Andesite Intrusions Research Articles

Mineralogy and Hydrothermal Fluids Characteristics of the Mekarbakti Low Sulfidation Epithermal Gold Prospect at Garut Regency, West Java, Indonesia

Abstract Mekarbakti prospect is situated in the western sector of Java Island which tectonically lies within the W-E trending Sunda-Banda magmatic arc. This arc hosts numerous prospects for ore mineralization with various types of mineral deposits, one of which is epithermal that found in the study area. The Mekarbakti prospect is located close to the Arinem prospect, about 8 km to the east, which is confirmed as a low sulfidation epithermal deposit type. Arinem is proven to be economical to mine, thus it becomes interesting to conduct study on the Mekarbakti prospect, to elucidate its mineralization model. This paper is dealing with the characteristics of hydrothermal alteration using petrographic and X-Ray Diffraction (XRD) to identify the alteration minerals, using ore microscopy analysis to identify the type of ore minerals, and fluid inclusion to reveal the hydrothermal fluid characteristics. The geological condition of the study area is composed of andesite lava, andesite breccia, tuff, lapilli, and andesite intrusion. The distribution of silicification alteration is controlled by geological structures with an NW-SE pattern, while argillic and propylitic alterations tend to have a gradient trend towards silicification alteration. Propylitic alteration is characterized by chlorite+illite+smectite±epidote. Argilic alteration is marked by the presence of clay minerals such as kaolinite+illite+smectite+quartz. Silicification alteration typified by quartz+kaolinite+diaspore+illite. Ore mineralization in the study area is associated with massive, brecciated, comb, cockade and, stockwork vein textures. Ore and metalliferous minerals are found as pyrite, hematite, goethite, chalcopyrite, sphalerite, galena, pyrrhotite, covellite, chalcocite, acanthite, native gold, and native silver. Generally, the mean homogenization temperature in the research area ranged from 248.05 – 266.12 °C, with salinity ranged from 1.01 – 1.68 wt.% NaCl eq. The epithermal deposit in the study area is classified as deep low sulfidation epithermal deposit specifically in transition between base metal and precious metal interval.

Integration Of 3G Data (Geomagnetic, Gravity And Geology) To Identify Geothermal System Controlled By Geological Structure Of Telomoyo Plateau (Study Case Of Candi Umbul Area)

As the technology development, there are more and more new innovations that utilize existing resources to support the energy needs and to fulfill the consumer needs. The energy source that is being discussed at this time is geothermal energy. Geothermal energy sources are considered effective because they are renewable and environmentally friendly when compared to other energies such as fossil energy. In Indonesia itself, geologically, it has a complex series of volcanoes that can be used as a heat source in this new energy innovation. One area that thought to have geothermal potential is the telomoyo mountain area, which is indicated by the manifestation of hot springs on the surface, precisely around Candi Umbul. However, it is necessary to conduct subsurface studies to identify the presence of subsurface structures such as faults or intrusions as heat sources, where this can be overcome using geophysical methods. This research was conducted using the integration of two geophysical methods, namely the geomagnetic method to determine the direction of the fault continuity and the gravity method to determine the regional heat source, and in the other side geological data as a reference for interpretation. From geomagnetic measurements, 163 rock magnetism data were obtained which indicated the existence of a fault continuity with northeast-southwest orientation in the direction of the river flow and cut the manifestation of hot springs. Meanwhile, from 176 gravity topex data, a high complete boguer anomaly value was obtained as an indication of an intrusion in the northeast that was cut by the fault recorded in the geomagnetic data. Based on geophysical data analysis and correlation with geological data from previous studies, it can be assumed that there is a geothermal system in the study area with andesitic intrusion as the heat source, fault structure with northeast-southwest orientation as a weak zone for meteoric water migration, andesite lava as caprock, unit tuff rock as a reservoir and the telomoyo plateau as a recharge area to supply meteoric water from the geothermal system.

