Geochemistry and isotope chemistry of Michigan Basin brines: Devonian formations

Detailed chemical and isotope analysis of 87 formation waters collected from six Devonian-aged units in the Michigan Basin are presented and discussed in terms of the origin of the dissolved components and the water. Total dissolved solids in these waters range from 200,000 to >400,000mg/1. Upper Devonian formations produce dominantly NaCaCl brine, while deeper formations produce CaNaCl water. Ratios of Cl/Br and Na/Br along with divalent cation content (MCl₂), indicate that these brines are derived from evapo-concentrated seawater. Other ion concentrations appear to be extensively modified from seawater values by water-rock reactions. The most important reactions are dolomitization, which explains the Ca content of the brines, and reactions involving aluminosilicate minerals. Stable isotope (δ¹⁸O and δD) compositions indicate that water molecules in the deeper formations are derived from primary concentrated seawater. Isotope enrichment by exchange with carbonates and perhaps gypsum cannot be discounted. Isotope values indicate water in the Upper Devonian formations is a mixture of seawater brine diluted with meteoric-derived water. Dilution has predominantly occurred in basin margins. Two scenarios are presented for the origin of the brines in the Devonian formations: (1) they originated when the Devonian sediments and evaporites were first deposited; or (2) they are residual brine liberated from the deeper Devonian and possibly Silurian salt deposits.     

Geochemistry of metal-rich brines from central Mississippi Salt Dome basin, U.S.A.

Oil-field brines are the most favored ore-forming solutions for the sediment-hosted Mississippi Valley-type ore deposits. Detailed inorganic and organic chemical and isotope analyses of water and gas samples from six oil fields in central Mississippi, one of the very few areas with high metal brines, were conducted to study the inorganic and organic complexes responsible for the high concentrations of these metals. The samples were obtained from production zones consisting of sandstone and limestone that range in depth from 1900 to 4000 m (70–120°C) and in age from Late Cretaceous to Late Jurassic. Results show that the waters are dominantly bittern brines related to the Louann Salt. The brines have extremely high salinities that range from 160,000 to 320,000 mg/l total dissolved solids and are NaCaCl-type waters with very high concentrations of Ca (up to 48,000 mg/l) and other alkaline-earth metals, but with low concentrations of aliphatic acid anions. The concentrations of metals in many water samples are very high, reaching values of 70 mg/l for Pb, 245 mg/l for Zn, 465 mg/l for Fe and 210 mg/l for Mn. The samples with high metal contents have extremely low concentrations (<0.02 mg/l) of H₂S. Samples obtained from the Smackover Formation (limestone) have low metal contents that are more typical of oil-field waters, but have very high concentrations (up to 85 mg/l) of H₂S. Computations with the geochemical code SOLMINEQ.87 give the following results: (1) both Pb and Zn are present predominantly as aqueous chloride complexes (mainly as PbCl₄²⁻ and ZnCl₄²⁻, respectively); (2) the concentrations of metals complexed with short-chained aliphatic acid anions and reduced S species are minor; (3) organic acid anions are important in controlling the concentrations of metals because they affect the pH and buffer capacity of the waters at subsurface conditions; and (4) galena and sphalerite solubilities control the concentrations of Pb and Zn in these waters.     

Origin and mixing history of brines,Palo Duro Basin,Texas, U.S.A.

Formation waters in the Palo Duro Basin, Texas, U.S.A. fall into four major groups based on integrated chemical and isotopic characteristics: (1) interbed brines within the major Permian evaporite aquitard; these are the most chemically concentrated and¹⁸O-rich fluids in the basin, and are interpreted as evaporatively concentrated sea water which has been hydrologically isolated since the Permian; (2) brines below the salt on the eastern side of the basin have ClBr, divalent cation, and isotopic systematics indicating a mixture of evaporatively concentrated sea water and meteoric water of δD= −20‰; (3) brines below the salt on the western side of the basin have chemical and isotopic systematics suggesting a mixture of two pulses of meteoric water, one with δD= −20‰ and the other with δD= −55‰; and (4) waters above the salt have the isotopic composition of meteoric waters. Diagenetic alteration of the cation chemistry has occurred for brines within and below the salt. Aquifers below the salt on the eastern side are interpreted as having been charged with dense Permian evaporite brines which subsequently mixed in various amounts with a basin-wide pulse of Triassic meteoric water. On the western side the descending Triassic meteoric waters became saline by dissolution of halite and are currently mixing with a Tertiary pulse of meteoric water initiated by the Laramide uplift to the west. The hydrochemistry suggests flow on the western side of the basin and static conditions on the eastern side. An unrecognized, approximately N-S permeability restriction, or discontinuity in the potentiometric flow surface, is inferred for major aquifers in the central area of the basin.     

