The Hokko prospect is located in the Minamikayabe area southwestern Hokkaido, Japan, where gold-bearing quartz veins of Pliocene
age are exposed at the surface. The alteration mineral assemblage is typical of low-sulfidation epithermal systems, with the
quartz veins associated with adularia alteration overprinted on Late Miocene propylitic alteration. Fluid inclusion studies
of the vein quartz reveal mean homogenization temperatures of approximately 220 °C, and the co-existence of low-salinity (<2
wt.% NaCl equivalent) and moderate salinity (2 to 12 wt.% NaCl equivalent) fluid inclusions within the same veins. The moderate
salinity fluid inclusions (2–12 wt.% NaCl equivalent) typically have relatively low homogenization temperatures between 150°
to 200 °C. The results obtained from stable isotope analysis of δ18O in quartz vein material showed a gradual decrease in δ18O signatures with increasing depth. The majority of the samples have calculated fluid source signatures (δ18OH2O) between −8.0 and −10.0‰, but there is a significant change in the composition above 185 m drill depth. The shallower samples
in particular show a wide range of oxygen isotope signatures that are associated with the moderate salinity fluid inclusions.
We interpret that low-salinity inclusions within the Hokko system represent the composition of the liquid phase of the fluid,
before boiling, and that the moderate-salinity inclusions are representative of the residual liquid phase, after extensive
non-adiabatic boiling and vapor loss in an open system. This mechanism resulted in the entrapment of fluids with variable
salinities at the same time, and in close proximity to each other. This is also reflected in the δ18OH2O values which become more variable and heavier where the moderate-salinity inclusions occur. Deposition of ore minerals within
the Hokko vein system also occurred at this time as a result of boiling and gas loss.
Received: 30 May 1997 / Accepted: 6 January 1998 相似文献
Seven rare-earth elements (La, Ce, Sm, Eu, Tb, Yb, Lu) and Co, Cr, Sc, Ba, Hf and Th have been determined by non-destructive neutron activation analysis on the Quaternary volcanic rocks in Hokkaido, Japan. The trace-element abundances are discussed in terms of the petrological problems, particularly the origin of calc-alkali magma. On the basis of the La/Sm ratio and the contents of K, Ba, Th and La, lateral variations in the contents of trace elements exist across the Kurile and the northern Honshū arcs. The calc-alkali rocks can be classified into three types which correspond to Kuno's three basalt-magma types. There is no essential difference in the rare-earth patterns between the basaltic rocks and the associated calc-alkali rocks in each petrographic province. This suggests that the calcalkali rocks may be derived from the basaltic magmas by fractional crystallization under certain conditions. 相似文献
Freezing was used in the jointing section of two shields (diameter 3.14 m) which ran across at right angles and at 25-m depth under the crossing of main roads for the construction of a 2.4-m diameter sewer tunnel. The object of freezing was to create a frozen barrier to prevent the inflow of water and fine sand into the area to be excavated.
Thirty-five freeze pipes (length about 7 m) were placed from one shield conically in the direction of the flank of another shield with freeze pipes attached to its inside wall. The cooling unit was placed on unoccupied land at about 60 m distant from the jointing section and the freeze pipes were connected with the cooling unit by supply and return pipelines.