GEOLOGY AND THE STUDY OF HEAVY METAL IMPACTS ON ENVIRONMENTAL QUALITY ASSESSMENT USING ARCGIS FOR INTERPRETATION DISTRIBUTION

The Boto area, Jatiroto District, Wonogiri Regency, Central Java, Indonesia, is a traditional gold mining site with inadequate waste management practices, particularly mercury usage. This study aims to assess mercury contamination and evaluate phytoremediation potential in the area. Geological mapping, petrographic analysis, fossil analysis, stereographic analysis, soil Hg measurement, soil and water chemistry analysis, and XRF analysis were conducted to characterize the geology, geomorphology, and heavy metal distribution. The study area is divided into four geomorphological units: Intrusive Hill Unit (V1), Homoklin Hill Unit (S1), Karst Hill Unit (K1), and Alluvial Plain Unit (F1). Stratigraphically, the area consists of the Nglanggeran Breccia Unit interbedded with early Miocene lava, the early Miocene andesite Intrusion Unit with a terrestrial depositional environment, the Wonosari Limestone Unit of Middle-Late Miocene age, and the Holocene Alluvial Deposit Unit. Geological structures include left-lateral normal faults trending relatively north-south (Sesar Boto), left-lateral normal faults trending southeast-northwest (Sesar Brenggolo), and right-lateral downthrown faults trending southeast-northwest (Sesar Mesu). Fractures and veins are also present. Mercury (Hg), manganese (Mn), arsenic (As), and lead (Pb) distribution mapping was conducted. The contaminated zone falls into TCLP-A, and waste management falls into category 1, located in Boto Village, which is a gold mining and processing area. Phytoremediation is proposed as a remediation strategy. Keywords: gold mining, heavy metals, XRF, phytoremediation, distribution

Structural control of igneous intrusions on fluid migration in sedimentary basins: the case study of large bitumen seeps at Cerro Alquitrán and Cerro La Paloma, northern Neuquén Basin, Argentina

Abstract Cooling subvolcanic igneous intrusions are known to have a tremendous impact on fluid flow in the shallow Earth's crust. However, the long-term post-cooling legacy of subvolcanic intrusions on fluid flow has received much less attention. Here we describe geological examples in the Andean foothills, Argentina, showing that igneous intrusions have long-term effects on fluid flow after their emplacement and cooling. The case study consists of ∼11 Myr-old eroded andesitic intrusions of Cerro Alquitrán and Cerro La Paloma, northern Neuquén Basin, Argentina, at the rims of which large volumes of bitumen are naturally seeping out at the Earth's surface. The intrusions exhibit laccolithic shapes with steep-sided contacts with the host rock. Near the intrusive contacts, the andesite is intensely broken along concentric breccia bands and fracture bands, interpreted as resulting from syn-emplacement brittle magma deformation, which represent high-permeability pathways for the migrating bitumen. Organic geochemical analyses of the bitumen show that the seeping oils were generated from incipiently mature Vaca Muerta sections located in a regional kitchen to the west, implying a lateral migration of ∼10–20 km. The Cerro Alquitrán and Cerro La Paloma intrusions are demonstrative examples highlighting how extinct subvolcanic intrusions have long-term consequences for subsurface fluid circulations in sedimentary basins.

