It has been estimated that 20-40 % of volatile elements (e.g. Bi, Pb, As, Sb) and up to 40-50 % of Cd and Hg are sourced from volcanic emanations annually (Nriagu, 1989, Nature). Some workers believe these ranges are too high (Hinkley, 1999, EPSL) or too low (Zreda-Gostynska and Kyle, 1997, JGR) leading to considerable differences in global inventory budgets of these metals and the degree to which they load the atmosphere. We investigate the degassing behavior of a wide variety of heavy metals with varying volatility from silicate liquids. Natural basalt, dacite, and synthetic rhyolite (Ab-Or-Qz minimum eutectic) were doped with a heavy metal cocktail (Bi, Pb, Tl, Au, Pt, Re, Sb, Sn, In, Cd, Mo, As, Cu). Diffusion experiments were conducted in air at 0.1 MPa and 1000 - 1400 ° C in open Pt capsules. During experiments of varying duration, trace element concentration gradients arise in the volatile trace metals due to their varying volatility, as measured (normal to the melt/gas interface) by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in quenched glasses after the experiment. Diffusion coefficients (Dx) were then extracted for elements using a semi-infinite one-dimensional diffusion model. We compare our results to previous experimental diffusion work on a more restricted set of elements and compositions (MacKenzie and Canil, 2006, Geochimica) and apply them to published trace metal concentrations in plumes measured over time at Kilaeuea and Erebus. This work may help better constrain the behavior/mobility of these metals in nature and their loading on the atmosphere and biosphere.
Volcanic Architecture of the Matagami Mining Camp: Implications for Mineral Exploration
Volcanogenic massive sulfide (VMS) deposits represent important sources of copper, gold, zinc, lead and silver in Canada. The Matagami mining camp, in the northern part of the Archean Abitibi greenstone belt, contains at least 18 zinc-rich VMS deposits, but the camp used to be considered "in decline" until recently; it is now seeing an important renewal of exploration activity. VMS deposits at Matagami are mainly located along two bands, the "South Flank" and the "North Flank", on the sides of a mafic intrusion. There is also exploration further west, in the so-called "West camp", but correlation of this area with the South Flank remains uncertain. The aim of the PhD project that started last year is to reconstruct the volcanic architecture of the area at a local scale (specific ore deposits) and regional scale (mining camp). We also aim to better understand the geological environment of mineralization and the emplacement processes of the VMS lenses. The reconstruction is based on volcanic facies variations and a chemo-stratigraphic approach. Drill holes from selected cross-sections on the South Flank have been described in detail during a first field session. The sections were chosen in the high-priority areas for exploration. It is now clear that some VMS deposits (Perseverance, McLeod…) occur at the contact between two rhyolites, rather than in a bimodal environment as was previous thought (Piche et al., 1993, Economic Geology 88: 1542-1558). The traditional exploration model at Matagami relied heavily on following an important exhalative horizon, called the Key Tuffite, since all the known deposits were located along this horizon. Other exhalites exist higher up in the sequence, with significant ore lenses now being intersected along these new horizons. The footwall rhyolite, named Watson Lake, does not seem to comprise any significant volcaniclastic rocks, and may represent a thick, large-volume, massive, low-viscosity lava flow, rather than a silicic dome or a series of such domes. The rhyolites from the hanging wall, called Bracemac for the southern part, and Dumagami in the north of the South Flank, are not continuous, and their deposition chronology is still unknown. Systematic downhole sampling has been performed in order to get chemical profiles of the different units. Four samples representing the three South Flank rhyolites and a Watson-like rhyolite from the West Camp will be dated using U-Pb on zircons. This will help to correlate the different units between the two sectors, and also clarify the age relationships within the South Flank.
