Knowledge Recovery for Continental-Scale Mineral Exploration by Neural Networks

This study is concerned with understanding of the formation of ore deposits (precious and base metals) and contributes to the exploration and discovery of new occurrences using artificial neural networks. From the different digital data sets available in BRGM's GIS Andes (a comprehensive metallogenic continental-scale Geographic Information System) 25 attributes are identified as known factors or potential factors controlling the formation of gold deposits in the Andes Cordillera. Various multilayer perceptrons were applied to discriminate possible ore deposits from barren sites. Subsequently, because artificial neural networks can be used to construct a revised model for knowledge extraction, the optimal brain damage algorithm by LeCun was applied to order the 25 attributes by their relevance to the classification. The approach demonstrates how neural networks can be used efficiently in a practical problem of mineral exploration, where general domain knowledge alone is insufficient to satisfactorily model the potential controls on deposit formation using the available information in continent-scale information systems.     

Subgrain-scale decoupling of Re and Os and assessment of laser ablation ICP-MS spot dating in molybdenite

Reproducibility of Re-Os molybdenite ages depends on sample size and homogeneity, suggesting that Re and Os are decoupled within individual molybdenite crystals and do not remain spatially linked over time. In order to investigate the Re-Os systematics of molybdenite at the subgrain (micron) scale, we report LA-ICP-MS Re-Os ages for an Archean molybdenite crystal from Aittojärvi, Finland, analyzed in situ in a white aplite matrix. A related Aittojärvi molybdenite (A996D), in the form of a very fine-grained mineral separate, is used as one of our in-house NTIMS standards, and thus its age of 2760 ± 9 Ma is well established. Measurements of (¹⁸⁷Re + ¹⁸⁷Os)/¹⁸⁵Re on micron scale spots along 200 μm traverses across the crystal yield a wide range of ages demonstrating that, in this case, microsampling of molybdenite does not produce geologically meaningful ages. Experimentation with mineral separations and sample size over a 7-yr period predicted that this would be the outcome. We suggest that ¹⁸⁷Os is more likely to be the mobile species, based on its charge and ionic radius, and that ¹⁸⁷Os becomes decoupled from parent ¹⁸⁷Re with time on the micron and larger scale. Incompatible charge and ionic radius for Os ions formed during reduction of molybdenite-forming fluids may explain the widely observed absence of common (initial) Os in molybdenite. Geologically accurate ages for molybdenite can only be obtained for fully homogenized crystals (or crystal aggregates) so that any post-crystallization ¹⁸⁷Re-¹⁸⁷Os decoupling is overcome.A growing number of geologically accurate ID-NTIMS ¹⁸⁷Re-¹⁸⁷Os ages for homogenized molybdenite suggest that postcrystallization mobility of radiogenic ¹⁸⁷Os must be limited to within the molybdenite mineral phase. We suggest that radiogenic ¹⁸⁷Os may be stored in micron scale dislocations, kink bands, and delamination cracks produced by deformation, and that the unusual structure and deformation response of molybdenite results in an increased chemical stability in this mineral. Migration of ¹⁸⁷Os into adjacent silicate phases is highly unlikely, but other contacting sulfides may take in Os. In an example from a Proterozoic skarn deposit at Pitkäranta (western Russia), we demonstrate minor loss of radiogenic ¹⁸⁷Os from molybdenite and a corresponding gain in adjacent chalcopyrite such that the molybdenite age is not perceptibly disturbed, whereas the resulting chalcopyrite ages are impossibly old. Therefore, it is unadvisable to perform Re-Os analytical work on any sulfide in contact or intimate association with molybdenite. In addition to large errors in the age, if the isochron method is employed, initial ¹⁸⁷Os/¹⁸⁸Os ratios could be erroneously high, leading to seriously errant genetic interpretations.     

