Loss Of CaO Research Articles

Petrogenesis of the post-collisional rare-metal-bearing Ad-Dayheen granite intrusion, Central Arabian Shield

At Hadhb't Ad-Dayheen, in the central Arabian Shield, a post-collisional igneous complex called the Ad-Dayheen intrusion is exposed. It was emplaced in the Early Ediacaran (613–625 Ma), during the final tectono-magmatic stage of Arabian Shield development. Despite limited and discontinuous ring-shaped outcrops due to alluvial cover and later faulting, three pulses of intrusion can be recognized in the field: an early pulse of monzogranite; a second pulse of syenogranite and alkali feldspar granite; and a final pulse of alkaline and peralkaline granite, mineralized microgranite, and pegmatite. Samples show distinctively low contents of CaO, MgO, and Sr in contrast to elevated concentrations of alkalis, Rb, Nb, Y, Ta, Hf, Ga, Zr and rare-earth elements (REE); these are common characteristics of post-collisional rare-metal-bearing A-type granites. The suite displays positive Nb–Ta anomalies and pronounced negative Eu anomalies (Eu/Eu* = 0.11–0.35). The alkaline/peralkaline granites and microgranite of the Ad-Dayheen intrusion feature disseminated mineralization, whereas mineralization is localized in the pegmatite. The primary magma feeding the Ad-Dayheen intrusion was mostly generated by partial melting of the juvenile crust of the Arabian Shield, with a minor mantle contribution. We argue that an episode of lithospheric delamination led to crustal uplift, erosional decompression, and generation of mantle melts that supplied heat to drive crustal melting. The anatectic deep crustal melts assimilated a F-bearing component that also added rare metals to the magma. Each pulse can be described by a fractional crystallization model, but the parental liquid of each subsequent pulse was first modified by further addition of fluorine and rare metals and loss of CaO, Sr, Ba, and Eu due to fluorite fractionation. Texture and morphology of the ore minerals indicate that mineralization (U, Th, Zr, Nb, Ta, Y, Hf and REE) took place in two stages: a magmatic stage coinciding with emplacement of the intrusion, followed by a hydrothermal stage. The magmatic process enriched the residual melt in high field strength elements (HFSE) and REE. The later hydrothermal stage further localized these elements and increased their concentrations to economic grades. The pegmatite is highly mineralized and contains high concentrations of U (81–179 μg/g), Th (244–600 μg/g), Zr (2397–14,927 μg/g), Nb (1352–2047 μg/g), Ta (96–156 μg/g), Y (828–2238 μg/g), Hf (131–377 μg/g) and ∑REE (1969–4761 μg/g).

The lithogeochemical signatures of hydrothermal alteration in the Waihi epithermal district, New Zealand

ABSTRACTWhole rock lithogeochemical data from the andesite rocks which host low-sulfidation epithermal gold veins in the Waihi district, New Zealand have been used to interpret hydrothermal alteration processes that variably altered the andesite rocks. The andesite host rocks are interpreted to have similar primary chemical compositions throughout the Waihi area, as reflected in similar ratios of immobile elements (eg Zr, Ti and other high field strength elements). The geochemical data reveal that elevated K/Al ratios occur in rocks proximal to low-Au–Ag deposits, reflecting adularia alteration in the host rocks associated with the upflowing hydrothermal fluids. Silicification of host rocks is achieved both by the addition of SiO2, as well as the destruction of plagioclase (reflected in the loss of CaO and Na2O). The rocks have been variably altered, with calcite and pyrite abundance reflecting the addition of carbon and sulphur. Pathfinder element (eg As, Sb) concentrations are variable across the district, with high pathfinder element concentrations interpreted to reflect proximity to hydrothermal fluid flow. Together, the results provide a district-scale overview of hydrothermal alteration at Waihi, and emphasise the utility of lithogeochemistry for classifying and quantifying hydrothermal alteration that can then be used to interpret hydrothermal processes and assist mineral exploration.

