Carbonate-rich Veins Research Articles

Mineralogical constraints on the genesis of an alkalic-type epithermal Au-Te deposit: Tuvatu, Fiji

To study the characteristics and genetic constraints on alkalic-type epithermal Au mineralisation, here we use the example of the Tuvatu Au-Ag deposit in Fiji, with an emphasis on detailed, quantitative mineralogy. Tuvatu mineralisation is hosted in a weakly altered potassic monzonite in parallel sided-veins of K-feldspar, biotite, sericite, calcite, and quartz, with epidote-bearing propylitic or sericite-rich selvages. Petrographic study of core and automated SEM-based mineralogical mapping of thin sections have been utilised to update previous parageneses of the deposit. Automated SEM techniques enable identification of small amounts of obscure minerals that form minuscule grains, which would otherwise be very difficult to identify and measure. As a result, our data show that gold fineness is extremely high, with the mean and median Au content of native-Au and Au-Ag alloy being 96.7% and 100% respectively, yet precious-metal tellurides make up the majority of the Au deportment. Tellurides show evidence of multiple phases and zoning with depth. For the first time at Tuvatu, Pt- and Pd-tellurides have been identified. Tuvatu has a number of features in common with alkalic systems elsewhere, including quartz-poor, carbonate-rich veins and alteration, abundant and varied telluride minerals, high gold grades, and Pt-Pd occurrences. We suggest these characteristics are a result of relatively high temperature (250–300 °C) fluids and immiscible semi-metal melts fluxing into the shallow epithermal environment. High pH fluids lead to quartz-poor alteration, but mildly acidic conditions dominate in areas of high fluid flux, where the lower pH causes precipitation of tellurides with quartz. Boiling of the fluids produces zonation of tellurides with depth but leaves relatively subtle textural evidence compared to boiling in most epithermal systems, in common with other quartz poor, carbonate-rich alkalic epithermal deposits around the world.

Episodic hydrofracturing and large-scale flushing along deep subduction interfaces: Implications for fluid transfer and carbon recycling (Zagros Orogen, southeastern Iran)

We investigate the late Cretaceous blueschist-facies (480 °C-1.8 GPa) segment of the Zagros suture zone, a well-preserved block-in-matrix paleo-subduction channel. We aim to determine the relative chronology, conditions of deformation, and potential fluid sources and processes associated with the widespread occurrence of lawsonite + clinopyroxene + glaucophane veins and aragonite-bearing hydraulic breccias. We use a multi-scale approach methodology to provide new insights into deep fluid flow mechanisms as well as to constrain possible sinks of CO2-bearing fluids in the subducting slab. Petrological analyses suggest that silicate-rich vein systems began precipitating during early burial and evolved with ongoing burial and shearing-related deformation in the blueschist-facies, while most carbonate-rich veins and hydrofractures formed at near-peak P-T conditions. In situ LA-ICP-MS trace element analyses reveal that: (i) individual silicate host-vein pairs have similar REE signatures, reflecting local-scale fluid-mediated element redistribution, (ii) carbonate-bearing veins and metasediments also have similar trace element signatures and (iii) lawsonite in blueschist-hosted veins exhibit REE enrichments along their rims, suggesting an increasing contribution of metasedimentary-derived fluids upon approaching peak P-T. Carbonate OC isotope compositions of the veins and metasedimentary rocks range from +13.6 to +17.9‰ (δ18OVSMOW) and − 1.0 to +3.1‰ (δ13CVPDB), demonstrating metasedimentary-derived fluid sources related to large-scale H2O homogenization with far-traveled mafic- ultramafic-derived fluids. SrNd isotopic ratios in carbonate veins and the adjacent host resemble their host composition indicating that host rock-buffered isotopic homogenization occurred between the infiltrating fluids and the rock matrix, possibly during episodic porous flow. Thermodynamic modeling predicts that decarbonation via fluid-assisted reactions is inefficient at blueschist-facies and that carbon release likely occurs deeper along the subduction interface (i.e., at eclogite-facies). We propose that deeply produced H2O-rich fluids interacted with the carbonate-bearing lithologies along the subduction interface facilitating fluid-mediated decarbonation and further fluid transport as hydraulic pulses (e.g., porosity waves) that traveled at the kilometer-scale parallel to the subduction interface, (i) contributing to the isotopic homogenization herein observed and (ii) triggering episodic hydrofracturing in the lawsonite-blueschist-facies (≈50-60 km depth). Veinsets in exhumed subducted rocks hence provide a unique opportunity to understand fluid-rock interaction processes in the region at which episodic tremor and slow slip events phenomena occur.

