Altered Country Rocks Research Articles

Tourmaline geochemical and B isotopic constraints on pegmatite Li mineralization and exploration

Pegmatite–related deposits represent one of the most significant types of mineral deposits housing rare–metal elements such as Li, Be, Nb, Ta, Rb, Cs, and Sn. Although extensively studied for almost two centuries, the mechanism controlling the rare–metal mineralization in pegmatites remains controversial. In addition to the enrichment of rare–metal elements in the source region, differentiation processes (e.g., fractional crystallization and liquid immiscibility) after emplacement may have also contributed to the concentration and mineralization of rare–metal elements. However, compared to fractional crystallization, the role of liquid immiscibility in pegmatite mineralization has received limited attention. In this study, the element and boron (B) isotopic compositions of tourmalines from different textural zones (Zones I–VI) of the rare–metal–mineralized Koktokay No. 3 pegmatite, Altai, NW China, as well as from the altered country rock and the border zone, were analyzed to evaluate the role of liquid immiscibility in the generation of Li–mineralized pegmatites. Tourmalines display a variety of compositions, ranging from schorl and elbaite in the outer zones to elbaite in the inner zones of the Koktokay No. 3 pegmatite. Tourmalines from Zones I–III exhibit no obvious internal textures, whereas some tourmalines from Zones IV–VI have replacement textures or abrupt zonations. The distinction is attributed to the absence and presence of exsolving fluids during their formation, respectively. Tourmalines in Zones I–III and VII–VIII display less variable δ11B values (–15.07 ‰ to –12.21 ‰ and –14.16 ‰ to –13.10 ‰, respectively), reflecting a negligible B isotope fractionation produced by fractional crystallization during the pegmatite evolution. By contrast, tourmalines in Zones IV–VII exhibit more significant variations in δ11B values (–14.83 ‰ to –8.09 ‰) compared to those in Zones I–III and VII–VIII. The high δ11B tourmalines in Zones IV–VII were most likely crystallized from the fluids exsolving from the highly evolved pegmatite–forming magma. Their occurrence indicates the fluid exsolution occurring between zones IV and V, where Li mineralization began in the Koktokay No. 3 pegmatite. The mineralization of rare–metal elements is closely linked to the evolution of magma into a coexisting magma–fluid system. In addition, Li–mineralized pegmatites are characterized by tourmalines with Fe3+Al-1 substitution and higher Zn, Li, Li/Sr, and V/Sc than barren pegmatites. These differences are believed to be due to the higher fO2 and greater extent of magmatic differentiation in Li–mineralized pegmatites compared to the barren ones. These findings provide new insights into using the geochemical compositions of tourmalines as a guide for exploring Li–mineralized pegmatites.

Origin of the Dashuigou Independent Tellurium Deposit at the Southeastern Qinghai-Tibet Plateau: Based on the Abundances of Trace Elements in the Country Rocks

Through a detailed study of the abundances and spatial-temporal distribution patterns of Te, Bi, As, Se, Cu, Pb, Zn, Au, and Ag in the rock types of different geological epochs in the Dashuigou independent tellurium deposit, and in combination with other research findings of previous researchers in this area, the authors conclude as follows: Abundances of the main ore-forming elements Te, Bi, As, Se, Au, and Ag are not high in the regional geological background, generally lower or close to their respective crustal Clark values, but almost all altered country rocks contain high levels of ore-forming elements. This indicates that the deposit’s ore-forming elements do not come from the country rocks. This also indicates that the geological thermal events that cause alteration and mineralization originate from depths and may be related to mantle plumes. Considering the distribution pattern of these ore-forming elements in the ore bodies’ hanging wall and footwall, the metallogenic mechanism may be as follows: Mineralization is not achieved through lateral secretion in the horizontal or near horizontal direction, but rather through the upward movement and emplacement of deep ore-forming elements driven by geological processes such as mantle plumes. In addition, the migration of deep ore-forming elements is not achieved through dispersed infiltration between overlying rock particles, but through non widespread concentrated penetrating channels. This type of channel is likely to be the expansion structures where faults from different directions intersect, or where linear faults intersect with circular structures.

Zircon double-dating, trace element and O isotope analysis to decipher late Pleistocene explosive-effusive eruptions from a zoned ocean-island magma system, Ascension Island

