Metasomatic Zones Research Articles

Extraction of gravity components related to W-Sn polymetallic deposits using Bi-dimensional empirical mode decomposition (BEMD) in the Laojunshan Ore Cluster Region, SW China

The Laojunshan ore cluster region is situated at the north Song Chay dome, which is the junction of the Yangtze, the Cathaysia, and the Indo-China Blocks. The north Song Chay dome characterized by the Laojunshan metamorphic core complex is an important ore-forming region for W-Sn-Pb-Zn-In elements related to the Late Cretaceous granitic magmatism. In this paper, the Bi-dimensional empirical mode decomposition (BEMD) is applied to separate gravity components from the original gravity data, combined with the known geological and mineralized information, to explore the relationships between the gravity components and various ore-controlling factors at different depths. Four intrinsic mode functions (IMFs) and a residue are obtained. They may imply the spatial distribution of the geological bodies with featured gravity components at different depths. The results are shown as follows:(1) The low-pass filtering gravity component (BIMF4) implies the deeply buried geological units in the study region. The Cretaceous Laojunshan granitic complex and the Nanwenhe gneissic granite display negative gravity components and weak positive gravity components, respectively. The former is closely related to Sn-Zn-In mineralization, the latter is closely related to W polymetallic deposits. Therefore, the negative gravity components showing the Laojunshan granitic complex are the prospecting targets for deeply buried Sn-Zn-In deposits. The weak positive gravity components displaying the Nanwenhe gneissic granite are the prospecting targets for deeply buried W polymetallic deposits.(2) The band-pass filtering gravity component (BIMF3) implies the geological units at middle depth. The skarn-type Sn-W polymetallic deposits are observed in the transition zones from the positive to the negative gravity components, which coincide with the contact metasomatic zones between the Cretaceous granite and the Cambrian-Devonian carbonate rocks. Some small quartz vein W deposits occur within the Cretaceous Laojunshan granite with negative gravity components, and some layered W deposits occur in the Silurian gneissic granite with a weak positive gravity component.(3) High-pass filtering gravity component (BIMF2) implies geological units at the shallow crust, which shows that most of the deposits (occurrences) are located within the positive gravity component zones. These positive gravity component zones should be ore-finding targets for further prospecting for the undiscovered Sn and W polymetallic deposits.(4) The gravity components separated by BEMD coincide with the spatial distribution of the granites associated with various types of mineralization and alteration zones at different depths, which will give insights for further prospecting for concealed ore bodies in this study region.

Metasomatic Transformation of Amphibolites into Corundum-Bearing Plagioclasites: Zoning, Numerical Model of the Process (on the Example of the Unique Khitostrov Mineral Deposit, Fennoscandian Shield)

The paper presents the results of a study of desilicated rocks of the middle crust at the level of amphibolite facies using the example of corundum-bearing plagioclasites developing over the metabasites at the unique Khitostrov mineral deposit in the Belomorian-Lapland orogen of the Fennoscandian Shield. The main attention is paid to new geological data, documentation and analysis of metasomatic zoning, determination of the P-T conditions of its formation, and a model of the metasomatic process.

Quartz-tourmaline metasomatites in gold deposits in Aktyuz-Boоrdinsky ore district (Northern Tien Shan, Kyrgyzstan)

In the Aktyuz-Boordinsky ore district in the Northern Tien Shan, numerous deposits and ore occurrences of gold, rare and rare earth metals and base metals are discovered. Gold deposits are characterized by the presence of a number of metasomatites: beresites, listvenites, K-feldspatites, etc. Many of them are also characterized by quartz-tourmaline gold-bearing metasomatites. A mineralogical and petrographic description of the metasomatic zoning of quartz-tourmaline metasomatites from weakly to highly altered zones is given. Quartz-tourmaline metasomatites form vein-like bodies and metasomatic breccias, where they act as cement, and the fragments can consist of intrusive rock, beresites, K-feldspatites, and listvenites. For the Aktyuz-Boordinsky ore district, quartz-tourmaline metasomatites are a prospecting feature for gold, and for other areas they are a prospecting criterion.

