Synvolcanic gold mineralization within a deformation zone: the Chevrier deposit, Chibougamau, Abitibi Subprovince, Canada

The Chevrier gold deposit is located in the northeastern part of the Abitibi Subprovince within the Chevrier volcanic centre (CVC). This calc-alkaline volcanic centre evolved from mafic to felsic in a submarine environment, and the Chevrier deposit is spatially associated with the uppermost unit composed mostly of massive dacite to rhyolite, dated at 2730 Ma. The Chevrier deposit is divided in two zones. The North Zone is composed of high-grade, discordant quartz-carbonate veins and disseminated pyrite mostly within melanocratic gabbro dykes, whereas, the South Zone is within a low-grade, concordant pyrite envelope associated with quartz-carbonate-pyrite veinlets. Both zones are highly deformed and show strong carbonate, sericite and chlorite alterations. Although these zones share many characteristics with orogenic deposits, the crosscutting of the Chevrier deposit by structures associated with the major deformation events suggests that mineralization was prior to these events. Furthermore, felsic dykes associated with the formation of the CVC crosscut auriferous veins and zones, therefore suggesting a synvolcanic timing for the gold mineralization. Characteristics of the zones, such as similar vein composition and alteration mineral assemblages and location within the volcanic pile, suggest that they are part of a single hydrothermal event where the South Zone represents the shallower part of the deposit. The Chevrier deposit is classified as a noncarbonate-hosted stockwork and disseminated deposit and shows many similarities with strata-bound gold deposits of the Andacollo mining district of Chile. Dating of mineralization at the Chevrier deposit indicates that the emplacement of gold occurred more than 25 Ma before the onset of the orogeny affecting this part of the Abitibi subprovince. It also illustrates how synvolcanic mineralization can be misinterpreted as an orogenic deposit without the presence of crosscutting relationships with dykes, faults or other structures.Contribution No. 2005-5130-01 of the Ministère des Ressources naturelles et de la Faune du Québec.     

Réflexions sur la formation des bandes de Forbes : l'instabilité de la fusion de la glace saleReflections on the formation of Forbes ogives: the instability of fusion of dirty ice

Forbes ogives show alternations of dark (ice + mineral dust) and light bands at the surface of certain glaciers. We propose to understand their origin by the influence of the content of mineral matter on the lowering of the temperature and pressure of ice fusion and upon the increase of fusion velocity. We are then in an unstable situation: a local increase in the mineral content being induced by the fusion process, which in turn increases; this creates a dark band. The movement of the glacier cannot keep up with the fusion: pressure is lowered again below the fusion point, and a white band is formed. To cite this article: B. Guy et al., C. R. Geoscience 334 (2002) 1061–1070.     

Disaggregating and mapping crop statistics using hypertemporal remote sensing

Governments compile their agricultural statistics in tabular form by administrative area, which gives no clue to the exact locations where specific crops are actually grown. Such data are poorly suited for early warning and assessment of crop production. 10-Daily satellite image time series of Andalucia, Spain, acquired since 1998 by the SPOT Vegetation Instrument in combination with reported crop area statistics were used to produce the required crop maps. Firstly, the 10-daily (1998–2006) 1-km resolution SPOT-Vegetation NDVI-images were used to stratify the study area in 45 map units through an iterative unsupervised classification process. Each unit represents an NDVI-profile showing changes in vegetation greenness over time which is assumed to relate to the types of land cover and land use present. Secondly, the areas of NDVI-units and the reported cropped areas by municipality were used to disaggregate the crop statistics. Adjusted R-squares were 98.8% for rainfed wheat, 97.5% for rainfed sunflower, and 76.5% for barley. Relating statistical data on areas cropped by municipality with the NDVI-based unit map showed that the selected crops were significantly related to specific NDVI-based map units. Other NDVI-profiles did not relate to the studied crops and represented other types of land use or land cover. The results were validated by using primary field data. These data were collected by the Spanish government from 2001 to 2005 through grid sampling within agricultural areas; each grid (block) contains three 700 m × 700 m segments. The validation showed 68%, 31% and 23% variability explained (adjusted R-squares) between the three produced maps and the thousands of segment data. Mainly variability within the delineated NDVI-units caused relatively low values; the units are internally heterogeneous. Variability between units is properly captured. The maps must accordingly be considered “small scale maps”. These maps can be used to monitor crop performance of specific cropped areas because of using hypertemporal images. Early warning thus becomes more location and crop specific because of using hypertemporal remote sensing.     

深海火山堆积期间从海底火山作用到富含硫化物石英脉型金成矿作用的演化

富含硫化物石英脉型金矿是加拿大太古宙金矿类型中重要的一种.关于它的成因仍是个谜.象Copper Rand和Geant Dormant这样一些金矿,其中的富硫化物含金石英脉的形成早于区域韧性变形,因而排除了变质成因的可能.Geant Do rmant金矿尤其适合于阐述金矿成因问题,因为其中的含金石英脉产于一层序关系保存完好的弱形变火山沉积序列中.本文论述了与该矿床有关的火山堆积及其热液系统的演化.从野外、岩相学与地球化学资料确定了火山演变及其形成的水下环境.热液事件则从野外分布特征、硫化物以及蚀变矿物组合和蚀变围岩的质量平衡计算等方面加以讨论.在分析火山与热液事件年代学和几何学关系基础上,认为含金石英脉是在火山成因热液系统的最后阶段形成的,该热液系统包括在深水火山堆积期间从贫金海底火山作用演变成富金矿脉型成矿作用的整个过程.本文探讨了在这种环境中含金石英脉的形成和金的富集机制以及所提出的金矿成因模式在勘探中的意义.     