In this first detailed study of zircon from Ascension Island, South Atlantic, we take a novel approach combining trace element and O isotope compositional data with double-dating (disequilibrium 238U–230Th and (U–Th)/He) to decipher timescales and dynamics of magmatic processes. The Echo Canyon (EC) sequence comprises small-volume explosive-effusive eruptions of trachyte that tapped a compositionally zoned magma system. Associated volcanic hazards may be constrained from the age of volcanism, duration of magma storage, and magma source and plumbing system character. Zircon U–Th–Pb dating of lithic lava clasts has revealed recurrent evolved volcanism at 1.34 and 0.6 Ma, and 95 ka. The (U–Th)/He zircon cooling ages indicate that most of the EC explosive-effusive sequence erupted in a brief episode at ca. 95 ka. Additionally, uniform 238U–230Th zircon crystallisation ages suggest moderately protracted magma storage with melt present at depth for at most 103–104 years before eruption. The enriched character of zircon trace element compositions, relative to MORB, in the absence of a continental crustal signature in the oxygen isotope values (δ18O range 2.67–5.63‰), suggests the presence of an enriched component in the EC magma source. Furthermore, low δ18O zircon compositions imply assimilation of high temperature hydrothermally altered country rock by the source magma. The mineral assemblage in crystal-poor pumices indicates equilibrium storage conditions: zircon saturation and Ti-in-zircon crystallisation temperatures are consistent with alkali feldspar-melt temperatures. Significantly, zircon crystals were preserved both as macrocryst inclusions and in the groundmass of EC explosive and effusive deposits. These rocks preserve evidence of magma evolution by fractional crystallisation. This process led to pre-eruptive compositional stratification, which is evidenced in the range of whole-rock major and trace element compositions and zircon Zr/Hf values. Notably, zircon crystallisation and cooling ages derived from pumice, lava, and accidental lithic lava clasts in highly explosive pyroclastic deposits, have revealed episodes of evolved magmatism that would otherwise have gone undetected. In addition, the zircon trace element and isotope compositions, in combination with the range of crystallisation ages, evidence progressively deeper tapping of less evolved magma stored in discrete lenses. Thus, a combined zircon geochronological-geochemical approach can place constraints on the ca. 0.6 Ma recurrence of past explosive-effusive pulses of millennial to decamillennial duration and their enriched magma sources. This information is relevant for assessing hazards and informing monitoring and forecasting efforts to assist in managing associated risks for small ocean island volcanoes with particularly vulnerable populations and infrastructure.

Deynekoite, Ca9□Fe3+(PO4)7 – a new mineral of the merrillite group from phosphide-bearing contact facies of paralava, Hatrurim Complex, Daba-Siwaqa, Jordan

AbstractDeynekoite, Ca9□Fe3+(PO4)7 (R3c, a = 10.3516(3)Å, c = 37.1599(17)Å, V = 3448.4(3)Å3 and Z = 6), a new mineral of the merrillite group was found in the contact facies of paralava of the Hatrurim Complex in the Daba-Siwaqa pyrometamorphic rock field, Jordan. The paralava, consisting of diopside, tridymite, anorthite, wollastonite and fluorapatite, is enriched in Fe-bearing phosphides and phosphates at the contact with the altered country rock. Cristobalite overgrowing tridymite has a fish-scales texture indicating that temperature of paralava could have reached 1500°C. Deynekoite with empirical formula (Ca8.90Na0.11K0.02)Σ9.03(Fe3+0.62Mg0.30Al0.05)Σ0.97P6.98V5+0.05O27.70(OH)0.30 forms transparent, light-yellow or light-brown grains up to 30–40 μm in size. Microhardness of deynekoite, VHN25 = 319(29) kg/mm2, corresponds to Mohs hardness = 4.5. Its density was calculated as 3.09 g⋅cm–3 on the basis of its empirical composition and structural data. Deynekoite is uniaxial (−), its refractive indices are ω = 1.658(3), ɛ = 1.652(3) (λ = 589 nm), and pleochroism is not observed. The formation of phosphides on the boundary of the paralava and country rock is connected with carbothermal reductive reactions and realised at temperatures above 1300°С. With decreasing temperature and increasing oxygen activity, phosphides are replaced by Fe2+-bearing phosphates. Deynekoite, which contains Fe3+ (substituting for Fe2+-phosphates) and a small amount of water, formed at temperatures of 600–800°C.

Integrated U–Pb, Lu–Hf and (U–Th)/He analysis of zircon from the Banxi Sb deposit and its implications for the low-temperature mineralization in South China

Low-temperature Sb (Au–Hg) deposits in South China account for more than 50% of the world’s Sb reserves, however, their genesis remains controversial. Here we report the first study that integrates U–Pb and Lu–Hf analysis by LA-(MC)-ICPMS and conventional (U–Th)/He analysis, all applied to single zircon crystals, in an attempt to constrain the origin and timing of world-class Sb (Au–Hg) deposits in Banxi (South China). Zircon separated from a quartz-stibnite ore and an altered country rock samples revealed similar U–Pb age spectra defining two major populations – Paleoproterozoic (~1900–2500 ​Ma) and Neoproterozoic (~770 ​Ma), which are characterized by variable εHf(t) values (–10.7 to 9.1 and –16.5 to 11.2, respectively) and Hf crustal model ages (TDMC) (2.48 to 3.24 ​Ga and 0.97 to 2.71 ​Ga, respectively). The U–Pb age and Hf isotopic features of the zircons are consistent with the Banxi Group in the region, indicating that the zircons involved in the low-temperature hydrothermal system were originally from the Banxi Group country rocks. Thirty-three mineralization-related zircon crystals yielded a mean (U–Th)/He age of 123.8 ​± ​3.8 ​Ma, which is interpreted to represent the timing of the latest low-temperature mineralization stage of the Banxi Sb deposit. The combined U–Pb, Lu–Hf and (U–Th)/He data suggest that Precambrian basement rocks were the major contributors to the low-temperature mineralization, and that Early Cretaceous (130–120 ​Ma) could be the most important ore-forming epoch for the Sb deposits in South China. This study also demonstrates the analytical feasibility of integrated U–Pb - Lu–Hf - (U–Th)/He “triple-dating”, all applied to single zircon crystals. This approach reveals the full evolution of zircon, from its origin of the magmatic source, through its crystallization and low-temperature cooling. Although this study demonstrates the usefulness of this integrated approach in dating low-temperature mineralization, it has great potential for zircon provenance and other studies that may benefit from the large amount of information that can be extracted from single zircon crystals.