The Geology, Geochemistry and Alteration of the Westwood Au-Zn-Cu Deposit, Abitibi Subprovince, Canada
The Westwood Au-Zn-Cu deposit, one of the latest major discoveries made in the Abitibi Subprovince, is hosted by the Bousquet Formation of the Archean Blake River Group, Abitibi Subprovince. The Bousquet Formation forms a south-facing, steeply dipping homoclinal volcanic sequence. Three mineralized corridors from the north to the south have been defined to date: 1) the Zone 2 Extension Corridor, 2) the North Corridor, and 3) the Westwood-Warrenmac Corridor. The Zone 2 Extension consists of auriferous quartz and sulphide veins. The North Corridor mineralization consists of various amounts of auriferous disseminated pyrite and sulphide veins. The Westwood-Warrenmac Corridor comprises semi-massive to massive sulphide lenses on a specific stratigraphic horizon. The Westwood deposit is interpreted, on a preliminary basis, to represent the transition between syngenetic vein systems (Zone 2 Extension) and subseafloor (North Corridor) and seafloor volcanogenic massive sulphide-style gold-rich mineralization (Westwood-Warrenmac). Careful inspection of drill core cross-cutting the mineralized sequence, and detailed geochemical analysis show that deposit-scale geology is dominated by coherent to volcaniclastic mafic to intermediate and tholeiitic to transitional flows towards the north (Bousquet Formation lower member), and by coherent and volcaniclastic intermediate to felsic, and transitional to calc-alkaline flows in the southern portion (Bousquet Formation upper member). The Zone 2 Extension ore zones occur in the lower Bousquet Formation and are hosted by mafic to intermediate units. The North Corridor ore zones are hosted by basalt and andesite. The Westwood-Warrenmac ore zone occurs most often in rhyodacitic to rhyolitic rocks of the upper Bousquet Formation. Depletion in Na, Ca, Mg, and Mn occur approaching peak metal and gold concentration, whereas enrichment in Fe is observed. This correlates to the increased concentration of sulphide minerals associated with the ore zones. There is relatively little change in Si, although an apparent enrichment may be present in some intervals. These major element trends reflect the abundances of alteration minerals such as garnet, biotite, chlorite, and sericite. The Westwood study contributes to better geological and geochemical exploration models for gold-rich VMS systems in Archean greenstone belts.
Volcanological and Metallogenic Study of a Prospective Segment of the Hebecourt Formation, Abitibi Subprovince, Canada A Preliminary Results
The Blake River Group is the youngest dominantly volcanic subalkaline package within the Archean Abitibi Greenstone Belt. The overwhelmingly tholeiitic and mafic Hebecourt Formation is located in the northern part of the Group and is among its oldest stratigraphic units, corresponding approximately to the age of the rocks hosting the world-class Horne volcanogenic massive sulphide (VMS) deposit in the Noranda camp. A collaborative research endeavour has been initiated to examine the top part of the Hebecourt Formation on either side of the Ontario-Quebec border. This sector was selected because VMS-style mineralisation is known from historical and recent drilling, but the area has seen much less exploration than, for example, the central Noranda camp. The study aims to improve the lithostratigraphy, chemostratigraphy and characterisation of the physical volcanology of the units and to understand and exploit the exhalative horizons as tools for mineral exploration. Improved chemostratigraphy will enable the development of vectors from the sulphide and mineral chemistry of the exhalative horizons by firmly establishing their stratigraphic position. Enhanced chemostratigraphy will also improve the positioning of syn-volcanic faults of the area. Detailed facies interpretation will indicate the location of volcanic vents, which are often associated with hydrothermal vents. In the study area, the top part of the Hebecourt Formation is characterised by two progressions from mafic to felsic lavas with an exhalative horizon capping each progression. The first progression consists of a basalt (predominantly pillowed), a rhyolite (subdivided into a lower and an upper unit), and an exhalite. Both rhyolitic units are increasingly brecciated up succession from massive intervals with local flow banding. The second progression begins locally with another basalt, followed by a variolitic basaltic andesite (hyaloclastite at the base and pillows with interstitial hyaloclastite at the top) and finally a rhyodacite (that has both massive and lapilli tuff facies), and the second exhalite. Locally there is a second rhyodacite, interpreted as intrusive due to its chemical affinity more typical of the overlying volcanic rocks in the Blake River Group. The exhalative horizons vary in thickness and composition, between horizons and laterally, from very finely laminated pyrite to massive chert with no visible sulphides, to sulphide stringers.
