Geochemical mass balance and weathering rates of ultramafic schists in Amazonia

The hydrochemical balance of the Yaou catchment in French Guiana was determined over a period of 1 year, combined with a detailed characterization of the primary minerals and their weathering products, in order to estimate ultramafic rock weathering rates in a rainforest environment. The time required to develop the main horizons of the laterite profile was obtained from estimations of the weathering rates and dissolution kinetics of some major parent minerals (chlorite, serpentine, biotite, calcite).

The specific transport of solute matter in the catchment is 205 kg/ha/a. The Cl and NO₃ net outputs shows that the system is in dynamic equilibrium, notably with respect to the biomass reservoir. The output flux of Mg in solution is mainly supplied from the weathering of primary minerals, whereas that of Ca comes both from atmospheric contributions and from the dissolution of carbonates. The fluxes of K and, more particularly, Na derive essentially from the atmosphere. Knowing the weathering rate of primary minerals, the total CO₂ consumption rate by silicate weathering is estimated at 1430 mol/ha/a.

The weathering rates of chlorite, serpentine and biotite range from 18 to 65 mol/ha/a, and those of talc and calcite are, respectively 51 and 153 mol/ha/a. Weathering rates normalized to mineral modal proportions give a decreasing order of stability resistance to weathering (calcitebiotitechlorite>serpentine>talc) that agrees with their vertical distribution in the weathering profile. The dissolution kinetics of chlorite, biotite and serpentine, expressed in relation to the Si release rate, were calculated using estimations of the mineral exchange surface by (a) optical microscope image processing of crystal outlines, and (b) BET specific surface measurements. The release rate of Si, computed for biotite, chlorite and serpentine, ranges around 10⁻¹³ and 10⁻¹⁴ mol/m²/s using microscope images on particle sizes. The estimated dissolution kinetics is two orders of magnitude lower when using the BET measured exchange surface, which is 100 times larger.

The saprolitization rate, calculated from the weathering rates of calcite, chlorite and biotite, is on average 7.5 m/Ma. The rate of latosol development, calculated from the weathering rate of serpentine at the saprolite–latosol interface, is estimated at 4.5 m/Ma. That means that the profile is chemically thickening at a rate of 3 m/Ma.     

The mineral chemistry of hydrothermally altered and metamorphosed wall-rocks at the Stollberg Fe-Pb-Zn-Mn(-Ag) deposit,Bergslagen, Sweden

The c. 1.9 Ga old Stollberg sulphide and Mnrich skarn iron ores and sulphide ores in Bergslagen, south-central Sweden are hosted by hydrothermally altered and metamorphosed felsic volcanic and volcaniclastic rocks. The ores are underlain by comformable alteration zones characterized by albite-gedrite-quartz and biotite-muscovite-plagioclase-K-feldspar-quartz +/– garnet assemblages. The present mineralogies are interpreted as medium-grade metamorphic equivalents to the original alteration mineral assemblages. PT-conditions during prograde regional metamorphism are semiquantatively determined to be 510 to 560 °C at approximately 3 kbar. With increasing modal content of gedrite and biotite in the alteration zones, the Mg/Fe ratios and X_Mg's in octahedral positions of these minerals also increase. In the gedrite-bearing strata, whole-rock Mg/Fe ratios remain constant, whereas in the biotite-rich unit the wholerock Mg/Fe trend is parallel to that of the biotites.The trends in the metamorphic mineral composition are interpreted to be a product of original changes in fluid composition during the evolution of a sub-seafloor hydrothermal system. During the initial stage of alteration, Fe-Mn-rich fluids altered the rocks, and during a later stage, the fluids became more Mg-rich, possibly due to entrainment of fresh seawater, and the alteration zones became relatively more Mg-rich. Sulphide precipitation was contemperaneous with Mg metasomatism, suggesting base metal precipitation was a function of the mixing of cool seawater with hydrothermal fluid. It is proposed that early hydrothermal alteration was associated with the deposition of areally extensive Fe-oxide formation, and that Mg metasomatism defines a second stage of hydrothermal activity during which sulphide mineralization overprinted the earlier formed Fe-oxide deposit.     

Chemical mineralogy of some cordierite-bearing rocks near Yellowknife,Northwest Territories,Canada

Meta-graywacke and meta-argillite of Archean age near Yellowknife contain biotite, cordierite, gedrite and sillimanite isograds towards the Sparrow Lake granite pluton. The chemistry of biotite, cordierite, gedrite and garnet in rocks that up-grade from the cordierite isograd indicate a small range of chemical composition, particularly with reference to Mg, Fe and Mn. The analyses show further that among the coexisting ferromagnesian minerals Fe/Fe+ Mg ratio decreases in the sequence: garnet, gedrite, biotite, cordierite while Mn/Fe+Mg+Mn ratio decreases in the sequence garnet, gedrite, cordierite, biotite. The same order is also observed in the distribution diagrams. The regular distribution of Mg, Fe and Mn among the coexisting phases demonstrate that chemical equilibrium was attained and preserved in these Archean rocks. Mg-Fe distribution between cordierite and biotite appears to be dependent on the temperature of crystallization or metamorphic grade.     

