Subvolcanic Bodies Research Articles

TWO PULSES OF MAGMATIC ACTIVITY DURING THE EARLY RIPHEAN KUONAMKA IGNEOUS PROVINCE FORMATION: PALEOMAGNETIC SUBSTANTIATION OF THE HYPOTHESIS

By the beginning of the 20th century, at least ten different generations of intrusive mafic bodies 1800–900 Ma were distinguished on the territory of the Anabar massif. The first modern age estimates, however, indicate the formation of the bulk of these bodies in a relatively short period of time ~1480–1500 Ma. Based on geochronological and paleomagnetic data, intrusive bodies with this age were combined into the Kuonamka large igneous province (LIP). However, a complex of paleomagnetic studies indicated the probable existence of at least two pulses of magmatic activity during the formation of this province, differing in age, possibly by ~20 Ma. We performed a paleomagnetic study of subvolcanic bodies of the main composition of Permo-Triassic (dykes) and Mesoproterozoic (sills) age, emerging to the surface in the valley of the middle reaches of the Kotui River. As expected, the studied dikes contain an ancient magnetization, the direction of which is typical for Permo-Triassic magmatic bodies of the region and the Siberian platform as a whole, and the paleomagnetic directions of the sills are close to those previously determined in the age-related subvolcanic bodies of the Kuonamka igneous province (1501±3 Ma) of the Anabar massif. The data obtained allow us to test the previously proposed hypothesis about the existence of two magmatic pulses ~1480 and 1500 Ma ago during the formation of the Kuonamka igneous province. The results of the analysis of already known and new paleomagnetic data obtained by us do not support this hypothesis. A refined paleomagnetic pole was proposed for the Siberian platform ~1500 Ma ago with coordinates 22.6° S, 65.0° E and A95=4.3°.

Serra Geral Group plumbing system and 3D geological framework in Morungava region, southern Paraná-Etendeka LIP (Brazil): Evidence from borehole, geochemical, and petrophysical data

The Morungava region (east of Porto Alegre-Rio Grande do Sul, Brazil), in the southern half of the Paraná-Etendeka Large Igneous Province (LIP), hosts Serra Geral Group subvolcanic intrusions (sill complex). These intrusions comprise various chemical compositions, including high-Mg rocks. So far, only a few examples of high-Mg rocks have been discovered in the Paraná-Etendeka LIP. Many investigations have been conducted in the Morungava region, yet several questions regarding the distribution, geometry, composition, and structural framework of the subvolcanic bodies in the subsurface remain unanswered. Therefore, we built a three-dimensional geological model of the Serra Geral Group sills and their associated stratigraphic framework in the Morungava region. This was achieved through the combined analysis of geological data (boreholes and geological maps), as well as petrophysical and chemical data, to improve the understanding of these subvolcanic bodies underground. The stratigraphic framework depicts the Precambrian basement topography, the distribution of the Paraná Basin units, and the Rio Grande do Sul Coastal Plain strata, which span the majority of the examined area. To categorize the Serra Geral Group rocks intersected by the boreholes, we established a validation and correlation technique using three separate data sources: laboratory geochemical data, portable X-ray fluorescence measurements, and gamma-ray logs. Using this technique, we classified the Serra Geral Group rocks in the Morungava region into three types (1, 2, 3), which share compositional similarities with Palmas, Vale do Sol, and Torres formations, respectively. Additionally, we identified a fourth type that could be compared to the Lomba Grande Complex, Silica Saturated Olivine Tholeiite-type dikes, and some high-Mg rocks from Namibia.

Intra-basin Early Miocene volcanism at the Northern Puna-Eastern Cordillera boundary, province of Jujuy, Argentina

The limit between the Puna and the Eastern Cordillera is an area of geological complexity where Cenozoic tectonic, volcanic and sedimentary processes overlap. In this contribution, we describe the Early Miocene volcanic and subvolcanic manifestations located on the border between Northern Puna and Eastern Cordillera, analyzing their relationship with the foreland basin sedimentary succession. We present a new age of 17.94 ± 0.17 Ma (40Ar/39Ar hornblende) for the volcanic rocks, located in the middle Member of the Moreta Formation, as well as new geochemical analysis, showing andesitic-trachydacitic compositions belonging to the high K-calcalkaline series. In addition, we have studied the subvolcanic bodies intruded in the Río Grande Formation, from we which propose an early post-depositional emplacement. In both cases, we present brief considerations about the chemical composition of both study sites, since in the case of Moreta Formation the rocks are high K-calcalkaline andesites while in the case of Río Grande Formation, they are somewhat more undersaturated in silica and enriched in K (shoshonitic series). The new data we presented evinces an event of intra-foreland basin volcanism, mafic to intermediate in composition, at 18 Ma in the Northern Puna-Eastern Cordillera transition.