ANALISIS STRUKTUR GEOLOGI BERDASARKAN DATA PERMUKAAN DAERAH BANJARHARJO, KABUPATEN BREBES, PROVINSI JAWA TENGAH

The research area is administratively located in Banjarharjo Village, Banjarharjo District, Brebes Regency, Central Java Province. Geographically, the research area is located at coordinates 108o 49 '15.4 "- 108o 52' 30.8" East Longitude and 06o 59 "21.8" - 07o 02 '04.8 "LS. The research area is included in The North Central Java Basin (The North Serayu Through/Basin) which is a Back Arc Basin. Based on the stratigraphic and sedimentological composition, the research area is classified into five geological units, (old to young), namely: (1) Sandstone and Claystone Interchange Unit, (2) Carbonate Claystone Unit, (3) Sand Limestone Unit, (4) Volcanic Breccia Unit and (5) Andesite Intrusion Unit. According to Pulonggono and Martodjojo (1994) Java Island, there are three straight lines of dominant structure, including: (1) East Laur - Southwest (Meratus Pattern), (2) North - South (Sundanese Pattern) and (3) West - East (Pattern Java). Based on the strike dip and stockiness data at the research location, the results of the force with the direction of the main stress are relatively Northeast-Southwestern, with the combination of Harding modeling, (1977) and Moody and Hill, (1956). The structural arrangements in the research area that are formed are (1) Cibuluh thrust fault, (2) Maibah sinistral strike-slip fault, (3) Cikuya sinistral strike-slip fault, and (4) Cikuya dextral strikeslip fault.

STRATIGRAPHY AND PETROGRAPHY OF ROCKS IN KARANGTENGAH VILLAGE AND SURROUNDINGS, BABAKAN MADANG SUB-DISTRICT, BOGOR DISTRICT, WEST JAVA PROVINCE

Stratigraphy and petrography, integral components of geological science, are crucial for identifying mineral composition, rock texture, and characteristics, as well as understanding relationships between layers and layer positions. This study is located in Karangtengah Village and its Nearby Surroundings, Babakan Madang District, Bogor Regency, West Java Province. Physiographically, the study area is included in the Bogor Zone (Van Bemmelen, 1949). The research site is also incorporated in the Geological Map of Bogor Sheet, West Java. And specifically this research was conducted in the Cileungsi River. This river is included in the Jatiluhur Formation (Sudjatmiko, 1972). Stating the purpose of geological mapping, such as understanding regional geology, identifying mineral resources, or assessing geological hazards. Then, data collection in the field, such as conducting detailed field surveys to observe and record geological features, using field notebooks, GPS for plotting, and cameras for documentation, collecting rock samples for laboratory analysis (Petrography analysis and microfossil analysis). As a result of this study, there are four rock units in the Karangtengah area, with the order of formation from the oldest to the youngest. Firstly, Claystone Unit (Tmbl) consists of mudstones and sandstones, with fossils of P. transitoria and G. peripheroronda indicating an Early Miocene age (N8) and the depositional environment is the Outer Neritic. Second, the Old Andesite Intrusion (At) consists of andesite igneous rocks with Middle Miocene age and deposited in a terrestrial environment. Third, the Tuff Unit (Qt) is dominated by pyroclastic tuff rocks with Pleistocene age and deposited in a terrestrial environment. Finally, the Andesite Unit (Am) consists of andesite igneous rocks younger than the Pleistocene, penetrating the Tuff Unit (Qt) and the Claystone Unit, and deposited in a terrestrial environment.

Gold mineralization in the Panikaban area, Banyumas, Central Java Province, Indonesia

Abstract Our research was conducted in Panikaban Area, Ponorogo Regency, Central Java, Indonesia. Based on the result of field data collection and analysis which has been carried out in the laboratory, stratigraphy study area was divided into 6 unofficial lithostratigraphy units and 2 litodem with the sequence of an old rock to young as follows: Halang volcanic breccia unit (Tmb), Halang sandstone unit (Tmbp), Kumbang andesite lava unit (Tma), Andesite intrusion (Tmi), Tapak volcanic breccia unit (Tpb), Tapak sandstone unit (Tpbp), Tapak limestone unit (Tpbg) and Alluvial (Qa). Rock lithology conditions in the research area are also included in the category in experiencing the process of alteration and minerallization, making the study area was being divided into three zones of alteration, namely: argillic zoning, propylitic zoning, and sub-propylitic zoning. Mineralization found in the research area is relatively associated to quartz veins (veins or veinlets) in the Halang sandstone unit, as well as on the intrusion body in the study area. Ore mineralization contained in research area such as sulfide minerals, such as: pyrite (FeS 2), chalcopyrite (CuFeS 2), galena (Pbs) and bornite (Cu 5 FeS 4). Results of the analysis of AAS (Atomic Absorption Spectroscopy) or atomic absorption spectrophotometry is used to determine the content of sulfide mineral elements contained in a sample. The analysis shows increase in sulfide mineralization which is characterized by the abundance of the elements Cu, Pb, Zn, Ag, Au, some places elements of Au increases compared with other elements.

