Generations Of Calcite Research Articles

Formation of the Auriferous and Barren North Dipper Veins in the Sigma Mine, Val d'Or, Canada: Constraints from Structural, Mineralogical, Fluid Inclusion, and Isotopic Data

The Sigma-Lamaque mine (>100 metric tons (t) of gold) is an example of an Archean greenstone belt gold deposit formed by processes related to multistage hydrothermal fluid circulation. The earliest recognized vein systems are steeply to moderately dipping fault-fill veins within shear zones and contemporaneous subhorizontal veins, which were overprinted by the late North Dipper vein system. The latter strikes east-west, dips moderately to the north, and exhibits complex internal geometry with features similar to fault-fill and extensional veins, and therefore is interpreted as extensional shear veins. This late vein system was considered one of the major sources of gold in the shallow underground and open-pit ore reserve estimations. These late North Dipper veins were studied in detail in the Sigma pit where they contain ore-grade gold and are hosted by the calc-alkaline tuffs of the Val d’Or Formation and in the North zone where they are barren and hosted by the tholeiitic volcanic rocks of the Jacola Formation. In the Sigma pit, these veins have apparent thicknesses of 0.8 to 3 m and very irregular contacts with the vein walls. They comprise irregular zones of milky quartz filling, disrupted blocks of layered quartz and tourmaline and slabs of foliated pyrite-rich wall rock. Layers with tourmaline and quartz crystals oriented perpendicular to the vein walls are observed close to the vein walls. The veins contain minor amounts of calcite, pyrite, rutile, and traces of chlorite, muscovite, pyrrhotite, chalcopyrite, galena, bismuth tellurides, and gold. Locally, the sulfide content can attain 10 modal percent of the vein mineralogy. In the veins, native gold (with 6 wt % Ag) occurs rarely associated with the main vein-filling minerals, commonly as inclusions in pyrite (main vein-filling stage), and most commonly filling fractures in pyrite, tourmaline, and quartz, where it is alloyed with 8 to 17 wt percent Ag and is associated with calcite and Bi tellurides (late auriferous stage). The proximal wall-rock alteration related to the auriferous North Dipper veins overprints the metamorphic assemblage and is characterized by abundant fine-grained plagioclase, calcite, quartz and minor chlorite, tourmaline, and pyrite in the tuffs and more abundant muscovite in the intrusive feldspar porphyry. In the wall rock, gold with 6 to 28 wt percent Ag is found locally filling fractures in pyrite, where it is associated with a late generation of calcite. The barren North Dipper vein also comprises mainly quartz and tourmaline but lacks calcite and gold, has a very low content of sulfides ( Aqueous-carbonic fluid inclusions with salinities less than 8 wt percent (NaCl equiv) and homogenization temperatures between 228° and 440°C, and carbonic fluid inclusions with less than 0.2 mol percent CH 4 coexisting in the same healed fractures, are common in the auriferous North Dipper veins but very rare in the barren vein where aqueous fluid inclusions have lower temperatures (dominantly 125°–225°C). Crush leaching and laser ablation time-of-flight inductively coupled mass spectrometry (LA-TOF-ICPMS) of fluids in the inclusions indicate elevated concentrations of Ni, Cu, Sb, Pb, and Ag in the fluids in the auriferous veins compared to the aqueous fluids in the barren veins. This suggests different sources for the metals in the fluids associated with the gold, possibly including both igneous and sedimentary rocks. Calculated oxygen isotope equilibration temperatures for quartz-tourmaline pairs from the auriferous main vein-filling stage are 323° to 370°C. This is within the range documented in fluid inclusions related to the late auriferous stage, indicating that the temperature of the auriferous hydrothermal fluids did not change appreciably during the evolution of the system. Calculated δ 18 O H 2 O values for the mineralizing fluids are 9.5 per mil, consistent with either low water/rock ratio systems associated with high-grade metamorphism or a magmatic origin for the fluids. Gold included in pyrite may have precipitated during the main vein-filling stage due to the decrease of H 2 Sconcentration in the hydrothermal fluid following the deposition of sulfides. However, gold in fractures in the main vein-filling minerals precipitated later, due probably to dilution of the auriferous aqueous-carbonic fluids by mixing with aqueous fluids. The absence of ore-grade gold in the barren North zone vein is probably due to the lack of circulation of auriferous aqueous-carbonic fluids in the North zone, located distal to the first-order Larder-Lake Cadillac fault, which may have served as the main regional conduit for the auriferous aqueous carbonic fluids.

Genesis of filamentary pyrite associated with calcite crystals

The calcite of the hydrothermal Surneshko Kladenche copper vein deposit from the Rossen ore field, Bulgaria, sometimes encloses peculiar filamentary pyrite crystals. Three successive calcite generations were observed belonging to a low-temperature (<235°C) carbonate paragenesis formed in open cavities of the ore veins after the main chalcopyrite mineralisation. Three generations of pyrite crystals are associated with these calcite crystals: pyrite 1, with (001) elongated columnar crystals which crystallised in open space; pyrite 2, in groups of long (up to 10 mm) sub-parallel tortuous filaments of varying thickness (3-20 µm) which are oriented nearly perpendicular to the surfaces of a transient w{3145} scalenohedral calcite crystal zone; and pyrite 3 ,m ade of slightly elongated small crystals, located in the outermost v{2131} zone of the same scalenohedral calcite crystals. The columnar pyrite 1 is formed by subsequent thickening of thin straight whiskers rapidly grown under diffusional regime, whereas the filamentary pyrite 2 and 3 grew contemporaneously with the enclosing calcite crystal.