Geodynamics and Early Cretaceous Magmatism of the Northern Volcanic-Plutonic Belt of Verkhoyan-Kolyma Fold Area (Northeastern Russia)

New data on the geological position, U‒Pb SIMS zircon ages, petro-geochemical features, Sr‒Nd isotopic composition and geodynamic setting of the granitoids and volcanites of the Northern volcanic-plutonic belt, Verkhoyan-Kolyma fold area, are presented. Magmatites of the belt include granitoids of Elikchan, Kuranakh, Bakyn plutons, composed of elikchansky granite-granodiorite complex, and volcanites of predominantly intermediate-felsic Tumusskaya sequence with subvolcanic bodies of the same composition. They form single Early Cretaceous (127–121 Ma) volcanic-plutonic assemblage. Granitoid plutons are elongated in sub-latitudinal-northwestern direction and are discordant to main fold and thrust structures. Granitoids intrude and metamorphose Jurassic terrigenous and Early Cretaceous volcanites of Tumusskaya sequence and are cut by younger Late Cretaceous subvolcanic bodies. Granitoids of Bakyn, Elikchan and Kuranakh plutons combine petro-geochemical features of I-, S- and A-type granites. Such diversity of petro-geochemical granitoid types as well as interrelations of major (\({\text{F}}{{{\text{e}}}_{{\text{2}}}}{\text{O}}_{3}^{{{\text{tot}}}}\)–TiO2–MgO) and rare (Ba/La–Nb × 5–Yb × 10) elements in granitoids and the same age volcanites of Tumusskaya sequence allow to refer them to magmatites of transform margin or plates translation boundaries. Collision between Chukotka microcontinent and Siberian continent with earlier accreted Kolyma-Omolon microcontinent in Barremian-Aptian time changed to post-collisional extension and formation of volcanic-plutonic assemblage of the Northern volcanic-plutonic belt. Post-collisional extension took place in the regime of plates translation boundaries. Sr–Nd isotopic characteristics of granitoids of all plutons indicate the interrelation of mantle and crustal sources of granitoid melts in this process.

PETROLOGICAL and GEOCHEMICAL CHARACTERISATION of the KGWEBE VOLCANIC FORMATION in the ghanzi-chobe BELT PORTION of the KALAHARI COPPER BELT, western Botswana.

A combination of petrography and geochemistry was utilised to investigate the magma sources, magma evolution and petrogenesis of the Kgwebe Volcanic Formation (KVF) rock units. Based on their texture, the KVF rock units comprise amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks, porphyritic subvolcanic bodies, quartz-feldspar porphyries and clastic sedimentary rocks, that are intercalated with amygdaloidal rocks. The amygdaloidal rocks are commonly crosscut by calcite veins and characterised by amygdales (filled vesicles) set in the hyalopilitic groundmass. The porphyritic mafic flows are composed of plagioclase laths set in an aphanitic groundmass. The pyroclastic rocks are generally characterised by up to 75% of both mineral and lithic fragments originating from explosive volcanic activities. Porphyritic subvolcanic bodies are characterised by high content of subrounded epidote (after plagioclase) set in a microcrystalline groundmass. The quartz-feldspar porphyries are characterised by quartz and feldspar phenocrysts in variable proportions set in a fine-grained groundmass of variable colour. Rock units of the KVF represent two geochemically contrasting series that are not co-genetic and are derived from different geotectonic settings. The amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks and porphyritic subvolcanic bodies are sub-alkaline and basaltic in composition, whereas the quartz-feldspar porphyries span from the rhyolite/dacite (mostly) to the andesite/andesite basalt (accessorily) field compositions. Likewise, while quartz-feldspar porphyries are consistently high-K calc-alkaline bodies, their spatially associated amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks and porphyritic subvolcanic bodies counterparts are calc-alkaline. Similarly, quartz-feldspar porphyries are within plate bodies, whereas the more mafic KVF units are arc-related. Furthermore, the ferroan quartz-feldspar porphyries generated from the crust and the magnesian amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks and porphyritic subvolcanic bodies are mantle-derived, with however, significant crustal contamination fingerprints. Textural evidence (resorbed, embayed and rounded phenocrysts) and the inferred high-water content (as supported by the abundance of plagioclase phenocrysts) also suggest a rapid magma ascent and magma mingling occurred during the KVF eruption, which was both effusive and explosive. Concomitant volcanism and sedimentary processes likely occurred as supported by the intercalation of clastic sedimentary rocks within volcanics of the KVF.

