Volcanoclastic Rocks Research Articles

Mineralogy and geochemistry of altered Emeishan basaltic volcaniclastics with respect to their critical element mineralization

Research on the mineralogical and geochemical characteristics of altered end-Guadalupian basaltic volcanoclastic rocks can increase the understanding of the alteration history of the overlying stratabound Nb(Ta)-Zr(Hf)-REY-Ga mineralization in the Late Permian coal-bearing sequences. In this paper, based on petrographic observations in addition to XRD, XRF, and ICP-MS analyses, we present and discuss in detail the relationships between primary and secondary minerals and outline the distribution of major and trace element chemistry in the middle/Late Permian basaltic volcaniclastics from the ELIP's zone, western Guizhou, southwest China. The primary clastic suite consists of plagioclase-group minerals, clinopyroxenes, feldspathoids, spinels, and basaltic glasses. Fragments of mafic and, less commonly, felsic and alkaline volcanic rocks are minor components of the studied samples. The alteration products are represented by various chlorite-group minerals (including abundant chamosite), quartz, calcite, albite, analcime, barite, and pyrite, along with relatively minor amounts of titanite, sanidine, magnetite, rutile, and copiapite. The mineralogical and geochemical characteristics of the studied volcaniclastics provide strong evidence correlating them with the high-Ti basalt group, widely distributed within the inner Emeishan large igneous province (ELIP) and coeval rift zone of the middle and outer ELIP. After deposition, the volcaniclastics reacted with complex solutions including heated meteoric waters and were periodically infiltrated by seawater and ascending hydrothermal fluids. As a result, the primary volcanic rocks partly lost alkalis, titanium, silica, and most of the trace elements. These elements, especially the incompatible elements, were probably enriched in the overlying tuffaceous and coal-bearing sequences in the middle and outer ELIP.

Secular change of tectonic setting in the Archean Takanen greenstone belt, northeastern Karelia Province, Fennoscandian Shield

The target of this study is the Takanen greenstone belt (TGB), a small 11-km-long and east-west oriented supracrustal belt in the northern Lentua complex of the Western Karelia Subprovince (eastern Finland). We present a new geological interpretation of the TGB based on lithology, geochemistry, stratigraphy, and two new zircon U-Pb age determinations and Lu-Hf-in-zircon data. The TGB forms a ���≥800-m-thick synclinal sequence composed of intercalated mafic-���komatiitic and felsic-intermediate volcanic and volcaniclastic rocks (lavas, tuffs, and tuffites) crosscut by granitoid dykes, and metamorphosed in lower amphibolite facies with few primary volcanic textures visible. The felsic-intermediate rocks are calc-alkaline and split into strongly and weakly REE-fractionated groups. The mafic-ultramafic rocks are tholeiitic with flat REE-patterns, but some samples show LREE-enrichment. Massive Fe-sulfide layers (≤��1 m) are associated with volcanoclastic rocks. Komatiites are Al-undepleted and comprise differentiated (peridotite-gabbro) and undifferentiated (dunite/serpentinite) cumulates and thin undifferentiated flows. Channel-facies komatiitic cumulates are found stratigraphically above Fe-sulfide-rich unites and have �potential for komatiite-hosted Ni-deposits. Our new isotope data from two felsic-intermediate volcanic rock samples shows that the TGBs stratigraphically lowermost and uppermost units are 2.96 Ga and 2.71 Ga old, respectively. Based on Hf isotope data, the older 2.96 Ga sample contains reworked crustal material, possibly originating from ~3.2 Ga crust.In contrast, the 2.71 Ga sample is isotopically juvenile (near-chondritic), indicating an episode of juvenile crust formation in the northern Lentua Complex. No unconformity is found between the stratigraphically upper and lower parts of the TGB, but the ~250 Ma age difference (and geochemical indicators) suggests they may have formed in different tectonic settings. The lower part of TGB stratigraphy is correlated with the ~2.94 Ga Luoma Formation in the nearby Suomussalmi greenstone belt. The 2.71 Ga volcanic unit does not have a known equivalent in greenstone belts in Finland, but a corollary is found in Russia, and a tentative correlation to a possible 2.75–2.70 Ga arc-system in Takanen–Khedozero-Bolshozero–Ilomantsi is suggested.

Petrographic composition of diamond-bearing Carnian formations of the Bulkur anticline the north-eastern Siberian Platform

Research subject. Diamondiferous explosive detrital and sedimentary volcanoclastic rocks of the Carnian base of the Upper-Triassic confined to the west flank of the Bulkur Anticline in the north-eastern Siberian Platform. Aim. To identify diamondiferous features of Carnian rocks. Materials and methods. The materials collected by the authors while performing prospecting and evaluation works by the OOO Arctic Mining Company in 2022-2023 were used. The materials were represented by field observation results and mineral-petrographic studies. Rock types were defined by petrographic analysis in thin sections using a Leica DM2700P microscope and known classifications. Diamonds and accessory minerals were distinguished from steam sediment samples and described at the central analytical laboratory of the Vilyuysk Geosurvey Expedition, PJSC Alrosa. The estimation of diamond and pyrope content in core- and bulk samples (CPT) was performed considering their direct correlation. Results. The highest concentrations of diamonds were found to be localized in rocks in the west flank of the Bulkur Anticline. These formations were detected in lapilli tuffs, xenotuff breccia and orthotuffites. Paratuffites and tuff aleuro-sandstones exhibit a lesser diamond potential. Conclusions. The diamond potential of rocks of the Carnian base correlates well with the pyrope content and saturation of volcanoclastic materials. Tuff aleuro-sand-stones of the Bulkur member in the Osipay base and overlying shell deposits are considered potentially diamondiferous. Diamondiferous formations possess considerable thickness and productive efficiency, thus assuming a substantial increase in the diamond prospective potential in the Russian Arctic.

