Calcite Veinlets Research Articles

Geology, Geochemistry, and Genesis of the Tongcun Reduced Porphyry Mo (Cu) Deposit, NW Zhejiang Province, China

The Tongcun Mo (Cu) deposit in Kaihua city of Zhejiang Province, eastern China, occurs in and adjacent to the Songjiazhuang granodiorite porphyry and is a medium‐sized and important porphyry type ore deposit. Two irregular Mo (Cu) orebodies consist of various types of hydrothermal veinlets. Intensive hydrothermal alteration contains skarnization, chloritization, carbonatization, silicification and sericitization. Based on mineral assemblages and crosscutting relationships, the ore‐forming processes are divided into five stages, i.e., the early stage of garnet + epidote ± chlorite associated with skarnization and K‐feldspar + quartz ± molybdenite veins associated with potassic‐silicic alteration, the quartz‐sulfides stage of quartz + molybdenite ± chalcopyrite ± pyrite veins, the carbonatization stage of calcite veinlets or stockworks, the sericite + chalcopyrite ± pyrite stage, and the late calcite + quartz stage. Only the quartz‐bearing samples in the early stage and in the quartz‐sulfides stage are suitable for fluid inclusions (FIs) study. Four types of FIs were observed, including 1) CO2‐CH4 single phase FIs, 2) CO2‐bearing two‐ or three‐phase FIs, 3) Aqueous two‐phase FIs, and 4) Aqueous single phase FIs. FIs of the early stages are predominantly CO2‐ and CH4‐rich FIs of the CO2‐CH4‐H2O‐NaCl system, whereas minerals in the quartz‐sulfides stage contain CO2‐rich FIs of the CO2‐H2O‐NaCl system and liquid‐rich FIs of the H2O‐NaCl system.For the CO2‐CH4 single phase FIs of the early mineralization stage, the homogenization temperatures of the CO2 phase range from 15.4 °C to 25.3 °C (to liquid), and the fluid density varies from 0.7 g/cm3 to 0.8 g/cm3; for two‐ or three‐phase FIs of the CO2‐CH4‐H2O‐NaCl system, the homogenization temperatures, salinities and densities range from 312°C to 412°C, 7.7 wt% NaCl eqv. to 10.9 wt% NaCl eqv., and 0.9 g/cm3 to 1.0 g/cm3, respectively. For CO2‐H2O‐NaCl two‐ or three‐phase FIs of the quartz‐sulfides stage, the homogenization temperatures and salinities range from 255°C to 418°C, 4.8 wt% NaCl eqv. to 12.4 wt% NaCl eqv., respectively; for H2O‐NaCl two‐phase FIs, the homogenization temperatures range from 230 °C to 368 °C, salinities from 11.7 wt% NaCl eqv. to 16.9 wt% NaCl eqv., and densities from 0.7 g/cm3 to 1.0 g/cm3. Microthermometric measurements and Laser Raman spectroscopy analyses indicate that CO2 and CH4 contents and reducibility (indicated by the presence of CH4) of the fluid inclusions trapped in quartz‐sulfides stage minerals are lower than those in the early stage.Twelve molybdenite separates yield a Re‐Os isochron age of 163 ± 2.4 Ma, which is consistent with the emplacement age of the Tongcun, Songjiazhuang, Dayutang and Huangbaikeng granodiorite porphyries. The δ18OSMOW values of fluids calculated from quartz of the quartz‐sulfides stage range from 5.6‰ to 8.6‰, and the δDSMOW values of fluid inclusions in quartz of this stage range from – 71.8‰ to 88.9‰, indicating a primary magmatic fluid source. δ34SV‐CDT values of sulfides range from +1.6‰ to +3.8‰, which indicate that the sulfur in the ores was sourced from magmatic origins.Phase separation is inferred to have occurred from the early stage to the quartz‐sulfides stage and resulted in ore mineral precipitation. The characteristics of alteration and mineralization, fluid inclusion, sulfur and hydrogen‐oxygen isotope data, and molybdenite Re‐Os ages all suggest that the Tongcun Mo (Cu) deposit is likely to be a reduced porphyry Mo (Cu) deposit associated with the granodiorite porphyry in the Tongcun area.

The formation conditions of labuntsovite-group minerals in the Kovdor massif, Kola Peninsula

In the Kovdor massif, labuntsovite-group minerals occur in dolomite carbonatite veins (labuntsovite-Mg), in a natrolite-calcite vein (lemmleinite-Ba and labuntsovite-Fe), and in calcite pockets and veinlets cutting fenites (late labuntsovite-Mg). They are closely intergrown with paragenetic carbonates, and this makes it possible to estimate their crystallization temperature from the fluid inclusions entrapped in dolomite (≥265°C) and calcite (175–225°C). The earlier labuntsovite-Mg was formed under relatively acidic conditions, whereas later labuntsovite-calcite mineralization was derived from alkaline solutions.