Lower crustal hydrothermal circulation at slow-spreading ridges: evidence from chlorine in Arctic and South Atlantic basalt glasses and melt inclusions

Hydrothermal circulation at slow-spreading ridges is important for cooling the newly formed lithosphere, but the depth to which it occurs is uncertain. Magmas which stagnate and partially crystallize during their rise from the mantle provide a means to constrain the depth of circulation because assimilation of hydrothermal fluids or hydrothermally altered country rock will raise their chlorine (Cl) contents. Here we present Cl concentrations in combination with chemical thermobarometry data on glassy basaltic rocks and melt inclusions from the Southern Mid-Atlantic Ridge (SMAR; ~ 3 cm year−1 full spreading rate) and the Gakkel Ridge (max. 1.5 cm year−1 full spreading rate) in order to define the depth and extent of chlorine contamination. Basaltic glasses show Cl-contents ranging from ca. 50–430 ppm and ca. 40–700 ppm for the SMAR and Gakkel Ridge, respectively, whereas SMAR melt inclusions contain between 20 and 460 ppm Cl. Compared to elements of similar mantle incompatibility (e.g. K, Nb), Cl-excess (Cl/Nb or Cl/K higher than normal mantle values) of up to 250 ppm in glasses and melt inclusions are found in 75% of the samples from both ridges. Cl-excess is interpreted to indicate assimilation of hydrothermal brines (as opposed to bulk altered rock or seawater) based on the large range of Cl/K ratios in samples showing a limited spread in H2O contents. Resorption and disequilibrium textures of olivine, plagioclase and clinopyroxene phenocrysts and an abundance of xenocrysts and gabbroic fragments in the SMAR lavas suggest multiple generations of crystallization and assimilation of hydrothermally altered rocks that contain these brines. Calculated pressures of last equilibration based on the major element compositions of melts cannot provide reliable estimates of the depths at which this crystallization/assimilation occurred as the assimilation negates the assumption of crystallization under equilibrium conditions implicit in such calculations. Clinopyroxene–melt thermobarometry on rare clinopyroxene phenocrysts present in the SMAR magmas yield lower crustal crystallization/assimilation depths (10–13 km in the segment containing clinopyroxene). The Cl-excesses in SMAR melt inclusions indicate that assimilation occurred before crystallization, while also homogeneous Cl in melts from Gakkel Ridge indicate Cl addition during magma chamber processes. Combined, these observations imply that hydrothermal circulation reaches the lower crust at slow-spreading ridges, and thereby promotes cooling of the lower crust. The generally lower Cl-excess at slow-spreading ridges (compared to fast-spreading ridges) is probably related to them having few if any permanent magma chambers. Magmas therefore do not fractionate as extensively in the crust, providing less heat for assimilation (on average, slow-spreading ridge magmas have higher Mg#), and hydrothermal systems are ephemeral, leading to lower total degrees of crustal alteration and more variation in the amount of Cl contamination. Hydrothermal plumes and vent fields have samples in close vicinity that display Cl-excess, mostly of > 25 ppm, which thus can aid as a guide for the exploration of (active or extinct) hydrothermal vent fields on the axis.

Using in situ SHRIMP U-Pb Monazite and Xenotime Geochronology to Determine the Age of Orogenic Gold Mineralization: An Example from the Paulsens Mine, Southern Pilbara Craton

Paulsens is a mesothermal orogenic gold deposit located in the Wyloo Inlier on the southern margin of the Pilbara craton of Western Australia. Gold occurs in quartz-sulfide veins hosted within a folded and faulted gabbro dike, from which baddeleyite yields a U-Pb crystallization age of 2701 ± 11 Ma. Monazite and xenotime in the veins and from hydrothermally altered country rocks yield three distinct U-Pb dates of ca. 2400, 1730, and 1680 Ma. Textural relationships between euhedral xenotime and pyrite with rounded native gold inclusions from within the quartz-sulfide veins show that the primary gold mineralization was synchronous with xenotime crystallization at 2403 ± 5 Ma, and coeval with pervasive alteration of the host rocks, which yield monazite ages of 2398 ± 37 and 2403 ± 38 Ma. Regional-scale hydrothermal events at ca. 1730 and 1680 Ma are linked to the growth of monazite within phyllitic rocks at 1730 ± 28 and 1721 ± 32 Ma, carbonate veining at 1655 ± 37 Ma, and gold remobilization or introduction of new gold at 1680 ± 9 Ma. The ca. 2400 Ma age for mineralization and hydrothermal alteration does not correspond with any known deformation event in the region, indicating a significantly different and more complicated low-temperature tectonothermal evolution for the southern Pilbara region than previously recognized. The in situ secondary ion mass spectrometry dating of monazite and xenotime employed here will lead to better targeting of orogenic gold deposits in the northern Capricorn Orogen, and these techniques can be utilized for orogenic gold exploration worldwide.

