Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO2 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 CO₂ 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 CO₂ 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 δ¹³C 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 K₂O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K₂O, Rb, and Ba, and are depleted in Na₂O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO₂ 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 CO₂ in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO₂ 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 × 10¹³ 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 × 10¹¹ cm²/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 CO₂ in the Early Archean.     

http://www.sciencedirect.com/science/article/pii/S1674987112000357

Field and remote sensing studies reveal that Au-bearing quartz±carbonate lodes in Romite deposit,in the extreme South Eastern Desert of Egypt,are controlled by NNE-striking shear zones that splay from the ca.660—550 Ma Hamisana Zone.Quartz in releasing bends with sinistral shear geometry and abundant boudinaged quartz-carbonate lodes with serrate ribboned fabrics suggest vein formation throughout a transpressive wrench system.Ubiquitous hydrothermal quartz,carbonate,and subordinate chlorite and sericite within the shear zones and as slivers in veins,indicate that gold deposition and hydrothermal alteration occurred under greenschist fades conditions.The Al（Ⅳ） in chlorite indicates a formation temperature of～300℃.comparable with temperatures estimated from arsenopyrite composition for grains intimately associated with gold in quartz veins. The new geological and geochemical data indicate that splays off the Hamisana Zone are potential gold exploration targets.Quartz veins along the high order（2nd or 3rd） structures of this crustal-scale shear zone are favorable targets.In the Romite deposit and in surrounding areas,a Au-As-Cu-Sb-Co-Zn geochemical signature characterizes mineralized zones,and particularly rock chips with>1000 ppm As and high contents of Cu,Zn,and Co target the better mineralized areas. The carbonateδ¹³C_pdb andδ¹⁸O_Smow isotope signatures preclude an organic source of the ore fluid,but metamorphic and magmatic sources are still valid candidates.The intense deformation and lack of magmatism in the deposit area argue for metamorphic dewatering of greenstone rocks as the most likely fluid source.The narrow ranges ofδ¹³C（-4.6‰to -3.1‰） andδ¹⁸O（11.9‰-13.7‰） in carbonate minerals in lodes imply a corresponding uniformity to the ambient temperature andδ¹³C_CO2(δ¹³C_∑C) of the ore fluids. The calculatedδ¹⁸Oh₂o values of 6.9‰—7.9‰for ore fluids,based onδ¹⁸O values of vein quartz further suggest a likely metamorphic origin.     

Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit,Quadrilátero Ferrífero,Brazil

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D₁, D₂ and D₃). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D₃. The orebodies constitute sulfide-bearing D₁-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H₂O-rich containing CO₂ + Na⁺ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H₂O is indicated. The CO₂ is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H₂O to CO₂-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.     

Application of short-wave infrared spectroscopy to define alteration zones associated with the Elura zinc–lead–silver deposit, NSW, Australia

Short-wave infrared reflectance spectra obtained from a Portable Infrared Mineral Analyser (PIMA) have been used to define alteration zones adjacent to base metal sulfide ore bodies at the Elura Mine, Cobar, Australia. The spectroscopic work identified white mica (sericite), chlorite and carbonates of hydrothermal origin in the alteration zones associated with the ore bodies. Weathering, alteration and ore zones can be discriminated by variations in the intensity and wavelength of relevant absorption features. Hydrothermal alteration is classified into four principal types, namely sericitic, silicic, chloritic and carbonate alteration. The first three types comprise overprinting hydrothermal assemblages of quartz, sericite, chlorite, ankerite, siderite, calcite and sulfides developed in strongly altered metasiltstone and slate of Early Devonian age, adjacent to the zinc–lead–silver mineralisation. An extensive zone of carbonate alteration, manifested as porphyroblasts of siderite in the host metasediments, is recognised beyond the zones of strong alteration. Spectral analysis is consistent with the mineralogical data obtained from XRD and XRF analyses and defines the limits of the alteration zones to distances of about 80 m around the ore bodies. The study demonstrates the potential for spectral analysis to assist with drill hole logging and the identification of alteration zones as part of mineral exploration and development programs.     

Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilátero Ferrífero, Brazil

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D₁, D₂ and D₃). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D₃. The orebodies constitute sulfide-bearing D₁-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H₂O-rich containing CO₂ + Na⁺ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H₂O is indicated. The CO₂ is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H₂O to CO₂-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.     