Characteristic Features of REE and Pb–Zn–Ag Mineralizations in the Na Son Deposit,Northeastern Vietnam

The Na Son deposit is a small‐scale Pb–ZnPb–Zn–Ag deposit in northeast Vietnam and consists of biotite–chlorite schist, reddish altered rocks, quartz veins and syenite. The biotite–chlorite schist is intruded by syenite. Reddish altered rocks occur as an alteration halo between the biotite–allanite‐bearing quartz veins and the biotite–chlorite schist. Allanite occurs in the biotite–allanite‐bearing quartz veins and in the proximal reddish altered rocks. Rare earth element (REE) fluorocarbonate minerals occur along fractures or at rim of allanite crystals. The later horizontal aggregates of sulfide veins and veinlets cut the earlier reddish altered rocks. The earlier Pb–Zn veins consist of a large amount of galena and lesser amounts of sphalerite, pyrite and molybdenite. The later Cu veins cutting the Pb–Zn veins include chalcopyrite and lesser amounts of tetrahedrite and pyrite. The occurrences of two‐phase H₂O–CO₂ fluid inclusions in quartz from biotite–allanite‐bearing quartz veins and REE‐bearing fluorocarbonate minerals in allanite suggest the presence of CO₂ and F in the hydrothermal fluid. The oxygen isotopic ratios of the reddish altered rocks, biotite–chlorite schist, and syenite range from +13.9 to +14.9 ‰, +11.5 to +13.3 ‰, and +10.1 to +11.6 ‰, respectively. Assuming an isotopic equilibrium between quartz (+14.6 to +15.8 ‰) and biotite (+8.6 ‰) in the biotite–allanite‐bearing quartz vein, formation temperature was estimated to be 400°C. At 400°C, δ¹⁸O values of the hydrothermal fluid in equilibrium with quartz and biotite range from +10.5 to +11.7 ‰. These δ¹⁸O values are consistent with fluid that is derived from metamorphism. Assuming an isotopic equilibrium between galena (+1.5 to +1.7 ‰) and chalcopyrite (+3.4 ‰), the formation temperature was estimated to be approximately 300°C. The formation temperature of the Na Son deposit decreased with the progress of mineralization. Based on the geological data, occurrence of REE‐bearing minerals and oxygen isotopic ratios, the REE mineralization is thought to result from interaction between biotite–chlorite schist and REE‐, CO₂‐ and F‐bearing metamorphic fluid at 400°C under a rock‐dominant condition.     

Cordierite-Cummingtonite Facies Rocks from the Gold Brick District, Colorado

The unusual association of cordierite and cummingtonite (? gedrite+ chlorite + biotite + ilmenite + plagioclase + quartz) definesa metamorphic facies within aluminous, low-Ca amphibolites fromthe Proterozoic rocks of the Gold Brick District, east of Gunnison,Colorado. More Fe-rich bulk chemistries in the same facies arecharacterized by assemblages consisting of cordierite+-gedrite+ garnet + chlorite + biotite + ilmenite + plagioclase + quartz,whereas more Mg-rich compositions are characterized by cordierite+ anthophyllite + chlorite + biotite + ilmenite ? plagioclase+ quartz. The assemblage gedrite 4- cummingtonite + chlorite+ biotite + ilmenite + plagioclase + quartz was also observed.Coexisting cordierite+ anthophyllite + cummingtonite was notobserved in any rocks, apparently because this assemblage isstable over only a very narrow range of bulk compositions. Metamorphosedpelitic rocks are more iron rich than the assemblage cordierite+ gedrite + garnet + chlorite + biotite + ilmenite + plagioclase+ quartz and consist of garnet ?cordierite ?staurolite ? chlorite? andalusite + biotite + ilmenite + plagioclase + quartz? microclineor muscovite. Mineral rim compositions from cordierite-bearing amphibolitesand metapelites determined by electron microprobe analysis showsystematic Fe/Mg partitioning and define assemblages that occupynon-overlapping regions of the compositional system SiO₂-TiO₂-Al₂O₃-MnO-FeO-MgO-CaO-Na₂O-K₂O-H₂Oas determined by algebraic and statistical methods developedby Braun & Stout (1975) and Fisher (1989). Graphical methods(projections) produced spurious overlaps not confirmed by themore rigorous algebraic tests. The spurious overlaps were generatedbecause standard projective analysis was not able simultaneouslyto account for the important effects of the components Na₂O,CaO, and MnO on the AFM topologies. The results of algebraicand statistical analysis are consistent with an equilibriumorigin at constant values of temperature and pressure. The cordierite-cummingtonite facies encompasses the relativelylow-pressure and moderate-temperature conditions associatedwith the stability field of andalusite. Garnet-biotite geothermo-metry,and garnet, aluminosilicate, silica, plagioclase (GASP) geobarometrysuggest that temperatures and pressures were nearly constantacross the study area at 550( ? 70) ?C and 3 kb, respectively,near the peak of metamorphism. Other geothermometers and geobarometers,and independent pressure and temperature estimates, are compatiblewith garnet-biotite thermometry and GASP geo-barometry. Gradientsin fO₂ or H₂O are not required to explain the compatibilityof these assemblages at constant T and P. Cordierite + cummingtonite-bearingrocks can apparently be derived from anthophyllite +garnet-bearingrocks by increasing temperature or decreasing pressure.     