An evolution model of large consequent bedding rockslides, with particular reference to the Jiweishan rockslide in Southwest China

Strata consisting of massive, dipping layers of hard rock interbedded with thin, soft interlayers are common in Southwest China, and this structure is conducive to the formation of consequent bedding rockslides. The huge Jiweishan rockslide is an example that mobilized 5.6 million m3 of massive, lower Permian limestone that was stratigraphically situated above a weak, 30cm-thick layer of bituminous shale with a downslope dip of 21°. Field data and historical observations indicate that the rupture surface of this rockslide weakened gradually from the rear, upper part to the front. This evidence includes (1) a decrease in the thickness and areal extent of fracture-parallel calcite veinlets along the thin rupture zone. (2) the development of a wide, vertical tension fracture at the upper boundary of the slide block, about 50years before the catastrophic failure; (3) a succession of small collapses that migrated from the upper to the lower parts of the slide block in the decade before the massive failure. This type of landslide is accordingly termed an advancing consequent bedding rockslide (ACB rockslide). An equation was developed to simulate the mechanical behavior of the rupture surface as a function of position and time. Finally, a computational model that incorporates this equation was used to simulate the evolutionary history using realistic mechanical parameters. The simulation results show how the size of an upper “driving block” progressively enlarges at the expense of a lower “ locking block” that restrains the slide mass, until the locking block becomes so small and weak that failure occurs. Good agreement between the simulation results and field data support the proposed evolution model, including the calculated size of the locking block prior to failure and the width of the upper tension fracture.

Petrography, geochemistry and U–Pb zircon age of the Matongo carbonatite Massif (Burundi): Implication for the Neoproterozoic geodynamic evolution of Central Africa

The Matongo carbonatite intrusion belongs to the Neoproterozoic Upper Ruvubu alkaline plutonic complex (URAPC), that is located in Burundi along the western branch of the East African Rift. Beside the Matongo carbonatite, the URAPC alkaline complex comprises feldspathoidal syenites, diorites, quartz-bearing syenites and granites.Three main facies have been recognized in the Matongo carbonatite: (1) Sövites represent the dominant facies. Two varieties have been recognized. A scarce coarse-grained sövite (sövite I), which is altered and poorly enriched in REE (4<ΣREE<8ppm), is encountered in highly fractured zones. A fine-grained sövite (sövite II), which is made of saccharoidal calcite, commonly associated with apatite, aegirine and amphibole, is abundant in the intrusion. Sövite II is enriched in LREE (442<ΣREE<1550ppm, 49<LaN/YbN<175). (2) Ferrocarbonatites, that form decimeter-wide veins crosscutting the sövites, are characterized by a LREE enriched patterns (225<ΣREE<1048ppm, 17<LaN/YbN<64). (3) K-feldspar and biotite-rich fenite facies (silicocarbonatites) have been recognized at the contact between the carbonatites and the country rock. They are likewise LREE-enriched (134<ΣREE<681ppm, 25<LaN/YbN<46). Additionaly, “late” hydrothermal MREE-rich carbonatite veinlets can be found in sövite I. They are characterized by moderate enrichment in REE (ΣREE=397ppm), with a MREE-humped pattern (LaN/YbN=3.7). The different facies represent the typical magmatic evolution of a carbonatite, while the silicocarbonatites are interpreted as resulting from the fenitisation of the country host-rocks. In addition, the most REE-depleted and fractionated facies, i.e. the coarse-grained sövite facies and the “late” calcite veinlets testify for hydrothermal processes that occurred after carbonatite emplacement and result from REE mobilization and redistribution.Large idiomorphic zircon crystals (megacrysts), found in the vicinity of the carbonatite can directly be related to the carbonatite evolution. They have been dated at 705.5±4.5Ma (U–Pb concordant age, LA–ICP–MS). Similar zircon megacrysts of the Lueshe carbonatite (DRCongo) have been dated and give a concordant age at 798.5±4.9Ma (U–Pb, LA–ICP–MS). Considering that an extensional tectonic regime occured at that time in Central Africa – what remains debated - both ages could relate to different stages of Rodinia breakup, with uprise of mantle-derived magmas along Palaeoproterozoic lithospheric zones of weakness.

Origin of Au-bearing rodingite in the Karabash massif of alpine-type ultramafic rocks in the southern Urals