Fatty acids in sparry calcite fracture fills and microsparite cement of septarian diagenetic concretions

Sparry calcite fracture fills and concretion body cements in concretions from the Flodigarry Shale Member of the Staffin Shale Formation, Isle of Skye, Scotland, entrap and preserve mineral and organic materials of sedimentary and diagenetic origin. Fatty acids are a major component of the lipids recovered by decarbonation and comprise mainly n-alkanoic and α-ω dicarboxylic acids. Two generations of fracture-fill calcite (early brown and later yellow) and the concretion body microspar yield significantly different fatty acid profiles. Early brown calcites yield mainly medium-chain n-alkanoic acids with strong even predominance; later yellow calcites are dominated by α-ω dicarboxylic acids with no even predominance. Both fracture fills lack the long-chain n-alkanoic and α-ω dicarboxylic acids additionally recovered from the concretion bodies. The absence of longer chain acids in the calcite spar fracture fills is inferred to result from the transport of fatty acids by septarian mineralising fluids whereby low-aqueous solubility of longer chain acids or their salts accounts for their relative immobility. Comparative experiments have been carried out using conventional solvent extraction on the concretion body and associated shales, both decarbonated and untreated. Extracted lipid yields are higher, but the fatty acids probably derive from mixed locations in the rock including both kerogen- and carbonate-associated lipid pools. Only experiments involving decarbonation yielded α-ω dicarboxylic acids in molecular distributions probably controlled mainly by fluid transport. Alkane biomarker ratios indicate very low thermal maturity has been experienced by the concretions and their host sediments. Septarian cracks lined by brown calcite formed during early burial. Microbial CO 2 from sulphate-reducing bacteria was probably the main source of mineralising carbonate. Emplacement of the later septarian fills probably involved at least one episode of fluid invasion.

The petrology of ikaite pseudomorphs and their diagenesis

Petrographic examination of pseudomorphs after ikaite (CaCO 3 .6H 2 O) from five localities around the world, with ages ranging from Permian to Quaternary, indicates many petrographic similarities, with up to three generations of calcite, plus other ‘late’ infilling or replacive cements. The similarities of the first generation of calcite (type 1) developed in all samples examined suggests that the initial ikaite decomposition followed a similar pattern in all these examples. Type 2 calcite is spherulitic and type 3 is calcite spar that infills the cavities between types 1 and 2. Types 2 and 3 are present in pseudomorphs from the Black Alley Shale (Permian, Australia), Carnavon Gorge (probably Permian, Australia) and the Fur Formation (Eocene, Denmark). Gennoishi (pseudomorphs from the Miocene, Japan) and jarrowites (pseudomorphs from Quaternary muds, River Tyne, UK) contain only type 1 calcite.

Rb-Sr and Nd-Sm isotopes in fluorite related to the granites of the Bushveld Complex

The Lebowa Granite Suite of the Bushveld Complex is a differentiated sheeted intrusion, between 1.5 and 3.5km thick, with an areal extent of some 30 000km2. The granite suite and the overlying Rooiberg Group felsic volcanics are near coeval with the mafic Layered Suite all being dated within 2057±3 Ma. Despite this narrow time frame of emplacement ages, a compilation of age dates ranges from ~2.06 to ~1.05 Ga and reported 87Sr/86Sr are often high and with large uncertainties. Because of hydrothermal alteration, a study of chemical and isotopic variations of the whole rocks is equivocal. Since fluorite has a low Rb/Sr ratio, and is stable at low temperatures, the back correction of the 87Sr/86Sr value has minimum uncertainty and the initial 87Sr/86Sr ratio (Ro) of the magma or fluid from which it crystallised can be determined. In this study Rb, Sr, Nd and Sm analyses of fluorite from a number of different types of occurrences are reported. In addition, strontium isotope analyses were made on ‘primitive’ granite from a vein cross-cutting the layered mafic suite in the eastern Bushveld and compared to data obtained for an ‘evolved’ feldspar from the Zaaiplaats granite. We recognise two distinct processes and events: a primary magmatic process that occurred at 2057±3 Ma and took place in a very short time span (<1my). Hydrothermal fluid was not retained and a much later secondary hydrothermal process possibly related to the marginal tectonism of the Kaapvaal craton in the Limpopo belt, which may have been a longer-lived event. The primary fluorite in stockworks, volcanic pipes and disseminated in some granites originated from late magmatic fluid that also infiltrated and engulfed country rock ( eg Buffalo). These fluorites have variable but low 87Rb/86Sr, low Ro and moderate Sm and Nd (>50 and >100ppm respectively). The samples from the Vergenoeg pipe have accessory fluorite with low Sr concentrations (32 to 70ppm) and Ro in the range 0.7166 to 0.7225. Fluorites from Buffalo, Zwartkloof and Spoedwel show a wide range of Sr content and Ro, from 39 to 231ppm and 0.727 to 0.794 respectively, resulting from mixing of a granitic fluid with assorted country rocks. These late magmatic fluids generated large fluorite bodies such as Vergenoeg and had a significant autometasomatic imprint as indicated by chloritisation of biotite such as in the Steelpoort Park granite veins. For the primary fluorite, most samples fall on an array indicating an age of ~2.05 Ga. but with a large scatter. A different and later hydrothermal fluid produced a totally different generation of fluorite and calcite in open vugs and pegmatites at Zaaiplaats and Buffalo. These secondary fluorites display very high Ro, have low abundances of Sm and Nd with Sm varying from 30.97 to 0.17ppm and Nd ranging from 69.59 to 1.22ppm. In particular, different coloured fluorite from one pegmatitic pod show an extreme range of Ro varying from ~ 0.7199 to ~0.8518, with even higher Ro in the calcite.