Characteristics of chemical weathering and water–rock interaction in Lake Nyos dam (Cameroon): Implications for vulnerability to failure and re-enforcement

For the first time, comprehensive study of hydrogeochemistry of water seeps, role of chemical weathering on dam failure, estimation of minimum width of dam to resist failure and simulation of changes in dissolved ions and secondary mineral was conducted on the Lake Nyos dam. The salient results and conclusions were; the dam spring water represented a mixture of 60–70% rainwater and 30–40% Lake water (from 0 to −40m). The chemistry of the observed waters was Ca–HCO3 for rainwater, Ca–Mg–HCO3 in boreholes, and Mg–Ca–HCO3− for spring water. The relative rate at which ions dissolved in water was HCO3−>Mg2+>Ca2+>Na+>SiO2>K+>NO3−>SO42−>Cl−. Weathering of rocks resulted in the formation of clay minerals such as kaolinite and smectite. Relative mobility of elements compared to Alumina (Al2O3) indicated that in monzonites there was a loss of CaO, Na2O, K2O, P2O5 and gain of SiO2, Fe2O3, TiO2, MnO and MgO, while in basalts there was a loss of SiO2, Fe2O3, Ca2O, NaO, MgO and gain of TiO2, K2O and P2O5. Values of chemical alteration index that ranged from 49 to 82 suggest a weak to intermediate categories of chemical weathering that occurred at a rate of 5.7mm/year. Paired to that rate, which suggests that the dam is not vulnerable to failure at the previously thought time scale, some other processes (physical weathering, secondary mineral formation and lake overflow) can cause instant failure. Hydrostatic pressure of 1.6GN generated by Lake water can be supported only when the width of the dam is greater than 19m. PHREEQC-based simulation for 10years indicates decoupling of Ca and Mg, and Na and Mg. Multidisciplinary monitoring of the dam is advocated.

Preservation of Blueschist-facies Minerals along a Shear Zone by Coupled Metasomatism and Fast-flowing CO2-bearing Fluids

AbstractTwo types of blue halo (types I and II) composed of blueschist-facies minerals are centered around a brittle, normal shear zone in greenschist-facies rocks on the island of Syros, Aegean Sea, Greece. The shear zone is steeply dipping and cuts a near-horizontal layer of greenschist-facies rocks (albite + epidote + actinolite + chlorite + quartz). Type I and II blue haloes are 0·3 m and c. 1 m wide respectively, and are seen on both sides of the shear zone. The inner type I haloes are composed of nearly pure glaucophane schist and were formed by metasomatic addition of Na2O and SiO2, and to a lesser extent of K2O and large ion lithophile elements (LILE), coupled with loss of CaO, Al2O3 and MnO. The outer type II haloes consist of a carbonated blueschist-facies assemblage (glaucophane + calcite + phengite + epidote + garnet + quartz). These experienced only slight metasomatic changes (i.e. addition of K2O and LILE), which cannot alone explain halo formation. We present petrological, geochemical and thermodynamic evidence that this assemblage was preserved at greenschist-facies conditions because XCO2 was elevated by flow of a CO2-bearing fluid along the shear zone, which was approximately contemporaneous with greenschist-facies hydration in the surrounding rocks. We further note that the flux of CO2-bearing fluid along the shear zone was rapid with respect to the fluid flux in the surrounding rocks. Mass-balance calculations reveal that the fluid flux within the shear zone was at least 100–2000 times greater than the fluid flux within the surrounding rocks. Mineral textures show greenschist-facies minerals replacing blueschist minerals in the type II haloes, supporting our interpretation that the blueschist-facies minerals were preserved during greenschist-facies retrogression. A simplified P–T vs XCO2 pseudosection confirms that preservation of carbonated blueschist can occur at greenschist-facies conditions in the presence of a CO2-bearing fluid.

The Origin of Etch Pits Recorded on Residual Grain Surfaces from Kaolinized Granitic Rocks West Region Cameroon

The weathering of some granitic rocks in the West Region of Cameroon (Central Africa) was examined by X-ray diffraction (XRD) and X-ray fluorescence Spectrometer (XRF) in conjunction with Scanning Electron Microscopy (SEM). Kaolinite and illite were the most predominant weathering products whereas gibbsite, goethite and maghemite were also found in small amounts; only residual quantities of quartz and potassium feldspar were observed. The geochemical study shows the mobility of major elements during the progressive kaolinization of granitic rocks. The main trend is the loss of elements, with high losses of CaO, Na2O and MgO, slight losses of SiO2 while Al2O3 and K2O appear to increase with weathering intensity. Chemical index of alteration (CIA) shows a high value (68-95%). The scanning electron microscope analysis reveals the existence of several surface features that reflect the effect of mainly chemical actions on the quartz and K-feldspar grains. Quartz and K-feldspar crystals from kaolinized granitic rocks show low to very high chemical etching attributed to weathering under moderate to severe environmental conditions such as low pH, high temperature and humidity and extreme leaching typical of tropical climatic conditions. Etched pit densities on quartz surfaces increase with weathering, as a result of increasing silica concentrations in pore fluids.