The rodingite belt in the Karabash massif situated 40 km north of Miass is continuously traced for 2.5 km along its central part. Rodingites bear up to 1% sulfide disseminations and gold particles with high Cu content (up to 40 wt %) throughout the belt. The central part of the rodingite belt is especially rich in gold, which was mined at the Zolotaya Gora (Gold Mountain) deposit. The Au-bearing rodingite belt is zonal and was formed during three stages. The inner zone is composed of chlorite-andradite-diopside rock of the first stage, which is crosscut by diopside veinlets of the second stage and calcite veinlets of the third stage. The intermediate zone consists of fine-grained chloritolite of the first stage and coarse-grained chlorite veinlets of the second stage. The outer zone of the metasomatic column is occupied by antigorite and chrysotile-lizardite serpentinites. No relict rocks or minerals of the replaced protolith have been established except sporadic Cr-spinel grains. Native gold was being deposited during all periods of rodingite formation. In terms of the currently adopted concept of evolution of the Ural Foldbelt, the Sm-Nd isochron age of rodingite estimated at 369.4 ± 8.8 Ma corresponds to the period of collisional compression of Silurian-Devonian oceanic and island-arc complexes and upward pushing out of a block of the melanocratic basement underlying these complexes. A proposed model of rodingite formation is based on ore mineralogy, REE geochemistry, and thermobarogeochemical and isotopic study of minerals. It is suggested that in contrast to the barren bimetasomatic rodingite replacing dikes, the studied rodingite are considered to be fissure veins accompanied by metasomatic alteration of host serpentinite. The estimation of initial isotopic composition of fluid components indicates that the ore-bearing fluid is of metamorphic origin (δDfl = −4 to −13‰ and δ18Ofl = 5.9 to 8.3‰). The fluid was formed as a product of dehydration of oceanic serpentinite at the base of melanocratic rocks and related gabbroids that moved out to the surface. These rocks were a source of gold and other components (Ca, Al, Ti, Cu, Ni, REE, P, etc.).

Multistage gold mineralization at the Lapa mine, Abitibi Subprovince: insights into auriferous hydrothermal and metasomatic processes in the Cadillac–Larder Lake Fault Zone

The Lapa gold deposit contains reserves of 2.4 Mt at 6.5 g/t Au and is one of the few deposits located directly within the Cadillac–Larder Lake Fault Zone (CLLFZ), a first-order crustal-scale fault that separates the Archean Abitibi Subprovince from the Pontiac Subprovince to the south. Gold mineralization is predominantly hosted in highly strained and altered, upper greenschist–lower amphibolite facies mafic to ultramafic rocks of the Piche Group. Auriferous ore zones consist of finely disseminated auriferous arsenopyrite–pyrrhotite ± pyrite and native gold disseminated in biotite- and carbonate-altered wall rocks. Native gold, which is also present in quartz ± dolomite–calcite veinlets, is locally associated with Sb-bearing minerals, especially at depth ≤1 km from surface where the deposit is characterized by a Au–Sb–As association. At vertical depth greater than 1 km, gold is associated with arsenopyrite and pyrrhotite (Au–As association). The mineralogy and paragenesis of the Lapa deposit metamorphosed ore and alteration assemblages record the superposition of three metamorphic episodes (M1, M2, and M3) and three gold mineralizing events. Spatial association between biotitized wall rocks and auriferous arsenopyrite indicates that arsenopyrite precipitation is concomitant with potassic alteration. The predominant Au–As association recognized across the deposit is related to gold in solid solution in arsenopyrite as part of a pre-M2 low-grade auriferous hydrothermal event. However, the occurrence of hornblende + oligoclase porphyroblasts overprinting the biotite alteration, and the presence of porous clusters and porphyroblasts of arsenopyrite with native gold and pyrrhotite indicate an auriferous metasomatic event associated with peak M2 prograde metamorphism. Late retrograde metamorphism (M3) overprints the hornblende–oligoclase M2 assemblage within the host rocks proximal to ore by an actinolite–albite assemblage by precipitation of free gold and Sb–sulfosalts at lower P–T. The complex relationships between ore, structural features, and metamorphic assemblages at Lapa are related to the tectonometamorphic evolution of the Cadillac–Larder Lake Fault Zone at different times and crustal levels, and varying heat and fluid flow regimes. The Lapa deposit demonstrates that early, low-grade gold mineralization within the Cadillac–Larder Lake Fault Zone has benefited from late gold enrichment(s) during prograde and retrograde metamorphism, suggesting that multi-stage processes may be important to form gold-rich orogenic deposits in first order crustal-scale structures.

Metamorphic alteration, mineral paragenesis and geochemical re-equilibration of early Archean quartz–amphibole–pyroxene gneiss from Akilia, Southwest Greenland

Quartz–amphibole–pyroxene gneiss from the island of Akilia, Southwest Greenland has been claimed to contain the earliest traces of life on Earth in the form of biogenic carbonaceous matter encapsulated as inclusions in apatite crystals. Various lines of evidence, including petrography, geochronology, field relations, and geochemistry, have, however, been presented that challenge this interpretation. Textural relationships and geochemical signatures in this controversial gneiss presented here manifest a complex, spatially variable metamorphic history that includes granulite- and amphibolite-facies overprints and metasomatism. A peak metamorphic, granulite-facies, quartz–orthopyroxene–clinopyroxene–amphibole–magnetite assemblage is preserved in only a few centimeter-scale layers within the 5-m-thick, quartz–amphibole–pyroxene gneiss unit. Calcite veinlets that appear to postdate the peak metamorphism occur in pyroxene. The quartz–amphibole–pyroxene gneiss unit has subsequently experienced isochemical (except hydration) amphibolite-facies alteration during which pyroxenes were retrogressed to amphiboles and magnetite, and calcite was consumed. Parts of the quartz–amphibole–pyroxene gneiss that contain texturally late hornblende have experienced metasomatic alteration by Al-carrying fluids. These fluids controlled the alteration of pyroxenes and amphiboles to hornblende, and modified the trace-element composition by remobilizing LREE and Eu. Apatite has variable REE composition and 87Sr/86Sr in the quartz–amphibole–pyroxene gneiss, but on the local scale (cm) is in equilibrium with co-existing silicates. Effective recrystallization of apatite crystals as well as co-existing silicates during several stages of the metamorphic history makes the intact preservation of diagenetic apatite with encapsulated primary carbonaceous matter implausible. Hence, it is highly unlikely that Akilia apatite could serve as repository of the earliest traces of life on Earth.