STABLE ISOTOPE (δ13C, δ18O) ANALYSIS OF SULFIDE-BEARING CARBONATE SAMPLES USING LASER ABSORPTION SPECTROMETRY

Hydrothermally altered country rocks surrounding hydrothermal ore deposits are commonly characterized by altered δ 18 O and δ 13 C values relative to distal, unaltered rocks. Stable isotopes have been recognized as a powerful tool to complement traditional geochemical and geophysical approaches during mineral exploration, particularly in carbonate replacement and Carlin-type deposits. However, wall-rock alteration and carbonate veining during ore formation is often accompanied by the precipitation of sulfide minerals, complicating stable isotope analysis. The classic phosphoric acid extraction technique for carbonate minerals results in the release of additional H 2 S gas, interfering with isotope ratio mass spectrometry (IRMS), and laser absorption spectrometry (LAS) analysis of CO 2 . Here we present the first results from a novel off-axis integrated cavity output spectrometer (OA-ICOS) instrument, capable of determining the stable isotope composition ( δ 13 C, δ 18 O) of mixed CO 2 -H 2 S gas evolved from carbonate-sulfide mixtures. Instrument calibration was performed by analyzing in-house and commercially available CaCO 3 stable isotope reference materials. Analyses of mixed CO 2 -H 2 S tank gas and sulfide-spiked stable isotope reference materials containing up to 40 wt % chalcopyrite and sphalerite demonstrate that the method outlined here is capable of effectively analyzing previously problematic high sulfide samples. The analytical capabilities, together with the robust nature of the OA-ICOS instrument, may be of direct practical use for mineral exploration in carbonate-hosted hydrothermal systems including ore deposits.

Two fluid sources and genetic implications for the Hatu gold deposit, Xinjiang, China

The Hatu gold deposit is located in the West Junggar domain, Xinjiang, China. It contains 56tonnes of gold. The deposit is hosted in the volcanic and sedimentary rocks of the Early Carboniferous Tailegula Group, and an ~E–W trending ductile shear zone controls the mineralization. The gold mineralization is hosted in quartz veins and hydrothermally altered country rocks with <3vol.% sulfides developed along the faults. The fluid inclusions are characterized by the presence of dominant CO2 and minor CH4 fluid. The δ13CVPDB values of CH4 (−18.7‰ to −35.4‰) and CO2 (−9.7‰ to −13.9‰) in fluid inclusions from quartz and their ∆13CCO2–CH4 values (5.5‰ to 24.6‰) equilibrated at a temperature range from 299°C to 1013°C; these data indicate that the carbon had a dominantly sedimentary source with a minor contribution from the mantle. The oxygen and hydrogen isotope compositions (δ18O=8.5‰ to 12.2‰, δD=−87‰ to −105‰) of water from fluid inclusions in quartz indicate that ore fluids comprised largely metamorphic water with a minor contribution of magmatic water. The δ34S values of sulfide from the mineral zones are between −2.3‰ and +2.5‰, with the exception of two values (−9.7‰ and +7.4‰), showing a heterogeneous sulfur source. Fluid inclusions in pyrite and arsenopyrite that are intergrown with auriferous quartz have 3He/4He ratios of 0.2 to 0.3 Ra, and their 40Ar/36Ar ratios range from 310 to 650, indicating a crustal source for much of the ore fluids. We propose that the Hatu deposit is a structurally controlled orogenic gold system which is characterized by two different fluids one dominantly derived from a crustal source with a minor component of mantle derived fluids.

Geology and ore fluid geochemistry of the Jinduicheng porphyry molybdenum deposit, East Qinling, China

Jinduicheng deposit is a giant Mesozoic porphyry Mo system deposit in the East Qinling molybdenum belt, Shaanxi Province, China. The mineralization is associated with the I-type Jinduicheng granite porphyry. Both the porphyry stock and country rocks underwent intense hydrothermal alteration. The alteration, with increasing distance from the parent intrusion, changes from silicification, through potassic and phyllic assemblages, carbonation, to propylitic assemblages. Molybdenite, the dominant ore mineral, occurs in veinlets, most of which are hosted by the altered country rocks, with less than 25% of the ore in the porphyry body. The hydrothermal system comprises four stages, including pre-ore quartz and K-feldspar; two ore stages of quartz, K-feldspar, molybdenite, and Pb- And Zn-bearing sulfides; and post-ore quartz and carbonate. Six main types of primary fluid inclusions are present in hydrothermal quartz, including two-phase aqueous, one-phase aqueous, three-phase CO2-bearing, CO2-dominated fluid inclusions, gas inclusions, and melt inclusions. The homogenization temperatures of fluid inclusions range from 210 to 290°C in the pre-ore stage, 150–310°C in ore stage I, 150–360°C in the ore stage II, and 195–325°C in the post-ore stage quartz. Estimated salinities of the ore-forming fluids range from 6.9 to 13.5, 4.3 to 12.3, 6.2 to 12.4, and 3.4 to 9.9wt.% NaCl equiv. in stages 1–4, respectively. The δ34S values of pyrite in the two ore stages range from 2.8‰ to 4.3‰, whereas the δ34S values of molybdenite range from 2.9‰ to 6.2‰. The data suggest both magmatic and crustal sources of sulfur. The δD and δ18O values for the hydrothermal fluids are −57.2‰ to −84.4‰ and 8.0‰ to −3.2‰, respectively. The fluid inclusion and stable data indicate that the pre-ore hydrothermal fluids were mostly of magmatic origin, but the fluids responsible for ore deposition were mixed magmatic and meteoric, and eventually meteoric water dominated the system in the post-ore stage.

