Volcanic Breccia Research Articles

Post-Cretaceous brittle tectonics in the Tunas Alkaline Complex, Paraná, Brazil

We present a study of the brittle tectonics of Tunas Alkaline Complex focusing on the kinematic and geometric analysis of faults and brittle shear zones deforming the complex. The data acquired and respective interpretations sought to contribute to the post-Cretaceous tectonic structural models of southern Brazil. The ~83 Ma-old alkaline complex is made up of syenites, alkali syenites, plutonic and volcanic breccias, as well as trachyte, microsyenite, and bostonite dykes. Morphostructural analysis using remote sensing combined with right dihedral kinematic analysis of fault-slip data and relations of intersections among structures allowed us to characterize four Cenozoic paleostress fields and their respective tectonic pulses: (I) Eocene-Oligocene NW-SE extension responsible for normal oblique-slip faults striking N60-70E; (II) Oligocene-Miocene NE-SW compression associated with tectonic accommodation of the South American Plate at circa 27-26 Ma, and responsible for WNW-ESE-trending sinistral and NE-SW-trending dextral strike-slip faults; (III) Plio-Pleistocene N-S to NNW-SSE compression related to dextral reactivation of NW-SE-striking faults, sinistral reactivation of NE-SW-trending faults, and generation of NNW-SSE-trending strike-slip faults; and (IV) Pleistocene-Holocene NW-SE to WNW-ESE compression that caused dextral reactivation of ENE-WSW-trending faults and sinistral strike-slip reactivation of NNW-SSE-trending faults.

Chronology and geochemical characteristics of the highly fractionated alkali feldspar granite from the Maodeng deposit in the southern Great Xing'an Range

毛登矿床位于内蒙古锡林浩特境内，是大兴安岭南段典型的钼铋锡铜矿床，产于阿鲁包格山杂岩体晚阶段侵位的碱长花岗岩和花岗斑岩以及下-中二叠统大石寨组火山角砾岩中。本文以碱长花岗岩为主要研究对象，开展了岩石地球化学、Sr-Nd-Hf同位素、锆石U-Pb定年和微量元素地球化学研究，以探讨成岩时代、岩浆结晶的物理化学条件及其对成矿的制约。研究显示，碱长花岗岩锆石结晶年龄为140.5±0.8Ma，侵位于早白垩世。岩体高硅，富碱，贫钙、镁、铝，富集Rb、F、Th、Nd、Sm、Zr和Hf等元素，亏损Eu、Ba、Sr、P、Nb、Ti和轻稀土等元素，轻、重稀土分馏较小，具稀土四分组效应。碱长花岗岩锆石Ti温度为654~817℃，Ce⁴⁺/Ce³⁺值为0.1~19，岩浆氧逸度lgf（O₂）为-25.7~-18.6，表明原生岩浆为高温、低氧逸度的熔体。岩体具较高的ε_Nd（t）值（-0.1~+2.9）和正的ε_Hf（t）值（+4.0~+9.3）以及年轻的二阶段模式年龄（t^Nd_DM2=699~940Ma；t_HfDM^C=600~936Ma），表明岩浆源区来自含大量幔源组分的新生地壳的部分熔融。碱长花岗岩经历了高度分异演化和熔体-流体作用，具高F、低氧逸度特征，是毛登矿区钼、铋、锡、铜金属矿化形成的重要先决条件。;Located in Xinlinhot of Inner Mongolia in the southern segment of Great Xing'an Range, the Maodeng deposit is a typical Mo-Bi-Sn-Cu deposit occurred in the alkali feldspar granite and granite porphyry in the late emplacement of Alubaogeshan complex, as well as the volcanic breccia of Lower-Middle Permian Dashizhai Formation. In this paper, whole-rock geochemistry, Sr-Nd-Hf isotope, zircon U-Pb age and trace element geochemistry of alkali feldspar granite have been studied, aiming to investigate the rock-forming age, the formation conditions, the significance for petrogenesis and metallogenesis. The zircon dating yields a weighted mean U-Pb age of 140±0.8Ma, which indicates the emplacement of the alkali feldspar granite took place during the Early Cretaceous. Geochemically, the alkali feldspar granite is characterized by high concentrations of SiO₂ and (Na₂O+K₂O), with low contents of CaO, MgO and Al₂O₃. The rock is enriched in Rb, F, Th, Nd, Sm, Zr and Hf, while depleted in Eu, Ba, Sr, P, Nb, Ti and LREE. It is especially characterised by a low LREE/HREE ratio, with a huge Eu depletion and a REE tetrad pattern. The zircon of the alkali feldspar granite shows high temperature (654~817℃) and low Ce⁴⁺/Ce³⁺ ratio (0.1~19), oxygen fugacity (lgf(O₂)=-19.6~-14.4) of the magma. The granite has relatively high ε_Nd(t) (-0.1~+2.9), positive ε_Hf(t) (+4.0~+9.3) and young two-stage Nd and Hf model ages (t^Nd_DM2=699~940Ma; t_HfDM^C=600~936Ma), suggesting that the rock is dominantly derived from the partial melting of a juvenile lower crust which was originated from source rocks or magma separated shortly from the upper mantle, followed by strong fractional crystallization during magma ascent. Based on the above reasearch, it is proposed that under the condition of relatively low oxygen fugacity and high temperature, the magma evolves by highly fractional crystallisation and melt-fluid interaction during the late stage. Therefore, it is believed that the alkali feldspar granite plays an important role in Mo-Bi-Sn-Cu mineralization in the Maodeng deposit.