Is the transition impact to post‐impact rock complete? Some remarks based on XRF scanning, electron microprobe, and thin section analyses of the Yaxcopoil‐1 core in the Chicxulub crater

Abstract The transition from impact to post‐impact rocks in the Yaxcopoil‐1 (Yax‐1) core is marked by a 2 cm‐thick clay layer characterized by dissolution features. The clay overlies a 9 cm‐thick hardground, overlying a 66 cm‐thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro‐conglomerate layers with litho‐ and bioclasts and the altered remains of impact glass, now smectite. The micro‐conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex‐zoned, idiomorphic overgrowth, with Mn‐rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K‐feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni‐rich spinels in the clay layer, and the absence of basal Paleocene biozones P0 and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yax‐1 core that can be used to argue against the synchroneity of the end‐Cretaceous mass‐extinctions and the Chicxulub crater.

The structural and diagenetic evolution of injected sandstones: examples from the Kimmeridgian of NE Scotland

Injected sandstones occurring in the Kimmeridgian of NE Scotland along the bounding Great Glen and Helmsdale faults formed when basinal fluids moved upward along the fault zones, fluidizing Oxfordian sands encountered at shallow depth and injecting them into overlying Kimmeridgian strata. The orientation of dykes, in addition to coeval faults and fractures, was controlled by a stress state related to dextral strike-slip along the bounding fault zones. Diagenetic studies of cements allow the reconstruction of the fluid flow history. The origin of deformation bands in sandstone dykes and sills was related to the contraction of the host-rocks against dyke and sill walls following the initial stage of fluidized flow, and these deformation bands are the earliest diagenetic imprint. Early non-ferroan calcite precipitated in injection structures at temperatures between 70 and 100 °C, indicating that it precipitated from relatively hot basinal fluids that drove injection. Coeval calcite-filled fractures show similar temperatures, suggesting that relatively hot fluids were responsible for calcite precipitation in any permeable pathway created by dextral simple shear along the faults. During progressive burial, percolating sea water was responsible for completely cementing the still relatively porous injected sandstones with a second generation of ferroan calcite, which contains fluid inclusions with homogenization temperatures below 50 °C. During this phase, depositional host sandstones were also cemented.

Origins, closed system formation and preservation of calcites in glaciated crystalline bedrock: evidence from the Palmottu natural analogue site, Finland

A study of the geochemistry of fracture filling calcite from the Palmottu research site, a U–Th deposit located about 100 km NW of Helsinki, Finland, was part of a comprehensive natural analogue research project considering the migration of U in the crystalline bedrock environment. Fracture filling minerals act as records of the paleohydrological and chemical conditions and therefore provide insight into magmatic processes, ore deposits and long-term stability of the crystalline bedrock environment in relation to deep disposal of radioactive waste. An earlier investigation of calcite and associated U minerals identified a recent origin for these fracture infillings. Uranium series disequilibrium data indicated that relatively recent, glacial water had penetrated to a depth of 62 m. A fluid inclusion study, combined with the isotope geochemistry of several generations of fracture calcite, was undertaken to further study this aspect and to better understand the thermal and fluid history in the crystalline rock environment in general. The study revealed that at least 3 fluids were recorded by fracture calcites: 1) A crystalline calcite precipitated at 139–238 °C from a low salinity, Na–Cl fluid of magmatic or metamorphic origin, 2) A massive calcite (with high salinity fluid inclusions) precipitated at 136–141 °C from a high salinity, Ca–Na–Cl fluid of magmatic or metamorphic origin and 3) A massive calcite (with low salinity fluid inclusions) precipitated at 43–286 °C from a low salinity, Na–Cl fluid of magmatic or metamorphic origin that underwent equilibrium fractionation during cooling in a rock dominated system. The preservation of high temperature infillings and the cooling trend, indicating isotopic re-equilibration of water due to a very low water/rock ratio, demonstrate that the downward percolation of geologically recent waters is limited, despite the reworking of highly soluble U compounds in the upper tens of metres of the bedrock that was shown by U series disequilibrium studies. The veins in which these calcites occur have not experienced a later phase of fluid activity that dissolved or altered the calcites. Therefore, it is likely that they have not been reactivated as transmissive fractures since that initial hydrothermal episode, even during the Quaternary history in which the formation was subjected to ice sheet loading and unloading.

