Oxygen and Hydrogen Isotopes of Waters in the Ordos Basin,China:Implications for Recharge of Groundwater in the North of Cretaceous Groundwater Basin

Hundreds of precipitation samples collected from meteorological stations in the Ordos Basin from January 1988 to December 2005 were used to set up a local meteoric water line and to calculate weighted average isotopic compositions of modern precipitation. Oxygen and hydrogen isotopes, with averages of-7.8‰ and -53.0‰ for δ^18O and δD, respectively, are depleted in winter and rich in spring, and gradually decrease in summer and fall, illustrating that the seasonal effect is considerable. They also show that the isotopic difference between south portion and north portion of the Ordos Basin are not obvious, and the isotope in the middle portion is normally depleted. The isotope compositions of 32 samples collected from shallow groundwater （less than a depth of 150 m） in desert plateau range from -10.6‰ to -6.0‰ with an average of-8.4‰ for δ^18O and from -85‰ to -46‰ with an average of-63‰ for δD. Most of them are identical with modern precipitation. The isotope compositions of 22 middle and deep groundwaters （greater than a depth of 275 m） fall in ranges from -11.6‰ to -8.8‰ with an average of -10.2‰ for δ^18O and from -89‰ to -63‰ with an average of -76‰ for δD. The average values are significantly less than those of modern precipitation, illustrating that the middle and deep groundwaters were recharged at comparatively lower air temperatures. Primary analysis of ^14C shows that the recharge of the middle and deep groundwaters started at late Pleistocene. The isotopes of 13 lake water samples collected from eight lakes define a local evaporation trend, with a relatively flat slope of 3.77, and show that the lake waters were mainly fed by modern precipitation and shallow groundwater.

Oxygen and hydrogen isotope ratios of chert from the Sixtymile Formation in Grand Canyon National Park,USA：a warm palaeoclimate,freshwater deposit

New oxygen and hydrogen isotope ratios of chert from middle, intraformational breccias, and upper breccia members of the Sixtymile Formation（SMF） in eastern Grand Canyon National Park（AZ） yield palaeoclimate estimates between 27 and 33℃. The isotopic compositions of cherts define a domain approximately parallel to the meteoric water line when plotted on a δD–δ-（18）O diagram; these data indicate that meteoric water was involved during formation of the chert. In thin section, the absence of interlocking mega quartz（〉35 lm） and silicafilled fractures and veins, along with preserved micromorphological silica fabrics, suggest that the chert has not been permeated by later hydrothermal fluids. Petrographic observations in thin section such as cyclic silica precipitation phases and glaebular micromorphologic fabrics lend support to the interpretation that meteoric waters were involved during chert precipitation. The post 742 Ma SMF has been correlated with diamictite（transition） beds of the Kingston Peak Formation（CA）, which in turn have been interpreted to have been deposited during the Sturtian Ice Age（-750–700 Ma）. Absence of facetted and striated clasts and other diagnostic glaciogenic features in the SMF,an unconformable contact with the stratigraphically older Chuar Group, coupled with warm palaeotemperature data inferred from stable isotope values of chert, tentatively suggest that deposition of sediment in the SMF likely did not take place during the Sturtian Ice Age.

Genesis of the Ciemas Gold Deposit and Relationship with Epithermal Deposits in West Java,Indonesia:Constraints from Fluid Inclusions and Stable Isotopes

The Ciemas gold deposit is located in West Java of Indonesia, which is a Cenozoic magmatism belt resulting from the Indo-Australian plate subducting under the Eurasian plate. Two different volcanic rock belts and associated epithermal deposits are distributed in West Java： the younger late Miocene-Pliocene magmatic belt generated the Pliocene-Pleistocene epithermal deposits, while the older late Eocene-early Miocene magmatic belt generated the Miocene epithermal deposits. To constrain the physico-chemical conditions and the origin of the ore fluid in Ciemas, a detailed study of ore petrography, fluid inclusions, laser Raman spectroscopy, oxygen-hydrogen isotopes for quartz was conducted. The results show that hydrothermal pyrite and quartz are widespread, hydrothermal alteration is well developed, and that leaching structures such as vuggy rocks and extension structures such as comb quartz are common. Fluid inclusions in quartz are mainly liquid-rich two phase inclusions, with fluid compositions in the NaCI-H2O fluid system, and contain no or little CO2. Their homogenization temperatures cluster around 240℃-320℃, the salinities lie in the range of 14-17 wt.% NaCI equiv, and the calculated fluid densities are 0.65-1.00 g/cm^3. The values of δ18OH2O-VSMOW for quartz range from ＋5.5%0 to ＋7.7‰, the δDVSMOW of fluid inclusions in quartz ranges from -70‰ to -115‰. All of these data indicate that mixing of magmatic fluid with meteoric water resulted in the formation of the Ciemas deposit. A comparison among gold deposits of West Java suggests that Miocene epithermal ore deposits in the southernmost part of West Java were more affected by magmatic fluids and exhibit a higher degree of sulfidation than those of Pliocene-Pleistocene.