Geochemistry of Brines from Salt Ore Deposits in Western Tarim Basin

In the geological evolution of the Tarim Basin, many transgressions and relictions happened. So there have been plentiful sources of salt. Moreover, because of uttermost drought, a lot of salt has been deposited. It is possible to find potash salt in this area. In our fieldwork, we have found salt and brine in western Tarim Basin. Based on a geological survey and the characteristics of sedimentary facies and paleogeography, this paper deals with the geochemical parameters and discusses the possibility of formation of potash salt in terms of the chemical analyses of samples collected from western Tarim Basin. Results of brine analysis lead to some conclusions: most of these salt brines have eluviated from very thick halite beds, mainly chloridetype salt and this kind of halite does not reach the stage of potash deposition in all aspects; WKSL (Wukeshalu) occupies a noticeable place, and we should attach importance to this district because there have been some indicators of the occurrence of potash deposits as viewed from the contents of Br and K. Finally, low Br contents are recognized in the Tarim Basin as a result of salt aggradation, and this point of view has been proved by the results of this experiment and the data available. It cannot depend upon the index of Br to judge the evolution stage of halite. We must look for other facies of potash except marine facies.     

Geological evolution of the Ishpeming Greenstone Belt,Michigan, U.S.A.

The Ishpeming Greenstone Belt is an Archean belt in the southern part of the Canadian Shield in the Upper Peninsula of Michigan, U.S.A. Two volcanic cycles are preserved in it. The oldest formation, and basal to the first cycle (the Kitchi Schist), consists of mafic metavolcanics, has a major serpentinized ultramafic body near its base, and grades upward to a coarse felsic volcanic breccia at the top of the cycle. This unit in turn is overlain by a sequence of mafic flows that grades upward to interbedded mafic flows and exhalites of the Mona Schist. This sequence has been intruded by the Dead River Pluton.The Ishpeming Greenstone Belt probably represents the keel of a previously much more extensive Greenstone Belt.Gold mineralization occurs associated with mafic basaltic volcanic rocks and serpentinized ultramafics low in the succession, and with carbonate-rich quartz-chlorite-sericite schists and exhalites higher in the sequence. No mineral deposits are now being exploited here.     

塔中志留系层序及其可容纳空间分析

根据经典层序地层学理论,综合研究野外露头、测井、地震等资料,将塔中地区志留系划分为5个层序,每个层序均由海侵和高位体系域两部分组成。除SQ₁为Ⅰ型层序外,其余4个层序均为Ⅱ型层序,而且SQ₃为一个三级中间体系域。受加里东运动影响,隆升后的塔中地区在晚奥陶世遭受剥蚀,地形变得平缓,在此基础上形成了以潮坪沉积为主的志留纪滨浅海沉积。在志留纪塔中地区构造稳定,海平面的升降决定了可容纳空间的大小。潮坪沉积相带的迁移反映了在全球海平面升降形成的异旋回中发育的潮坪自旋回沉积,可容纳空间动态变化是外在的表现形式。     

A strontium, oxygen and hydrogen isotopic composition of brines, Michigan and appalachian basins, Ontario and Michigan

⁸⁷Sr/⁸⁶Sr ratios of brine from samples from the Michigan and Appalachian Basins, in Ontario and Michigan, covering the stratigraphic interval from the Cambrian to Mississippian, vary from 0.708 to 0.711. With the exception of the salt units of the Salina Formation (Silurian), most values are greater than seawater for the time in question, indicating water-rock interaction. The sources of the radiogenic Sr has not been identified. All samples plot below the GMWL in δ¹⁸O−δ²H space, with the Cambrian and Ordovician samples closest to the line. Mixing of brines meteoric and glacial (Pleistocene) water is indicated in some cases. The more concentrated brines from each stratigraphic unit show a very narrow spread in values. All the Ordovician brines show a narrow range over a 200 km area for Sr, O and H isotopes, indicating extensive lateral migration of the fluids.Strontium in the brine has not equilibrated isotopically with its host rock. In some cases the late-stage minerals saddle dolomite, calcite and anhydrite have the same ⁸⁷Sr/⁸⁶Sr ratios as the brine, indicating that they precipitated from the brine in isotopic equilibrium.     

