Epigenetic Veins Research Articles

Discriminating Superimposed Alteration Associated with Epigenetic Base and Precious Metal Vein Systems in the Rouyn-Noranda Mining District, Quebec; Implications for Exploration in Ancient Volcanic Districts

Abstract The Rouyn-Noranda mining district of Quebec contains 20 Cu-Zn (±Au ±Ag) volcanogenic massive sulfide (VMS) deposits, including the giant and gold-rich Quemont and Horne deposits. Mineralized epigenetic veins are also present, but their origin and relative timing remain enigmatic. The nature and extent of their alteration signatures and the effect of their superposition on district-scale alteration patterns is unknown. The VMS-related quartz-sulfide Cu-Zn-Ag veins have δ18Oquartz values of 8.5 ± 0.8‰, reflecting δ18Ofluid compositions of –0.4 to 3.1‰ (250°–350°C) that are typical of Archean seawater. They are associated with a proximal Fe-rich chlorite alteration and marginal spotted sericite-chlorite alteration with whole-rock δ18O values of 2.9 to 5.9‰ and are interpreted to have formed within the structurally controlled discordant upflow zones of a VMS hydrothermal system. Younger gold-bearing quartz-carbonate veins were emplaced along mechanical anisotropies created by mafic dikes during north-south compression and the formation of regional E-trending faults, folds, and cleavage. They are characterized by δ18Oquartz values of 11.3 ± 0.8‰, reflecting δ18Ofluid compositions of 2.4 to 5.9‰ (250°–350°C), typical of a metamorphic fluid, possibly mixed with a lower δ18O upper crustal fluid. They are associated with ankerite, calcite, muscovite, chlorite, albite, and quartz ± hematite alteration with whole-rock δ18O values of 5.8 to 10.3‰. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon ages for two tonalite intrusions constrain the maximum age of the Cu-Zn-Ag veins to 2697.6 ± 0.7 Ma and the minimum age to 2695.3 ± 1.0 Ma, which is also the maximum age of the gold quartz-carbonate veins. Superposition of alteration related to the gold quartz-carbonate veins on previously chlorite- and sericite-altered rocks has resulted in mixed alteration signals with whole-rock δ18O values of ~6 to 8‰ that have perturbed and masked regional alteration patterns related to older VMS mineralization, such as those found in the Quemont and Horne deposits. These results indicate that defining alteration vectors in camps that have superimposed hydrothermal systems requires full consideration of the hydrothermal history of the camp, and if such constraints are lacking, whole-rock δ18O values should not be used as a stand-alone exploration method.

Structural-morphological features and crystallization environment of quartz in shungite rocks of the Onega basin

The article reports the results of a study of quartz in shungite rocks of the Shunga, Zazhogino and Maksovo deposits in the Onega Paleoproterozoic Basin located in the Karelian Craton in the southeastern part of the Baltic Shield. Silica is one of the main components of shungite rocks (Corg 35–80 %) and lydites (Corg up to 5 %), in which it forms two morphological types: epigenetic vein quartz and authigenic quartz. The rocks retain signs of delayed diagenesis of organic and siliceous matter: high dispersion of mineral segregations, dyscrystalline mineral matter, globular and other metacolloidal forms of segregations, which indicates the colloidal origin of authigenic quartz. The values of the (O/Si)at ratio were determined for morphologically different segregations of quartz in shungite rocks. The results of the study suggest that the structural features of authigenic quartz arose during the early stages of lithogenesis in the context of active interaction with organic matter. Most likely, the organic matter partially or fully retained its bondage to the siliceous matter of shungite rocks in the process of catagenetic transformations.

New perspective on trace element (Re, Ge, Ag) hosts in the Cu-Ag Kupferschiefer deposit, Poland: Insight from a LA-ICP-MS trace element study

The Kupferschiefer deposit in SW Poland provides an opportunity to study an ore system where besides factors such as temperature, salinity and pH, redox conditions play a major role in controlling trace element incorporation in sulphides. Chalcocite, djurleite, bornite, chalcopyrite, sphalerite, covellite and pyrite from the Cu-Ag deposit in the Lubin-Sieroszowice district were analysed with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to study the distribution of trace elements and to identify the main carriers of by-products such as silver and rhenium. Trace element anomalies are not only associated with certain minerals but also linked to specific mineralization styles. Chalcopyrite in epigenetic veins has significantly higher content of Ge, As and Sb while sulphides adjacent to hematitic “red spots” are enriched in redox sensitive elements: djurleite is enriched in V, Se, Re, Au, Hg and Tl, chalcopyrite in V, Se, Hg and Tl, and pyrite in Tl in comparison with bulk Cu-Ag mineralization. Elevated and substantially more variable data for V, Mn, Ni, Co, Ga, Mo, Sn in shale hosted sulphides can be attributed to submicron inclusions of Kupferschiefer silicate matrix which might skew low concentration values even if careful petrographic observations and precise spot selection are applied. Silver concentrations form a linear pattern from chalcocite/djurleite > bornite > chalcopyrite > sphalerite. Silver distribution in the deposit on a broad scale could be linked to the general Fe3+-Cu-Pb-Zn-Fe2+ zonation as well as mineral sequence (hematite-chalcocite-bornite-chalcopyrite-galena-sphalerite-pyrite) observed in the majority of sediment hosted stratiform copper deposits. Epigenetic sulphide-calcite veins associated with local faults and tectonic zones contain chalcopyrite exceptionally rich in germanium (up to 4806 μg/g of Ge) suggesting that the potential of Kupferschiefer Cu-Ag ores in terms of Ge grades and resources could be underestimated. The majority of analysed sulphides do not contain rhenium above detection limits (0.1 μg/g Re) with measurable quantities found almost exclusively in djurleite (up to 3.9 μg/g Re). This indicates that copper sulphides, in particular djurleite, could be a preferred host and an important carrier of rhenium in the deposit.