Stratigraphic and structural setting of the Dunraine Shear Zone, Val d'Or, Québec: metallogenic implications
The Dunraine shear zone is a sub-vertical, ENE-WSW trending deformation corridor crosscutting the Val d'Or Formation. The Val d'Or Formation is interpreted as the construction phase of an Archean island arc massif characterised by andesitic to rhyolitic lavas and associated volcaniclastic rocks. In the study area, the bedrock mainly consists of massive feldspar-phyric, intermediate lavas (andesite-dacite) and intermediate/felsic lapilli tuffs to tuff breccias with minor felsic lava flows. Red horizons of fine-grained pyrite beds with sericite and quartz, interpreted as exhalites, are locally found near the mineralized section of the Dunraine shear zone. To the south, this volcanic sequence is intruded by a syn-volcanic diorite (e.g. Dunraine sill). The regional schistosity (S1) varies between N245° and N260° and dips 75° to 90° northwestward. Two generations of lineations are hosted by this regional schistosity. L1 is a down-dip stretching lineation indicating a N-S oriented compression deformational phase associated with reverse faulting as locally observed along the Dunraine shear zone. L2 is a sub-horizontal, faintly-developed lineation with plunges varying from 10° to 40° westward. It appears to be associated with late dextral shearing. Dragging structures affecting S1 are consistent with dextral movement. Subvertical kink bands, oriented N360° to N015° are also locally observed in the field and are indicative of dextral shearing as well. Centimetre- to meter-wide, flat-lying extensional veins are present along the Dunraine shear zone, their orientation varies from N165° to N200° and dip values are 20° to 30° West or SW. These veins are gently folded, suggesting a late-tectonic origin. The relationship between bedding (S0) and S1 is difficult to establish clearly, mainly because both structures are frequently parallel to sub-parallel. The northward-directed way-up of bedding is however clearly indicated by load cast structures and channelling in the volcaniclastic sequence exposed on the D3 stripping. The sulphide mineralisation is marked by a narrow (1-10 metres) but laterally extensive (more than 2 km-long), chalcopyrite (Cpy)- and pyrite (Py)-rich (± sphalerite) stringers and horizons of disseminated sulphides with traces of gold. The mineralisation is hosted by dacitic to rhyodacitic lapilli tuff and/or lava. The stringers are 1 mm to 3 decimetres- wide with lengths varying from 10 cm to several metres-long. The occurrence of Cpy and Py is commonly accompanied by an intense silicification/chloritisation of host rocks whereas sericitisation and carbonatation are the dominant alterations elsewhere. The sulphides stringers are affected by S1 and commonly transposed subparallel to the schistosity. Evidence for crystal deformation of the sulphides suggests that the mineralisation occurred before the regional deformation and is therefore likely of syn-volcanic origin.
