Altered Rocks Research Articles

Alteration enrichment of nitrogen in the gabbroic oceanic crust: Implications for global subducting nitrogen budget and subduction-zone nitrogen recycling

Uptake of nitrogen (N) during hydrothermal alteration of oceanic crust makes the altered oceanic crust (AOC) a significant N reservoir in parallel to seafloor sediments. While N enrichment in the upper (basaltic) oceanic crust during low-temperature alteration has been widely observed, the N characteristics of moderate- to high-temperature altered gabbroic oceanic crust, which accounts for ∼70 vol% of the crustal material in subducting slabs, have been rarely studied. The lack of these data resulted in large uncertainties in calculating the N input flux of global subducting slabs and modeling the N quantities released through the arc and subducted into the deep mantle. To fill this gap, we examined the N concentrations and isotope compositions of 38 altered gabbroic rocks recovered by ODP/IODP drillings from three oceans, i.e., Hole 735B at the Atlantis Bank in the Southwest Indian Ridge, Hole 1309D at the Atlantis massif in the Mid-Atlantic Ridge, and Hole 1415P at the Hess Deep in the East Pacific Rise. All the gabbroic samples show significant N enrichment with mostly positive δ15N values (in average: 7.4±3.8 ppm and +0.6±2.0‰ for Hole 735B, 5.3±2.1 ppm and +1.4±1.3‰ for Hole 1309D and 7.9±1.9 ppm and +2.0±1.8‰ for Hole 1415P), which are comparable to those of altered basalts from global oceanic crust. The N concentrations and δ15N values of these gabbroic rocks can be readily explained by mixing between minor inherited mantle N and mainly secondary N derived from seawater. The secondary N-hosting minerals in altered gabbroic rocks are likely plagioclase, amphibole and chlorite formed during moderate- to high-temperature alteration stages. Using these new data, we obtained a global nitrogen input flux of 20.6-1.3+0.9 × 109 mol·yr−1 to 29.9±2.2 × 109 mol·yr−1 for subducting AOC, in which 50% – 64% is contributed by the altered gabbroic oceanic crust. Integrating with previous estimate of N input flux from seafloor sediments, a total N input flux of 74.9-1.3+0.9 – 84.2±2.2 mol·yr−1 was estimated for global subducting slabs. Mass balance between this input N flux and various estimates of N output flux in global arcs gave a large range (from 47% – 53% to 10% – 20%) for the fraction of slab N to be subducted beyond the sub-arc depth to the deeper mantle. The upper end (47% – 53%) of this range is more consistent with the results from other constraints, such as thermal structural control and comparison between altered gabbros and meta-gabbros.

Weathering and mineral alteration of metamorphic rocks and genesis of Planosols along a rainfall gradient in Borborema Province, Northeast Brazil

Soils in Northeast Brazil have a correlation with soils in the Northwest Africa, as Planosols are predominant in these regions. However, the weathering and genesis of these soils are little understood, compromising their use and conservation. The present study aimed to investigate weathering and mineralogical changes as well as genesis of Planosols along a pluviometric gradient, considering their formation from gneiss rocks, and analyze climate influence on soil formation. We collected three profiles in different climatic regions of the Brazilian northeastern region and described and classified the samples morphologically. We also performed the physical, chemical, mineralogical, and micromorphological analyses. The results indicated that external material added to soil surface contributed to the formation of surface horizons rich in coarser fractions. Subsurface horizons however were rich in clay (e.g.: P1–2Bn: 433 g kg−1; P2-2B: 427 g kg−1; P3-2Bn1: 429 g kg−1) and were formed from changes of the parent material and the clay formation in situ. Biotite, plagioclases, K-feldspars, and hornblende are the main primary minerals sources for the formation of clay minerals (mainly smectite and kaolinite) from transformation processes and neoformation. Pedoplasmation, monosiallitization, bisiallitization, redoximorphism, and alkalization contributed to the formation of the Planosols studied. Despite incipient soil evolution, the Planosols displayed variation according to the climatic condition. Nevertheless, the incipient mineral alteration favored important soil attributes that could influence agricultural productivity, even in the region with higher precipitation rates.

Effects of Hydrothermal Alteration and Mineralization on the Paleomagnetic Properties of Rocks from IODP Expedition 376 at Brothers Volcano