Sr isotopes in metasomatized fault rocks constrain the age of juxtaposition of oceanic and continental lithosphere (Dun Mountain Ophiolite, New Zealand)

The juxtaposition of oceanic and continental lithosphere along major fault zones is a rare but globally significant process that is commonly associated with the closure of an ocean basin. However, it is often difficult to determine the timing in the absence of a high-grade metamorphic sole that can be dated. We show that the timing of mantle and crust juxtaposition along the base of the Dun Mountain Ophiolite in New Zealand can be constrained using Sr isotope data from metasomatic rocks that developed due to the extreme chemical contrast between ultramafic (mantle) and quartzofeldspathic (crustal) lithologies along the basal Livingstone Fault. Fluids that migrated through ultramafic rocks into greenschist facies quartzofeldspathic metasediments resulted in formation of actinolite, diopside, hedenbergite, epidote and garnet, as well as the disappearance of quartz and white mica, and the near-complete removal of Rb, in a rodingite and albite-rich zone up to c.10 m wide. Major and trace element compositions indicate that albite-rich rocks and rodingites have quartzofeldspathic protoliths equivalent to rocks outside of the metasomatic zone. Due to loss of the main Rb-bearing phase (mica), the timing of metasomatism can be broadly constrained by age correction of the 87Sr/86Sr ratios of unaltered quartzofeldspathic rocks to those of the strongly metasomatized equivalents. The calculated age, c. 173 Ma, represents juxtaposition of continental and oceanic lithosphere along the Livingstone Fault. Although obduction of the Dun Mountain Ophiolite may have initiated prior to the dated event, the new age is consistent with published paleogeographic reconstructions, and indicates that a >2000 km long fore-arc ocean basin that existed along the Gondwana margin was closed by the middle Jurassic. Removal of Rb from metasedimentary rocks in contact with ultramafic rocks in ophiolite settings provides an opportunity to constrain the timing of juxtaposition at lower-greenschist facies conditions.

Textural and Mineralogical Evolution of the Little Nahanni Pegmatite Group (NWT, Canada) with Implications for Metasomatism, Rare-Metal Mineralization, and Pegmatite–Wall Rock Interaction

Abstract Situated in the Northwest Territories (Canada), the Cretaceous (ca. 85 Ma) Little Nahanni Pegmatite Group is an LCT-type pegmatite swarm enriched in Li, Sn, and Ta. Displaying intensive albitization associated with rare-metal mineralization (i.e., Nb, Ta, Sn), having a notably high surface-area-to-volume ratio (i.e., hundreds of thin dikes), and being particularly well exposed in a suite of thirteen glacial cirques, the Little Nahanni Pegmatite Group provides an ideal opportunity to investigate some unresolved aspects of pegmatite formation and evolution, such as metasomatism, pegmatite–wall rock interaction, and the nature and origin of rare-metal mineralization (i.e., magmatic versus metasomatic). Using an integrated approach combining field observations with detailed textural and mineralogical studies, two different stages of pegmatite evolution are documented: primary magmatic followed by intense metasomatism. Preservation of primary magmatic features, such as coarse oriented high aspect-ratio crystals (spodumene, K-feldspar), banded aplites, anisotropic fabrics, and skeletal textures, highlight the importance of undercooling (ΔT) of the melt with consequent disequilibrium crystallization and boundary layer effects. The metasomatic stage is dominated by the formation of domains of secondary albite and is attributed to interaction with a residual highly fluxed sodic melt and is responsible for most of the rare-metal mineralization; less abundant micaceous units and cafemic domains reflect incursion of acidic magmatic-sourced and alkaline wall rock derived fluids. Our findings are significant and have widespread application to all pegmatite types. Particularly significant is the finding that the rare-metal mineralization is mostly localized to zones of metasomatism.

Mineralization Styles and Alteration Paragenesis of Metasomatic Zones in the Highly Fractionated Granite of Gabal Gattar, Northern Eastern Desert, Egypt

Mineralization Styles and Alteration Paragenesis of Metasomatic Zones in the Highly Fractionated Granite of Gabal Gattar, Northern Eastern Desert, Egypt