Dyke or pipe? Contributions of magnetic fabrics to the reconstruction of the geometry of an eroded subvolcanic body (Cadí basin, Pyrenees)

The reconstruction of the geometry of igneous bodies in inverted basins is often a difficult task, because most of their volume can be either below or above the topographic surface. Indirect methods, such as the determination of their internal fabric can give clues to constrain geometrical reconstructions. The Sant Salvador intrusion, located in the Late Carboniferous-Permian Cadí basin (Central-Eastern Pyrenees), is a good example for testing the application of magnetic fabrics to shallow igneous bodies because of (i) its characteristics in terms of crystal composition/orientation, showing a homogeneous petrofabric controlled by plagioclase crystals 0.4 mm in size, (ii) its good, though partial, preservation, (iii) its position in the stratigraphical succession, between volcaniclastic sediments and unconformably overlying red beds, and (iv) the many unknown factors related to the intrusion, including the geometry of the body and its depth and the amount of erosion after intrusion. Furthermore, the Sant Salvador intrusion provides opportunity for checking the correspondence between the petrofabric and the magnetic fabrics when the magnetic carriers of susceptibility (with an average value of 5217·10−6 SI) are mainly ferromagnetic. In this case the main magnetic carriers are magnetite and also hematite, the latter resulting from oxidation processes at the paleosurface. The results of Anisotropy of Magnetic Susceptibility show a bimodality in the K1 disposition after bedding restitution: vertical in the deeper areas of the intrusion and subhorizontal, E-W, in the uppermost stratigraphic positions with an intermediate necking zone in which transition from vertical to horizontal flow is interpreted to occur. Bands deflecting the mineral foliation at the micro scale have also a tectonic or thermal origin but do not seem to interfere with the magnetic ellipsoid axes. Columnar jointing is interpreted to be parallel to the long axis of the intrusion (and therefore to magma flow, and perpendicular to the paleosurface. All these features indicate that the Sant Salvador intrusion can be interpreted as a subvertical pipe feeding a sill that was eroded before the sedimentation of the overlying red beds.

Emplacement dynamics of the plumbing system and lava pile of the Paraná Magmatic Province in Morro da Igreja, Santa Catarina, Brazil

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

Metasomatized mantle under the Eastern Mexican Alkaline Province: evidence from the Oligocene Rincón Murillo Gabbro, Sierra de San Carlos-Cruillas, NE México

The Cenozoic Sierra de San Carlos-Cruillas (SSCC) magmatic complex belongs to the Eastern Mexican Alkaline Province (EMAP). The SSCC mainly consists of plutonic and subvolcanic bodies displaying diverse lithological compositions. These rocks exhibit both OIB- and arc-like geochemical characteristics. In this contribution, we present new petrographic, geochemical, isotopic, and geochronological data of the Rincón Murillo Gabbro (RMG), situated in the southern part of the SSCC. The RMG is nepheline normative and displays an OIB-like intraplate character. Laser ablation ICP-MS U-Pb analyses on titanites yielded Oligocene (Rupelian) crystallization ages of 31.8 ± 1.8 and 31.0 ± 1.7 Ma. The 87Sr/86Sr(i) (0.703400–0.704159) and 143Nd/144Nd(i) (0.512732–0.512815) ratios indicate a mantle origin. Trace element data indicate the participation of different enriched mantle components in the source. One component is related to previous subduction processes, generating the HFSE-depleted arc-like magmas of San José Gabbro in the northern part of SSCC. The second component is related to deep mantle melts with a slightly carbonatitic signal and HFSE-enriched fluids derived from the subducted Hess conjugate plateau generating different enriched sources in the upper mantle under the SSCC. The passage, eclogitization, detachment and foundering of the Hess plateau gave rise to an asthenospheric upwelling that triggered the low to moderate partial melting degrees (3–15%) of an amphibole-bearing garnet peridotite, generating the OIB-like parental magmas of RMG, located in the southern part of SSCC. Two-component mixing, and AFC models indicate that variable degrees of crystal fractionation and minor crustal assimilation are involved in the evolution of Rincón Murillo Gabbro.