Geochemical assessment of soils in districts of fluoride-rich and fluoride-poor groundwater, north-central Sri Lanka

High contents of fluoride in groundwater are a controversial issue in the dry zone of Sri Lanka. This study describes the geochemistry of residual soils from relatively fluoride-rich (<8mg/L; mean 2.0mg/L) and fluoride-poor (<1mg/L; mean 0.4mg/L) groundwater sites in the dry zone to identify possible sources for fluoride. Abundances of 22 major and trace elements were determined in 74 soil samples using X-ray fluorescence. The results show that soil fluoride is lower than average upper continental crust and basement rocks in both the fluoride-rich (<411mg/kg) and fluoride-poor (<277mg/kg) groundwater sites. Negative linear correlation exists between fluoride in the soil and the groundwaters, suggesting that fluoride is readily leached to water rather than being retained in the unconsolidated sandy clay loam soils. Weathering of heavy minerals such as zirconium, apatite, fluorite, monazite and garnet are the main source for the soil in the fluoride-rich groundwater districts. In these areas Zr, Nb and Th are immobile relative to the basement, and F, CaO and P2O5 are depleted, suggesting that the loss of CaO provides favorable conditions for the leaching of F to water. Conversely, soils in the relatively fluoride-poor district are enriched in TiO2, Fe2O3, MnO, Cr, V and Sc, denoting the weathering of biotite, hornblende, garnet and pyroxenes in the basement. Primary minerals present in the soils are the main cause for the enrichment of those elements. Further, fluoride levels in the soils and subsequently in the groundwaters show links with original magmatic contrast between the basement formations in each area. Soil geochemistry suggests that the meta-igneous rocks in the fluoride-rich districts may have been influenced by a fluoride-rich residual melt, whereas the fluoride-poor districts are associated with acidic meta-igneous rocks and meta-sedimentary rocks.

Large calcite and bulk-rock volume loss in metacarbonate xenoliths from the Quérigut massif (French Pyrenees)

Chemical mass transfer was quantified in a metacarbonate xenolith enclosed within the granodiorite of the Querigut massif (Pyrenees, France). Mass balance calculations suggest a strong decrease of CaO, SrO and CO2 contents (up to −90%), correlated with a decrease of modal calcite content as the contact is approached. Most other chemical elements behave immobile during metasomatism. They are therefore passively enriched. Only a small increase of SiO2, Al2O3 and Fe2O3 contents occurs in the immediate vicinity of the contact. Hence, in this study, skarn formation is characterized by the lack of large chemical element influx from the granitoid protolith. A large decrease of the initial carbonate volume (up to −86%) resulted from a combination of decarbonation reactions and loss of CaO and CO2. The resulting volume change has potentially important consequences for the interpretation of stable isotope profiles: the isotope alteration could have occured over greater distances than those observed today.

Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO 2 sink in the Early Archean

Early Archean (3.46 Ga) hydrothermally altered basaltic rocks exposed near Marble Bar, eastern Pilbara Craton, have been studied in order to reveal geological and geochemical natures of seafloor hydrothermal carbonatization and to estimate the CO 2 flux sunk into the altered oceanic crust by the carbonatization. The basaltic rocks are divided into basalt and dolerite, and the basalt is further subdivided into type I, having original igneous rock textures, and type II, lacking these textures due to strong hydrothermal alteration. Primary clinopyroxene phenocrysts are preserved in some part of the dolerite samples, and the alteration mineral assemblage of dolerite (chlorite + epidote + albite + quartz ± actinolite) indicates that the alteration condition was typical greenschist facies. In other samples, all primary minerals were completely replaced by secondary minerals, and the alteration mineral assemblage of the type I and type II basalts (chlorite + K-mica + quartz + carbonate minerals ± albite) is characterized by the presence of K-mica and carbonate minerals and the absence of Ca-Al silicate minerals such as epidote and actinolite, suggesting the alteration condition of high CO 2 fugacity. The difference of the alteration mineral assemblages between basalt and dolerite is probably attributed to the difference of water/rock ratio that, in turn, depends on their porosity. Carbonate minerals in the carbonatized basalt include calcite, ankerite, and siderite, but calcite is quite dominant. The δ 13C values of the carbonate minerals are −0.3 ± 1.2‰ and mostly within the range of marine carbonate, indicating that the carbonate minerals were formed by seafloor hydrothermal alteration and that carbonate carbon in the altered basalt was derived from seawater. Whole-rock chemical composition of the basaltic rocks is essentially similar to that of modern mid-ocean ridge basalt (MORB) except for highly mobile elements such as K 2O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K 2O, Rb, and Ba, and are depleted in Na 2O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO 2 enrichment is recognized in the basalt due to the ubiquitous presence of carbonate minerals, but there was essentially neither gain nor loss of CaO. This suggests that the CO 2 in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO 2 ratios of the basalt are generally the same as that of pure calcite, indicating that Ca in the basalt was almost completely converted to calcite during the carbonatization, although Mg and Fe were mainly redistributed into noncarbonate minerals such as chlorite. The carbon flux into the Early Archean oceanic crust by the seafloor hydrothermal carbonatization is estimated to be 3.8 × 10 13 mol/yr, based on the average carbon content of altered oceanic crust of 1.4 × 10 -3 mol/g, the alteration depth of 500 m, and the spreading rate of 1.8 × 10 11 cm 2/yr. This flux is equivalent to or greater than the present-day total carbon flux. It is most likely that the seafloor hydrothermal carbonatization played an important role as a sink of atmospheric and oceanic CO 2 in the Early Archean.

