Majid Ghaderi

Trace element analysis of scheelite by excimer laser ablation-inductively coupled plasma-mass spectrometry (ELA-ICP-MS) using a synthetic silicate glass standard

Abstract Concentrations of W and trace elements in scheelites (CaWO 4 ) associated with Archean lode-gold deposits in Western Australia were determiined by ArF (193 run) excimer laser ablation-ICP-MS with external calibration against a silicate glass standard reference material, NIST 610. The excimer laser beam drilled well-defined pits in both scheelite and silicate glass. With internal standardization against Ca alone, W measurements for the scheelites fall within 5% of the concentrations expected from electron microprobe measurements. A matrix effect between scheelite and silicate glass is apparent in the behaviour of W during; laser ablation: W is progressively fractionated from Ca in the glass but not in the scheelite. Proper data reduction, therefore, requires use of the early maximum count rates for W and Ca rather than the mean count rates during ablation. W isotopic ratios measured in the scheelites and an in-house generated silicate glass, ANU 252, are both within ∼2.5% of the accepted values for the natural ratios and are reproducible to ∼1%, if data for the first ∼11 s of ablation are excluded. Using the same data reduction techniques employed for W, concentrations of Sr, Y, Mo, REE and Pb, present at ppm levels in the scheelites, were measured with a precision of 4% or less. Measurements on Th and U, present at the 5–10 ppb level, and P, Mn, Nb and Ta are less precise (∼5–40%) and concentrations of Rb, Zr, Ba, Sn, Hf, TI and Bi are, for the most part, below detection limits. For Sr, Sm and Nd, replicate ELA-ICP-MS measurements made on 80-μm wide spots in scheelite largely encompass the concentrations determined by ID-TIMS on bulk samples. REE patterns determined by ELA-ICP-MS for the scheelites vary smoothly as a function of atomic number. Most of the patterns are hump-shaped but others are rather flat except for positive Eu anomalies. This suggests that the hydrothermal fluids that formed the scheelites did not have a common composition and source.

ORIGINS OF ND–SR–PB ISOTOPIC VARIATIONS IN SINGLE SCHEELITE GRAINS FROM ARCHAEAN GOLD DEPOSITS, WESTERN AUSTRALIA

Accessory gangue scheelite (CaWO4) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique. Type I scheelite is strongly enriched in middle REE (MREE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial 87Sr/86Sr and eNd values ranging from 0.70141 to 0.70163 and +2.5 to +3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have 87Sr/86Sr<0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching of a Rb-rich mineral during hydrothermal alteration of the greenstone. The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial 87Sr/86Sr ratios and eNd values between 0.70130 and 0.70146, and +1.1 to +2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that 87Sr/86Sr ratios in Type II fluids were closer to those of the host dolerite (0.7008–0.7013), due to more extensive fluid interaction with the dolerite. A positive correlation between Na and REE suggests that REE3+ are accommodated by the coupled substitution REE3++Na+=2 Ca2+ into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small (<m) to medium (<km) scale processes involving mixing between “fresh” batches of hydrothermal fluid with fluids that had already been involved in extensive wall-rock alteration. The very high-eNd values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high Sm/Nd ratios. However, tiny (<20 μm) grains of secondary hydroxyl-bastnasite were found within micro-fractures of one scheelite grain containing an extremely high-eNd signature. The hydroxyl-bastnasite probably formed during recent REE redistribution within the scheelite as a result of meteoric fluid circulation. The scale of this cryptic low-temperature alteration is sufficient to explain the anomalously high-eNdi values observed in scheelite from Western Australia.

Application of fuzzy AHP method to IOCG prospectivity mapping: A case study in Taherabad prospecting area, eastern Iran

Abstract Using fuzzy analytical hierarchy process (AHP) technique, we propose a method for mineral prospectivity mapping (MPM) which is commonly used for exploration of mineral deposits. The fuzzy AHP is a popular technique which has been applied for multi-criteria decision-making (MCDM) problems. In this paper we used fuzzy AHP and geospatial information system (GIS) to generate prospectivity model for Iron Oxide Copper-Gold (IOCG) mineralization on the basis of its conceptual model and geo-evidence layers derived from geological, geochemical, and geophysical data in Taherabad area, eastern Iran. The FuzzyAHP was used to determine the weights belonging to each criterion. Three geoscientists knowledge on exploration of IOCG-type mineralization have been applied to assign weights to evidence layers in fuzzy AHP MPM approach. After assigning normalized weights to all evidential layers, fuzzy operator was applied to integrate weighted evidence layers. Finally for evaluating the ability of the applied approach to delineate reliable target areas, locations of known mineral deposits in the study area were used. The results demonstrate the acceptable outcomes for IOCG exploration.