Sedimentation and welding processes of dilute pyroclastic density currents and fallout during a large-scale silicic eruption, Kikai caldera, Japan

Sedimentation and welding processes of the high temperature dilute pyroclastic density currents and fallout erupted at 7.3 ka from the Kikai caldera are discussed based on the stratigraphy, texture, lithofacies characteristics, and components of the resulting deposits. The welded eruptive deposits, Unit B, were produced during the column collapse phase, following a large plinian eruption and preceding an ignimbrite eruption, and can be divided into two subunits, Units B l and B u. Unit B l is primarily deposited in topographic depressions on proximal islands, and consists of multiple thin (< 1 m) flow units with stratified and cross-stratified facies with various degrees of welding. Each thin unit appears as a single aggradational unit, composed of a lower lithic-rich layer or pod and an upper welded pumice-rich layer. Lithic-rich parts are fines-depleted and are composed of altered country rock, fresh andesite lava, obsidian clasts with chilled margins, and boulders. The overlying Unit B u shows densely welded stratified facies, composed of alternating lithic-rich and pumice-rich layers. The layers mantle lower units and are sometimes viscously deformed by ballistics. The sedimentary characteristics of Unit B l such as welded stratified or cross-stratified facies indicate that high temperature dilute pyroclastic density currents were repeatedly generated from limited magma-water interactions. It is thought that dense brittle particles were segregated in a turbulent current and were immediately buried by deposition of hot, lighter pumice-rich particles, and that this process repeated many times. It is also suggested that the depositional temperature of eruptive materials was high and the eruptive style changed from a normal plinian eruption, through surge-generating explosions (Unit B l), into an agglutinate-dominated fallout eruption (Unit B u). On the basis of field data, welded pyroclastic surge deposits could be produced only under specific conditions, such as (1) rapid accumulation of pyroclastic particles sufficiently hot to weld instantaneously upon deposition, and (2) elastic particles' interactions with substrate deformation. These physical conditions may be achieved within high temperature and highly energetic pyroclastic density currents produced by large-scale explosive eruptions.

Oxygen isotope evidence for magmatic variability and multiple alteration events in the Proterozoic St. Francois Mountains, Missouri

The St. Francois Mountains contain at least two Mesoproterozoic caldera complexes, the Butler Hill and the Taum Sauk calderas, each with diameters greater than 20 km. Igneous zircons from intrusive and extrusive units within the calderas have a range of δ 18O(zircon) from 5.8 to 8.2‰ V-SMOW. These zircons are similar to other Proterozoic zircons from igneous rocks around the world and suggest a magma source from partial melting of older crust with variable mantle component. There is no significant difference in the magmatic oxygen isotope ratios between the Taum Sauk and Butler Hill calderas. Values of Δ 18O(quartz–zircon) = 2.1–2.4‰ indicating that most quartz from samples in the two calderas still preserves near-magmatic oxygen isotope ratios. The average δ 18O(quartz) from the two calderas is 8.6 ± 2.6‰. In contrast, felsic volcanic units within the Taum Sauk caldera exposed at Johnson Shut-Ins suggest interaction with water at low temperature resulting in δ 18O(quartz) as high as 16.7‰. Quartz phenocrysts within a single sample from Johnson Shut-Ins range from 8.7 up to 16.7‰. The isotope heterogeneity, coupled with evidence of healed microcracks in cathodoluminescence images, suggests low temperature hydrothermal alteration affected the volcanics during or shortly after eruption. After caldera collapse the hydrothermally altered rocks were partially re-melted and incorporated into the magma chamber as evidenced by the slight increase in δ 18O(zircon) of the resurgent dome intrusions. In contrast, the Butler Hill caldera preserves evidence of high temperature hydrothermally altered country rock assimilated into the Stono Mountain granitic ring intrusion based on low δ 18O quartz values and visible zoning in CL. The two calderas experienced different alteration histories suggesting the lack of any homogenizing regional fluid influx.