Geological and Geochemical Characteristics of the Zhulazhaga Gold Deposit in Inner Mongolia, China

Located in Alxa Zuoqi (Left Banner) of Inner Mongolia, China, the Zhulazhaga gold deposit is the first large- scale gold deposit that was found in the middle-upper Proterozoic strata along the north margin of the North China craton in recent years. It was discovered by the No. 1 Geophysical and Geochemical Exploration Party of Inner Mongolia as a result of prospecting a geochemical anomaly. By now, over 50 tonnes of gold has been defined, with an average Au grade of 4 g/ t. The ore bodies occur in the first lithological unit of the Mesoproterozoic Zhulazhagamaodao Formation (MZF), which is composed mainly of epimetamorphic sandstone and siltstone and partly of volcanic rocks. With high concentration of gold, the first lithological unit of the MZF became the source bed for the late-stage ore formation. Controlled by the interstratal fracture zones, the ore bodies mostly appear along the bedding with occurrence similar to that of the strata. The primitive ore types are predominantly the altered rock type with minor ore belonging to the quartz veins type. There are also some oxidized ore near the surface. The metallic minerals are composed mainly of pyrite, pyrrhotite and arsenopyrite with minor chalcopyrite, galena and limonite. Most gold minerals appear as native gold and electrum. Hydrothermal alterations associated with the ore formation are actinolitization, silicatization, sulfidation and carbonation. A total of 100 two-phase H 2 O-rich and 7 three-phase daughter crystal-bearing inclusions were measured in seven gold- bearing quartz samples from the Zhulazhaga gold deposit. The homogenization temperatures of the two-phase H 2 O-rich inclusions range from 155 to 401°C, with an average temperature of 284°C and bimodal distributions from 240 to 260°C and 300 to 320°C respectively. The salinities of the two-phase H 2 O-rich inclusions vary from 9.22wt% to 24.30wt% NaCl equiv, with a mode between 23 wt% and 24wt% NaCl equiv. Comparatively, the homogenization temperatures of the three- phase daughter crystal-bearing inclusions vary from 210 to 435°C and the salinities from 29.13wt% to 32.62wt% NaCl equiv. It indicates that the ore-forming fluid is meso-hypothermal and characterized by high salinity, which is apparently different from the metamorphic origin with low salinity. It suggests a magmatic origin of the gold-bearing fluid. The δ 18 O values of quartz from auriferous veins range from 11.9 to 16.3 per mil, and the calculated values in equilibrium with quartz vary from 1.06 to 9.60 per mil, which fall between the values of meteoric water and magmatic water. It reflects that the ore-forming fluid may be the product of mixing of meteoric water and magmatic water. Based on geological and geochemical studies of the Zhulazhaga gold deposit, it is supposed that the volcanism in the Mesoproterozoic might make gold pre-concentrate in the strata. The extensive and intensive Hercynian tectono-magmatic activity not only brought along a large number of ore-forming materials, but also made the gold from the strata rework. It can be concluded that the ore bodies were mainly formed in late hydrothermal reworking stage. Compared with typical gold deposits associated with epimetamorphic clastic rocks, the Zhulazhaga deposit has similar features in occurrence of ore bodies, ore-controlling structure, wall-rock alterations and mineral assemblages. Therefore, the Zhulazhaga gold deposit belongs to the epimetamorphic clastic rock type.

Isotopic and Chemical Characteristics of Lagoon Waters in Niigata Prefecture, Japan

In order to investigate the isotopic and chemical characteristics of lagoon waters in Niigata Prefecture in recent years, oxygen and hydrogen stable isotope ratios （i.e., 8180 and 6D）, the concentrations of DOC （dissolved organic carbon）, DO （dissolved oxygen） and pH, etc. in water samples of Sakata and Toyanogata were measured. Samples were generally taken monthly at the fixed sampling points from these lagoons. Consequently, the following matters have been mainly clarified： （1） ~D value of water samples in Sakata was generally larger than that in Toyanogata similarly to the case of ~180, though remarkable large difference among samples was not found; （2） the pH value of lagoon water samples is almost 6.5-8.5 （which is generally larger than that of river water）, and pH at the spot of SI （downstream point of Lower Lagoon （Shitakata）） is remarkably high （9.0-9.5）; （3） Lagoon water has the chemical characteristics contrasting to groundwater with a focus on river water from the viewpoint of pH （acidity or alkalinity） and DO. These matters can be closely related to the biological activity such as photosynthesis due to aquatic plant and phytoplankton and the activity of Crustacea plankton etc. in lagoon.

Seasonal patterns in water uptake for Medicago sativa grown along an elevation gradient with shallow groundwater table in Yanchi county of Ningxia,Northwest China

Lucerne （Medicago sativa L.） is a deep-rooted perennial leguminous forage with high evapo-transpiration rate exceeding the annual precipitation in semi-arid areas of Northwest China. Groundwater might be the potential water sources of lucerne in the area with shallow groundwater table. In this study, stable isotopic compositions of oxygen and hydrogen （6180 and 8D） of different water sources and xylem sap were analyzed to determine the seasonal （April, June, July and August） and topographic （three slope positions） variations in water sources for lucerne growing in Ningxia eastern semi-arid area adjacent to Mu Us Desert characterized by shallow groundwater table. IsoSource software was used to calculate the probable contribution of different water sources to the total plant water uptake. Stomatal conductance, stem water potential, carbon isotope discrimination （A13C） of whole plant were also determined for evaluating the water status of lucerne growing at different slope positions. The results showed that soil water content increased as the elevation decreased. Oxygen compositions of soil water in the 0-40 cm profile fluctuated considerably. Soil water 8180 values in deep profile （〉3.5 m） were similar to those in groundwater, implying the recharge of groundwater to this soil layer. Highest water utilization rate from deep soil profile （below 350 cm） was recorded for lucerne grown at the slope position 1 （groundwater table depth of 3.5-3.9 m） in April, June and July. The lucernes at slope position 2 （groundwater table depth of 5.8-6.4 m） and slope position 3 （groundwater table depth 7.1-8.3 m） mostly used water from deep soil layers （below 350 cm） during dry period, and turned to use water from superficial soil layer in wet period. Higher yield, A13C value of whole plant and stomatal conductance were observed for lucerne grown at the slope position 1 than those at other slope positions. These results indicated that groundwater is a significant water source for transpiration of lucerne grown in Ningxia semi-arid area with shallow groundwater table where lucerne grassland is suggested to be established so as to obtain better yield performance.