深基坑变形监测方案设计及数据分析研究

结合某深基坑监测实践,探讨了基坑安全监测点的布设,并对支撑轴力和地下水位监测数据进行了深入分析,研究表明,混凝土持续的体积收缩以及在荷载下混凝土发生徐变被认为是影响轴力异常的最主要因素.对基坑工程监测实践有一定的借鉴意义.     

多角度高光谱CHRIS/Proba植被模式数据大气校正

遥感数据大气校正是进行定量遥感的前提,大气校正的好坏直接影响到后期的定量分析、信息提取和遥感应用.选择长白山地区2008年8月30日的多角度高光谱CHRIS/Proba数据为研究对象,根据多角度高光谱CHRIS数据植被模式的特点,利用滤波进行条带噪声去除;根据MODTRAN模型原理进行多角度高光谱CHRIS/Proba...     

Multistage gold mineralization at the Lapa mine, Abitibi Subprovince: insights into auriferous hydrothermal and metasomatic processes in the Cadillac–Larder Lake Fault Zone

The Lapa gold deposit contains reserves of 2.4 Mt at 6.5 g/t Au and is one of the few deposits located directly within the Cadillac–Larder Lake Fault Zone (CLLFZ), a first-order crustal-scale fault that separates the Archean Abitibi Subprovince from the Pontiac Subprovince to the south. Gold mineralization is predominantly hosted in highly strained and altered, upper greenschist–lower amphibolite facies mafic to ultramafic rocks of the Piché Group. Auriferous ore zones consist of finely disseminated auriferous arsenopyrite–pyrrhotite?±?pyrite and native gold disseminated in biotite- and carbonate-altered wall rocks. Native gold, which is also present in quartz ± dolomite–calcite veinlets, is locally associated with Sb-bearing minerals, especially at depth ≤1 km from surface where the deposit is characterized by a Au–Sb–As association. At vertical depth greater than 1 km, gold is associated with arsenopyrite and pyrrhotite (Au–As association). The mineralogy and paragenesis of the Lapa deposit metamorphosed ore and alteration assemblages record the superposition of three metamorphic episodes (M1, M2, and M3) and three gold mineralizing events. Spatial association between biotitized wall rocks and auriferous arsenopyrite indicates that arsenopyrite precipitation is concomitant with potassic alteration. The predominant Au–As association recognized across the deposit is related to gold in solid solution in arsenopyrite as part of a pre-M2 low-grade auriferous hydrothermal event. However, the occurrence of hornblende?+?oligoclase porphyroblasts overprinting the biotite alteration, and the presence of porous clusters and porphyroblasts of arsenopyrite with native gold and pyrrhotite indicate an auriferous metasomatic event associated with peak M2 prograde metamorphism. Late retrograde metamorphism (M3) overprints the hornblende–oligoclase M2 assemblage within the host rocks proximal to ore by an actinolite–albite assemblage by precipitation of free gold and Sb–sulfosalts at lower P–T. The complex relationships between ore, structural features, and metamorphic assemblages at Lapa are related to the tectonometamorphic evolution of the Cadillac–Larder Lake Fault Zone at different times and crustal levels, and varying heat and fluid flow regimes. The Lapa deposit demonstrates that early, low-grade gold mineralization within the Cadillac–Larder Lake Fault Zone has benefited from late gold enrichment(s) during prograde and retrograde metamorphism, suggesting that multi-stage processes may be important to form gold-rich orogenic deposits in first order crustal-scale structures.     

Geochemistry and zircon U-Pb ages of adakitic rocks from the Dulan area of the North Qaidam UHP terrane, north Tibet: Constraints on the timing and nature of regional tectonothermal events associated with collisional orogeny

Geochemistry and U-Pb ages of leucosomes and tonalites in a high pressure granulite unit of the Dulan area have been determined to constrain the tectonothermal evolution related to collision and thickening of lower crust in the North Qaidam Mountains (NQD). Leucosomes and tonalites show a marked chemical resemblance to adakites: (1) high La/Yb and Sr/Y, and low Y and HREE; (2) high Al₂O₃ and low Mg^# values with obvious positive Eu anomalies; and (3) slightly positive ε_Nd(t) values. Zircon U-Pb analysis of leucosomes and tonalites yielded ²⁰⁶Pb/²³⁸U ages of 428-437 Ma and 436-437 Ma respectively, which constrain the emplacement ages of the adakitic rocks. Petrological, geochronological and geochemical characters indicate that the adakitic rocks may have been derived from partial melting of a thickened mafic lower crust (> 50 km), suggesting a dominating source regime of synchronous high-pressure mafic granulites. Contemporary magmatism in other units of the NQD shows evidence of a widespread tectonothermal event during early Silurian (420-450 Ma) that includes metamorphism, magmatism and anatexis related to collision and thickening of lower crust.     