ANALISIS BAHAYA, LINTASAN, DAN SISTEM PROTEKSI TERHADAP POTENSI LONGSORAN TIPE JATUHAN BATU PADA LERENG BANGUNAN PELIMPAH BENDUNGAN TUGU, JAWA TIMUR

The construction of the Tugu Dam spillway does not escape the problem of slope instability, especially the rock fall type landslide as a result of the rock slope cutting work at STA+80. The purpose of this study was to determine the characteristics of the rock discontinuity area and the solutions needed to address the potential hazards of rock fall on the slopes of spillway structure. In this study, a semi-quantitative method conducted based on the Rockfall Hazard Rating System (RHRS) which is carried out by identifying outcrops on rock slopes. Determination of the rock fall trajectory, was conducted by statistical methods on rock mass based on changes in velocity when rocks roll, slide, and bounce. Geologically, the research area belongs to the Mandalika Formation. Based on the RHRS weighting, the total score on the STA+80 slope is 399, which means that the slope needs to be repaired or given safely with a moderate level of urgency. The rock fall trajectory modeling at the measurement location X = 121,875 has a kinetic energy of 973.14 kJ andesite and 72.59 kJ of volcanic breccia, for high results of 0.43 meters of andesite reflection and 2.04 meters of volcanic breccia, and velocity results translational velocity obtained at 33.8 m/s andesite and 8.67 m/s volcanic breccia. The potential for rock fall requires a safety system with a type of retained flexible barriers with a height of 5 meters that can be applied to the toe of the slope.Keywords: rock fall, discontinuity, trajectory, protection system, Tugu Dam

Submarine volcanic activity and giant amygdale formation along the Panama island arc as a precursor to 6000-year-old agate exploitation on Pedro González Island

AbstractAn extensive deposit of agate occurs in Pedro González Island in the Gulf of Panama. Previous archaeological research showed that the agate was exploited between 6200 and 5600 cal BP to make stone tools found at the oldest known Preceramic human settlement in the Pearl Island archipelago. We constrain here the origin and geological context of the agate through a geological and geochemical study of the island. We show that it includes primary volcanic breccias, lavas, and tuffaceous marine deposits with sedimentary conglomerates and debris flow deposits, which we define as the Pedro González Formation. This formation records submarine to subaerial volcanic activity along an island arc during the Oligo-Miocene, confirming previous regional models that favour progressive emergence of the isthmus in the early Miocene. The igneous rocks have an extreme tholeiitic character that is interpreted to reflect magmatic cessation in eastern Panama during the early Miocene. The agate is hosted in andesitic lavas in unusually large amygdales up to 20–40 cm in diameter, as well as small amygdales (0.1–1.0 cm) in a bimodal distribution, and in veins. The large size of the agates made them suitable for tool manufacture. Field evidence suggests that the formation of large amygdales resulted from subaqueous lava–sediment interaction, in which water released from unconsolidated tuffaceous deposits at the base of lava flows rose through the lavas, coalesced, and accumulated below the chilled lava top, with subsequent hydrothermal mineralization. These amygdales could therefore be regarded as an unusual result of combined peperitic and hydrothermal processes.