Stratigraphy of the Agnew-Wiluna Greenstone Belt: review, synopsis and implications for the late Mesoarchean to Neoarchean geological evolution of the Yilgarn Craton

Our ability to understand the geological evolution of Archean cratons is fundamentally dependent on stratigraphic reconstructions across greenstone belts. Here, we integrate published stratigraphic, geochemical and geochronological data to produce a revised stratigraphic model for the Agnew-Wiluna Greenstone Belt of the Yilgarn Craton. The Agnew-Wiluna Greenstone Belt records ca 130 m.y. of magmatism that occurred between ca 2820 and 2690 Ma and can be divided into three cycles. Cycle 1 is marked by ca 2820–2740 Ma mafic–ultramafic volcanic rock successions. Cycle 2 is associated with ca 2740–2720 Ma felsic volcanic rocks, associated fragmental deposits and contemporaneous mafic–ultramafic intrusions. Cycle 3 consists of ca 2720–2690 Ma mafic–ultramafic volcanic rock successions and co-magmatic intrusions. Exhumation and reworking of these continental rift basin successions after ca 2690 Ma resulted in two younger, additional, depositional cycles in the Agnew-Wiluna Greenstone Belt. Cycle 4 is associated with intermediate to felsic volcanoclastic rocks and epiclastic rocks erupted/deposited between ca 2690 and 2665 Ma. Cycle 5 is associated with the deposition of polymict conglomerates and sandstones in localised depocentres between ca 2665 and 2655 Ma. The stratigraphic scheme identified at the scale of the Agnew-Wiluna Greenstone Belt is the local expression of craton-wide tectono-thermal events. Our revised stratigraphic model for the Agnew-Wiluna Greenstone Belt and proposed correlations at the craton scale support a para-autochthonous model whereby crust from a proto-Yilgarn continental mass was extended/thinned, favouring the activation of translithospheric pathways associated with the ca 2720–2690 Ma Kalgoorlie Large Igneous Province. Inversion of this intra-cratonic rift basin after ca 2680 Ma led to the protracted activation of shear zones and multiple shifts in sedimentary depocentres. The widespread occurrence of previously unidentified fragments of ca 2820–2720 Ma substrate within the Kalgoorlie-Kurnalpi Rift has significant implications for craton-scale stratigraphic reconstructions and challenges existing terrane and domain boundaries. KEY POINTS We propose a revised, coherent and testable stratigraphic model for the Agnew-Wiluna Greenstone Belt of the Kalgoorlie Terrane. We compare our revised stratigraphic model for the Agnew-Wiluna Greenstone Belt with that of the Murchison and Southern Cross domains of the Youanmi Terrane in order to build a new space–time–integrated stratigraphic cross-section across the western half of the Yilgarn Craton. We demonstrate for the first time the existence of a magmatic continuum (i.e. no significant magmatic hiatus) between ca 2820 and 2690 Ma, providing the missing link between the geological record of the Youanmi Terrane and that of the Eastern Goldfields Superterrane.

Polymetallic nanoparticles in pyrite from massive and stockwork ores of VMS deposits of the Iberian Pyrite Belt