Epigenetic-Hydrothermal Fluorite Veins in a Phosphorite Deposit from Balaton Highland (Pannonian Basin, Hungary): Signatures of a Regional Fluid Flow System in an Alpine Triassic Platform

The middle Anisian extensional tectonics of the Neotethyan realm developed a small, isolated carbonate platform in the middle part of the Balaton Highland (western Hungary), resulted in the deposition of uranium-bearing seamount phosphorite on the top of the drowned platform and produced some epigenetic fluorite veins in the Middle Triassic sequence. The stable C-O isotope data of carbonates are shifted from the typical Triassic carbonate ranges, confirming the epigenetic-hydrothermal origin of veining. Primary fluid inclusions in fluorite indicate that these veins were formed from low temperature (85–169 °C) and high salinity NaCl + CaCl2 + H2O type (apparent total salinity: 15.91–22.46 NaCl wt%) hydrothermal fluids, similar to parent fluids of the Alpine-type Pb-Zn deposits. These findings indicate that the Triassic regional fluid circulation systems in the Alpine platform carbonates also affected the area of the Balaton Highland. This is also in agreement with the previously established palinspatic tectonic reconstructions indicating that the Triassic carbonate and basement units in the Balaton Highland area were a part of the Southern Alpine. Similar fluorite veining in phosphorite deposits is also known in the Southern Alpine areas (e.g., Monte San Giorgi, Italy). Raman spectroscopic analyses detected H2 gas in the vapor phase of the fluid inclusions and a defect-rich fluorite structure in violet to black colored growth zones. This unique phenomenon is assumed to be the result of interaction between the uranium-rich phosphorite and the parent fluids of the epigenetic fluorite veins.

Geochronology, geochemistry and zircon Hf-O isotopic composition of ore-bearing volcanic rocks at Dapingzhang VMS Cu-Zn deposit, SW China: Petrogenetic, metallogenic and tectonic implications

The Dapingzhang Cu-Zn deposit is exposed in the eastern Proto-Tethys Ocean (PTO) tectonic domain. The mineralization is hosted by felsic volcanics that occur in bimodal association with minor basalt. Styles of mineralization include syngenetic banded massive and fragmented sulfides and epigenetic veins, stockworks and disseminations. The deposits are classified as bimodal-felsic (Kuroko type) VMS deposit. Two rhyolite rock samples yielded zircon U-Pb ages of 413.2 ± 1.7 Ma and 430.7 ± 4.1 Ma, revealing that the ore-related magmatism took place in the mid-late Silurian. Geochemically, the submarine sodium-rich volcanic rocks are strong peraluminous and belong to the calc-alkaline series. The rocks are characterized by enrichment in large-ion lithophile elements (LILEs, Ba, Th, and U), depletion in high-field-strength elements (HFSEs, Nb, Ta, Ti, Th, and P) and LREEs, and have affinities with subduction-related arc setting. The zircons yield εHf(t) values of +8.17 to +15 and two-stage model ages of 457–881 Ma. Oxygen isotopes of zircon grains show a mean δ18Ozircon of 5.20 ± 0.23‰ with variations from 3.67‰ to 7.77‰, most of which is similar to the mantle-like value of 5.3 ± 0.6‰ (2σ). The felsic volcanic rocks of the Dapingzhang deposit can be attributed to partial melting of the juvenile lower crust, while minor amounts of asthenospheric mantle components were introduced into their magma sources. Taking into account previous data, we propose that the volcanic rocks were generated in a back-arc extension setting, which provides reliable evidence for the Early Paleozoic an-back-arc system in the northeastern margin of Gondwana. The PTO may have been bipolar-subducted beneath the Sibumasu and Indochina blocks.

Diagenetic and Epigenetic Mineralizing Events in the Kalahari Copperbelt, Botswana: Evidence from Re-Os Sulfide Dating and U-Th-Pb Xenotime Geochronology