Gold mineralization without quartz veins in a ductile-brittle shear zone, Macraes Mine, Otago Schist, New Zealand

Greenschist facies schist which hosts the Macraes Mine in East Otago, New Zealand has been pervasively altered by post-metamorphic (lower greenschist facies) fluids over a 120 m thick section perpendicular to foliation. Metamorphic titanite has been replaced by rutile, and epidote has been replaced by a variety of metamorphic minerals including siderite, chlorite, muscovite and calcite. The early stages of this alteration occurred during development of a ductile cleavage associated with kilometre scale recumbent folding. The cleavage was widely overprinted by a subparallel set of spaced (mm scale) microshears which are locally enriched in rutile and hydrothermal graphite. Strain was then concentrated into narrow (m scale) zones where more intensely deformed portions of the rock are crossed and highly disrupted by closely spaced (100 μm scale) microshears. The highly strained rocks show a combination of mylonitic and cataclastic microstructures, including crystal-plastic grain size reduction and recrystallization of micas to form a new foliation. Muscovite has grown at the expense of albite in the mylonitic cataclasites. Hydrothermal alteration was accompanied by addition of pyrite, arsenopyrite and gold without development of quartz veins. Gold precipitated with sulphides during reduction of the fluid by hydrothermal graphite. The whole altered rock sequence was later cut sporadically by mesothermal quartz veins which contain gold, scheelite, rutile, pyrite and arsenopyrite. This deposit displays a continuum of post-metamorphic processes and hydrothermal fluid flow which occurred during uplift of the schist belt. Received: 4 December 1997 / Accepted: 21 September 1998     

Hydrothermal carbonates in altered wall rocks at the Uwamuki Kuroko deposits, Japan

Magnesite, siderite and dolomite are characteristic alteration minerals occurring in Miocene hanging wall rocks of dacitic composition which host the Kuroko orebodies. These carbonates generally occur in a more stratigraphically upper horizon than chlorite alteration zone surrounding the orebodies. The Mg/(Mg+Fe) ratios of the carbonates decrease from the central alteration zone to marginal zone. The Mg/(Mg+Fe) ratios of carbonates and chlorite positively correlate. The δ¹⁸O and δ¹³C values of magnesite, siderite and dolomite positively correlate with each other and lie between the igneous and marine carbonate values. The petrographic, isotopic and fluid inclusion characteristics and thermochemical modelling calculations indicate that magnesite and dolomite formed in the central zone close to the orebodies due to the interaction of hydrothermal solutions with the biogenic marine carbonates. Calcite formed further from the orebodies from hydrothermal fluids which did not contain a biogenic marine carbon component. The compositional and textural relationships indicate that superimposed alterations (chlorite alteration and carbonate alteration) occurred in hanging wall rocks. The mode of occurrences and the Mg/(Mg+Fe) ratios of magnesite and dolomite occurring in hanging wallrocks are useful in the exploration for concealed volcanogenic massive sulfide-sulfate deposits. Received: 9 September 1997 / Accepted: 23 September 1997     

Alteration at the Olympic Dam IOCG–U deposit: insights into distal to proximal feldspar and phyllosilicate chemistry from infrared reflectance spectroscopy