Karakteristik Mineralisasi Endapan Emas Epitermal Sulfidasi Rendah di Daerah Mekarbakti, Kecamatan Bungbulang, Kabupaten Garut, Provinsi Jawa Barat

The southern mountains of western Java have been known to have large gold deposit potential. The Mekarbakti area, located in Garut regency, is known to have gold-bearing quartz veins, but the distribution and characteristics of these veins are not well recorded. The purpose of this study is to determine the geological conditions of the deposits and the characteristics of the epithermal deposits in the study area, in term of alteration, ore mineralization, vein texture, and ore geochemistry. Field work includes random sampling of veins from surface outcrops and cores. Laboratory analysis includes mineragraphic, geochemical analysis by means of Fire Assay Atomic Absorption Spectrophotometry (FA-AAS) and Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) . The lithology of the Mekarbakti area is composed of volcanic rocks including andesite lava, andesite intrusion and pyroclastic rocks such as breccia and tuff. The mapping results show that the hydrothermal alteration that developed is divided into three zones of alteration type, namely the silicification zone (silica + - clay), the argillic zone (kaolinite + - smectite - illite) and the propylitic zone (chlorite - smectite - epidote - illite). Quartz texture is generally dominated by brecciated, colloform, comb, lattice bladed and cockade textures. The texture interpretation of the veins shows a crystalline quartz zone, which is enriched in base metals such as galena, sphalerite and chalcopyrite and a crustiform – colloform zone which usually contains gold and silver. Veins in the study area contain the elevated grades of 3.21 ppm Au and 75.5 ppm Ag.

Petrological, mineralogical, and geochemical compositions of coal in the Ombilin Basin, West Sumatra, Indonesia

Coal deposits have received considerable attention in recent years owing to their by-product waste being a promising alternative source of critical elements. The Ombilin Basin is well known as one of the largest coal-producing basins in Indonesia. This study investigated the petrological, mineralogical, and geochemical compositions of four Eocene coal seams in Ombilin Basin basement rock, and their relevance regarding critical element enrichment. Samples of coal and basement rock from the Ombilin Basin were analyzed using thin- and polished-section scanning electron microscope energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, and inductively coupled plasma mass spectrometry. Coals from the Ombilin Basin are high-volatile bituminous C coals with mean random vitrinite reflectance values of 0.58%–0.66%. Vitrinite content is the most dominant maceral (61.06 vol% mmf), followed by liptinite (24.95 vol% mmf), and inertinite (13.75 vol% mmf). Minerals observed in the coal samples comprised pyrite, quartz, and clay. Pyrite was present as discrete crystals, massive crystals, framboidal pyrite, and cell-filling on other macerals. Both syngenetic and epigenetic pyrite was found in the Ombilin coal. Based on the petrological and mineralogical analyses, basement rock in the Ombilin Basin can be divided into four groups: granodiorite, andesite-basaltic, marble, and meta-sediment. In comparison with World Hard Coal and common Chinese coals, some layers/partings in the coal seams in the Ombilin Basin are enriched in U (37.6 ppm), V (175 ppm), and total rare earth elements and yttrium (REY; 126.15 ppm). REY-bearing minerals in the Ombilin coal samples were attributed to clay and sulfide minerals, while others were found as dispersed and scattered, indicating that REY are organically bound. Inorganic materials in Ombilin coal deposits have mainly intermediate-felsic provenance, are derived from Cretaceous granitoid rocks and andesite intrusions, and have been subjected to freshwater conditions.