Abstract The ages of sedimentation and copper-silver mineralization in the late Meso- to Neoproterozoic Kalahari Copperbelt in Botswana, an economically significant copper province, have previously been poorly constrained within an ~600 m.y. period that spans the Neoproterozoic from the assembly and breakup of Rodinia to the assembly of Gondwana. Rhenium-osmium geochronology of molybdenite and copper sulfide minerals and U-Th-Pb laser ablation split-stream inductively coupled plasma-mass spectrometry (LASS ICP-MS) analysis of xenotime grains are utilized to provide absolute and relative age data on the host rocks and mineralizing events within the Ghanzi Ridge region of the Kalahari Copperbelt. The data reveal a prolonged history of events, which is partially comparable with depositional and mineralizing events in the neighboring Central African Copperbelt. Abundant disseminated molybdenite is located within a shale layer near the base of the Proterozoic D’Kar Formation at the Northeast Mango Two deposit. Unusual molybdenite textures suggest organic matter may have been a precursor. Two molybdenite separates from a small calcite-molybdenite stringer in a wall-rock fragment that is enclosed within an epigenetic quartz-calcite-chalcopyrite vein with ill-defined and mismatched margins yielded Re-Os ages of 981 ± 3 and 981 ± 7 Ma. These ages indicate an early hydrothermal mineralizing event in the basin. A xenotime inclusion intergrown with molybdenite and chalcopyrite within the epigenetic vein yielded a younger U-Th-Pb age of 538 ± 8 Ma, suggesting two mineralizing events are preserved in a complex 6-cm-wide vein. Based on vein texture and alteration, the ages represent an ~981 Ma calcite-molybdenite mineralization event overprinted by an ~538 Ma quartz-chalcopyrite-molybdenite mineralization event, perhaps during reopening of the vein. Re-Os and U-Th-Pb geochronology were utilized at the Zone 6 deposit on minerals associated with a hydrothermal quartz-calcite-chalcocite-idaite-bornite vein. Several authigenic xenotime grains that occur along the margin of the vein yielded three concordant U-Th-Pb ages that indicate xenotime growth at ~950 to 925 Ma while other xenotime grains in a similar position yielded mostly discordant data, suggesting disturbance of the isotopic system in the xenotime grains. A coprecipitated chalcocite-idaite mixture within the hydrothermal vein produced an Re-Os age of 549.0 ± 11.2 Ma. Re-Os analysis obtained from a coprecipitated molybdenite-bornite mixture at the Northeast Fold deposit yielded an age of 515.9 ± 2 Ma. Together, the earliest Neoproterozoic Re-Os molybdenite and U-Th-Pb xenotime ages provide both a minimum depositional age constraint for the lowermost D’Kar Formation and clear evidence that diagenetic hydrothermal mineralizing events took place within the Ghanzi basin. The timing of this mineralizing event corresponds with a poorly documented regional thermal event that affected the northern margin of the Kalahari craton during the final stages of the assembly of Rodinia at ~980 Ma. The lower to middle Ghanzi Group of the Kalahari Copperbelt is at least 100 m.y. older than the host rocks within the neighboring Central African Copperbelt, which are associated with the breakup of Rodinia. The latest Neoproterozoic to Cambrian Re-Os and U-Th-Pb ages indicate that hydrothermal copper-silver mineralizing events occurred during the Pan-African (~600–480 Ma) fold-thrust evolution of the Ghanzi-Chobe zone and were broadly synchronous with widespread epigenetic hydrothermal copper-cobalt mineralizing events in the adjacent Central African Copperbelt.

Cu(-Ag) mineralizace z Tismic u Českého Brodu (perm blanické brázdy, Česká republika)

A mineralogical study of samples newly collected from dump material at the formerly mined locality Tismice near Český Brod (central Bohemia) revealed the presence of covellite/yarrowite, acanthite, malachite and azurite. In addition, psammitic to aleuritic detrital material originated from host Permian sandstones/arkoses is a common compound of the studied ore samples. The Cu carbonates clearly prevail among ore minerals, whereas sulphide phases are accessories. The silver-enriched covellite/yarrowite, strongly replaced by malachite and azurite, has a coarse-grained texture implying that its primary hydrothermal or late hydrothermal origin cannot be excluded. The other recorded ore minerals are clearly supergene in origin. Although the nature of the mineralization from Tismice is in general similar to other occurrences of epigenetic vein Cu mineralization hosted by the Permian sediments in the vicinity of the town of Český Brod, the enrichment in silver is reported for the first time here.

Geochemistry, Mineralogy and Petrogenesis of the Vein-Type Barites of Some Portions of the Benue Trough, Nigeria

Several barite deposits have been reported to occur in Nigeria within the Benue Trough, one of the West Central African Rift Systems (WCARS) considered variously as an aulacogen, strike-slip or pull-apart basin. In spite of these occurrences and the high local and international demand for barites, Nigerian barite is yet to find a place even in its local market probably due to the poor knowledge of the fundamental properties of the barites. To throw more light on this some deposits that are mined on artisanal basis in the Trough were studied alongside an overview of the evolution of the basin and the geochemistry and petrogenesis of the barite. Barites in the area of study occur as white, reddish-brown and clear varieties with specific gravity varying between 3.3966 and 4.3432 based on pycnometric and immersion methods. They occur as epigenetic veins of between about 0.1 m to more than 1.5 m, which encourages open cast mining. Chemical analysis show that the samples studied contain between 72.61 and 72.12% Ba corresponding to a calculated total BaO of 48.37 and 60.1%. The other relatively significant elements but of less value in the samples include Si, Fe, Sr, Ca, Al, Pb, Mg, Mn, P, and Ni. XRD results also show barite mineralogical association with wenkite and plumbian barite of insignificant quantity. The barites likely formed from intermingling of infiltrating solutions containing barium with soluble sulphates arising within the rocks passing through fractures created during the Late Cretaceous inversion in the basin. The inversion is suggested herein to have resulted from magmatic underplating and consequent peneplanation in the middle Benue Trough.