Geological Setting of the Meadowbank Gold Deposits, Woodburn Lake Group, Nunavut

The Meadowbank gold deposits are hosted by a series of polydeformed and metamorphosed auriferous iron formations located in the Neoarchean Woodburn Lake group, western Churchill province, Nunavut. The supracrustal rocks in the Meadowbank area consist of a thick sequence of intermediate-composition volcanic rocks with intercalated iron formation, ultramafic volcanic rocks, and quartzite to quartz arenite. U-Pb geochronology gives an interpreted age of 2711 ± 3 Ma for the host strata at Meadowbank. The intermediate volcanic rocks are predominately volcaniclastic and have a geochemical composition that is consistent with an active continental margin setting. Iron formation deposition was coeval with the intermediate volcanism, as indicated by incorporation of volcanic detritus in the chemical precipitate. The geochemistry of the iron formation indicates that it precipitated from a fluid that at one time was at high temperature (>250°C), reducing, and likely acidic. Intercalated ultramafic rocks rarely show spinifex textures and have trace element ratios that are consistent with mantle plume-related undepleted mantle asthenosphere, likely recording episodic mantle upwelling associated with rifting in an active continental margin. The contact between the ultramafic volcanic rocks and the overlying quartz arenite is a disconformity marked by a quartz pebble conglomerate. However, similar geology on the structural hanging wall and footwall of the contact suggests that the conglomerate does not represent a significant hiatus in deposition, and is more likely a prograding terriginous siliciclastic unit. Three metamorphic events are recognized. The first, M1, is a cryptic greenschist facies event that is pre-D2. The second event, M2, is a mid-greenschist to amphibolite facies, syn-D2 event. M3 is the last event recognized, and is a post-tectonic greenschist facies event that is regional but inhomogeneous in extent, possibly reflecting thermal aureoles around post-tectonic 1.7 to 1.8 Ga Nueltin granites. The structural geology of the area is complex with four phases of deformation recognized, two of which had a significant effect on the geometry of the deposit. All of these regional events are interpreted as Paleoproterozoic in age. Relationships between deformation fabrics and mineralization, as well as the overall geometry of the mineralized envelopes, suggest that the deposit formed during syn- to late-D2. Superimposed on the mineralization are D4 structural elements that postdate gold mineralization. The main control on gold mineralization is replacement of magnetite by pyrrhotite and pyrite in high-strain corridors. The composition of amphiboles and chlorites associated with gold mineralization is remarkably consistent and shows no spatial or temporal variation, suggesting that it was buffered by the iron-rich nature of the host rocks. The bulk metasomatic effect on the intermediate volcanic rocks is characterized by the addition of K2O and the loss of CaO, Na2O, and MgO, with little variation in the total iron content. The elemental losses are likely the result of destruction of feldspars, stilpnomelane, and ferroactinolite, which are relatively common in the unmineralized rocks, and the formation of sericite, chlorite, and grunerite. Textural and timing relationships suggest that the mineralization is syn- to late-D2, and by inference M2. Fluid inclusions, chlorite, and arsenopyrite geothermometry all suggest that mineralization took place at about 325° to 375°C and 1.3 kbar pressure. These are lower P-T conditions than is indicated by M2 mineral assemblages, suggesting that the deposit formed during the waning stages of metamorphism. Strain partitioning due to the mechanical contrast of the iron formation preferentially localized dilational settings during D2 deformation. This allowed fluid influx, sulfidation of the iron formation, and resulting precipitation of gold to be concentrated in high-strain corridors. Geologic relations and geochronology collectively suggest that a Paleoproterozoic (ca. 1.8–1.9 Ga) deformation event was responsible for the introduction of gold into the Neoarchean supracrustal sequences.