Multistage Gold Mineralization at The Wanagon Gold Prospect, Ertsberg District, Mimika Regency, Papua Province, Indonesia

DOI:10.17014/ijog.9.3.279-290The Wanagon Gold prospect area located in the Ertsberg District, Papua Province, the eastern part of Indonesia, is predominantly underlain by Upper Mesozoic to Cenozoic sedimentary rocks intruded by the Wanagon Sill monzodiorite-diorite and andesite intrusion (Wanagon Dike). The study by previous researchers at Wanagon Gold prospect was based on pre-2005 exploration data and did not contain extensive additional drillings in 2007-2011. This paper aims to elucidate the genesis of the Wanagon Gold deposits based on the latest exploration data. The mineralogy was determined by using thin section and polish section analyses, a scanning electron microscope with an energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), and Near-Infrared spectroscopy (NIR). The chemical composition of the rock was identified using an X-ray fluorescence spectrometer (XRF). Gold mineralization is associated with pyrite, of which there are three types of gold-bearing pyrite: (1) massive pyrite, (2) disseminated pyrite, and (3) fine-grained pyrite associated with clay minerals. Only massive pyrite contains visible gold-bearing minerals such as native gold, electrum, and gold-telluride minerals, while in nonmassive pyrite, gold is only detected by the assay. The deposit is interpreted as structurally controlled distal gold skarn. Instead, its local association of gold with arsenic also indicates a minor component of more-distal sediment-hosted type gold mineralization.

Geothermal System in Parang Wedang, Yogyakarta, Indonesia

Geothermal manifestations in Parangtritis, Indonesia, known as Parang Wedang Hot Spring, indicate a geothermal system in the subsurface. This circumstance motivated our research to model the Parang Wedang geothermal system in order to determine its subsurface conditions. Geological mapping, the geophysical method, and geochemical analysis were integrated to produce a conceptual model of the Parang Wedang geothermal system. The study area consists of structural hills, karst hills, and eolian plains with andesite breccias, limestone, andesite, and sand deposits as lithological variations. The results of magnetic modeling indicate that the research area is associated with the presence of an andesite intrusion and shows a NE-SW trending geological structure that is thought to be a path for hydrothermal fluid to the surface. Geochemical analysis was performed at two hot springs with temperatures of 47 °C and 49 °C. Geothermometer calculation showed that the geothermal reservoir in Parang Wedang has a temperature of 100 to 120 °C, a depth of about 180 to 285 m, and can be classified as a low enthalpy geothermal system.

Evidence of a Magmatic Intrusion in the Tim Mersoi Basin in Relation With Uranium Mineralizations

The Tim Mersoi Basin is a very rich uranium basin. It is a basin that belongs to the Iullemenden syncline located in the northern part of Niger. Research work in recent years in this basin has revealed significant mineralization. The present study in this basin is based on the QEMSCAN and petrographic analysis of samples particularly highlighted during recent exploration campaigns. This study has shown that the basement of the sector is granitic in nature but the presence of an andesite intrusion has been revealed in this studied area. The presence of this andesite dyke has also shown a close link with uranium mineralization.

Magnetotelluric data analysis using 2D resistivity modelling in Gondang region, Bojonegoro

Previous studies in Gondang Subdistricthave identified underground mud reservoirnear a rock intrusion, while another study near Gondang Subdistrict has identified deep fault structures. This study will identify the distribution and characteristics of possible geological features in the area using magnetotelluric (MT) method to further describe the relationship between the geological features related to local geology. MT data measurements were conducted on 7 stations alonga north-south line, then modelled in 2D using nonlinear conjugate gradient algorithm. The model was used to describe the subsurface resistivity distribution and to identify the geological features. The results show 5 resistive zones (20–1250 Ω⋅m) and 4 conductive zones (≤10 Ω⋅m). The former consist of 4 vertical zones, 1 vertical zone deeper than 5 km, and 1 horizontal zone near the surface. 2 conductive zones surround a resistive zone, while 2 others stretch below the horizontal resistive zone. The vertical resistive zones are interpreted as andesite intrusions, and the horizontal one as volcanic breccia. The conductive zones are interpreted consisting of tuff and marl with possible saline water content. 3 vertical intrusions are thought to have the same source, and all vertical intrusions are suspected to co-occurr with the Pandan Volcano intrusion.