Geochemistry and geochronology of VHMS mineralization in the Sangkaropi district, central-West Sulawesi, Indonesia: Constraints on its tectono-magmatic setting

The Sangkaropi district in central-West Sulawesi is one of the few known volcanic hosted massive sulfide (VHMS) districts in Indonesia. It comprises several clusters of mostly small mineral occurrences, including Sangkaropi, Billolo and Rumanga. The mineralization is hosted by felsic volcanics that occur in bimodal association with basalt. Styles of mineralization include syngenetic banded massive and fragmented sulfides and epigenetic veins, stockwork and disseminations. The deposits are classified as zinciferous bimodal-felsic (Kuroko type) VHMS.We present for the first time preliminary results for whole rock and trace element analysis and U-Pb age dating for the bimodal volcanic hosts, together with sulfur isotope data for the mineralization and associated barite alteration. Combined with previous data, this new information suggests the mineralization in the Sangkaropi district took place around 34 Ma (boundary Eocene-Oligocene) in a continental margin arc setting, when the stress regime changed from near-neutral to extensional and volcanism changed from being andesite-dominated to bi-modal. The sulfur isotope data suggest that the mineralizing fluids had both significant magmatic and seawater components.

Applied statistical functions and multivariate analysis of geochemical compositional data to evaluate mineralization in Glojeh polymetallic deposit, NW Iran

Various genesis of epithermal veins as well as host rock cause complication in the modeling process. Thus LINEST and controlling function were applied to improve the accuracy and the quality of the model. The LINEST is a model which is based on multiple linear regression and refers to a branch of applied statistics. This method concerns directly to the application of t-test (TINV and TDIST to analyses of variables in the model) and F-test (FDIST, F-statistic to compare different models) analysis. Backward elimination technique is applied to reduce the number of variables in the model through all the borehole data. After 18 steps, an optimized reduced model (ORM) was constructed and ranked in order of importance as Pb>Ag>P>Hg>Mn>Nb>U>Sr>Sn>As>Cu, with the lowest confidence level (CL) of 92% for Cu. According to the epigenetic vein genesis of Glojeh polymetallic deposit, determination of spatial patterns and elemental associations accompanied by anomaly separation were conducted by K-means cluster and robust factor analysis method based on centered log-ratio (clr) transformed data. Therefore, 12 samples (cluster 2) with the maximum distance from centroid, indicates the intensity of vein polymetallic mineralization in the deposit. In addition, an ORM for vein population was extracted for Sb>Al>As>Mg>Pb>Cu>Ag elements with the R2 up to 0.99. On the other hand, after 23 steps of optimization process at the host rock population, an ORM was conducted by Ag>Te>Hg>Pb>Mg>Al>Sb>As represented in descending order of t-values. It revealed that Te and Hg can be considered as pathfinder elements for Au at the host rock. Based on the ORMs at each population Ag, Pb, and As were often associated with Au mineralization. The concentration ratio of (tSb × tAl)vein/(tSb × tAl)background as an enrichment index can intensify the mineralization detection. Finally, Glojeh deposit was evaluated to be classified as a vein-style Au (Ag, Pb, As)-polymetallic mineralization.

Submarine hydrothermal processes, mirroring the geotectonic evolution of the NE Hungarian Jurassic Szarvaskő Unit

The Jurassic pillow basalt of the NE Hungarian Szarvaskő Unit is part of an incomplete ophiolitic sequence, formed in a back-arc- or marginal basin of Neotethyan origin. Different, often superimposing hydrothermal processes were studied aiming to characterise them and to discover their relationship with the geotectonic evolution of the region. Closely packed pillow, pillow-fragmented hyaloclastite breccia and transition to peperitic facies of a submarine lava flow were observed. The rocks underwent primary and cooling-related local submarine hydrothermal processes immediately after eruption at ridge setting. Physico-chemical data of this process and volcanic facies analyses revealed distal formation in the submarine lava flow. A superimposing, more extensive fluid circulation system resulted in intense alteration of basalt and in the formation of mostly sulphide-filled cavities. This lower temperature, but larger-scale process was similar to VMS systems and was related to ridge setting. As a peculiarity of the Szarvaskő Unit, locally basalt may be completely altered to a grossular-bearing mineral assemblage formed by rodingitisation s.l. This unique process observed in basalt happened in ridge setting/during spreading, in the absence of known large ultramafic blocks. Epigenetic veins formed also during Alpine regional metamorphism, related to subduction/obduction. The observed hydrothermal minerals represent different steps of the geotectonic evolution of the Szarvaskő Unit, from the ridge setting and spreading till the subduction/obduction. Hence, studying the superimposing alteration mineral assemblages can be a useful tool for reconstructing the tectonic history of an ophiolitic complex. Though the found mineral parageneses are often similar, careful study can help in distinguishing the processes and characterising their P, T, and X conditions.