Mineral resource potential of Kaalamo magmatic system

As a region with favorable geographic and climatic conditions and the developed infrastructure, Karelia is attractive for commercial-scale mining of mineral resources which enjoy high demand in the modern economy. Such resources include platinum group metals and gold. Promising occurrences of these minerals are detected in the Ladoga region, in associations with intrusive rocks of Kaalamo clinopyroxenite–gabbro norite–diorite system aged as 1888.3±5.2 million years. With respect to the age and petrochemistry, this rock system is somewhat similar to Kotalahti and Vammala Nickel Belts in Finland. Noble metals in Kaalamo intrusions belong to the syngenetic and epigenetic types. The syngenetic ore genesis started at the late magmatic stage (~800 °C) of intrusions, evolved and finished at the hydrothermal–metasomatic stage (<271 °C). The syngenetic mineralization (Araminlampi, Saouth Kaalamo, Keinoset, Ninimyaki) represents strata (to 10–11 m thick) impregnated with sulfides (pyrrhotine, chalcopyrite) and with noble metals in meta pyroxenite and hornblendite at the contents: ΣPd, Pt, Au—0.2–1.1 g/t; Сu—0.1–1.0%, Ni—0.03–0.1%, Co—0.01–0.03%. The epigenetic mineralization originated at the temperature of ~500–<230 °C in the zones (to 3.5 m thick) of intense shear deformation and low-temperature metasomatism (actinolite, tremolite, chlorite, prehnite, albite, quartz, calcite) and, among other things, transformed the older ore shoots. The ore content of the largest occurrences of this type (Surisuo, Rantamyaki) varies greatly: ΣPd, Pt, Au—0.1–11 g/t; Сu—0.2–2.0%, Ni—0.03–0.2%, Co—0.01–0.05%. The test ores contain up to 50 noble metals; both mineralization types are dominated by palladium bismuth–tellurides and sperrylites. The geological, mineralogical, physicochemical and geochemical data are reflective of the probable effect exerted on the ore genesis by the Svekofennian regional metamorphism and granite formation. The undiscovered potential resources of ΣPt, Pd, Au in four out of seven detected ore occurrences total approximately 9 tons. The overall mineral resource potential of Kaalamo magmatic system embraces 20–30 t of noble metals, 50–60 kt of copper and 10 kt of scandium (all meta pyroxenites, including nonmetallic, have steadily consistent contents of Sc—70–90 g/t).The study was carried out under the state contract with the Karelia Science Center of the Russian Academy of Sciences, R&D Topic No. 121040600173-1.

Petrographic composition and ore potential of low-temperature metasomatites of the Middle-Dniprean mega-block of the Ukrainian Shield

Purpose. To substantiate the formation conditions, distribution patterns and metallogenic specialization of low-temperature metasomatites of the Middle Dnipro megablock of the Ukrainian Shield based on the research results obtained. Methodology. The authors used a set of field and laboratory methods. Petrographic, mineragraphic, petrochemical studies were carried out. Interpretation of the results of chemical, laser microspectral, X-ray diffraction, scintillation spectral, thermal analyses is carried out. Findings. A generalized petrological description of low-temperature metasomatites of the Middle Dnipro megablock of the Ukrainian Shield is carried out, taking into account the composition of the original rocks, characteristics of newly formed mineral parageneses, metallogenic specialization and patterns of distribution of metasomatic formations. The study on occurrences of vertical and horizontal metasomatic zoning within the greenstone structures of the Middle-Dniprean region was carried out. The composition of telescoped metasomatites spatially related to the intersection of deep faults is characterized. The metallogenic specialization of low-temperature metasomatic formations is determined and the process of formation of ore concentrations of complex composition in the nodes of intersection of deep faults has been characterized. Based on the results of studies on the composition and ore content of metasomatites, an insignificant level of erosional truncation of vertical ore-metasomatic columns within greenstone structures is substantiated. Originality. The aspects of petrology and ore content of the group of low-temperature metasomatic formations of the Middle Dnipro megablock of the Ukrainian Shield have been revealed. The spatial relationship of the metasomatic areas with the systems of deep faults has been substantiated, which justifies using the identified metasomatic formations as geological indicators of deep fault zones. The features of ore content of low-temperature metasomatic formations of the Middle Dnipro megablock are characterized. It is shown that formations of telescoped metasomatites with a complex composition of mineralization are spatially associated with the intersection of deep faults. Practical value. The metallogenic specialization of low-temperature metasomatic formations has been determined, which can be used as a search criterion for various types of deposits of metallic and non-metallic minerals within the Ukrainian shield.