3D gravity data inversion modeling for subsurface identification of Gondang area, Bojonegoro regency

Gondang area had experienced an earthquake of a 4.2 magnitude in the mid-2015, showing that tectonics in the area are active. Also, the Gondang area has a mountain categorized as one of the eleven earth thermal prospects in East Java, i.e., Pandan Mountain. It is exciting to study the subsurface structures of the Gondang area. To date, precedent studies only present geological reviews of the surface. This study presents information on subsurface structure conditions based on gravity. The technique used was 3D inversion modeling with Single Value Decomposition (SVD) inversion. The study ’ s primary data were gravity data from 61 points with a study area of 6.9 km x 5.5 km from 3-10 September 2019. Secondary data utilized were Bojonegoro geological map and local geological studies. From 3D modeling results, slicing was conducted for five cross-sections based on density contrast. Then, the subsurface 2D cross-section results were interpreted for each section. Subsurface model results illustrated density distributions in south, northeast, and northwest areas with high densities ranging from 2.8 g/cc to 3.0 g/cc in >500 m depths. Meanwhile, the north, southwest, and southeast areas had low densities ranging from 1.70 g/cc to 2.0 g/cc in >500 m depths. Furthermore, it indicates the continuity of andesite intrusion from the south to the study site in >500 m depths.

Characteristics of Kedondong Trass and Bobos Trass as Cement Raw Material, Cirebon, West Java, Indonesia

The use of cement materials in construction continues to increase every year, consumes lots of raw material and emits CO2 from clinker production. To eliminate this negative effect, alternative materials are needed. Trass is natural pozzolan which is formed from silica-alumina rich volcanic rocks. As supplementary cementitious material, trass is sufficiently durable and reduce clinker proportion in cement mixture, thus more environmentally friendly.
 This research aims to determine characteristics and composition of Kedondong trass and Bobos trass, Cirebon, West Java as raw material for pozzolan cement. The study was conducted using petrography and XRD analysis to determine mineralogy of rocks. XRF analysis was carried out to determine chemical composition as well as other tests to determine trass quality.
 Kedondong trass is originated from andesite intrusion and andesitic breccia, while Bobos trass is formed from hypersthene-andesite intrusion. Based on mineralogy analysis, trasses have similar mineral composition consist of plagioclase, quartz, pyroxene, hornblende, and sanidine. XRD analysis shows abundance of cristobalite and tridymite from each samples. This mineralogy is confirmed by geochemistry result, which is the samples contain more than 70% SiO2 + Al2O3 and less than 4% SO3. Other chemical characteristics that have been tested are moisture content, ignition loss, and clay content in which all of those parameters meet the industrial standard for cement material.

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.

Geology, hydrothermal alteration and mineralization of the Carlin-type gold deposit at South Ratatotok, Southeast Minahasa Regency, North Sulawesi Province, Indonesia