The Origin of Carbonatites from Amba Dongar within the Deccan Large Igneous Province

Abstract There are disparate views about the origin of global rift- or plume-related carbonatites. The Amba Dongar carbonatite complex, Gujarat, India, which intruded into the basalts of the Deccan Large Igneous Province (LIP), is a typical example. On the basis of new comprehensive major and trace element and Sr–Nd–Pb isotope data, we propose that low-degree primary carbonated melts from off-center of the Deccan–Réunion mantle plume migrate upwards and metasomatize part of the subcontinental lithospheric mantle (SCLM). Low-degree partial melting (∼2%) of this metasomatized SCLM source generates a parental carbonated silicate magma, which becomes contaminated with the local Archean basement during its ascent. Calcite globules in a nephelinite from Amba Dongar provide evidence that the carbonatites originated by liquid immiscibility from a parental carbonated silicate magma. Liquid immiscibility at crustal depths produces two chemically distinct, but isotopically similar magmas: the carbonatites (20% by volume) and nephelinites (80% by volume). Owing to their low heat capacity, the carbonatite melts solidified as thin carbonate veins at crustal depths. Secondary melting of these carbonate-rich veins during subsequent rifting generated the carbonatites and ferrocarbonatites now exposed at Amba Dongar. Carbonatites, if formed by liquid immiscibility from carbonated silicate magmas, can inherit a wide range of isotopic signatures that result from crustal contamination of their parental carbonated silicate magmas. In rift or plume-related settings, they can, therefore, display a much larger range of isotope signatures than their original asthenosphere or mantle plume source.

REE mineralisation within the Ditrău Alkaline Complex, Romania: Interplay of magmatic and hydrothermal processes

The Ditrău Igneous Complex (north-east Romania) is a tilted Mesozoic alkaline intrusion (~19 km diameter), with enrichments in rare earth elements (REE), niobium, and molybdenum. It has the potential to contribute to a secure and sustainable European REE mining industry, ensuring supply security for these critical metals. The complex comprises a sequence of ultramafic rocks, alkali gabbros, diorites, syenites, nepheline syenites and alkali granites. These units have been significantly modified by sub-solidus interaction with late-stage magmatic fluids and are cut by secondary mafic dykes. The complex was subsequently cut by REE-mineralised carbonate-rich veins. Geochemical and petrological data, including apatite mineral chemistry, from the alkaline igneous rocks, dykes and veins within the Ditrău Complex, have been used to assess the interplay of magmatic processes with late-stage magmatic and hydrothermal fluids, and the effects of these processes on element remobilisation and concentration of critical metals. Only limited critical metal enrichment was achieved by magmatic processes; the REE were preferentially incorporated into titanite and apatite in ultramafic cumulates during primary crystallisation, and were not enriched in evolved magmas. A hydrothermal system developed within the Ditrău Complex magma chamber during the later stages of magmatic crystallisation, causing localised alteration of nepheline syenites by a sodium-rich fluid. Mafic dykes subsequently acted as conduits for late stage, buoyant potassic fluids, which leached REE and HFSE from the surrounding syenitic rocks. These fluids percolated up and accumulated in the roof zone, causing the breakdown of nepheline to K-rich pseudomorphs and the precipitation of hydrothermal minerals such as zircon and pyrochlore within veins. REE mineralisation within the Ditrău Complex is hosted in the latest hydrothermal phase, mineralised carbonate-rich veins, which cross-cut the complex. Monazite is the main REE-bearing phase, it crystallised from a late REE- and carbonate-rich fluid with pH controlled REE deposition.

