Iodine-rich waters involved in supergene enrichment of the Mantos de la Luna argentiferous copper deposit,Atacama Desert,Chile

Recent studies have suggested the involvement of highly saline deep formation waters that modified preexisting Cu assemblages to form atacamite during supergene oxidation of Cu deposits in the Atacama region. In this report, we document the occurrence of (Ag–I) inclusions hosted by supergene chalcocite from Mantos de la Luna, an argentiferous Upper Jurassic stratabound Cu deposit in the Coastal Range of northern Chile. The presence of this unusual mineral assemblage indicates that iodargyrite precipitated from reducing iodine-rich waters, suggesting that the fluids involved in supergene enrichment of Cu deposits in the Coastal Range were more complex than previously thought. This suggests the prevalence of hyperarid conditions during the latest stages of supergene enrichment of the Mantos de la Luna Cu deposit in the Atacama region, supporting the notion that supergene enrichment processes in hyperarid areas are dynamic in nature and do not exclusively require the presence of meteoric water.     

Hydrothermal circulation beneath Mount Pelée inferred by self potential surveying. Structural and tectonic implications

Self-potential (SP) surveys were made on Mount Pelée volcano (Martinique Island, French West Indies) in 1991 and 1992 in order to recognize its hydrothermal system, the associated groundwater channeling and the main superficial structures of the massif. Almost 70 km of profiles were carried out with an average sample spacing of 50 m. Measurements essentially reveal negative SP anomalies, down to −1700 mV, with high gradients (−1.83 mV/m) due to the infiltration of meteoric water into the massif. Rims of summit calderas Morne Macouba and Etang-Sec present sharp negative SP anomalies on the western, northern, and eastern flanks. Negative SP anomalies indicate no upward water flow beneath Mount Pelée summit. On the southwestern volcano flank, a 3.5×6 km horseshoe-shaped structure corresponding to a southwest flank collapse event, older than 25,000 years BP, is clearly identified by the SP mapping. High gradients border the inner southern rim from Morne Calebasse to St Pierre town and the Caribbean Sea. Along the northern rim of the horseshoe-shaped structure the negative SP anomalies give place to a positive SP anomaly, up to 200 mV, of SW–NE trend. This zone covers the area of two active hot springs (Sources Chaudes and Puits Chaud: 40–65°C). Marine magnetic surveys and bathymetry show that the horseshoe-shaped structure spreads into the Caribbean Sea up to about 10 km from the coast. Buried structural discontinuities are evidenced inside the flank collapse structure. The upper one deviates the groundwater flow coming from the summit toward the south flank where the flow finds an indentation to expand again downwards. This discontinuity is either an old hypothetical caldera rim partly destroyed by the collapse of the south–southwestern flank and covered by recent pyroclastic deposits, or more probably the trace of a bulge landslide. A circulation model of the hydrothermal waters is proposed. Rainfall (5–6 m/year) is partly drained inside the summital calderas and the flank collapse zone through pyroclastic flows down to an impermeable basement. There the groundwater constitutes perched aquifers at the contact of the bulge landslide, or of the hypothetical old caldera rim. Along the inner northern border of the flank collapse structure the phreatic water is reheated. Warm groundwater flows along the northern avalanche structure rim and discharges near the coast in ground and marine outcrops, of medium temperature. Finally, the main part of the meteoric water is channeled along the old caldera rim, or along the bulge landslide towards the south flank of Mount Pelée, where some gaps in the rim exist. There the groundwater finds again a subhorizontal gravitational circulation along Mount Pelée slopes into the Caribbean Sea.     

New data on the Lower Mesozoic basal series of the Traras mounts (Tlemcen,northwestern Algeria)

The lower Mesozoic of the Traras mount starts with the so-called Red Series, which lays uncomformably on the folded and granitized Paleozoic basement. This series is overlain by Middle Liassic limestones. On the basis of its lower and upper limits and the lack of dating evidence, former authors attribute a Permian to lower Liassic (Sinemurian) age for this series. Palynological results allow to range it in the upper Triassic probably Norian-Rhetian. The Red Series is studied from five sections that depict important thickness and facies variations from southwest to the northeast, inferring the irregularities of the Hercynian erosional surface. In the Central Traras, this series shows over 130-m-thick micaceous sandy conglomeratic units, deposited within alluvial fans, where the bedrock source corresponds to granite of Nedroma or its equivalent. In the southern and eastern Traras, where the series’ thickness respectively does not exceed 30 m and averages 40–80 m, besides alluvial deposits issued from hercynian basement erosion, it exhibits volcanic evidences interbedded by lacustrine deposits. Vertical and spatial distribution depicts synsedimentary infilling of narrow elongated rift basins, typically half-grabens, bounded on one side by a normal fault or a series of normal faults trending NNE-SSW to NW-SE, inherited from the hercynian event. The study of tectonostratigraphic units permitted to put forward geodynamics of the infilling basins.     

Regional debris flow susceptibility analysis based on principal component analysis and self-organizing map: a case study in Southwest China

A method was developed to analyze the susceptibilities of 541 regional basins affected by debris flows at the Wudongde Dam site in southwest China. Determining susceptibility requires information on source material quantity and occurrence frequency. However, the large number of debris flows can hinder the individual field investigation in a each small basin. Factors that may trigger debris flows can be identified using remotely sensed interpretation information. Susceptibility analysis can then be conducted based on these factors. In this study, SPOT5 satellite imagery, digital elevation models (DEM), a lithology distribution map, and rainfall monitoring data were used to identify 12 debris flow trigger factors: basin relief ratio, slope gradient in the initiation zone, drainage density, downslope curvature of the main channel, vegetation coverage, main channel aspect, topographic wetness index, Melton’s ruggedness number, lithology, annual rainfall, form factor, and cross-slope curvature of the transportation zone. Principal component analysis was used to obtain the eight principal components of these factors that contribute to susceptibility results. Then, a self-organizing map method was adopted to analyze the principal components, which resulted in a debris flow susceptibility classification. Field validation of 26 debris flow basins was used to evaluate the errors of the susceptibility classification, as well as assess the causes of such errors. The study found that principle component analysis and self-organizing map methodologies are good predictors of basin susceptibility to debris flows.