South Ratatotok in North Sulawesi, Indonesia is one of the regions that was once a gold-producing region in North Sulawesi. Mesel gold deposit in this region is approved as Carlin type gold deposit with a total of resources of 12.25 Mt @5.21 g/t Au. Near the Mesel deposit, gold mineralization is currently discovered prospects such as Leon, Bulex Hill, Heind’s Find and Monkey Forest. This study is aimed to elucidate the types and distribution of hydrothermal alteration and ore mineralization encountered in these four prospect areas. Research methods including field work (including geological and hydrothermal alteration mapping, rocks and ore sampling) and various laboratory works consisting of petrographic analysis, ore microscopy, X-Ray Diffraction (XRD) and fire assay were performed. As a result, the study area is composed of several lithological units including limestone, volcanic breccia, porphyritic andesitic intrusion units as well as colluvial deposit. The wall rocks have been suffered by various hydrothermal alteration as follows: silicification in limestone, carbonatization in volcanic breccia, argillic alteration in volcanic breccia and propylitic alteration developed in porphyritic andesitic intrusion, minor decarbonatization associated with limestone and minor oxidation in volcanic breccia. Gold mineralization is present as a replacement type in very fine size of sulphides and gold is closely associated with silicified limestone. Ore chemistry by fire assay analysis indicates that gold and silver contents vary from 0.012 g/t to 2.41 g/t Au, and <0.5 g/t to 2.1 g/t Ag, respectively. The increase of gold content is geochemically followed by high values of Ag, As, Sb, Cu, Pb, Zn and by low value of Hg because of oxidation. Supergene process was also the key factor in the enrichment of gold where the oxidized ore has relatively higher gold grade than in non-oxidized ore.

DETERMINATION OF LANDSLIDE SLIP PLANE USING GEOLOGY AND GEOELECTRICAL ANALYSIS AT MOUNT GEGER PULUS LEGOK EMO SLOPE SEGMENT, CILILIN, WEST JAVA

Mount Geger Pulus in Cililin District, West Java shows a cone geomorphology composed of an Andesitic intrusion in which part of the slope is covered by volcanic rock. Small scale landslides have occurred several times indicating that the surrounding area is at risk of landslides. The slope segment of Legok Emo of this mount has a steep to very steep slope, covered by secondary and mixed forests. Settlement lies around the bottom of this slope. The results of geoelectrical measurements and geological analysis of this slope segment indicate that the slope has a potential to landslide that is controlled by tuff acting as a slip plane and can be triggered by rainfall. The layer that has the potential to landsliding are weathered volcanic breccias with a thickness of about 3 meters with a maximum of 5 meters that is overlying tuff which is relatively impermeable.

Volcanic record of the arc-to-rift transition onshore of the Guaymas basin in the Santa Rosalía area, Gulf of California, Baja California

AbstractThe Gulf of California is an archetype of continental rupture through transtensional rifting, and exploitation of a thermally weakened arc to produce a rift. Volcanic rocks of central Baja California record the transition from calcalkaline arc magmatism, due to subduction of the Farallon plate (ca. 24–12 Ma), to rift magmatism, related to the opening of the Gulf of California (&amp;lt;12 Ma). In addition, a suite of postsubduction rocks (&amp;lt;12 Ma), referred to as “bajaites,” are enriched in light rare-earth and other incompatible elements (e.g., Ba and Sr). These are further subdivided into high-magnesian andesite (with 50%–58% SiO2 and MgO &amp;gt;4%) and adakite (&amp;gt;56% SiO2 and MgO &amp;lt;3%). The bajaites correlate spatially with a fossil slab imaged under central Baja and are inferred to record postsubduction melting of the slab and subduction-modified mantle by asthenospheric upwelling associated with rifting or slab breakoff. We report on volcanic rocks of all three suites, which surround and underlie the Santa Rosalía sedimentary rift basin. This area represents the western margin of the Guaymas basin, the most magmatically robust segment of the Gulf of California rift, where seafloor spreading occurred in isolation for 3–4 m.y. (starting at 6 Ma) before transtensional pull-apart basins to the north and south ruptured the continental crust. Outcrops of the Santa Rosalía area thus offer the opportunity to understand the magmatic evolution of the Guaymas rift, which has been the focus of numerous oceanographic expeditions.We describe 21 distinct volcanic and hypabyssal map units in the Santa Rosalía area, using field characteristics, petrographic data, and major- and trace-element geochemical data, as well as zircon isotopic data and ten new 40Ar-39Ar ages. Lithofacies include lavas and lava domes, block-and-ash-flow tuffs, ignimbrites, and hypabyssal intrusions (plugs, dikes, and peperites). Calcalkaline volcanic rocks (13.81–10.11 Ma) pass conformably upsection, with no time gap, into volcanic rocks with rift transitional chemistry (9.69–8.84 Ma). The onset of rifting was marked by explosive eruption of silicic ignimbrite (tuff of El Morro), possibly from a caldera, similar to the onset of rifting or accelerated rifting in other parts of the Gulf of California. Epsilon Hf zircon data are consistent with a rift transitional setting for the tuff of El Morro. Arc and rift volcanic rocks were then juxtaposed by normal faults and tilted eastward toward a north-south fault that lay offshore, likely related to the north-south normal faults documented for the early history of the Guaymas basin, prior to the onset of northwest-southeast transtenional faulting. Magmatism in the Santa Rosalía area resumed with emplacement of high-magnesian andesite lavas and intrusions, at 6.06 Ma ± 0.27 Ma, coeval with the onset of seafloor spreading in the Guaymas basin at ca. 6 Ma.The 9.69–8.84 Ma rift transitional volcanic rocks underlying the Santa Rosalía sedimentary basin provide a maximum age on its basal fill. Evaporites in the Santa Rosalía sedimentary basin formed on the margin of the Guaymas basin, where thicker evaporites formed. Overlying coarse-grained clastic sedimentary fill of the Santa Rosalía basin and its stratiform Cu-Co-Zn-Mn sulfides may have accumulated rapidly, coeval with emplacement of 6.06 Ma high-magnesian andesite intrusions and the ca. 6 Ma onset of seafloor spreading in the Guaymas basin.