Twenty thousand metres of diamond drill core representing a 14 km cross-section from weakly to intensely altered Roxby Downs Granite through the Olympic Dam Breccia Complex, host to the Olympic Dam iron-oxide–copper–gold–uranium deposit in South Australia, was analysed using the HyLogger-3 spectral scanner. Thermal and shortwave infrared spectroscopy results from 30 drill holes provide insight into the spatial relationships between quartz, orthoclase–microcline, albite–oligoclase and progressively changing sericite and chlorite compositions. The relative proportions of quartz, feldspars and phyllosilicates were mapped with thermal infrared spectroscopy. Variations in the chemistry of sericite and chlorite were extracted by proxy from their shortwave infrared spectral response, together with their relative spatial distribution. HyLogger scanning has revealed four deposit-scale mineralogical trends, progressing from least-altered Roxby Downs Granite into mineralisation where most of the feldspar has been replaced by sericite + hematite + quartz: (1) a progressive Al–OH wavelength shift of 2205 nm to 2210 nm for sericite, followed by a spatially rapid reversal corresponding to lower phengite/muscovite abundance ratios; (2) progressive Mg/Fe–OH wavelength shift of 2248 nm to 2252 nm reflecting an increase in the Fe:Mg ratio of chlorite; (3) increasing ratio of microcline to orthoclase followed by a rapid decrease; and (4) slightly decreasing ratio of albite to oligoclase followed by plagioclase destruction prior to albite replacement by sericite. The HyLogger feldspar results support recent petrographic evidence for hydrothermal albite and K-feldspar at the Olympic Dam deposit, not previously reported. The spectral results from continuous HyLogger scans also show that the microscopic observations and proposed feldspar replacement reactions are not locally isolated phenomena, but are applicable at the deposit and regional-scale. A modified quartz–K-feldspar–plagioclase ternary diagram utilising mineralogy interpreted from HyLogger thermal infrared spectra (QAP_TIR) diagram along with supporting data on the abundance ratios of orthoclase/microcline and albite/plagioclase, and the wavelength shifts in characteristic absorption features for sericite and chlorite, can be used as empirical vectors towards mineralisation within the Olympic Dam mineral system, with potential application to other IOCG ore-forming systems. Intrusion of Gairdner Dyke Swarm dolerite dykes into sericite ± hematite altered Roxby Downs Granite results in retrograde albite–chlorite–magnetite alteration envelopes (up to tens of metres thick) overprinting the original sericite ± hematite alteration zone and needs to be carefully evaluated to ensure that such areas are not falsely downgraded during exploration.     

Geochemistry and mineral chemistry of lode gold mineralisation, SE Egypt: implications for ore genesis and exploration

Orogenic, lode gold mineralisation in the South Eastern Desert of Egypt is related to quartz veins spatially and temporally associated with conjugate NW- and NE-trending brittle–ductile shear zones. These structures are assumed to be linked to a regional transpression deformation which occurred late in the tectonic evolution of the area. In the Betam deposit, gold is confined to quartz(±carbonate) veins cutting through tectonised metagabbro and metasedimentary rocks in the vicinity of small granite intrusions. The ore bodies contain ubiquitous pyrite and arsenopyrite, in addition to minor disseminated chalcopyrite, pyrrhotite, galena, tetrahedrite and rare gold/electrum. New ore microscopy and electron microprobe studies indicate that most free-milling Au is intimately associated with the late-paragenetic galena–tetrahedrite–chalcopyrite assemblage. An early Fe–As sulphide assemblage, however, shows minor traces of refractory gold. New mineralogical and geochemical data are used to better constrain on possible element dispersions for exploration uses. This study indicates that parameters that most consistently define primary dispersion of gold in the mine area include pervasive silicification, sericite and carbonate alteration. The trace element data of gold lodes reflect a systematic dispersion of gold and certain base metals. Low-cost, extensive exploration programs may use elevated concentrations of Ag, Sb, Cu and Pb as tracers for Au ore zones in the Betam mine area and surroundings.     

The geochemistry of wall rock alteration in turbidite-hosted gold vein deposits, central Victoria, Australia

Gold deposits hosted in Ordovician to Devonian turbidites in central Victoria, Australia, consist of steeply dipping quartz vein systems hosted mainly by reverse faults. Wall rock alteration of the host metasandstones, metasiltstones and shales (or slates) extends at least 20 m from the veins in the Bendigo-Ballarat zone (BBZ) and 10 m in the Melbourne zone (MZ) deposits. Alteration minerals include carbonates (ankerite, siderite and dolomite) chlorite, seriate, arsenopyrite, pyrite, chalcopyrite and sphalerite, with less common barite, albite and biotite in the BBZ and carbonates (siderite, ankerite, dolomite and ferromagnesite), sericite, chlorite, arsenopyrite, pyrite, and stibnite with less common chalcostibite in the MZ. SiO₂, Na₂O, MgO and Sr with P₂O₅ commonly decreasing during alteration while CO₂, S, As, Sb, Au, V, Al₂O₃, Ga, K₂O and Rb with Ni and Cr commonly increase. TiO₂, FeO, Fe₂O₃, MnO, Th, U, Nb, La, Ce, and Sc showed little change. Concentrations of Zn, Cu, Pb, and Ca are variable.The relatively large decrease of SiO₂ could account for most, if not all, quartz present in the ore veins. The Na₂O and MgO could have crystallized in the veins in the very minor albite and chlorite present. The addition of V in all and Ni and Cr in most deposits probably reflects a source enriched in these elements such as underlying greenstones. The source of both the volatile (S, As, Sb) and lithophile (K₂O and Rb) elements as well as Au is unknown, but they could have been derived from a magmatic source or from the metamorphism of Cambrian greenstones. CO₂, present as carbonate, was derived mainly by the reaction of graphite, originally present in the sediments, with the ore solutions. Al₂O₃, the only other major element after SiO₂, probably increased mainly due to the decrease of the latter.     