Methane-related carbonates formed at submarine hydrothermal springs: a new setting for microbially-derived carbonates?

Numerous small calcite mounds, up to 2.5 m in diameter and 0.75 m in height, accompanied by Ba, Hg and Tl mineralisation, occur in shallow submarine hydrothermal vents on the sea bottom, at 10 m depth, near Punta Mita, on the western coast of Mexico. The hydrothermal activity consists in water and gas (mainly nitrogen and methane) venting at 85°C, through a 100-m-long fissure hosted in basaltic rocks and partially covered by a thin layer of unconsolidated detrital sediments. The mounds consist of travertine-like calcite aggregates that develop around the main submarine hot springs amidst a hydrothermally altered basaltic host rock. Two main calcite generations are texturally recognisable: the first generation shows a radial-fibrous texture; the second is fine-grained calcite, which cements detrital grains and fills the pore spaces. The δ 13C analyses of calcite reveal a strong depletion in 13C, with values as low as −39.2‰ (Vienna PeeDee Belemnite), which suggest that microbial communities may have induced calcite precipitation through microbial methane oxidation. Barite, sulphides (mainly pyrite and cinnabar) and phosphates (carbonate–hydroxylapatite) are also present in the mounds in lower concentrations and form by direct precipitation from the hydrothermal fluid. The Punta Mita hydrothermal carbonate mounds represent a potentially novel environment for microbially induced carbonate mineralisation, which is characterised by high temperatures not encountered in areas of cold seep carbonate formation. Stable isotope results suggest that microorganisms responsible for the oxidation of methane may be present and active at temperatures near 85°C at the Punta Mita vents.

Carbonate-Hosted Zn-Pb Deposits in Upper Silesia, Poland: Origin and Evolution of Mineralizing Fluids and Constraints on Genetic Models

Microthermometric and crush-leach analyses of fluid inclusions in ore and gangue minerals of the Upper Silesian Zn-Pb deposits, Poland, along with first results of Rb-Sr geochronology on sulfides, provide important constraints on the paleohydrogeologic and metallogenetic models for the origin of these ores. The analyzed samples comprise two generations of dolomite, two generations of sphalerite, galena, and late calcite. The two dolomite generations and the late calcite were also analyzed for their oxygen and carbon isotope compositions, allowing a characterization of the mineralizing fluids. The ore-forming fluids represent highly saline (20–23 wt % CaCl 2 equiv) Na-Ca-Cl brines, episodically introduced into the Triassic host carbonates. They had an oxygen isotope composition of ~0 per mil V-SMOW. Their Na-Cl-Br content (molar Na/Br and Cl/Br ratios between 99 and 337 and between 248 and 560, respectively) suggests that they originated by evaporation of seawater, which most likely occurred in the Permian-Triassic. The relative concentrations of potassium (molar K/Cl between 0.0147 and 0.0746) and lithium (molar Li/Cl between 0.0004 and 0.0031) further indicate that the fluids significantly interacted with siliciclastic rocks. The ionic and calculated oxygen isotope compositions of the fluids indicate that they were more evolved than present-day brines in the Upper Silesian coal basin, and the present-day brines show more extensive mixing with low-salinity fluids. The first results of direct Rb-Sr dating of ore-stage sulfides yield an isochron model age of 135 ± 4 Ma for the mineralizing event. This is consistent with hydrothermal activity and ore formation in Upper Silesia occurring in response to Early Cretaceous crustal extension preceding the opening of the northern Atlantic Ocean. The data presented support a model in which bittern brines migrated down into the deep subsurface and evolved into mineralizing fluids owing to extensive water-rock interaction. They were episodically expelled along deeply penetrating faults during the Early Cretaceous to form Zn-Pb deposits in the overlying Mesozoic carbonate rocks.

Cross dating (Th/U- 14C) of calcite covering prehistoric paintings in Borneo

We present the first application of cross-dating (Th/U measured by thermo-ionization mass spectrometry (TIMS) and 14C measured by accelerator mass spectrometry (AMS)) of calcite covering prehistoric paintings. Th/U age estimates of cave drapery range from 9800 to 27,300 yr B.P. while conventional 14C age is estimated between 9900 and 7610 yr B.P. depending on the dead carbon correction. The age discrepancy is attributed to a disturbance of Th/U and/or 14C geochemical systems, showing the limits of the geochronological approach applied to this kind of material. For the Th/U system, the poor consistency of U data (U content, 234U/ 238U activity ratios) and apparent ages argue for open system conditions. For 14C system, variation of the dead carbon fraction (dcf) and a possible mixing of successive generations of calcite could account for age discrepancy. Nevertheless, one sample shows concordant ages for the two methods. Compatible ages through corrections for open system conditions are assumed for other samples. Then, the cross-dating suggests 9900 yr as the minimum age of the piece of drapery; the underlying painting must be older. This study of rock art demonstrates the presence of a Pleistocene population before 9900 yr in the southeast of Borneo, whereas previously the only population in evidence in this area was of Austronesian type from ∼5000 to 6000 yrs ago.