Finite-element modelling of folding and faulting

The paper is concerned with the finite-element modelling of geologic phenomena. More specifically, the paper presents numerical models of folding and sait doming. A (penalty-) finite-element model based on the equations governing a viscous incompressible fluid is used to simulate the folding is used to simulate the folding of a viscous layer in a less viscous matrix and the salt dome formation, and to predict the location of active faulting and deformation in prograding delta systems on continentalmargins. The initiation and development of drape folding and associated faulting are also modelled. The United States continental margin of the Gulf of Mexico is used as an example of prograding delta system. The finite-element predictions seem to agreewith the published results.     

The magnetic effects of brecciation and shock in meteorites: I. The Ll-chondrites

The complex brecciation and shock history of amphoterite (LL-) chondrites is well reflected in their diverse natural magnetic remanence (NRM) behavior: Most LL-chondrites have a multicomponent, undemagnetizable NRM, analogous to that of lunar breccias. Only one meteorite among those studied, namely Dhurmsala (LL6), meets the criteria of NRM stability and directional coherence with progressive AF cleaning, indicative of a useful paleoremanence. Ancient field paleointensity determinations for Dhurmsala (LL6) of 0.03 and 0.1 Oe, agree well with our earlier estimates of 0.01 and 0.08 Oe for the LL6 Jelica and Vavilovka, respectively. In light of their petrographic structure, cooling rates, radiometric ages and shock indicators, it appears likely that the NRM may have been thermally imprinted, during cooling following shock-metamorphism. The closely similar saturation remanence (IRM_s) behavior for LL-chondrites, in contrast to the intragroup scatter in NRM characteristics, implies that - although formed by similar process from the same starting material, - the LL-chondrites have suffered widely different degrees of shock/metamorphic reheating.     

Holocene paleoenvironmental implications of diatom and organic carbon records from the southeastern Kara Sea (Siberian Margin)

Diatom assemblages and organic carbon records from two sediment cores located within an estuarian bay of the inner Kara Sea trace changes in Yenisei River runoff and postglacial depositional environments. Paleosalinity and sea-ice reconstructions are based on modern relationships of local diatom assemblages and summer surface-water salinity. Approximately 15,500 cal yr B.P., rivers and bogs characterized the study area. When sea level reached the 38- to 40-m paleo-isobath approximately 9300 cal yr B.P., the coring site was flooded. From 9300–9100 cal yr B.P., estuarine conditions occurred proximal to the depocenter of fluvially derived material, and salinity was <7–8. Paleosalinity increased to 11–13 by 7500 cal yr B.P., following postglacial sea-level rise and the southward shift of the Siberian coast. Sharp decreases in diatom accumulation rates, total sediment, and organic carbon also occurred, suggesting the presence of brackish conditions and greater distance between the coast and study site. Maximum paleosalinity (up to 13) was recorded between 7500 and 6000 cal yr B.P., which was likely caused by the enhanced penetration of Atlantic waters to the Kara Sea. Stepwise decreases to modern salinity levels happened over the last 6000 cal yr.     

The Vendian record of Sr and C isotopic variations in seawater: Implications for tectonics and paleoclimate

New Sr and C isotopic data, both obtained on the same samples of marine carbonates, provide a relatively detailed record of isotopic variation in seawater through the latest Proterozoic and allow, for the first time, direct correlation of these isotopic changes in the Vendian ( 540–610 Ma). The strong isotope variations determined in this study record significant environmental and tectonic changes. Together with a fairly poorly constrained Nd isotopic record, the Sr and C isotopic records can be used to constrain rates of erosion, hydrothermal alteration and organic C burial. Further, comparison of these records with those of the Cenozoic permit investigation of the general relationship between global tectonics and continental glaciation. In particular, results of this study show a very large change in the ⁸⁷Sr/⁸⁶Sr of marine carbonates from low pre-Vendian ( > 610 Ma) values ( 0.7066) to high Middle Cambrian values ( 0.7090). This change is greater in magnitude than the significant increase in seawater ⁸⁷Sr/⁸⁶Sr through the Cenozoic. Both changes are attributed to high erosion rates associated with continent-continent collisions (Pan-African and Himalayan-Tibetan). In the latest Proterozoic these high erosion rates, probably coupled with high organic productivity and anoxic bottom-water conditions, contributed to a significant increase in the burial rate of organic C. Ice ages mark both the Neoproterozoic and Cenozoic, but different stratigraphic relationships between the Sr isotopic increase and continental glaciation indicate that uplift-driven models proposed to explain Cenozoic climatic change cannot account for the latest Proterozoic ice ages.     