Genesis of carbonate-rich veins in the serpentinites at the Calabria-Lucania boundary (southern Apennines)

In the northern sector of the Pollino massif (southern Apennines, Italy) well-exposed lens-shaped bodies of serpentinites and sediments of the Frido Unit belonging to the Liguride Complex crop-out. Most serpentinite rocks are cross-cut by carbonate and quartz-carbonate veins with different thickness. This study focuses on petrographic, mineralogical, geochemical, and isotopic analysis of the carbonate veins embedded in serpentinite rocks, collected at the Fosso Arcangelo site and at the Pietrapica quarry (Calabria-Lucanian boundary). The paragenesis of the carbonate veins from the Fosso Arcangelo site is dominantly made by calcite and aragonite with minor amount of rhodochrosite. In quartz-carbonate veins from Pietrapica quarry, dolomite, Mg-calcite, quartz, with minor ankerite were observed.The δ13CV-PDB isotope ratios of carbonates in the carbonate veins are in the range from +2.16‰ to -3.66‰ and corresponding δ18OV-SMOW values are between +15.02‰ and +21.53‰. The δ13CV-PDB values of carbonates in quartz-carbonate veins are in the range from -3.60‰ to -1.78‰ and the corresponding δ18OV-SMOW average value is around +21.3‰. The results suggest that the carbonate at both site occurredunder the same environmental conditions from crustal-derived andlow-moderate temperatures hydrothermal fluids having different chemical composition.

In-situ assimilation of mantle minerals by kimberlitic magmas — Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa)

The lack of consensus on the possible range of initial kimberlite melt compositions and their evolution as they ascend through and interact with mantle and crustal wall rocks, hampers a complete understanding of kimberlite petrogenesis. Attempts to resolve these issues are complicated by the fact that kimberlite rocks are mixtures of magmatic, xenocrystic and antecrystic components and, hence, are not directly representative of their parental melt composition. Furthermore, there is a lack of direct evidence of the assimilation processes that may characterise kimberlitic melts during ascent, which makes understanding their melt evolution difficult.In this contribution we provide novel constraints on the interaction between precursor kimberlite melts and lithospheric mantle wall rocks. We present detailed textural and geochemical data for a carbonate-rich vein assemblage that traverses a garnet wehrlite xenolith [equilibrated at ~1060°C and 43kbar (~140–145km)] from the Bultfontein kimberlite (Kimberley, South Africa). This vein assemblage is dominated by Ca–Mg carbonates, with subordinate oxide minerals, olivine, sulphides, and apatite. Vein phases have highly variable compositions indicating formation under disequilibrium conditions. Primary inclusions in the vein minerals and secondary inclusion trails in host wehrlite minerals contain abundant alkali-bearing phases (e.g., Na–K bearing carbonates, Mg-freudenbergite, Na-bearing apatite and phlogopite). The Sr-isotope composition of vein carbonates overlaps those of groundmass calcite from the Bultfontein kimberlite, as well as perovskite from the other kimberlites in the Kimberley area. Clinopyroxene and garnet in the host wehrlite are resorbed and have Si-rich reaction mantles where in contact with the carbonate-rich veins. Within some veins, the carbonates occur as droplet-like, globular segregations, separated from a similarly shaped Si-rich phase by a thin meniscus of Mg-magnetite. These textures are interpreted to represent immiscibility between carbonate and silicate melts.The preservation of reaction mantles, immiscibility textures and disequilibrium in the vein assemblage, suggests quenching, probably triggered by entrainment and rapid transport toward the Earth's surface in the host kimberlite magma. Based on the Sr-isotope systematics of vein carbonate minerals, and the close temporal relationship between carbonate-rich metasomatism and kimberlite magmatism, we suggest that the carbonate-rich vein assemblage was produced by the interaction between a melt genetically related to the Bultfontein kimberlite and wehrlitic mantle wall rock. If correct, this unique xenolith sample provides a rare snapshot of the assimilation processes that might characterise parental kimberlite melts during their ascent through the lithospheric mantle.