AbstractThe 3-D subseafloor architecture of submarine hydrothermal systems is largely unknown, particularly at arc volcanoes. The alteration of volcanic rocks in these systems produces dramatic changes in their magnetic properties. Here, we present the first comprehensive study of paleomagnetic measurements from oriented samples of hydrothermally altered dacites from Brothers volcano (Kermadec arc), drilled during International Ocean Discovery Program (IODP) Expedition 376. These data have enabled insight into the progressive evolution of magnetic minerals in subseafloor volcanic rocks affected by variable types and degrees of hydrothermal alteration in response to varying fluid temperatures, chemistry, and associated mineralization; from initial chloritization typical of relatively low-temperature interaction with seawater to extremely altered rocks affected by higher-temperature, very acidic magmatic fluids.Hydrothermally altered samples show a significant reduction in natural remanent magnetization intensity (10–4 to 10–2 A/m) compared with unaltered samples (1–10 A/m), suggesting that primary titanomagnetite grains are destroyed during the hydrothermal alteration process. Except for a small region in proximity to the mineralized stockwork zone, no chemical remanent magnetization is observed in association with hydrothermal alteration, consistent with the widespread formation of diamagnetic and/or paramagnetic minerals such as pyrite, rutile, and leucoxene, which do not carry any natural remanent magnetization.Demagnetization experiments show that most of the oriented samples possess a stable characteristic remanent magnetization induced by the residual primary magnetic minerals formed at the time the rocks cooled on the sea floor. Partially chloritized dacites, however, are characterized by large magnetic susceptibilities, low Koenigsberger ratios, and very low magnetic coercivities, consistent with initial dissolution of smaller, singledomain magnetic grains, indicating that intensely hydrothermally altered rocks are better paleomagnetic indicators than initially chloritized samples at the periphery of the hydrothermal systems.The significant magnetic contrast between fresh and hydrothermally altered rocks, in addition to a thick layer (>300 m) of demagnetized rocks observed at Brothers volcano, confirms the empirical results that magnetic anomalies are important geophysical tools to determine the geometry of hydrothermal systems at submarine arc volcanoes.

Polymetallic Uranium Mineralisation in Rohil, Rajasthan, Western India: Insights from Mode of Occurrences, Structural Controls, Alteration Geochemistry and Exploration

Geological and radiometric studies of outcrops aided by extensive subsurface exploration through drill holes in an otherwise soil-covered terrain revealed the existence of low grades, medium tonnage, and metasomatite types of polymetallic uranium deposits at Rohil in India. Microscopic studies, electronprobe micro analyses, and geochemical analyses of samples from lodes indicate the polymetallic nature of mineralisation involving copper and molybdenum, in addition to uranium. Wide variations in the composition of fluid (S-, F-, P-, and O-rich) led to the formation of sulphides, fluorite, U-phosphosilicate, quartz, and magnetite, respectively, and are associated with uraninite. Litho-geochemical analyses from the Rohil deposit indicate multifarious metasomatic alterations associated with polymetallic mineralisation occurring in veins. The major mineralogical and metasomatic controls on rock compositions and the extent of material transfer processes that influenced the host rocks and mineralisati on are quantified by molar element ratio studies and alteration plots. General element ratio (GER) diagrams on chemical analyses of rock samples reveal albitisation and chloritisation as major and microclinisation, sericitisation, carbonatisation, and silicification as minor wall rock alterations associated with ore mineralisation. The alteration box plot between the chlorite–carbonate–pyrite index (CCPI) and the Ishikawa alteration index (AI) indicates the influence of hydrothermal activity and dominance of both albitisation and chloritisation. The ore zone is controlled by meso- and microstructures and the geometry of the soda- and potash-metasomatised zone around hydrothermal veins. This zone contains several anastomosing mineralised veins defined by a prominent joint that is set in quartzite that strikes subparallel to the axial surface of the F2 isoclinal folds and the pervasive schistosity S1 in the quartz–feldspar–biotite schist. Aventurisation of albite and microcline, established through electron probe micro analyses, can be considered as a pathfinder for uranium mineralisation. The close association of uranium and metallic sulphide mineralisation with microstructural, mineralogical (albitisation, chloritisation, and microclinisation), and geochemical variations can be applied as suitable exploration guides in a similar geological set-up worldwide.

Clays as indicators of paleoclimate and source rocks in The Chu-Sarysu Basin (Kazakhstan)

Newly formed smectite and palygorskite and their association are good proxies of a subtropical climate alternating dry and warm/ humid seasons during the late Cretaceous during the formation of the Chu-Syrasu basin. The association of fine-grain clays, smectite and fibres (palygorskite) and the occurrence locally of grains of albite, and natrolite, indicate they formed from water, slightly alkali-rich, and enriched in silica and magnesium. These clays may result partly from the alteration of volcanic rocks (glass) either in situ in case of volcanic emissions during sedimentation or close as smectite are euhedral and palygorskite well preserved. The flood plain may have been submitted during the hot season to drying, favouring the formation of brines which interacted with volcanic glass. Evaporation processes could have thus triggered the oversaturation with respect to smectite and palygorskite.
 Besides, muscovite as coarse grain particles, illite and chloritized biotites attest to a second source compatible with the coarse grain microcline and quartz, which can derive from granites. Source rocks could be, therefore, dual, with acid plutonic series (peraluminous granites probably) releasing coarse-grained detrital phyllosilicates (muscovite and biotite-chlorite) transported together with quartz and feldspars by rivers, and volcanic series, altered into newly formed clays (smectite and palygorskite).

