Pyroclastic Volcanic Rocks Research Articles

ANALYSIS OF SUBSURFACE STRUCTURE OF BANDAR LAMPUNG CITY BASED ON GRAVITY ANOMALIES

Bandar Lampung City is located at the southeastern tip of Sumatra Island, an area highly prone to earthquake and tsunami disasters. Along the Sumatra Island, there are seismic faults stretching along the Bukit Barisan Mountains. In the Bandar Lampung region, one of these faults is the Panjang Lampung fault. Gravity methods are commonly used to identify subsurface structures based on variations in rock density. This study aims to identify the subsurface structure of Bandar Lampung City based on gravity anomaly modeling, both 2D and 3D models. The research consists of three main stages: data correction, data processing including spectrum analysis, moving average, second vertical derivative analysis, and subsurface structure modeling. The complete Bouguer anomalies in the study area range from 41.9 mGal to 73.3 mGal. Modeling results indicate the presence of structures such as the Panjang Lampung fault in the Northeast and a graben in the Central region, verified through SVD analysis and geological information. The existence of the Panjang Lampung fault, classified as an active fault, along with volcanic pyroclastic rocks and significant sediment layers in the central region, makes the research area potentially susceptible to the impact of amplification in the event of an earthquake disaster.

Re-evaluation of a geothermal system model based on high-resolution gravity field inversion: A case study of the Maritaing geothermal field, Alor Regency, East Nusa Tenggara Province, Indonesia

The Maritaing region in East Nusa Tenggara Province, Indonesia, offers significant geothermal potential for direct power generation and use. This study aims to re-evaluate the geothermal system model in the Maritaing geothermal field using high-resolution gravity inversion. High-resolution gravimetric data were collected using satellite observations and data processing techniques, allowing a better understanding of the subsurface geological structure and density distribution of rocks associated with the geothermal reservoir. This study applied data corrections to acquire complete Bouguer anomalies, gravity gradient analysis to detect geological structures, and inversion modeling of residual gravity anomalies to predict subsurface rock density distribution. The complete Bouguer anomalies exhibit positive values ranging from 70 to 140 mGal and have a northwest–southeast orientation, indicating the existence of andesitic lava and pyroclastic volcanic rocks. Gravity gradient analysis determines the geological structures, such as faults or fractures, used as conduits to create Kura Hot Springs. The model provides useful insights into geological occurrences, such as faults, rock layers, and reservoir accumulations that support geothermal fluids. Based on the present study’s findings, we provide recommendations for developing and managing geothermal resources in the Maritaing area.

Stability analyses of diversion tunnel portal slopes at the Cidanau Dam, Banten, Indonesia

The inlet and outlet portal slopes of the Cidanau Dam diversion tunnel were designed with 55° and 46° overall slope angles, respectively, based on stability chart method static analyses. However, the performance of tunnel portal slopes under earthquake loads has not been verified whereas the research area is located in a high earthquake hazard zone. This research was conducted to verify the slope performance under static and earthquake load using limit equilibrium (LE) and finite element (FE) methods. The engineering geological mapping and rock cores evaluation showed that both portal slopes comprised four materials: residual soil, very poor-lapilli tuff, poor-lapilli tuff, and fair-tuff breccia. The stability analyses results showed that the designed tunnel portal slopes were in a stable state under static and earthquake loads. The lowest safety factor (FS) values at the inlet section under static and seismic load are 1.9 and 1.24, and at the outlet section are 2.29 and 1.69, respectively. However, the weathered pyroclastic volcanic rocks comprising the slope is required to be protected from severe degradation upon exposure during and after construction. Results of this study are expected to contribute to the safe design of rock slopes in tropical areas, particularly in Indonesia.