Diffusion in coronas around clinopyroxene: modelling with local equilibrium and steady state, and a non-steady-state modification to account for zoned actinolite-hornblende

Retrograde coronas of Caledonian age, between clinopyroxene and plagioclase in the Jotun Nappe Complex, Norway, illustrate the effects of diffusion kinetics on mineral distributions among layers and on the compositions of hornblende-actinolite. One corona type comprises a symplectite of epidote + quartz adjacent to plagioclase, and a less well-organized intergrowth of amphibole + quartz replacing clinopyroxene. The observed mineral proportions imply an open-system reaction, but the similarity of Al/Si ratios in reactant plagioclase and product symplectite indicates approximate conservation of Al2O3 and SiO2. The largest inferred open-system flux is a loss of CaO, mostly derived from consumption of clinopyroxene. The approximate layer structure, Pl|Ep + Qtz|Hbl + Qtz|Act±Hbl + Qtz|Cpx, is modelled using the theory of steady-state diffusion-controlled growth with local equilibrium. To obtain a solution, it is necessary to use a reactant plagioclase composition which takes into account aluminous (epidote) inclusions. The results indicate that, in terms of Onsager diffusion coefficients Lii, Ca is more mobile than AL (LCaCa/LAlAl≳3.) (where ≳ means greater than or approximately equal to). This behaviour of Ca is comparable with that of Mg in previously studied coronas around olivine. Si is non-diffusing in the present modelling, because of silica saturation. Oxidation of some Fe2+ to Fe3+ occurs within the corona. Mg diffuses towards its source (clinopyroxene) to maintain local equilibrium. Other coronas consist of two layers, hornblende adjacent to plagioclase and zoned amphibole + quartz adjacent to clinopyroxene. In the zoned layer, actinolitic hornblende forms relict patches, separated from quartz blebs by more aluminous hornblende. A preliminary steady-state, local-equilibrium model of grain-boundary diffusion explains the formation of low-Al and high-Al layers as due to Al immobility. Zoning and replacement are qualitatively explained in terms of evolution of actinolite to more stable aluminous compositions. This is modelled by a non-steady-state modification of the theory, retaining local equilibrium in grain boundaries while relatively steep zoning profiles develop in grain interiors through slow intracrystalline diffusion. Replacement of actinolite by hornblende does not require a change in P−T conditions if actinolite is a kinetically determined, non-equilibrium product. The common preservation of a sharp contact between hornblende and actionolite layers may be explained by ineffectiveness of intracrystalline diffusion: according to the theory, given sufficient grain-boundary Al flux, a metastable actinolite + quartz layer in contact with hornblende may be diffusionally stable and may continue to grow in a steady state.

Carbonate alteration in basalt, Total Erickson gold mine, Cassiar, northern British Columbia, Canada

The Total Erickson gold mine is in a sequence of upper Paleozoic to lower Mesozoic mafic volcanic, ultramafic, and sedimentary rocks of the Sylvester allochthon in the McDame lode gold camp, approximately 12 km southeast of Cassiar, British Columbia. Gold and silver occur in quartz veins that are mostly within mafic volcanic rocks. Rocks within the mine were affected by four types of alteration: synvolcanic hydrothermal, regional metamorphic, epigenetic hydrothermal, and contact metamorphic. The most common alteration type is epigenetic carbonation of basalt, which occurs as well-developed envelopes around gold-silver-bearing white quartz veins and carbon-rich veins. Younger dolomite veins also are surrounded by carbonate alteration envelopes. Disseminated and fracture-controlled elemental carbon is present locally in white quartz veins and locally is sufficiently abundant to form carbon veins.Rocks within carbonate alteration envelopes are composed of ankerite, siderite, dolomite, quartz, sericite, kaolinite, pyrite, carbon, and minor titanium oxides. A generalized model of mineral species distribution throughout the envelopes around white quartz veins, carbon veins, and dolomite veins is developed. Mass balance calculations indicate the following compositional changes around a gold-silver-bearing white quartz vein: gain of K 2 O and SiO 2 ; loss of Na 2 O, FeO, and MgO; and loss of CaO from the outer portion of the envelope and gain in the inner. Loss on ignition and the alteration mineral assemblage indicate gain of volatile components including, at least, CO 2 , H 2 O, and S.This redistribution of elements during alteration processes that are genetically related to gold quartz vein formation defines a primary dispersion halo, the documentation of which is a sound basis for confident interpretation of exploration lithogeochemical data.

