Geology of the Victor Kimberlite, Attawapiskat, Northern Ontario, Canada: cross-cutting and nested craters

The pipe shapes, infill and emplacement processes of the Attawapiskat kimberlites, including Victor, contrast with most of the southern African kimberlite pipes. The Attawapiskat kimberlite pipes are formed by an overall two-stage process of (1) pipe excavation without the development of a diatreme (sensu stricto) and (2) subsequent pipe infilling. The Victor kimberlite comprises two adjacent but separate pipes, Victor South and Victor North. The pipes are infilled with two contrasting textural types of kimberlite: pyroclastic and hypabyssal-like kimberlite. Victor South and much of Victor North are composed of pyroclastic spinel carbonate kimberlites, the main features of which are similar: clast-supported, discrete macrocrystal and phenocrystal olivine grains, pyroclastic juvenile lapilli, mantle-derived xenocrysts and minor country rock xenoliths are set in serpentine and carbonate matrices. These partly bedded, juvenile lapilli-bearing olivine tuffs appear to have been formed by subaerial fire-fountaining airfall processes.

The Victor South pipe has a simple bowl-like shape that flares from just below the basal sandstone of the sediments that overlie the basement. The sandstone is a known aquifer, suggesting that the crater excavation process was possibly phreatomagmatic. In contrast, the pipe shape and internal geology of Victor North are more complex. The northwestern part of the pipe is dominated by dark competent rocks, which resemble fresh hypabyssal kimberlite, but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias±volcaniclastic kimberlite. Current evidence suggests that the hypabyssal-like kimberlite is, in fact, not intrusive and that the northwestern part of Victor North represents an early-formed crater infilled with contrasting extrusive kimberlites and associated breccias. The remaining, main part of Victor North consists of two macroscopically similar, but petrographically distinct, pyroclastic kimberlites that have contrasting macrodiamond sample grades. The juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically. The nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different, indicating that they derive from different magma pulses, or phases of kimberlite, and thus represent separate eruptions. The initial excavation of a crater cross-cutting the earlier northwestern crater was followed by emplacement of phase (i), a low-grade olivine phenocryst-rich pyroclastic kimberlite, and the subsequent eruption of phase (ii), a high-grade olivine phenocryst-poor pyroclastic kimberlite, as two separate vents nested within the original phase (i) crater. The second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade. Thus, the final pyroclastic pipe infill of the main part of the Victor North pipe appears to consist of at least three geological/macrodiamond grade zones.

In conclusion, the Victor kimberlite was formed by several eruptive events resulting in adjacent and cross-cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal-like kimberlite, which is now interpreted to be of probable extrusive origin. Within the pyroclastic kimberlites of Victor North, there are two nested vents, a feature seldom documented in kimberlites elsewhere. This study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism.     

Secondary geochemical dispersion in the Precambrian auriferous Hutti-Maski schist belt, Raichur district, Karnataka, India. Part I: anomalies of As, Sb, Hg and Bi in soil and groundwater

The nature of secondary geochemical dispersion of As, Sb, Hg and Bi in soil and ground water of the semi-arid, tropical, Archaean, auriferous, Hutti-Maski greenstone belt has been investigated for identification of appropriate geochemical techniques for Au exploration in similar terrains.Results indicate that the <180 μm size-fraction of C-horizon soil is an appropriate sampling medium for delineating pedogeochemical anomalies of As, Sb, Hg and Bi related to gold mineralisation. These pedogeochemical anomalies along with anomalous values of alkalinity, chloride, sulphate, As and Sb in groundwater are controlled significantly by primary mineralisation located along shear zones in the greenstone belt. Arsenic anomalies in soil are broad, whereas, those of Sb and Bi are restricted to narrow zones directly over mineralised areas. In contrast, Hg anomalies around known mineralised areas are irregular and do not clearly demarcate the mineralised areas. The study indicates that anomalies of As, Sb and Hg in soil are principally hydromorphic, whereas those of Bi are clastic.The study recommends use of groundwater sampling at 2–3 km spacing with routine analysis of chloride, sulphate and alkalinity along with As and Sb in the first phase. This may be followed up with sampling of C-horizon of soils on a 1 km square grid for As-anomalies. Arsenic-anomalous areas may be sampled for As, Sb, Hg and Bi on a 500 m square grid for detailed exploration.     