Mineralogical and petrographic characteristics of the host rocks of the Pervaya Rudnaya Gorka Fe-skarn ore occurrence

The article discusses the mineralogical and petrographic features of the metamorphic formations of the poorly studied and promising the Pervaya Rudnaya Gorka Fe-scarn ore occurrence (Kongor ore region), localized in the Voikar, and the typification of host rocks by mineral composition. In order to clarify the ore-forming affiliation of the Pervaya Rudnaya Gorka ore occurrence, the comparative analysis was carried out with the reference to well-studied Novogodnee-Monto Au-Fe-skarn deposit (Toupugol-Khanmeishor ore region), which showed similar mineral composition, structural and texture features and geological structure of ore objects. According to the results of studies, a close composition of the enclosing volcanic-sedimentary rocks, in particular the skarn formations, containing the gold-sulfide-magnetite mineralization at the Novogodnee-Monto ore deposit; connection with the same intrusive complexes (Sob, Kongor, and Musyur) and faults of the north-western direction. Using the microscopic method of research, the changes in the propylite formation area at the Pervaya Rudnaya Gorka ore occurrence, indicating a large-scale regional pre-ore metasomatism, were first identified and described in detail. The following metasomatic zonation (from the inner to the outer zone): actinolite + epidote chlorite + actinolite + epidote chlorite + epidote was been established at the ore field. Based on the obtained research results, it is concluded that the poorly studied territories of the Polar Urals are promising.

Apatite–Albite Metasomatites (Aceites) of the Eastern Aldan–Stanovoy Shield

This paper summarizes the findings of research on Riphean ore-bearing apatite–albite metasomatites (aceites) identified in metamorphic, volcanic and intrusive rocks in the eastern Aldan–Stanovoy Shield. The characteristic features of lithological and structural control of aceites, their mineral and petrochemical composition, geochemical associations, ontogeny, metasomatic zoning, and geochemical specialization are outlined. Aceites in metamorphic rocks are assigned to the albite–chlorite–apatite facies and in igneous rocks to the albite–apatite facies. Apatite–albite metasomatites host mineralization of two types: uranium (in aceites after metamorphic rocks) and uranium–rare earth element–rare metal (in aceites after volcanic and intrusive rocks).

GEOLOGY, MINERAL COMPOSITION OF ORE AND ALTERED HOST ROCKS OF MASSIVE SULPHIDE MINERALIZATION OF THE AKZHARSKY ORE FIELD (SOUTH URALS)

The article describes the geological structure of the Akzharsky ore field, of five individual massive sulphide mineralization: Yuzhnoye, Yuzhno-Akzharskoye, Novoye, Bezymyannoye and Burozheleznyakovoye. The results of a microscopic study of the mineral composition of ores and altered host rocks are discussed. It is shown that all ore mineralizations are concentrated at a single lithological-stratigraphic level, but differ from each other in details of the structure, scale and mineral composition of mineralization. Massive ores prevail, and embedded and densely embedded ores are subordinate. The composition of the ore is mainly chalcopyrite-pyrite, to a lesser extent sphalerite-chalcopyritepyrite and pyrite. In the direction from south to north of the ore field, there is a change in the direction of increasing «polymetallicity» in ore mineralogy. Texture and structure of the ores of the Akzharsky ore field are rather uniform. Linear structures predominate (from banded to schistosely banded). Massive structures have a sharply subordinate distribution; brecciated ones are noted in isolated cases. Hydrothermal alterations formed mainly in rhyodacite-basalt strata from foot wall of ore bodies and, according to mineral composition, belong to propylites. They are characterized by an unstable set of mineral facies, weak contrast of metasomatic zones, and extremely uneven of rocks alteration. It has been established that the volcanic complexes of the ore field contain deposits not typical for the Ural massive sulphide ore province, differing in the mineral composition of ores, the conditions of their geological setting and the nature of the rocky alterations. This made it possible to concretize the concept of the relationship between the considered mineralization and volcanism and significantly supplement the already known typical models of pyrite-bearing ore-magmatic systems.

Subduction zone sulfur mobilization and redistribution by intraslab fluid–rock interaction