Mineralization and genesis of the orogenic gold system in the Kalamaili area, East Junggar, Xinjiang, northwestern China

There are many lode gold deposits and occurrences in the Kalamaili area of the East Junggar, northwestern China. However, little is known about the specifics of ore geology and geochemistry of these gold deposits because of very limited exploration and research work in this region. Field geology, fluid inclusions, stable isotopes, trace elements in quartz, and hydrothermal zircon U-Pb dating are combined in this study to constrain the nature and source of ore fluids, the timing of mineralization, and the mechanism of gold precipitation in Kalamaili. The gold deposits are confined to a narrow zone between two regional NW- to NWW-trending shear zones and are structurally controlled by secondary, high-angle faults of the shears. The orebodies occur in the Middle Devonian and Lower Carboniferous, zeolite to lower greenschist facies clastic sedimentary and pyroclastic rocks. Gold mineralization occurs as auriferous quartz-sulfide ± tourmaline veins/veinlets and disseminated ores in the immediate altered wall rocks. The ore mineralogy is relatively simple and dominated by quartz with minor to trace amounts of sulfides (pyrite and arsenopyrite), sericite, calcite, and native gold. Quartz of various generations contains three types of fluid inclusions, including predominant H2O-CO2-NaCl inclusions and subordinate H2O-NaCl inclusions and CO2 ± CH4 ± N2 inclusions. The mineralizing fluid is uniformly characterized by a medium to high homogenization temperature (mostly 240−330 °C), low salinity (typically <6 wt% NaCl equivalent), reduced, and CO2-rich-H2O-NaCl ± CH4 fluid. The hydrogen and oxygen isotope data (δ18OH2O = +5.6‰ to +14.3‰, δDH2O = −99‰ to −62‰) indicate a metamorphic origin for the mineralizing fluid. The majority of sulfide δ34S values range between 0‰ and +10‰, indicative of a largely sedimentary rock reservoir of sulfur in the ore-forming fluids. Geochemically, the auriferous quartz is characterized by low concentrations of most trace elements including Ti, Al, Li, Ge, and Sb. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb isotope dating of hydrothermal zircons from the auriferous quartz vein yielded a weighted mean 206Pb/238U age of ca. 313 Ma. Phase separation of ore fluids and fluid-rock interaction are suggested as key mechanisms for the gold precipitation. Integrated geological and geochemical evidence indicates that formation of the orogenic gold system in Kalamaili is related to the transition from compressional to transcurrent deformation during the Late Carboniferous. Target gold exploration in this region should focus on the northeast side of the Kalamaili fault zone, where there exist suitable structural and stratigraphic trap sites with high fluid flux and potential gold mineralization.

Remote sensing and geochemical investigations of sulfide-bearing metavolcanic and gabbroic rocks (Egypt): Constraints on host-rock petrogenesis and sulfide genesis

Field geology, geochemical analyses and multi-sensor remotely sensed data of Landsat-8, ASTER and Sentinel-2 were used to detect various types of hydrothermal alteration and sulfide-gold mineralization in Neoproterozoic metavolcanic and gabbroic rocks from Darhib and El-Beida areas in the South Eastern Desert (SED) of Egypt. We also used RADAR data of Sentinel-1 using PCI Geomatica software to decipher the structural lineaments that control this mineralization. Different styles of disseminated to massive sulfide deposits include: 1) undeformed magmatic disseminated ores in Darhib gabbros, metavolcanic rocks and metabasic dykes; 2) deformed (remobilized) massive ores mostly of a magmatic-hydrothermal origin in Darhib tremolite talc rocks along E-W shear zones; 3) hydrothermal sulfide ores along El-Beida NW–SE shear zones. Cu-Zn massive ores represent the main sulfides in the Darhib silicified tremolite-talc rocks and meta-andesites, and occur as bands, veins and patches along Darhib E-W shear zones. Discontinuous occurrences of sulfide ores-bearing tremolite-talc rocks indicate that major shear zones act as structural controls on mineralization. Sulfide ores consist chiefly of chalcopyrite, sphalerite, pyrite and galena as well as minor covellite and bornite. These ores are highly concentrated along E-W striking Darhib shear zones and resulted from granite-derived hydrothermal fluids that form/and or accumulate sulfide minerals from the sulfide-bearing metavolcanic rocks around shear zones. Two stages of hydrothermal alterations are recorded in the Darhib talc mine including pre- (talcification, amphibolitization and carbonatization) and syn- (silicification and chloritization) sulfide ore alterations. Disseminated sulfide mineralization hosted in Darhib gabbros, metavolcanic rocks and metabasic dykes is of a magmatic origin, similar to Cu-Ni sulfide deposits hosted in gabbroic varieties in worldwide mafic–ultramafic intrusions. On the other hand, El-Beida gold-sulfide-bearing quartz and quartz carbonate veins are hosted in ferruginated and silicified metavolcanics/ metavolcaniclastic rocks along NW–SE shear zones (Najd Fault System trend). These are spatially associated with regional convergent structures (high-angle convergent wrench structures), suggesting orogenic gold type mineralization. The E-W with minor WNW-ESE shear zones in Darhib and NW-SE trending shear zones in El- Beida structurally controlled sulfides and gold mineralization in the SED of Egypt. El-Beida sulfide mineralization occurs as disseminated ores that consist mainly of pyrite, chalcopyrite, covelite and chalcocite with occasional occurrence of gold. The spatial occurrence of native gold in ferrigenous alteration zones indicates the role of iron in gold precipitation through buffering of gold-bearing complexes. Mineralogical and geochemical data of Darhib and El-Beida sulfide-bearing rocks indicate that they crystallized from depleted mantle sources in a back-arc basin setting, starting with the generation of arc-like volcanic rocks in a nascent BAB stage followed by the emplacement of plutonic equivalent of MORB/OIB-like rocks such as gabbroic intrusions in a mature BAB.