Gold Mineralization in Banded Iron Formation in the Amalia Greenstone Belt,South Africa: A Mineralogical and Sulfur Isotope Study

The Blue Dot gold deposit, located in the Archean Amalia greenstone belt of South Africa, is hosted in an oxide (± carbonate) facies banded iron formation (BIF). It consists of three stratabound orebodies; Goudplaats, Abelskop, and Bothmasrust. The orebodies are flanked by quartz‐chlorite‐ferroan dolomite‐albite schist in the hanging wall and mafic (volcanic) schists in the footwall. Alteration minerals associated with the main hydrothermal stage in the BIF are dominated by quartz, ankerite‐dolomite series, siderite, chlorite, muscovite, sericite, hematite, pyrite, and minor amounts of chalcopyrite and arsenopyrite. This study investigates the characteristics of gold mineralization in the Amalia BIF based on ore textures, mineral‐chemical data and sulfur isotope analysis. Gold mineralization of the Blue Dot deposit is associated with quartz‐carbonate veins that crosscut the BIF layering. In contrast to previous works, petrographic evidence suggests that the gold mineralization is not solely attributed to replacement reactions between ore fluid and the magnetite or hematite in the host BIF because coarse hydrothermal pyrite grains do not show mutual replacement textures of the oxide minerals. Rather, the parallel‐bedded and generally chert‐hosted pyrites are in sharp contact with re‐crystallized euhedral to subhedral magnetite ± hematite grains, and the nature of their coexistence suggests that pyrite (and gold) precipitation was contemporaneous with magnetite–hematite re‐crystallization. The Fe/(Fe+Mg) ratio of the dolomite–ankerite series and chlorite decreased from veins through mineralized BIF and non‐mineralized BIF, in contrast to most Archean BIF‐hosted gold deposits. This is interpreted to be due to the effect of a high sulfur activity and increase in fO₂ in a H₂S‐dominant fluid during progressive fluid‐rock interaction. High sulfur activity of the hydrothermal fluid fixed pyrite in the BIF by consuming Fe²⁺ released into the chert layers and leaving the co‐precipitating carbonates and chlorites with less available ferrous iron content. Alternatively, the occurrence of hematite in the alteration assemblage of the host BIF caused a structural limitation in the assignment of Fe³⁺ in chlorite which favored the incorporation of magnesium (rather than ferric iron) in chlorite under increasing fO₂ conditions, and is consistent with deposits hosted in hematite‐bearing rocks. The combined effects of reduction in sulfur contents due to sulfide precipitation and increasing fO₂ during progressive fluid‐rock interactions are likely to be the principal factors to have caused gold deposition. Arsenopyrite–pyrite geothermometry indicated a temperature range of 300–350°C for the associated gold mineralization. The estimated δ³⁴S_ΣS (= +1.8 to +2.5‰) and low base metal contents of the sulfide ore mineralogy are consistent with sulfides that have been sourced from magma or derived by the dissolution of magmatic sulfides from volcanic rocks during fluid migration.     