Subduction zones mediate the sulfur exchange between Earth’s interior and surface, and play a critical role in the long-term global sulfur cycle. Dehydration of the subducted slab releases aqueous fluids, which in turn interact with the surrounding wall-rocks during reactive flow through the slab. Fluids are regarded as the key agent for sulfur transfer within the slab and subsequently into the mantle wedge. However, the mechanisms underlying sulfur mobilization and redistribution during fluid–rock interaction are not fully understood. Here, we investigate three high-pressure blueschist/eclogite–selvage–vein systems from the Southwestern Tianshan metamorphic belt in northwestern China to explore sulfur behavior during fluid–rock interaction along channelized fluid pathways and within their alteration halos. Petrologic observations on mineral parageneses and δ34S compositions of sulfides reveal two different scenarios of sulfur mobilization and redistribution during fluid–rock interaction at HP conditions. Fe-rich and medium-fS2 (near the pyrite–pyrrhotite buffer) fluids favor pyrrhotite stability over pyrite and can trigger the pyrite–to–pyrrhotite transition in the reaction halo. In this case the bulk-rock sulfur budget may not vary but the sulfur isotope exchange is still effective, producing a δ34S mixing trend in the metasomatic zones. In contrast, Fe-poor and high-fS2 fluids cause conversion of pyrrhotite to pyrite, significant sulfur addition, and sulfur isotope exchange in alteration selvages. Medium-fS2 fluids can transport sulfur and thus are effective in transferring sulfur out of the slab, whereas high-fS2 fluids cause sulfur sequestration along fluid pathways and considerably reduce long-distance sulfur transfer. Multiple fluid infiltration events with different fluid sources result in several pyrite overgrowth sequences, recorded by contrasting Co–Ni element distributions and in-situ δ34S zoning of pyrite. The heaviest δ34S value (+25‰) thus far reported in HP rocks has been found in vein pyrite, suggesting that a seawater sulfate-derived δ34S signature can be transported to great depths in a subduction zone, even though the timing and mechanism of sulfate–to–sulfide reduction in the subduction zone remain unconstrained. Our study provides natural evidence for fluid-mediated sulfide transformation, sulfur transport/storage, and sulfur isotope exchange during fluid–rock interaction, and illuminates the role these processes play in sulfur release from the subducting slab and sulfur subduction into the deep mantle.

Geological position and mineral composition of kyanite amphibolites of the Chupinskiy paragneiss belt (Belomorian province of Eastern Fennoscandia)

The Lyagkomina occurrence of kyanite amphibolites (the Belomorian Province of the Fennoscandian Shield) is confined to an amphibolite boudin among kyanite-garnet-biotite and garnet-biotite gneisses of the Chupa Sequence. The main rock types at the Lyagkomina occurrence are meso- and melanocratic garnet-kyanite amphibolites with large kyanite (more than 1 cm in length) and kyanite-garnet amphibolites with small kyanite. There are also leucocratic garnet-biotite-amphibole-plagioclase rocks with kyanite, melanocratic epidote-staurolite-amphibole rocks, zoisite plagioclasites with staurolite and kyanite. The country gneisses around the boudin were metasomatically altered: quartz disappears in kyanite gneisses, and amphibole appears in garnet-biotite gneisses. Calcic amphiboles in the kyanite amphibolites are represented by high-alumina tschermakite; biotite — by intermediate-alumina phlogopite; garnet belongs to the pyrope-almandine series with a high grossular content. Plagioclases have an intermediate and basic composition. Intermediate plagioclases are common in kyanite amphibolites without staurolite and in kyanite amphibolites with staurolite rims, while the basic plagioclases are found in staurolite amphibolites. Many rocks are characterized by reaction structures, representing the change of the mineral assemblages. Metasomatic zoning has been reconstructed in the occurrence with a regular sequence of the mineral assemblages: Hbl + Grt + Pl + Qtz ± Bt (garnet amphibolite) → Hbl + Grt + Pl + Ky + Qtz ± Bt → Hbl + Grt + Pl + Ky ± Bt → Hbl + Grt + Pl + Ky + St ± Bt → Hbl + Grt + Ky + Pl + St + Czo ± Bt → Hbl + Grt + Pl + St + Czo ± Bt → Hbl + St + Czo + Pl ± Bt. In the described series, the alumina content increases and the Mg content of the main rock-forming minerals decreases, and desilication of the rock occurs. The described mineral zoning was formed during metamorphism as a result of the reaction between the initial amphibolite and fluid flow in the shear zone (i. e., at synmetamorphic metasomatism). Petrological and isotope data indicate that the Lyagkomina kyanite amphibolites are genetically related to the occurrences of corundum-bearing rocks in the Belomorian Province of the Fennoscandian Shield.

АПАТИТ-АЛЬБИТОВЫЕ МЕТАСОМАТИТЫ (ЭЙСИТЫ) ВОСТОКА АЛДАНО-СТАНОВОГО ЩИТА

The paper summarizes the findings of research on Riphean ore-bearing apatite-albite metasomatites (aceites) identified in metamorphic, volcanic and intrusive rocks in the eastern Aldan-Stanovoy shield. The characteristic features of lithological and structural control of aceites, their mineral and petrochemical composition, geochemical associations, ontogeny, metasomatic zoning, and geochemical specialization are outlined. Aceites in metamorphic rocks are assigned to the albite-chlorite-apatite facies and in igneous rocks to the albite-apatite facies. Apatite-albite metasomatites host mineralization of two types: uranium (in aceites after metamorphic rocks) and uranium – rare earth element – rare metal (in aceites after volcanic and intrusive rocks).