Palaeohydrogeological Evidences from Fracture Filling Minerals - Results from the Äspö/Laxemar Area

ABSTRACTTwo different EU projects: the EQUIP project (Evidences from Quaternary Infills from Palaeohydrogeology) running from 1997 to 2000 and the ongoing PADAMOT-project (Palaeohydrogeological Data Analysis and Model Testing) have addressed the possibility of using fracture minerals as recorders of past hydrochemical and hydrogeological environments. The Swedish contribution in these two projects has concentrated on studies of fracture filling calcites from the Äspö/Laxemar area, southeast Sweden.The analyses applied in the two projects comprise stable isotope ratios of carbon and oxygen and strontium together with trace element contents and textural studies. The results obtained so far (mainly from the EQUIP project) show that several generations of calcite can be identified, chemical zoning is common, and the influence on calcite precipitation of fresh and marine water decreases with depth. It can be concluded that fine scale zoning, possible dissolution/redistribution of calcite, and the disturbances caused by drilling (where loose material which may contain young calcite precipitates were lost), introduce uncertainties in the recognition of different calcite generations. However, despite these problems it is indicated that calcite fracture fillings in Precambrian bedrock environments can add information e.g. concerning changes in groundwater chemistry and redox conditions in the past. The possibilities and problems with palaeohydrogeological interpretations are discussed based on the available data.

Carbonate-Hosted Zinc-Lead Deposits in the Lower Cambrian of Hunan, South China: A Radiogenic (Pb, Sr) Isotope Study

Zinc-lead deposits hosted by Lower Cambrian carbonate rocks near Huayuan in the Hunan-Guizhou orogenic belt, southern China, have been studied by means of Pb and Sr isotopes. The deposits are characterized by two mineralization stages: (1) a main economic stage comprising cockade-style sphalerite, galena, pyrite-marcasite, fluorite, barite, and calcite, and (2) a subsequent noneconomic stage with nodular (rosette) sphalerite, saddle dolomite, and bitumen. Both ore stages are postdated by a generation of late calcite. Main-stage sulfides (206Pb/204Pb = 18.162–18.225) display a steep linear trend on conventional Pb-Pb diagrams which indicates binary mixing of lead components bearing the imprint of ancient, 207Pb-dominated crustal rocks and reservoirs with elevated Th/U ratios, possibly located in the lower crust. Nodular rosette sphalerites fall outside this line but reflect a second mixing trend between mineralizing fluids characterized by 206Pb/204Pb > 18.3 and lead isotopically similar to specific main-stage cockade sphalerites, suggesting either remobilization of Pb from main-stage ores or contemporaneous mixing of three distinct lead components in the course of an evolving hydrothermal event. The 87Sr/86Sr ratios of diagenetic and hydrothermal carbonate cements and barites range from 0.70905 to 0.71006 and increase systematically with the paragenetic sequence, indicating progressive mixing of marine Sr derived from the Lower Cambrian carbonate host rocks with radiogenic Sr supplied by external mineralizing fluids. In order to identify possible local metal sources, U-Pb and Rb-Sr analyses were performed on six whole- rock samples from the Lower Cambrian sequence and their time-integrated Pb and Sr isotope evolution was compared to the isotopic signature of the mineralization. Whereas shales and black shales of the Cambrian Balang and Niutitang Formations underlying the ore-bearing Qingxudong carbonate unit appear to be possible external sources for the radiogenic Sr found in hydrothermal carbonates, the U-Pb data do not permit these shales to be considered as primary source rocks for the ore lead. Both the Pb and the Sr isotopes indicate mineralizing fluids that had interacted with various external metal sources, some of them most probably being located in sedimentary or crystalline basement rocks. This refutes prevailing genetic models favoring the hydrocarbon- and metal-rich Lower Cambrian Niutitang black shales as the most likely source reservoir for the Zn and Pb in the deposits of the Huayuan district. The Pb and Sr isotope mixing trends observed for the ore and gangue preciptates reflect small-scale heterogeneous fluid systems that were not capable of producing large deposits. This may indicate that the Huayuan district is not a favorable target for future exploration of economic ore deposits. It also limits the prospects of successful radiometric dating of the mineralization, since conventional geochronologic models require initial isotopic homogeneity.

Variability of petroleum inclusions in vein, fossil and vug cements—a geochemical study in the Barrandian Basin (Lower Palaeozoic, Czech Republic)