Petrographic and geochemical characteristics of dolomite types and the origin of ferroan dolomite in the Trenton Formation, Ordovician, Michigan Basin, U.S.A.

Three major types of dolomite occur in the Trenton Formation (Mid-Ordovician) of the Michigan Basin. These are: (1) ‘regional dolomite’ which is confined to the extreme western edge of the basin; (2) ‘cap dolomite’ which occurs in the upper portion of the Trenton and is confined to the basin's southern margin; and (3) ‘fracture-related’ dolomite which occurs in association with both large- and small-scale faults and fractures. These three dolomite types can be distinguished from one another by their major element chemistry, oxygen isotope ratios and rock texture. The regional dolomite is fine-grained, has <0.34 mol% FeCO₃, and mean δ¹⁸O of ?6·8‰_OPBD. The cap dolomite is texturally similar to regional dolomite but contains 3–13·0 mol% FeCO₃ and has a mean δ¹⁸O of ?7·7‰. Fracture-related dolomites are coarse-grained, low in iron, and have the most depleted δ¹⁸O ratios (x?=–9·0%_PDB). Petrographic relationships imply that the regional dolomite, formed prior to the cap dolomite probably during early diagenesis. The cap dolomite formed at relatively shallow depths as a result of the interaction of the overlying Utica Shale and the Trenton Limestone. Fracture-related dolomites post-date the cap dolomite and formed during deeper burial. A temperature of precipitation of approximately 80°C was calculated for fracture-related dolomites using oxygen isotope data. The distribution of the cap dolomite was controlled by the availability of Fe^2? which was in turn controlled by the availability of S^2?. In the centre of the basin Trenton-Utica deposition was continuous. The upper Trenton contained relatively high concentrations of organic matter which was used by sulphate reducing bacteria to produce H_2S from seawater sulphate. The precipitation of iron sulphides (pyrite + iron monosulphide) followed and used up most of the available Fe^2?. As a result only small amounts of ferroan dolomite formed. On the periphery of the basin, subaerial exposure resulted in the oxidation of most of the available organic matter. Sulphate reducing bacteria were therefore limited and produced limited amounts of H₂S. As a result only a minor amount of iron sulphide (iron monosulphide) formed. The remaining Fe^2- was then available for the formation of the ferroan cap dolomite. This model is supported by the following: (1) In the southern margin of the basin, the contact between Trenton cap dolomite and the overlying Utica Shale is sharp and probably unconformable. In the centre of the basin the contact is gradational. (2) In the centre of the basin, the total organic carbon content in the upper Trenton is an order of magnitude higher than in the cap dolomite. (3) The whole-rock concentration of iron is high in both the cap dolomite and in slightly dolomitized equivalent beds in the basin centre. (4) Iron sulphides are abundant in the centre of the basin and mostly in the form of pyrite. In the cap dolomite, iron sulphide is minor and primarily in the form of iron monosulphide.     

Reef-capping laminites in the Upper Silurian carbonate- to-evaporite transition, Michigan Basin, south-western Ontario

Silurian pinnacle reefs in the subsurface of the south‐western Ontario portion of the Michigan Basin display a variety of laminated carbonates (laminites) within predominantly muddy reef‐capping facies in the upper part of the Guelph Formation and the overlying A‐1 Carbonate of the Salina Group. Laminites, which are limestone, dolomite or partially dolomitized limestones, have a range of morphologies, from simple planar to a variety of wavy and serrated forms. Individual laminae are composed mainly of micrite, microspar or replacive dolomite, and vary internally from isopachous and continuous over the diameter of the core to non‐isopachous and often discontinuous. Clotted and peloidal micrite, sometimes defining small knobs and chambers, is interpreted as being microbial in origin and occurs within all types of laminites. Fibrous cement locally comprises laminite clasts in breccias or coats clasts in breccias, and also occurs as spherulites in the interparticle spaces in breccias. Although similar laminites have been described from elsewhere in the Michigan Basin and interpreted as caliche, travertine and abiotic subtidal stromatolites, the laminites in south‐western Ontario are most realistically regarded as microbial. The causes for the variations in morphology and characteristics of the constituent laminae are uncertain, although fluctuations in local microenvironmental conditions would have been important, set against a backdrop of an increasingly restricted overall setting. Caliche or travertine origins for these laminites are unlikely in general, except perhaps locally at the subaerial exposure surface at the tops of pinnacle reefs.     