The Peña do Seo W-Sn deposit, NW Iberia: Petrology, fluid inclusions and O-H-S isotopes

The Peña do Seo W-Sn ore deposit in NW Iberia consists mainly of quartz veins hosted in schists. Vein mineralogy comprises wolframite, cassiterite and minor molybdenite. Peraluminous S-type granites and minor metaluminous granitoid breccias crop out in the same location. Whole rock geochemistry, mineral characterization, fluid inclusions and stable isotope geochemistry have been combined to infer hydrothermal mineralization conditions. A pre-ore stage (0) involving the alteration of host rocks, three stages of mineralization (I, II and III), and a supergene stage (IV) have been identified. Stage I (oxide-halide-sulfide stage) consisted of a cassiterite-pyrite-fluorite mineralization in granitoid breccias. Stage II (main oxide stage) involved wolframite-rich selvages and wolframite-cassiterite-(molybdenite) quartz veins. Stage III (main sulfide stage) consisted of a sulfide mineralization in quartz veins comprising pyrite and minor arsenopyrite and chalcopyrite. Three types of fluid inclusions were found in vein quartz: (1) aqueous two-phase inclusions, with homogenization temperatures (Th) between 445 °C and 280 °C and moderate salinities (9-14 wt.% NaCl eq.), (2) aqueous-carbonic three-phase fluid inclusions, with Th from 340 °C to 260 °C and low salinities (2-7 wt.% NaCl eq.), and (3) aqueous two-phase fluid inclusions, with Th from 270°C to 155 °C and low salinities (0-6 wt.% NaCl eq.). δ18O values in quartz from mineralized veins range from +11.2‰ to +13.4‰, and between +15.0‰ and +15.4% in quartz from mineralized granitoid breccias. δ34S values in sulfides (pyrite, arsenopyrite and chalcopyrite) range between +13.0 ‰ and +37.1 ‰, thus suggesting a marine source of sulfate and possible equilibration with host-rock sulfides. δD values in muscovite and chlorite from quartz veins range between -105.7 ‰ and -71.5 ‰ and between -69.4 ‰ and -67.1 ‰, respectively, indicating a transition from magmatic to magmatic-metamorphic conditions. An aqueous (H2O–NaCl) magmatic-hydrothermal fluid led to the W-Sn mineralization, as deduced from the studied fluid inclusions and isotopic signatures. High W and Sn contents in the peraluminous granites indicate that the W-Sn mineralization in Peña do Seo could partially be related to the granites and granitoid breccias. Whereas the Sn (± W) likely derived from hydrothermal fluids exsolving from the crystallizing magmas, host quartz schists would have supplied other elements required for wolframite deposition such as Fe and Mn. Sulfide mineralization in stage III was probably driven by fluid dilution.

Sources, transport, and deposition of metal(loid)s recorded by sulfide and rock geochemistry: constraints from a vertical profile through the epithermal Profitis Ilias Au prospect, Milos Island, Greece

Drill core samples from the Profitis Ilias Pb-Zn-Cu-Ag-Au vein mineralization on Milos Island, Greece provide new insights into (i) the metal sources, (ii) the primary vertical metal(loid) distribution, and (iii) the supergene enrichment processes in a transitional shallow-marine to subaerial hydrothermal environment. Metal contents of unaltered and altered host rocks combined with Pb isotope analyses of hydrothermal sulfides suggest that most metal(loid)s were derived by leaching of basement rocks, whereas the distinct enrichment of Te is related to the addition of Te by a magmatic fluid. The trace element contents of base metal sulfides record decreasing Au, Te, Se, and Co, but increasing Ag, Sb, and Tl concentrations with increasing elevation that can be related to progressive cooling and fluid boiling during the hypogene stage. The formation of base metal veins with porous pyrite hosting hessite inclusions at ~ 400 m below the surface was triggered by vigorous fluid boiling. By contrast, the enrichment of native Au associated with oxidized Fe and Cu phases in the shallower part of the hydrothermal system resulted from supergene remobilization of trace Au by oxidizing meteoric water after tectonic exhumation to subaerial levels. Disseminated pyrite with higher Tl/Pb ratios and locally elevated Hg concentrations relative to vein pyrite reflects infiltration of the host rocks by boiled liquids and condensed vapor fluids. The vertical and temporal evolution of the Profitis Ilias mineralization, therefore, provides unique insights into the transport and precipitation of Au, Ag, Te, and related metal(loid)s by multiple fluid processes.