This paper reports the first-ever study on nanoscale mineralogy in pyrite from the volcanogenic massive sulfide (VMS) deposits of the Iberian Pyrite Belt, southwestern Iberian Peninsula. It targeted colloform-textured grains formed at low temperature in the distal part of a polymetallic (Pb-Zn) massive sulfide lens hosted in felsic volcanoclastic rocks from the Masa Valverde deposit, and euhedral-textured grains (re)-deposited by higher temperature fluids in the Co-Au rich stockwork hosted in black shales of the Filón Norte orebody of the Tharsis deposit. The results acquired by a combination of techniques for mineral microanalysis and characterization (i.e., reflected light, FE-SEM, EPMA, LA-ICP-MS, HRTEM-STEM and TEM-EDS) show that trace amounts of metals (Au, Ag, As, Pb, Sb, Cu, Co) are incorporated as both lattice-bound and into nanoparticles (NPs). The mode of occurrence is strongly related with the evolutionary history of the mineralization. In the colloform pyrite collected from the massive sulfide lens, a rhythmic banding/oscillatory zonation with up to 3 wt% As, 5,000 ppm Pb, 1,070 ppm Sb and 750 ppm Cu is defined by the coexistence of several nano-sized layers (5 to 100 nm) and NPs (<100 nm) containing all these metals. The NPs include galena [PbS], tetrahedrite [(Cu,Fe)12Sb4S13)] and arsenopyrite [FeAsS] that exhibit euhedral and less frequently anhedral (i.e., droplet-like) morphologies being both randomly and preferentially oriented with respect to As-rich pyrite bands they are usually associated with. These features suggest formation of the NPs via direct deposition from the hydrothermal fluid(s) or low-temperature melts entrained in them as well as exsolution of trace elements originally dissolved in the As-rich pyrite structure. Additionally, some of these NPs are connected to late fractures disrupting the chemical zoning in colloform pyrite documenting a third genetic type of NPs related to late infiltration of fluids post-dating pyrite formation. In contrast, euhedral pyrite from the stockwork form well-developed homogeneous grains with discrete porous areas relatively depleted in Fe (45.20 wt%), and As (8,800 ppm) but enriched in Co (5,900 ppm). At the nanoscale, Co-enriched domains show patchy zoning defined by irregular distribution of Co– and As-rich bands of 200–500 nm in thickness. These nanometer Co– and As-rich bands are often disrupted by micron-to-nano-sized polycrystalline Au-Ag-Hg particles that fill voids in porous areas. Contact morphology anatomy between Co-rich pyrite and inclusions suggests that the Au-Ag-Hg particles are negative crystals occupying spaces originated in pyrite by coupled dissolution-reprecipitation reaction. Likewise, HRTEM observations along such pyrite-inclusion contacts show the existence of polycrystalline matrices in both pyrite and Au-Ag-Hg inclusions, the former consisting of nano-sized domains of arsenian pyrite and/or arsenopyrite in As-free pyrite and the Au-Ag-Hg inclusions made up of multiple crystal domains including nano-crystallites of Au0/Ag0 or electrum. Recognition of crystalline nanodomains and NPs in these polycrystalline matrices raises the possibility that Au NPs or nanomelts already present in the hydrothermal fluid catalyzed the formation of these heterogeneous crystals.

SAR data and field surveys combination to update rainfall-induced shallow landslide inventory

The Campania region has been recurrently hit by severe landslides in volcanoclastic deposits. The city of Naples, and in particular the Camaldoli and Agnano hills (Phlegraean Fields), also suffered several landslide crises in weathered volcanoclastic rocks as a consequence of intense rainfalls or wildfires. To identify slope failures phenomena occurred in the winter season 2019–2020 an innovative procedure has been proposed. The purpose of this procedure is to highlight areas where major land cover changes occurred within our area of study, which can be potentially related to mass movements. The amplitude of spaceborne SAR images has been exploited for the change detection analysis and the output derived from the segmentation procedure has been compared with field observations. The amplitude-based method has been already applied in the detection of landslides, but never on the event with limited extensions, such as for this application. The achieved outcomes allowed the mapping of 62 new landslides that have been used to update the current landslide inventory database. This type of information is expected to help decision-makers with land planning and risk assessment.

Ore-formation at the Halasheng Ag-Pb-Zn deposit, northeast Inner Mongolia as revealed by trace-element and sulfur isotope compositions of ore-related sulfides

The Early Cretaceous Halasheng Ag-Pb-Zn deposit in NE China, which is mainly hosted in Jurassic rhyolitic tuff, andesite, and volcanoclastic rocks, comprises both proximal skarns and more distal hydrothermal discordant ore veins and stockworks with associated Fe-Mn-carbonate, illite, and silica alteration in NNW-trending extensional faults. Suggestions of a magmatic-hydrothermal relationship are tested by LA-ICP-MS trace-element and sulfur isotope studies of ore minerals, particularly sphalerite which is a major ore mineral.Sphalerite has at least three types: SpS, anhedral, mainly dark red sphalerite in skarns; Sp1, euhedral, normally pale yellow coarse-grained, and rarely oscillatory zoned sphalerite formed in Stage 2 of hydrothermal vein mineralization; and Sp2, fine-grained, red, non-zoned sphalerite formed in Stage 3 of hydrothermal mineralization. Sphalerite with different textures has As, Ag, Cd, Sb, and Pb contents that increase from proximal skarn to distal hydrothermal mineralization consistent with the increase in sulfosalt minerals in the late hydrothermal stage. In situ time-resolved depth profiles, together with tangential profiles, and LA-ICP-MS element mapping of sphalerite indicate that trace elements are largely in solid solution in Halasheng sphalerite with substitution mechanisms including: Zn2+ ↔ Fe2+, Cd2+, Mn2+, and 3Zn2+↔ (As3+, Sb3+) + Ag++Pb2+. Enrichment of In and Sn, and depletion of Ga and Ge, together with In/Sn ratios indicate the high-temperature ore-forming fluids at Halasheng that are most likely closely related to intrusions. In situ mean δ34S values of SpS, Sp1, and Sp2 sphalerites are 5.3‰, 7.6‰, and 8.3‰, respectively, indicating that sulfur was derived both from magmatic-hydrothermal fluids and from wall rocks with higher (>10‰) values.These data help confirm that the Halasheng Ag-Pb-Zn deposit is an intrusion-related deposit with local proximal skarn mineralization at the contact of Tamulangou Formation andesite with Ergunahe Formation crystalline limestone, and more distal vein-type volcanic-hosted hydrothermal mineralization, with both ore and gangue mineral assemblages and available fluid inclusion data suggesting an intermediate-sulfidation epithermal association. As ore-forming hydrothermal fluids cooled and evolved they became enriched in silver, with silver-bearing sulfosalt minerals and silver-rich sphalerite precipitated in the latter stages of deposition.In terms of future exploration, there is potential for the concealed Lower Cretaceous Pb-Zn skarns and intermediate sulfidation Ag-Pb-Zn epithermal deposits at Halasheng to be part of a larger porphyry-related mineral system with porphyry Cu-Au-Mo deposits, similar to those of Middle Triassic to Early Jurassic age in the same metallogenic belt, at deeper crustal levels than the known deposits.