Mineralogical and geochemical evidence for multi-stage origin of mineral veins hosted by teschenites at Tichá, Outer Western Carpathians, Czech Republic

Numerous mineral veins are hosted in a body of teschenite which is situated within the Lower Cretaceous flysch siliciclastics of the Silesian Unit at Tichá. Mineralogy, fluid inclusions, stable isotopes and trace elements have been studied in order to assess the origin of this mineralization. Three stages of vein cementation have been recognized, each of them being characterized by distinct mineral composition and genetic conditions. The first stage is composed of titanite, aegirine-augite to aegirine, annite, analcime and strontian apatite. These minerals originated from NaCl-rich, CaCl 2-poor magmatic brine (total fluid salinities range between 47 and 57 wt%), leaving after crystallization of host teschenite in low-pressure (<1 kbar) environment. Crystallization temperatures reached ∼390–510 °C for early phases, titanite and aegirine-augite. The second stage is formed by calcite, chlorite, dolomite, siderite, strontianite, quartz, pyrite and sphalerite. The parent fluids were low-salinity (0.5–4.5 wt% NaCl eq.) aqueous solutions with low content of strong REE-complexing ligands, that were progressively cooled during mineral precipitation (up to ∼190 °C at the beginning, ∼90 °C at the end of crystallization). These fluids are interpreted to be predominantly of external origin, derived from surrounding sedimentary sequences during diagenetic dewatering of clay minerals. The highly positive δ 18O and near-zero δ 13C values indicate an interaction of fluids with sedimentary carbonates. The third stage is formed by a dense net of calcite veinlets, which probably originated during tectonic deformations connected with orogenetic movements during the Tertiary. The source of strontium for first stage mineralization was probably related to the special conditions of magmatic evolution of the host teschenite, whereas strontium for second stage minerals could have been remobilized during hydrothermal alteration from earlier teschenite-hosted mineral phases and/or limestone.

The Chailag-Khem fluorite-barium-strontium rare earth carbonatite occurrence, the Western Sayan Range, Russia

The geological and mineralogical data on the Chailag-Khem F-Ba-Sr-REE occurrence in the Western Sayan Range, Russia, are discussed. The chemical compositions of rocks, ores, and minerals (ICP-MS, Link) are reported. The occurrence is localized in a tectonic crush zone composed of Cambrian quartz-sericite slates intruded by quartz syenite porphyry. Ore mineralization occurs as veins, cement of tectonic breccia, and metasomatic disseminations in host rocks. Massive ore consists of calcite, strontianite, and quartz; impregnations of euhedral fluorite, ankerite, and bastnaesite crystals; and fine-grained barite aggregate. Accessory minerals include parisite, synchysite, barytocelestine, sulfides, rutile, and uraninite. Late metasomatic calcite and strontianite segregations and veinlets are abundant. In genetic, mineralogical, and geochemical features, the Chailag-Khem occurrence is similar to the Late Mesozoic carbonatite deposits of Central Tuva, of which the Karasug Fe-F-Ba-Sr-REE deposit is the largest and best known. All carbonatite deposits and occurrences are located within a longitudinal zone transverse to the major tectonic elements of the region.

Fluid-induced disturbance of the monazite Th–Pb chronometer: In situ dating and element mapping in pegmatites from the Rhodope (Greece, Bulgaria)