MANIFESTASI PANAS BUMI SERTA REKOMENDASI PEMANFAATANNYA DI DESA KOTO SANI DAN SEKITARNYA, KECAMATAN X KOTO SINGKARAK, KABUPATEN SOLOK, PROVINSI SUMATRA BARAT

The volcanic arc or volcanic arc that extends along the western side of Sumatra Island. Koto Sani Village, Kec. X Koto Singkarak, Solok Regency, West Sumatra Province is one of the areas on Sumatra Island that has geothermal potential. In terms of the developing tectonic order, namely in the form of a geological structure in the form of the Sumani segment of the Sumatra Fault which moves in a strike slip to the right or dextral. This geological structure is one of the main components forming the geothermal system apart from the heat source, reservoir rock, stratigraphic characteristics and the cap layer. This study aims to determine the geological setting, any geothermal manifestations found in the study area and estimate the temperature below the surface for recommendations for its use. The research method used was the interpretation of satellite imagery on the 1:25,000 scale of the Indonesian Rupa Bumi map (RBI), surface geological mapping, measurement of the physical characteristics of geothermal fluids, geochemical analysis of geothermal fluids (anion-cation) and then plotting Giggenbach's triangle diagram (1991). and geothermal geothermal calculations, as well as analysis of 18O and 2H isotopes. The research area generally forms a landscape of elongated hills with quite steep valleys as a result of tectonic activity in the form of geological structures as migration routes of hot fluids from the reservoir to the surface. This is reinforced by the results of the interpretation of river flow patterns, namely meandering or sub-dendritic and elongated or called parallel as an indication of disturbance from faults or faults. Geothermal manifestations found are hot springs, sintered silica and hydrothermal alteration rocks. The estimated temperature below the surface is 407.64°C with a high temperature category and is recommended for direct use, such as the tourism and fisheries sector, as well as indirect use, such as geothermal energy power plants.

Mal'javr – the first gold prospect in the Archean conglomerates, the Kola region

A new type of prospects of gold mineralization in the Kola region is considered in the paper. The Mal'javr prospect is located in the south-western flank of the Uragubsky greenstone belt in biotite gneiss – the metamorphosed sedimentary rocks with interlayering of polymictic conglomerate, gravelite, and sandstone. Gold-bearing sulfide mineralization was found in altered rocks, which form a series of lens-shaped bodies, concentrated along a shear zone of NNE strike. The bodies of altered rocks are zonal: the central zone makes 50–80 % of the lens volume, it consists of garnet and quartz, the intermediate zone is of garnet-biotite mineral composition, and in the outer zone, which is often reduced, the main minerals are hedenbergite, hornblende, and grunerite. The metasomatic alteration is connected with an increase of iron content, decrease of Al, Si, and alkaline metals Na and K, redistribution of calcium to the outer zone of metasomatic lenses; and magnesium is inert. If compared to the unaltered gneiss, the altered rocks are depleted in Rb, Cs, Sr, Ba, Zr, Nb; and content of Cu, Zn, Mn, As, Ag, Te, W, Bi (all these elements are known to associate with gold) increases. All zones of altered rocks are rich in sulfide mineralization. Pyrrhotite and arsenopyrite are the main sulfide minerals, minor minerals are chalcopyrite, pentlandite, magnetite, tochilinite, native gold, and late pyrite and marcasite. Rock alteration and formation of sulfide mineralization happened at a high temperature > 600 °С, and the late alteration processes at ~400 °C. The arsenopyrite-gold mineralization probably formed with As and Au mobilized from the host biotite gneiss during regional metamorphism or due to pegmatite vein intrusion.

Geological constraints on volcanic-fluid pathways at the Maruyamasawa-Fumarolic-Geothermal-Area, and its relation to the present magmatic-hydrothermal activity in Zao Volcano, Tohoku, Japan