ОСОБЕННОСТИ РАЗМЕЩЕНИЯ И УСЛОВИЯ ФОРМИРОВАНИЯ КОШКАРЧАЙСКОГО МЕДНО-ПОРФИРОВОГО МЕСТОРОЖДЕНИЯ (МАЛЫЙ КАВКАЗ, АЗЕРБАЙДЖАН)

The paper deals with peculiarities of location and evolution features of Koshkarchay copper-porphyritic field of Murovdagh ore province. Major factors of concentration of copper-porphyritic mineralization in rocks of gabbro-diorite-granodiorite formation has been revealed. It has been established that the structure of this ore region was formed as a result of successive alteration of some deformation stages accompanied by development of fractures in intrusive massive, by dike intrusions, blocks displacements along faults and the fractures composed of various mineral associations. It has been made clear that the copper-porphyritic mineralization is related to the wide halo of hydro-thermal-deformed rocks, frequently observed with clearly outlined ore-metasomatic zones. The general column of metasomatic zonation within the limits of ore region and fields are expressed by successive alteration of the following metasomatic zones: quartz → kalifeldspath → quartz-sericite → argillizated → propylitic. Based on structural-morphological features it can be stated that mineralization in Koshkarchay field is stockwork-vein deposit of embedded type in which the impregnations dominate over veins. The major useful component is copper. Its percentage within stockwork body varies within the broad range – from 0.2 to 2.5 %, with average 0.41 %. Among the mineral resources significantly impacting the importance of the field the molybdenum and precious metals can be indicated. In some intervals the average gold content is 2.0 grams/ton and higher. High silver content was also identified and its value reaches 30-45 grams/ton and this may positively impact the whole value of the field. Results of factor analysis of well data allowed identifying geochemical associations for localization of the areas of mineralization at various stages of evolution of porphyritic-epithermal system. Values of Ф1 factor correlate with quantity of chalcopyrite, bornite and pyrite and from this it can be inferred the tie of geochemical association of Cu (Mo, Pb, Sb) to the early chalcopyrite-bornite-pyrite mineralization of porthyric stage of structure evolution. Values of Ф2 factor referring to the association Mo (Cu, Pb, Co), statistically are related to the content of chalcopyrite and molybdenite, which are the main mineral paragenesis of copper-molybdeneporphyritic mineralization. Thermal-pressure and chemical methods applied for researches on impregnations of ore of pyrite-chalcopyrite, galenitesphalerite-chalcopyrite stages in quartz made it possible to describe the fluid mode of ore formation in this field. Ore-forming hydrothermal solutions by the data of researches on fluid impregnations were the chloride-sodium type, salt concentrations varied from 20 to 30 mass % – eqv. NaCl. Ore components were transferring by these solutions in a form of complex ions containing chlorides of Na and K, rarely sulfates and carbonates. Ore forming process was going under temperatures of 350-2000 С. The certain range of physical and chemical parameters and chemical composition of ore forming fluid corresponds to each type of mineralization. The study results enabled us to define temperatures of homogenization of impregnations and evaluate temperatures of fluids during mineralization at each stage, as well as to define concentrations of major salt components. Complexity of targets evidence high perspectives of copper-porphyritic areas and areas outlined in a range of perspective ore fields.

Examination of alteration and metasomatic zones based on the tectonic setting of the Kamoo region (northeast of Isfahan, Iran)

Kamoo region (northeast of Isfahan) has an anticline structure with a northwest-southeast trend. The exposed part of this anticline consists of siltstone-sandstone and Jurassic shale, which is located below the Cretaceous formation as an unconformity. Penetration by magmatic masses (dikes) and mineralization events are mainly related to fault systems and fractures in the region and follow the trend of the anticline. In addition, the intensity and variety of alterations are affected by tectonic factors, with alteration and metasomatic haloes having developed next to crushed areas. The alteration process is the main controller of mineral mass grade in the study area; therefore, iron grade in the Kamoo ore has increased by the effects of alteration. The alteration effects include propylitic haloes, iron hydroxides (limonite), and clay mineralization (argillic). The host rock consists of Jurassic shales with siltstone, sandstone, and Cretaceous limestone. Calcareous formations adjacent to granodiorite masses are the main hosts of iron skarns. The results of this study showed that the mineral samples from Kamoo were relatively rich in Light Rare Earth Elements (LREEs); the average La/Yb ratio is about 18.61 and Eu anomaly is between 0.52 and 1.94. Based on the findings of this study, the origin for Kamoo skarn was consistent with the model presented by Meinert, and the region’s mineralization and alteration characteristics correspond to the conditions prevailing in oceanic subduction and back-arc basin environments.