Burial of reefs by shallow-water carbonates,Silurian Gower formation,Iowa, U.S.A.

The Gower Formation was deposited in two main stages. In the first, crinoid-coelenterate reef complexes, consisting of numerous closely spaced mounds, developed to maximum relief in water about 30 metres deep initially. Complexes are asymmetrical: the well defined windward Marginal Zone has pronounced local relief; the flatter, more extensive Leeward Zone fans out behind a high central area. Equivalent off-reef beds, where correlation is clear, consist of relatively fine skeletal dolomite.The second-stage deposits record a fall in sea level and the filling of first-stage topographic lows by progressively shallower-water sediments. These were mainly laminated carbonate muds and fine sands (Brady/Anamosa facies group). In the Marginal Zone these were deposited at first as steep wedge-beds on earlier mound flanks, together with a distinctive fauna and scattered stromatolites (Brady facies). Mounds expanded laterally while developing a platform on top. Poorly fossiliferous, flatter-lying beds (Laminate-Anamosa) subsequently filled remaining inter-mound hollows. Equivalent deposits in the Leeward Zone consist almost entirely of Laminate-Anamosa. Off-reef equivalents are probably generally similar.The second stage of Gower deposition ended with the accumulation across inter-complex basins of extensive uniform sheets of stromatolites and subordinate laminated muds (Crenulate-Anamosa). Near the reefs Crenulate-Anamosa overlies Laminate-Anamosa; in basin centres it lies directly on beds of the first depositional stage. The stromatolites are believed to have originated in very shallow, perhaps tideless, water with restricted circulation and high salinity.The shallow-water origin of these sediments suggests that reef tops were exposed during second-stage deposition, and consequently accumulated little or no sediment. Filled caves and a slightly earlier erosion surface tend to support this interpretation.

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Zusammenfassung Die Sedimente der silurischen Gower-Formation im Staate Iowa wurden in zwei Phasen abgelagert. Während der ersten entwickelte sich in Krinoiden-Coelenteraten Riffkomplexe aus vielen nah benachbarten Kuppen ein Relief in ursprünglich 30 m Wassertiefe. Die Riffkomplexe sind asymmetrisch: die gut definierte Randzone in Windrichtung hat ein ausgeprägtes Detailrelief; hinter einer zentralen Erhebung erstrecken sich die weniger steilen und ausgedehnteren Fächer der Lee-Zone. Soweit korrelierbar, bestehen die riffernen Sedimente aus relativ feinem skeletalem Dolomit.Die Ablagerungen der zweiten Phase repräsentieren eine Senkung des Meeresspiegels; die topographisch tieferen Teile des vorhandenen Reliefs werden mit Sedimenten aus immer geringerer Wassertiefe gefüllt, hauptsächlich mit laminierten Karbonatschlammen und Feinsanden (Brady/Anamosa-Fazies-Gruppe). In der Randzone wurden sie zunächst als steile keilförmige Schichten auf Kuppenhängen abgelagert (Brady/Fazies); diese enthalten eine charakteristische Fauna und einzelne Stromatolithe. Die Kuppen verbreiterten sich entsprechen zu Plateaus. Fossilarme, flacher liegende Schichten (laminierte Anamosa-Fazies) füllten dann die Restsenken zwischen den Kuppen. Die entsprechenden Ablagerungen der Lee-Zone gehören fast ausschließlich zur letzteren Fazies, und rifferne Sedimente sind wahrscheinlich im allgemeinen ähnlich.Am Ende der zweiten Phase der Gower-Formation wurden ausgedehnte einförmige Schichten von Stromatolithen und — untergeordnet — laminierten Schlammen (gewellte Anamosa-Fazies) in den Becken zwischen den Riffkomplexen abgelagert. In Riffnähe wird die laminierte von der gewellten Anamosa-Fazies überdeckt, im Beckenzentrum folgt die letztere direkt auf die Ablagerungen der ersten Phase. Für die Stromatolith-Bildung wird sehr flaches, vielleicht gezeiten-loses Wasser mit beschränkter Zirkulation und hoher Salinität angenommen.Bei Ablagerung dieser Sedimente im Flachwasser müssen die Riffoberflächen während der zweiten Phase trocken gelegen haben, weshalb sie mit wenig oder keinem Sediment überdeckt wurden. Hohlraumfüllungen und eine wenig ältere Erosionsfläche scheinen diese Interpretation zu unterstützen.

