Textural variations within different representative types of dune sediments in Kuwait

Samples were collected from the surfaces of four types of typical dunes in order to identify variations in textural characteristics over their bodies. These dunes are barchan, climbing dune, falling dune, and nabkha. Statistical parameters vary from position to another and show that each dune has its own characteristics. It is well recognized that all the sediments of the studied dunes tend to be finer from borders toward the mid dune. Histograms and bivariate diagrams successfully differentiate between different localities within all studied dunes. The climbing dune shows high uniformity where medium sand represents the mean grain size of 91% of collected samples. Samples from barchan and falling dune show lowest variability in statistical parameter values compared to other dunes. On the other hand, nabkha sediments are more variable and show higher values of average statistical parameters. All studied dunes are coarser than surrounding dunes in regional areas and other comparable dunes. But particularly, the barchan sediments in Kuwait are characterized by larger grain size, better sorting than other comparable dunes in the upwind (Iraq) and downwind (Saudi Arabia) and other parts of the world.     

Textural and isotopic variations in graphite from plutonic rocks,South-Central New Hampshire

Graphite occurs in two distinct textural varieties in syntectonic granitoids of the New Hampshire Plutonic Series and in associated metasedimentary wall rocks. Textural characteristics indicate that coarse graphite flakes were present at an early stage of crystallization of the igneous rocks and thus may represent xenocrystic material assimilated from the wall rocks. The range of ¹³C values determined for flake graphite in the igneous rocks (–26.5 to –13.8) overlaps the range for flake graphite in the wall rocks (–26.0 to –16.7), and spatial correlation of some ¹³C values in the plutons and wall rocks supports the assimilation mechanism. The textures of fine-grained irregular aggregates or spherulites of graphite, on the other hand, indicate that they formed along with secondary hydrous silicates and carbonates during retrograde reactions between the primary silicates and a carbon-bearing aqueous fluid phase. Relative to coexisting flake graphite, spherulitic graphite shows isotopic shifts ranging from 1.9 higher to 1.4 lower in both igneous and metasedimentary samples.The observed isotopic shifts and the association of spherulitic graphite with hydrous silicates are explained by dehydration of C-O-H fluids initially on or near the graphite saturation boundary. Hydration of silicates causes dehydration of the fluid and drives the fluid composition to the graphite saturation surface. Continued dehydration of the fluid then requires coprecipitation of secondary graphite and hydrous silicates and drives the fluid toward either higher or lower CO₂/CH₄ depending upon the inital bulk composition. Isotopic shifts in graphite formed at successive reaction stages are explained by fractionation of ¹³C between secondary graphite and the evolving fluid because ¹³C is preferentially concentrated into CO₂ relative to CH₄.Epigenetic graphite in two vein deposits assiciated with the contacts of these igneous rocks is generally enriched in ¹³C (–15.7 to –11.6) relative to both the igneous and wall-rock ¹³C values. Values of ¹³C vary by up to 3.4 within veins, with samples taken only 3 cm apart differing by 2.0 These variations in ¹³C correlate with textural evidence showing sequential deposition of different generations of graphite in the veins from fluids which differed in proportions of carbon species or isotopic composition (or both).     

Age and geochemistry of the Newania dolomite carbonatites, India: implications for the source of primary carbonatite magma