A robust age model for the Cryogenian Pocatello Formation of southeastern Idaho (northwestern USA) from tandem in situ and isotope dilution U-Pb dating of volcanic tuffs and epiclastic detrital zircons

Abstract Tandem in situ and isotope dilution U-Pb analysis of zircons from pyroclastic volcanic rocks and both glacial and non-glacial sedimentary strata of the Pocatello Formation (Idaho, northwestern USA) provides new age constraints on Cryogenian glaciation in the North American Cordillera. Two dacitic tuffs sampled within glacigenic strata of the lower diamictite interval of the Scout Mountain Member yield high-precision chemical abrasion isotope dilution U-Pb zircon eruption and depositional ages of 696.43 ± 0.21 and 695.17 ± 0.20 Ma. When supplemented by a new high-precision detrital zircon maximum depositional age of ≤670 Ma for shoreface and offshore sandstones unconformably overlying the lower diamictite, these data are consistent with correlation of the lower diamictite to the early Cryogenian (ca. 717–660 Ma) Sturtian glaciation. These 670–675 Ma zircons persist in beds above the upper diamictite and cap dolostone units, up to and including a purported “reworked fallout tuff,” which we instead conclude provides only a maximum depositional age of ≤673 Ma from epiclastic volcanic detritus. Rare detrital zircons as young as 658 Ma provide a maximum depositional age for the upper diamictite and overlying cap dolostone units. This new geochronological framework supports litho- and chemostratigraphic correlations of the lower and upper diamictite intervals of the Scout Mountain Member of the Pocatello Formation with the Sturtian (716–660 Ma) and Marinoan (≤650–635 Ma) low-latitude glaciations, respectively. The Pocatello Formation thus contains a more complete record of Cryogenian glaciations than previously postulated.

A Fundamental Study on the Potential of Alkali-Aggregate Reaction according to KS F 2545 and ASTM C 1260 Test Methods

Chemical experiment KS F 2545 and Physical experiment ASTM C 1260 has been accomplished to estimate the potential of alkali aggregate. Used for testing aggregate samples are forest aggregate and recycled aggregate which collected in Gangwon province Samcheok and Pyeongchang, Jeollabuk province Gimje and Kochang, and Gyeongsangnam province Goryeong. As the results of chemical experiment confirmed that if silicate rock and carbonate rock are mixed, reduction in alkalinity is increase. So it has been identified that case makes a disturb at the result of alkali aggregate reaction. In 9 out of the 62 aggregate samples check dissolved silica exceeding 100 mmol/ℓ. and mortar bar length increase rate confirmed that 5 of 9 chemical method aggregates were 0.1~0.2% and 2 aggregates were 0.2%. As a result of the alkaline aggregate reaction test using the chemical method and the mortar bar method, the aggregates showing alkali aggregate reaction are sandstone and tuff aggregates. Therefore, Alkali aggregate reaction tests are required to use clastic sedimentary rocks and volcanic pyroclastic rocks aggregates.

Geochemical characteristics of Late Carboniferous mineralization in the East Tianshan: A case study of the Meiling deposit in the Kalatage area

The Meiling deposit in the Kalatage area of the East Tianshan, which is the most recently identified Late Carboniferous hydrothermal vein deposit, is located in a series of basic-intermediate-acidic volcanic-pyroclastic rocks and contains ore bodies composed of single quartz-sulfide veins and quartz-sulfide vein groups in a stockwork/en-échelon arrangement with Cu as the primary ore-forming element and Zn, Au and Ag as secondary ore-forming elements. This study examines the geochemical characteristics of sub-volcanic rocks and analyses fluid inclusions and stable isotopes (H, O and S) to determine the metallogenic characteristics of the Meiling deposit and discusses the pattern and geological significance of the mineralization in the Kalatage area. The quartz porphyry, dacite porphyry and granite porphyry from the Meiling deposit, which are closely related to the mineralization, are calc-alkaline magmatic rocks that formed in an island arc setting. Most of the primary quartz fluid inclusions are liquid-rich two-phase FIs with homogenization temperatures mainly of 154–251 °C and salinities of 3.1–5.6% NaCl equivalent and the sphalerite fluid inclusions are liquid-rich two-phase FIs characterized by omogenization temperatures mainly of 156–263 °C and salinities of 2.4–5.3% NaCl equivalent, which are low temperature and low salinity. H and O isotope analysis show that in the polymetallic sulfide-quartz stage, the δD values ranged from −79.3‰ to −54.0‰, the δ18OH2O values ranged from −8.99‰ to −2.57‰, and the primary ore-forming fluids were mixtures of a dominant meteoric water phase with minor amounts of magmatic fluid. S isotope analysis shows that in the polymetallic sulfide-quartz stage, the δ34S values of the main sulfides were distributed between −1.6‰ and 5.2‰, similar to the range of a homogenous sulfur source. These results, together with the S-isotopic characteristics of the wall rocks from the Meiling deposit, demonstrate that the ore-forming metals might have originated from the volcanic-subvolcanic wall rocks. These results show that the metallogenic characteristics of the Meiling deposit are consistent with those of low temperature epithermal deposits and clearly demonstrate that the Meiling deposit is a low temperature epithermal deposit that formed in an island arc setting. The Kalatage area provides a natural laboratory for studying the characteristics of geological mineralization during the arc-basin system transformation process.