Tourmalines from the Koktokay No.3 pegmatite, Altai, NW China: spectroscopic characterization and relationships with the pegmatite evolution

Mossbauer spectra (MS), infrared spectra (IR) in the OH stretching region, and reflectance spectra (RS) were obtained for natural tourmalines from the Koktokay No.3 pegmatite, the contact zone, and the altered country rock, Altai, NW China. Black Fe-rich dravite in the altered country rock and black Fe-rich dravite + schorl-foitite have the lowest degree of Fe 2+ ordering at Y-site and a high Fe 3+ /Fe 2+ ratio (0.35 and 0.30, respectively). Black elbaite-schorl in zones I to IV has a moderate degree of Fe 2+ ordering at Y-site and a low Fe 3+ /Fe 2+ ratio (0.11–0.18). Green elbaite in the inner pegmatite has the highest degree of Fe 2+ ordering at Y-site and contains no ferric iron. Tourmalines from the altered country rock, the contact zone, and zones I to IV of the pegmatite contain only OH − and no molecular water, whereas those from zones V to VIII of the pegmatite contain both OH − and molecular water. Variations of degrees of Fe 2+ ordering at Y-site of tourmaline crystal structure are generally consistent with the magmatic to hydrothermal evolution of the Koktokay No.3 pegmatite. However, the lowest degrees of Fe 2+ ordering at Y-site of tourmalines from the altered country rock and the contact zone are probably caused by rapid consolidation rates. The presence of molecular water in tourmalines from zones V to VIII reflects an increase of water fugacity during fractionation evolution of the pegmatite. The fact that molecular water is absent in tourmaline from the altered country rock and the contact zone suggests that there was a very low water fugacity during the interaction between fluids from the pegmatite and the country rock. The variation of Fe 3+ /Fe 2+ ratio suggests a decreasing oxidation state from the altered country rock across the contact zone through zones I–IV to zones V–VIII of the pegmatite, and generally consistent with previous investigations.

Origin and Evolution of the Um Egat and Dungash Orogenic Gold Deposits, Egyptian Eastern Desert: Evidence from Fluid Inclusions in Quartz

Shear zone-related, mesothermal gold deposits of Um Egat and Dungash in the Egyptian Eastern Desert are hosted by greenschist facies metavolcanic and/or metasedimentary rocks of Pan-African age. Both deposits comprise boudinaged quartz veins that show evidence of incipient recrystallization, and are similar in alteration style, structural control, and mineralogy. The ore mineralogy includes pyrite, arsenopyrite ± pyrrhotite ± chalcopyrite ± galena; gold occurs both in the veins, usually included in arsenopyrite, pyrite, or pyrrhotite next to fragments of altered country rocks, or disseminated in the alteration haloes. Arsenic in arsenopyrite and Aliv in chlorite geothermometers indicate that wall-rock alteration and ore mineral precipitation occurred at temperatures between 400° and 250°C. Fluid inclusions in vein quartz occur in clusters, or along trails. Three types of fluid inclusions were identified based on petrography and laser micro-Raman spectroscopy: (1) two-phase carbonic inclusions with CO2 + CH4 ± N2 ± H2O, (2) two-phase aqueous inclusions, and (3) three-phase aqueous-carbonic inclusions with CH4. Final melting of ice (Tmf) for most inclusions occurs at temperatures between −4° and 0°C, indicating a low salinity ( 300°C, but cluster into two distinct groups for each type of inclusion. Inclusions from the same trail or cluster are commonly characterized by different degrees of fill or different Th values. Field, petrographic, and microthermometric data suggest that low-salinity aqueous-carbonic fluids interacted with graphite-bearing metasedimentary rocks to form CH4 at T >400°C and P >3 kbars. These reduced fluids leached gold as they circulated through the metavolcanic rocks, carrying it in the form of bisulfide complexes. Interaction of these aqueous-carbonic fluids with the country rocks caused hydrothermal alteration and precipitated gold-bearing sulfides in the alteration zones. A drop of pressure during the migration of these fluids to shallower depths along the shear zones led to phase separation at T ≤300°C and P ≤2.3 kbars. Quartz crystallizing over a range of lower temperatures and pressures trapped carbonic and aqueous fluids as separate inclusions in clusters along pseudosecondary and secondary trails. Postdepositional deformation caused decrepitation of some inclusions, and the stretching and leakage of others, increasing Th to >250°C. Deformation also remobilized the gold, depositing it as globules of higher fineness in secondary sites.

CHEMICAL AND TEXTURAL FEATURES OF TOURMALINE FROM THE SPODUMENE-SUBTYPE KOKTOKAY No. 3 PEGMATITE, ALTAI, NORTHWESTERN CHINA: A RECORD OF MAGMATIC TO HYDROTHERMAL EVOLUTION

The Koktokay No. 3 pegmatite, Altai, northwestern China, is a spodumene-subtype granitic pegmatite. In this study, we report textural and chemical features of tourmaline from the altered country-rock, the contact zone, and the pegmatite. The tourmaline ill the altered country-rock, Ca- and Fe-rich dravite, shows an obvious chemical heterogeneity within individual grains. Tourmaline in the contact zone consists of two generations: zoned Ca- and Fe-rich dravite and in interstitial foitite-schorl solid solution. Tourmaline from the altered country-rock and the contact zone reflects interaction between the country rock (metagabbro) and the pegmatite-forming melt or fluids derived from it. The chemical variation of these tourmalines depends on various contributions of components from the country rock and the pegmatite. The tourmaline in the outer zones (zones I to IV) of the pegmatite is elbaite-schorl solid solution with all intermediate composition between the end members; it is generally homogeneous within individual grains. In the inner zones (zones V, VI, and VIII), the tourmaline is dominantly elbaite with rare rossmanite in zone V. Elbaite is either abruptly zoned within individual grains, or has a replacement texture. Chemically, elbaite in the inner zones has a higher proportion of X-site vacancy than elbaite-schorl in the outer zones. Chemical trends of tourmaline compositions in the spodumene-subtype Koktokay No. 3 pegmatite are generally similar to those in other pegmatite subtypes (lepidolite, petalite, and elbaite subtype). Systematic variations in the internal textures of tourmaline from the outer zones to the inner zones suggest that exsolution of fluids occurred between zone IV and zone V. The outer zones crystallized from a volatile-unsaturated pegmatite-forming magma, whereas the inner zones crystallized from a hydrothermal system. This evolution process is consistent with the London model of internal evolution of granitic pegmatites.