Characteristics and Distribution of Mineral Textures and Fluid Inclusions in the Epithermal Ag-Au Deposits at Guanajuato, Mexico
Fluid inclusion and mineralogical features indicative of boiling have been characterized in 855 samples from epithermal precious metals deposits along the Veta Madre at Guanajuato, Mexico. Features associated with boiling that have been identified include colloform texture silica, plumose texture silica, moss texture silica, ghost-sphere texture silica, lattice-bladed calcite, lattice-bladed calcite replaced by quartz and pseudo-acicular quartz after calcite and coexisting liquid-rich and vapor-rich fluid inclusions. Samples were assayed for Au, Ag, Cu, Pb, Zn, As and Sb, and were divided into high-grade and low-grade samples based on the gold and silver concentrations. For silver, the cutoff for high grade was 100 ppm, and for gold the cutoff was 1 ppm. The feature that is most closely associated with high grades of both gold and silver is colloform texture silica, and this feature also shows the largest difference in grade based on the presence or absence of that feature (178.8 ppm Ag versus 17.2 ppm Ag, and 1.1 ppm Au versus 0.2 ppm Au). For both Ag and Au, there is no significant difference in average grade in samples that contain coexisting liquid-rich and vapor-rich fluid inclusions and those where this feature is absent. The textural and fluid inclusion data were analyzed using the binary classifier within SPSS Clementine. The models that correctly distinguish between high and low grade samples most consistently (˜ 70-75% of the tests) for both Ag and Au were the neural network, the C5 decision tree and Quest decision tree models. For both Au and Ag, the presence of colloform silica texture is the variable with the greatest importance, i.e., the variable that has the greatest predictive power. Boiling features are absent or rare in samples collected along a traverse perpendicular to the Veta Madre. This suggests that if an explorationist observes these features in samples collected during exploration, an environment favorable to precious metal mineralization may be nearby. Good evidence for boiling is observed in the deepest levels of the Veta Madre that have been sampled in the mines and drill cores, suggesting that additional precious metal reserves are likely beneath the deepest levels sampled.
Major and trace element geochemistry of Archean sulfidic black shale horizons as a potential vectoring tool for VMS exploration
Metalliferous black shale horizons are a common but minor component of many subaqueous volcanic successions. These horizons are commonly drilled during volcanogenic massive sulfide (VMS) deposit exploration programs. Although ore metal enrichment can be identified by conventional assay methods, matrix dilution and post-burial hydrothermal and metamorphic activity may obscure information on the type and mechanism of metal addition to the shale. We used a combination of geochemical investigations at a variety of scales to discriminate between VMS-prospective and VMS-barren horizons. In addition, element signatures associated with hydrothermal plume fallout were identified and used and to determine relative direction to the palaeo-venting centre. Portable x-ray fluorescence (pXRF) analysers were used to identify and correlate prospective horizons within exploration drill cores. pXRF is a rapid and relatively inexpensive method of analysis that can deliver quantitative geochemical information at a cm-scale and help to identify intervals meriting further, more costly and time-consuming analyses. Subsequently, laser-ablation ICP-MS analysis of metal sulfides was used to constrain hydrothermal, hydrogenous and diagenetic end-member compositions, and to quantify element remobilization during post-burial alteration. These data were then used to refine the pXRF survey methodology and develop primary vectors toward potential concealed base metal deposits.
Geology and Geochemistry of the Lac Cinquante Uranium Deposit, Nunavut
The Lac Cinquante Uranium Deposit, is located in the Kivalliq district of Nunavut approximately 350 km west of Rankin Inlet, and is centered on approximately Latitude 62°34'33"N, Longitude 98°41'41"W. Geologically it is located within the Baker Lake Basin of the Churchill province, one of several northeast- trending Proterozoic basins in the Western Churchill Province that lie unconformably on top of Archean volcanics. Mineralization is found within basement volcanics that have undergone hydrothermal alteration and mineralization in fault zones. Previous studies of this area document two major zones of mineralization: the Main zone and the South zone; with three dominant styles of mineralization in the Archean greenstones including: disseminated pitchblende with base metals in tuffaceous metasediments, discrete pitchblende veins that cut across the metasediments, and quartz, carbonate, sulphides, and pitchblende in gash veins on 040 to 060 trending cross fractures. Additionally, mineralized zones hosting uranium are also present in the overlying Proterozoic sediments. Field based mapping completed in the summer of 2008 at 1:5000 has revealed a more detailed and complicated geological history than previously reported. The newly acquired map and historical data have been combined in an attempt to develop a comparative data collection. Geochemical data has aided in a more developed interpretation for the formation of the greenstone belt within which the Lac Cinquante uranium deposit is hosted. Current analytical techniques complementing the geological observations include X-ray diffraction to determine mineral assemblages and X-ray fluorescence for major and trace element information. Other analytical techniques will be utilised including electron microprobe to understand precise mineral chemistry of uranium bearing minerals, oxygen stable isotopes to understand fluid migration and ore forming reservoirs, and further stable isotope analyses to understand the temperature and mineral-fluid interactions leading to uranium mineralization are planned.