Geochemistry of Gneiss-Granulite transformation in the ?incipient charnockite? zones of southern India

Upper amphibolite facies gneisses in the southern Indian Shield show local transformation into veins, clots and patches of orthopyroxene-bearing dry granulites (“incipient charnockites”). Depending upon the protolith composition, these desiccated zones are classified into ortho- and para-charnockites and have developed within rocks of distinct mineralogy and chemistry at different time intervals through the structurally-controlled influx of carbon dioxide-rich fluids. Our geochemical investigations at five critical quarry sections indicate that the incipient charnockites have undepleted chemistry and very low K/Rb values. In the paracharnockite localities, where granulite formation is characterized by consumption of garnet, biotite and quartz to produce orthopyroxene, loss of Rb and Ba and enrichment of Ti are observed. In contrast, the orthocharnockite localities show marked LILE enrichment with gain of K, Rb and Ba and loss of CaO, suggesting extensive replacement of plagioclase in the gneisses by K-feldspar in the charnockite through K-Na-Ca exchange reactions with influxing carbonic fluids. The marked depletion in Fe, Mg, Ti and P in these rocks correlates with progressive dissolution of hornblende, biotite, magnetite and accessory apatite. Our study indicates that gneiss to granulite transformation, even if on a local scale, is not an isochemical phenomenon, but attended by distinct element mobilities, although they are contrastingly different from the geochemical trends in some regional high grade terrains.

Stratigraphy and alteration of the host rocks, United Verde massive sulfide deposit, Jerome, Arizona

The early Proterozoic volcanogenic United Verde massive sulfide orebody is situated within the Cleopatra Member of the Deception Rhyolite. The Cleopatra Member, a porphyritic rhyodacite, has been subdivided into an upper unit and a lower unit. The United Verde orebody, previously interpreted to lie at the top of the Cleopatra Member, is now recognized as lying within the basal section of the upper unit of the Cleopatra Member.The upper unit and lower unit are differentiated on the basis of alteration mineralogy, mineral and rock geochemistry, and lithologic peculiarities. The contact between the two units is marked by sediments at the base of the upper unit, including a distinct green calcitebearing feldspar tuff, chert and hematitic chert layers, and a quartz porphyry conglomerate.The Cleopatra Member contains six distinct alteration types. Five alteration types characterize the lower unit and include two types of moderate chloritic and sericitic alteration (designated B1 and B2), two spatially distinct types of sericitic and silicic alteration (designated S1 and S2), and an area of intense chloritic alteration (designated C). These five alteration types record the effects of hydrothermal alteration by the massive sulfide ore-forming system and of a regional greenschist facies metamorphic event. The upper unit contains the least altered rocks of the Cleopatra Member and the bulk of the sixth alteration type, designated H. The rocks included within H-type alteration have disseminated hematite and/or hematitic chert veining.Whole-rock and mineral geochemistry of the lower unit may be used to establish position within the palcohydrothermal system. The distal area of moderate alteration (B1) contains samples enriched in MgO, slightly depleted in alkalies, and having ratios of Fe 2 O (sub 3 (sub (total)) ) /Fe 2 O (sub 3 (sub (total)) ) + MgO for whole rocks and Fe (sub (total)) /Fe (sub (total)) + Mg for chlorites that are low relative to other Cleopatra Member samples. These attributes are suggestive of the low-temperature seawater-rock interaction expected in the domain of fluid recharge. Rocks of alteration type C experienced significant additions of iron and magnesium, and losses of CaO, Na 2 O, and K 2 O, presumed to reflect alteration in the main discharge conduit for the hydrothermal fluid. Chlorite and whole-rock iron to iron-plus-magnesium ratios decrease within the area of alteration type C toward the sediment-water interface. This is consistent with mixing of an evolved ore-forming fluid with seawater in the upper levels of the area of alteration type C. Samples from the area of alteration type B2, which is intermediate between the recharge (B1) and discharge areas (C), are greatly depleted in CaO and Na 2 O, and are enriched in iron. Alteration types S1 and S2 are interpreted to reflect localized sites of fluid discharge where SiO 2 and K 2 O have been added and other major elements, except for Al 2 O 3 , have been removed.Whole-rock and mineral chemistry of the alteration types record progressive changes in the hydrothermal fluid from seawater to an ore-forming fluid. The fluid that was discharging and depositing ore apparently had a log a (sub O 2 ) between -41.7 and -35.5, a log a (sub H 2 S) between -5 and -2.6, and was saturated with respect to iron sulfides, magnesium silicates, and ore-forming metals.

Proterozoic contact relationships between gneissic basement and metasedimentary cover in the Needle Mountains, Colorado, U.S.A.