REE in the Great Whale River estuary, northwest Quebec

We report rare earth element (REE) concentrations of a longitudinal profile within the estuary of the Great Whale River in northwest Quebec and in Hudson Bay. All of the measured REE have concentrations less than those predicted by conservative mixing of seawater and river water, demonstrating removal of the REE from solution. REE removal is rapid, occurring primarily at salinities less than 2‰. Removal of the REE is greatest for the light REE, and ranges from about 7% for the light REE to no more than 40% for the heavy REE. Fe removal is essentially complete at low salinity. The shape of the Fe and REE vs. salinity profiles is not consistent with a simple model of destabilization and coagulation of iron and REE-bearing colloidal material. A linear relationship between the activity of free ion REE³⁺ and pH is consistent with a simple ion-exchange model for REE removal.Surface and subsurface samples of Hudson Bay seawater are characterized by high REE concentrations and high La/Yb relative to average seawater. The subsurface sample has a Nd concentration of 100 pmol/kg and an ε_Nd of −29.3 ± 0.3. These characteristics are consistent with the high REE concentration, high La/Yb, and low ε_Nd of river inputs into Hudson Bay. These results indicate that rivers draining the Canadian Shield are a major source of non-radiogenic Nd and REE to the Atlantic Ocean. We estimate that outflow of water from Hudson Bay to the Labrador Sea could supply ≈ 30% of the non-radiogenic Nd in North Atlantic Deep Water.     

Authigenic zeolites and their relation to silica diagenesis in ODP Site 661 sediments (Leg 108, Eastern Equatorial Atlantic)

At ODP Site 661 from the continental margin of Northwest Africa, authigenic clinoptilolites were investigated in detail. Based on chemical data, crystal size and shape and their intergrowth with associated mineral phases, two types of clinoptilolite (type A and B) occur representing different diagenetic conditions under which the formation of zeolites took place. Clinoptilolite type A was found in a sediment section comprises numerous hiatuses and stratigraphically condensed sediments and could have been formed during early diagenesis. In contrast, clinoptilolite type B was precipitated in Campanian sediments during later diagenesis, after the transformation from opal-A to opal-CT had taken place. At Site 661, the silica source for the authigenesis of both clinoptilolite types is biogenic opal. Volcanic ash material occurs, but in low amounts and thus is not a prerequisite for the formation of clinoptilolite at Site 661.

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Zusammenfassung In der ODP-Bohrung 661 (Leg 108) vom Kontinentalrand vor Nordwest-Afrika wurden authigene Klinoptilolith-Kristalle mit Hilfe der Rasterelektronenmikroskopie, Röntgendiffraktometrie und Mikrosonde im Detail untersucht. Anhand der Kristallgrößen und -formen, ihren Verwachsungen mit anderen Mineralphasen und ihren chemischen Zusammensetzungen wurden 2 Klinoptilolithtypen (Typ A und B) unterschieden. Typ A wurde in stratigraphisch kondensierten Sedimenten nachgewiesen und könnte im Zuge von frühdiagenetischen Porenwasserreaktionen entstanden sein. Klinoptilolithtyp B, welcher vorwiegend in den campanen Sedimenten der ODP-Bohrung 661 vorkommt, wurde während der späteren Silikatdiagenese nach der Transformation von Opal-A in Opal-CT gebildet. Als Silikatquelle für die Kristallisation beider Klinoptilolithtypen wurden überwiegend biogene Opalskelette benutzt. Vulkanisches Aschenmaterial, welches ebenfalls ein Potential für die Klinoptilolithentstehung bildet, wurde nur in einem Horizont nachgewiesen und hatte keinen wesentlichen Einfluß auf die Klinoptilolithentstehung.