Crystal structure and Raman spectroscopic studies of OH stretching vibrations in Zn-rich fluor-elbaite

Abstract Zinc-rich fluor-elbaite from Piława Górna, Poland, was studied by electron microprobe (EPMA), single-crystal X-ray diffraction (SREF), and Raman spectroscopy (RS) to check the possibility of the application of RS to draw crystal-chemical conclusions for Al-rich and Li-bearing tourmalines on basis of the O–H stretching vibrations in the spectral range 3400–3800 cm–1. This tourmaline, forming a thin metasomatic zone around gahnite, features varying compositions with a ZnO content reaching in the studied fragment of 5.70(12) wt%. The crystal structure of this Zn-rich fluor-elbaite [a = 15.921(1), c = 7.127(1) Å] was refined with a R1 value of 1.67%. Its formula was determined on the basis of electron-microprobe and structure refinement as (Na0.84☐0.14Ca0.01)XΣ1.00(Al1.06Li0.84Zn0.69Fe0.322+Mn0.09)YΣ3.00AlZ6(BO3)3(Si6TO18)(OH)3V(F0.65OH0.26O0.09)W. The deconvolution of the O–H stretching vibration bands, performed by fitting of an input model of component bands with Gaussian function shapes for the empirical spectrum, indicates that each of the three maxima assigned for VOH bonded to YAl3+, Y2+, and YLi+ and with the total integral intensity of at least 75% of the total OH content could be resolved into 1 to 3 bands, depending on the X-site occupation (vacancies, Na+, and Ca2+). The deconvolution indicates further that several low intense bands of WO–H modes above a Raman shift of 3600 cm–1, totally reaching ≤25%, are dependent on the occupation of triplets of YYY cations bonded to the hydroxyl. These WO–H modes are also influenced by the X-site occupation. Due to ordering of all octahedral cations (except Al) at the Y site and a complete occupation of the Z site by Al and the V site by OH, it seems possible to evaluate the Li and OH contents in a Al-rich and Li-bearing tourmaline directly from the Raman spectrum. By using the ratio VOHIYAlZAlZAl/(VOHIYZZ + WOHIYYY) as evaluated from RS, corresponding to the ratio YAl/V+WOH in the crystal, the formula of the Zn-rich fluor-elbaite can be calculated as (Na0.85☐0.14Ca0.01)XΣ1.00(Al1.11Y1.112+Li0.78)YΣ3.00AlZ6(BO3)3(Si6O18)(OH)3(F0.65OH0.13O0.22), where Y 2+ = Zn + Fe + Mn. The formula, determined only on basis of EPMA and deconvolution of RS in the O–H stretching bands, corresponds very well (≤1 SD range of EPMA) to the formula determined on basis of EPMA and SREF. This result implicates that the O–H stretching vibrations, measured by Raman spectroscopy, could be applied for Al-rich and Li-bearing tourmalines as a useful tool for providing additional information for determining the crystal-chemical formula. It is also very helpful when crystal structural data are not available.

The Patterns of the Ore–Metasomatic Zoning of the Svoboda District, the Malmyzh Au–Cu Porphyry Deposit

The ore–metasomatic zoning of the Svoboda locality, which is one of the centers of mineralization on the Malmyzh Au–Cu porphyry deposit, is reported. The central part of the ore bodies is composed of quartz–sericite and biotite–feldspar metasomatic rocks that bear relatively rich Au–Cu mineralization. The flank zones of ore bodies are mostly composed of phyllisite and propylite. These zones are characterized by high concentrations of Pb, Mn, and Zn. A positive correlation was detected for two groups of elements: Mo–Cu–Au and Pb–Zn–Mn. Based on analysis of the vertical change in the contents of ore components and their relationships, the modern erosion level at the upper part of the ore body has been proposed.