Timing of the Erlangping back‐arc basin in the Qinling orogen, central China and its tectonic significance

AbstractBack‐arc basins hold the key in understanding the geodynamics of orogenic processes. The Qinling–Dabie orogenic belt in central China is one of the most important orogenic belts constraining the tectonic framework of eastern Asia. However, its Palaeozoic accretionary processes remain equivocal, mainly derived from the age uncertainty of the back‐arc basin in the Qinling orogen. We carried out zircon U–Pb geochronology for two pyroclastic volcanic rocks intercalated within the Erlangping back‐arc basin basalts. They yield U–Pb ages of 435.8 ± 4.2 Ma and 435.7 ± 3.8 Ma, which precisely constrain the timing of the back‐arc basin opening. The opening of the Erlangping back‐arc basin might have been triggered by the rollback of the Proto‐Tethyan oceanic slab due to the southward migration of arc magmatism at ca. 440 Ma. The Palaeozoic tectonic evolution and orogen‐scale geodynamic processes of the Qinling orogen are thus reconstructed.

Geodynamic, Tectonic, and Magmatic Indicators of Large Uranium Clusters of the Transbaikal–Mongolia–North China Province

The Transbaikal–Mongolia–North China province of East Asia is ranked as the most productive uranium province. Its area contains hundreds of occurrences, dozens of deposits of different (endo-, exo-, and polygenic) types, and several large uranium-bearing clusters and districts. The largest F–Mo–U clusters were revealed in the Mongol–Argun and Inshan–Liaohe volcanoplutonic zones (VPZ) in the western and southwestern periphery of the Great Xingan belt, respectively. Tectonic data on the Late Mesozoic (J2–K) Tulukuev (Southeastern Transbaikalia), Dornod (East Mongolia), and Guyuan–Duolung (Inshan–Liaohe, China) depressions of the above-mentioned VPZ are analyzed to distinguish their important structural details, which are responsible for the distribution of uranium-bearing fields and deposits in the volcanic rocks. Some of the deposits are associated with paleovolcanic necks, extrusive rocks, subvolcanic bodies, and dikes and have complex morphology. Other deposits are localized in stratified volcanic–pyroclastic rocks or within the basement rocks of the depressions. Recent geochronologic, geophysical, and seismotomographic data indicate not only the synchronous formation of the ore clusters in the Early Cretaceous (Valanginian), but also their localization within a crust of moderate (36–42 km) thickness above the periphery of a stagnant slab in the mantle transition zone. The coincidence of the slab boundary projections and area of their influence with the spatial position of large F–Mo–U ore clusters (Streltsovka, Dornod, and Guyuan–Duolung) separated from each other by hundreds and thousands of kilometers is regarded as evidence for the possible influence of deep geodynamics on the formation of the corresponding mineralization. Some of the aforementioned and similar clusters of the province seem to be insufficiently studied. This refers, in particular, to the exploration of the thick volcanic–pyroclastic fields associated with volcanic paleocenters within the clusters, as well as the basement rocks of the basin.

Structural control on overprinting high-sulfidation epithermal on porphyry mineralization in the Chodarchay deposit, northwestern Iran