Sedimentary environment and tectonic setting of the clastic formation of the Tanjianshan Group in the Tuomoerrite area, North Qaidam

柴北缘构造带断续出露的寒武-奥陶纪滩间山群火山-沉积岩系是早古生代时期原特提斯洋俯冲造山作用的产物，也是中国西部块状硫化物和造山型金矿床的重要含矿岩系。前人已对滩间山群火山岩组开展了大量的地球化学和同位素年代学研究，而碎屑岩组的沉积序列和沉积相研究相对薄弱。我们在柴北缘托莫尔日特地区地质填图过程中，对该地区的滩间山群碎屑岩组开展了系统的沉积组合序列和沉积相研究。托莫尔日特地区滩间山群碎屑岩组是由富含火山碎屑物质的含砾砂岩、砂岩、粉砂岩、泥岩、沉凝灰岩、硅质凝灰岩、硅质岩及少量砾岩共同组成。可分为下岩性段和上岩性段，其中下岩性段为一套形成于海底扇外扇环境以沉凝灰岩为主的沉积组合；上岩性段包含海底扇内扇、中扇和外扇沉积，且以中扇环境的沉积为主。垂向上，它们呈现出粒度向上逐渐变细特征；空间上，具有向北西方向火山物质成分逐渐增多、砾岩和砂岩厚度变薄且砾岩消失的变化特征。砂岩主要为岩屑长石杂砂岩，其中岩屑主要是安山岩和少量玄武岩、英安岩、凝灰岩；长石主要为斜长石；石英碎屑相对缺失。砾岩中砾石以安山岩和硅质岩为主，并含有少量灰岩和凝灰岩砾石。底冲刷面、正粒序、平行层理和波纹层理等沉积构造普遍发育，呈现出典型的浊流沉积特征；同时在局部露头见有滑塌构造和波痕。区域上，这套沉积组合序列位于寒武-奥陶纪岛弧火山岩的南侧；古水流分析显示，它们的碎屑物质主要来自于其北东方向。这些结果表明，柴北缘构造带东段的滩间山群碎屑岩组是一套成熟度极低且与寒武-奥陶纪岛弧火山活动密切相关的沉积组合。;Volcano-sedimentary system of the Cambrian-Ordovician Tanjianshan Group in the northern margin of Qaidam basin was formed by the subduction of the Proto-Tethys Ocean during Early Paleozoic and hosted massive sulfides and orogenic gold deposits in western China. Much attention had been paid to geochemistry and isotopic dating of the volcanic Formation of the Tanjianshan Group, but the sedimentary sequence and facies of the associated clastic rocks received less care. Systemic studies on the sedimentary sequence and facies in the processes of the geological mapping demonstrate that the clastic Formation in the Tuomoerrite area consists of pebbly sandstone, sandstone, siltstone, mudstone, tuffite, siliceous tuff, chert and minor conglomerate. This formation can be subdivided into two lithologic units. The lower unit is mainly dominated by tuffite deposited in the lower submarine fan environment, whereas the upper unit is deposited in the upper, mid, and lower fan environments. In contrast, the rock assemblages of the upper unit are mainly composed of the mid-fan deposits. The upper unit is characterized by fining-upward sequence and increasing abundance of volcanic composition northwestward, but the thickness of sandstones decreases and conglomerate disappears to northwest. Sandstones mainly comprise lithic greywacke. Lithic fragments contain andesite, basalt, dacite, and minor tuff and limestone. Feldspar fragments are dominated by plagioclase grains, and less monocrystalline quartz grains with an igneous origin occur in several samples. Clasts of conglomerates are dominated by angular andesite, chert and minor limestone and tuff. Irregular basal erosional surface, normal grading, parallel laminations and ripples are widespread show typical of turbidite. Slumping folds and wave marks can also be observed in the local outcrops. Regionally, volcano-clastic rocks distributed in the southern margin of Cambrian-Ordovician Keke island arc with a southwestward paleocurrent. These results demonstrate that the clastic formation is closely related to the Cambrian-Ordovician island arc.

Geochronology and geochemistry of the Early-Middle Permian metamorphic volcano-clastic rocks, northern Liaoning Province: Implications for the tectonic evolution of the eastern segment of the northern margin of the North China Craton