Although no commercial oil or gas occurrences have been found in the Barrandian Basin, residual oils and petroleum inclusions give evidence about the petroleum history of this Lower Palaeozoic Basin. Petroleum inclusions are hosted in six generations of calcite and quartz cements that can be attributed to different stages of a basinal cycle and associated diagenetic events. They were analysed using an on-line crushing and an off-line crush-and-leach approach. Five different groups (PI-A to PI-E) and intermediate mixtures (PI-M) could be differentiated based on the relative distribution of n-alkanes. All oils had a calculated aromatic maturity Rc ranging between 0.9 and 1.6%. PI-A shows a molecular weight maximum in the range of n-C8–11 with a constant attenuation towards higher carbon numbered n-alkanes. This distribution is interpreted as the pristine precursor oil for other petroleum inclusion groups. PI-B has a maximum weight abundance at n-C15–20. The genetic relationship between PI-B and PI-A can tentatively be explained by mixing effects or by gas stripping. PI-C is bimodal and characterised by a molecular weight maximum at n-C32–34, and also a molecular weight maximum at n-C15–20 similar to that of group PI-B. PI-D shows a normally distributed molecular weight maximum in the range n-C25–28 and is interpreted as a wax precipitate from ascending gas saturated in n-alkanes. PI-E contains predominantly gaseous compounds with only a few higher hydrocarbons. Thompson's aromaticity values are elevated for condensates of group PI-E, and also for group PI-C that is dominated by long-chained n-alkanes. This gives evidence that fault-bound petroleum migration in the Barrandian Basin was associated with evaporative fractionation for group PI-C and PI-E. Samples of group PI-E yielded gas-range compounds only, and all come from a last generation of vein-filling whitish calcite that was formed in a late uplift phase of the basin. For other petroleum inclusion groups, only a vague preferential occurrence in specific mineralisation phases or stratigraphic intervals of the host rock has been found. This reflects the predominance of fracture-bound migration in the Barrandian Basin. Aromatic maturity values also showed no correlation between either crystal type or PI-group. In six of the investigated petroleum inclusion samples alk-1-enes are present. These terminal olefins are interpreted to be the result of the natural pyrolysis of petroleum due to the intrusion of volcanic dykes or hydrothermal processes. An artificial generation of olefins during sample work-up and analysis is unlikely. The preservation of alk-1-enes from Palaeozoic times was possible due to the protecting environment of petroleum inclusions.

Rift valley sedimentation and diagenesis, Tanzanian Examples - A review

The main diagenetic events of polyphase rift valley successions are controlled by provenance geology, tectonic evolution and subsidence, weathering related to different episodes of fresh water flushing, and topographic and climatic effects. Based on the Tanzanian examples as case studies and comparisons with published studies on rift valley sedimentation and diagenesis, the main diagenetic phases of Tanzanian rift valley successions can be presented: 1. Early diagenetic precipitation of evaporites, hematite and calcrete along with feldspar dissolution, and in some cases formation of pore-filling clay minerals, zeolites and feldspar, may take place. 2. Middle diagenetic formation of albite, zeolites, chlorite, recrystallization of calcite and some quartz precipitation have been reported from several rift valley settings. Continuous subsidence results in enhanced recrystallization of calcite, with a trend towards larger crystal size and more poikilotopic configurations, and new generations of quartz and clay minerals such as e.g . illite. The precipitation of dolomite, ankerite and siderite have commonly been observed during this stage. 3. Late diagenetic processes is related to uplift and renewed tectonic activity. Under such conditions new generations of hematite, calcite and clay minerals were formed, partly as a result of feldspar dissolution caused by meteorically-induced fresh water flushing.

Assessing the past thermal and chemical history of fluids in crystalline rock by combining fluid inclusion and isotopic investigations of fracture calcite

Fluid inclusion studies combined with the isotope geochemistry of several generations of fracture calcite from the Olkiluoto research site, Finland, has been used to better understand the past thermal and fluid history in the crystalline rock environment. Typically, fracture mineral investigations use O and C isotopes from calcite and an estimate of the isotopic composition of the water that precipitated the calcite to perform δ 18O geothermometry calculations to estimate past temperature conditions. By combining fluid inclusion information with calcite isotopes, one can directly measure the temperature at which the calcite formed and can better determine past fluid compositions. Isotopic, petrologic and fluid inclusion studies at the Olkiluoto research site in Finland were undertaken as part of an investigation within the Finnish nuclear waste disposal program. The study revealed that four fluids were recorded by fracture calcites. From petrologic evidence, the first fluid precipitated crystalline calcite at 151–225°C with a δ 13C signature of −21 to −13.9‰ PDB and a δ 18O signature of 12.3–13.0‰ SMOW. These closed fracture fillings were found at depths greater than 500 m and were formed from a high temperature, low salinity, Na–Cl fluid of possible meteoric water altered by exchange with wallrock or dilute basinal origin. The next fluid precipitated crystalline calcite with clay at 92–210°C with a δ 13C signature of −2.6 to +3.8‰ PDB and a δ 18O signature of 19.4–20.7‰ SMOW. These closed fracture fillings were found at depths less than 500 m and were formed from a moderate to high temperature, low to moderate salinity, Na–Cl fluid, likely of magmatic origin. The last group of calcites to form, record the presence of two distinct fluid types. The platy (a) calcite formed at 95–238°C with a δ 13C signature of −12.2 to −3.8‰ PDB and a δ 18O signature of 14.9–19.6‰ SMOW, from a high temperature, low salinity, Na–Cl fluid of possible magmatic origin. The platy (b) calcite formed at 67–98°C with a δ 13C signature of −13.0 to −6.2‰ PDB and a δ 18O signature of 15.1–20.1‰ SMOW, from a low temperature, high salinity, Ca–Na–Cl fluid of possible basinal brine origin. The two calcites are related through a mixing between the two end members. The source of the fluids for the platy grey (a) calcites could be the olivine diabase dykes and sills that cut through the site. The source of fluids for the platy (b) calcites could be the Jotnian arkosic sandstone formations in the northern part of the site. At the Olkiluoto site, δ 18O geothermometry does not agree with fluid inclusion data. The original source of the water that forms the calcite has the largest effect on the isotopic signature of the calcites formed. Large isotopic shifts are seen in any water by mineral precipitation during cooling under rock–water equilibrium fractionation conditions. Different calcite isotopic signatures are produced depending on whether cooling occurred in an open or closed system. Water–rock interaction, at varying W/R ratios, between a water and a host rock can explain the isotopic shifts in many of the calcites observed. In some cases it is possible to shift the δ 18O of the water by +11.5‰ (SMOW) using a realistic water–rock ratio. This process still does not explain some of the very positive δ 18O values calculated using fluid inclusion data. Several other processes, such as low temperature recrystallization, boiling, kinetic effects and dissolution of calcite from fluid inclusion walls can affect isotopic signatures to varying degrees. The discrepancy between fluid inclusion data and δ 18O geothermometry at the Olkiluoto site was most likely due to poor constraint on the original source of the water.