The Chodarchay high-sulfidation epithermal-porphyry deposit is located in the Tarom volcano-plutonic subzone of Western Alborz structural zone in NW Iran. In addition to Chodarchay, the Tarom subzone hosts several other epithermal deposits. Lithologic units in Chodarchay consist of volcanic-pyroclastic rocks and intrusive bodies. Petrographical studies have identified different rocks which host the mineralization. Mineralization has taken place in both volcanic-pyroclastic units and intrusions. Detailed mineralogical and alteration data gathered from drill cores and thin sections provides support for the hypothesis that the high-sulfidation epithermal mineralization is underlain by a porphyry copper system. This deposit is the first reported porphyry-epithermal mineralization in this subzone. The main structure of the Chodarchay area is a fault zone that formed parallel to the general Tarom NW-SE direction. Based on remote sensing studies, on a wider scale, lineaments generally show NE-SW trends, but the Chodarchay fault zone has a NW-SE strike and is associated with another W-E trending fault zone. These fault zones are correlated with argillic alteration. There are two sets of reverse fault fractures with a strike-slip component, based on slickenlines in the field and core observations. A normal sense of fault movement remains in the NW part of the structural system. These new data sets indicate two main extensional and compressional mechanisms for the fault. The evolution of the fault system from normal to reverse in the deposit characterizes the present day structural system of the Chodarchay area. Plutonism and porphyry mineralization are related to the extensional stage, whereas the epithermal part of the deposit is associated with the compressional stage of fault activation.

Telluromandarinoite, A New Tellurite Mineral From the El Indio-Tambo Mining Property, Andes Mountains, Chile

Abstract Telluromandarinoite, a tellurite, is a new mineral species from the Wendy open pit, Tambo mine, El Indio-Tambo mining property, Coquimbo Province, Chile. The ideal endmember telluromandarinoite formula is Fe 3+ 2 Te 4+ 3 O 9 ·6H 2 O and it is the Te 4+ analogue of the selenite mineral mandarinoite, Fe 3+ 2 Se 4+ 3 O 9 ·6H 2 O. These deposits are located in rhyolitic and dacitic pyroclastic volcanic rocks of Tertiary age (8–11 Ma) that are strongly hydrothermally altered. The mineralization in the Tambo area is characterized by high-level epithermal veins and breccias located along roughly east–west structures. Hydrothermal breccias consisting of silicified clasts of dacite tuffs cemented by a silica/barite/alunite matrix are common at the occurrence. In fact, all studied specimens containing tellurite mineralization are associated with alunite. Telluromandarinoite is translucent, pale green, with a white streak and vitreous luster. It forms as individual platy crystals, 0.2 mm or less in size, but more commonly as aggregates of platy crystals. The crystals are too small to allow a Mohs hardness determination; they are brittle with an uneven fracture and no observed cleavage or parting. Telluromandarinoite is biaxial positive with α = 1.750(3), β = 1.807(3), and γ = 1.910(5), with a calculated 2 V = 76.9°. The optical orientation is Y = b , c ˆ Z = 10° in obtuse β. No dispersion was noted and no pleochroism was observed. An average of 10 electron microprobe analyses gave SeO 2 22.91, TeO 2 44.30, Fe 2 O 3 26.43, and H 2 O (calc.) 17.59, total 111.23 wt.%. The mineral loses H 2 O in vacuum, so the high totals obtained were expected. The empirical formula (based on 15 O atoms) is Fe 3+ 2.03 (Te 4+ 1.71 Se 4+ 1.27 ) Σ2.98 O 9 ·6H 2 O with Z = 4, and D calc = 3.372 g/cm 3 . Spot analyses gave stoichiometries that range from telluromandarinoite Fe 3+ 2.03 (Te 4+ 2.12 Se 4+ 0.86 ) Σ2.98 O 9 ·6H 2 O to mandarinoite Fe 3+ 2.07 (Se 4+ 1.64 Te 4+ 1.31 ) Σ2.95 O 9 ·6H 2 O. A crystal-structure analysis shows the mineral to be monoclinic, space group P 2 1 / c , with a 16.9356(5), b 7.8955(3), c 10.1675(3) A, β 98.0064(4)°, and V 1346.32(13) A 3 . The strongest lines in the X-ray powder pattern [ d in A,( I ),( hkl )] are: 8.431(44)(200), 7.153(100), 3.5753(41), 3.4631(21), 2.9964(34), 2.8261(19)(412). The crystal structure of telluromandarinoite is similar to that of emmonsite, Fe 3+ 2 Te 4+ 3 O 9 ·2H 2 O.