Magmatic and geodynamic evolution of Urumieh–Dokhtar basic volcanism, Central Iran: major, trace element, isotopic, and geochronologic implications

Basic volcanic rocks from the West Nain area of the Urumieh–Dokhtar Magmatic Assemblage demonstrate significant subduction-related geochemical characteristics; these along with the new age data obtained for the volcanic rocks shed new light on the geodynamic evolution of the Iranian segment of Alpine–Himalayan orogeny. The late Oligocene (26.5 Ma) high-Nb basic volcanic rocks are likely to represent a transient rather enriched asthenospheric mantle underlying the otherwise dominantly Eocene–early Oligocene West Nain island arc. Lithospheric mantle geochemical signatures of the low-Zr volcanic rocks (20.6 Ma) and high-Th volcanic rocks (19.7 Ma) imply replacement of the underlying mantle. The substitution of asthenospheric mantle by a lithospheric mantle wedge might have been associated with – or perhaps caused by – an increase in the subduction rate. Culmination of the West Nain magmatism into slab melting that produced the early Miocene (18.7 Ma) adakitic rocks is compatible with subsequent ascent that triggered slab decompression melting.

Oxygen isotope and fluid inclusion study of the Sorkhe-Dizaj iron oxide-apatite deposit, NW Iran

The Sorkhe-Dizaj orebody is located 32 km southeast of Zanjan within the Tarom subzone of the Alborz-Azarbaijan structural zone. It is hosted mainly in quartz monzonite-monzodiorite and, to a lesser extent, in volcanoclastic rocks. Mineralization occurs in the form of stockwork and veins, comprising predominantly magnetite and actinolite, with minor pyrite and chalcopyrite. Two generations of magnetite and apatite are inferred: the first as disseminations in the host rock and the second mainly as an alteration product of actinolite, secondary K-feldspar, silica, sericite, chlorite and epidote. Fluid inclusion studies were carried out on second-generation apatite, and late-stage quartz to understand the geochemical evolution of the ore-bearing fluids. Fluid inclusions are of three types, i.e. primary, secondary, and pseudo-secondary. These inclusions are liquid or vapour single-phase, two-phase rich in liquid or vapour, and three-phase. Homogenization temperatures of second-generation apatite are inferred to be between 209°C and 520°C (mostly between 290°C and 320°C), indicating salinities of 9.08–21.61 wt.% NaCl equiv. At 342°C, the δ18O values range from 9‰ to 11.32‰ for the second-generation magnetite associated with coeval apatite. Fluid inclusions in the late-stage quartz veins are inferred to have homogenized between 186°C and 263°C, with δ18O values ranging between 2.5‰ and 7.4‰ at 220°C. Oxygen isotopes in the late-stage carbonate veins have values of 3.28–6.14‰ at 100°C. These data in the late-stage veins imply introduction of a cooler, less saline, isotopically depleted fluid, probably meteoric water. Field observations, mineral parageneses, and fluid inclusion + oxygen isotope data suggest that the magnetite-apatite veins formed from a predominantly magmatic-derived fluid. Introduction of cooler meteoric water in the final stage of mineralization reduced δ18O values, facilitating precipitation of sulphides, quartz, and carbonate veins.

Oligocene–Miocene geodynamic evolution of the central part of Urumieh-Dokhtar Arc of Iran

Basic volcanic rocks from Tafresh, west Kashan, and west Nain volcanic successions in the central part of Urumieh-Dokhtar Magmatic Assemblage (UDMA) of Iran yield K–Ar ages ranging from 26.8 to 18.2 Ma. These ages indicate significant Late Oligocene–Early Miocene basic volcanism in the UDMA. These ages, combined with K–Ar ages of 26.0 and 14.1 Ma, respectively, for associated low-silica and high-silica adakites, help constrain reconstructions of the UDMA geodynamic evolution. Late Oligocene–Early Miocene slab roll-back associated with an asthenospheric mantle influx are suggested as the major processes responsible for concurrent volcanism showing Nb–Ta-depleted, Nb–Ta-enriched and low-silica adakite signatures. Slab roll-back, the likely consequence of a decrease in subduction velocity, led to partial melting of the subducted slab and produced Early–Middle Miocene high-silica (dacitic) adakites. Oligocene to Miocene volcanic rocks do not conform to the Oligocene continental collisional model for the UDMA, rather they suggest a decrease in the subduction rate that prompted the asthenospheric mantle influx.

