Geochemical Characteristics of Granitoids of the Erdenet Porphyry Copper Deposit, Mongolia

Abstract. The Erdenet porphyry copper deposit is one of the major mineral deposits in Mongolia. The geochemical data of granitoids acquired for the Erdenet and its surrounding areas are re-examined. The granitoids of the Erdenet deposit with hypogene type mineralization show the lowest TiO₂ content. Although Ti was possibly lost through the pyritization, it is also possible that the hypogene type mineralization occurred accompanied with the most differentiated granitoids. The variation of the element contents related to the mineralization of the Erdenet deposit shows the decrease of MgO and CaO contents, rather constant K₂O content, rather constant to decrease of Na₂O content, with respect to the Cu contents. The rather constant Na₂O in the mineralized zone is owing to the residual albite against the sericitic alteration. The granitoids of the Erdenet area show an increase of Na₂O content and a decrease of K₂O content with an increase of SiO₂ content. This trend makes clear contrast to granitoids in the surrounding areas. The granitoids of the Erdenet area might have the adakitic nature based on the Sr and Y contents.     

Relationship between atmospheric conditions at Dhaka, Bangladesh, and rainfall at Cherrapunjee, India

To improve flood forecasting, the understanding of the atmospheric conditions associated with severe rainfall is crucial. We analysed the atmospheric conditions at Dhaka, Bangladesh, using upper-air soundings. We then compared these conditions with daily rainfall variations at Cherrapunjee, India, which is a main source of floodwater to Bangladesh, and a representative sample of exceptionally heavy rainfall events. The analysis focussed on June and July 2004. June and July are the heaviest rainfall months of the year at Cherrapunjee. July 2004 had the fourth-heaviest monthly rainfall of the past 31 years, and severe floods occurred in Bangladesh. Active rainfall periods at Cherrapunjee corresponded to “breaks” in the Indian monsoon. The monsoon trough was located over the Himalayan foothills, and strong westerly winds dominated up to 7 km at Dhaka. Near-surface wind below 1 km had southerly components, and the wind profile had an Ekman spiral structure. The results suggest that rainfall at Cherrapunjee strongly depends on the near-surface wind speed and wind direction at Dhaka. Lifting of the near-surface southerly airflow by the Meghalaya Plateau is considered to be the main contributor to severe rainfall at Cherrapunjee. High convective available potential energy (CAPE) also contributes to intense rainfall.     

Initial behavior of granite in response to injection of CO2-saturated fluid

To understand the initial reactions of granite in a CO₂-saturated hydrothermal system, experiments were conducted using a batch-type autoclave over a temperature range of 100–350 °C at up to 250 bar and numerical computations of phase equilibria based on the experimental results were carried out. The experiments showed that the dissolution of granite and the deposition of secondary minerals were encouraged by the addition of CO₂. Solution chemistry and examination of the granite’s surface texture suggested that its initial dissolution is characterized by the release of Na and Ca (from the dissolution of plagioclase) and that initial precipitation occurs by deposition of some secondary minerals on to plagioclase and/or biotite in the CO₂-saturated system. However, the effect of CO₂ was small at 350 °C owing to the low activity of H₂CO₃. According to EDX analysis and numerical phase equilibrium calculations, the secondary minerals formed might be kaolinite, muscovite, smectite and calcite. That is, the granite as a whole might have the potential to take-up dissolved CO₂. The results suggest that the alteration of granite under CO₂-saturated hydrothermal conditions has the potential to capture CO₂ when it is injected at moderate temperatures (150–250 °C) into granite-hosted rock masses.     

Stable Isotopic Information on Calcareous Pelitic Rocks in the Tizapa Volcanogenic Massive Sulfide Deposit Area, the United Mexican States

Abstract: Tizapa volcanogenic massive sulfide (VMS) deposit is hosted in greenschist facies metamorphic rocks; footwall is green schist of felsic to mafic metavolcanic rocks and hanging wall is graphite schist of metasedmentary pelitic rock. Pb-Pb dating of ore samples indicates 103. 4Ma to 156. 3Ma for the age of mineralization (JICA/MMAJ, 1991).     

Rainfall on the Meghalaya plateau in northeastern India—one of the rainiest places in the world

Monthly and daily variations in rainfall over Cherapunjee and Mawsynram on the Meghalaya plateau of northeastern India are analysed. Cherapunjee and Mawsynram are well known as two of the places with the heaviest rainfall in the world. The daily rainfall variation is attributed to the influence of synoptic scale disturbances, with a periodicity of 10–20 days, and the orographic interaction. The annual and monthly highest rainfalls over Cherapunjee during the 31 years from 1973 to 2003 were much larger than mean values.     

Fine S wave velocity structure beneath Iwate volcano, northeastern Japan, as derived from receiver functions and travel times

A genetic algorithm inversion of receiver functions derived from a dense seismic network around Iwate volcano, northeastern Japan, provides the fine S wave velocity structure of the crust and uppermost mantle. Since receiver functions are insensitive to an absolute velocity, travel times of P and S waves propagating vertically from earthquakes in the subducting slab beneath the volcano are involved in the inversion. The distribution of velocity perturbations in relation to the hypocenters of the low-frequency (LF) earthquakes helps our understanding of deep magmatism beneath Iwate volcano. A high-velocity region (dV_S/V_S=10%) exists around the volcano at depths of 2–15 km, with the bottom depth decreasing to 11 km beneath the volcano’s summit. Just beneath the thinning high-velocity region, a low-velocity region (dV_S/V_S=−10%) exists at depths of 11–20 km. Intermediate-depth LF (ILF) events are distributed vertically in the high-velocity region down to the top of the low-velocity region. This distribution suggests that a magma reservoir situated in the low-velocity region supplies magma to a narrow conduit that is detectable by the hypocenters of LF earthquakes. Another broad low-velocity region (dV_S/V_S=−5 to −10%) occurs at depths of 17–35 km. Additional clusters of deep LF (DLF) events exist at depths of 32–37 km in the broad low-velocity zone. The DLF and ILF events are the manifestations of magma movement near the Moho discontinuity and in the conduit just beneath the volcano, respectively.