海水交代过程中细碧角斑岩的矿物组合和矿物成分的变化规律——以陕西略阳细碧角斑岩为例

对陕西略阳细碧角斑岩的矿物学、岩石学、岩石地球化学、以及全岩O同位素的研究，认为细碧角斑岩是海水与玄武安山岩的交代产物；交代作用初期岩石中的Na2O含量升高，而MgO、CaO降低；交代作用后期岩石中的MgO升高而Na2O含量降低，即在整个过程中岩石的Na2O含量是一个最大值，而MgO有一个最小值。矿物组合由钠长石、绿帘石、少量绿泥石组成，最终产物为绿泥石和石英。     

Yerranderie a Late Devonian Silver–Gold–Lead Intermediate Sulfidation Epithermal District, Eastern Lachlan Orogen, New South Wales, Australia

Felsic volcanic units of the Early Devonian Bindook Volcanic Complex host the Yerranderie epithermal silver–gold–lead district 94 km west–southwest of Sydney. Mineralization in the district forms part of a fault‐controlled, intermediate sulfidation, epithermal silver–gold–base metal vein system that has significant mineral and alteration zonation. Stage 1 of the mineral paragenesis in the veins developed quartz and carbonate with early pyrite, whereas stage 2 is a crustiform banded quartz–pyrite–arsenopyrite assemblage. Stage 3, the main stage of sulfide deposition, comprises early sphalerite, followed by a tetrahedrite–tennantite–gold assemblage, then a galena–chalcopyrite–native silver–pyrite assemblage, and finally a pyrargyrite–polybasite–pearceite assemblage. Stage 4 involves the deposition of quartz veins with minor (late) pyrite and stage 5 is characterized by siderite that infilled remaining voids. Mineral zonation occurs along the Yerranderie Fault, with bornite being restricted to the Colon Peaks–Silver Peak mine area, whereas arsenopyrite, which is present in both the Colon Peaks–Silver Peak and Wollondilly mine areas, is absent in other lodes along the Yerranderie Fault. The Yerranderie Fault, which hosts the major lodes, is surrounded by a zoned alteration system. With increasing proximity to the fault the intensity of alteration increases and the alteration assemblage changes from an outer quartz–muscovite–illite–(ankerite) assemblage to a quartz–illite–(pyrite–carbonate) assemblage within meters of the fault. ⁴⁰Ar/³⁹Ar dating of muscovite from the alteration zone gave a 372.1 ± 1.9 Ma (Late Devonian) age, which is interpreted to be the timing of the quartz–sulfide vein formation. Sulfur isotope values for sulfides range from 0.1 to 6.2‰ with one outlier of ?5.6 δ³⁴S‰. The results indicate that the initial ore‐forming fluids were reduced, and that sulfur was probably sourced from a magmatic reservoir, either as a direct magmatic contribution or indirectly through dissolution and recycling of sulfur from the host volcanic sequence. The sulfur isotope data suggest the system is isotopically zoned.     

Le Chaˆtelet gold-bearing arsenopyrite deposit,Massif Central,France: mineralogy and geochemistry applied to prospecting

The abundant data available on the geochemistry and mineralogy of the Herynian gold deposit of Le Chaˆtelet (Massif Central, France) formed the basis for an analysis of the complex signature of a gold deposit and its hydrothermal alteration haloes. This study describes the deposit and its environment at various scales.The sulfide paragenesis of the Le Chaˆtelet deposit resulted from several stages, one of which was an event responsible for the deposition of Au-bearing arsenopyrite quartz veins; the Au is contained within the arsenopyrite lattice. Hydrothermal activity associated with these veins produced two superimposed alteration haloes: an outer halo of kaolinite + tosudite + siderite alteration that developed at the expense of the plagioclase minerals, and an inner halo of illite±siderite alteration that affected all the minerals. The hydrothermal alteration was followed by silicification of the wall rocks and then deposition of the Au-bearing arsenopyrite.The hydrothermal alteration related to the event also generated geochemical signatures such as Li, B, LREE, As, Sb, Au and W anomalies. Mineralogical studies and geochemical analyses of the hydrothermal facies show that: (1) the anomalous Li is trapped mainly in the tosudite lattice and to a lesser degree in the illite lattice; and (2) the B is probably located in tosudite and illite lattices. Arsenic, Sb and Au are contained in the arsenopyrite lattice.During weathering, the lithochemical signatures (determined by the analytical techniques used in this study) lose some of their specific characteristics, depending on the original concentrations. At deposit scale (0.25–1.0 km²), B and Li in the soill profiles indicate zones of hydrothermal alteration, whereas As and Au indicate zones of mineralization. At a regional scale, the association of Au, Li and B in stream sediments is characteristics of this type of mineralization. Therefore, in the hydrothermal setting described at Le Chaˆtelet, Au, As, Li and B appear, at all scales, to be dependable pathfinders for Au-bearing hydrothermal systems of possible economic interest.     