新识别的"下二台"构造杂岩作为华北板块北缘东段分布的构造混杂岩带重要组成部分，其物质组成、形成时代和构造属性仍需进一步研究，这将为探讨华北板块北缘东段晚古生代构造演化提供重要依据。作者在"下二台"构造杂岩中识别出一套早-中二叠世变质火山-碎屑岩，其以变质碎屑岩为主，并夹变质火山岩，二者在野外产出上混杂在一起。变质火山岩原岩类型包括流纹岩、英安岩、安山岩、玄武安山岩，为一套钙碱性火山岩，属于准铝质-弱过铝质岩石。根据岩相学和地球化学特征，将其分为变质酸性火山岩和变质中-基性火山岩；二者均相对富集轻稀土元素，亏损重稀土元素，轻重稀土元素分馏明显，Eu负异常不明显，但变质酸性火山岩明显亏损P、Ti元素，结合高场强元素相关性特征，认为二者不是同一基性岩浆分异的产物。变质火山岩锆石LA-ICP-MS U-Pb同位素年龄为272~288Ma，代表其原岩结晶年龄，时代为早二叠世；变质酸性火山岩原始岩浆来源于地壳物质的部分熔融，变质中-基性火山岩原始岩浆来源于岩石圈地幔（俯冲带附近），并遭受了地壳物质的混染，二者均形成于活动大陆边缘火山弧环境。变质碎屑岩原岩恢复为泥砂质沉积岩和砂泥质沉积岩，相对亏损轻稀土元素，富集重稀土元素，轻重稀土元素分馏较明显，Eu异常不明显。两件碎屑岩样品锆石LA-ICP-MS U-Pb同位素年龄主要介于267~347Ma，推断其沉积下限为267Ma和269Ma，均为中二叠世；泥砂质沉积岩可能来源于再旋回的以长英质岩石为母岩的沉积岩，砂泥质沉积岩可能来源于再旋回的以长英质和镁铁质岩石为母岩的沉积岩，二者分别形成于活动大陆边缘大陆岛弧和大洋岛弧环境。下二台地区早-中二叠世变质火山-碎屑岩为"下二台"构造杂岩重要组成部分，它表明二叠纪时期华北板块北缘东段经历了三个构造演化阶段：早二叠世古亚洲洋加速俯冲，形成新的大陆弧阶段；中二叠世古亚洲洋持续俯冲，大陆弧和大洋弧碰撞阶段；晚二叠世陆-陆碰撞前阶段。

A preliminary study on the element abundance in the Hulusimpang Formation, Way Kalianda, Pesawaran, Lampung, Indonesia

Hulusimpang Formation.has known as Oligocene-Miocene rocks that consisted of volcanoclastic rock. Its scope was wide, especially in southern Sumatra. This formation is supposed as a prospect host of various hydrothermal mineralization. A preliminary study was carried out to examine the abundance of elements located in Way Kalianda River, Pesawaran, Lampung. This study was conducted by easuring its stratigraphy and analyzing its composition using an X-Ray fluorescence analyzer. The lithologies generally consist of lapilli tuffs, volcanic breccias, interbeded by claystone and sandstone; in addition, it is also frequently found petrified wood and andesitic-lithic fragments. As a result, the significant abundant elements are Fe (35.5%), Si (27.9%), Al (17.4%), K (6.7%), Cl (5.5%), Ti (1.7%), and Ca (1.5%) and also Mn, Ag, P, Mg, Sr, Zr, and Co. It also presents trace elements such as Rb, Zn, Pb, Te, V, Ba, Cr, Sn, Ni, Ga, Nb, Mo, and Eu. These elements are suggested from distal facies of intermediates-magma series Tertiary volcano.

40Ar/39Ar AGES OF HYDROTHERMAL FLUIDS OF CABAÇAL Au-Zn-Cu DEPOSIT: IMPLICATIONS OF THE CACHOEIRINHA OROGENY ON MINERALIZATION EVENT IN SW AMAZONIAN CRATON

The Cabaçal deposit is located in SW Amazonian Craton, Mato Grosso State, Brazil, with important Au-Zn-Cu mineralization. The ore is hosted by the Alto Jauru orogenic rocks (1.79-1.74 Ga) comprised of felsic volcanic and volcanoclastic rocks and mineralizations occur as bands concordant with the mylonitic foliation, breccias, quartz-carbonate veins and rock-disseminated, and the polymetallic ore comprises chalcopyrite, pyrite, marcasite, pyrrhotite, sphalerite and galena. The mineralization is related to hydrothermal alteration and includes quartz, chlorite, carbonate, sericite and biotite. Six sericite grains from the hydrothermal zones yielded 40Ar/39Ar ages from 1521.3 ± 1.3 Ma to 1510.4 ± 1.2 Ga. These ages are evidence that hydrothermal solutions originated related to the Cachoeirinha orogen (1.58-1.52 Ga) cutting the Alto Jauru orogen (1.79-1.74 Ga) host rocks. Detailed geology and petrography indicate banded iron formation played an important role in the metal deposition as a chemical barrier for the mineralizing fluids.

Perlite deposits of the Central Slovakia Volcanic Field (Western Carpathians): Geology and properties