Petrology, Geochemistry and Tectonics of the Extrusive Sequence of Fannuj-Maskutan Ophiolite, Southeastern Iran

The Fannuj-Maskutan ophiolite (FMO) contains all the components of a typical ophiolite sequence: metamorphic tectonites, cumulates, isotropic gabbros, plagiogranites, diabasic sheeted dikes, extrusives and sedimentary cover. The extrusive sequence of FMO comprises of alternate pillow lavas and sheet flows, in which the aphyric types are more abundant than phyric facies. The majority of extrusive rocks are basalt and minor basaltic andesite. The FMO extrusives are similar to back-arc basin basalts (BABB). These rocks show many similarities with mid-ocean ridge basalts (MORB), such as marked Fe, Ti, V enrichment from less fractionated to fairly fractionated rocks, relatively high Ti, P, Y contents, and significant Nb depletion. However, they show higher Th/Ta and LREE/HREE ratios compared to MORBs. In addition, the FMO extrusive rocks are generally more enriched in large ion lithophile elements (Rb, Ba, Sr, and K) than average normal-MORB, which suggests that the mantle beneath the FMO was modified by subducted slab-derived components. So, the Fannuj-Maskutan extrusives can be regarded as representative of back-arc basin located between the Bajgan-Durkan arc and the continental margin of the Lut block. These geochemical features, support the idea that the extrusive sequence of Fannuj-Maskutan ophiolite are formed in an environment related to suprasubduction zone (SSZ).

Detecting and mapping different types of iron mineralization in Sangan mining region, NE Iran, using satellite image and airborne geophysical data

The Sangan mining region is the largest Fe skarn in western Asia has emplaced into the Khaf-Kashmar-Bardaskan volcano-plutonic belt in the NE Iran. In this region, carbonate rocks of Jurassic skarnified and hosted different epigenetic types of iron minerals, including magnetite, hematite, goethite, and limonite. The combination of remote sensing and airborne geophysical data is a powerful tool for mapping and interpreting iron mineralization in some area with intensely rugged topography or a broad expanse area, where systematic sampling and conventional geological mapping has some limitation and time consuming. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite data and airborne geophysical magnetometry data were used for evaluating and mapping different types of iron mineralization in the Sangan mining region. Preprocessing of the datasets involved band ratio (BR), principal component analysis (PCA), spectral angle mapper (SAM), and constrained energy minimization (CEM) of the visible-near infrared and short wave infrared ASTER data were used to map four types of iron minerals (magnetite, hematite, goethite, and limonite). For preparing a lithological mapp of this region, an RGB image produced by combination of BR and PCA, (R:(5+7)/6, G:PC3, B:PC 5). Implementing SAM and CEM technique were useful for mapping and detecting magnetite, hematite, goethite, and limonite. The Reduce To the Pole (RTP) map of the airborne geophysical magnetometry data is a practical tool for iron ore exploration that were used in this region for enhancing high anomalous signature of magnetite after extracting granitoid rocks. The integration of the extracted information from the ASTER image processing algorithms and geophysical magnetometry, mapped iron mineralization and identified new potential of high magnetite mineralization. The results verified by geological map and comprehensive fieldwork. This integration model can generalize to other arid and semi-arid regions with iron potential for both regional and district scales.

Heavy metal assessment in stream sediments from the rivers passing through the mining area

Stream sediment samples in two sizes of sand and clay/silt from the Chodarchay and Gilankesheh rivers which pass through the Chodarchay copper deposit, northwestern Iran, were measured for their metal concentrations using a sequential extraction procedure. The average concentrations of cadmium (18.22) in sediments from the rivers exceed the world average shale and continental upper crust values. Based on Geo-accumulation Index, cadmium is intensely elevated (in the clay fraction greater than 5 and in the sand fraction between 3 and 5), arsenic and lead are slightly elevated in a few stations and others are not-elevated. The zinc and copper values are almost equal to or lower than Geo-accumulation Index; thus, the sediments are unpolluted with respect to zinc and copper. Enrichment factor values confirm the risk of cadmium in the environment. Comparison of the mean heavy metal concentrations in the sediments with threshold effect concentration and probable effect concentration values shows that cadmium (clay/sand = 15.23/7.77) is higher than the threshold effect concentration and probable effect concentration values, while copper (13.41/7.71), lead (6.38/9.18), zinc (18.26/8.19), and arsenic (3.60/1.14) are lower than the threshold effect concentration values. The cadmium and lead could cause serious danger for river biota. Based on pollution intensity, sediment samples from both rivers are divided into low to highly enriched in lead, zinc, and copper, very high to intensely enriched in arsenic, and intensely enriched in cadmium and arsenic. The majority of metals are observed in the clay/silt fraction.