Formation of the Tocantinzinho 1989 Ma gold-only deposit (Pará State, northern Brazil), based on sulfur, oxygen, hydrogen, and carbon isotope data

The Tocantinzinho gold deposit in the Brazilian Amazon region, approximately 1989 Ma in age, contains 16.7 Mt of ore in syenogranite and monzogranite breccias and microbreccias, most with gold in pyrite, silicates and quartz, and a small part in quartz vein pyrites. Three hydrothermal phases are recognised, H1, H2, and H3, where H1, the first hydrothermal phase, is the oldest. The H1 transitional, late-magmatic hydrothermal phase altered feldspar to sericite and mafic minerals to chlorite. During the H2 hydrothermal phase, pyrite, gold, and minor galena were precipitated, followed by intense sericite alteration. The last hydrothermal phase, H3, was coeval with a large number of andesite intrusions that generated high-pressure fluid-driven cataclasis. This event opened veins, tension gashes, and pull aparts that were filled with carbonate, worm-like chlorite, prismatic quartz, galena, pyrite, and rutile, which likely displaced and concentrated the gold near the carbonate-filled structures.The minerals sulfide δ34S values vary between −4.4 and + 5.6‰, which is compatible with a sulfur magmatic source, and indicate that the sulfur and likely the gold are both magmatic. The δ18OSMOW values of carbonates vary between +7.63 and + 21.23‰ in a narrow range of δ13CPDB values between −3.94 and −1.69‰. Two explanations are possible for these values. If that the carbonate fluids were genetically related to the andesite magmatism, this fluid mixed with meteoric water, and degassing of this mixture occurred in the near-surface to produce dissolved inorganic CO2 with high δ18O value. Alternatively, when fluid mixing occurred, the oxidation state of the environment increased, and the δ13C values of the carbonates remained relatively uniform, whereas the δ34S values of the sulfides varied. The δ18O values of the quartz, sericite, and chlorite of the mineralized and barren rocks are consistent with δ18O values > +8 for the fluids that crystallized the chlorites and δ18O > + 4‰ for those that crystallized the muscovite. High δD values of these fluids, estimated from mineral δD values range between −2 and −25‰, and differentiate them from the fluids that form porphyry-type Cu–Au and orogenic deposits. The isotope data for Tocantinzinho present characteristics of an intrusion-related gold deposit that combines the features of equigranular, intrusion-hosted, mesozonal deposits, such as those hosted in reduced granite, with Au-rich porphyry deposits hosted in oxidized granites.