Perlites in the Central Slovakia Volcanic Field are associated with with rhyolite dykes, cryptodomes, extrusive domes, coulées and volcanoclastic rocks of the Jastrabá Fm. (12.3–11.4 Ma). From numerous occurrences only the Lehôtka pod Brehmi (LPB) and Jastrabá (JST) represent deposits of economic interest. The LPB deposit exploits a pile of extruded hyaloclastite breccia composed of grey porous and dark dense fragments. The JST deposit exploits glassy rhyolite breccia composed of grey porous fragments associated with an extrusive dome/coulée. The perlites at both deposits are peraluminous, calc-alkaline of the high-K type, poor in phenocrysts (around 5 %) of plagioclase, biotite and minor amphibole (LPB) or sanidine/anorthoclase (JST). Glass at both deposits is silica rich (75.4–79.5 wt. % dry) with Al2O3, K2O and Na2O as other major constituents. It is inhomogeneous showing domains enriched in Na2O or K2O. Glass water content (3.0–6.0 wt. %) shows a weak positive correlation with its silica content and a negative correlation with its Na2O content. Perlites show porosities of 5–16 % (dark dense), 16–30 % (grey porous) and 30–44 % (pale grey ­pumiceous). Narrow stretched pores represent remnants after outgassing of ascending magma while open undeformed pores grew at a low pressure before quenching. The transformation of volcanic glass into perlite took place owing to the hydration by heated fluids of meteoric origin. The hydration was supported by a significant porosity with inter­connected pores and by sustained elevated temperature. Perlites at both deposits show a low content of tightly-bound water and a low Na/K ratio. These properties are responsible for their relatively low degree of expansion. On the other hand, due to the same reason, the perlites have a good mechanical stability.

Ordovician volcanic rocks record rifting, Variscan metamorphism and gold mineralization processes (Truchas Syncline, NW Iberia, Spain)

Ordovician volcanic rocks outcrop in several locations of the NW Iberian Variscan belt. Their composition is mainly basaltic (with less acid types) and occur as volcanic-volcanoclastic layers within a shale-slate succession. This work focuses on volcanic and related rocks within a prominent Variscan structure, the Truchas Syncline. We studied field relations, petrography, mineralogy, geochemistry and conducted thermodynamic modelling to review the petrogenesis and establish the evolution of these volcanic rocks classified as within-plate alkaline basalts (high Ti/Y, Nb/Y and Nb/Yb). Crustal contamination is absent given the elevated Nb/La ratio (1–1.5). These features indicate low melting degrees of the upper mantle and a continental rifting environment. The finding of Ordovician orthid brachiopods in some of the volcanoclastic rocks suggests a shallow marine environment for the volcanic deposition. Variscan metamorphism occurred at lower greenschist conditions with chlorite-temperatures of 374 ± 6 °C. Quartz + carbonate veins indicate that H2O–CO2 metamorphic fluids traversed some volcanic rocks, reacting with Ca–Fe–Mg phases to produce carbonates (Mg-calcite–Fe-dolomite). For this event, T-XCO2 modelling indicates temperatures below 350–360 °C and fluid XCO2 between 0.10 and 0.45. Such fluids can be important carriers of Au and might explain gold deposits in adjacent quartzites. Metasomatic shales (Fe-chlorite + quartz) outcrop nearby and were derived from a mixed protholith of shales and minor volcanic components. Its geochemistry shows Fe enrichment and high peraluminosity. Variscan deformation further modified its geochemistry causing Si-depletions and relative increases of other elements (K, Na, Ti, Al, Rb, Sr, Ba and LREE) in shear zones domains.

Hydrothermal evolution and ore genesis of the Jiamante Au deposit in Western Tianshan, Xinjiang, China: Evidence from the geology, fluid inclusions, and C–H–O–S–Pb isotopes

The Jiamante Au deposit is one of the important deposits in the Axi ore cluster of the Western Tianshan, Xinjiang. The orebodies occurring mainly as quartz‐sulphide veins are hosted in Late Devonian volcanoclastic rocks and granitic porphyries, and structurally controlled by a group of NNW‐ and NNE‐ trending extensional faults and volcanic breccias. Primary metallic minerals are chalcopyrite, galena, pyrite, sphalerite, and native gold, with minor amounts of bismuthinite and bismuth. Nonmetallic minerals are dominated by quartz, sericite, chlorite, and calcite. Medium to low temperature hydrothermal alterations are well developed and characterized by silicification, beresitization, phyllic alteration, and chloritization. Two hydrothermal stages were identified: (a) the main‐ore quartz‐sulphide stage; and (b) the post‐ore quartz‐calcite stage. Microthermometric measurements indicate that the primary biphase liquid‐rich aqueous fluid inclusions within quartz of the main‐ore stage homogenize at temperatures (Th) of 176 to 251°C and have salinities of 2.1 to 7.4 wt% NaCleqv. A lower range of homogenization temperatures (169 to 212°C) and salinities (1.6 to 5.2 wt% NaCleqv) were obtained in calcite of the post‐ore stage. The fluid characteristics of moderate to low temperatures and salinities imply that the ore‐forming process occurred in an epithermal environment. In addition, there is a positive relationship between Th and salinities consistent with the fluid mixing trend. Together with H–O isotopic data (δDH2O = −118 to −97‰ and δ18OH2O = −5.5 to −1.6‰) and fluid characteristics, it is suggested that the ore‐forming fluids were a mixture of magmatic and meteoric waters. The δ13C values for CO2 in the hydrothermal fluids range from −3.2 to −2.7‰, and the sulphides have δ34S values of −5.2 to 2.9‰ with relatively uniform Pb isotopic compositions (206Pb/204Pb = 18.228 to 18.287, 207Pb/204Pb = 15.574 to 15.617, and 208Pb/204Pb = 38.151 to 38.251), indicating that the ore‐forming materials were magmatic sources that were derived directly from magmatic fluids and/or leached from volcanic‐subvolcanic rocks by hydrothermal fluids. Combined with the evidence from the mineralization and alteration, fluid inclusions, and C–H–O–S–Pb isotopes suggest that the Jiamante deposit is an intermediate‐sulphidation epithermal Au deposit. A combination of fluid mixing and fluid–rock interactions is inferred as the important factors causing ore deposition. According to the close spatial and temporal relationship between intermediate‐epithermal deposits and porphyry deposits, it is further suggested that porphyry‐type mineralization should have a good potential at the deep sites of the Jiamante deposit and its vicinity.