A field survey based on outcrop and hand-dug trench observations was conducted in the Maruyamasawa-Fumarolic-Geothermal-Area (MFG) of the Zao Volcano. Powder X-ray diffraction and microscopic analyses of samples collected from the MFG provide new possible links between underground magmatic–hydrothermal activity and fumaroles in the volcanic system. The MFG is a fumarolic area (solfatara) with evidence of intense geothermal activity, such as hot and cold springs, fumaroles precipitating native sulfur, and ongoing fumarolic alteration. The MFG occurs on a talus slope that mainly comprises detritus from Late Cretaceous granitoids mixed with blocks of the ca. 1 Ma Robanomimiiwa Pyrocalstics (Rmp). The Rmp is well exposed on a cliff behind the talus slope, and it unconformably overlies the granitoids with a NW-striking, SW-dipping contact. The southwestward extension of the Rmp-granitoids unconformity is projected laying approximately 500 m below the Okama crater, which is 800–1000 masl. The mineral assemblages of the MFG altered rocks are mainly 1) cristobalite + native sulfur ± pyrite, 2) cristobalite + quartz + alunite + kaolin-group minerals + smectite, and 3) quartz-only (intensely silicified) rocks. These mineral associations indicate a typical shallow acidic environment between steam-heated and fumarolic alteration zones. Three sandy layers (Layers 1 to 3 from bottom to top) are recognized in the hand-dug trench: Layer 1 is brown, poorly sorted and well-consolidated, Layer 2 is pale gray to gray and moderately sorted, and Layer 3 is pale gray and poorly sorted. The 14C dating of a paleosol underlying Layer 3 constrains the deposition event to around 437 ± 20 yrBP (2σ: 1429–1470). The presence of gray basaltic andesite (Rmp), granitic fragments (granitoids), and altered rocks (MFG constituent) in these layers suggests detrital remobilization from the MFG talus slope by rainfall (snowmelt) that emplaced thin lahar deposits. Furthermore, the detrital cristobalite and alunite in the L1-L3 matrix are minerals of fumarolic origin contained in the MFG altered rocks. These results indicate that the fumarolic activity and alteration of the MFG occurred before the deposition of these layers.Based on this information, we propose the following model for the MFG activity: (1) The Rmp-granitoids unconformity behaves as an open pathway for ascending volcanic fluids and controls the MFG geothermal features. (2) The MFG was formed over 600 years ago, probably during early Okama crater activity at the 1200s CE. (3) Volcanic fluids have ascended through the unconformity over the last several hundred years. Therefore, careful monitoring of the MFG is crucial for detecting surface anomalies that could indicate imminent eruptions of the Zao Volcano.

Post-Subduction Granite Magmatism and Gold-Sulfide Mineralization in the Abu Zawal (Fatira) Area, Eastern Desert, Egypt

Gold-sulfide mineralization in the Abu Zawal (Fatira) mine area, North Eastern Desert of Egypt, is related to porphyritic felsite dikes and elongate silicification zones in granitic rocks. These felsite dikes and the host granitic rocks exhibit major and trace element geochemical features typical of calc-alkaline and metaluminous I-type granites, likely originated in a late-orogenic setting. Their geochemical characteristics along with their fractionated LREE relative to HREE patterns imply either formation in a subduction-related environment or generation from subduction-modified source materials. Partial melting of subduction-metasomatized lower crustal rocks during extension following the lithospheric thickening may account for the production of such fertile, high Sr/Y and La/Yb magmas. In the Abu Zawal (Fatira) area, NE-trending altered felsite dikes, hydrothermal breccias, quartz enclaves, and wall-rock replacements are characterized by disseminations of chalcopyrite, pyrite, and rare gold. Alteration mineralogy, dominated by sericite, drusy quartz, kaolinite, calcite, and specular hematite, combined with the available fluid inclusion data suggests moderate to low temperature and near neutral pH conditions. The geochemical data of the altered wallrocks and mass balance calculations indicate significant mass losses in the altered rocks consistent with fluid/wallrock ratios higher than unity and near neutral pH conditions. Considering that the silica-rich host rocks, hydrothermal alteration, and sulfide-bearing hydrothermal quartz breccia in Fatira mine area were intuitively related to sulfur-saturated, oxidized felsic magmatism and associated hydrothermal systems, they are most likely linked to the post-subduction felsite porphyries (post-Hammamat felsites ~ 607 Ma), or pertaining to the late phases of the subaerial high-K calc-alkaline volcanics (Dokhan Volcanics ≤ 620 Ma).

Aqueous formate solution for enhanced water imbibition in oil recovery and carbon storage in carbonate reservoirs

This paper presents an experimental study of the injection of aqueous formate solution into oil-wet carbonate porous media at different formate concentrations and acidity levels. The research was motivated by the potential use of aqueous formate solution as a wettability modifier in enhanced oil recovery and/or a carbon carrier in geological carbon storage in carbonate reservoirs. However, the wettability alteration of carbonate rocks by formate has not been tested at elevated concentrations of formate or in any coreflood. The experimental program in this research consists of aqueous stability, wettability alteration, and three dynamic imbibition experiments with fractured carbonate cores with varying formate concentrations up to 30 wt% and initial pH between 6 and 7.With an excess amount of calcite powder, the 20 wt% formate solution in 15000 ppm NaCl brine showed essentially the same pH history as the base NaCl brine, where calcite dissolution caused the solution pH to increase to a stable value near 9. None of the samples studied in this research showed solid precipitation. Material balance of dynamic imbibition data indicated that the imbibition of formate into the matrix was most significant in coreflood #1, in which 30 wt% formate solution was injected into a fractured carbonate core. A large gradient in formate concentration between the fracture and the matrix likely caused the rapid mass transfer. Then, calcite dissolution and the resulting formate species caused wettability alteration to enhance water imbibition, which in turn expelled the oil in the matrix.The incremental oil recovery factor was 9.1 % for 30 wt% formate (pH = 7; coreflood #1), 2.9 % for 5 wt% formate (pH = 7; coreflood #2), and 7.0 % for 5 wt% formate + HCl (pH = 6; coreflood #3). A greater oil recovery factor resulted from a greater concentration of formate and a reduced pH. This is the first time corefloods were performed with aqueous formate solution at elevated concentrations up to 30 wt%.