The Newania carbonatite complex of India is one of the few dolomite-dominated carbonatites of the world. Intruding into Archean basement gneisses, the rocks of the complex have undergone limited diversification and are not associated with any alkaline silicate rock. Although the magmatic nature of the complex was generally accepted, its age of emplacement had remained equivocal because of the disturbed nature of radioisotope systems. Many questions about the nature of its mantle source and mode of origin had remained unanswered because of lack of geochemical and isotopic data. Here, we present results of our effort to date the complex using ¹⁴⁷Sm–¹⁴³Nd, ²⁰⁷Pb–²⁰⁶Pb and ⁴⁰Ar–³⁹Ar dating techniques. We also present mineral chemistry, major and trace element geochemistry and Sr–Nd isotopic ratio data for these carbonatites. Our age data reveal that the complex was emplaced at ~1,473 Ma and parts of it were affected by a thermal event at ~904 Ma. The older ²⁰⁷Pb–²⁰⁶Pb ages reported here (~2.4 Ga) and by one earlier study (~2.3 Ga; Schleicher et al. Chem Geol 140:261–273, 1997) are deemed to be a result of heterogeneous incorporation of crustal Pb during the post-emplacement thermal event. The thermal event had little effect on many magmatic signatures of these rocks, such as its dolomite–magnesite–ankerite–Cr-rich magnetite–magnesio-arfvedsonite–pyrochlore assemblage, mantle like δ¹³C and δ¹⁸O and typical carbonatitic trace element patterns. Newania carbonatites show fractional crystallization trend from high-Mg to high-Fe through high-Ca compositions. The least fractionated dolomite carbonatites of the complex possess very high Mg# (≥80) and have similar major element oxide contents as that of primary carbonatite melts experimentally produced from peridotitic sources. In addition, lower rare earth element (and higher Sr) contents than a typical calcio-carbonatite and mantle like Nb/Ta ratios indicate that the primary magma for the complex was a magnesio-carbonatite melt and that it was derived from a carbonate bearing mantle. The Sr–Nd isotopic data suggest that the primary magma originated from a metasomatized lithospheric mantle. Trace element modelling confirms such an inference and suggests that the source was a phlogopite bearing mantle, located within the garnet stability zone.     

Textural,chemical and isotopic variations induced by hydrothermal fluids on mesozoic limestones in northwestern Sicily

The results are given of textural and compositional investigations carried out on carbonate materials outcropping in various localities in northwestern Sicily where fluorite, barite and calcite mineralizations of hydrothermal origin occur. Observation of the textural features indicate variations in the degree of calcite recrystallization and silicification that appear to be more marked in rocks that show more evident effects of mineralization. The geochemical behaviour of the minor elements indicate variations in chemical composition (increase of Mn, Fe, and Sr and removal of Mg) as a result of interactions between mineralizing fluids and host-rocks. Comparison between the isotopic compositions of oxygen and carbon in mineralized and unmineralized limestones shows that isotopic exchange took place between CaCO₃ and hydrothermal fluids. The values obtained support relatively short limestone/water contact times and moderate temperature values. It is suggested that the ore-forming solutions were essentially heated meteoric waters. Precipitation of the fluorite was caused by temperature decrease, reaction with Ca-rich wall rock, and increase in pH of initially rather acid hydrothermal solutions.This work was supported by a grant from the Ente Minerario Siciliano.     

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland,USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (～1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e₁ and e₂). The better-developed e₁ sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e₂ lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e₁ and e₂ strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.     

Macro‐and Microstructural,Textural Fabrics and Deformation Mechanism of Calcite Mylonites from Xar Moron‐Changchun Dextral Shear Zone,Northeast China

The calcite mylonites in the Xar Moron-Changchun shear zone show a significance dextral shearing characteristics. The asymmetric(σ-structure) calcite/quartz grains or aggregates, asymmetry of calcite c-axes fabric diagrams and the oblique foliation of recrystallized calcite grains correspond to a top-to-E shearing. Mineral deformation behaviors, twin morphology, C-axis EBSD fabrics, and quartz grain size-frequency diagrams demonstrate that the ductile shear zone was developed under conditions of greenschist facies, with the range of deformation temperatures from 200 to 300°C. These subgrains of host grains and surrounding recrystallized grains, strong undulose extinction, and slightly curved grain boundaries are probably results of intracrystalline deformation and dynamic recrystallization implying that the deformation took place within the dislocation-creep regime at shallow crustal levels. The calculated paleo-strain rates are between 10~(–7.87)s~(–1) and 10~(–11.49)s~(–1) with differential stresses of 32.63–63.94 MPa lying at the higher bound of typical strain rates in shear zones at crustal levels, and may indicate a relatively rapid deformation. The S-L-calcite tectonites have undergone a component of uplift which led to subhorizontal lifting in an already non-coaxial compressional deformation regime with a bulk pure shear-dominated general shear. This E-W large-scale dextral strike-slip movement is a consequence of the eastward extrusion of the Xing'an-Mongolian Orogenic Belt, and results from far-field forces associated with Late Triassic convergence domains after the final closure of the Paleo-Asian Ocean.     