Relationships between the occurrence of accessory Ge-minerals and sphalerite in Variscan Pb-Zn deposits of the Bossost anticlinorium, French Pyrenean Axial Zone: Chemistry, microstructures and ore-deposit setting

The presence of unique accessory Ge-minerals (containing up to 70 wt% Ge) is a widespread phenomenon in Pb-Zn deposits of the Variscan Pyrenean Axial Zone (PAZ). Such a mode of occurrence is, however, rare worldwide with germanium more typically occurring as a trace component of sulfides, notably sphalerite (<3200 ppm), or in coal deposits (<5570 ppm). The PAZ Pb-Zn deposits are thus an excellent target to unravel the processes and key factors controlling formation of mineralization highly concentrated in Ge. Three Pb-Zn deposits are studied here: Argut-dessus; Pale Bidau; and Pale de Rase. All three are located in Late Ordovician rocks within the PAZ Bossost anticlinorium. The main mineralization (Type 2) is localized in epigenetic veins with different geometries relative to host rocks and deformation. Textural analysis of sphalerite mineralization shows evidence for both recrystallization and deformation. Sphalerite is associated with muscovite or graphite and is the unique host for the discrete Ge-minerals in the studied samples. Brunogeierite (GeFe2O4) and argutite (GeO2) occur almost universally at sphalerite grain boundaries. Laser ablation inductively coupled plasma-mass spectrometry analysis of sphalerite and brunogeierite show that sphalerite is relatively depleted in Ge (mean ∼13 ± 3 ppm) and other trace elements such as Sb (∼21 ± 2 ppm), Cu (∼153 ± 23 ppm) and Ga (∼127 ± 10 ppm) compared to brunogeierite (29–30 wt% Ge), which is also richer in Ga (∼2406 ± 270 ppm) and Cu (∼2438 ± 550 ppm). Collectively, these observations on this exceptional example of Ge enrichment highlight the role of oxidizing, moderate temperature (∼<450 °C) metamorphic fluids on the partitioning of Ge between discrete Ge-minerals and their sulfide hosts. Comparison with other Ge-deposits worldwide supports the hypothesis that low grade metamorphism and deformation may play a key role in the genesis and concentration of Ge-mineralization, especially in low-grade metamorphic Kipushi-type deposits in which discrete Ge-minerals are located immediately adjacent to recrystallized sulfides.

Lead and sulfur isotope constraints on the genesis of the polymetallic mineralization at Oued Maden, Jebel Hallouf and Fedj Hassene carbonate-hosted Pb–Zn (As–Cu–Hg–Sb) deposits, Northern Tunisia

The Oued Maden, Jebel Hallouf and Fedj Hassene Pb–Zn (Ba–Sr–F–Fe–Hg) hydrothermal ore deposits are located in the Nappe zone of Northern Tunisia. These ore deposits occur as epigenetic veins, karst and stockwork fillings in Upper Cretaceous limestones. The ore mineralogy consists mainly of galena and sphalerite, accompanied by minor amounts of jordanite, pyrite, chalcopyrite, orpiment, realgar, and other sulfosalts. Sulfur isotope data from sphalerite and galena indicate that the reduced sulfur was derived through thermochemical and/or bacterial reduction of dissolved sulfate resulting in metal precipitation. Lead isotope ratios and corresponding calculated age models indicate that the Pb in galenas has been derived from a homogenous crustal source during Upper Miocene time.

Dellaventuraite, NaNa2(MgMn23+Ti4+Li)Si8O22O2, a new anhydrous amphibole from the Kajlidongri Manganese Mine, Jhabua District, Madhya Pradesh, India

Dellaventuraite is a new amphibole species from the Kajlidongri manganese mine, Jhabua District, Madhya Pradesh, India. It occurs with leakeite, kornite, albite, braunite, and bixbyite associated with cross-cutting epigenetic veins that have reacted with regionally metamorphosed rocks containing Mn-rich minerals (braunite, bixbyite, jacobsite, spessartine) to produce Mn-rich amphiboles, Mnrich pyroxenes, Mn-rich mica, piemontite, and manganophyllite. Dellaventuraite occurs as anhedral grains, the color of which varies from pink to red, depending on Mn content. It is brittle, H = 5, Dcalc = 3.184 g/cm 3 , has a pale pink streak, vitreous luster, and does notfluoresce in ultraviolet light; it has perfect cleavage on {110} and conchoidal fracture. In transmitted plane-polarized light, dellaventuraite is strongly pleochroic, X = pale mauve-brown, Y ~ Z = dark red-brown; Y ^ a = 20° (in β obtuse), Z = b, with absorption X &lt; Y ~ Z. It is biaxial positive, η α = 1.688 ± 0.003, η β = 1.692 ± 0.005, η γ = 1.721 ± 0.003, 2V (obs) = 49 ± 3°, 2V (calc) = 41°. Dellaventuraite is monoclinic, space group C2/m, a = 9.808(1), b = 17.840(2), c = 5.2848(5) Å, γ = 104.653(1)°, V = 894.6(2) Å 3 , Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d(I,hkl)]: 2.697(10,151), 2.542(9,.202), 3.127(8,310), 3.378(7,131), 2.154(7,261), 1.434 (7,.661), 4.450(6,021), 8.459(5,110), 2.727(5,.331), 2.328(5,.351). Analysis by a combination of electron microprobe, SIMS and crystal-structure refinement gives SiO 2 = 54.22, Al 2 O 3 = 0.81, TiO 2 = 5.45, Fe 2 O 3 = 6.44, Mn 2 O 3 = 7.57, ZnO = 0.12, NiO = 0.16, MgO = 8.26, Li 2 O = 1.53, CaO = 1.85, Na 2 O = 8.12, K 2 O = 2.12, H 2 O = 0.80, Cr, V, F, Cl not detected, sum 97.41 wt%. The formula unit, calculated on the basis of 24(O,OH,F) is (K 0.40 Na 0.61 )(Na 1.71 Ca 0.29 ) (Mg 1.81 Zn 0.01 Ni 0.02 -Li 0.90 Fe 3+ 0.71 Mn 3+ 0.85Ti 4+ 0.60Al 0.10 )(Si 7.96 Al 0.04 )O 22 [(OH) 0.80 O 1.20 ]; the ideal end-member composition NaNa 2 (MgMn 3+ 2 LiTi0 4+ )Si 8 O 22 O 2 . The crystal structure of dellaventuraite was refined to an R index of 3.8% using MoKα X-ray intensity data. The M1 site is occupied by Ti 4+ , Mn 4+ , and Mg in approximately equal amounts, the M2 site is occupied primarily by Mg and Fe 3+ , and M3 is occupied by Li with minor Mg and Mn 2+ . Local bond-valence considerations suggest that O 2- at O3 is linked to Ti 4+ Mg or Mn 3+ Mn 3+ at the adjacent M1 sites, and that OH at O 3 is linked to MgMg at the adjacent M1 sites.