Genesis of the world-class Dachang gold deposit, northern Qinghai-Tibet Plateau: A multiproxy approach

Dachang is a world-class gold deposit that occurs in the Bayan Har Terrane, northern Qinghai-Tibet Plateau, with Au resources of 121 t at an average grade of 3.0 g/t. Although this deposit was investigated in some previous studies, its genesis remains debated. The deposit is controlled by the Gande-Maduo anticlinorium and thrust fault zone, and the orebodies occur in subsidiary interlayer faults or fractures in thin-bedded, stratatoid, lenticular and vein forms. The host strata belong to the Lower Triassic Bayan Har Shan Group consisting of metasandstone, metasiltstone, slate, and carbonaceous slate interlayered with phyllite. The mineralization of Dachang can be divided into the following four stages. Stage 1 gold-sulfide-quartz veins, stage 2 gold-sulfide-carbonate-quartz veins, stage 3 stibnite-quartz veins and stage 4 quartz-carbonate veins. There are two types of ores: altered rocks and quartz veins. The main ore minerals are pyrite, arsenopyrite, stibnite and native gold, with trace amounts of tetrahedrite, chalcopyrite, galena and sphalerite. The wallrock alteration comprises mainly sulfidation, silicification, carbonatization, and sericitization. The element association is As, Sb, Au, Te, Bi, Ag, W and Pb in order of enrichment factors, whereas Cu, Mo, and Sn are depleted relative to their crustal abundances.In this study, we identified three types of primary fluid inclusions in quartz from veins of the four mineralization stages as follows: type 1 liquid-dominated two-phase aqueous inclusions; type 2 aqueous-carbonic inclusions, including aqueous-dominated (type 2a) and carbonic-dominated inclusions (type 2b); and type 3 two-phase hydrocarbon inclusions. Microthermometric measurements of fluid inclusion assemblages (FIAs) and bulk fluid inclusion analyses show that the ore-forming fluids had similar low-salinity compositions (1.7 to 6.9 wt% NaCl equiv.) in the H2O-CO2-N2 system with trace amounts of hydrocarbons. The mineralization temperatures range from 150 to 200 °C, and estimated depths are approximately 5 to 6.5 km. Fluid/wallrock reactions and immiscibility or boiling during mineralization in response to pressure fluctuations in a brittle faults were the main triggers for gold precipitation at Dachang.The hydrogen, oxygen, carbon and He-Ar isotopic compositions indicate that the ore-forming fluids originated from metamorphic devolatilization of immediate- and deep-level country rocks and no meteoric water or mantle volatiles were involved. The organic carbon proportions account for 42 to 53% of the total carbon in ore-forming fluid, and carbon fractions from sedimentary carbonates range from 47 to 58%. The gold mineralization followed immediately after the regional metamorphism (240–225 Ma) and occurred when the host rocks had exhumated into a brittle environment. The Dachang deposit is an epizonal orogenic gold deposit.

Elastic response of porous rock to accumulated slip on strike slip fault networks in geo-reservoirs

Subsurface reservoirs are generally highly fractured, whereby fractures constitute a natural fluid flow path and define the preferential flow direction. Slip accumulated during the faulting process alters also the petrophysical properties of the host rock. Although the mechanical alteration of the host rock, and related porosity and permeability changes, due to fault slip has been previously described, a predictive physics-based model has not been scaled with fault length in reservoirs to provide an initial porosity permeability alteration model yet. In this study, we develop a predictive model to quantify how accumulated fault slip changes porosity and permeability in a porous medium, by combining deformation modeling based on triangular dislocations and linearized poro-elasticity equations. We applied our model the Ghawar field fault map and rock-types. We conducted a Monte-Carlo simulation, varying fault roughness and accumulated slip, to quantify the corresponding variation in porosity and permeability using a 5 km long strike-slip fault and three different rock-types. Our Monte-Carlo simulation shows that long-term accumulated slip on rough strike-slip fault surfaces change porosity by ±1%, leading to an absolute permeability change of up to 22.5%. We further used these results as a benchmark for the elastic response of porous rocks to accumulated slip scaled to certain fault length. Using these benchmark results for Ghawar field reservoir rocks, we determined the slip-related porous medium permeability changes for every fault on the Ghawar fault map, accounting for their length, location, and orientation. In doing so, we found that fault roughness, slip amount, and shear sense all affect the medium’s permeability, creating substantial permeability anisotropies. Locally, these anisotropies are further enhanced by superposition of permeability changes of individual faults that constitute the fault system. We suggest the resulting permeability distribution model should be used as the initial permeability model for porous media in fractured reservoirs.

Genesis and Prospect Evaluation of Sinongduo Skarn Type Lead-zinc Deposit in Tibet

Tibet Sinongduo lead-zinc mine is one of the largest lead-zinc mines in Tibet Autonomous Region, located in Gongjue County, Changdu City, Tibet Autonomous Region. The deposit mainly produces lead, zinc and other metal elements, which is one of the important metal resources in Tibet. This paper mainly introduces the geological characteristics, ore body characteristics, ore types and wall rock alteration of the Sinongduo lead-zinc deposit in Tibet, further analyzes the genesis of the deposit in the region, sorts out the prospecting indicators in the region, and makes a prospect prediction. It is found that there is a prospect of expanding the scale of the deposit in the deep, southern and northern abnormal areas of the No.1 ore belt in the mining area.