Diapirism during regional compression: The structural pattern in the Chibougamau region of the archean Abitibi belt,Québec

The major fold pattern of the Chibougamau region in the Archean Abitibi Belt, Quebec, is the result of two fold phases. The principal F₁-synclines first formed as subsiding fault troughs, and were shortened in NS direction and molded around volcanic centers that had been consolidated by large synvolcanic plutons; they have a general easterly direction but curve locally to the NE or SE. N-trending F₁-folds are locally present in the anticlinoria and are thought to have been produced by the upwelling of basement gneiss and/or synvolcanic plutons. The F₂-folds have rather regular easterly trends; their axial plane schistosity S₂ is the predominant planar fabric element. Basin margin faults were transformed into east-trending thrust faults during the F₁- and F₂-folding.The superposed schistosities S₃ and S₄ are conjugate shear planes related to NS shortening. They and the kink bands produce only minor strain except at a few localities. Synkinematic and late kinematic diapiric plutons pierce the structural pattern. The F₁-folds, derived from the paleogeographic pattern during the uplift of volcanic islands and during the subsidence of fault basins, may also be interpreted as diapiric. Thus, diapirism took place during the whole period of NS compression.ENE trending left-lateral shear belts and their associated first and second order faults, displaced the Kenoran structures in late Archean and early Proterozoic time.

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Zusammenfassung Der Faltenbau des Chibougamaugebietes im Archaischen Abitibi Belt, Quebec, ist das Resultat zweier Faltungsphasen. Die gro\en F₁-Synklinalen bildeten sich zuerst als absinkende Verwerfungströge, die später in NS-Richtung eingeengt wurden und gleichzeitig dabei an die verfestigten Vulkanzentren gepre\t wurden. Die gro\en synvulkanischen Plutone konsolidierten die vulkanischen Zentren. Die F₁-Falten streichen generell OW, biegen aber lokal in NO- und SO-Richtung um. Nord-streichende F₁-Falten finden sich lokal in den Antiklinorien und werden auf das Aufsteigen der basalen Gneisse und/oder der synvulkanischen Plutone zurückgeführt. Die F₂-Falten haben ein regelmä\iges Ost-Streichen; ihre der Faltenachsenfläche parallele Schieferung S₂ ist das deutlichste planare Strukturelement. Die Randbrüche der Verwerfungsbecken wurden während der F₁- und F₂-Faltungsphase in Oststreichende überschiebungen umgewandelt. Die S₃ und S₄-Schieferungen sind konjugierte Scherflächen in NS-Kompression. S₃ und S₄ und die Knickbänder verursachen nur lokal eine erhebliche Verformung. Synkinematische und spätkinematische Plutone durchbrechen diese Strukturen und die F₁-Falten, erzeugt durch den Aufstieg von Vulkaninseln und das Absinken von Verwerfungsbecken können ebenfalls als diapirische Strukturen gedeutet werden. Der Diapirismus überdauerte die ganze Periode der NS-Kompression.Linkshändig rotierte ONO-streichende Scherzonen und die dazugehörigen Verwerfungen erster und zweiter Ordnung versetzen die Kenorischen Strukturen während des späten Archaikums und des frühen Proterozoikums.

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Résumé Les plis majeurs de la région de Chibougamau dans la ceinture Archéenne de l'Abitibi au Québec sont le résultat de deux phases de plissement. Les synclinaux F₁ majeurs prenaient naissance comme bassins de faille subsidents et furent comprimés en direction NS. Ils furent moulés autour des centres volcaniques qui avaient été consolidés par des plutons synvolcaniques; leur direction générale est est-ouest mais ils s'incurvent localement vers le NE ou vers le SE. Des plis F₁ de direction N sont présents localement dans les anticlinoria et nous pensons qu'ils sont le résultat de l'ascension diapirique du gneiss du socle et des plutons synvolcaniques. Les plis F₂ ont une direction EO assez régulière; leur schistosité de plan axial (S₂) est l'élément structural planaire prédominant. Les failles en bordure des bassins de subsidence furent transformées en chevauchement lors des phases de plissement F₁ et F₂.Les schistosités superposées S₃ et S₄ sont des surfaces de cisaillement conjugées en compression NS. Ces schistosités et les plis en chevron ne sont pas responsables d'une déformation substantielle, sauf à quelques endroits. Des plutons diapiriques d'âge syn-cinématique à tardi-cinématique percent ce patron structural. Le patron des plis F₁, déterminé par le patron paléogéographique durant le soulèvement d'Îles volcaniques et la subsidence des bassins de faille peut aussi Être interprété comme une structure diapirique. Des ceintures de cisaillement senestre en direction ENE et leur cortège de failles de premier et deuxième ordre ont déplacé les structures Kénoréennes tard dans l'Archéen et tÔt dans le ProtérozoÏque.

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