Contrasting sources for lead in Cu-polymetallic and Zn–Pb mineralisation in Ireland: constraints from lead isotopes

Devonian red bed-hosted deposits in SW Ireland occur either as low-grade stratiform disseminations and minor veinlets with bornite and djurleite, or as major quartz veins with copper-polymetallic deposits. Pb isotopic ratios vary according to the style of mineralisation. For disseminated sulphides, there is a considerable scatter of lead isotope compositions. These appear to define a mixing line between a basement source and a more radiogenic component, which may have been granite-derived detrital minerals in the red beds. For minor veinlets there is a more restricted distribution of lead isotopic data, which could be interpreted in terms of fluid flow homogenising the wide spread of Pb isotope ratios from the disseminated sulphides. This is consistent with published sulphur isotope data. Major quartz veins show a clustered array of Pb isotopic ratios and it is suggested that the bulk of the metals were derived after Variscan compression, from a basement source with a minor contribution from red beds. Lower Carboniferous limestone-hosted deposits form three groups: Cu–Ag ± Hg–As–Sb–U epigenetic vein-hosted deposits; Cu–As ± Zn, Pb, Ag, Co, Ni stratabound deposits; and Cu–As ± Pb, Ag, Mo, Co, Ni epigenetic vein deposits. Lead isotope compositions for the stratabound deposits have a very restricted range for each locality and plot close to the same mixing line as the Carboniferous Zn–Pb deposits of Central Ireland. It is suggested that the lead in both areas has been derived from a similar basement source. The model age of ca. 350±10 Ma for stratabound mineralisation is close to that of their host rocks. Lead isotope data for the epigenetic Cu–As vein deposits suggest a similar source, but a younger age of 280±10 Ma for mineralisation. This 280±10-Ma age has important implications for the timing of the Variscan orogeny in SW Ireland. Because chalcopyrite–tennantite occurs along pressure solution cleavage planes that have been deformed by thrusting, the emplacement of the mineralised tectonic Ross Island sheet, west of Killarney, must post-date 280 Ma. The difference in the ore assemblage between the limestone-hosted Cu-polymetallic deposits of Ireland, and the penecontemporaneous Zn–Pb deposits of Central Ireland may be because of differences in the underlying lithology. In the SW, up to 6 km of red beds underlie the limestone-hosted stratabound Cu-polymetallic deposits whereas, in Central Ireland, the Zn–Pb ore-bearing carbonate rocks are underlain by lower Palaeozoic greywackes with a minimal thickness of late Devonian red beds. However, the isotopic evidence only indicates a major red bed source of metals at Ardtully. Elsewhere, unless the metals in the red beds originated from a basement rather than a granite source, a significant red bed contribution to the metals seems unlikely. An alternative possibility is that copper ores were derived from an extension of the Ordovician island arc of SE Ireland.

Impact of sulphur and trace element geochemistry on the utilization of a marine-influenced coal—case study from the South Wales Variscan foreland basin