Hydrothermal alteration of surficial rocks at Los Humeros geothermal field, Mexico: a magnetic susceptibility approach

Utility of the geothermal surface manifestations (GSMs; thermal springs, geysers, fumaroles, and zones of hydrothermal alteration) in the studies related to the geothermal exploration is widely recognized. The identification of hydrothermally altered rocks and zones of alteration is very important because their presence indicates the type and size of the geothermal reservoir and existing thermal conditions. The use of traditional methods (i.e., geochemistry, mineralogy, and petrography) requires expensive equipment, time-consuming, and laborious sample preparation methods. Some of the rock magnetic parameters, like magnetic susceptibility (χlf) and percentage of frequency-dependent magnetic susceptibility (χfd%), are potential to become effective additional tools in identification of the hydrothermal rocks during the initial stages of geothermal exploration. Three chemical methods, Chemical Index of Alteration (CIA), loss-on-ignition (LOI), and the binary plot (CaO + Na2O + K2O) vs. (Fe2O3 + MnO + MgO), along with two rock magnetic methods, χlf and the binary plot (χlf vs. χfd%), are applied to nine intensively altered andesite reference rocks. All the five methods have correctly identified that 99 out of the total 350 studied rocks are altered. More altered rocks are distributed surrounding the several faults in the study area. Various faults (e.g., Los Humeros fault and the Loma Blanca fault) favor fluid flow and present strong hydrothermal alteration at the surface. However, there are no altered rocks on the surface region between the E-W trending Las Papas and Las Viboras faults. The presence of only the deeper fluid pathway toward the east in the surroundings of these two faults result into the almost absence of hydrothermal alteration along their strike at the surface. Consequently, there are not many altered rocks observed surroundings these two faults at the surface. These features suggest that the surface hydrothermal alteration at Los Humeros Geothermal Field (LHGF) is controlled by faults. χlf and χfd% are reliable, simple to measure, fast, cost-effective, and have the potential to become reliable additional tools for future exploration studies.

Neoproterozoic ophiolite exotic blocks in the Outer Western Carpathians, southern Poland: a record of the fast ocean-floor cooling and alterations

Ophiolitic blocks, represented by metagabbro and serpentinite containing relict pyroxene, olivine, baddeleyite, zirconolite, Ni-pyrite and pyrrhotite, were found as exotic blocks in an olistostrome in the Magura Nappe, Outer Western Carpathians. The geochemical and isotopic features of the blocks suggest they represent mantle-derived rocks, with within-plate geochemistry signatures, modified by subduction, with lithospheric mantle input. A U-Pb apatite cooling age (614 ±3 Ma) is within age uncertainty of a published U-Pb zircon magmatic crystallization age (~614 Ma) implying rapid post-crystallization cooling. Pervasive alteration with replacement of primary minerals by low-temperature assemblages is observed in all rock fragments and is interpreted as contemporaneous with shearing. The secondary mineral assemblages and temperature modelling allow the interpretation that the pervasive ocean-floor metamorphism is the alteration in these meta-mafic rocks. These ophiolitic fragments can be linked to the Neoproterozoic break up of Rodinia/Pannotia.

P-T parameters of metamorphism of the rocks of the differentiated body of the Misaelga complex (western slope of the Southern Urals)

The article presents materials on the P-T parameters of metamorphism of the rocks of the differentiated body of the Misaelga complex. Based on the study of metamorphogenic minerals and mineral associations: amphibole, amphibole-plagioclase, amphibole-garnet, epidote-garnet, muscovite, chlorite, ilmenite-titanomagnetite, a gradual decrease in temperature and pressure was established from the final stages of the magmatic stage of formation of rocks of the Misaelga complex to the beginning of the metamorphogenic stage.
 It is shown that the process of amphibolization begins immediately when the temperature of the residual melt drops by ~100°С (from 800°С to 700°С), which indicates the autometamorphic character of rock alteration. At the same time, the decomposition of the solid solution in titanomagnetite and ilmenite begins (766–588°С). Further changes include alteration (albitization) of plagioclase (550–>400°С), sericitization (~ 300°С) and chloritization (333–157°С).
 The association of minerals with garnet, distributed in a narrow interval of the section at a depth of 341.5 m with a horizon thickness of no more than 2–2.5 m, is a zone in which dynamothermal metamorphism manifested itself, due to fluid working out of the substrate of the Taratash complex in shear deformation zones, identified in the interval 1400–1200 Ma. The P-T parameters of dynamothermal metamorphism, determined by the garnet-epidote association, correspond to 550–580°С at a pressure of 2 kbar, and according to the garnet-amphibole geobarometer – 300–400°С. At the same time, it should be emphasized that these associations characterize the regressive stage of metamorphism.