微生物白云岩模式研究进展

白云石（岩）问题一直是沉积学领域长期关注的研究主题之一。近年来,在研究含有白云石的现代自然环境和促进原生白云石的沉淀实验中,都加入了微生物因素,并取得了令世人关注的效果,这无疑为白云石（岩）的成因研究提供了新思路与新途径。在前人研究基础上,总结现有的观察资料和实验结果,将微生物促进白云石沉淀的机理模式归纳为厌氧模式和需氧模式2种,并分别介绍这两种模式中硫酸盐还原细菌、产甲烷菌和嗜盐好氧细菌促进白云石沉淀的机理;与微生物相关的矿物形态学特征中,球形和哑铃形白云石及白云石最初的成核阶段所形成的纳米球粒状结构具有一定代表意义,尤其是纳米球粒状结构可以作为生物矿物学上微生物白云石的标志性结构。通过这些特殊的形态特征来寻找微生物作用的证据,或可为古代相似成因白云石（岩）的成因研究提供一种标志。     

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.     

Investigations of an intrusive contact,northwest Nelson,New Zealand—I. Thermal,chronological and isotopic constraints

In the Nelson area of New Zealand, intrusion of the Separation Point Batholith 114 Ma ago caused thermal metamorphism in the adjacent Devonian Rameka Gabbro that allowed radiogenic ⁴⁰Ar to diffuse partially out of hornblendes in the gabbro.Comparison of mineral ages in the batholith (obtained by the K-Ar, ${}^{40}\text{Ar}{}^{39}\text{Ar}$, Rb-Sr, U-Pb and fission track methods) with closure temperature estimates for the various isotopic systems has allowed thermal histories of several samples to be resolved. These curves indicate that, following emplacement, the batholith cooled at an exponentially decreasing rate reaching 100°C by about 75 Ma ago.Oxygen isotope measurements on minerals from the gabbro and granodiorite give calculated equilibrium water compositions of +7 and +9 of δ¹⁸O_SMOW respectively. Measured porosities from samples of the gabbro yield a model permeability of 3 × 10^?17 m¹². These results confirm that heat transfer from the batholith to the gabbro was predominantly by conduction. A numerical heat flow model has been developed for the region, and shows that samples in the gabbro near the contact reached ~600°C. whereas 5 km away from the contact, the samples were only raised ~ 50°C above the ambient temperature.Excess ⁴⁰Ar was identified as causing a significant scatter in the K-Ar ages of hornblendes from the gabbro. Though obscured by this effect, these data suggest a crystallization age of the Rameka Gabbro of 367 Ma, which is consistent with geological evidence.     

Microstructures in carbonates from the Alnø and Fen carbonatites

The nature of carbonates and related second phase minerals in carbonatite samples from the Alnø and Fen regions has been studied by optical and transmission electron microscopy, aided by X-ray diffraction and electron microprobe analysis.Calcite, dolomite and ankerite have been found to exhibit two phase microstructures. Coherent ribbons of calcitic material occur in dolomite and ankerite, while the calcite grains commonly contain a fine dispersion of plate-like and sometimes rod-like precipitates. Calcite and dolomite frequently occur in contact with the same crystallographic lattice orientation, the small lattice mismatch being indicated by moiré patterns at the grain boundaries and occasional misfit dislocations.The two-phase structure of calcite in dolomite reported for the first time in this paper is thought to be a high temperature analogue of the modulated microstructures reported recently in calcian sedimentary dolomites (Wenk et al. 1983).     

Sr and Nd isotope data of apatite,calcite and dolomite as indicators of source,and the relationships of phoscorites and carbonatites from the Kovdor massif,Kola peninsula,Russia