Rock Slope Stability Evaluation on The Construction of New Road Shortcut 4 Border City of Singaraja – Mangwitani, Bali

Shortcut 4 new national road development project on Singaraja - Mangwitani section Bali found a potential rock slide slope problem. An outcrop of igneous rock with an intensive joint was not expected to be encountered previously. The excavation work in road construction had to pay attention to the stability of the resulting rock slope considering that apart from the potential for slope failure, rock slope could also threaten the bridge abutment building in front of it. The location of the rock slope was on the edge of Lake Bratan which is geologically part of the early Holocene volcanic rocks, namely mountain rocks composed of tuff, lava and volcanic breccia. Anisotropic andesite slope was controlled by a discontinuous plane with a certain pattern. Rock Quality Assessment was carried out by the Rock Mass Rating (RMR) method and Slope Mass Rating (SMR) for slope stability evaluation. The planar, tople and wedge potensial slope failure were evaluated. The potential for planar slope failure has a value of SMR 30.18 (Unstable), 57.6 (Partialy Stable) for wedge slope failure potential and 47.6 (Partialy Stable) for tople slope failure potential. The SMR value indicated that the rock slope requires engineering threatment to become stable.

Lower to Middle Jurassic volcanism and Au–Ag mineralization at Cerro Moro District, Deseado Massif, Argentine Patagonia

In the Cerro Moro Mine area, a horst block exposes a volcanic sequence with ages ranging from 191 Ma to 167 Ma, extending the Jurassic volcanic history of the Deseado Massif back to the Lower Jurassic (Sinemurian-Pleinsbachian boundary). Three volcanic sequences are recognized: a lower volcanic complex comprising welded tuffs, rhyolitic volcanic breccias, andesite lavas and intercalated continental sediments from 191 to 186 Ma, an intermediate sequence of welded ignimbrites, block-and-ash flow deposits and epiclastic sediments with ages ranging from 182 to 181 Ma, and an upper sequence of rhyolitic domes and welded ignimbrites from 180 to 167 Ma. Epithermal vein mineralization was formed in a series of pulses beginning at 180 Ma and culminating in a bonanza event at 171 Ma, coincident with the eruption of the upper volcanic sequence. Increasing grades of precious and base metals with time leading up to the bonanza event are interpreted as the result of repeated fertilization of the hydrothermal system by successive fluid pulses. Detrital, entrained and inherited zircons in the Jurassic rocks have ages ranging from the Triassic to Paleoproterozoic.

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.

Andesite prospect at West Sungkai of North Lampung: Its distribution based on electrical resistivity tomography

Infrastructure construction made andesite’s demand has increased, particularly in Lampung Province. In this research, its distribution in West Sungkai of North Lampung is mapped based on Electrical Resistivity Tomography (ERT) data from 6 lines, each of them was 186 m in length. Due to its excellent vertical resolution, Wenner configuration is performed. The research area is part of Quarter Holocene Volcanic (Qhv) formation. Lajur Barisan members consist of volcanic breccia, lava, and andesite-basalt tuff; thus, resistivity modeling is built within this aisle. Subsurface resistivity model has been created using the non-linear inversion method with promising low error at the third to fifth iterations, which marks an acceptable value. Using 2D and 3D ERT modeling, it is estimated that there are three mains of rocks based on their resistivity value: sandy tuff with 65 – 212 Ω m; tuff with 212 – 655 Ω m; and andesite with resistivity more than 655 Ω m. Andesite within this area is likely lava andesite which spread from the middle to the West and north. It is located at 5 – 35 m in depths with the reserve estimation of andesite is about 1.65 million tons.

