Soil geochemistry of Mother Lode-type gold deposits in the Hodson mining district, central California, U.S.A.

The Hodson mining district is in the westernmost foothills of the Sierra Nevada in California, about 17 km west of the town of Angels Camp. This district is part of the West Gold Belt, which lies about 12–16 km west of, and generally parallel to, the better known Mother Lode Gold Belt in central California. The district produced several million dollars worth of Au between about 1890 and 1940.     

Fe-rich tholeiitic liquids and their cumulate products in the Pleasant Bay layered intrusion, coastal Maine

Fe-rich tholeiitic liquids are preserved as chilled pillows and as the chilled base of a 27 meter thick macrorhythmic layer in the Pleasant Bay mafic-silicic layered intrusion. The compositions of olivine (Fo₁) and plagioclase (An₁₃₋₈) in these extremely fine grained rocks suggest that they represent nearly end stage liquids that formed by fractionation of tholeiitic basalt. Their major element compositions (∼17.5 wt% FeO_T and 54 wt%SiO₂) closely resemble highly evolved glasses in the Loch Ba ring dike and some recent estimates of end-stage liquids related to the Skaergaard layered intrusion, and are consistent with recent experimental studies of tholeiite fractionation. Their trace element compositions are consistent with extensive earlier fractionation of plagioclase, olivine, clinopyroxene, ilmenite, magnetite and apatite. The mineral assemblage of the chilled rocks (olivine, clinopyroxene, quartz, ilmenite and magnetite), apatite saturation temperatures, and very low Fe³⁺/Fe²⁺indicate conditions of crystallization at temperatures of about 950 °C and f _{O 2} about two log units below FMQ. Cumulates that lie about 3 meters above the chilled base of the macrorhythmic layer contain cumulus plagioclase, olivine, clinopyroxene, ilmenite, apatite and zircon. This mineral assemblage and the Fe-Mg ratio in clinopyroxene cores suggest that this cumulate was in equilibrium with a liquid having a composition identical to that of the chilled margin which lies directly beneath it. The high FeO_T and low SiO₂ concentrations of this cumulate (23.3 and 45.8 wt%, respectively) are comparable to those in late stage cumulates of the Skaergaard and Kiglapait intrusions. This association of a chilled liquid and cumulate in the Pleasant Bay intrusion suggests that late stage liquids in tholeiitic layered intrusions may have been more SiO₂-rich than field-based models suggest and lends support to recent experimental studies of tholeiite fractionation at low f _O2 which indicate that saturation of an Fe-Ti oxide phase should cause FeO_T to decrease in the remaining liquid. Received: 17 January 1997 / Accepted: 10 June 1997     

Carbon,oxygen and sulphur isotope variations in concretions from the Upper Lias of N.E. England

Carbon, oxygen and sulphur isotope data for transects across two pyrite-bearmg carbonate concretions, and their host sediments, from the Upper Lias of N.E. England show symmetrical zonation. δ¹³C_PDB values of the calcite cement (?12.9 to ?15.4%.) indicate that most of it originated from organic matter by bacterial reduction of sulphate, augmented with marine and, to a lesser extent, fermentation derived carbonate. Organic carbon (δ¹³C_PDB = ?26.1 to ?37.0%.). reflects the admixture of allochtho-nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation.Two phases of pyrite are present in each concretion. The earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has δ³⁴S_CD values of ?22 to ?26%. indicating formation by open system sulphate reduction. The later euhedral phase is more abundant and reaches values of ? 2.5 to ? 5.5%. at concretion margins. This phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system. The δ¹³C and δ³⁴S data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface. The δ₁₈O values of the calcite in one concretion (δ¹⁸O_PDB = 2.3 to ?4.8%.) indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater. The other concretion is isotopically much lighter (δ¹⁸O_PDB?8.9 to ?9.9%.) and large δ¹⁸O differences between concretions in closely-spaced horizons imply that local factors control the isotopic composition of pore waters.     

Geologic sources and geographic distribution of sand tempers in prehistoric potsherds from the Mariana Islands

Temper sands in prehistoric potsherds of the Mariana Islands include terrigenous detritus derived from Paleogene volcanic bedrock and calcareous grains derived chiefly from modern fringing reefs, but also in part from uplifted Neogene limestones overlying volcanic bedrock. Calcareous sands are nondiagnostic of island of origin, but volcanic sands and the terrigenous component of hybrid sands composed of mixed terrigenous and calcareous grain types can be traced to geologic sources on Saipan and Guam, the only occupied islands where volcanic bedrock is extensively exposed. Quartzose tempers of several types were derived exclusively from dacitic volcanic rocks on Saipan. Nonquartzose tempers of andesitic parentage derive from both Saipan and Guam, but abundance of orthopyroxene as well as clinopyroxene is diagnostic of Saipan andesitic tempers, the presence of olivine is diagnostic of selected tempers from Guam, and placer temper sands rich in heavy ferromagnesian minerals occur only in sherds on Guam. Temper analysis documents widespread ceramic transfer from Saipan to other islands throughout Mariana prehistory, and more restricted ceramic transfer from Guam to nearby Rota, although the origin of some andesitic temper types is petrographically indeterminate. © 2001 John Wiley & Sons, Inc.     