The cooling unit had a cooling capacity of 79,000 kcal/h at an evaporation temperature of −27°C and a condensation temperature of +40°C. Freezing was continued for 180 days and the CaCl2 brine temperature was −25 to −30°C. These works were accomplished without the sinking of shaft and the use of injection, in complete safety with no traffic restriction on the road surface. 相似文献
Near-liquidus phase relations in one-atmosphere dry and water-saturated high pressure conditions were experimentally determined on products of three historic andesitic eruptions. Run conditions ranged from 900 to 1100°C, at pressures up to 1500 bars with fO2 controlled close to the nickel-nickel oxide (NNO) buffer. In order to represent the compositions of the true liquid parts of the erupting magmas, groundmass portions were mechanically separated from the porphyritic andesites. Such groundmass materials should lie exactly on the liquidus field boundary between the phases precipitating from the magma just prior to eruption under the prevailing P-T condition.All the samples showed a crossing of the plagioclase and orthopyroxene liquidi in the pressure-temperature range from 1 to 800 bar and 950–1090°C. The crossing condition approximates the magmatic condition immediately prior to eruption. In the case of the 1970 eruption of Akita-komagatake, the crossing point is at 150 bar and 1090°C, matching closely the observed explosive gas pressure and the temperature. In both cases of the 1783 eruption of Asama and the 1914–1915 eruption of Sakurajima volcanoes, the crossing point shift from higher water pressure and lower temperature for the earlier erupting magmas to lower pressure and higher temperature for the later magmas. This regularity may be explained by a vertical gradient of the temperature and water content within the magma column prior to eruption. 相似文献
An earthquake swarm, and the major pumice eruptions in August 1977 which followed, marked the start of the dacitic doming activity of Usu volcano in southwestern Hokkaido, Japan. The sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data. The distribution of the abundant hypocenters shows clearly an earthquake-free zone beneath the summit crater. The hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations, considered due to magma intrusion into this earthquake-free zone. The earthquake mechanism solutions are mostly of dip-slip type and are interpreted in terms of the doming deformations. The existence of earthquake families (earthquakes with similar waveforms) is the main cause of the peculiar occurrence of earthquakes in space, time and magnitude. The concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm, and the observed episodic deformations. 相似文献
The relationship between island-induced cyclonic eddies and chlorophyll a (chl-a) was investigated using field data and satellite images in the eastern channel of the Tsushima Straits. The maximum chl-a concentration around the leeward eddy of the Tsushima Islands was two or three times greater than that of outside the eddy. Two different mechanisms of chl-a enhancement associated with island-induced cyclonic eddies were found in the post-bloom periods. In summer, when nutrients were depleted in the surface layer, eddy pumping increased the nutrient supply in the euphotic zone, resulting in enhanced chl-a around the shallow thermocline near the eddy core. In late autumn, when the mixed layer deepened over the euphotic zone, the mixed layer depth became shallow due to the doming effect of the cyclonic eddy, therefore, the improved irradiance condition led to an increase in the chl-a concentration in the surface mixed layer. Nighttime satellite visible images showed a number of fishing vessels in the lee region of the Tsushima Islands, implying that the enhanced phytoplankton biomass may have resulted in good feeding conditions for fishes and squids in the Tsushima Straits. 相似文献
Deep-water corals form unique ecosystems, yet very little is known about factors that regulate their distribution and growth. The abundance and size of two deep-water gorgonian coral species, Paragorgia arborea and Primnoa resedaeformis, and their relationship with depth and substratum cover, were investigated at Northeast Channel, off Nova Scotia, in July 2006. This is the first study to measure abundance and size of these two coral species at depths >500 m in the Canadian Atlantic region. A total of 5 transects between 500 and 1000 m depth were examined using video collected by the remotely operated vehicle ROPOS. Abundance of both species was patchy, but higher at these deeper depths than at <500 m. Abundance generally declined with depth, and was moderately correlated with cover of hard substratum (cobble, boulder, bedrock). These relationships were stronger and less variable for P. resedaeformis than for P. arborea, suggesting that factors such as topographic relief may play an additional role in regulating distributions of P. arborea. Maximum colony height was 125 and 240 cm for P. resedaeformis and P. arborea, respectively, and much greater than recorded for depths <500 m. Overall, colony height and depth relationships were strong for both species, but variable among transects. P. resedaeformis showed a negative relationship with depth, while the opposite was observed for P. arborea, suggesting that the two species are affected differently by factors that vary with depth (e.g. temperature, fishing disturbance). Relationships between colony size and size of attachment stone were stronger for P. arborea, especially for overturned colonies, than for P. resedaeformis, suggesting that availability of suitably coarse substrate may be more important for the long-term persistence of P. arborea colonies. 相似文献