Study of Hydrothermal Alteration and Mineralization in the Lahbako Field, Jember Regency, East Java Province

Administratively, the research location is in Lahbako Field, Jember Regency, East Java Province. This study discusses the geological characteristics, hydrothermal alteration, and mineralization in the study area. The method used is field data collection by mapping and laboratory analysis in the form of petrographic analysis and mineragraphic analysis. The research area is divided into three geomorphology units, namely a unit with steep hills, a unit with undulating hills, and a unit with sloping plains. The stratigraphy of the study area is divided into five-rock units, while the order of rock units from oldest to youngest is the volcanic Breccia unit, the intercalated Sandstone, and Claystone unit with Tuff insertion, the Diorite Intrusion Unit, the Granodiorite Intrusion Unit, and the youngest unit, namely Alluvium Unit. The research area has a high straightness density value and a straight lineage pattern that tends to be dense. The area with high lineament density is assumed to be the area with the best prospect of alteration and mineralization because the lineament reflects the geological structure which is the path of hydrothermal fluid passage that causes alteration and mineralization. Based on petrographic analysis, the research area has undergone alteration with the type of Philic alteration (Chlorite-Quartz-Feldspar), Prophiliic Alteration (Chlorite-Quartz-Feldspar) Feldspar-Quartz-Epidote) and Argillic Alteration (Alunite-Quartz-Biotite). The study area is indicated as an area with high sulfide epithermal mineral deposits where the indication is based on the discovery of vein textures in the form of Vuggy Quartz and ore minerals in the form of Silver, Copper, and Iron in mineragraphic analysis.

Bouguer Anomaly of Geothermal Reservoir at Tiris Area, Probolinggo, East Java, Indonesia

Research related to the geothermal system in the Tiris geothermal area (TGA) Probolinggo Regency has been conducted using the gravity method. This study aims to investigate the subsurface structure, with a target on estimating geothermal reservoir rocks from the study area. This study utilized the Gravity meter La Coste & Romberg type G-1503 on 116 acquisition points in an area of 2.16 km2, covering all geothermal manifestation points in TGA. The gravity measurement data obtained is then processed through gravity corrections, which include: conversion into milli-Gals (mGal) units, tidal correction, drift correction, latitude correction, free air correction, Bouguer correction, and terrain correction. These corrections to obtain a complete Bouguer anomaly (CBA) value. The study area shows the CBA value on a horizontal plane which ranges from 0.1 mGal to 4.2 mGal. The separation of the regional and residual Bouguer anomaly from the CBA on a horizontal plane employed the Moving Average method through spectrum analysis. The value of residual Bouguer anomaly ranges from -0.7 mGal to 2.7 mGal. The low anomalies are scattered in the northwest, and a small number are spread in the northeast and southeast, while the high anomalies are in the middle of the study area. The result of 3D inversion modeling finds that the study area's subsurface structure consists of four rock layers, namely lapilli tuff, tuffaceous breccia, volcanic breccia, and basalt. Volcanic breccia is approximated as geothermal reservoir rocks at a depth of 700 to 1000 meters below the acquisition point. In contrast, basalt is supposed to be intrusive igneous rock because it tends to break through the surface at a depth of 348 to 350 meters below the acquisition point. The presence of these intrusive rocks can be predicted through spectrum analysis result, which shows a regional anomaly source at a depth of 348 meters below the acquisition point. This intrusion rock is suspected to be a heat source rock in the geothermal system in the study area.