Volcanology and geochemistry of the Lundberg volcanogenic massive sulphide zone, Buchans, Newfoundland
The Buchans mining camp is characterized by a number of unusually high grade low-tonnage volcanogenic massive sulphide deposits (VMS) hosted by a bimodal calc-alkaline continental arc sequence. The mining camp produced a total of 16.2 Mt of ore at an average grade of 14.5% zinc, 7.6% lead, 1.3% copper, 126g/t Ag, and 1.37g/t Au rivalling most other VMS districts. Recent exploration has been focused on the Lundberg zone located in the hanging wall of the Lucky strike orebody, providing an excellent opportunity to explore the relationships between volcanology, sedimentation and mineralization in closely spaced drillcore. Four main volcanic and volcano-sedimentary units have been identified including two different rhyolites (both geochemical and textural constraint), dacite, basalt and late diabase. These units can be correlated across the entire Lundberg zone and provide an excellent testable stratigraphy. The stratigraphy of the Lundberg zone is characterized by a lower basaltic hyaloclastite with locally developed scoreaceous textures. Overlying the basalt is a volcano-sedimentary unit consisting of a rhyolite>dacite>siltstone>basalt framework breccia with a pyritic sandstone matrix, and rare sphalerite, galena, pyrite clasts and isolated massive sulphide (i.e., 20cm) and barite beds. These are structurally or stratigraphically overlain by a feldspar>>quartz phyric, green- pale yellow-red, +/- flow banded, coherent rhyolite and quartz>feldspar phyric, pale beige to red-dark green rhyolite with large quartz phenocrysts. The sequence is intruded by diabase sills. The basalt is extensively altered and cut by stockwork chlorite-pyrite, polymetallic and chalcopyrite rich veins which may form the stockwork to the overlying massive and transported sulphides. Some of the polymetallic veins contain extensive bladed calcite and quartz, suggesting local boiling may have occurred. Determination of the detailed volcanic stratigraphy, geochemical and mineralization characteristics of the Lundberg zone provides a predicative model that can be applied to exploration in this mining camp. On a broader scale, the presence of epithermal characteristics in the Buchans VMS camp provides clues to the origin of these high-grade, low tonnage deposits.
Geochemical and isotopic (Sr, O, C, S) evidence for multiple fluid sources for the Muþ barite deposits, SE Anatolia, Turkey
Barite mineralizations are observed in recrystalized and dolomitic limestones relating to Devonian aged Ground Formation, and in forms of veins, clearance filling ; and they are mostly hydrothermal characterized. It was defined by the results of field observations and analysis that barite mineralization is accompanied by the elements like copper, lead and zinc. That 18O and 87Sr/86Sr isotope values of limestones and barites which are wall rocks are very close reminds same origin formation. According to the relations of ore and wall rock, the most natural source that may cause mineralization is Devonian sea water, but mineralization has higher trace element and rare earth element concentrations. That Eu characterizing continental climate from rare earth elements was found high and Ce, characteristic of sea water, was negative preoccupy that mineralization in the area did not create any formation. In addition, considering that high 34S (‰ 17-44) and 18O values (‰ 12-20) and 87Sr/86Sr isotopes characterize the continental crust, a complicate formation mechanism is seen. It is thought that the solution causing mineralization is the Devonian sea water affected from continental crust. 34S ve 18O are enriched when Devonian sea water, in continental crust, enters a hydrothermal cycle which is permanently heatened by granitic intrusion and in consequence of bacterial activities occured during this process. It dissolves and incorporates Ba, Sr, Pb, Cu and Zn during this cycle. Eventually, mineralization occured through impregnation of these elements into piling in parts sedimentation continued.