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Résumé Les sédiments de la formation silurienne Gower dans l'état Iowa furent déposés en deux phases. Pendant la première phase, il se forma un relief, à partier de nombreux pitons sous-marins voisins, dans les complexes de récifs de crinoïdes-coelentérés à l'intérieur d'une profondeur d'eau originelle de 30 m. Les complexes de récifs sont asymétriques. La zone de bordure, bien définie dans la direction du vent, possède un relief de détails marqué; derrière une élévation centrale s'étendent les surfaces moins escarpées et allongées de la zone »Lee«. S'il est possible de les comparer, les sédiments des récifs se composent de dolomites relativement fines.Les dépôts de la deuxième phase représentent un affaissement du niveau de la mer. Les parties topographiques profondes du relief présent seront comblées par des sédiments provenant de profondeurs d'eau toujours plus petites, principalement avec des vases de carbonates en lamelles et des sables fins (groupe de faciès »Brady/Anamosa«). Dans la zone de bordure, ces sédiments furent d'abord déposés sous forme de strates abruptes et cunéiformes sur les versants des coupoles (faciès Brady); ils contiennent une faune caractéristique et des »Stromatolites« particulières. Les coupoles s'étendirent pour se transformer en plateau. Puis, les couches plus basses, pauvres en fossiles (faciès en lamelles Anamosa), remplirent les depressions restantes entre les coupoles. Les dépôts correspondants de la zone »Lee« appartiennent, d'une manière presque exclusive, au dernier faciès et les sédiments loins des récifs sont en général, vraisemblablement, similaires.A la fin de la deuxième phase de la formation Gower, des couches de »Stromatolites« étendués et uniformes ainsi que des vases en lamelles subordonnées (faciès ondulés Anamosa) furent déposées dans les bassins entre les complexes de récifs. A proximité des récifs, le faciès en lamelles sera recouvert par le faciés ondulé Anamosa; dans le centre du bassin, le dernier faciès suit immédiatement les dépôts de la première phase. Pour la formation de »Stromatolites« on présume l'action d'eau à bas niveau, peut être sans marées, mais avec des courants limités et de la salinités élevée.Pendant la sédimentation de ces couches dans l'eau basse, les surfaces des récifs doivent avoir été à sec durant la deuxième phase. C'est pourquoi elles ne sont couvertes que de peu de sédiment ou pas du tout. Le remplisage de cavernes et une surface d'érosion un peu plus ancienne semblent supporter cette interprétation.

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Petrochemistry of the Fish Cove rhyolite,Keweenaw peninsula,Michigan, U.S.A.

A small (360 × 180 m) rhyolitic intrusive body in the lower portion of the Portage Lake Lava Series of Michigan's Keweenaw peninsula was mapped and sampled in detail. The rhyolite is one of a number of similar bodies which make up less than 1% of the total volume of this thick Late-Precambrian plateau basalt pile. The rock is a low-calcium rhyolite with fine-grained homogeneous texture and sparse phenocrysts of plagioclase and quartz. Analyses of selected trace and major elements for 21 samples taken from the body reveal a chemical zonation consisting of a core zone enriched in K, Rb and Ba, and a border zone relatively poor in these elements. Little areal difference is found with respect to other elements tested (Mn, Sr, Zr, Ca, Ti, and Fe). This apparently primary zonation seems to result from the migration of K, Rb and Ba during crystallization of the shallow intrusive. Though zoned, the trace-element chemistry of the Fish Cove body is distinct from that of eight other rhyolites in the Portage Lake Lava Series, and suggests that fingerprinting by trace elements might be a fruitful method for identifying and correlating the sources of numerous rhyolitic pebbles in conglomerates interbedded with the basaltlava flows of the Portage Lake Series.     

Geochemistry of Precambrian mafic dikes,central Bighorn Mountains,Wyoming, U.S.A.