Mineralogy and genesis of soils developed from ultrabasic and alkaline rocks of Lages Alkaline Complex, Brazil

In the Plateau of Lages, intrusions of alkaline and ultrabasic rocks occur, rocks of rare occurrence in Brazil, whose soils derived from them are practically unknown. The study aimed to evaluate the chemical composition, mineralogy, genesis, and classification of the soils developed from the rocks Nepheline Sienite (P1), Volcanic Breccia (P2), Kimberlitic Breccia (P3), Phonolitic Tephrite (P4), and Phonolite (P5). After the morphological description (field) and the collection of soil samples, chemical analyses of the soils, mineralogical of the clay fraction and the total elements of the soils and the corresponding rocks that gave rise to them were carried out. Three Humudepts (Cambissolos), a Hapludolls (Chernossolo), and a Kandiudox (Nitossolo) were identified. Except for the P3 profile, the others are acidic soils, with high levels of exchangeable aluminum, poor in basic cations, with low sum and base saturation, showing a high degree of weathering and strong leaching. In the clay fraction, the presence of 2:1 clay minerals predominated with kaolinite with interlayered hydroxy-Al polymers. The P3 profile showed a slightly acidic reaction, high sum and base saturation, and practically zero exchangeable aluminum, a situation compatible with mineralogy, where trioctahedral vermiculite predominated, revealing that it was a young, slightly weathered soil.

STRUKTUR GEOLOGI AREA BENCANA TANAH LONGSOR DI DESA TUMENGGAL, KECAMATAN PENGADEGAN-PURBALINGGA

The Ligung Formation which is predominantly composed of claystone and the Kalibiuk Formation which is composed of volcanic breccias, both of which landslide simultaneously on the hillsides in the Tumenggal area, Pengadegan District, Purbalingga-Central Java. How the influence of the geological structure that has worked to compose the geological order of this area, which has an effect on the occurrence of the landslide, is the study that is revealed in this paper. Field observations of landslide locations were carried out by determining the position of the landslide, determining the type of bedrock, measuring the direction of the slope, measuring the dipping of rock layers and describing the distribution of landslides in the study area. The geological structures found in the study area are rock layers of the Ligung and Kalibiuk Formation and rock fractures/joints. Little dipping structure of the bed was observed in the field while intensive jointing was found in claystone. This structure has accelerated the weathering of claystone to form a thick soil. Andesite breccia found at the bottom of the slope has undergone intensive weathering. Rocks in the study area have a slope to the south and landslides to the north or towards the fore-slope, this is formed due to thick soil and intensive fractures/joints in the bedrock.

Genesis and fluid evolution of the Huangtan Au-Cu deposit in the Kalatag district, Eastern Tianshan, NW China: Constraints from geology, geochronology, fluid inclusions, and H-O-S-Pb isotope geochemistry

The newly discovered Huangtan Au-Cu deposit is located in the central Dananhu - Tousuquan arc of Eastern Tianshan, southern Central Asian Orogenic Belt (CAOB). It is the first Au-dominated volcanogenic massive sulfide (VMS) polymetallic deposit in the Eastern Tianshan. Veinlet-disseminated and massive orebodies are hosted within Early Silurian pyritic phyllic tuff and volcanic breccia and controlled by a secondary fracture zone of the Kalatag fault with extensive hydrothermal alteration. Four primary alteration/mineralization stages have been recognized as follows: (1) Early ore stage (S1), forming mainly ore-barren fine quartz veins with minor gold-bearing sulfides; (2) main ore stage (S2), forming mainly thick quartz veins with abundant coarse-grained subhedral pyrite (S2-1), and plentiful chalcopyrite, sphalerite, barite and Au-bearing sulfide veins (S2-2); (3) late ore stage (S3), which is characterized by plenty of barren quartz–calcite veins with few sulfides; and (4) supergene stage (S4), accompanied by abundant oxide mineralization, including malachite, jarosite and other supergene minerals. From S1 to S3, microthermometric data of fluid inclusions show homogenization temperatures of 263–379 °C (mean = 308 °C), 188-292 °C (mean = 240 °C), and 118-198 °C (mean = 158 °C), respectively, and salinities of 5.3–14.2 (mean = 10.8) wt.% NaCl equiv., 2.8–10.7 (mean = 7.6) wt.% NaCl equiv., and 0.3–14.1 (mean = 2.6) wt.% NaCl equiv., respectively. The ore-forming fluids are characterized by middle-low temperature, low salinity, relatively reduced condition, and an H2O-NaCl ± CO2 ± CH4 system.The δ34S values (−5.25‰ to 0.50‰) and Pb isotopic ratios (206Pb/204Pb = 17.868–19.495, 207Pb/204Pb = 15.446–15.575, and 208Pb/204Pb = 37.350–38.491) suggest that the ore-forming materials came predominantly from a deep-seated magma source with a minor contribution of lower continental crust. The δ18OH2O and δDV-SMOW values of fluid inclusions in each metallogenic stage range from −6.1 to 5.6‰ and −66.8 to −53.9‰, respectively, suggesting the dominant role of magmatic water mixed with convectively circulating heated seawater during fluid evolution. Fluid cooling dilution, local boiling, and fluid mixing were considered as the main mechanisms of metal precipitation. The 40Ar-39Ar plateau age of hydrothermal muscovite from the late ore stage (S3) is 414.4 ± 0.4 Ma, which represents the upper limit age of shallow hydrothermal alteration and Au-Cu mineralization. It is also consistent with the Early Silurian polymetallic metallogenic event in the Kalatag district. The auriferous Huangtan and adjacent Cu-Zn-rich Huangtupo VMS deposits show obvious ore-forming element differences, and constitute a unique VMS metallogenic system in the Eastern Tianshan Orogenic Belt (ETOB), which provides an important research object and new insight for ore prospecting in the peripheral Gobi Desert area.