Orthopyroxene–sillimanite–sapphirine granulites from the Bamble granulite terrane, southern Norway

Silica-deficient sapphirine-bearing rocks occur as an enclave within granulite facies Proterozoic gneisses and migmatites near Grimstad in the Bamble sector of south-east Norway (Hasleholmen locality). The rocks contain peraluminous sapphirine, orthopyroxene, gedrite, anthophyllite, sillimanite, sapphirine, corundum, cordierite, spinel, quartz and biotite in a variety of assemblages. Feldspar is absent.
Fe²⁺/(Fe²⁺+ Mg) in the analysed minerals varies in the order: spinel > gedrite ≥ anthophyllite ≥ biotite > sapphirine>orthopyroxene > cordierite.
Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene-sillimanite-sapphirine-cordierite-corundum developed during a high-pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium-pressure granulite facies generated various sapphirine-bearing assemblages. The diagnostic assemblage of this stage is sapphirine-cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum.
The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H₂O-poor conditions.     

Phase Equilibria of Amphibolites from the Post Pond Volcanics, Mt. Cube Quadrangle, Vermont

Amphibolites of the Post Pond Volcanics, south-west corner ofthe Mt. Cube Quadrangle, Vermont, are characterized by a greatdiversity of bulk rock types that give rise to a wide varietyof low-variance mineral assemblges. Original rock types arebelieved to have been intrusive and extrusive volcanics, hydrothermallyaltered volcanics and volcanogenic sediments with or withoutadmixtures of sedimentary detritus. Metamorphism was of staurolite-kyanitegrade. Geothermometry yields a temperature of 535 ± 20°C at pressures of 5–6 kb. Partitioning of Fe and Mg between coexisting phases is systematic,indicating a close approach to chemical equilibrium was attained.Relative enrichment of Fe/Mg is garnet > staurolite >gedrite > anthophyllite cummingtonite hornblende > biotite> chlorite > wonesite > cordierite dolomite > talc;relative enrichment in Mn/Mg is garnet > dolomite > gedrite> staurolite cummingtonite > hornblende > anthophyllite> cordierite > biotite > wonesite > chlorite >talc. between coexisting amphiboles varies as a function ofbulk Fe/Mg, which is inconsistent with an ideal molecular solutionmodel for amphiboles. Mineral assemblages are conveniently divided into carbonate+ hornblende-bearing, hornblende-bearing (carbonate-absent)and hornblende-absent. The carbonate-bearing assemblages allcontain hornblende + dolomite+ calcite + plagioclase (andesineand/or anorthite) + quartz with the additional phases garnetand epidote (in Fe-rich rocks) and chlorite ± cummingtonite(in magnesian rocks). Carbonate-bearing assemblages are restrictedto the most calcic bulk compositions. Hornblende-bearing (carbonate absent) assemblages occur in rocksof lower CaO content than the carbonate-bearing assemblages.All of these assemblages contain hornblende + andesine ±quartz + Fe-Ti oxide (rutile in magnesian rocks and ilmenitein Fe-rich rocks). In rocks of low Al content, cummingtoniteand two orthoamphiboles (gedrite and anthophyllite) are common.In addition, garnet is found in Fe-rich rocks and chlorite isfound in Mg-rich rocks. Several samples were found that containhornblende + cummingtonite + gedrite + anthophyllite ±garnet +chlorite + andesine + quartz + Fe-Ti oxide ±biotite. Aluminous assemblages contain hornblende + staurolite+ garnet ± anorthite/bytownite (coexisting with andesine)± gedrite ± biotite ± chlorite ±andesine ± quartz ± ilmenite. Hornblende-absentassemblages are restricted to Mg-rich, Ca-poor bulk compositions.These rocks contain chlorite ± cordierite ± staurolite± talc ± gedrite ± anthophyllite ±cummingtonite ± garnet ± biotite ± rutile± quartz ± andesine. The actual assemblage observeddepends strongly on Fe/Mg, Ca/Na and Al/Al + Fe + Mg. The chemistry of these rocks can be represented, to a firstapproximation, by the model system SiO₂–Al₂O₃–MgO–FeO–CaO–Na₂O–H₂O–CO₂;graphical representation is thus achieved by projection fromquartz, andesine, H₂O and CO₂ into the tetrahedron Fe–Ca–Mg–Al.The volumes defined by compositions of coexisting phases filla large portion of this tetrahedron. In general, the distributionof these phase volumes is quite regular, although in detailthere are a large number of phase volumes that overlap otherphase volumes, especially with respect to Fe/Mg ratios. Algebraicand graphical analysis of numerous different assemblages indicatethat every one of the phase volumes should shift to more magnesiancompositions with decreasing µ_H2O. It is therefore suggestedthat the overlapping phase volumes are the result of differentassemblages having crystallized in equilibrium with differentvalues of µ_H2O or µ_CO2 and that the different valuesmay have been inherited from the original H₂O and CO₂ contentof the volcanic prototype. If true, this implies that eithera fluid phase was not present during metamorphism, or that fluidflow between rocks was very restricted.     