Precambrian quartz dolerites and metadolerites of the central Bighorn Mountains form dikes that intrude a Precambrian metamorphic and igneous terrane typical of the Laramide uplifts of the middle Rocky Mountains. They have a restricted range of major- and trace-element compositions and are typical of basalts in the middle stages of tholeiitic fractionation. Fractionation in the direction of iron enrichment occurred by removal of plagioclase. Average element concentrations of the two groups are nearly identical to one another, are comparable to those in Archean metabasalts from numerous shield areas, and are intermediate between those of modern oceanic tholeiites and continental tholeiites. These average concentrations suggest a depth of magma generation and thickness of crust intermediate between those for the oceanic and continental environments.     

Groundwater quality variations in glacial drift and bedrock aquifers,Barry County,Michigan, U.S.A.

Groundwater samples from 288 domestic wells in Barry County, Michigan, were analyzed for 33 inorganic chemical parameters. Variations in chemical composition were investigated by considering the possible effects of human impact, aquifer type (bedrock vs glacial drift), chemical evolution along groundwater flow paths, and glacial landform type (moraine vs outwash). Approximately 25 percent of the glacial drift wells were classified as degraded by human impact and were excluded from further analysis of chemical variation. Two-sample tests comparing individual concentrations from drift and bedrock aquifers suggest that groundwater in the Marshall Sandstone aquifer is derived from local recharge through the glacial drift. This conclusion is supported by generalized groundwater flow patterns recognized for the two aquifers.Concentrations in both aquifers were examined in relation to generalized flow paths derived from water level data and also by classification of wells as recharge, transition, and discharge. No spatial concentration trends in major ions were detected, although iron concentrations do appear to increase from recharge to discharge areas. Declining redox potential along groundwater flow paths may explain this trend.The possible influence of glacial landform type was investigated by comparing concentrations of wells in moraines with those in outwash deposits. Wells in moraines have significantly higher concentrations of most parameters, perhaps due to higher content of finer, more chemically reactive sediment grains.     

Geochemistry of natural gases in deep strata of the Songliao Basin, NE China

Chemical composition and stable carbon isotopic studies were undertaken for 27 gas samples from deep strata of the Xujiaweizi Depression in the Songliao Basin to investigate their origin. Gas molecular and carbon isotopic compositions show great variety. Methane is the main component for all studied samples and its content ranges from 57.4% to 98.2% with an average of 90.1%. Gas wetness ranges from 0.8% to 16.7% with an average of 2.7%. The main non-hydrocarbon gases are carbon dioxide and nitrogen with an average of 4.0% and 3.2%, respectively. Carbon isotope data suggest that these deep strata gases are mainly coal-type gases mixed with minor amounts of associated (oil-type) gases. Coal-type gases are characterized by heavier carbon isotopic values and drier chemical compositions. These gases were generated from the Lower Cretaceous Shahezi Formation coals interbedded shales with type III kerogen during the postmature stage of hydrocarbon generation. Oil-type gases are characterized by lighter carbon isotope and higher wetness, which were generated from the Lower Cretaceous shales with type II kerogen in the shallow strata during the early mature stage of hydrocarbon generation. Mixing of two different gases causes unusual carbon isotopic distribution patterns, with lighter isotopic values in higher numbered carbons in most gases. The discovery of coal-type gases in the Songliao Basin provides new prospects for the exploration in this region.     

Degradation and Metamorphic Differentiation of the Keweenawan Tholeiitic Lavas of Northern Michigan, U.S.A.