The monazite Th–Pb chronometer is widely used in a variety of geological contexts but may be affected by interaction with fluids. In the central part of the Rhodope Massif (Greece, Bulgaria), synfolial pegmatite veins were emplaced within two major synmetamorphic shear zones. In the Chepelare shear zone (CZS), which displays no evidence of post-metamorphic fluid flow, consistent monazite Th–Pb ages of ∼ 36 Ma are interpreted as dating vein emplacement. Higher in the metamorphic pile, zircon and monazite give U–Th–Pb ages of ∼ 42 Ma for vein emplacement. In the Nestos Shear Zone (NSZ) pegmatites show evidence of post-emplacement interaction with fluids during greenschist facies ductile deformation. Evidence for this includes the precipitation of calcite in microscale tension gashes within feldspar porphyroclasts, and the oxygen isotope disequilibrium between quartz and feldspar. Aqueous carbonic fluids deriving from carbonate lithologies in the footwall of the NSZ are identifiable in the pegmatites by the high δ 18O value of quartz (up to 21.8‰) and the presence of calcium in calcite veinlets. In these samples, some of the monazite grains show large intragrain scattering of Th–Pb age spanning up to ∼ 12 Ma, with intergrain scattering reaching ∼ 16 Ma (from ∼ 39 to ∼ 55 Ma). Within individual grains, age domains correlate with chemical heterogeneities, and some show a characteristic Ca-excess. These chemical and isotopic alterations are interpreted to be caused by interaction with fluids derived from carbonate lithologies. Complementary U–Pb data on zircons from the NSZ pegmatites yield mostly Mesozoic ages related to an older metamorphic cycle, and an age of ∼ 48 Ma for one grain with typical magmatic zoning. The Th–Pb ages of 49.3 ± 1.6 to 54.9 ± 1.7 Ma probably relate to the emplacement of the pegmatites. The spread of younger Th–Pb ages (from 38.6 ± 1.1 to 46.2 ± 1.6 Ma) probably reflects the period of fluid circulation during progressive cooling to greenschist facies conditions, or variable perturbations of the monazite isotope system during fluid-assisted greenschist facies deformation. This is consistent with 39Ar– 40Ar mica ages of ∼ 32 to 34 Ma, interpreted as a tighter constraint for the timing of the greenschist facies metamorphism. This study illustrates the capacity of monazite to record distinct events in a single rock and highlights the need for identifying the potential involvement of fluids in order to interpret monazite ages.

Fluid regime and origin of gold-bearing rodingites from the Karabash alpine-type ultrabasic massif, Southern Ural

The rodingites of the Karabash Massif are distinguished by the presence of native cupriferous gold. This zonal hydrothermal-metasomatic complex was formed in three stages. The inner zone of rodingite proper is made up of chlorite-andradite-diopside rocks of stage 1, which are cut by diopside veinlets of stage 2 and calcite veinlets of stage 3. The intermediate zone consists of chloritolites, which give way to the antigorite and chrysotile-lizardite serpentinites of the outer zone. Thermometric and cryometric studies and gas chromatography showed that the gold-bearing rodingites of stages 1 and 2 were formed at t = 420–470°C, P = 2–3 kbar, and \(X_{CO_2 } \) = 0.001–0.02, i.e., under conditions typical of rodingite formation. The final stage was accompanied by a decrease in P-T parameters (0.5–1.0 kbar and 230–310°C) and an increase in \(X_{CO_2 } \) up to 0.04. The rodingite-forming fluid was extremely rich in water (\(X_{H_2 O} \) = 0.942–0.981) and contained hydrogen as the major gas component (\(X_{H_2 } \) = 0.012–0.023); its composition was essentially chloride-magnesium with minor amounts of CaCl2 and FeCl2 and a low salinity of 2.6–8.0 wt % NaCl equiv. The rodingite minerals showed the following isotopic characteristics (‰): δ18O from 5.5 to 6.6 and δD from 42.8 to −44.3 for chlorite, δ180 from 2.0 to 3.8 for andradite, δ18O from 6.0 to 6.6 for diopside, and δ18O from 10.6 to 11.4 and δ13C from 0.1 to −1.8 for calcite. The chloritolite is characterized by δ18O from 5.9 to 6.6 and δD from −49.8 to −64.4; the antigorite serpentinite shows δ18O=6.5 and δD=−65.2; and the antigoritized chrysotile-lizardite serpentinite shows δ18O from 6.8 to 6.9 and δD from −127 to −128. The calculated isotopic composition of fluid in equilibrium with various rocks suggested its metamorphic origin. It was formed from the water released during dehydration of oceanic serpentinites, from the components of ultrabasic and basic magmatic rocks, and, at the final stage, from marine carbon.

The Ore-forming Fluid of the Gold Deposits of Muru Gold Belt in Eastern Shandong, China - a Case Study of Denggezhuang Gold Deposit

Abstract. Denggezhuang gold deposit is an epithermal gold-quartz vein deposit in northern Muru gold belt, eastern Shandong, China. The deposit occurs in the NNE-striking faults within the Mesozoic granite. The deposit consists of four major veins with a general NNE-strike. Based on crosscutting relationships and mineral parageneses, the veins appear to have been formed during the same mineralization epochs, and are further divided into three stages: (1) massive barren quartz veins; (2) quartz-sulfides veins; (3) late, pure quartz or calcite veinlets. Most gold mineralization is associated with the second stage. The early stage is characterized by quartz, and small amounts of ore minerals (pyrite), the second stage is characterized by large amounts of ore minerals. Fluid inclusions in vein quartz contain C-H-O fluids of variable compositions. Three main types of fluid inclusions are recognized at room temperature: type I, two-phase, aqueous vapor and an aqueous liquid phase (L+V); type II, aqueous-carbonic inclusions, a CC2-liquid with/without vapor and aqueous liquid (LCO2+VCC2+Laq.); type III, mono-phase aqueous liquid (Laq.). Data from fluid inclusion distribution, microthermometry, and gas analysis indicate that fluids associated with Au mineralized quartz veins (stage 2) have moderate salinity ranging from 1.91 to 16.43 wt% NaCl equivalent (modeled salinity around 8–10 wt% NaCl equiv.). These veins formatted at temperatures from 80d̀ to 280d̀C. Fluids associated with barren quartz veins (stage 3) have a low salinity of about 1.91 to 2.57 wt% NaCl equivalent and lower temperature. There is evidence of fluid immiscibility and boiling in ore-forming stages. Stable isotope analyses of quartz indicate that the veins were deposited by waters with δO and δD values ranging from those of magmatic water to typical meteoric water. The gold metallogenesis of Muru gold belt has no relationship with the granite, and formed during the late stage of the crust thinning of North China.