Multiple sources and genesis of the Suoerkuduke Cu-Mo deposit during the Carboniferous, East Junggar: Insights from zircon U-Pb age and C-O-S-Pb isotopes

The Suoerkuduke Cu-Mo ore bodies are closely associated with skarns, but this deposit shows distinct characteristics from typical contact metasomatic skarn deposits. No carbonate rocks and coeval magmatic activities were found in the oredistrict. Thus, there is no unidirectional zonation sequence from proximal to distal, but a bidirectional skarn zonation as follow: garnet skarn in the center, and epidote-garnet skarn, epidote skarn and diopside-hedenbergite skarn (with no symmetrical distribution) in outer parts. The sulfides are pyrite, chalcopyrite, molybdenite, sphalerite, galena and pyrrhotite, and no obvious sulfide zonation was observed.The multiple sources of ore-forming fluids and ores imply a complicated and distinct metallogenic process of the Suoerkuduke deposit. The ore-forming fluids mainly consist of magmatic water, meteoric water and seawater. The coeval magmatic-hydrothermal activities took place in the nearby area of the Suoerkuduke oredistrict, but not within the oredistrict, indicating that magmatic water was most likely derived from a batholith at depth. This hidden batholith provided magmatic fluids, most of the sulfur and a portion of Cu metal for the Suoerkuduke deposit. The Mo metal was derived from the leaching of andesitic rocks and tuffaceous sandstone by meteoric water. The ascending magmatic fluids mixed with deeply circulated meteoric water and seawater, and then leached volcanic-pyroclastic rocks to enrich Ca, Fe and Cu metals. These fertile fluids subsequently metasomatised andesitic porphyry, pyroxene andesite and tuffaceous sandstone to form the skarn. The decrease in the temperature and fO2 values facilitated the deposition of sulfides.

Spheroidal halloysites from Patagonia, Argentina: Some aspects of their formation and applications

Halloysite deposits in Argentina have been identified in the province of Río Negro (Patagonia). The mineralized area occurs as altered zones in Eocene volcanic–pyroclastic rocks, dacitic to rhyolitic in composition. A comprehensive study of these deposits was carried out by means of mineralogical, geochemical and isotopic analyses.Intense weathering has transformed the whole rock to a white mass composed of 75% –90% halloysite+kaolinite, with cristobalite, tridymite, and quartz, as the main non-clay minerals. Ferruginous beidellite and titanium minerals are also present in minor amounts.The halloysite–kaolinite ratio ranges from 75 to 25 to 100–0. Due to the alteration of very dense rocks, halloysite morphology is predominantly spheroidal. Tubular halloysite is the main constituent in more porous rocks, but the latter are scarce in the area.Because of the predominance of the spheroidal type, the use of halloysite as HNT (halloysite nanotube) is not feasible. Nonetheless, this mineral can be modified by different organic molecules, and used to remove pollutants such as emulsified hydrocarbons and heavy metals. A special product made with this halloysite is used as sunscreen when sprayed on fruits in areas of intense solar radiation. The whiteness of this mineral is not very high due to the amount of titanium oxide present in its composition (about 1%). Nevertheless, titanium oxide is present as individual particles, so it could be removed by a mechanical process.New applications for spheroidal halloysite are currently being investigated. Moreover, depending on the price and demand, the tubular halloysite of low grade deposits could be exploited.

Soft rocks in Argentina

Soft rocks are a still fairly unexplored chapter in rock mechanics. Within this category are the clastic sedimentary rocks and pyroclastic volcanic rocks, of low to moderate lithification (consolidation, cementation, new formed minerals), chemical sedimentary rocks and metamorphic rocks formed by minerals with Mohs hardness less than 3.5, such as limestone, gypsum, halite, sylvite, between the first and phyllites, graphitic schist, chloritic shale, talc, etc., among the latter. They also include any type of rock that suffered alteration processes (hydrothermal or weathering). In Argentina the study of low-strength rocks has not received much attention despite having extensive outcrops in the Andes and great impact in the design criteria. Correlation between geomechanical properties (UCS, deformability) to physical index (porosity, density, etc.) has shown promising results to be better studied. There are many studies and engineering projects in Argentina in soft rock geological environments, some cited in the text (Chihuído dam, N. Kirchner dam, J. Cepernic Dam, etc.) and others such as International Tunnel in the Province of Mendoza (Corredor Bioceánico), which will require the valuable contribution from rock mechanics. The lack of consistency between some of the physical and mechanical parameters explored from studies in the country may be due to an insufficient amount of information and/or non-standardization of criteria for testing materials. It is understood that more and better academic and professional efforts in improving techniques will result in benefits to the better understanding of the geomechanics of weak rocks.