MINERALOGY OF THE SNAP LAKE KIMBERLITE, NORTHWEST TERRITORIES, CANADA, AND COMPOSITIONS OF PHLOGOPITE AS RECORDS OF ITS CRYSTALLIZATION

The Cambrian Snap Lake kimberlite, in the Northwest Territories, Canada, is a rare example of an economic, near-horizontal kimberlite dyke deposit with an average vertical thickness of ~2.5 m. The diamond deposit consists of hypabyssal serpentine monticellite kimberlite and contains volumetrically minor hypabyssal kimberlite breccias. The kimberlite is composed of serpentinized olivine set in the groundmass of serpentinized monticellite, serpentine, phlogopite, dolomite, spinel, apatite, rutile and ilmenite. The Snap Lake kimberlite is similar in major-element composition to Group-I kimberlite worldwide, yet has exceptionally low CaO and P 2 O 5 . The low Ca character of the kimberlite is evident in the absence of perovskite, low proportion of carbonate, and the occurrence of dolomite rather than calcite as the dominant groundmass carbonate. Three textural types of phlogopite present in the kimberlite (macrocrystic–microphenocrystic, large poikilitic groundmass phlogopite and small groundmass phlogopite) have distinct patterns of composition and zonation. Detailed studies of the zonation reveal a complex evolution of the phlogopite compositions from an early deep xenocrystal to two stages of groundmass crystallization. Superimposed on this evolution common to all kimberlites worldwide is an additional early stage of groundmass crystallization in which barian phlogopite was precipitated only in one part of the dyke. The crystallization of Ba-rich phlogopite is accompanied by Sr-rich apatite, Mn-rich ilmenite and Fe 3+ -rich rutile, and may be related to a highly localized influx of residual oxidized carbonatitic fluid. The deuteric origin of the fluid explains the Ca-poor character of the Snap Lake kimberlite and the formation of late carbonate-rich veins pervasively cross-cutting the dyke.

Petrology of ultramafic lamprophyres from the Beaver Lake area of Eastern Antarctica and their relation to the breakup of Gondwanaland

Mesozoic melilite-bearing ultramafic lamprophyres are developed as sill, dyke and plug-like intrusive bodies in the East Antarctic Beaver Lake area. They consist of varying amounts of olivine, melilite, phlogopite, nepheline, titanomagnetite and perovskite as major phases, accompanied by minor amounts of apatite, carbonate, spinel, glass and, rarely, monticellite. The rocks are mineralogically and geochemically broadly similar to olivine melilitites, differing in higher CO2 and modal phlogopite and carbonate contents. The ultramafic lamprophyres are MgO-rich (13.4–20.5 wt%) and SiO2-poor (32.8–37.2 wt%), indicative of a near-primary nature. Major and trace element features are consistent with minor fractionation of olivine and Cr-spinel from melts originating at depths of 130–140 km. Primary melts originated by melting of upper mantle peridotite which had been veined by phlogopite + carbonate + clinopyroxene-bearing assemblages less than 200 Ma before eruption. The presence of the veins and their time of formation is required to explain high incompatible trace element contents and growth of 87Sr/86Sr, leaving 143Nd/144Nd unaffected. The major element, compatible trace element, and most radiogenic isotope characteristics are derived from melting of the wall-rock peridotite. The depth of about 130 km is indicated by the presence of phlogopite rather than amphibole in the veins, by control of the REE pattern by residual garnet, by the high MgO content of the rocks, and by the expected intersection of the rift-flank geotherm with the solidus at this depth. The higher CO2 contents than are characteristic for olivine melilitites favoured the crystallization of melilite at crustal pressures, and suppressed the crystallization of clinopyroxene. The Beaver Lake ultramafic lamprophyres are a distal effect of the breakup of Gondwanaland, too distal to show a geochemical signature of the Kerguelen plume. Upward and outward movement of the asthenosphere-lithosphere boundary beneath the Lambert-Amery rift led first to the production of phlogopite- and carbonate-rich veins, and later to the generation of the ultramafic lamprophyres themselves.