The Portage Lake Lava Series of the Keweenaw Peninsula, northernMichigan, is composed of over 5000 m of tholeiitic lava flows.Chemical and petrologic study of thin undifferentiated flowsfrom this sequence shows that the bulk of the flows are olivinetholeiites; highly ironenriched tholeiites comprise up to 10per cent of the volume. In addition, a few small rhyolitic intrusives,exceedingly rich in alkalis, are exposed in the region. Duringburial the amygdular tops of each flow acted as channelwaysfor migrating fluids during a regional metamorphic event thatproduced a progressive sequence of secondary phases within theflow tops. In the upper part of the stratigraphic section, laumontite,analcime, albite, and chlorite dominate assemblages characteristicof the zeolite facies. Stratigraphically deeper. flow tops arecharacterized by metamorphic assemblages of the prehnite-pumpellyitefacies. Within the higher rank facies, rocks along fractureshave been transformed to calcium-rich monomineralic domains(metadomains) of epidote or pumpellyite. These metadomains gradeoutward through incompletely reconstituted rocks containingalbitized feldspars, and finally, in flow centers, to basaltdisplaying few signs of mineralogical readjustment. Extremechemical disparity exists within the altered flow tops, particularlywith respect to calcium and sodium content. Calculations revealthat bulk compositions of metamorphically adjusted flow topsare similar to unaltered basalts. Thus, the chemical variationdisplayed by the metamorphic rock types resulted from localizedmetamorphic differentiation. The Keweenawan sequence displays progressive dehydration fromthe top to the base of the stratigraphic section. At the topchlorite (H₂O = 12 per cent) and pumpellyite (H₂O = 6 per cent)metadomains formed by dehydration. The fact that most of theserocks were hydrated strongly suggests that fluid pressures wereless than total pressures during the metamorphic event. In rocksundergoing hydration in the uper part of the stratigraphic section,therefore, water pressures decreased away from channelways.As a result, the most hydrous rock types formed in these regions.Conversely, in rock undergoing dehydration water pressures increasedaway from channelwasys, so that the most dehydrated rocks formedin such regions. Water content of an individual rock is dependenton both position in the stratigraphic pile and position withrespect to fluid channelways.     

Geochemistry and petrogenesis of lamproites,late cretaceous age,Woodson County,Kansas, U.S.A.

Lamproite sills and their associated sedimentary and contact metamorphic rocks from Woodson County, Kansas have been analyzed for major elements, selected trace elements, and strontium isotopic composition. These lamproites, like lamproites elsewhere, are alkalic (molecular $\text{K}{}_2\text{O + Na}{}_2\text{O}\text{Al}{}_2\text{O}{}_3\text{= 1.6–2.6}$), are ultrapotassic $\text{(}\text{K}{}_2\text{O}\text{Na}{}_2\text{O}\text{= 9.6–150)}$, are enriched in incompatible elements (LREE or light rare-earth elements, Ba, Th, Hf, Ta, Sr, Rb), and have moderate to high initial strontium isotopic compositions (0.7042 and 0.7102). The silica-saturated magma (olivine-hypersthene normative) of the Silver City lamproite could have formed by about 2 percent melting of a phlogopite-garnet lherzolite under high $\text{H}{}_2\text{O}\text{CO}{}_2$ ratios in which the Iherzolite was enriched before melting in the incompatible elements by metasomatism. The Rose Dome lamproite probably formed in a similar fashion although the extreme alteration due to addition of carbonate presumably from the underlying limestone makes its origin less certain. Significant fractional crystallization of phases that occur as phenocrysts (diopside, olivine, K-richterite, and phlogopite) in the Silver City magma and that concentrate Co, Cr, and Sc are precluded as the magma moved from the source toward the surface due to the high abundances of Co, Cr, and Sc in the magma similar to that predicted by direct melting of the metasomatized Iherzolite.Ba and, to a lesser extent, K and Rb and have been transported from the intrusions at shallow depth into the surrounding contact metamorphic zone. The Silver City lamproite has vertical fractionation of some elements due either to volatile transport or to variations in the abundance of phenocrysts relative to groundmass most probably due to flow differentiation although multiple injection or fractional crystallization cannot be conclusively rejected.     

Saline brines and metallogenesis in a modern sediment-filled rift: the Salton Sea geothermal system, California, U.S.A.