The Charrarruca porphyry-type alteration zone: New evidence of mineralizing Late Cretaceous-Eocene magmatism in the Southern Central Andes of Argentina

The Charrarruca porphyry-type alteration zone is located in the northern part of the Andacollo Mining District (AMD), placed in the northern Neuquén Precordillera, (Argentina). Potassic, chloritic-carbonatic, sericitic, and propylitic alteration halo of ∼1.5 km2 affects an andesitic/dacitic subvolcanic body. These rocks, with arc affinity and Upper Cretaceous-Paleogene age (65 ± 2.5 Ma), derived from a magma evolved under normal crust thickness conditions, allowing the correlation with the intrusive facies of the Naunauco Group. The petrogenetic characteristics of these rocks attest to indications of the fertility of this magmatism, turning into a prospective guide for the unexplored Upper Cretaceous–Eocene Neuquén porphyry copper metallogenic belt.

Геохимические особенности Ag-Au рудопроявления Кыплатап (центральная Чукотка)

The article considers the geochemical features of unconventional mineralization in secondary quartzite’s of the Kyplatap ore occurrence, located on the southeastern flank of the volcanic dome structure (VDS) of the same name, complicating the Palyavaam-Pykarvaam volcanotectonic depression (VTD) in the Chaun zone of the Central Chukotka sector of the Okhotsk-Chukotka volcanic Belt (OChVB). The Kyplatap VDS was formed due to the intrusion into the Late Cretaceous Alkaquun volcanites of a large laccolith-like subvolcanic body composed of rhyolite-trachyriolite-trachydacites. Based on the geochemical studies of the Kyplatap ore occurrence, the authors compiled a representative database of geochemical data, including the results of analyses of express-RFA, PKSA and XSA with coordinates and composition of ores and rocks. Correlation analysis of the database showed a positive relationship between Au and Ag, as well as Sb for ore mineralization, and a negative correlation of these elements with the association of the rock group of elements (Ti, Y, Zr, Nb and Yb) that were removed during ore formation. The authors also distinguished the second rock group of elements (K, Ga, Sn, Ba, V, W, Pb, Sc, La and B), characterized by medium and weak correlation of the first group of elements. They calculated the coefficients of enrichment and deficiency of ore-forming elements for the samples taken within the ore zones of the Kyplatap ore occurrence. The extreme enrichment group includes elements Au, Ag, Sb, Bi, As, the average contents of which are 100 times or more, and the minimum concentrations are at least 2-10 times higher than the average contents in the upper crust. The normal enrichment group includes elements S, Sn, Cu, Zn, Mn, Mo and Pb, the average contents of which are 2-10 times, and the maximum concentrations are 5-100 times higher than the average concentrations of the upper crust. The obtained values of the enrichment coefficients indicate a sufficiently intensive metensomatoses and ore formation within the Kyplatap ore occurrence.

Peculiarities of the Magnetic Field of Geothermal Systems of the Pauzhetsky Area (South Kamchatka)

Based on many years of research, maps of magnetic field anomalies ΔTa for large geothermal systems of the Pauzhetsky region of South Kamchatka were constructed. Magnetic fields have both general characteristics and individual features for each research object. In the area of Nizhne-Koshelevsky steam-dominated geothermal field identified a system of linear negative magnetic field anomalies, confined to thermal controlling discontinuous tectonic disturbances. Pauzhetsky geothermal field is characterized by a heterogeneous structure of the anomalous magnetic field ΔTa: NW region is marked by a quiet weakly negative magnetic field, indicating the predominance in this part of the field lateral spreading of hydrotherms from the upper aquifer; SE – a large number of alternating magnetic anomalies of high intensity, confined to subvolcanic bodies of acidic to moderate composition. The South Kambalnaya group of thermal fields is characterized by a lower magnetic induction modulus T as compared to the Pauzhetsky and Lower Koshelevsky geothermal fields, indicating a more intense alteration of the Kambalnaya Ridge rocks by hydrothermal-metasomatic processes, apparently as a result of long-term exposure to convective heat flow.