Fenites of the Miaskite–Carbonatite Complex in the Vishnevye Mountains, Southern Urals, Russia: Origin of the Metasomatic Zoning and Thermodynamic Simulations of the Processes

Mineral zoning in fenites around miaskite intrusions of the Vishnevye Mountains complex can be interpreted as a magmatic-replacement zonal metasomatic aureole (in D.S. Korzhinskii’s understanding): the metasomatic transformations of the fenitized gneisses under the effect of deep alkaline fluid eventually resulted in the derivation of nepheline syenite eutectic melt. Based on the P–T–fO2 parameters calculated from the composition of minerals coexisting in the successive zones, isobaric–isothermal fO2–aSiO2 and µNa2O–µAl2O3 sections were constructed with the Perplex program package to model how the fenites interacted with H2O–CO2 fluid (in the Na–K–Al–Si–Ca–Ti–Fe–Mg–O–H–C system). The results indicate that the fluid–rock interaction mechanisms are different in the outer (fenite) and inner (migmatite) parts of the zonal aureole. Its outer portion was dominated by desilication of rocks, which led, first, to quartz disappearance from these rocks and then to an increase in the Al# of the coexisting minerals (biotite and clinopyroxene). In the inner part of the aureole, fenite transformations into biotite–feldspathic metasomatic rocks and nepheline migmatite were triggered by an increase in the Na and Al activities in the system alkaline H2O–CO2 fluid–rock. As a consequence, the metasomatites were progressively enriched in Al2O3 and alkalis, and these transformations led to the development of biotite in equilibrium with K–Na feldspar and calcite at the sacrifice of pyroxene. The further introduction of alkalis led to the melting of the biotite–feldspathic metasomatites and the origin of nepheline migmatites. The simulated model sequence of metasomatic zones that developed when the gneiss was fenitized and geochemical features of the successive zones (differences in the LILE and REE concentrations in the rocks and minerals of the fenitization aureole and the Sm–Nd isotope systematics of the rocks of the alkaline complex) indicate that the source of the fluid responsible for the origin of zonal fenite–miaskite complexes may have been carbonatite, a derivative of mantle magmas, whereas the miaskites were produced by metasomatic transformations of gneisses and subsequent melting under the effect of fluid derived from carbonatite magmas.

Variability of Alunite Quartzite Composition as a Reflection of the Characteristics of Its Genesis

Based on 5000 analyses of geological prospecting samples from four alunite deposits (Began’, Ukraine; Haft Sanduq, Iran; Bolshaya Kremenyukha and Kachar, Kazakhstan) and 6000 microprobe analyses of alunite, the features of changes in three parameters of alunite quartzite composition have been studied: (1) alunite content, (2) mole fraction of K in alunite, and (3) correspondence of the amount of K and Na to that of alunite sulfur. All three parameters are highly variable. A linear correlation between them has been identified in over half of the exploration workings. This correlation is sometimes very strong, approaching a functional relationship, especially for two: the alunite content and mole fraction of K in alunite. In the first three of the above-mentioned deposits, this correlation is almost always positive, and in the last, it is always negative. In other words, as alunite content increases, it becomes higher-K in the first case and higher-Na in the second case. Alunite quartzite makes up a regular zone of the infiltration metasomatic column of sulfuric acid alteration in aluminosilicate rocks under conditions apparently close to isothermal. However, the highly variable contents of the basic minerals (alunite and quartz) and alunite composition point to a difference between the alunite quartzite formation model and the theoretical model for isothermal infiltration metasomatic rock. According to the theoretical model, the solution and rock compositions mainly change at the boundaries of the metasomatic zone, whereas the rock and mineral compositions are not characterized by significant variations within the zone. During alunite quartzite formation, the solution and rock compositions change not only at the boundaries, but also within the zone due to ongoing oxidation of sulfur compounds in the solution with the sulfate formation. An increasing sulfate concentration leads to the solution becoming supersaturated in relation to alunite and causes its precipitation, with filling of voids and displacement of quartz. Cooling, heating, or a change in the composition of the hydrothermal system solutions lead to temporal variations in the K/Na ratio in precipitated alunite. Alunite formation inside the alunite zone, accompanied by changes in the mole fraction of K in deposited alunite, is the reason for the high variability of the alunite quartzite composition and for the correlation between the alunite content and its mole fraction of K. If the contribution of higher-K alunite to the variability of its total content is higher than that of higher-Na alunite, a positive correlation is likely to appear, otherwise a negative correlation is more probable.