The Salton Sea geothermal system (SSGS) is the site of active hydrothermal metamorphism and metallogenesis in the delta of the Colorado River, which partially fills the Salton Trough rift zone at the head of the Gulf of California. Growth of the delta across the rift has isolated the northern part of the Salton Trough since the Pleistocene, forming the evaporative Salton Sea basin whose sediments host the SSGS. More than 70 commercial geothermal wells, including a 3.2 km deep research borehole drilled as part of the Salton Sea Scientific Drilling Project (SSSDP), are yielding a wealth of new data from this system.Within the SSGS, active greenschist facies metamorphism is occuring at temperatures 365°C at only 2–3 km depth, by reaction of NaCaKFeMnCl brines with the deltaic and lacustrine Pilo-Pleistocene sediments. Two kinds of base metal ore mineralization occur at depth: stratabound diagenetic Fe sulfides, and epigenetic vertical veins containing FeZnCuPb sulfides and Fex oxides. The vein mineralization occurs as two types: an older, reduced carbonate-sulfide assemblage, and a modern, oxidized, silicate-hematite-sulfide assemblage. The earlier assemblage formed at temperatures up to 100°C higher than the ambient temperatures measured in the wells today, implying that cooler, oxidized fluids have displaced hot, reduced fluids.A sharp interface between shallow fluids containing <12 wt % TDS and deep hypersaline brines containing 15–27 wt % TDS exists in the SSGS. The deeper hypersaline brines are rich in base metals (Fe 1500 ppm, Mn 1000 ppm, Zn 500 ppm) whereas the overlying lower-salinity fluids contain less than 100 ppm each of Fe, Mn and Zn. The modern silicate-hematite-sulfide vein assemblage is precipitating where the two fluids appear to be mixing. The two fluid types also have distincty different δ¹⁸O and δD relations. The shallow lower-salinity fluids have only partially-exchanged oxygen with deltaic and lacustrine host rocls. The deep hypersaline brines have exchanged oxygen extensively at 250°C with the deltaic sediments. δD values of the hypersaline brines are typically lighter than either the lower-salinity fluids or the modern evaporating groundwaters occupying the Salton Trough, implying a different source for the original fluids.The high salinities of the geothermal brines are derived from a combination of evaporation of fossil lakewaters, groundwater dissolution of shallow lacustrine evaporites, and subsurface hydrothermal metamorphism of buried lacustrine evaporites. Episodic filling and desiccation of the closed Salton Sea basin has allowed cold saline brines to form and percolate down into the sedimentary section. In some wells anyhydrite meta-evaporites and interbedded solution collapse breccias occur at 1 km depth. The anyhydrite contains fluid inclusions that are saturated in halite at their homogenization temperatures of 300°C, recording the hydrothermal dissolution of bedded salt. Based on Sr and Pb isotopic data and whole-rock chemical data, the source of the metals in the hypersaline brines is from leaching of the host sediments. However, the origin of reduced S for ore mineralization remains an enigma. δ³⁴S values for vein sulfides cluster uniformly around zero per mil, implying that an isotopically-homogeneous source of reduced sulfide exists in the brines. It is possible that the vein sulfides receive a constribution from a magmatic S source.Movement and mixing of brines of different chemistry and oxidation states play a major role in ore genesis. Our model envisages an early stage in which a deep brine pool accumulated at depth in the sedimentary section by partial section by partial evaporation of basin and dissolution of bedded salts. Intrusion of rift-related basaltic magma into the base of the sedimentary caused heatingand fracturing of the sediments, resulting in precipitation of the carbonate-sulfide veins during pore fluid expulsion. Heating also caused a diapir of the hypersaline brine to rise and displace colder, less saline, shallower pore fluids. This brine intrusion was accompanied by pervasive and extensive mineralization. As this diapir cooled it began to move downward, drawing in shallow, more oxidized fluids and causing the formation of the modern silicate-hematite-sulfide vein ore zone.     

Environmental control of carbon isotope variations in Pennsylvania black-shale sequences, Midcontinent, U.S.A.

A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ¹³C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ¹³C_kerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ¹³C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO₂-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO₂” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.     

Geochemistry and genesis of brine emanations from Cretaceous strata of the Mamfe Basin, Cameroon

The geochemistry of 5 salt springs in the southwestern Mamfe Basin was investigated in order to infer the mineral content of their source and to relate the genesis of the springs to the local geology. Field observations revealed that, they are cold springs (23–28 °C), and are composed of secondary brines that are neutral to alkaline with pH values ranging from 7 to 8.7. Results of chemical analysis show that the springs contain major ions that form evaporite minerals, as well as chalcophile elements. The dominant cation is Na⁺ (>96%), and the dominant anion is Cl⁻ (>99%). Based on correlation coefficients between ions that form evaporites and field occurrence of efflorescences of halite, it is suggested that the ancient evaporites in the Mamfe Basin are composed entirely of carbonate and chloride salts. Meteoric and convective fluid flow processes are responsible for the dissolution of ancient evaporites and subsequent migration of brines to the surface from underground. The brines migrate through permeable strata with migration pathways resulting from a combination of fracture porosity created by post––Cretaceous tectonism and intergranular porosity enhanced by the chemically aggressive migrating brines.