Laser-Ablation SF-ICP-MS Analyses of MPI-DING Glass Reference Materials: Procedures and First Results from the KU Element2 Laboratory
We present the first laser ablation data of the KU Element2 laboratory, obtained on the MPI-DING glass reference materials, to test precision and reproducibility of the setup for in-situ trace element analysis. This data is compared to the compilation of results from other laboratories published by Jochum et al. (2006) and tested to calibrate trace element thermometry in quartz and rutile. We are also examining element fractionation and signal stability under different ablation and gas flow conditions. The KU Element2 laboratory at the Department of Geology is equipped with a high-resolution sector-field ICP- MS and a Photon Machines 193nm short pulse excimer laser. This laser enables largely color-independent ablation of different materials. Heating of the sample during the ablation process is minimal due to the short pulse, which helps to eliminate matrix effects. The homogenized laser beam creates flat bottom craters of 5 to ca. 175 microm diameter. A new 3 inch, small volume ablation cell is tested for washout efficiency and location independence of analysis. The main areas of research pursued with this setup are trace element analysis of major and accessory minerals and U-Th-Pb dating of accessory minerals for application to metamorphic and igneous petrology as well as detrital studies.
High Precision 142Nd/144Nd and 143Nd/144Nd Isotope Ratio Measurements in Rock Samples
The long-lived 147Sm-143Nd system with a half-life (T1/2) of 106 Gyr is generally used for geochronology. The short-lived 146Sm-142Nd system (T1/2= 103 Myr) is used as a geological tracer to track early (∼500 Ma) silicate differentiation  events in different planetary bodies. The isotope composition measurements by thermal ionization mass spectrometry (TIMS) require purification of Nd using chemical separation methods. This is important as an impure sample will give both a very poor ion yield and cause beam instability in the mass spectrometer, potentially resulting in a poor analysis . The separation of Nd for 143Nd isotope measurement is, fairly straightforward because there is no isobaric interference of any other REE. While 142Nd isotope analysis needs chemically separated Nd fraction to be ∼100% Ce-free as latter is composed of a substantial amount of 142Ce isotope. A 4-steps technique, modified from Caro et al., , for the separation of Nd is established at the Cosmochemistry Laboratory of University of Toronto, Canada and applied to the measurement of Nd isotope ratios in geological reference sample BCR-2 (USGS, Columbia River basalt) using TIMS. Results of the isotopic ratios obtained for BCR-2 are in good agreement with published values [e.g., 4]. Analytical work on the samples discovered as the oldest rocks on Earth  from Nuvvuagittuq greenstone belt in Québec, Canada and various meteorites is in progress. An account of the procedures involved is briefly described here. All working solutions and acids were prepared using >18.2 MØmega.cm-1 H2O from a Milli-Q water system. Experiments were performed under Class 100 clean work bench with acid-cleaned apparatus and plastic-ware. The whole rock powders were weighed (20-30 mg) and dissolved in a mixture of HF and HNO3 using PFA vials and heated at 110°C. Further decomposition was done in Teflon bomb in the oven at 205°C. Later on contents of the Teflon bomb were transferred to vials and fluorides were removed with a mixtures of HClO4 and HCl. Finally the digested samples were dissolved in 4N HCl prior to the column chromatography. The separation of alkalis and REE was achieved with 2 ml BioRad column using AG®50W-X12 resin; following which the separation of Nd and Sm fractions was achieved using Ln-Spec resin in PFA column. Results: Triple filament geometry was used to measure Nd as a metal in multi-dynamic mode using Isoprobe-T TIMS. About 600x10-9 g of JNdi-1 standard  produced a 142Nd beam strength of ~5×10-11 A; 400 cycles constituted one measurement, where each cycle consisted of 4 sequences of 10 second counting time. A set of ∼10 measurements of JNdi-1 gave extremely accurate and precise ratios for 142/144, 143/144 and 145/144 with internal precision better than 4 ppm and an external precision of less than 7 ppm in all cases. The BCR-2 samples were loaded ∼200 ng (factor of 4 less than JNdi-1) and therefore their operating signal strength for 142Nd was ∼1×10-11 A. Based on our analyses we conclude that the internal precision for BCR-2 samples in the range of 8-9 ppm and their external precision is comparable to JNdi-1. References:  Caro et al, (2008) Nature 452, 336-339;  Míková & Denková, (2007) Geosciences 52, 221-226;  Caro et al, (2003) Nature 432, 428-432;  Raczek et al., (2003) Geostandards Newsletter 27, No.2, 173-179;  O'Neil et al, (2008) Science 321, 1828-1831;  Tanaka et al., (2000) Chem Geol, 168, 279-281.