Stratigraphy, volcanic processes and evolution of the Martin Vaz archipelago

The volcanic Martin Vaz Archipelago consists of four Plio-Pleistocene small islands (total area of 0.37 km2) in the South Atlantic Ocean in front of Brazil's coast. Together with Trindade Island, the archipelago corresponds to the easternmost portion of the Vitória-Trindade Seamount Chain (VTC).We performed geological mapping, coupled with X-ray diffraction, petrography, scanning electron microscopy, and lithogeochemistry analyses at Martin Vaz samples. Four lithostratigraphic units were identified (i) Martin Vaz Formation records older explosive phonolitic activity. It is interpreted as resulting from hydromagmatic volcanism; (ii) Subvolcanic phonolites consist of a dome (cryptodome) and plug morphologies, which represent the most voluminous rocks at Martin Vaz Archipelago; (iii) Mirante Formation records hyper-concentrated density currents of ultrabasic rocks; and (iv) Atobá Formation is composed of ultrabasic to intermediate lavas of basanitic to phonolitic composition, interpreted as a result of Strombolian (Fountain Fed)/Hawaiian volcanism.The volcanic rocks described at Martin Vaz Archipelago and Trindade Island represent the final members of the nephelinite-phonolite series, with a significant absence of intermediate phases. Basanites of the Martin Vaz, Mirante, and Atobá formations are geochemically similar to the nephelinites of Trindade Island. In the same way, the geochemical characteristics of phonolites from subvolcanic bodies and the Bandeira Member are similar to those observed on the phonolites of Trindade Island. Geochemical and thermodynamic modeling suggest cogeneticity between ultrabasic and phonolitic magmas. Fractional crystallization about 0.5 GPa and at low water pressures explain the chemical evolution trends observed. Geochemical characteristic points to melts of a metasomatized source composed of garnet wehrlite and/or hornblendite.The new data allowed for the elaboration of a novel conceptual model for the geological evolution of the Martin Vaz Archipelago, one of the last volcanic manifestations in the Brazilian territory.

GEOCHEMISTRY AND AGE OF MAFIC DYKES OF THE Be-DEPOSIT SNEZNOE: LATE PALEOZOIC MANTLE PLUME MAGMATISM EVIDENCE IN EASTERN SAYAN

Within the East Sayan rift zone of the Late Paleozoic Barguzin salic large igneous province (SLIP), along with alkaline-peralkaline granitoids with ore mineralization, mafic subvolcanic bodies (gabbroids, mafic dykes) are common. One of the typomorphic mafic – peralkaline granitoid association is the Snezhnoye beryllium deposit. Using the U–Pb LA–ICP–MS method, mafic dykes of the Snezhnoye beryllium deposit were dated. The age of zircons from the diabase (dolerite) dyke was 301 ± 6 Ma, and from the microgabbro one, 297 ± 2 Ma. The obtained dates are close to the age of fluorite-beryl mineralization (306 Ma) and peralkaline granites with Ta-Nb mineralization attributed to the Ognite complex (311–295 Ma). The geochemical features of mafic dykes suggest the generation of mafic magmas from an asthenospheric mantle source, which, taking into account the intracontinental setting, testifies in favor of the plume-mantle nature of mafic – alkaline-peralkaline granitoid magmatism of the East Sayan rift (rare-metal) zone.

Digital geologic map, petrography and U-Pb geochronology of the Río Santiago Shear Zone, southernmost Sierra Madre Occidental, western Mexico

The boundary between the Sierra Madre Occidental (SMO) and the Trans-Mexican Volcanic Belt (TMVB) is well exposed along the Santiago River, on the border of the states of Nayarit and Jalisco. This region includes a polydeformed belt produced by the relative motion between the Jalisco Block and the SMO, here defined as the “Rio Santiago Shear Zone” (RSSZ). In this work, we present detailed geological cartography, petrographic descriptions, and new zircon U-Pb ages that allow us to establish the stratigraphy of the region surrounding the RSSZ, providing temporal constraints to its tectonic evolution. In the mapped area, the base of the SMO stratigraphic column consists of a succession of massive andesitic lava flows intruded by felsic dikes and a felsic subvolcanic body of 27.5 and 27.2 Ma, and it is covered by a series of ignimbrites of 26.8 and 25.3 Ma. A red beds succession separates a second package of ignimbrites of ∼20-19 Ma, in turn capped by rhyolitic lava domes of 19.7 and 18.6 Ma and a rhyodacitic lava flows of 15.6 Ma, this Miocene succession is also intruded by a felsic body of 18.3 Ma west of the La Yesca dam. The SMO succession is covered by rocks belonging to the bimodal volcanism of the TMVB, emplaced between the end of the Miocene and the Pleistocene. The RSSZ developed through several deformation phases that occurred since the end of the Oligocene. The Oligocene succession of the SMO is affected by NE-SW and WNW-ESE trending lateral faults, NW-SE open folds and WNW-ESE and E-W trending reverse oblique faults. Some of the reverse oblique faults were later reactivated as normal faults. WNW-ESE trending normal faults associated with the Tepic-Zacoalco Rift affected the area south of the Santiago River during the Pliocene and Pleistocene. Normal faulting produced an accumulated vertical offset up to 700 m between the northern and southern regions of the Santiago River.