A detailed Sr−Nd isotopic study of primary apatite, calcite and dolomite from phoscorites and carbonatites of the Kovdor massif (380 Ma), Kola peninsula, Russia, reveals a complicated evolutionary history. At least six types of phoscorites and five types of carbonatite have been identified from Kovdor by previous investigators based on relative ages and their major and accessory minerals. Isotopic data from apatite define at least two distinct groups of phoscorite and carbonatite. Apatite from the earlier phoscorites and carbonatites (group 1) are characterized by relatively low⁸⁷Sr/⁸⁶Sr (0.70330–0.70349) and¹⁴³Nd/¹⁴⁴Nd initial ratios (0.51230–0.51240) with F=2.01–2.23 wt%, Sr=2185–2975 ppm, Nd=275–660 ppm and Sm=31.7–96.2 ppm. Apatite from the second group has higher⁸⁷Sr/⁸⁶Sr (0.70350–0.70363) and¹⁴³Nd/¹⁴⁴Nd initial ratios (0.51240–0.51247) and higher F (2.63–3.16 wt%), Sr (4790–7500 ppm), Nd (457–1074 ppm) and Sm (68.7–147.6 ppm) contents. This group corresponds to the later phoscorites and carbonatites. One apatite sample from a carbonatite from the earlier group fits into neither of the two groups and is characterized by the highest initial⁸⁷Sr/⁸⁶Sr (0.70385) and lowest¹⁴³Nd/¹⁴⁴Nd (0.51229) of any of the apatites. Within both groups initial⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios show negative correlations. Strontium isotope data from coexisting calcite and dolomite support the findings from the apatite study. The Sr and Nd isotopic similarities between carbonatites and phoscorites indicate a genetic relationship between the two rock types. Wide variations in Sr and Nd isotopic composition within some of the earlier carbonatites indicate several distinct intrusive phases. Oxygen isotopic data from calcite and dolomite (δ¹⁸O=+7.2 to +7.7‰ SMOW) indicate the absence of any low-temerature secondary processes in phoscorites and carbonatites, and are consistent with a mantle origin for their parental melts. Apatite data from both groups of phoscorite plot in the depleted quadrant of an ε_Nd versus ε_Sr diagram. Data for the earlier group lie along the Kola Carbonatite Line (KCL) as defined by Kramm (1993) and data from the later group plot above the KCL. The evolution of the phoscorites and carbonatites cannot be explained by simple magmatic differentiation assuming closed system conditions. The Sr−Nd data can best be explained by the mixing of three components. Two of these are similar to the end-members that define the Kola Carbonatite Line and these were involved in the genesis of the early phoscorites and carbonatites. An additional component is needed to explain the isotopic characteristics of the later group. Our study shows that apatite from rocks of different mineralogy and age is ideal for placing constraints on mantle sources and for monitoring the Sr−Nd evolution of carbonatites. Editorial responsibility: W. Schreyer     

Calcite solubility and speciation in supercritical NaCl-HCl aqueous fluids

The solubility of calcite in NaCl-H₂O and in HCl-H₂O fluids was measured using an extraction-quench hydrothermal apparatus. Experiments were conducted at 2 kbar, between 400° C and 600° C. Measurements in NaCl-H₂O were conducted in two ways: 1) at constant pressure and NaCl concentration, as a function of temperature; and 2) at constant pressure and temperature, as a function of NaCl concentration. In both the NaCl-H₂O and the HCl-H₂O systems, the solubility of calcite increases with increasing chlorine concentrations. For example, the log calcium molality in equilibrium with calcite increases from –3.75 at 2 kbar and 500° C, in pure H₂O to –3.10 at 2 kbar and 500° C at log NaCl molality=–1.67. At fixed pressure and NaCl molality, the solubility of calcite is almost constant from 400° C to 550° C, but increases somewhat at higher temperatures. The results can be used to determine the dominant calcium species in the experimental solutions as a function of NaCl concentration and to obtain values for the second dissociation constant of CaCl_2(aq). At 2 kbar, 400° C, 500° C, and 600° C, we calculate values for the log of the dissociation constant of CaCl⁺ of –2.1, –3.2, and –4.3, respectively. The 400° C and 500° C values are consistent with those obtained by Frantz and Marshall (1982) using electrical conductance techniques. However, our 600° C value is 0.8 log units higher than that reported by Frantz and Marshall. The calcite solubilities in the NaCl-H₂O and HCl-H₂O systems are inconsistent with the solubilities of calcite in pure H₂O reported by Walther and Long (1986). They are, however, consistent with the measurements of calcite solubilities in pure H₂O presented in this study. These results allow for the calculation of the solubilities of calcium silicates and carbonates in fluids that contain CO₂ and NaCl.     