The Main Types of Paleogene Sedimentary Rocks and Conditions of their Formation on the Guyots of the Magellan Seamounts (Pacific Ocean)

he Paleocene–Eocene sedimentary complex is most widely developed among the sedimentary cover deposits of the Magellan Seamounts. It is composed of pelagic (nanoforaminiferal) and reef limestones, edaphogenic breccias and volcanoclastic rocks. The most widespread pelagic limestones form narrow (0.5–1.5 km) and extensive (up to 20 km) ribbon-like bodies that cover the upper parts of the slopes and the rim of the summit plateau. The maximum area of their distribution, which covers 315 km2, is noted on the Fedorov Guyot. Pelagic limestones and edaphogenic breccias outcrop over large areas and have reciprocal facies transitions. The Oligocene hiatus in sedimentation has been established in the sections of the Cenozoic sedimentary cover deposits of the studied guyots.

Petrology and geochemistry of volcanic and volcanoclastic rocks from Zhob ophiolite, North-Western Pakistan

The Zhob ophiolite is divided into Ali Khanzai, Omzha, and Naweoba blocks. The ophiolite geology comprises various lithological units including basalt–chert and hyaloclastite mudstone units. The basalt–chert and hyaloclastite mudstone units consist of thick lava and pelagic sediments. On the basis of petrology and geochemistry, the lavas of basalt–chert unit can be divided into tholeiitic basalt, trachybasalt, basaltic andesite and dacite, and that of hyaloclastite mudstone unit into more alkaline foidite, picrobasalt, and tephrite-basanite. The tholeiitic rocks have a flat N-MORB-normalized pattern with enrichment of Th and depletion of Nb compared to other immobile elements and thus indicate a subduction zone component in the rocks. They have chondrite-normalized REE patterns typical of N-MORB. The alkaline rocks have depleted chondrite-normalized HREE compared to N-MORB similar to those of OIB. Our geochemical results suggest that the tholeiitic rocks may have formed in a supra-subduction zone setting while the alkaline rocks are intraplate setting that was influenced by a subduction component. The Zhob lavas therefore are likely to represent the floor of a branch of the Ceno-Tethys Ocean and may have obducted over the Indian Plate passive continental margin during the Late Cretaceous.

ОСНОВНЫЕ ТИПЫ ОСАДОЧНЫХ ПОРОД ПАЛЕОГЕНА И УСЛОВИЯ ИХ ОБРАЗОВАНИЯ НА ГАЙОТАХ МАГЕЛЛАНОВЫХ ГОР (ТИХИЙ ОКЕАН)

The Paleocene-Eocene sedimentary complex is most widely developed among the sedimentary cover sediments of the Magellan Seamounts. It is made up of pelagic (nanoforaminiferal) and reef limestones, edaphogenic breccias and volcanoclastic rocks. The most widespread pelagic limestones form narrow (0.5–1.5 km) and extensive (up to 20 km) ribbon-like bodies which cover the upper parts of the slopes and the rim of the summit plateau. The maximum area of their development is marked on Fyodorov Guyot – 315 km2. Pelagic limestones and edaphogenic breccias are exposed over large areas and have mutual facies transitions. The Oligocene hiatus in sedimentation was established in the sections of the Cenozoic sedimentary cover deposits of the studied guyots.

Geochemical evolution of soils developed from pyroclastic rocks of Trindade Island, South Atlantic

The geochemical behavior of the major, minor, trace and rare earths elements (REEs) in soil profiles from ultramafic volcanoclastic rocks of the Vulcão do Paredão and Morro Vermelho Formation from Trindade Island (TI) was analyzed in this study. Losses and gains of chemical elements were calculated through the mass balance for two profiles along the slope: one located at higher altitude (460 m) and the other at lower altitude (258 m). In all profiles, Al, Fe and Ti accumulate due to their low mobility, whereas Ca, Na, K and Mg are the most intensely leached. Soils located at lower altitude show higher K and Mg values in the surface due to the contribution of saline sprays. Leaching of the REEs from higher to the lower slope led to the enrichment of these elements, especially the light REEs, in the soil at the lowest altitude (258 m). The high altitude profile showed Ce positive anomaly due to longer exposure to weathering. The geochemical balance shows a relative enrichment of Ti, Mn, Fe, Co, Cr, Ni, V, Zr, S related to the loss of mobile elements during the soil formation process, despite the youthful nature of these volcanic rocks.