Geochemical processes in the saltwater–freshwater transition zone: comparing results of a sand tank experiment with field data

Geochemical processes, occurring in a stable transition zone between saltwater and freshwater, were simulated in a 2D, multi-layer flow chamber experiment. Mixing, calcite dissolution, and oxidative degradation of organic matter were identified as the main controlling factors. The results of the chamber experiment were compared to field data and verified by thermodynamic modeling. Similarity in most ion distributions suggests the general applicability of the experimental method. Differences in the redox conditions between field and experiment were reflected by the oxidants involved in the mineralization of organic carbon; while field data show evidence of sulfate reduction, the presence of oxygen in the laboratory experiment resulted in the reoxidation of sulfides.     

La tectogenèse hercynienne dans la partie occidentale du bassin de l'Ahnet et la région de Bled El-Mass, Sahara Algérien: un continuum de déformation

The study area is located in the north of the Hoggar Basement. It is bounded to the east by the Idjerane-Azaz north-south-trending structural system, the Azzel-Matti Ridge to the south and to the north by the Timimoun Basin. Distribution of the structural elements and the determination of deviatoric stress tensors indicate two main directions for major stress (σ1), one striking northeast-southwest and the other west-northwest-east-southeast. Structural evolution during the Hercynian Orogeny can be considered as a continuous strain rather than as two contrasting tectonic events. The structural features can be attributed to the association of wrench faulting and thrusting, which is compatible with a strain regime of combined pure and simple shear. The basement fabric features exerted a very strong control on the structural evolution during the Hercynian deformation.     

Reduction of inorganic compounds in autoclaves by using suspended cellulose

Autoclaves are closed vessels in which liquids can be heated above their ‘normal' boiling points. As a consequence the pressure will rise. This equipment has permitted a variety of processes like the syntheses of organics and the leaching of metal ores. Pressure technologies can be used for accelerating reactions, for specific syntheses or they offer an environmental or economical alternative for existing processes. Wet pressure oxidation of sewage sludge, an organic waste product, has become a proven technology. If less oxygen is added than stoichiometrically required for a complete oxidation, cellulose will break down by a different reaction mechanism, releasing electrons. Under these circumstances reduction of inorganic compounds is possible.     

Recruitment of juvenile marine fishes to seagrass habitat in a temperature Australian estuary

We investigated the relationship between distance from the ocean and the recruitment of ocean-spawned juvenile fish to seagrass shallows within marine dominated Lake Macquarie, a coastal barrier lagoon in Southeast Australia. Samples were taken by seine net every 6 wk between June 1986 and June 1987, at 20 sites established at various distances from the entrance channel. The fish fauna was diverse: 80 species from 39 families were caught, with the Gobiidae, Monacanthidae, Syngnathidae, Tetraodontidae, Mugilidae, Atherinidae, Clupeidae, Mullidae, Sparidae, and Blenniidae being well represented. Ambassis jacksoniensis, Atherinomorus ogilbyi, and Gerres subfasciatus accounted for 46% of all individuals. Thirty-two species were classified as ocean spawners, 38 as lagoon spawners, and 10 as unknown in terms of spawning area. Newly settled juveniles of ocean spawners were concentrated near the lagoon's entrance, where most recruited in spring. This pattern occurred in the absence of a salinity gradient: distance from the ocean coupled with weak internal water circulation appears to limit larval distribution and hence juvenile recruitment. Small juveniles of Rhabdosargus sarba were sufficiently abundant for their subsequent dispersal to be directly traced. On the basis of results for this species, and indirect evidence of dispersal for several others, it is suggested that ocean-spawned juveniles, after settling near the entrance, gradually disperse as they grow and change their habitat. Thus, further from the entrance, their recruitment to seagrass shallows is later, at larger sizes, and in smaller numbers. Newly settled juveniles of lagoon spawners, however, were widely distributed within the lagoon. The optimal recruitment of ocean-spawned juveniles to similar coastal lagoons may depend on suitable habitat being available near the entrance. *** DIRECT SUPPORT *** A01BY081 00003