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Résumé Des cristaux de clinoptilolite authigène, provenant du site ODP 661 (marge continentale nord-ouest-africaine), ont fait l'objet d'une étude détaillée par microscopie électronique, diffractométrie X et microsonde. Les données chimiques, la taille et la forme des cristaux ainsi que leur intercroissance avec d'autres phases minérales permettent de distinguer 2 types (A et B) de clinoptinolite, qui correspondent aux conditions diagénétiques différentes dans lesquelles les zéolites se sont formées. La clinoptilolite de type A a été rencontrée dans une succession de sédiments stratigraphiquement condensés; elle pourrait s'être formée au début de la diagenèse. La clinoptilolite de type B, par contre, rencontrée surtout dans les sédiments campaniens a précipité au cours des stades tardifs de la diagenèse, après la transformation de l'opale A en opale CT. Au site 661, la source de silice nécessaire à la formation authigène des deux types de clinoptilolite est constituée par l'opale biogénique. Les cendres volcaniques qui pourraient éventuellement être à l'origine de la formation de clinoptilolite, n'ont été rencontrées que dans un seul horizon et n'ont joué aucun rôle en l'occurence.

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Some assessments of the potential 2 × CO2 climatic effects on water balance components in the eastern mediterranean

General circulation model (GCM) coarse evaluations of the climatological impact in the Eastern Mediterranean due to global doubling of the atmospheric CO₂ concentration were used as input for apreliminary estimation of modifications in local processes affecting the water balance in this region. It is suggested that: (i) in the 2 × CO₂ climate theaverage regional change of precipitation associated with typical mid-winter cyclonic systems is relatively small, however, it is associated with redistribution of the regional rainfall; (ii) in the elevated terrain in the northern part of the region, daytime snowmelt due to warm air advection may be enhanced, as much as 2.8 cm per day; and (iii) transpiration in the coastal area of the Eastern Mediterranean may increase by ~ 13% of its current level in the summer and somewhat more in the winter.     

Oxygen isotope compositions of selected laramide-tertiary granitoid stocks in the Colorado Mineral Belt and their bearing on the origin of climax-type granite-molybdenum systems

Quartz phenocrysts from 31 granitoid stocks in the Colorado Mineral Belt yield ¹⁸O values less than 10.4, with most values between 9.3 and 10.4. An average magmatic value of about 8.5 is suggested. The stocks resemble A-type granites; these data support magma genesis by partial melting of previously depleted, fluorine-enriched, lower crustal granulites, followed by extreme differentiation and volatile evolution in the upper crust.Subsolidus interaction of isotopically light water with stocks has reduced most feldspar and whole rock ¹⁸O values. Unaltered samples from Climax-type molybdenumbearing granites, however, show no greater isotopic disturbance than samples from unmineralized stocks. Although meteoric water certainly played a role in post-mineralization alteration, particularly in feldspars, it is not required during high-temperature mineralization processes. We suggest that slightly low ¹⁸O values in some vein and replacement minerals associated with molybdenum mineralization may have resulted from equilibration with isotopically light magmatic water and/or heavy isotope depletion of the ore fluid by precipitation of earlier phases.Accumulation of sufficient quantities of isotopically light magmatic water to produce measured depletions of ¹⁸O requires extreme chemical stratification in a large magma reservoir. Upward migration of a highly fractionated, volatile-rich magma into a small apical Climax-type diapir, including large scale transport of silica, alkalis, molybdenum, and other vapor soluble elements, may occur with depression of the solidus temperature and reduction of magma viscosity by fluorine. Climax-type granites may provide examples of ¹⁸O depletion in magmatic systems without meteoric water influx.