The Lightning Creek Sill Complex, Cloncurry District, Northwest Queensland: A Source of Fluids for Fe Oxide Cu-Au Mineralization and Sodic-Calcic Alteration

The Lightning Creek Cu-Au prospect is hosted by a cogenetic suite of plutonic, I-type granitoids. The dominant rock type is a porphyritic quartz monzodiorite that is intruded by more fractionated rocks, including monzogranite and alkali feldspar granite. A series of flat-lying sills are interpreted to be late-stage differentiates, based on their timing, mineralogy, and chemistry. In parts of the prospect there is pervasive sodic-calcic alteration (pyroxene after amphibole, albite after K feldspar and oligoclase) of the plutonic rocks. This alteration predates sill emplacement and is unrelated to veining or fracturing of any kind. The presence of small amounts of carbonate in the altered rocks suggests that the fluids were CO2 bearing. Quartz and feldspar separates from these altered rocks have oxygen isotope compositions similar to those from fresh quartz-monzodiorite, suggesting that the fluids were hot and of magmatic composition. Sodium and Ca were added and K, Fe, Cl, and Cu were stripped during what is interpreted as an autometasomatic event. The sills display considerable textural and mineralogical complexity and evolved from equigranular, quartzofeldspathic rocks (aplites), with magmatic chemistry, to unusual Fe-rich rocks (albite-magnetite-quartz) that exhibit a range of bizarre spherulitic textures. Some of the albite and magnetite in the sills is secondary. Albite forms pseudomorphs after K feldspar (Na-Fe ± Ca alteration) along sill margins and within sills, at the contacts between different textural zones. Halos of disseminated magnetite + clinopyroxene (Fe-Ca ± Na alteration) are developed adjacent to early magnetite veins. Fluid inclusion studies indicate that these rocks crystallized at temperatures in excess of 500°C and at pressures in excess of 1.5 kbar. The range of spherulitic textures is taken to indicate crystallization under hydrous conditions with the episodic release of a fluid phase. This magmatic fluid phase was dominated by H2O, CO2, and chlorine and underwent phase separation into a CO2-rich vapor and a hypersaline brine (33–55 wt % NaCl equiv). The hypersaline fluid was enriched in Fe (~10 wt %) and Cu (~1 wt %, PIXE analysis), in addition to Na, K, and Ca. Where this fluid was retained within Fe-rich portions of the sills, it caused Ca-Fe ± Na alteration (pyroxene-albite ± magnetite growth at the expense of quartz). Where the fluid was expelled from the sills, it produced quartz-magnetite ± clinopyroxene ± albite veins (broadly coeval with the early magnetite veins). Although rich in Cu, these granitoid-derived magmatic fluids did not generate significant Cu(-Au) mineralization, perhaps because of the high temperatures involved and/or a lack of reduced sulfur in the fluids or host rock. However, the amount of iron present is estimated (from the aeromagnetic anomaly) to be in excess of 2,000 million tonnes (Mt). A later generation of calcite ± chlorite ± pyrite ± chalcopyrite veins contain traces of Cu-Au mineralization. Fluid inclusion and stable isotope work indicate that these veins probably crystallized from cooler (<200°C), more dilute (15–28 wt % NaCl equiv) fluids, perhaps generated by the admixture of a meteoric component. The conclusions reached in this study have implications for understanding the genesis of Fe oxide Cu-Au deposits and related sodic-calcic alteration. The study indicates the potential for CO2-rich granitoid magmas to evolve hypersaline, Fe- and Cu-rich fluids capable of causing intense magnetite veining and Cu(-Au?) mineralization. Autometasomatic sodic-calcic alteration of the granitoids may be an important precursor to mineralization, contributing Fe, K, Cu, and Cl to the magmatic fluids.

Timing of cement precipitation and fault activity in the Eastern and Southern Alps (Austria, Germany, Italy and Slovenia)

Five carbonate cement generations in faults and associated fractures were studied in Permian to Tertiary sedimentary rocks in the Drau Range, in the Southern Alps and in different nappes of the Northern Calcareous Alps (NCA). Timing of precipitation was deduced from the stratigraphical distribution of the cements. The first major cementation phase is represented by clear saddle dolomite (CSD), which is of Triassic to Cretaceous age. The second major cementation phase, characterized by another saddle dolomite and three generations of blocky calcite, is of Miocene age. The distribution of the different carbonate cements in faults and fractures shows, which fault/fracture sets were main conduits for fluid-flow during the evolution of the Alps.