Evidence of iron oxide-copper–gold mineralization in the Torud-Chahshirin Magmatic Belt, northern Iran: Insight from the Robaie area

The Torud-Chahshirin Magmatic Belt in northern Iran is defined by Tertiary porphyry, IOCG, Skarn, Manto, MVT, and Au, Ag and base metal-rich epithermal systems, and it is a part of the eastern Alborz Orogenic Belt of Iran. Mineralization in the Torud-Chahshirin Magmatic Belt occurs as veins, veinlet, and disseminated mineralization hosted by the Eocene-Oligocene volcanic, subvolcanic, and volcanoclastic rocks. Mineralization prospects of the Torud-Chahshirin Mgmatic Belt are spatially and temporally accompanied with magmatic rocks of early to middle Eocene (∼55–37 Ma.) calc-alkaline and early to late Oligocene (∼34–24 Ma.) alkaline rocks that have continually evolved through assimilation and fractional crystallization (AFC) processes at shallow crustal levels. The Robaie area is located in the Torud-Chahshirin Magmatic Belt. The dominant stratigraphic unit is composed of Eocene volcanic rocks (trachyandesitic to andesitic in composition), which are intruded by subvolcanic rocks. The subvolcanic rocks have features typical of shoshonitic affinity and are metaluminous to weakly peraluminous. These rocks are characterized by enrichment in large-ion-lithophile elements (LILEs) and light rare-earth-elements (LREEs), depletion of heavy rare-earth-elements (REEs, LaN/YbN ≈ 9.65–24.34), and high-field-strength-elements (HFSEs). Values of εNd(i) commonly show a small variation between +0.2 and +0.7, whilst 86Sr/87Sr (i) goes from 0.7056 to 0.7063. The whole set of evidence agrees with derivation of intrusive rocks of Robaie from primitive magma resulting from partial melting in a mantle wedge above a subduction zone. A sample from a dyke of biotite-hornblende diorite porphyry, with a negative εNd(i) value (−1.7), probably corresponds to a magma that underwent assimilation of crustal material. U-Pb data of zircons from dioritic stock and dyke show ages of 49.99 Ma and 50.43 Ma, respectively. Mineralization in Robaie is related to the biotite-hornblende diorite intrusions and is associated with hydrothermal alterations including argillic, argillic-silicification, argillic-sericitic, and propylitic alteration. The ore-bearing veins, veinlets, and disseminated mineralization are controlled by structures. There are two types of veins and veinlets mineralization in the Robaie area including: 1) quartz-chalcopyrite-pyrite-specularite ± gold veins, and 2) quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets. The highest amount of copper (>1%) and gold (>5 g/t) is associated with quartz-chalcopyrite-pyrite-specularite ± gold veins. Microthermometric study of fluid inclusions shows that homogenization temperatures are 181–422 °C and salinities of ore-forming fluids ranged from 11.1 to 17 wt% NaCl equivalent. The temperature and salinity of ore-forming fluids were lower in the formation of the quartz-chalcopyrite-pyrite-specularite ± gold veins than in the quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets, and fluid mixing was the most important mechanism for mineralization in the area. The mineralogy, alteration, geochemistry, fluid inclusion studies, and petrogenesis of subvolcanic rocks of the Robaie area indicate that the Robaie deposit could be classified as a member of the Iron Oxide-Copper-Gold (IOCG) deposits.

Mineralogical and geochemical properties and genesis of kaolin and alunite deposits SE of Aksaray (Central Turkey)

The kaolin and alunite deposits, situated approximately 20 km southeast of Aksaray in Central Anatolia, were formed by hydrothermal alteration related to extensional tectonics from the Upper Miocene to Quaternary, mostly from rhyolitic volcanoclastic rocks. The mineralogical and geochemical properties of the deposits were investigated to explain the stages of hydrothermal alteration and the origin of the deposits. X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical and stable isotope (O, H, and S) studies were carried out on the alteration minerals (e.g., kaolinite, gypsum, alunite, native sulfur and smectite) of the deposits.Kaolinite/halloysite minerals were determined along with mostly alunite- and silica-group minerals (α-quartz, opal-CT, and tridymite), feldspar, native (elemental) sulfur, and partial hematite, goethite, gypsum, cinnabar and realgar. The δ34S isotope values for alunite, gypsum and native sulfur are in a fairly narrow range and vary from 5.03 to 6.62‰, 4.5–5.24‰ and 5.97–7.49‰, respectively. The δ18O values of the kaolinites and smectites vary from +1.8 to +13.3‰ and from −98 to −152‰, respectively, and their δD values range from −4.2 to 15.7‰ and from −93 to −191‰, respectively, showing a spread that is very close the primitive magmatic water line. From the δ18O values, the calculated formation temperatures of the kaolinites and smectites range from 38 to 135 °C and from 29 to 189 °C, respectively. The δ18O values of the alunites and amorphous silica vary from 4.4 to 8.9‰ and from −1.1 to 8.4‰, respectively, and the δD values vary from −72 to −87‰, and from −104 to −191‰, respectively. In addition, the formation temperatures of alunite and amorphous silica minerals range from 57 to 100 °C and from 139 to 281 °C, respectively. Hydrothermal alteration progressed in different stages between 29 and 281 °C according to the calculated modal formation temperatures. The isotopic mineral data suggest that the original hydrothermal waters were a mixture of magmatic and meteoric waters. Oxygen and deuterium analyses indicate that kaolinite and alunite formed at lower temperatures than smectite and silica minerals. The determined stable isotopic compositions of the minerals indicate that hydrothermal solutions at different compositions and temperatures played important roles in the formation of minerals. The determined neoformed minerals may have been formed by steam-heated hydrothermal solutions sourced from rhyolitic magma and are mostly of hypogene origin.