Incremental channel samples through two high volatile A bituminous coal seams, the Amman Rider and Bute seams, below and above the Vanderbeckei Marine Band at the Westphalian A/B boundary, have been analysed using proximate, petrographic, inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence (XRF) and scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) techniques. The results show that both coals are vitrinite-rich and indicate a similar history of palaeomire development, with a period of falling water tables towards the middle plies of each seam. Mean sulphur contents in the coal with the marine roof, the Amman Rider seam (4.19 wt.%) are almost 4 times higher than in the coal with a non-marine roof, the Bute seam (1.09 wt.%), implying that high sulphur contents are derived from sea water sulphate. In both coals, sulphur is predominantly in the form of pyritic sulphur. Both syngenetic, framboidal and euhedral crystals, and epigenetic vein and cleat-fill pyrite are present. Relatively constant levels of organic sulphur at around 2% have been recorded in the Amman Rider seam. Sulphur levels in the basal and top plies of each seam are at least twice that of the ply with the lowest sulphur content in the seam, reflecting the stages in palaeomire development most strongly influenced by sulphate-containing waters. Thorium/Uranium ratios are below 2.9 in the coal with a marine roof, the Amman Rider seam, indicating an enrichment of U from sea water, most marked at the top and bottom of the seam. The seam without a marine roof shows Th/U ratios between 2.5 and 4.9, indicating mild marine enrichment of U. The concentrations of environmentally significant trace elements (ESEs) are correlated with either the ash or sulphur contents, or both. The correlation between S and Pb, As, Cu, Ni in the coal with a marine roof implies an origin of these ESEs from sea water and their mode of occurrence in sulphide minerals. Most ESEs in this coal are present at levels at least twice that of the global average in coals, the exceptions being Pb and Zn which have concentrations less than half the global average. By contrast, the coal without a marine roof and with lower sulphur contents has most ESEs present at concentrations less than a third of the global average, the exceptions being Ni and Cu. Diagenetic hydrothermal activity is documented in the middle ply of the coal with a marine roof. This hydrothermal system redistributed the sulphur, Pb and As in this ply. The principal utilization of bituminous coals in the South Wales coalfield is as feed coals in power station boilers. The resultant concentration of most ESEs in the solid residues from power plant coal combustion theoretically can be alleviated by careful blending of feed coals with low and high ESE contents.

Proterozoic Pb isotope evolution in the Belt-Purcell Basin; constraints from syngenetic and epigenetic sulfide deposits

Sediment-hosted massive sulfide and Ag-Pb-Zn veins are two major deposit types which contain large concentrations of lead in the Middle Proterozoic Belt-Purcell basin. New Pb isotope data on the Sullivan sediment-hosted massive sulfide deposit and the St. Eugene Ag-Pb-Zn vein deposit are combined with existing data to provide a comprehensive overview of Proterozoic Pb isotope evolution in the Belt-Purcell basin. The Pb isotope ratios for sulfides in the Sullivan deposit form a linear array, the Sullivan array. Bedded sulfides plot within this array and no systematic change in Pb isotope ratios with stratigraphic position (time) is observed. The Sullivan array results from the mixing of leads from the upper crustal and/or orogen Pb reservoirs, represented by metasedimentary rocks of the lower Aldridge Formation in the footwall of the Sullivan deposit, with, perhaps, leads derived from the Precambrian basement of the Belt-Purcell basin. A zone within the massive sulfides of the Sullivan deposit has been altered to chlorite-pyrite-calcite. The center of the alteration zone is comprised of massive pyrite-calcite which has more radiogenic Pb isotope compositions, the Sullivan alteration. The Sullivan alteration apparently formed from the same Pb reservoirs as the Sullivan array, perhaps as a product of the East Kootenay orogeny (<1370 Ma), after a period of up to 108 m.y. which allowed radiogenic Pb growth. Ag-Pb-Zn occurrences in the Belt-Purcell basin form an array in Pb isotope space, the Coeur d'Alene array, which is not collinear with the Sullivan array; it is characterized by Pb isotope compositions less radiogenic than those of the Sullivan array. The Coeur d'Alene array is a mixture of leads derived from the upper crustal and mantle Pb reservoirs. It is proposed that mantle lead was introduced into the Belt-Purcell basin at 1468 Ma by the mafic Moyie sills. Leads in sediment-hosted massive sulfide deposits in middle Aldridge rocks and leads of the Coeur d'Alene array in epigenetic veins formed after intrusion of the Moyie sills but before East Kootenay orogeny metamorphism.

Origin of hydrothermal CuZnPb mineralisation in the Olary Block, South Australia: evidence from fluid inclusions and sulphur isotopes

Styles of sulphidic mineralisation recognised in the Olary Block, South Australia, include disseminated to massive stratiform and stratabound occurrences, as well as post-peak metamorphic vein-type mineralisation, corresponding to the range of deposit types in the adjacent Broken Hill Block. However, it is still unclear as to whether the epigenetic veins are genetically related to stratabound mineralisation in either of these two portions of the Meso- to Palaeoproterozoic Willyama Inliers. A fluid inclusion and sulphur isotope study was undertaken to examine the nature of fluids associated with epigenetic mineralisation in the Olary Block, to determine possible genetic relationships between sulphides in epigenetic veins and stratabound mineralisation, and to evaluate possible sources of sulphur in each of these occurrences. Fluid inclusions in epigenetic assemblages can be classified in general terms of CO 2-rich, CH 4-rich, NaCl-rich and H 2O-rich types. Dense and moderately saline CO 2 and CH 4-rich fluids prevailed during or shortly after peak metamorphism, possibly as a result of metamorphic decarbonatisation and dehydration reactions at deeper levels as well as oxidation of graphite during retrograde metamorphism in shallower levels. Due to rehydration reactions and infiltration of metamorphic fluids from deep-seated hydrothermal reservoirs during retrogression and uplift, these fluids became increasingly saline and thus suitable to transport significant amounts of metals in solution. Near surface-derived fluids entered fluid conduits during further uplift and cooling, resulting in compositional dilution of the brines and decreasing fluid temperatures. Sulphur isotopic signatures in stratabound and epigenetic mineralisation in the Olary Block suggest a continuous supply of hydrothermal sulphur-bearing fluids from a deep-seated reservoir. Large-scale leaching of volcaniclastics and other sedimentary rocks or convective homogenisation of a mixed source (e.g., evaporitic or seawater sulphate and igneous sulphides) resulted in relatively uniform δ 34S values of +5 to 10‰ in the hydrothermal fluids. Mixing with sedimentary sulphur in stratabound occurrences is indicated by lighter sulphur isotopic compositions ( δ 34S ⩾ − 15.6) in sulphides associated with carbonaceous pelites. The distinct sulphur isotopic signature of epigenetic vein mineralisation in the Olary Block ( δ 34S inpyrrhotite between + 5.8 and + 7.6‰) precludes simple derivation from pre-existing deposits by remobilisation processes. It rather implies extraction from the same source region at a later stage, possibly induced by tectonic pumping and hydrothermal venting of a deep-seated crustal reservoir.