In the Needle Mountains of southwestern Colorado, Proterozoic siliciclastic metasedimentary rocks of the Uncompahgre Group occupy a synclinorium bounded by 1690 + Ma gneissic and plutonic basement rocks. Stratigraphic facing of the Uncompahgre Group away from basement and local preservation of erosional features along the basement—cover interface imply that the contact is an unconformity. A ≤12 m thick zone of phyllite, which commonly contains textural evidence of derivation from basement lithologies, occurs everywhere along the contact. Tectonic fabrics within and near the contact zones locally record non-coaxial deformation that resulted from upward movement of the cover relative to basement on each side of the synclinorium. Comparison of bulk chemistries of basement and contact zone samples indicates that hydration and the loss of CaO, MgO, SiO 2 and Na 2O accompanied alteration of plagioclase + quartz + biotite-bearing basement rocks to muscovite + quartz + Fe/Ti-oxide ± choritoid ± andalusite ± chlorite phyllites. The micaceous zone along the contact is interpreted as a metamorphosed regolith that localized out-of-synform movement during folding of the basement and its parautochthonous cover. Localization of intense deformation within mechanically weak lithologic horizons, such as regoliths, along basement—cover contacts can occur in situations where there is little relative displacement of the adjacent rocks and the stratigraphic significance of the boundary is preserved. Detailed petrographic and kinematic analysis may help distinguish such contacts from major fault boundaries.

The greenschist metamorphism of Archaean synvolcanic stocks near Wawa, Ontario, Canada

Abstract Two Archaean synvolcanic stocks with contact aureoles occur in the Wawa greenstone belt near Wawa, Ontario, Canada. The Gutcher Lake and Jubilee stocks consist mainly of granitoid trondhjemite with feldspar laths mottled by white mica + calcite + epidote and rimmed by clear albite. Biotite is partly or wholly pseudomorphosed by chlorite + sphene; some epidote is partly altered to calcite + chlorite. The granitoid phase grades into a foliated phase of quartz + albite + white mica + calcite + chlorite near fracture zones traversing the stocks.The alteration of the Gutcher Lake stock along its foliated margin involved addition of K2O, H2O + CO2, MnO, plus Rb; loss of CaO plus Sr; and a shift in Fe+2/Fet from 0.66 to 0.81. The alteration of the Jubilee stock along the Darwin Shear involved addition of H2O + CO2; loss of Sr; and no significant shift in Fe+2/Fet. The greenschist alteration also modified the contact aureoles bordering both stocks.One interpretation is that regional metamorphism in the Archaean overprinted a greenschist assemblage on both stocks. The alteration was intense near fracture zones and sporadic remote from fractures. Lower integrated water to rock ratios along the Darwin Shear compared to the margin of the Gutcher Lake stock may explain the comparatively lower perturbation of the element abundances and redox state of iron.

Serpentinization reaction responsible for rodingite formation of the Ashidachi ultramafic complex, Southwest Japan.

The Ashidachi ultramafic complex, highly serpentinized, is emplaced into the prehnite-pumpellyite grade rocks in the Sangun metamorphic belt, Japan. A number of small gabbroic bodies enclosed in the serpentinite are ubiquitously rodingitized along their margins. Progressive chemical change of rodingite zone indicates introduction of CaO from ultramafic rocks. Selected serpentinites, derived from clinopyroxene-bearing harzburgitic peridotite, were nalyzed for CaO content and density. The relationship between the CaO content and density suggests that the serpentinization proceeded, at least, in two different reaction steps; an early step of serpentiniza-tion occurred without much loss of CaO, whereas in a later step CaO was depleted to a significant extent. The depletion rate of CaO in the later step of serpentinization is ascribed to the olivine-clinopyroxene decomposition reaction. Ca-OH type fluids roduced by the reaction are respon-sible for the rodingite formation. The Ca-OH type fluids, essentially incompatible with a CO2 component, are in harmony with the fact that rodingite minerals typically are suggestive of CO2-poor fluids. Such CO2-poor metasomatic fluids might have been produced as well as maintained by the CO2-buffering fluid regimes of the serpentinization with formation of talc-carbonate selvages under the regional metamorphism of the Sangun belt.

Properties of refractories after exposure to high pressure gases: VI-vaporvs liquid contact

The properties of several cement-bonded castables and a phosphate-bonded ramming mix were determined before and after exposure to saturated vapor or immersed in liquid in pure steam, CO-steam, and a gas mixture containing H2O, CO, CO2, CH4, H2, and H2S, at pressures and temperatures anticipated at the cold face of the refractory lining in coal gasifiers. Chemical reactions occurring in the refractories were determined by XRD, DTAITGA and correlated with properties. No major difference in properties was found between castables immersed in liquid and those exposed to saturated vapor. However, the loss of CaO which occurred both in liquid and saturated vapor, seemed to be more in liquid than vapor and should be considered in long term service performance.