Western flanks of the Central Asian late paleozoic continental rift system and uranium mineralization

Introduction. Available information on the occurrences of high alkaline magmatic rocks fixing the position of the western flanks of the southern branches (Gobi Altai and Gobi Tien-Shan) of the Central Asian Late Paleozoic continental rift system in the territory of the North-West of Chingiz (the eastern part of Central Kazakhstan), the Kendyktas-Chu-Ili-Bet-Pak-Dal uranium-bearing province of the Southern Kazakhstan and the Chatkal-Naryn zone of Tien-Shan (Uzbekistan) is reviewed. Data on the confinement of uranium mineralization to these occurrences and its paragenetic relationship with the Late Paleozoic alkaline volcanic-plutonic association is provided.Aim. To study the uranium mineralization of Late Paleozoic alkaline magmatism to clarify metallogenic representations and justify prospects for the ore-bearingness of the western flanks of the southern branches of the Central Asian continental rift system.Materials and methods. The largest part of materials was obtained in the process of geological surveys and prospecting works of various scales in the regions of Central Kazakhstan in the period 1965–1985 with the participation of the author. Published materials concerning the subdivision of magmatic formations in the uranium ore fields of Southern Kazakhstan and the Chatkal-Naryn zone were also used.Results. Subvolcanic bodies of comendites and small masses of alkaline granites of the Late Paleozoic were identified across the area of uranium ore occurrences in the South-Western Chingiz and some deposits of South Kazakhstan confined to Devonian volcanic structures, which indicates their relationship with the southern branches of the Central Asian continental rift system. Rare-metalalbite, uranium-phosphorus and uranium-molybdenum formations are confined to alkaline rocks. Rare-metal mineralization is closely associated with alkaline rocks, while uranium mineralization occurred after the introduction of the most recent microgabbrodiorite and lamprophyre dikes. Similar correlations of rare-metal and uranium mineralization with alkaline granites and of microgabbrodiorite and lamprophyre dikes are observed in the uranium deposits of the Chatkal-Naryn zone located among the early Permian bimodal trachybasalt-trachyolite formation.Conclusion. The establishment of ore mineralization on the western flanks of the Gobi Altai and Gobi Tien-Shan branches in the territory of Southern and Central Kazakhstan allows these areas to be distinguished as metallogenic zones, promising in terms of rare-metal and uranium mineralization.

Mineralogical and geochemical peculiarities and PT conditions of ores from the Kyzyl-Tashtyg VMS polymetallic deposit, Eastern Tuva: Fluid inclusion and S, O, C isotopic data