Phase equilibria in the hornblende-bearing basic gneisses of the Uvete area, central Kenya

Abstract The hornblende-bearing basic gneisses in the Uvete area, central Kenya, were metamorphosed under a narrow range of P and T (6.5 ± 0.5kbar and 530 ± 40°C) of the staurolitekyanite zone in the Mozambique metamorphic belt. They show a wide variety of divariant and trivariant mineral assemblages consisting of hornblende, cumminatonite, gedrite, anthophyllite, chlorite, garnet, epidote, clinopyroxene, plagio-clase and quartz. The bulk and mineral chemistries and the graphical representation of phase relations show that each mineral assemblage approaches chemical equilibrium and defines a unique composition volume in the A'(Al + Fe³⁺− (13/7)Na)-F(Fe²⁺)-M'(Mg)-C'(Ca-(3/7)Na) tetrahedron. The composition volumes are distributed quite regularly and do not overlap each other.
The phase relations in the Uvete area are in contrast with those in the staurolite-kyanite zone amphibolites in the Mt. Cube quadrangle, Vermont. The amphibolites there contain low-variance mineral assemblages formed under different values of μ_H2O and μ_CO2. These assemblages define overlapping composition volumes in the A'-F'-M'-C'tetrahedron.
The mineral assemblages in the Uvete area are interpreted as having formed in equilibrium with fluid at a high and nearly constant μH₂O value. Such a fluid composition was externally controlled by the supply of H₂O-rich fluid expelled from the surrounding pelitic and psammitic rocks. The body size of the basic gneisses in the Uvete area (less than 400m in thickness) was small enough for the fluid to migrate completely.     

Metamorphism of the Basement of the Qilian Fold Belt in the Minhe-Ledu Area, Qinghai Province, NW China

The basement of the central Qilian fold belt exposed along the Minhe-Ledu highway consists of psammitic schists, metabasitic rocks, and crystalline limestone. Migmatitic rocks occur sporadically among psammitic schist and metabasitic rocks. The mineral assemblage of psammitic schist is muscovite + biotite + feldspar + quartz ± tourmaline ± titanite ± sillimanite and that of metabasitic rocks is amphibole + plagioclase + biotite ± apatite ± magnetite ± pyroxene ± garnet ± quartz. The migmatitic rock consists of leucosome and restite of various volume proportions; the former consists of muscovite + alkaline feldspar + quartz ± garnet ± plagioclase while the latter is either fragments of psammitic schist or those of metabasitic rock. The crystalline limestone consists of calcite that has been partly replaced by olivine. The olivine was subsequently altered to serpentine. Weak deformations as indicated by cleavages and fractures were imposed prominently on the psammitic schists, occasionally on me     

P-T-X (CO2) conditions in mafic and calc-silicate hornfelses from Oberon, New South Wales, Australia

Abstract The Rockley Volcanics from near Oberon, New South Wales occur within the aureole of the Carboniferous Bathurst Batholith and have been contact metamorphosed at P ∼ 100 ± 50MPa (10.5kbar) and a maximum T ∼ 565°C in the presence of a C–O–H fluid. Prior to contact metamorphism the volcanics were regionally metamorphosed and altered with the extensive development of actinolite, chlorite, plagioclase, quartz and calcite. The contact metamorphosed equivalents of these rocks have been subdivided into: Ca-poor (cordierite + gedrite), Mg-rich (amphibole + olivine + spinel), mafic (amphibole + plagioclase) and Ca-rich (amphibole + garnet + diopside; diopside + plagioclase; garnet + diopside + wollastonite) rocks.
The chemistry of the minerals in the hornfelses was controlled by the bulk rock chemistry and fluid composition. Pargasites and hastingsites as well as an unusual phlogopite with blue green pleochroism, are found in Ca-rich hornfelses. A comparison of the assemblages with experimentally derived equilibria suggests that the fluid phase associated with the Ca-rich hornfelses was water-rich (X_co2= 0.1 to 0.3) while that associated with the Mg-rich hornfelses was enriched in CO₂ (X_co2 > 0.7). The different hornfels types have reacted to contact metamorphism independently in both their solid and fluid chemistries.     