Interpretation of Magnetic Anomaly Data in the Andesitic Rock Prospect Area of Kutasari Subregency, Purbalingga Regency, Central Java, Indonesia

DOI:10.17014/ijog.8.3.345-357Interpretation of magnetic anomaly data has been carried out in the andesitic rock prospect area, Kutasari Subregency, Purbalingga Regency, Central Java, Indonesia. Geographically, this area is located within 109.2788° - 109.3072°E and 7.3032° - 7.3319°S. The study has been done in April – September 2019 with the purpose to map the distribution of andesitic rocks based on the local magnetic anomaly data. The data that are acquired in this study have the values ranging between -1,238.13 - 1,892.40 nT. The results of qualitative interpretation on the local magnetic anomaly data having been reduced to the pole show the distribution of strong anomalous sources in the northwest area interpreted as massive andesitic rocks. Whereas the results of quantitative interpretation through 2D-forward modeling on the local magnetic anomaly data show six anomalous bodies, with magnetic susceptibility values ranging from 0.0025 to 0.0350 cgs and depths range between 7.16 - 505.97 m. The highest magnetic susceptibility is 0.0350 cgs interpreted as a massive andesite intrusion forming a very dense dike; whereas the lowest magnetic susceptibility is 0.0025 cgs interpreted as undifferentiated igneous rocks, volcanic breccias, lava, and tuff. Based on the study results, the correlation between the results of qualitative and quantitative interpretations occurs.

Microzonation of Cisarua District Using Horizontal Vertical Spectral Ratio

The Bandung region is part of the framework of the Indonesian tectonic system, namely the tectonic plate meeting zone, where the Indo Autralia plate is infiltrated under the Eurasian plate in a convergent manner. The subduction process produces an effect in the form of an active fault geological structure in the Bandung area. One of these active faults is the Lembang Fault, which has a length of ± 29 kilometers and a shear acceleration of 3 to 5.5 millimeters per year. The microtremor measurement method is a passive geophysical method that utilizes natural subsurface vibrations so that it can provide dominant frequency data and amplification factors for soil layers. Based on the results of seismic susceptibility research using microtremor measurements using the HVSR method in the Lembang Fault zone in Cisarua Sub-District, it can be seen that the distribution of the dominant frequency values tends to be influenced by lithology and topography. In the research area, it is known to have a dominant frequency value that varies due to the different types of lithological units. In general, the dominant frequency ranges from 1-3 Hz because it is dominated by tuff sand and tuff pumice, and areas composed of volcanic breccias have a dominant frequency value between 3-6 Hz. Meanwhile, the amplification factor value will be influenced by rock deformation and weathering. The area that has a very high amplification factor value is in the southeast of the study area with an A0 value greater than 5. This indicates that the area is composed of a layer of thick and not dense tuff sand