A multi-stage, multi-process genetic hypothesis for greenstone-hosted gold lodes

Abundant new data about the geology and geochemistry of greenstone-hosted gold lodes have emerged during the worldwide, intensive search for these ores for the last decade. Unfortunately, neither the resulting descriptions of their field relationships nor the data from fluid inclusion, light stable isotopic and radiometric age studies are definitive of a single, unique genetic process. Instead, some of these relationships are best explained by an epigenetic process, others by a synsedimentary one, and many are equivocal and explainable by either. A generally acceptable genetic hypothesis for the many, various forms of these lodes must account not only for all field and laboratory data but, also, must explain the sources of the various components of the ores, and the mechanisms of their mobilization, transport and deposition. A multi-stage, multi-process hypothesis is necessary to provide such a comprehensive explanation. The initial genetic stage involved deuteric reactions between seawater and thick, subaqueous tholeiitic basalts low in the greenstone belt successions. These reactions did not involve deep, sub-seafloor convective circulation of seawater, as in the generation of volcanogenic massive sulfide deposits higher in the successions, but occurred at shallow depths under the strongly reduced conditions of Archean time. They leached gold, silica, carbonate and other components of the ores from the lavas and moved upward, carbonatizing and variably altering the sea floor strata that they penetrated. They were discharged passively over broad areas to the sea floor, where their leached components were deposited as auriferous exhalites, variably sulfidic, ankeritic, magnetitic and siliceous. In places these were pure chemical precipitates, but elsewhere were mixed with variable amounts of clastic or pyroclastic detritus.This accounts for the widespread low, but anomalous gold content of Algoma type iron-formations and for the absence of highly altered rocks marking focused discharge vents beneath the few exhalites that contained enough gold to be ore. Reworking of the auriferous exhalites and enclosing altered lavas along the flanks of growing Archean volcanic complexes shed rounded clasts of auriferous sulfide and ankeritized, fuchsitic rocks into flanking basins, forming pre-metamorphic conglomerates that are essentially coeval with, but locally disconformable or unconformable on their source greenstones. These weakly auriferous strata were important pre-enriched source-rocks during multiple, superimposed stages of metamorphic remobilization, re-deposition and further concentration of gold. Successively, these stages include; burial diagenesis involving compaction, dehydration and lithification; syn-kinematic deformation during subsidence; intrusion of small, sub-volcanic porphyritic plutons that were late but co-magmatic in greenstone belt evolution; orogenic plutonism that accompanied deep-seated granulitization of the belts, and; post-orogenic intrusions. All these formed late, discordant, epigenetic veins, vein systems or stockworks that cut all earlier rocks and one another. Most fluid inclusion and light stable isotopic studies are of minerals from these late, epigenetic veins, not from the earlier, fine-grained syn-sedimentary strata; and their results cannot be generally applied to the latter. The studies show that the late veins were formed by CO 2-rich fluids at 200°–400°C and depths of 3–4 km. The same fluids formed not only mineralized veins in the pre-enriched source-rocks but, also, barren veins widespread throughout most Archean terranes where the source-rocks were absent. The source of these fluids remains problematical. They may have originated from dehydration of rocks undergoing deep-seated granulitic metamorphism, from reactions between fluid and earlier carbonate-rich rocks of the greenstone belts at lower metamorphic rank, by magmatic crystallization-differentiation, or from all of these.

The geological setting of the southern Bathgate Hills, West Lothian, Scotland

ABSTRACTThe southern Bathgate Hills, in the eastern part of the Midland Valley basin of Scotland, were the site of a volcanic rise during late Dinantian to early Silesian times and a sequence of basaltic lavas and tuffs up to 600 m thick accumulated. The volcanic pile interrupted the regional sedimentary deposition, which involved a cyclical sequence of marine limestones and mudstones followed by estuarine, lagoonal and deltaic clastic deposits. During the Brigantian Stage of the Dinantian, freshwater terrestrial environments developed locally on the volcanic rise between eruptive phases, but later in the Brigantian the rise was transgressed by marine limestones. Intermittent basaltic eruptions continued into the Amsbergian Stage of the Silesian, accompanied by intrusion of high-level alkaline doleritic sills and associated with strata-bound Zn—Pb mineralisation. Folding later in the Silesian was followed by the intrusion of a suite of quartz-dolerite sills and dykes. These latter were commonly intruded along penecontemporaneous E—W trending faults. The intrusion of the quartz-dolerites may have resulted in remobilisation of earlier strata-bound mineralisation into epigenetic veins.