Sediment-infill volcanic breccia from the Neoarchean Shimoga greenstone terrane, western Dharwar Craton: Implications on pyroclastic volcanism and sedimentation in an active continental margin

We report sediment-infill volcanic breccia from the Neoarchean Shimoga greenstone belt of western Dharwar Craton which is associated with rhyolites, chlorite schists and pyroclastic rocks. The pyroclastic rocks of Yalavadahalli area of Shimoga greenstone belt host volcanogenic Pb–Cu–Zn mineralization. The sediment-infill volcanic breccia is clast-supported and comprises angular to sub-angular felsic volcanic clasts embedded in a dolomitic matrix that infilled the spaces in between the framework of volcanic clasts. The volcanic clasts are essentially composed of alkali feldspar and quartz with accessory biotite and opaques. These clasts have geochemical characteristics consistent with that of the associated potassic rhyolites from Daginkatte Formation. The rare earth elements (REE) and high field strength element (HFSE) compositions of the sediment-infill volcanic breccia and associated mafic and felsic volcanic rocks suggest an active continental margin setting for their generation. Origin, transport and deposition of these rhyolitic clasts and their aggregation with infiltrated carbonate sediments may be attributed to pyroclastic volcanism, short distance transportation of felsic volcanic clasts and their deposition in a shallow marine shelf in an active continental margin tectonic setting where the rhyolitic clasts were cemented by carbonate material. This unique rock type, marked by close association of pyroclastic volcanic rocks and shallow marine shelf sediments, suggest shorter distance between the ridge and shelf in the Neoarchean plate tectonic scenario.

Lithologic controls on mineralization at the Lagunas Norte high-sulfidation epithermal gold deposit, northern Peru

The 13.1-Moz high-sulfidation epithermal gold deposit of Lagunas Norte, Alto Chicama District, northern Peru, is hosted in weakly metamorphosed quartzites of the Upper Jurassic to Lower Cretaceous Chimu Formation and in overlying Miocene volcanic rocks of dacitic to rhyolitic composition. The Dafne and Josefa diatremes crosscut the quartzites and are interpreted to be sources of the pyroclastic volcanic rocks. Hydrothermal activity was centered on the diatremes and four hydrothermal stages have been defined, three of which introduced Au ± Ag mineralization. The first hydrothermal stage is restricted to the quartzites of the Chimu Formation and is characterized by silice parda, a tan-colored aggregate of quartz-auriferous pyrite–rutile ± digenite infilling fractures and faults, partially replacing silty beds and forming cement of small hydraulic breccia bodies. The δ34S values for pyrite (1.7–2.2 ‰) and digenite (2.1 ‰) indicate a magmatic source for the sulfur. The second hydrothermal stage resulted in the emplacement of diatremes and the related volcanic rocks. The Dafne diatreme features a relatively impermeable core dominated by milled slate from the Chicama Formation, whereas the Josefa diatreme only contains Chimu Formation quartzite clasts. The third hydrothermal stage introduced the bulk of the mineralization and affected the volcanic rocks, the diatremes, and the Chimu Formation. In the volcanic rocks, classic high-sulfidation epithermal alteration zonation exhibiting vuggy quartz surrounded by a quartz–alunite and a quartz–alunite–kaolinite zone is observed. Company data suggest that gold is present in solid solution or micro inclusions in pyrite. In the quartzite, the alteration is subtle and is manifested by the presence of pyrophyllite or kaolinite in the silty beds, the former resulting from relatively high silica activities in the fluid. In the quartzite, gold mineralization is hosted in a fracture network filled with coarse alunite, auriferous pyrite, and enargite. Alteration and mineralization in the breccias were controlled by permeability, which depends on the type and composition of the matrix, cement, and clast abundance. Coarse alunite from the main mineralization stage in textural equilibrium with pyrite and enargite has δ34S values of 24.8–29.4 ‰ and $$ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $$ values of 6.8–13.9 ‰, consistent with H2S as the dominant sulfur species in the mostly magmatic fluid and constraining the fluid composition to low pH (0–2) and logfO2 of −28 to −30. Alunite–pyrite sulfur isotope thermometry records temperatures of 190–260 °C; the highest temperatures corresponding to samples from near the diatremes. Alunite of the third hydrothermal stage has been dated by 40Ar/39Ar at 17.0 ± 0.22 Ma. The fourth hydrothermal stage introduced only modest amounts of gold and is characterized by the presence of massive alunite–pyrite in fractures, whereas barite, drusy quartz, and native sulfur were deposited in the volcanic rocks. The $$ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $$ values of stage IV alunite vary between 11.5 and 11.7 ‰ and indicate that the fluid was magmatic, an interpretation also supported by the isotopic composition of barite (δ34S = 27.1 to 33.8 ‰ and $$ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $$ = 8.1 to 12.7 ‰). The Δ34Spy–alu isotope thermometry records temperatures of 210 to 280 °C with the highest values concentrated around the Josefa diatreme. The Lagunas Norte deposit was oxidized to a depth of about 80 m below the current surface making exploitation by heap leach methods viable.