Storage space, pore–throat structure of igneous rocks and the significance to petroleum accumulation: An example from Junggar Basin, western China

Igneous rocks in deep basins are important unconventional petroleum reservoirs. Storage space and pore–throat structure of igneous rocks are crucial to be considered as effective reservoirs. There developed large-scale petroleum reservoirs in the Carboniferous igneous rocks of Hongshan-Zhongguai Bulge in the Junggar Basin, western China. The storage space and pore–throat structures of various types of igneous rocks and their significance to petroleum accumulation were studied based on the analysis of core samples. The methods include macroscopic core observation, microscopic observation under microscopes, scanning electron microscopy, micro-resistivity imaging logging and mercury injection experiments. Results show that the storage space is made up of primary gas pores, secondary dissolution pores and various types of fractures. The fillings of fractures are mainly calcite, followed by asphalt, chlorite and gypsum. The igneous rocks have different pore-throat structures, which govern the reservoir properties. Four types of igneous rock reservoirs were recognized, and the most favorable type is volcanic breccia and part of andesite. Dissolution plays an important role in remodeling the pore-throat structure and storage space. Tuff is the most susceptible to dissolution and present greatest alteration in storage space among the studied igneous rocks. An igneous rock weathering crust with high porosity formed near the unconformity capping the Carboniferous rocks, and oil is most abundant within 150 m across the unconformity surface. Faults and the weathering crust jointly control the formation of igneous rock reservoirs. The fault-controlled reservoirs commonly form in structural high position, and stratigraphic reservoirs generally developed in slope area.

Rock characterisation for a road tunnel in Mumbai (India)

The Mumbai Coastal Road is a freeway currently being constructed along the western coast of Mumbai. A key element of this route is a twin-bore tunnel with total length of about 3km (a 2km TBM-excavated central section, plus cut-and-cover ramps at both ends). The tunnel alignment underpasses Malabar Hill and then runs under Chowpatty Beach for most of its length. The central section is located below the sea level with ground covers ranging from a maximum of 70m at Malabar Hill to less than 15m under Chowpatty Beach. Each bore, excavated by Tunnel Boring Machine (TBM), has an internal dimeter of 11m and includes a section with a horizontal radius of curvature tighter than 400m. This paper describes (i) the desk studies and the geological survey which provided the basis for an additional ground investigation plan, (ii) the results of the additional boreholes, in situ test and laboratory test, (iii) the overall characterisation of the rock mass (and of the overlying soils) based on the entire set of old and new data. The rock characterisation provided the fundamental information for the TBM type selection, and for the design of all the underground works. The launching chamber and the first part of the TBM drive are located within a Basaltic lava flow, while the second part of the alignment is set in Volcanic Breccia underlying the fine-grained soils and made ground of Chowpatty Beach.

GROUNDWATER MODELING IN SANGON HAMLET AND SURROUNDING AREAS USING FINITE DIFFERENCE METHOD

Sangon Hamlet is included in the mountains which are dominated by andesite rocks and volcanic breccias which cross each other and have a distribution of joints and faults forming a fracture. The fracture is a medium for draining water in the area. There is a difference in ground water level as seen from the well. To re-detail the groundwater flow that has been formed, a groundwater flow pattern modeling is carried out. The modeling is done by numerical method assisted by Modflow Flex Software. For building the model, it is necessary to prepare a concept model, calibration and validation. In this area there are 2 aquifer systems, namely unconfined aquifers and aquitards. Furthermore, the model was calibrated using the sensitivity analysis method. The calibration results are obtained, namely the RMS value of 8.74% with a standard error estimate of 1.27 m and a correlation coefficient of 0.98. From this model, it is known that the flow of water moves from the northwest-north area in the form of a plateau by heading to the east-southeast area which is a lower plain and towards the Plampang River.