Chemical analysis of palaeogroundwaters: a new frontier for fluid inclusion research

Using a combination of fluid inclusion and mineralogical techniques, chemical data have been obtained that help characterise the hydrogeochemistry of palaeogroundwaters in the onshore section of the Eastern Irish Sea Basin, northwest England. The study area, Sellafield, provided an excellent suite of fracture-controlled carbonate cements deposited from shallow to deeply circulating late Triassic to Recent groundwaters. Methodologies developed for the analysis of hydrothermal fluid inclusions were extended to encompass a wide range of low temperature aqueous inclusions. These included single inclusion chemical analysis by UV laser ablation ICP mass spectrometry (Na, K, Mg, Sr, Li, Mn, Fe) and high precision salinity measurements (±1000ppm TDS) by microthermometric analysis. Closely integrated with these measurements were calcite morphology and cathodoluminescence (CL) studies that were undertaken to provide a relative chronology ‘stratigraphy’ of carbonate cementation and information on redox conditions and salinity with depth.The results demonstrate a clear distinction between the chemistry of the late Triassic groundwaters and the present day, deep, saline groundwaters. Though both have TDS values >100,000ppm, the former are Ca–Na–Cl brines with Na/Ca wt ratios of 2:1, whilst the latter are Na–Cl brines with Na/Ca wt. ratios greater than 20. For younger generations of calcite, attributable to deposition from groundwater during the Quaternary, TDS values are <100,000ppm with salinities typically less than 20,000ppm. Laser ablation analyses of aqueous inclusions in these younger calcites for Na, Sr and Mg plot exactly within the fields defined by the present day saline to brackish groundwaters, and display similar trends. CL patterns for the Quaternary calcites are primarily a function of trace element impurities (Fe and Mn), but all show a marked contrast at the fresh water–saline water transition. This contrast is also reflected in the morphology of the calcite crystals; from c-axis shortened ‘nailhead’ forms in the fresh groundwater zone to c-axis elongated ‘scalenohedra’ forms in the deeper saline zones. Implications for the evolution of the palaeogroundwaters in response to Tertiary uplift with respect to present day groundwater regimes are briefly discussed.

Relationship between fluid flow and faulting in the Alpine realm (Austria, Germany, Italy)

Abstract Five carbonate cement generations in faults and associated fractures, which precipitated during two major cementation phases, were studied in Permian to Tertiary sedimentary rocks in the Drau Range, in the Southern Alps and in different nappes of the Northern Calcareous Alps (NCA). The first major cementation phase is represented by clear saddle dolomite (CSD) that can be further subdivided by cathodoluminescence (CL) into five CL-zones. A non-carbonate zone, consisting of chlinochlorite (chlorite zone), is intercalated between CL-zones II and III. CL-zones I–IV occur in rocks of Permian to Early Jurassic age, while CL-zone V is also present in Upper Jurassic strata. The second major cementation phase, characterized by another saddle dolomite and three generations of blocky calcite, is of Miocene age. Dedolomitized CSD, which occurs in a synsedimentary normal fault of Early Jurassic age contains a zone of pyrite. The trace element composition of this pyrite zone and of an Upper Hettangian Fe–Mn crust (Marmorea crust) were compared with the trace element composition of the chlorite zone. Similar trace element patterns within all three non-carbonate zones suggest the same fluid source for these zones during Upper Hettangian time. This fluid could be of hydrothermal origin, as indicated by the geochemical characteristics of the Fe–Mn crust. Hydrothermal activities started during the beginning of rifting in the Alpine realm in the Late Triassic/Early Jurassic. Fluid inclusion data reveal variable homogenization temperatures for the five CL-zones in the clear saddle dolomite. Formation temperatures between 146 and 201°C were calculated, considering the overburden pressure during the formation of the CL-zones. Calculated equilibrium oxygen isotopic fluid compositions indicate δ 18 O values between 4.0 and 9.0‰ SMOW for the fluid from which the different saddle dolomite zones precipitated. The origin of the fluid can be related to brines present in the hinterland (Vindelician high, Bohemian massif). The stratigraphic occurrence of CSD and its paragenetic relationship to the non-carbonate zones enables a more exact dating of the CL-zones. CL-zones I and II precipitated between Norian and Late Hettangian time, and CL-zones III and IV are of Early Sinemurian age. The formation period of CL-zone V is Late Jurassic to Early Cretaceous. The distribution of the carbonate cements was investigated within different fault sets and in associated subparallel fractures in the NCA, i.e. the Allgau-, Lechtal- and Inntal-nappe, which show a polyphase deformation history. CSD occurs in normal faults, N–S trending high angular faults and WNW-trending strike-slip faults, indicating minimum an Early Jurassic age for these faults. The only carbonate cements that are present within NE-trending sinistral strike-slip faults are of the second major cementation phase, implicating a Miocene age for these faults. Fibrous shaped crystals of the first calcite cement of the second major cementation phase preferentially occur within WNW-trending strike-slip faults. Reactivation of these faults as conjugated shear pairs in connection with the formation of NE-trending strike-slip faults, is assumed.