The paper is devoted to the mineralogical and geochemical peculiarities and ore-forming conditions of the formation of the Kyzyl-Tashtyg pyrite-polymetallic deposit in Eastern Tuva. The ore mineral assemblages of the deposit were formed due to aqueous fluids containing Na, Mg, and K chlorides at the pressure of 900 bar and the variations of fO2, fS2, fSe2, and fTe2. The earliest pyrite ores in the central parts of the ore bodies were formed at relatively high temperatures of 400–260 °C due to a fluid of the composition NaCl-H2O and MgCl2-NaCl-H2O with the salinity of 4–11.3 wt% NaCl eq. The copper-zinc ores were deposited at 377–145 °C from the NaCl-H2O fluid with the salinity of 2.6–10 NaCl eq. and the copper ores – at 200–165 °C from the NaCl-KCl-H2O fluid with the salinity of 3.5–6.5 NaCl eq. The latest polymetallic and barite-polymetallic ores were formed at 330–130 and 270–140 °C, from the NaCl-KCl-H2O ± MgCl2-H2O fluid with the salinity of 3.5–8.8 and 2–8.5 wt% NaCl eq., respectively. The calculated δ34SH2S values of the fluid range from + 0.2 to + 6.6 ‰, which indicates the participation of sulfur of magmatic origin and borrowed from the host rocks. The δ34O isotopic composition of the fluid suggests that the fluid of magmatic genesis (+8.7… + 7.1 ‰) participated in the early stages of ore mineralization, and at the final stages, it was mixed with meteoric waters (-4.0…-0.7 ‰). The temperature decrease of mineral formation and fluid salinity from early to latest stages confirms the participation of heated meteoric waters. The results of the study of the geological structure, mineral assemblages, fluid inclusions, and stable isotope ratios provided to conclude that the formation of the Kyzyl-Tashtyg deposit was initiated by hydrothermal-magmatic activity, associated with the introduction of subvolcanic bodies of rhyolite and rhyolite-dacite and involvement of fluids separated during crystallization of felsic magma, as well as heated meteoric waters enriched with oxygen, which contributed to the growth of fO2 and variations fS2, fSe2, fTe2 and determined the diversity of mineral types of ores.

Conditions of localization and patterns of distribution of gold-poly-metal mineralization of the Dagkesaman deposits (Lesser Caucasus, Azerbaijan)


 
 
 We studied the structural-morphological types and mineral composition of ore bodies, stages of ore development and conditions of development of the deposits. The research revealed significant elements of the corresponding conditions of localization of gold-polymetallic mineralization, the most characteristic being albitophyre subvolcanic bodies and steeply-dipping damages related to them. According to the structural-morphologic peculiarities, the ore bodies should be united under three categories: 1) simple vein; 2) complex vein; 3) veinlet-impregnation types. The positions of gold ore bodies and their morphological peculiarities were found to be determined by structural factors. Presence of small ruptures, differently-oriented systems of fractures and faults, which run across the area, gentle curves of these systems and other structural elements –all of this promotes gold ore development. There research determined aureoles of ore-containing altered rocks (serialization, chloritization, kaolization and calcinations, and limonitization from hypergenical zones) and presence of zonal structure. Gold-productive areas were identified as the quartz-pyrite association with disperse gold, and especially, the second mineral association where there is a notable close paragenetic connection between noticeable gold and chalcopyrite-galena parageneis of polymetallic stage. In the process of ore development, the physical-chemical conditions have presumably changed from average deep (quartz-pyrite-sericite) to shallow (quartz-galena- sphalerite-gold ore) and close-to-the-surface (quartz-chalcocite-hematite). Gold that occurs with all the mentioned minerals of this stage has developed in the temperature interval of 220–160 °C. The study found the ores to be complex, containing the following dditional elements: Fe, Cu, S, Bi, Zn, Pb, As, Se, Te. The main fossil component is gold. Industrially valuable are also cadmium, zink, copper and silver, found in the ores. We found the following morphological types of grains of native gold: homogenous, zonal, mono-granular, spotted, which are of great significance for concluding on genesis of the deposits, prognoses, explorations and assessments.
 
 

Mineralizations related to the Tocota pluton. Southern Colangüil batholith, Cordillera Frontal, San Juan, Argentina

The southern portion of the Colangüil Batholith is formed by units of the Tocota Pluton, of Permian age, which intrude late Carboniferous-Permian (Asselian) sandstones. Sub-volcanic bodies with andesitic composition and tuffs of Cenozoic age, modern layers of piedmont sediments and both alluvial and glacier deposits also crop out. The region exhibits a set of metalliferous deposits (Cu-Au-As) and occasionally with bismuth. These are mostly filonian type. These are spatially associated with granodiorites, granite and sandstone. Their development is interpreted as belonging to the Gondwanan cycle.Within granitic rocks, mineralizations were formed between 139 ºC and about 400 °C, probably after the consolidation of the Tocota pluton and by the upward circulation of hydrothermal fluids, probably mixed with meteoric waters. In the host rock, mineralization occurred in an interval between 200 ºC and 350 °C, through hydrothermal activity detected in areas of phyllic-argillic alteration. The gangue constitutes mainly of quartz, tourmaline and and sometimes potassium feldspar and calcite. The mineralization pattern, in both the pluton and the sandstone, has been controlled by 1) a strong domain of ore filled faults and joints, with ENE and WNW as the more common strikes and NNW subordinated, 2) fault breccias and hydrothermal breccias, and 3) dykes of andesite composition with low metal content. The host rock mineralization is commonly emplaced in joints as well as finely disseminated, related to intense silicification. Such deposits contain hypogene minerals whose isotopic 34S/32S ratios are compatible with a granite-like filiation but other provenances cannot be ruled out.