Trace element and Nd–Sr isotopic composition of ultramafic lamprophyres from the East Antarctic Beaver Lake area

The trace element and Nd–Sr isotopic compositions of Cretaceous (110–117 Ma) ultramafic lamprophyres from the Beaver Lake area in East Antarctica, which are developed as sill, dyke and plug intrusions, have been investigated. Rare earth elements of lamprophyres are strongly fractionated, with LREE >100 times chondrite, whereas HREE are <10 times chondrite, presumably indicating the presence of residual garnet in the source region. A characteristic feature of the Beaver Lake rocks is low concentrations of Zr and Hf (Zr, 50–250 ppm; Hf, 1.0–5.0 ppm) and to a lesser extent Nb (17–90 ppm) which cause strong negative anomalies in the normalized trace element patterns. Eruption age-corrected Nd isotope values vary within narrow ranges ( εNd (t)=+2.1 to +4.3), whereas Sr isotope values show more variation ( 87Sr/ 86Sr (t)=0.704336–0.706431), similar to the isotopic compositions of other ultramafic lamprophyres. The genesis of the ultramafic lamprophyres is explained as a result of the gradual widening of the Lambert-Amery rift during the Phanerozoic. An increase in the geothermal gradient from cold continental conditions beneath the Archean block to the west may have resulted in grazing of the peridotite solidus at depths greater than 110 km. The initial carbonate-rich melts migrated upwards and froze as carbonate-bearing veins in the overlying lithosphere. Later upward and outward migration of the asthenosphere beneath the rift caused remelting of the veined region, resulting in melts of ultramafic lamprophyre composition with a mixed geochemical abundance and isotope signature from carbonate-rich veins and depleted peridotite wall-rock. The CO 2-rich component of the lamprophyres is derived principally from the vein assemblage, from which phlogopite gives rise to the variable Sr isotopes, and baddeleyite to the differing Zr and Hf abundances. The difference between the lamprophyre facies of the Beaver Lake intrusions can be explained by minor amounts of olivine and Cr-spinel fractionation.

Factors controlling the origin of gas in Australian Bowen Basin coals

Open system pyrolysis (heating rate 10°C/min) of coal maturity (vitrinite reflectance, VR) sequence (0.5%, 0.8% and 1.4% VR) demonstrates that there are two stages of thermogenic methane generation from Bowen Basin coals. The first and major stage shows a steady increase in methane generation maximising at 570°C, corresponding to a VR of 2 – 2.5%. This is followed by a less intense methane generation which has not as yet maximised by 800°C (equivalent to VR of 5%). Heavier (C 2+) hydrocarbons are generated up to 570°C after which only the C 1 (CH 4, CO and CO 2) gases are produced. The main phase of heavy hydrocarbon generation occurs between 420 and 510°C. Over this temperature range, methane generation accounts for only a minor component, whereas the wet gases (C 2–C 5) are either in equal abundance or are more abundant by a factor of two than the liquid hydrocarbons. The yields of non-hydrocarbon gases CO 2 and CO are greater then methane during the early stages of gas generation from an immature coal, subordinate to methane during the main phase of methane generation after which they are again dominant. Compositional data for desorbed and produced coal seam gases from the Bowen show that CO 2 and wet gases are a minor component. This discrepancy between the proportion of wet gas components produced during open system pyrolysis and that observed in naturally matured coals may be the result of preferential migration of wet gas components, by dilution of methane generated during secondary cracking of bitumen, or kinetic effects associated with different activations for production of individual hydrocarbon gases. Extrapolation of results of artificial pyrolysis of the main organic components in coal to geological significant heating rates suggests that isotopically light methane to δ 13C of −50‰ can be generated. Carbon isotope depletions in 13C are further enhanced, however, as a result of trapping of gases over selected rank levels (instantaneous generation) which is a probable explanation for the range of δ 13C values we have recorded in methane desorbed from Bowen Basin coals (−51 ± 9‰). Pervasive carbonate-rich veins in Bowen Basin coals are the product of magmatism-related hydrothermal activity. Furthermore, the pyrolysis results suggest an additional organic carbon source from CO 2 released at any stage during the maturation history could “mix” in varying proportions with CO 2 from the other sources. This interpretation is supported by C and O isotopic ratios of carbonates that indicate mixing between magmatic and meteoric fluids. Also, the steep slope of the C and O isotope correlation trend suggests that the carbonates were deposited over a very narrow temperature interval basin-wide, or at relatively high temperatures (i.e., greater than 150°C) where mineral-fluid oxygen isotope fractionations are small. These temperatures are high enough for catagenic production of methane and higher hydrocarbons from the coal and coal-derived bitumen. The results suggests that a combination of thermogenic generation of methane and thermodynamic processes associated withCH 4/CO 2 equilibria are the two most important factors that control the primary isotope and molecular composition of coal seam gases in the Bowen Basin. Biological process are regionally subordinate but may be locally significant.