Intracontinental basaltic magmatism in the sedimentary succession of the Tucuruí Group: An important record of the final evolution of the Araguaia Belt

The region of Tucuruí, northeastern Pará, is located in the transition between the Amazon Craton and the Araguaia Belt. In this region, there is an extensive graben filled by a volcano-sedimentary succession, 35 km wide that extends over 100 km in the NNW-SSE direction and which is represented by the Tucuruí Group. This group consists of a succession of basaltic flows and diabase sills interbedded with sub-arkose and siltstones. Together with the metasedimentary rocks of the Tocantins Group, these rocks represent the latest records of the Araguaia Orogen in the extreme North of Brazil. The volcanic deposits consist of basalt flows and volcanic breccias associated with diabase sills. The flows are predominantly composed of massive and locally amygdaloidal basalts and basaltic breccias. The basalts are aphanitic, fine-grained or glassy, holo-to hypocrystalline, and are made up of labradorite, augite, Fe–Ti minerals oxides, sulfides, and apatite. The massive basalts have intergranular or intersertal texture, whereas the amygdaloidal basalts are characterized by fibro-radial zeolites, epidote, and chlorite. The basaltic breccias are composed of elongated, hypovitreous basalt fragments encompassed by a volcanoclastic matrix with siltstone films and anastomosed siliceous-zeolitic materials. The basalts are subalkaline with tholeiitic Ocean Island Basalts affinity, intraplate, and are interpreted as products of fissural volcanism in a subaerial environment. Deformation is predominantly ruptile, and fluid remobilization occurs along faults and fractures, generating veins of quartz, carbonate, chlorite, and epidote by hydrothermal processes at low temperatures (anki-metamorphism). The sedimentary succession of the Tucuruí Group represents a paleo-segment formed in a shallow marine environment influenced by storm waves. The evolution of the succession is related to the installation and fill of a rift basin during the final stage of evolution of the Araguaia Belt at the Neoproterozoic/Paleozoic transition. This event is related to the extensional tectonics that marked the transition from a ductile to ruptile regime in the late stage of evolution of the Araguaia Orogen, and it represents a rare record of the end of the Brasiliano Cycle on the Brazilian Platform.

Evidence of a Palaeoproterozoic SLIP, northern Amazonian Craton, Brazil

The Orocaima SLIP consists of an association of acid-intermediate volcanic-plutonic rocks. The volcanic rocks were generated in explosive eruptions through low eruptive columns, probably associated with fissural volcanism in the north of the Amazonian Craton, Brazil, between 2.0 and 1.98 Ga. It generated ignimbrites, whose facies (volcanic breccia rich in lithic, lapilli-tuff and lithic lapilli-tuff) show the proximity of the source. The extensive area of ca. 200.000 km2 of ignimbrite, rhyolite and dacite deposits, as well as the age range (2.0–1.98 Ga) and geochemical signatures suggest that the Orocaima volcano-plutonism may correspond to one of the oldest silicic LIPs in the world. The silicic volcanism is essentially subaerial and characterized by high-grade ignimbrites (densely welded) and subordinate lava, the ages of which indicate the longevity of the volcanic event in the Orosirian. They are included in the Surumu Group and comprise rocks with high-K calc-alkaline affinities and were emplaced in a subduction-related setting, similar to the rocks that extend through Venezuela, Guyana and Suriname (Cuchivero-Surumu-Iwokrama-Dalbana metavolcanic belt - CSID). The occurrence of mafic fragments disseminated in volcanic and granitic rocks in the north of Roraima, Brazil and in other segments of the CSID belt suggests the coexistence of acid and basic magmas. Except for one sample (εNd(t) = -2.3), the Nd isotopic data of analyzed Surumu Group volcanic rocks yielded positive εNd(t) values (0.5–4.48; TDM = 2.0–2.47 Ga), suggesting generation from magmas derived from the mantle or from the melting of new juvenile crust. The Orocaima volcanism bears no evidence of involvement of Archean sources in the generation of the rocks. Thus, the Orocaima volcano-plutonism may represent one of the most significant ignimbrite eruption events during the Palaeoproterozoic in the world.