Sources and evolution of arc magmas inferred from coupled O and Hf isotope systematics of plutonic zircons from the Cretaceous Separation Point Suite (New Zealand)

Coupled O and Hf isotopic compositions of zircons from Early Cretaceous (113–124 Ma) granitoids of the Separation Point Suite (SPS), New Zealand, obtained by cathodoluminescence imaging-guided micro-beam methods (SIMS, LA-ICPMS), are used as a record of evolving magma compositions in a prominent Mesozoic arc system. Eight representative SPS samples from individual plutons in the Nelson and Fiordland regions yield magmatic zircons with initial Hf isotope ratios (expressed in ε Hf) ranging from − 4 to + 11 (Nelson) and + 5 to + 12 (Fiordland), respectively. Initial Hf isotope ratios of zircons are extremely heterogeneous within individual samples, with the vast majority of values distinctly less radiogenic than depleted mantle at ~ 120 Ma ( ε Hf ~ + 16). O isotope ratios are likewise variable, with δ 18O zircon (SMOW) values of 2–8‰ (Nelson) and 0–7‰ (Fiordland). The within-rock variability in both zircon Hf and O isotope ratios is testimony to open-system processes that operated during magma evolution and zircon crystallisation. Average δ 18O zircon for cores and rims allow constraints to be placed on the O isotopic composition of magmas from which zircon precipitated ( δ 18O magma ~ 4–8‰). Elevated δ 18O magma (> 6.5‰) require involvement of 18O-enriched supracrustal material (weathered continental crust or low- T seawater-hydrothermally altered oceanic crust), while δ 18O magma < 5.5‰ imply contribution from a 18O-depleted crustal component. Whole rock Sr isotope, Nb/Ta and Nd/Pb systematics are inconsistent with 18O-depleted slab melts ( δ 18O ~ 0–6‰) as a source component for SPS magmas. Instead, low δ 18O values suggest incorporation of high- T meteoric-hydrothermally altered country rocks similar to those of the Largs terrane presently exposed in northern Fiordland. In diagrams of SiO 2 versus inferred δ 18O magma the most primitive SPS samples from the Nelson and Fiordland regions plot close to the expected composition for primitive arc magmas. More evolved granitoids, however, show strongly divergent trends of O isotope composition as a function of SiO 2, suggestive of assimilation coupled to fractional crystallisation (AFC) in contrasting crustal environments. Emplacement-level contamination by local crust is supported by age distributions of inherited zircons, which indicate a predominance of Palaeozoic and Mesozoic zircons in Nelson and Fiordland granitoids, respectively.

Interpretasi Mineralisasi Epitermal Berdasarkan Studi Ubahan Hidrotermal dan Tekstur Urat Kuarsa di Kawasan Hutan Lindung Taliwang, Nusa Tenggara Barat

http://dx.doi.org/10.17014/ijog.vol2no3.20072Taliwang conservation forest, West Nusa Tenggara, is particularly covered by volcanic rocks consisting of alternated breccia and tuff. The breccia is composed of andesitic – dioritic clasts and matrix of fine-grained to lapilli tuff which had partially been hidrothermally altered and invaded by gold bearing quartz veins. Outcrops of quartz veins are commonly limonitized, trending nearly east-west and north-south with respectively dip of 70o and 85o. Identification of quartz veins originated from people mining’s waste leads to a prediction that there are quartz stockworks beneath the earth surface (50 – 100 m depth) in the conservation forest area, from which quartz veins penetrated the illite-paragonite-calcite-siderite- nacrite altered country rock. Textures of quartz veins were identified such as comb with some coarse- grained euhedral crystals, sugary/saccharoidal/fine grained crystalline quartz and ghost-bladed. Veins and host rocks generally contain disseminated and spotted pyrites. Evaluation of quartz textures, altered rocks analysis (PIMA method), fluid inclusion studies and chemical analysis (AAS method) of selective altered rocks/quartz vein samples exhibits that the alteration and mineralization processes might occur in an epithermal system, connecting with a change of hydrothermal fluids from near neutral into acid conditions at a temperature ranging from 231 to 185oC. Alteration of illite-paragonite-kalsit-siderite is suggested as a result of reaction between host rock and a near neutral fluid, whilst nacrite (kaolin group) or argillic is a result of reaction between host rock and an acid fluid, within a mixing zone of meteoric fluid and condensed acid gas released during boiling process of hydrothermal fluid in the depth. On the basis of salinity ranging from 0.9 to 2.2 equivalent wt.% NaCl of fluid inclusion, it is predicted that the deposition of gold bearing quartz and associated ore minerals takes place in a depth ranging from 293 to 120 m beneath paleosurface.