Structural overview of selected Group II kimberlite dyke arrays in South Africa: implications for kimberlite emplacement mechanisms

Group II kimberlite dykes occur in small, dominantly en-echelon dyke-fracture arrays, with individual dyke-fractures showing small angular variations from their array trends (5° to 15°). The analysed dyke systems are characterized by closely matching opposing dyke contacts, “ in-situ ” breccia, multiple kimberlite stringers within a dilated dyke-parallel fracture cleavage, wedge-shaped apophyses in bent bridges at dyke-fracture offsets/overlaps, kimberlite-free offset/overlap areas and calcite vein fibres orthogonal to dyke contacts. Commonly found microscopic structures include synemplacement/syncrystallization calcite veinlets, containing high aspect ratio stretched fibrous calcite, and elongate phlogopite phenocrysts and serpentinized olivine phenocrysts growing across the width of these veins. Both macro- and microscopic structures support a model of orthogonal host rock dilation during kimberlite emplacement. Terminations of dyke-fracture segments show minimal curvature or overlap, suggesting that remote horizontal stresses dominated during their emplacement (“passive” intrusion), as opposed to magma overpressured systems wherein dyke or dyke-fracture overlaps curve strongly towards each other (“active” intrusion). The application of Mohr diagrams suggests that low differential stresses, with no or only a very minor shear component, prevailed at the time of emplacement. The dominance of remote horizontal forces, imparting small differential stresses to the brittle portions of the crust, a closely-spaced, dilating dyke-parallel fracture cleavage ahead of the dyke tip (imparting a local suction) and the low-volume, low-viscosity, highly volatile nature of kimberlitic magmas may explain their empirically-constrained high emplacement velocities. This, in turn, explains the means by which such magmas may entrain significant volumes of high specific-gravity mantle material. Mobile hydrofracturing in the fringe zones around dilated craton-scale jointing is proposed to be a viable mechanism for kimberlite emplacement.

Upper Triassic and Lower Jurassic beds in the Podutik area near Ljubljana (Slovenia)

In the Podutik area near Ljubljana in a very shallow lagoon environment of the restricted shelf on the Dinaric carbonate platform the Main dolomite formation originated in Norian and Rhaetian. In the northern part of the carbonate platform on the opened shelf area the Dachstein limestone has been deposited. In the studied area both above-mentioned lithologic units occur in the typical Lofer development. Due to lack of fossils the Rhaetian stage is not separated from the Norian one. The boundary between the Triassic and the Jurassic period is marked chiefly by the gradual lithologic transition. It is focused into the zone, where the Lofer limestone with large megalodontids passes into the limestone with characteristically dark red calcite veinlets, intercalations of oolitic and onkolitic limestone and algae Palaeodasycladus mediterraneus Pia.

Fault controlled sequential vein dilation: competition between slip and precipitation rates in the Austin Chalk, Texas

Multi-layered calcite veins in a dilatant jog of a left-stepping, left-slipping shallowly buried fault segment are composed of alternating millimeter- to submillimeter-thick calcite veinlets and host lithons forming a coarse ‘crack–seal’ texture. The grain fabrics in calcite veinlets are mostly equant or irregular, suggesting face-controlled grain growth in a fluid-filled cavity. The relatively thick veinlets can be developed by progressive fault slip and veinlet opening simultaneously with calcite precipitation under low effective stress. Continuous changes in the oxygen isotopic compositions of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed from the footwall to the hanging wall. There is no particular evidence that these veins represent excursions in fluid pressure or instantaneous fracture opening related to episodic fault slip; the fracture formation and filling cycle could have taken place along a continuously slipping fault contained within a porous rock with normal fluid pressure.

A Fluid Inclusion Study of Au-Bearing Quartz Vein Systems inthe Central and North Deborah Deposits of the Bendigo Gold Field, CentralVictoria, Australia