A Model of Magmatic Crystallization

A computer model simulating fractional crystallization at oneatmosphere pressure incorporates nine broadly-defined minerals—magnetite,olivine, hypersthene, augite, quartz, plagioclase, orthoclase,leucite, and nepheline. The crystallization temperature of eachmineral is considered to be a smooth function of the compositionof the magmatic liquid. These mineral temperature equationsare obtained by multiple linear regression analysis of informationfrom published silicate systems and rock melting experiments.The nine equations are solved for any primary liquid, withinthe broad range of common magma types, to select the crystallizingmineral or minerals. Partition ratios from published experimentsand analyses of lavas and phenocrysts permit calculation ofthe composition of the crystallizing mineral assemblage. Subtractionof a small amount of that composition from the primary liquidyields a new liquid, which may be recycled to yield a sequenceof liquids during fractional crystallization. The crystallizationmodel handles assemblages of co-precipitating minerals, andcan trace progressive saturation in new minerals, substitutionof a new mineral for an old mineral, and cessation of crystallizationof a mineral. The sequences of minerals and liquids derivedfrom a broad set of primary liquids are geologically realistic,so the model is useful in predicting phenocrysts in volcanicrocks and events during crystallization of shallow intrusions.     

中亚与亚洲中部晚白垩世的陆生脊椎动物组合对比

通过对前人建议的26个生物地层标志化石存在与否的简约分析,中亚与亚洲中部晚白垩世的陆生脊椎动物组合的相对层位得到了更清楚的揭示。此区最古老的组合是乌兹别克斯坦克孜勒库姆沙漠的Khodzhakul组合(早塞诺曼期),其次为蒙古戈壁沙漠东部BaynShire组的下部和上部的组合(塞诺曼期至桑顿期)。中国内蒙古二连达布苏动物群与中亚的土伦期—桑顿期动物群属于同一类群,因为它们均具龟鳖类Khunnuchelys,前者时代可能为桑顿期。三个中亚的组合(Bissekty、Yalovach和Bostobe组合)中有两个地方性的鳄形类(Kansajsuchus和Tadzhikosuchus)和一个出现于戈壁沙漠的鳄形类(Shamosuchus)化石。戈壁沙漠的坎潘期至马斯特里赫特期组合与北美同期动物群为同一类群。Djadokhta组与BarunGoyot组的坎潘期脊椎动物组合具有高度的地方性,并反应了半干旱的古环境。产自Nemegt组的组合生存于比较潮湿的环境。在组成上,这一组合与其他河流相沉积环境(Bissekty、二连达布苏以及北美Judithian期和Lancian期的组合)相似。具顶饰的鸭嘴龙Saurolophus的存在,支持了Nemegt组合为马斯特里赫特期时代。戈壁沙漠的这三个组合(Djadokhta、Barun Goyot和Nemegt组合)被归为一类,因为它们共同拥有地方性的龟类Mongolemys和兽脚亚目的小驰龙类。亚洲中部和北美的坎潘期至马斯特里赫特期组合与亚洲更加古老的组合不同在于存有暴龙科、肿头龙亚目和鸭嘴龙科。在中亚,由于地区性的海侵,这一时间段内的陆生脊椎动物组合多不清楚。     

Petrology of Lamproites from Smoky Butte, Montana

Hyalo-armalcolite-phlogopite lamproites and sanidine-phlogopitelamproites occurring at Smoky Butte, Montana are rocks formedfrom rapidly quenched, high temperature, uncontaminated lamproiticmagma. Petrographic variations are attributable to differentcooling histories of several batches of compositionally identicalmagma. Compared with other occurrences of lamproite, the rocksare unusually rich in TiO₂ and are characterized by the presenceof abundant armalcolite and the most TiO₂-rich phlogopites yetfound in this paragenesis. Compositional data are given fortitanian phlogopite, olivine, diopside, titanian potassian richterite,armalcolite, sanidine, analcite, and glass. The mafic mineralsare Al-deficient and exhibit very little compositional variation.Original leucite has been pseudomorphed by sanidine or analcite.The latter mineral was probably formed at the same time thatthe glass lost K, and gained Na, during alteration by groundwater.All of the lamproites are strongly enriched in Ta, Hf, and thelight REE (La /Yb = 162–280), and have high MgO and Crcontents. Mineralogical, geochemical, and previously publishedisotopic data are combined in developing a petrogenetic modelwhich suggests that these lamproites were derived from an ancient(2.5 Ga) doubly metasomatized harzburgitic source, and thatthey represent relatively primitive lamproites which were intrudedat near-liquidus temperatures.     