Geochemistry of late (~ 1.1 Ga) fluid inclusions in rocks of the Kapuskasing Archean crustal section

Three outcrops, well constrained by geochronological and structural studies, and representing a traverse running from tonalite-dominated outcrops in the eastern Wawa gneiss terrane to high-grade granulites of the Kapuskasing structural zone, were mapped and sampled in detail in order to study the trapped fluids. All fluid inclusions in quartz are secondary and consist mostly of CO2-dominated (type II) and saline aqueous (type IIIa) fluids usually occurring on separate healed fractures but also coexisting on some fractures. Healed fractures in quartz contain fluid inclusions but are associated with carbonate–sericite alteration where they pass into adjacent mineral grains. Homogeneous H2O–CO2–salt fluid inclusions (type Ia) in carbonate-rich veins of probable Keweenawan (~ 1.1 Ga) age were trapped at 400–550 °C and ambient pressures of 1.5–2 kbar (1 kbar = 100 MPa). As these fluids cooled on penetration into cool (~ 200 °C) country rocks along fractures they underwent open-system H2O-CO2 phase separation from ~ 350 °C down to ~ 190 °C, producing a range of fluid compositions, including physically segregated CO2-rich (type II) and H2O–salt–rich (type IIIa). Combined gas and ion chromatographic bulk fluid inclusion analyses show that fluid types II and IIIa are not related to shield brines. Br−/Cl− ratios of samples containing phase-separated fluids are similar to the Br−/Cl− ratio of fluids in the carbonate-rich vein. The results of this study show that Keweenawan alkalic magmatism caused widespread carbonate alteration throughout the Kapuskasing structural zone and Wawa gneiss domain. The CO2 component of the fluids is probably magmatic in origin, whereas the aqueous part could also be magmatic or, alternatively, formation waters activated by Keweenawan magmatism.

Mineralogy and petrology of the rauhaugites of the Mt Weld carbonatite complex of Western Australia

The circa 2.06 Ga Mt Weld carbonatite complex of Western Australia intrudes an Archean greenstone sequence dominated by basic and ultrabasic metamorphosed igneous rocks. Carbonatites form the core of the complex and are surrounded by glimmerites. The dominant carbonatite is sovite and is intruded by rauhaugites and carbonate-rich veins. The present investigation examines the mineral chemistry and petrology of the layered rauhaugites. They are essentially composed of ferroan dolomite, mica, magnetite and apatite, with accessory amounts of pyrochlore, ilmenite, sphalerite, baddeleyite, pyrite, galena and minerals enriched in the REE. The micas consist of titan-phlogopite, low-Ti phlogopite and “tetraferriphlogopite”. It is proposed that the parental magma of the Mt Weld complex was a potassic, aillikitic lamprophyre.