庐江沙溪斑岩型铜金矿床绿泥石的地球化学特征及找矿指示

文章在前人对岛弧环境斑岩型矿床绿泥石主微量元素找矿指示研究的基础上,以陆内环境的长江中下游成矿带中的沙溪斑岩型铜金矿床为对象,利用LA-ICP-MS技术对沙溪斑岩型铜金矿床中绿泥石进行了主微量元素研究。结果显示,绿泥石中Ti、Ba、Co、K、Pb、Sr、Fe、V/Ni靠近矿化中心的位置含量高,Mn、Mg元素远离矿化中心的位置含量高。沙溪斑岩型铜金矿床中绿泥石元素含量分布主要受温度、被交代矿物、流体pH值和氧化还原环境、围岩性质影响。受交代矿物的影响,绿泥石中的有些元素(Si、Na、Mg、K、Al)的含量高低不能直接对矿化中心进行指示,但沙溪斑岩型铜金矿床绿泥石部分元素(Ti、Ba、Co、Pb、Sr、Fe)和元素比值(V/Ni)具有指示矿化中心的作用。     

Phyllosilicate minerals in the hydrothermal mafic–ultramafic-hosted massive-sulfide deposit of Ivanovka (southern Urals): comparison with modern ocean seafloor analogues

We have studied textural relationships and compositions of phyllosilicate minerals in the mafic–ultramafic-hosted massive-sulfide deposit of Ivanovka (Main Uralian Fault Zone, southern Urals). The main hydrothermal phyllosilicate minerals are Mg-rich chlorite, variably ferroan talc, (Mg, Si)-rich and (Ca, Na, K)-poor saponite (stevensite), and serpentine. These minerals occur both as alteration products after mafic volcanics and ultramafic protoliths and, except serpentine, as hydrothermal vein and seafloor mound-like precipitates associated with variable amounts of (Ca, Mg, Fe)-carbonates, quartz and Fe and Cu (Co, Ni) sulfides. Brecciated mafic lithologies underwent pervasive chloritization, while interlayered gabbro sills underwent partial alteration to chlorite + illite ± actinolite ± saponite ± talc-bearing assemblages and later localized deeper alteration to chlorite ± saponite. Ultramafic and mixed ultramafic–mafic breccias were altered to talc-rich rocks with variable amounts of chlorite, carbonate and quartz. Chloritization, locally accompanied by formation of disseminated sulfides, required a high contribution of Mg-rich seawater to the hydrothermal fluid, which could be achieved in a highly permeable, breccia-dominated seafloor. More evolved hydrothermal fluids produced addition of silica, carbonates and further sulfides, and led to local development of saponite after chlorite and widespread replacement of serpentine by talc. The Ivanovka deposit shows many similarities with active and fossil hydrothermal sites on some modern oceanic spreading centers characterized by highly permeable upflow zones. However, given the arc signature of the ore host rocks, the most probable setting for the observed alteration–mineralization patterns is in an early-arc or forearc seafloor–subseafloor environment, characterized by the presence of abundant mafic–ultramafic breccias of tectonic and/or sedimentary origin.Editorial responsibility: J. Hoefs     

Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite

The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K₂O content in muscovite, but in biotite it reflects an increase of not only the K₂O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.     

Hydrothermal alteration in metasedimentary rock-hosted orogenic gold deposits,Reefton goldfield,South Island,New Zealand