Uranium Tailings Samples UTS-1 to UTS-4 of CCRMP

The Canadian Certified Reference Materials Project announces the availability of reference uranium tailings sample UTS-1, -2, -3 and -4. Eighteen laboratories participated in the interlaboratory program for total iron, titanium, aluminium, calcium, barium, uranium, thorium, total sulphur, sulphate and arsenic. Eight laboratories participated in the inter-laboratory program for thorium-230, radium-226, lead-210 and polonium-210 in all four samples and for thorium-232, radium-228 and thorium-228 in UTS-1 and UTS-2.     

Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano,Panama

The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites. The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics. We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano is related to the subduction of relatively hot lithosphere.     

1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield

At least 14 small (1–11 km across) 1.8 Ga Svecofennian post-collisional bimodal intrusions occur in southern Finland and Russian Karelia in a 600-km-long belt from the Åland Islands to the NW Lake Ladoga region. The rocks range from ultramafic, calc-alkaline, apatite-rich potassium lamprophyres to peraluminous HiBaSr granites, and form a shoshonitic series with K₂O+Na₂O>5%, K₂O/Na₂O>0.5, Al₂O₃>9% over a wide spectrum of SiO₂ (32–78%). Although strongly enriched in all rocks, the LILE Ba and Sr and the LREE generally define a decreasing trend with increasing SiO₂. Depletion is noted for HFSE Ti, Nb and Ta. Available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded. Initial magmas (Mg#>65) in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post-collisional uplift some 30 Ma after the regional Svecofennian metamorphic peak. However, prior to the melting episode, the lithospheric mantle was affected by carbonatite metasomatism; more extensively in the east than in the west. The melts generated in the more carbonate-rich mantle are extremely enriched in P₂O₅4%, F12,000 ppm, LILE: Ba9000 ppm, Sr7000 ppm, LREE: La600 ppm and Ce1000 ppm. The parental magma underwent 55–60% fractionation of biotite+clinopyroxene+apatite+magnetite+sphene whereupon intermediate varieties were produced. After further fractionation, 60–80%, of K-feldspar+amphibole+plagioclase±(minor magnetite, sphene and apatite), leucosyenites and quartz-monzonites were formed. In the west, where the source was less affected by carbonatite metasomatism, calc-alkaline lamprophyres (vogesites, minettes and spessartites) and equivalent plutonic rocks (monzonites) were formed. Removal of about 50% of biotite, amphibole, plagioclase, magnetite, apatite and sphene produced peraluminous HiBaSr granites. The impact of crustal assimilation is considered to be low. At about 1.8 Ga, the post-collisional shoshonitic magmatism brought juvenile material, particularly enriched in alkalis, LILE, LREE and F, into the crust. Although areally restricted, the regional distribution of the post-collisional intrusions may indicate that larger volumes of 1.8 Ga juvenile material resides in unexposed parts of the crust.     

Contrasted melting relations in a pyrolite upper mantle under mid-oceanic ridge,stable crust and island arc environments

The pyrolite model composition provides a satisfactory source composition for mantle-derived magmas insofar as major elements and “compatible” trace elements are concerned but there is evidence for mantle inhomogeneity in the abundances of “incompatible” minor and trace elements (e.g., K, Ti, P, Rb, Sr, light rare earth elements etc.). The composition of a magma, assuming a constant source composition, varies according to the pressure, temperature and water pressure or water content of the source region. The latter two variables essentially determine the degree of partial melting of the source region and in considering the chemical composition of the melt and nature of the residual phases, this parameter is of prime importance.For high degrees (> 20% approx.) of partial melting of a pyrolite source region, magmas are of tholeiitic character but are of increasingly undersaturated and alkaline type for lower degrees of partial melting and high pressures. For any chosen degree of melting and fixed water content of the source region, magmas are more olivine-rich at higher pressures. For any chosen pressure and chosen degree of partial melting, magmas are less olivine-rich at high water contents (and thus lower temperatures). Quartz tholeiite magmas may be derived by ~ 30% melting of pyrolite under water-saturated conditions at pressures up to between 17 kbar and 20 kbar. These generalizations may be applied to understand the characteristic magmatism of mid-oceanic ridges, island chains, oceanic islands and orogenic regions.     

Fluids in granulites of the Southern Marginal Zone of the Limpopo Belt, South Africa

The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO₂(-CH₄) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H₂O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H₂O activity (0.2-0.3). The absence of water in CO₂-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO₂ inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H₂O and CO₂-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.