The Syama and Tabakoroni goldfields,Mali

Gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km. The deposits are interpreted to have formed during a craton-wide metallogenic event during the Eburnean orogeny. In the Syama goldfield, gold mineralisation in 9 deposits is hosted in the hanging-wall of the Syama-Bananso Shear Zone in basalt, greywacke, argillite, lamprophyre, and black shale. Gold is currently mined primarily from the oxidised-weathered zone of the ore bodies. In the Syama deposit, mineralisation hosted in altered basalt is associated with an intense ankerite–quartz–pyrite stockwork vein systems, whereas disseminated style mineralisation is also present in greywackes. In contrast, the Tellem deposit is hosted in quartz–porphyry rocks.In the Tabakoroni goldfield, gold mineralisation is hosted in quartz veins in tertiary splay shears of the Syama-Bananso Shear Zone. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolite) veins, with pyrite and lesser amounts of arsenopyrite. There are four main styles of gold mineralisation including silica-sulphide lodes in carbonaceous fault zones, stylolitic quartz reefs in fault zones, quartz–Fe–carbonate–sulphide lodes in mafic volcanics, and quartz–sulphide stockwork veins in silicified sediments and porphyry dykes. The several deposit styles in the goldfield thus present a number of potential exploration targets spatially associated with the regional Syama-Bananso Shear Zone and generally classified as orogenic shear-hosted gold deposits.     

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.     

Alteration associated with the gold-bearing quartz veins at Middagsberget,northern Sweden

The mesothermal Au deposit at Middagsberget in northern Sweden is associated with irregular stock-works of quartz veins occurring in shear zones across a dioritic intrusion. Alteration in the shear zones is characterized by sericitization, chloritization, the presence of sulphides and several generations of veins and small-scale fractures. Small fractures which are filled with variable amounts of quartz, carbonate, sericite, chlorite, albite and sulphides, are particularly abundant in zones having a high Au content. In general, these fractures are younger than the major quartz veins and were apparently important for strong Au-enrichment. Au is associated with arsenopyrite and it occurs as droplets or interfillings in the arsenopyrite or at the crystal surface; together with pyrrhotite as inclusions in arsenopyrite; as free grains in silicates but close to arsenopyrite; or in very small fractures in carbonates. The Au-mineralization is associated with elevated contents of As, S, Sb and W and depletion of Cu, Zn, Ge and Bi. The entire altered areas are enriched in Au compared with the < 1 to 15 ppb found in the host intrusion. During alteration an initial increase in Na or K was accompanied by a weak enrichment of Au and associated elements together with a depletion of elements such as Ca and Mg. In Au-rich samples this alkali enrichment was overprinted by the carbonate-and sulphide-bearing fractures, which often led to an increased Ca-content. An clear zonation of alteration types has not been found. The host rock has been altered by several generations of fluids: the shear zones were repeatedly ruptured and new small-scale fractures were opened. The different fluids did not, therefore, flow through identical paths although they penetrated the same major zones. This resulted in a complex pattern of variously superimposed alterations.     

A Clinopyroxene Paragenesis of Albite--Epidote--Amphibolite Facies in Meta-Syenites from the South-East Tauern Window, Austria

Mineral assemblages and textures are described from clinopyroxene-bearingmeta-syenites and related rocks from a small area in the PenninicBasement Complex of the south-east Tauern Window. Evidence from mineral textures, mineral compositions and geobarometryindicate that the clinopyroxene, a sodic salite, crystallizedas part of an equilibrium albite-epidote-amphibolite faciesparagenesis in the 35–40 Ma meso-Alpine metamorphic event.Phase relations in co-facial quartz + albite + K-feldspar +sphene-bearing meta-syenites and meta-granites are examinedusing a projection from these minerals onto the plane (A1₂O₃+ Fe₂O₃)-CaO-(MgO + FeO + MnO). The projection demonstratesthat salitic clinopyroxene can only be a stable phase in suchrocks if the bulk-rock Al/Na + K ratios are low. This is confirmedby comparing the whole-rock analyses of clinopyroxene-bearingmeta-syenites with those of clinopyroxene-free meta-syenitesand meta-granites. Mineral assemblages in a variety of lithologies from the south-eastTauern Window are used to construct a generalized AKM diagramfor magnesian albite + epidote + quartz-bearing rocks of thealbite-epidote-amphibolite facies. Thermochemical calculations indicate that the meta-syeniteswere metamorphosed at temperatures close to 500 C and at a pressureof 6+2 –4 kb. Fluids in equilibrium with meta-syeniteand meta-granite mineral assemblages had X_H2O values of 0–95,assuming X_H2O + X_CO2O= 1.0.     