Orogenic or mesothermal quartz lodes in lower Palaeozoic Greenland Group metasedimentary rocks of the Reefton area have produced 67 tonnes (t) of gold prior to 1951, and recent exploration has identified new gold resources in several deposits, including the largest past producers, Blackwater and Globe-Progress. The metasedimentary rocks consist of alternating sandstone and mudstone beds that were metamorphosed to lower greenschist facies prior to being hydrothermally altered adjacent to the quartz lodes. The sandstones are feldspathic litharenites averaging Q₆₅-F₁₀-R₂₅, with detrital grains of quartz, rock fragments, muscovite, and plagioclase and biotite that were altered to albite and chlorite, respectively, during metamorphism. Accessory minerals are graphite, apatite, zircon, tourmaline and titanite. Hydrothermal alteration of the sandstones has developed a mineral assemblage of K-mica, carbonate (dolomite, ankerite, ferroan magnesite and magnesian siderite), chlorite, pyrite and arsenopyrite. The abundance of hydrothermal chlorite is greater at Blackwater than at the other prospects studied. Hydrothermal alteration associated with the quartz lodes is marked by bleaching, magnesian siderite spots, disseminated arsenopyrite and pyrite and thin carbonate, quartz and sulphide veins. These trends are accompanied by increasing concentrations of S, As and Sb and decreasing Na, and a decrease of Fe and Mg in K-mica. The alkali alteration indices 3K/Al (representing K-mica) and Na/Al (representing albite) generally show antipathetic trends, with 3K/Al increasing near the lodes and Na/Al decreasing. These trends reflect the replacement of albite by K-mica. Carbonate alteration indices CO₂/(Ca + Mg +Fe) and CO₂/[Ca + Mg + Fe -0.5(S + As)] quantify the abundance of hydrothermal carbonates, but they show variable correlation with the lodes. They increase the width of the alteration halo in the hanging wall of the lodes at the Globe-Progress and General Gordon prospects, but the peak values are as far as 150 m from the lodes. By contrast, peak values of the carbonate alteration indices are within 10 and 2 m of the lodes, respectively, at the Merrijigs and Blackwater deposits. Data show that for deposits with wide hydrothermal alteration halos, such as at the Globe-Progress and General Gordon prospects, the use of a suite of geochemical indicators can assist exploration by indicating trends in hydrothermal alteration that provide vectors to mineralisation. They also increase the size of the exploration target. By contrast, the alteration halo of the Blackwater deposit is restricted to within less than 5 m of the quartz lode and, therefore, the geochemical indicators are of more limited assistance to exploration.     

Reactions to define the biotite isograd in the Ryoke metamorphic belt,Kii Peninsula,Japan

Two types of biotite isograd are defined in the low-grade metamorphism of the Wazuka area, a Ryoke metamorphic terrain in the Kii Peninsula, Japan. The first, BI₁, is defined by the reaction of chlorite+K-feldspar= biotite+muscovite+quartz+H₂O that took place in psammitic rocks, and the second, BI₂, by the continuous reaction between muscovite, chlorite, biotite and quartz in pelitic rocks. The Fe/Mg ratios of the host rocks do not significantly affect the reactions. From the paragenesis of pelitic and psammitic metamorphic rocks, the following mineral zones were established for this low-pressure regional metamorphic terrain: chlorite, transitional, chlorite-biotite, biotite, and sillimanite. The celadonite content of muscovite solid solution in pelitic rocks decreases systematically with the grade of metamorphism from 38% in the chlorite zone to 11% in the biotite zone. Low pressure does not prohibit muscovite from showing the progressive change of composition, if only rocks with appropriate paragenesis are chosen. A qualitative phase diagram of the AKF system relevant to biotite formation suggests that the higher the pressure of metamorphism, the higher the celadonite content of muscovite at BI₁, which is confirmed by comparing the muscovites from the Barrovian and Ryoke metamorphism.     

造山带背景铜镍矿床蚀变过程与水的来源——东天山黄山南矿床氢氧同位素的指示

造山带背景铜镍矿床以富水为主要特征, 但水(流体)在成矿中的作用以及其对岩石的改造过程仍不明确。本文以黄山南铜镍矿床为例, 通过辉橄岩和橄辉岩等超镁铁岩的蚀变矿物组合和H-O同位素变化规律, 限定蚀变过程与流体性质及来源。黄山南超镁铁岩原生矿物主要有尖晶石、橄榄石、斜方辉石、单斜辉石和少量填隙状角闪石、云母, 蚀变矿物有角闪石(浅闪石、阳起石、透闪石、普通角闪石以及镁闪石)、滑石、绿泥石和蛇纹石等。根据岩石结构与蚀变矿物比例, 将超镁铁岩分为弱蚀变、中等蚀变和强蚀变3类。蚀变矿物组合与角闪石成分指示超镁铁岩经历了高温蚀变阶段(>700℃), 形成了镁质闪石+滑石+绿泥石; 中温蚀变阶段(700~550℃)和低温蚀变阶段(< 550℃), 分别形成了钙质闪石+滑石+绿泥石+蛇纹石与滑石+碳酸盐+蛇纹石的矿物组合。蚀变岩石普遍以中低温蚀变为主, 可能与中低温阶段的叠加-改造作用相关。岩石随蚀变程度增加, Si、Na、K、Mn等主量元素和Rb、Ba等微量元素呈现明显降低趋势, 表明大多数元素在流体改造过程中从岩石中迁出, 说明蚀变过程为开放体系。中-强蚀变岩石中, 硫化物矿物边部的形态呈锯齿状, 但主体仍为磁黄铁矿-镍黄铁矿-黄铜矿的矿物组合, 指示蚀变过程可能造成了部分Ni、Cu、S元素的溶解与迁移, 但是改造程度相对有限。不同蚀变程度超镁铁岩均富集轻H同位素(δD: -86.6‰~-128.6‰)且O同位素变化较大(δ18O: 1.7‰~10.8‰)。虽然部分样品的H-O同位素接近岩浆水与大气降水的混合趋势线, 但是大多数强蚀变样品则呈现截然不同的趋势, 即δ18O值相似但δD值变化较大, 指示蚀变过程以岩浆水主导且经历了不同程度的去气作用。不同程度的岩浆去气作用可能与岩浆的结晶分异过程密切相关。因此, 本文认为黄山南矿床蚀变岩石为岩浆阶段"自蚀变"作用的产物。不同构造背景铜镍矿床的H-O同位素对比指示, 岩浆"自蚀变"作用在超镁铁侵入岩中普遍发生, 而造山带背景铜镍矿床的蚀变程度可能相对更高。     