Characterization of prehispanic cosmetics found in a burial of the ancient city of Teotihuacan (Mexico)

The present paper reports the chemical data obtained on samples of pigmenting materials contained in 31 miniature vessels found in a burial found in Teopancazco, a multiethnic neighborhood center located in the southeastern sector of the archaeological site of Teotihuacan (Central Mexico) and the analytical protocol established for the complete characterization of these archaeological materials. For this purpose a multi-technique approach based on the combination of several non destructive and micro-destructive instrumental techniques, namely, light microscopy (LM), scanning electron microscopy-X-ray microanalysis (SEM–EDX), transmission electron microscopy (TEM), voltammetry of microparticles (VMP), X-ray microdiffraction (μXRD), X-ray diffraction (XRD), UV–Vis spectrophotometry, FTIR spectroscopy and gas chromatography–mass spectrometry (GC–MS) has been developed in an attempt to completely characterize these materials. These instrumental techniques provide reliable and complementary data including elemental and mineralogical composition, identification of functional groups as well as speciation studies of electroactive species. Among the most relevant findings, volcanic pyroclastic rocks and clayey soils, most probably, from the Teotihuacan valley as well as galena, jarosite and mica from other provenances in Mesoamerica, have been identified. Charcoal and Ocote resin have also been identified. The chemical composition of the samples is discussed with a view to the possible function as cosmetic materials and their use in funerary rites. The results have been critically compared with those others obtained in other ancient civilizations. The study carried out on the materials found in the burial 105 is especially relevant insofar it represents the first evidence, scientifically characterized, of the use of cosmetics in Mesoamerica at this early period (AD 200–550).

Radon concentration levels in 16th and 17th century’s churches and convents

The preliminary results of indoor radon concentration measurements taken in some of Mexico City’s colonial churches and convents are presented. Measurements were taken in the churches of Santa Catarina, La Conchita, San Juan Bautista, San Antonio Panzacola, San Diego and San Mateo and the church and convent complex of El Carmen. These structures are all located in Coyoacan, a borough of Mexico City. Indoor radon concentrations in churches and convents constructed in the 17 th and 18 th centuries are interesting for several reasons. Most of these buildings were built using the stones of ancient Mexican pyramids, mainly blocks of basalt and volcanic pyroclastic rocks, and possess walls between 40 cm and 70 cm thick and naves with large volumes of air and relative low ventilation. The churches are public places with people most of the time. The indoor radon concentrations were measured using nuclear track detectors consisting of a closed-end cup containing CR-39 Lantrack ® polymer as detector material. The measurements were taken over four periods of three consecutive months. The results show indoor radon concentrations of between 82 and 165 Bq . m –3 , below to the United States Environmental Protection Agency (USEPA) indoor radon action level for workplaces. Using these results, the radiological risks were calculated and found to be negligible.

A Raman, IR and XRD analysis of the deterioration on historical monuments: Case study from Mexico

Raman, infrared and XRD analysis have been applied to the examination of deterioration on historical monuments (Morelia, Mexico). Forty-three samples pyroclastic volcanic rocks (ignimbrites) have been studied, all originating from two ex-convents San Francisco and San Agustin, which were the first architectonic complexes in the city of Morelia. Several new mineral neoformation such as sulfates, carbonates, halides, and phosphates were identified in these samples with spectrometric and XRD technique. The observed Raman and infrared spectra are reported and some mineral compound assignments in unaltered and deteriorated volcanic rocks have been made. This survey is the first Raman and infrared spectrometric examination of the environmental mineralogy in Mexico under conditions of urban weathering that are characterized mainly by one secondary alteration formations of low scale.