Back-reacted saponite in Jurassic mudstones and limestones intruded by a Tertiary sill, Isle of Skye

AbstractThe Lòn Ostatoin stream section, Trotternish Peninsula, Isle of Skye, exposes a sequence of Middle Jurassic mudstones and limestones which have been locally metasomatized by a transgressive sill of Tertiary age. Limestones in the sequence, including some previously reported as bentonite, have been altered to an unusual assemblage of grossular garnet and saponite clay. The mudstones also contain large proportions of saponite together with pyroxene and zeolites. Saponite also occurs within the basalt intrusion. Grossular and pyroxene represent artifacts of relatively high-temperature assemblages that formed during an early phase of alteration. As the intrusion and adjacent altered country rocks cooled, lower-temperature fluids flowed through a late set of contraction (micro)fractures. Back-reacted saponite, analcime and clinoptilolite were formed, possibly as alteration products of the unstable higher-temperature minerals. The lower-temperature mineral assemblage eventually sealed the late fracture system.This paper highlights an important concept for the study of analogue sites used to investigate thermal effects on engineered liners or barrier host rocks for the landfill and radioactive-waste industries. This is that the original thermally altered mineral assemblage may be overprinted by later, lower-temperature back-reactions. A detailed understanding of both processes is necessary in order to construct a sensible model for the thermal and mineralogical evolution of the site.

Origin of chromite in mafic–ultramafic-hosted hydrothermal massive sulfides from the Main Uralian Fault, South Urals, Russia

Mafic–ultramafic-hosted hydrothermal Fe–Cu–(Ni–Co) sulfide ores from the Main Uralian Fault Zone (MUFZ), South Urals (Ivanovka and Ishkinino ore fields), contain a relatively large (up to 3%) proportion of chromite. This association is common for magmatic Fe–Ni–Cu sulfides, but definitely unusual for hydrothermal sulfides. Textural, morphological and compositional data are used here to gain an insight into the origin and significance of this unusual chromite–sulfide association. The studied chromites occur both as broken fragments and as euhedral or subhedral crystals, which are included in the sulfides or scattered in their talc±chlorite±saponite±quartz±carbonate matrix. They are characterized by high Cr/(Cr+Al) ratios (0.58–0.85) and range in composition from magnesiochromite to chromite sensu stricto. Textural, morphological and compositional features, as well as the occurrence of relatively high-silica, low-Ti, low-K melt inclusions in some of the crystals, indicate that the ore-associated chromites (i) are a mixed population of grains derived from mafic–ultramafic mantle and crustal magmatic rocks and mantle peridotite melting residua, (ii) have no genetic relation with the host sulfides and (iii) represent relicts derived from the hydrothermally altered country rocks. The compositions of the chromites and of the melt inclusions denote a clear supra-subduction zone signature. The melts parent to the cumulitic chromites had an arc tholeiitic to, possibly, boninitic affinity. These data suggest that the host mafic–ultramafic complexes formed in an early arc or forearc setting and do not represent obducted portions of MORB oceanic lithosphere. Hence, contrary to previous interpretations, the associated massive sulfides could not originate on a mid-ocean ridge, but rather in an early arc or forearc environment. Given the relatively short life of the western Uralian arc system, the most probable time window for sulfide ore deposition is confined to Early to Middle Devonian time.

Neoproterozoic shearing and auriferous hydrothermalism along the lithospheric N–S East In Ouzzal shear zone (Western Hoggar, Algeria, North Africa)

The East In Ouzzal shear zone (EIOSZ) is a dextral N–S lithospheric lineament that contains a 100 km×5 km gold vein-type district including the two worldclass Amesmessa and Tirek goldfields. The 40Ar/ 39Ar laser-probe dating of ferrotschermakite and biotite from both goldfields confirms the Neoproterozoic age (627–613 Ma) of major syn- to late collisional ductile deformation. Gold-rich (>20 ppm) quartz veins at Amesmessa and at Tirek are oriented N–S and mostly confined to the longitudinal shear corridors. Later veins were diachronously emplaced under a ductile/brittle shear regime. Hydrothermal muscovite from the altered country rocks of a N–S gold-quartz-bearing vein is older at Amesmessa (611 Ma) than at Tirek (575 Ma). At Amesmessa, a second set of E–W-trending, undeformed and gold-poor (<5 ppm) quartz veins lies in the eastern mylonitic part of the EIOSZ. Dating hydrothermal muscovite from the altered country rocks of an E–W undeformed quartz vein suggests that ductile deformation had at least ended by 577 Ma at Amesmessa, whereas shearing was still active at Tirek. Primary gold deposition in the N–S quartz veins occurred during the earliest hydrothermal event. This primary gold was bound to the crystal lattice of hydrothermal pyrites which contain up to 24 ppm Au. However, the economic gold ores are younger than 611–575 Ma which is the timing for deposition of the hydrothermal muscovites from the N–S gold-rich quartz veins at Amesmessa and Tirek. Gold concentration could be related to a late Neoproterozoic reactivation of the shear zone.