Genesis of Palaeoproterozoic iron skarns in the Misi region, northern Finland

Sodic alteration is widespread in Palaeoproterozoic greenstone and schist belts of the northern Fennoscandian shield. In the Misi region that forms the easternmost part of the Peräpohja schist belt, several small magnetite deposits show intimate spatial relationships with intensely albitised gabbros, raising the possibility that regional sodic alteration released iron, which was subsequently accumulated into deposits. Two of these magnetite deposits, Raajärvi and Puro display a typical paragenesis as follows (from oldest to youngest): (1) diopside, (2) actinolite/tremolite-magnetite ± chlorite, biotite, and (3) serpentine ± hematite, chlorite. Mass balance calculations suggest that significant amounts of Fe, Ca, Mg, K, Cu, V, and Ba were lost, and Na and Si gained during the albitisation of the gabbro, at near-constant Al, Ga, Ti, and Zr. Significant amounts of Si, Ca, Fe, and Na were enriched in the formation of skarn related to magnetite deposits. Fe and V leached from country rocks deposited during the skarn-alteration and formed the vanadium rich iron deposits while Cu passed through the system without significant precipitation due to low sulphur fugasity. Variations in Na, Ca, Mg, K, and Ba contents reflect the composition of the infiltrating fluid during alteration. Conventional heating-freezing measurements and proton-induced X-ray emission (PIXE) analyses of the fluid inclusions related to actinolite/tremolite-magnetite stage alteration indicate that the fluids that caused the alteration and the Fe-mineralisation were complex, oxidised, highly saline H₂O ± CO₂ fluids that contained high amounts of Na, Ca, K, Fe, and Ba as well as elevated concentrations of Cu, Zn, and Pb. The oxygen isotope thermometry suggest that temperature during the Fe-mineralisation stage was between 390 and 490°C. Calculated δ¹⁸O_fluid values of 6.1–9.8‰ SMOW and δ¹³C values of calcites in the ores and skarns were between ?7.7 and 10.9‰ PDB and most likely reflect admixture of ¹³C depleted, possibly magmatic fluids with the marble wall rocks that show δ¹³C_calcite values of 13‰ PDB. The SIMS U–Pb data on the zircons in the albitised gabbro next to the Raajärvi and Puro deposits suggest that intrusion of the gabbro took place at 2123±7 Ma and was accompanied by the formation of diopside skarn. The TIMS data on the metasomatic titanites related to sodic alteration yielded ages of 2062±3 and 2017±3 Ma. Iron was probably stripped from the mafic country rocks by sodic alteration between 2123 and 2017 Ma, driven by repeated brine influxes. Subsequently, the metal-rich brine was focused by a fault system and the iron was precipitated from this fluid by a combination of wall rock reaction, fluid mixing, and a drop in the temperature.     