Vostok-2 gold-base-metal-tungsten skarn deposit,Central Sikhote-Alin,Russia

Vostok-2—East Russia’s largest skarn deposit of high-grade sulfide-scheelite ore with substantial base-metal and gold mineralization—was formed during the Mesozoic orogenic epoch of evolution of the Far East marginal continental system as an element of the gold-tin-tungsten metallogenic belt. The deposit is related to the multistage monzodiorite-granodiorite-granite complex pertaining to the ilmenite series and spatially associated with a minor granodiorite porphyry (?) stock, which bears petrological features transi- tional to those of intrusive rocks occurring at Au-W and Au deposits. The hydrothermal metasomatic alteration of host rocks evolved from pyroxene skarn via retrograde postskarn and propylitic (hydrosilicate) metasomatic rocks to the late, low-temperature quartz-sericite metasomatic rocks often with albite, chlorite, carbonate, and apatite. The mineral assemblages of skarn and postskarn metasomatic rocks correspond to those at the reduced-type tungsten skarn deposits. Zoning of the postskarn metasomatic rocks is controlled by granodiorite stock. The hydrothermal metasomatic alteration was accompanied by development of mineralization from scheelite via sulfide-scheelite with pyrrhotite and chalcopyrite to the gold-base-metal-scheelite assemblage with arsenopyrite, Bi-Sb-Te-Pb-Zn sulfides and sulfosalts. Several scheelite generations are recognized. Scheelite of the late generations is enriched in Eu, as is typical of gold deposits. The associated gold mineralization comprises both native gold varying in fineness and Au-bearing arsenopyrite. The significant gold mineralization emphasizes genetic links of this deposit with intrusion-related Au-W and Au deposits of the reduced type.     

Geochemical and Mineralogical Characteristics of Pleistocene Lignites and Associated Sediments of Marathousa Coal Field,Central Peloponnese,Greece

The mineralogy and geochemistry data are presented for thirty-seven shales,four concretions,two carbonate sediments and seven lignites from the Marathousa coal field of the Megalopolis Basin in Greece.The argillaceous rocks consist of chlorite,illite,kaolinte,albite,quartz.opal-A,calcite and dolomite;the concretions of aragonite,gypsum and pyrite;and the carbonate rocks of calcite,quartz and illite.The mineral matter in the lignites consists of gypsum,quartz,albite,chlorite,illite,opal-A,dolomite,pyrite,and rarely calcite and kaolinite Athree-factor model explains the total variaition of major and trace elements in the argillaceous sediments.The first factor is an aluminosilicate factor and involves the following elements:Al,Si,Mg,Na,K,Ti,Mn,Nb,Y,Rb,Zn,Cu,Ni,Cr,Nband V,associated with chlorite,albite and illite.The second factor involves the elements Ca,Sr,Ba,Znand Sc and is related to carbonate lithology and mainly the carbonate concretions with gypsum.The third factor involves Fe and Ce with a weak association with Mn.The diagenesis of the Marathousa sediments and lignites was not very advanced as indicated by (a) the total thickness of the sequence (500m),(b) the presence of biogenic silica(opal-A) and (c) the age of the deposit(Pleistocene).FOr these reasons the rpresence of chlorite,illite and kaolinite in the sediments and lignite is due not to diagenetic reactions but to weathering of the flysch and metamorphic rocks at the edges of the Megalopolis Basin and transport of the weathering products(illite,chlorite,kaolinite)into the basin of deposition.The diagenetic minerals of the Marathousa sequence include pyrite,gypsum,dolomite and aragonite.