The Central and North Deborah gold deposits, with a production of about 7 tons Au, are located in one of 15 domes in the Bendigo gold field, central Victoria, Australia, an important mesothermal lode gold province. The gold-bearing quartz vein systems are hosted predominantly by reverse faults in Lower Ordovician turbidites, which underwent lower greenschist facies metamorphism. In the Central and North Deborah mines, there are six stages of quartz veins that developed in the following sequence: (1) laminated veins, with a metal association of Au-As-Sb-Pb-Zn-Ni; (2) spurs (Au); (3) massive barren veins (main dilation stage); (4) brecciated veins, with Au-As; (5) quartz-ankerite veinlets; and (6) late, pure quartz or calcite veinlets. Of the three mineralized vein stages (1, 2, and 4), only the first one predated or was coeval with the main deformation event, whereas stages 2 and 4 postdated the main deformation. The vein structures indicate that vein development was controlled by repeated fluctuations of fluid pressure inducing hydraulic fracturing. Fluid inclusions in vein quartz contain C-O-H fluids of variable compositions. Three main types of fluid inclusions are recognized at room temperature: type I, two-phase, primary (CH4)V + (H2O)L fluid inclusions; type II, two- or three-phase, primary-pseudosecondary (CO2)V or L + (CH4)V or L + (H2O)L fluid inclusions, including IIa, CH4-CO2-H2O, and IIb, CO2-H2O fluid inclusions; and type III, (H2O)V + (H2O)L fluid inclusions. Type III fluid inclusions are further divided into three groups: IIIa, primary-pseudosecondary, vapor-rich; IIIb, primary-pseudosecondary, liquid-rich; and IIIc, secondary, liquid-rich. Data from fluid inclusion distribution, microthermometry, and Raman spectroscopy indicate that fluids associated with Au mineralized quartz veins (stages 1, 2, and 4) have moderate salinity ranging from 0 to 12 wt percent NaCl equiv (modeled salinities around 7–8 wt % NaCl equiv). These veins formatted at temperatures from 325° to 375°C, and pressures of 200 to 300 MPa, with emplacement depths of about 8 to 12 km. Fluids associated with barren quartz veins (stages 3, 5, and 6) have a low salinity of about 0 to 3 wt percent NaCl equiv (modeled salinities about 2–3 wt % NaCl equiv) and lower temperatures. There is evidence of fluid immiscibility in all vein stages and mixing between fluids of the same souce, but differing fO2 may also have influenced gold deposition (Cox et al., 1995). Hydrothermal ore-forming fluids responsible for gold mineralization are in the CH2-CO2-H2O-NaCl system, whereas those of massive barren veins (stage 3) and veinlets (stages 5 and 6) are in the CO2-H2O-NaCl and H2O-NaCl systems, respectively. The carbonic phases of type I and II fluid inclusions range from pure CH4, through mixed CH4 and CO2, to pure CO2. Gold ore-forming fluids were weakly acidic, with f O2 estimated to have been 10–25 to 10–37 bars, and generally above the CO2/CH4 and below the SO2/H2S buffer boundaries throughout gold mineralization. The relatively reduced mineralizing fluids are similar to those of other turbidite-hosted lode gold deposits and counterparts in volcanic-plutonic terranes.

Genesis of Vein Stockwork and Sedimentary Magnesite and Hydromagnesite Deposits in the Ultramafic Terranes of Southwestern Turkey: A Stable Isotope Study

Vein stockworks and lacustrine developments of cryptocrystalline magnesium carbonates of Neogene and Quaternary age occur within the partially serpentinized, discontinuous ultramafic belts of southwestern Turkey. They are comparable to the Neogene cryptocrystalline magnesite bodies elsewhere in the Alpine orogen to the northwest and southeast. Our previous work (Fallick et al., 1991) suggested that cool (less than or equal to 100 degrees C) modified meteoric water was the mineralizer, that ultramafic rock was the source of the magnesium, but that there were three separate sources of the (bi)carbonate. These sources were distinguishable by their stable isotope composition as follows: (1) low-temperature carbonate with delta(18)O((SMOW)) values of similar to 36 per mil and delta(13)C((PDB)) values of similar to 4 per mil, from CO2; (2) moderate-temperature carbonate with delta(18)O((SMOW)) values of +28 per mil and delta(13)C((PDB)) values of -15 per mil, by decarboxylation of organic-rich sediments; and (3) higher temperature carbonate with delta(18)O((SMOW)) values of similar to 19 per mil and delta(13)C((PDB)) values of similar to 3 per mil, assumed to have been generated by contact metamorphism of Paleozoic marine limestone at depth. In general these magnesite deposits were found to fall into two groups, comprising carbonate generated on two mixing lines. The first group spanned the putative mixing line from the atmospheric source (1) to organically derived source of CO2 (2). The second group extended between source (1) and the thermal source (3), although there were concentrations either around the end, or precisely at the contact metamorphic end of the line. In the present study we found that large stockwork deposits at Helvacibaba and Koyakci Tepe have delta(13)C((PDB)) and delta(18)O((SMOW)) values averaging similar to-12 and similar to+27 per mil, respectively, indicating a derivation mainly by oxidation of organic-rich metasediments perhaps underthrust at depth (end-member 2), with some involvement of carbon dioxide as bicarbonate in the circulating, hot, and modified meteoric water (end- member 1). Calcite veinlets in a meta-argillite of the Cambro- Ordovician Seydisehir Formation, most likely to have been underthrust beneath the stockworks, yielded delta(13)C((PDB)) values of -20 per mil, consistent with, though not proving, oxidized organic carbon being one of the sources of carbonate. The delta(18)O((SMOW)) values of these same veinlet carbonates are also rather low (22 parts per thousand), indicating precipitation from heated ground water, though their age is unknown. The major stratiform magnesite deposit at Hirsizdere in the center of the Menderes graben has delta(13)C((PDB)) and delta(18)O((SMOW)), values averaging similar to 3 and similar to 25 per mil, respectively and thus appears to be an example of the hydrothermal-sedimentary (i.e., exhalative) type (Ilich, 1968). In contrast, the hydromagnesite stromatolites presently growing in Salda Golu (Lake Salda) are apparently developing at cool ground-water seepages. The gross morphology of the Salda Golu stromatolites and the hydromagnesite sediments therefrom is reminiscent of that revealed in the Bela Stena magnesite pit in Serbia. These lacustrine deposits have mean delta(13)C((PDB)) values of similar to 4 and similar to 2 per mil and mean delta(18)O((SMOW)) values of similar to 36 and similar to 33 per mil, respectively, i.e., they both plot broadly over the CO2-meteoric water field (end- member 1), consistent with microbially mediated precipitation at cool ground-water seepages in enclosed evaporating lakes.