陕西山阳柞水矿集区燕山期岩体矿物学特征:对岩浆性质及成矿作用的指示

山阳-柞水矿集区位于中秦岭晚古生代弧前盆地中,矿集区内出露有泥盆、石炭系地层,同时发育大量印支期和燕山期的岩浆侵入体。目前的研究显示,矿集区内出露的燕山期岩体在其内部及其与地层的接触带附近发育有强烈的热液蚀变和CuMo矿化。通过对区域内燕山期岩体的矿物学研究表明：岩体发育有大量的钾长石和斜长石,部分已发生蚀变;黑云母是原生的镁质黑云母;角闪石主要属于浅闪石和镁角闪石;绿泥石主要是铁叶绿泥石,同时岩体中还发育有榍石、磷灰石等矿物。根据黑云母和角闪石成分特征可判定山柞地区的燕山期岩体是形成于造山带环境具有壳幔混溶特征的I型花岗岩;各岩体的结晶温度大致相同,在701~789℃之间;岩浆具有较高的氧逸度。根据绿泥石估算出岩体的热液蚀变温度在250~355°C,说明形成蚀变的热液流体属于中高温/中温热液流体。通过对比可以发现：山柞地区燕山期岩体在形成环境、岩浆源区及氧逸度方面与典型的大型、超大型斑岩、斑岩-矽卡岩型CuMoAu矿床的成矿岩体具有相似的特征,而且山柞地区燕山期岩体的黑云母在TiO2、Al2O3和CaO含量及K/Na、Mg/Fe、Mg/(Mg+Fe3++Fe2++Mn)及(Fe3++ Fe2+)/(Mg+Fe3++Fe2+)比值与典型性。但是山柞地区出露的燕山期岩体的黑云母的Ti、Si与AlIV+AlVI、Fe3++Fe2+的特征与典型矿床的成矿岩体中的黑云母成分具有一定的差异;磷灰石中F、Cl和S等挥发份成分,尤其是其中SO3含量明显低于典型矿床成矿岩体,这对于Cu/Mo等成矿元素的富集沉淀具有十分不利的影响;同时山柞地区燕山期岩体普遍形成于较大的深度,也是不利于其形成大规模矿化。总体上,山阳-柞水矿集区内燕山期岩体与典型的斑岩、斑岩-矽卡岩型CuMoAu矿床相比,既有部分有利的成矿条件,也有一些不利条件,虽然难以形成大型矿床,但仍具有形成中小型矿床的潜力,在今后的找矿勘查工作中依然需要重视。     

Mineralogy, Lithogeochemistry and Elemental Mass Balance of the Hydrothermal Alteration Associated with the Gold-rich Batu Hijau Porphyry Copper Deposit, Sumbawa Island, Indonesia

This paper discusses the mineralogy, whole-rock geochemistry and elemental mass balance of the hydrothermal alteration zones within the Batu Hijau porphyry copper-gold deposit, Sumbawa Island, Indonesia. The hydrothermal alteration and mineralisation developed in four stages, namely (i) the early stage consisting of a central copper-gold-bearing biotite (potassic), proximal actinolite (inner propylitic) and the distal chlorite-epidote (outer propylitic) zones; (ii) the transitional stage represented by the chlorite-sericite (intermediate argillic) zone; (iii) the late stages distinguished into the sericite-paragonite (argillic) and pyrophyllite-andalusite (advanced argillic) zones; and (iv) the very late stage typified by the illite-sericite zone. In general, major elements (particularly Ca, Mg, Na and K) and some minor and rare earth elements decrease from the least altered rocks towards the late alteration zones as a consequence of the breakdown of Ca-bearing hornblende, biotite and plagioclase. Chemical discrimination by means of millicationic R₁-R₂ diagram indicates that R1 [4Si − 11(Na + K) − 2(Fe + Ti)] increases while R₂[6Ca + 2Mg + Al] decreases with increasing alteration intensity, from least-altered, through early, transitional, to late alteration zones. Rare earth elements-chondrite (C₁) normalised patterns also exhibit the depletion of the elements through the subsequent alteration zones. These results are consistent with the elemental mass balance calculation using the isocon method which shows that the degree of mass and volume depletion systematically increases during alteration. A decrease of the elements as well as mass and volume from early, through transitional to late alteration stages may imply a general decrease of the element activities in hydrothermal fluids during the formation of the alteration zones.     

Redistribution and fractionation of rare-earth and other elements in a weathering profile

A weathering profile on a uniform Lower Cretaceous volcanogenic sandstone from southern Victoria, Australia is enriched in rare-earth elements (REE), Y and other elements including Ba, Sr and Rb. Enrichments of REE of up to 7 times parent-rock values are associated with Fe-leached members of alteration couplets with little or no enrichment in adjacent Fe-rich members. These alteration couplets are similar in appearance to Liesegang banding. The REE have been fractionated during accumulation, leading to relative enrichment of the light rare earths (LREE).

The formation of an alteration couplet from fresh rock at the weathering front involves redistribution of only Fe (and P) and does not involve redistribution of REE, Y. Ba, Sr and Rb. Breakdown of original minerals in the present soil and degradation of vermiculite in the upper part of the weathering profile releases RE and related elements into solution. This solution moves down and along the profile until it comes into contact with neutral to alkaline conditions at the narrow weathering front. Here REE particularly are absorbed and apparently fixed in vermiculite forming at the expense of biotite and chlorite. (La + Ce + Nd + Y) up to 10.1 wt.% is recorded in degraded biotite grains with similar but lower contents in the degraded diagenetic chlorite cement.

Such accumulation and fractionation have important implications for REE studies involving sedimentary rocks or for that matter outcrop samples of any rock type.