Evolution of the Lower Permian Rochlitz volcanic system, Eastern Germany: reconstruction of an intra-continental supereruption

Extensional tectonics in the Late Paleozoic Central Europe was accompanied by rift magmatism that triggered voluminous intracontinental caldera-forming eruptions. Among these, the Lower Permian Rochlitz Volcanic System (RVS) in the North Saxon Volcanic Complex (Eastern Germany, Saxony) represents a supereruption (VEI 8, estimated volume of 1056 km3) of monotonous rhyolites followed by monotonous intermediates. Mapping, petrography, whole-rock geochemistry along with mineral chemistry and oxygen isotopes in zircon display its complex eruption history and magma evolution. Crystal-rich (> 35 vol%), rhyolitic Rochlitz-α Ignimbrite with strong to moderate welding compaction erupted in the climactic stage after reheating of the magma by basaltic injections. Due to magma mixing, low-volume trachydacitic-to-rhyolitic Rochlitz-β Ignimbrite succeeded, characterized by high Ti and Zr-values and zircon with mantle δ18O. Randomly oriented, sub-horizontally bedded fiamme, and NW–SE striking subvolcanic bodies and faults suggest pyroclastic fountaining along NW–SE-oriented fissures as the dominant eruption style. Intrusion of the Leisnig and the Grimma Laccoliths caused resurgence of the Rochlitz caldera forming several peripheral subbasins. In the post-climactic stage, these were filled with lava complexes, ignimbrites and alluvial to lacustrine sediments. Significant Nb and Ta anomalies and high Nb/Ta ratios (11.8–17.9) display a high degree of crustal contamination for the melts of the RVS. Based on homogenous petrographic and geochemical composition along with a narrow range of δ18O in zircon Rochlitz-α Ignimbrite were classified as monotonous rhyolites. For the Rochlitz-β Ignimbrites, underplating and mixing with basic melts are indicated by Mg-rich annite–siderophyllite and δ18O < 6.0 in zircon. The wide spectrum of δ18O on zircon suggests an incomplete mixing process during the formation of monotonous intermediates in the RVS.

Melilite-olivine neрhelinites of Mt. Tabaat (Makhtesh Ramon, Israel): Geological, petrographic and geochemical characteristics and conditions of genesis

Research subject. The melilite-olivine nephelinite subvolcanic body Tabaat, which includes melilite rocks found for the first time on the territory of Levant.Materials and methods. The chemical composition of minerals (about 400 analyzes) was determined out on a CAMECA SX-100 microanalyzer equipped with five wave spectrometers with crystal analyzers TAP, LPET and LLIF. The elemental composition was measured at an accelerating voltage of 15 kV, an electron beam current of 40 nA. The oxygen concentration was calculated from the condition of stoichiometric composition of silicate minerals and chromite. In addition, the results of earlier studies of minerals (150 analyses) and data on melt inclusions were used.Results. The Tabaat subvolcanic body, which is part of the Early Cretaceous olivine-basalt-basanite-nephelinite association Makhtesh Ramona (Negev, Israel), has a complex concentrically-zonal structure, with olivine melanephelinites in the peripheral zone, melilite-olivine melanephelinites in the central and connecting zones normative and, less often, melilite-containing melanephelinites. Prismatic separation is widely manifested in the rocks. The fold-like bending and concave-curved edges of the prisms are a reflection of the plastic state of the cooling body and its ability to compress and accommodate a high fluid pressure, which develops during the formation of melilite nephelinites.Conclusion. All mineral diversity of rocks of the Mt. Tabaat is a derivative of a single portion of magmatic melt under conditions of its adiabatic cooling at the place of stabilization. A special role in the course of crystallization of the massif belongs to minerals with a high water content – analcime, zeolites, iddingsites, bowlingites and saponite-celadonites, which indicate the deuteric stage of its development. The study of melt inclusions in olivine and clinopyroxene showed the continuity of their composition with the composition of host melilite nephelinites and the importance of incongruent melting during the formation of melilite, which is a product of the reaction of nepheline with olivine or clinopyroxene.