Sequential vein growth with fault displacement: An example from the Austin Chalk Formation, Texas

To determine the opening and precipitation history and characteristics of vein‐forming fluids, analyses of oxygen and carbon isotopes and trace elements were carried out on multilayered crack‐seal calcite veins in the Austin Chalk Formation near San Antonio, Texas. The veins developed within the normal fault zones possessing unique chemical and textural characteristics which indicate sequential vein development. They are composed of alternating millimeter‐ to submillimeter‐thick calcite veinlets and host lithons, occasionally crosscut by coarse, equant‐grained secondary calcite veins. Regular changes in δ18O (e.g., −2.6 to −5.6‰, Pee Dee belemnite (PDB)) of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed. A systematic δ18O decrease in the vein opening direction primarily resulted from a continuous increase in temperature of the ascending fluids delivered to the Austin Chalk. Relatively constant δ13C (approximately +1.4±0.4‰, PDB) for the multilayered veins and most secondary veins indicates that the composition of fluids from which the calcite veins precipitated was initially buffered by the bulk chalk. There is no spatial variation in trace element composition of the calcite veinlets along the length of veins. Low Sr concentrations in both calcite veinlets and secondary veins relative to those of the host chalk reflect a low partition coefficient of Sr in calcite during vein formation. Normal faults in the Cretaceous Austin Chalk were conduits to upwardly mobile vein‐forming fluids.

Occurrence and morphology of carbonate concretions in the Beulah-Zap coal bed, Williston Basin, North Dakota

Carbonate concretionary bodies were encountered during mining of the Beulah-Zap lignite seam in the Coteau Properties' Freedom mine, Mercer County, North Dakota. Preliminary studies show that areal and vertical distribution of the concretions are variable. All concretions examined are composed almost entirely of calcite. They occur as thin tabular bodies, as more or less elliptical forms, or as tear shaped bodies, and may occur individually or as clusters of buff-colored, poorly consolidated to solidly crystalline material. The carbonate masses vary in size from a few millimeters to tens of centimeters. Bedding in the lignite may display some compactional folding over dense spheroidal to elliptical concretions, indicating formation of the concretions prior to compaction. Internal morphology of the concretions is complex, and includes cone-in-cone structure, cross-cutting calcite veinlets, and multiple generations of calcite. Carbon isotope values suggest the concretions are composed of biogenic carbonate, probably related to early diagenesis and decomposition of organic matter (peat); oxygen isotope values are light, and consistent with a freshwater origin.

Petrographic studies of Permian carbonates in southern Thailand

Detailed petrographic studies were conducted on Permian carbonates from four areas: Ratburi, Prachuab Khirikhan, Ko Ang Thong, and Surat Thani-Phang Nga. Six carbonate facies were identified in these areas—mudstone, wackestone, packstone, grainstone, algal boundstone, and intramicrite. The allochems of these carbonates are similar and composed of bioclastic and non-bioclastic grains. The bioclastic grains are blue-green algae (stromatolite), green algae (dasycladacean), tabulate corals, solitary corals, crinoids, foraminiferas ( Shanita sp. and Hemigordius sp.), brachiopods, bryozoan, gastropods and unidentified skeletal fragments. The non-bioclastic grains are ooids, peloids, and intraclasts. Three phases of paragenesis were observed: micritization and neomorphism, dolomitization and non-ferroan/ferroan calcite fractures and veinlets. Micritization, neomorphism, and dolomitization are extensive. Rhombic and mosaic-crystalline dolomites had replaced carbonate mud, sparry calcite cement as well as the allochems. Abundant non-ferroan calcite veinlets cross cut allochems, mud matrix, and dolomite rhombs which suggests they postdate dolomitization. These carbonate facies were interpreted as interior-platform carbonate with a generally well developed shoaling-upward sequence starting from mudstone to wackestone, packstone, and grainstone. Local algal boundstone or algal mound and intramicrite could indicate low energy, regressive cycles.