Geochemistry and Stable Isotope Composition of Altered Rocks at the Golden Cross Epithermal Au-Ag Deposit, New Zealand

Golden Cross is a late Miocene volcanic-hosted epithermal Au-Ag deposit associated with quartz ± calcite ± adularia ± illite assemblages located in the Coromandel peninsula, New Zealand. Gold- and Ag-bearing quartz veins occur in intensely altered flows and lesser pyroclastic volcanic rocks of andesitic and dacitic composition. The primary control of whole-rock geochemistry at Golden Cross, as at other volcanic-hosted epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages, is the destruction of igneous plagioclase by hydrothermal alteration and K metasomatism from hydrothermal fluids. Quantitative mass change calculations indicate that, irrespective of the rock type (andesite versus dacite), hydrothermally altered rocks are enriched in K, Rb, As, and Sb around the Empire vein system and stockwork. In contrast, Na and Sr are strongly depleted around the veins, with Fe locally depleted. Silicon, Ca, Mg, and Ba are variably enriched and depleted, with Si enriched immediately adjacent to the veins and Ca only locally enriched. The enrichment and depletion trends for these elements are directly related to the degree of alteration and the alteration mineralogy. Intensely altered rocks show addition of K and Rb, coupled with loss of Na and Sr, which reflect the replacement of plagioclase by adularia and/or illite. Calcium depletion also reflects the destruction of plagioclase, although Ca loss is locally offset by the formation of hydrothermal calcite. Progressively higher Na and Sr away from the veins reflect greater preservation of plagioclase and the transition into less altered rocks on the margins. Potassium, Rb, Na, and Sr all define anomalies that extend 200 to >350 m east of the veins and are more widespread than As and Sb, which are anomalous within 150 to 200 m of the veins. The geochemical zonation patterns identified at Golden Cross are very similar to those at other epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages. Potassium, Na, Rb, Sr, and Ca enrichment and depletion patterns provide insight into the intensity and type of alteration, thereby complementing traditional analyses of Au, Ag, and other pathfinder elements, such as As and Sb, that are used to explore for epithermal Au-Ag deposits. Potassium, Na, Rb, Sr, and Ca enrichment and depletion patterns at Golden Cross are larger than halos defined by precious metals and other trace element pathfinders, thereby providing a larger target area for exploration. Potassium metasomatism is particularly important because the Empire vein system occurs in altered rocks that are significantly enriched in K. Potassium metasomatism can be evaluated by raw K 2 O data or, more effectively, by K mass changes, molar element ratios of (2Ca + Na + K)/Al versus K/Al (wt % K 2 O * 100)/ppm Sr, Rb/Sr ratios, and K numbers [K/(2Ca + K + Na) molar]. Whole-rock oxygen isotope patterns at Golden Cross are similar to those described at other epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages. However, the area defined by low δ 18 O values is smaller than the area that can be defined by field mapping of alteration minerals or whole-rock major and trace element geochemistry.

Evidence for a differentiated ignimbritic activity ending the building-stage of Tahiti-Nui (French Polynesia)

A thick columnar-jointed ignimbrite-like formation occupies the upper part of the Orohena Massif on Tahiti-Nui. Differentiated products related to the ignimbrite were sampled at the base of the Orohena cliff and have been dated using the unspiked K/Ar Cassignol technique at 502 ± 7 ka . These pyroclastic volcanic rocks show chemical similarities with Ne-syenitic rocks from a shallow magma reservoir presently exposed farther south because of the dissection of the island by deep erosion. The presence of less-evolved magma blebs in the ignimbrite products suggests an incomplete magma mixing process. This likely indicates a late injection of a more basic magma into the reservoir, probably triggering the ignimbritic eruption, unique in the generally non-explosive eruptive history of Tahiti-Nui. To cite this article: A. Hildenbrand, P.-Y. Gillot, C. R. Geoscience 338 (2006).