大同地区早中生代煌斑岩—碳酸岩岩墙群

大同地区密集分布的岩墙群由基性－超基性煌斑岩和碳酸岩组成,侵入石炭－二叠纪煤层中。与地层的接触关系和Rb-Sr,K-Ar同位素年代学数据表明墙群的侵位于晚三叠世。Sr,Nd同位素比值显示,碳酸岩和煌斑岩都来源于壳幔混合型岩浆源区。稳定元素比值和相容,不相容元素同等富集则表明是碳酸岩和硅酸盐混合的原始母岩浆液态不混熔的产物。 岩墙群形成的地质条件和构造背景表明,它们是在伸展构造背景下,受到不同级别的构造控制,大同岩墙群的研究对进一步认识华北早中生代岩石圈演化有重要的意义。     

A numerical simulation of residual circulation in Tampa Bay. Part I: Low-frequency temporal variations

The residual (time-average) salinity and circulation in a numerical ocean model of the Tampa Bay estuary are shown to experience significant temporal variation under realistic forcing conditions. A version of the Estuarine Coastal Ocean Model developed for Tampa Bay with 70 by 100 horizontal grid points and 11 sigma levels is examined for the years 2001–2003. Model output variables are averaged over the entire time of the simulation to generate long-term residual fields. The residual axial current is found to be dominated by the buoyancy-driven baroclinic circulation with an outflow (southwestward) at the surface and to the sides of the shipping channel, and an inflow (northeastward) usually occurring subsurface within or above the shipping channel. Averages over 30 d are used to examine variations in the residual fields. During the simulation the average surface salinity near the head of Tampa Bay varies with the freshwater inflow, from 12‰ to 33%. At the bay mouth salinity varies from 30%. to 36%.. A localized measure of the baroclinic circulation in the shipping channel indicates the residual circulation can vary strongly, attaining a magnitude triple the long-term mean value. The baroclinic circulation can be disrupted, going to near zero or even reversing, when the buoyancy-driven flow is weak and the surface winds are to the northeast. Three time periods, representing different environmental conditions, are chosen to examine these results in detail. A scaling argument indicates the relative strength of buoyancy versus wind as ΔρgH²(LC _Dω²)⁻¹, where δρ is head-to-mouth density difference across the bay,g is gravitational acceleration,H is depth,L is bay length,C _D is the surface wind drag coefficient, andw is wind speed. Tampa Bay is usually in the buoyancy dominated regime. The importance of winds in the weak-buoyancy case is demonstrated in an additional simulation without wind stress.     

Textural and geochemical alternations in Late Cenozoic Bahamian dolomites

Dolomitized intervals of a core from San Salvador Island, Bahamas, exhibit variations of two texturally and geochemically distinct end-members. In the Pliocene section of the dolomitized interval, the two end-members alternate in a pattern that may reflect originally and/or diagenetically modified depositional facies. Formerly mud-free intervals, locally capped by exposure surfaces are massive crystalline, mimetic dolomites (CM). Muddier sediments are replaced by friable microsucrosic dolomites (MS). CM and MS dolomites also differ in porosity (< 10% vs > 30%), permeability (< 10 md vs > 100 md), mol% MgCO₃ (44–9 vs 47–7) mol%), oxygen isotopic composition (1–7 vs 2–7‰) and strontium content (241 vs 106 ppm). These data indicate that depositional and diagenetic fabric are the principal controls governing the distribution of dolomite types. Differences in texture and geochemistry are suggested as arising through differential rates of crystallization produced as a result of variations in permeability and reactivity of the precursor sediments and rocks.     

Effusive and intrusive complexes in west of Sarysu-Teniz watershed

The Sarysu-Teniz watershed is on a large, east-west trending arch in western Kazakhstan, within the Caledonian folded province. The lithologies, thickness, and ages of the main rock units are tabulated. The data are interpreted as supporting the hypothesis that diverse cycles in the development of mobile zones may be identified on the basis of the sequence of igneous formations (both effusive and intrusive), their geochemistry and the associated metallogenesis, as well as by sedimentary relationships. Some data appear to suggest the existence of 1) separate, basic and acidic magma chambers for certain effusive formations, .2) common sources for effusive and intrusive acidic magmas, and 3) notable hybridization and differentiation. Geologic environment also appears to control or be reflected by the composition of magma. Metallogenesis similarly correlates with magma composition including hybridization and differentiation modifications. It seems on the basis of the now available data that prospecting for certain types of mineral deposits should be continued.—R. M. Dietrich.