Gravity Survey of Socorro Island, Revillagigedo Archipielago, Eastern Pacific Ocean
Results of a gravity survey of the southern sector of Socorro Island in the Pacific Ocean are presented. Socorro is the largest of four volcanic islands of Revillagigedo Archipielago, which is located at the northern sector of N- S trending Mathematician ridge and intersection with E-W Clarion fracture zone in the Pacific plate. Socorro Island is formed by a large basaltic shield volcano, with the subaerial volcanic cone of Mount Evermann rising some 1400 m a.s.l. Volcanic products are dominantly silicic-peralkaline, giving Revillagigedo is unique character among Pacific plate volcanic islands, which appear closer to the slow spreading rates more differentiated islands in the Atlantic than to the fast spreading large melt volumes observed in the Pacific. Gravity survey aimed to investigate on the shallow crustal structure, tectonics and volcanic stratigraphy of the island. Survey profiles cross the Evermann volcano and the collapsed summit caldera rim and the Lomas Coloradas sector with cones, domes, lava flows and ignimbrites. Major Bouguer gravity anomalies are observed over the Evermann summit cone, partial caldera rim and Lomas Coloradas trachitic volcanic central part. Gravity 2.5- D inversion and forward models show a local andesitic basement that in parts rises up to 200 m depth, which is covered by peralkaline basalt units with thickness up to 200 m. Basaltic units from Evermann extend up to 3 km down the volcano flank. In the eastern side of Lomas Coloradas the basaltic units are some 300 m thick and up to 2 km length. Subaerial volcanism has been divided into three groups corresponding to pre-caldera, syn-caldera and post-caldera formations, with pre- and syn-caldera activity spanning the past 550 to 350 ka. Post-caldera volcanic activity continues to present, with flank and submarine activity, cinder cone eruptions and intense hydrothermal activity at summit cone.
A GIS-Based Approach to Multiscale Network Analysis: Method and Applications Regarding Volcanic Seismicity Patterns in Hawaii
While space-time seismicity patterns represent significant information sources concerning volcanic processes, gaining a useful characterization of their complex variability is notoriously challenging. This paper presents a method designed to capture relationships among seismic events, and to provide a description of the evolution of such relationships over time. Earthquake event networks are generated for successive temporal windows. The network nodes consist of event locations, while the weights of edges connecting the nodes are calculated based on the spatial distance that separates the nodes, the time interval between their moments of occurrence, and the magnitude of the events. Network properties - weighted degree distribution, edge distribution, tortuosity exponent - are established for temporally successive windows using procedures supported by the ArcGIS Network Analyst. Results include the identification of network scaling properties and of their variation in time. This methodology is applied to seismicity patterns in Hawaii. We show that the studied system exhibits long-range scaling properties and experiences phases of consistent change in terms of network connectedness, which can be correlated to significant volcanic processes. This approach proves to offer a fertile analysis instrument that could be usefully added to the current investigation tools applied in volcanic seismology.