跡津川断層に伴なう断層内物質 Ｉ

Intrafault materials derived from two localities within the Atotsugawa fault of Central Japan have been investigated to elucidate the characteristics of the fault. In each fractured zone within the fault, two different zones are clearly recognized; the one zone is composed mainly of very fine fragments and developing limitedly both side of the fractured zone in contact with parent rocks, and the other is composed mainly of coarse fragments and constituting the main part of fractured zone.Textures of the intrafault materials are in random arrangement of fragments showing rounded-angular shape, or in complete arrangement of fragments showing lenticular-anastomising shape. The grain-size frequency curves indicate no peakes or the remarkable peak of d=10-3 μm. Quartz grains within the intrafault materials under the scanning electron microscope have smooth surfaces keeping breakage or fracture surface, or small undulated, a little corroded surface.The mineralogy of 2μ<fractions from the intrafault materials is principally including illite, chlorite and montmorillonite, and occasionally with quartz and feldspar. Geochemically, within the zone composed mainly of very fine fragments, loss of SiO2, Na2O and K2O, addition of H2O+, H2O- and Total Fe, addition or loss of CaO have been relatively found, comparing with composition of parent rocks.

Low temperature fluid alteration of oceanic layer 2 basalts, DSDP Leg 37

Penetration of cold seawater into layer 2 of the oceanic crust occurs to at least 600 m, the maximum depth drilled below the sediment-acoustic basement boundary during Leg 37 of the DSDP. The main alteration phases are Mg-saponite, Fe-K rich celadonite, phillipsite, calcite, and hematite. The only exception to this occurs in the form of intensely hydrothermally altered gabbro breccias from an intrusive complex at site 334. There is no mineralogical evidence to suggest an increase in metamorphic grade with depth; however, a variation in mineralogy and alteration intensity, occurs as a function of structure, permeability, and the chemical nature of rock-fluid interaction. On this basis three types of alteration are defined as follows: (A) fracture focussed, oxidative; (B) palagonitic; and (C) non-oxidative, pervasive.Electron microprobe analyses reveal that alteration of fresh glass to palagonite involves the addition of H2O, an increase in total Fe, K2O, TiO2, and possibly SiO2, and the loss of CaO, MgO, Na2O, and MnO.Detailed examination of the mineralogy and chemistry of oxidative alteration suggests that during low temperature sea water–basalt interaction, basalt experiences a net gain in CaO, total Fe and K2O, while SiO2 and MgO appear to have been locally remobilized. These trends are generally consistent with the distribution of secondary phases. The distributions of Mn, Cu, Ni, Zn, Co, and Sr do not appear to have been significantly affected during this process.It is apparent that seawater must have experienced significant Eh lowering during this interaction; however, the occurrence of hematite–magnetite and absence of sulphide in this particular environment requires that seawater was either highly depleted in total sulfur or experienced a significant rise in pH.

SOME PYROXENES ASSOCIATED WITH PYROMETASOMATIC ZINC DEPOSITS IN MEXICO AND NEW MEXICO

Optical properties and chemical analyses are presented for 15 pyroxenes and amphiboles from zinc mines in Mexico and New Mexico. These analyses indicate that the characteristic pyroxenes in the pyrometasomatic zinc deposits examined by the writers in Mexico and New Mexico are manganoan hedenbergite and manganoan magnesian hedenbergite. At some localities johannsenite and ferroan johannsenite were formed, and later these were replaced by rhodonite coming from surges of manganese that were supplied from the invading magma of stocks, visible or concealed. Zoned birefringent garnets (andradite) were formed early in the silicate stage. On the basis of the temperature at which andradite from the Pewabic mine loses its birefringence on heating, the early stage of silicate deposition began at a temperature below 860°C. Later, at a lower temperature, possibly about 550°C., uralitization of the pyroxenes took place with a great loss of CaO and a gain in SiQ 2 , FeO, Al 2 O 3 , and MgO. The variety of uralite formed at the three mines where samples were analyzed is too low in CaO to be actinolite, and its chemical composition suggests cummingtonite. This is the first reported occurrence of cummingtonite formed by the uralitization of a calcium pyroxene. The sulfide ore minerals, sphalerite and pyrite, and also some magnetite are later than the silicate minerals and were formed by hydrothermal solutions at a somewhat lower temperature than that of the cummingtonite.