Geologic features and durability of limestones at the sphinx

Geologic features considered to influence durability of the limestone at the sphinx are depositional history, evolution of porosity, and development of joints. These strata have two orthogonal groups of vertical joints. Where these joints intersect, wedge-shaped blocks are separating, causing loss of material from the core. Major reduction of the sphinx, however, is occurring due to salt crystallization aided by the ink-bottle pore systems prevailing in these rocks.The weathering profile exhibits alternating layers, less and more highly weathered. The less weathered rock is a biomicritic grainstone with smaller quantities of halite and gypsum and a larger large-pore-to-throat ratio than the indented, predominantly micritic, layers with larger concentration of salts and a smaller large-pore-to-throat ratio. Based upon poresize distributions, pressures generated in the pores have been calculated using thermodynamic principles, and an equation has been derived that provides a quantitative measure of durability of these rocks.     

Depositional environment in and around Tamiraparani estuary,and off Tuticorin,Tamil Nadu,India: clues from grain size studies

The sediments of the estuaries and offshore Tuticorin along the southern coast of Tamil Nadu were studied for their textural variation. Ninety sediment samples were collected from three sectors (river, estuary, offshore) on the basis of prevailing energy conditions and oceanographic parameters. Frequency curves show unimodal to bimodal in nature. Offshore samples also fall in bimodal with medium and fine sand where sediment is additionally deposited by the Tamiraparani River. The mean values ranging from 1.69 to 2.01 φ with medium to fine sand. The fine sand in the riverine region indicates the depositional nature of the sediments. In pre-monsoon, the standard deviation ranges from 0.29 to 0.81 φ, which falls in the very well sorted to moderately well sorted. Estuary ranges from 0.63 to 1.30 φ, with moderately sorted, whereas in the marine sediments ranges from 0.4 to 1.01 φ, with moderately to moderately well- sorted and some patches of poorly sorted is observed. In both monsoons, the grain size variations are mostly influenced in river and offshore samples compared to the estuaries. The CM pattern indicates the deposition of sediments in graded suspension. In estuary, the sediments are medium sand with moderately to poorly sorted, fine skewed which are indicative of denudation processes taking place there. In marine, the sediments are medium-to-fine, moderately-to-well sorted, and fine skewed to very fine skewed sediments, probably as a result of the influence of palaeo-sediments deposited by rivers from inland as well as by waves and currents from offshore. Further, the marine samples also highlight depositional processes.     

Lahnsteinite, Zn4(SO4)(OH)6 · 3H2O, a new mineral from the Friedrichssegen Mine, Germany

A new mineral, lahnsteinite, has been found in the dump of the Friedrichssegen Mine, Bad Ems district, Rhineland-Palatinate (Rheinland-Pfalz), Germany. Lahnsteinite, occurring as colorless tabular crystals in the cavities of goethite, is associated with pyromorphite, hydrozincite, quartz, and native copper. The Mohs’ hardness is 1.5; the cleavage is perfect parallel to (001). D _calc = 2.995 g/cm³, D _meas = 2.98(2) g/cm³. The IR spectrum is given. The new mineral is optically biaxial, negative, α = 1.568(2), β = 1.612(2), γ = 1.613(2), 2V _meas = 18(3)°, 2V _calc = 17°. The chemical composition (wt %, electron microprobe data; H₂O was determined by gas chromatography of ignition products) is as follows: 3.87 FeO, 1.68 CuO, 57.85 ZnO, 15.83 SO₃, 22.3 H₂O, total is 101.53. The empirical formula is (Zn_3.3Fe_0.27Cu_0.11)_Σ3.91(S_0.98O₄)(OH)₅ · 3H_2.10O. The crystal structure has been studied on a single crystal. Lahnsteinite is triclinic, space group P1, a = 8.3125(6), b = 14.545(1), c = 18.504(2) Å, α = 89.71(1), β = 90.05(1), γ = 90.13(1)°, V = 2237.2(3) ų, Z = 8. The strong reflections in the X-ray powder diffraction pattern [d, Å (I, %)] are: 9.30 (100), 4.175 (18), 3.476 (19), 3.290 (19), 2.723 (57), 2.624 (36), 2.503 (35), 1.574 (23). The mineral has been named after its type locality near the town of Lahnstein. The type specimen of lahnsteinite is deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, registration number 4252/1.     

Structure of magnetite lodes at the Estyunino iron deposit in the central Urals

The structure of magnetite lodes is determined by iron and sulfur distribution, as well as texture and structure of ore. These features have been revealed by documentation of cores from ore intervals in exploration boreholes penetrating two main lodes 21 and 22 of the Estyunino iron deposit. The documentation of cores was accompanied by sampling for microscopic examination of texture and structure of ore and selection of Fe and S contents in ore. Then these data were summarized as sections of the lodes. It was established that the structure of magnetite lodes is characterized by conformable ore layers distinguished by texture, structure, and Fe and S contents. Banded and spotty ores containing less than 50% magnetite are predominant. Layers of homogeneous massive ore are infrequent. The textural pattern indicates a volcaniclastic nature of host rocks. The spotty texture is characteristic of hyaloclastites with vitreous shards. The banded texture with oriented distribution of fiamme is inherent to volcaniclastic rocks. In both cases, magnetite selectively replaces dark-colored vitreous fragments and is also dispersed in the salic matrix and lava fragments. No indications of crosscutting superposed relationships are observed. The available data can be satisfactorily explained by an impregnation-metasomatic mode of ore deposition.     

Metallogenic mechanisms of super-large Cu and Au deposits in the Dexing region and simulating experiments

Based on the results of previous studies and under the direction of the theory of “ore deposit genesis”, the authors made use of high temperature and high pressure experimental facilities and conditions at the Tectono-geochemistry Research Room under the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, and put the focus on the multi-source of tectonically controlling ore-forming materials, the characteristics of multi-stage and multi-episode hydrothermal activities and mineralization and the characteristics of multi-genesis and multi-ore deposition so as to shed light on the metallogenic mechanisms of super-large Cu and Au deposits. In addition simulating experiments were made on multi-stage and multi-episode tectonic activities and rock and ore deformation, multi-stage and multi-episode tectonic activities and mobilization and migration of ore-forming materials, and multi-stage and multi-episode tectonic activities and superimposition and enrichment of ore-forming materials. The experimental results showed that under the action of multi-stage and multi-episode tectonic stress the deformation and fragmentation of not only rocks and ores have been intensified, and but also the ore-forming materials originally disbursed in the rocks and ores have been mobilized and migrated and superimposed and enriched. The experimental results also provided the scientific experimental data and grounds for deep-going research on the rules of metallogenesis and geneses of super-large ore deposits in the Dexing region, Jiangxi Province.     

Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites

La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km³. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (76–77 wt % SiO₂) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (66–70 wt % SiO₂), dense (40–60% vesicularity)and crystal rich (30–40 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770–790°C with melt water contents of 3·1–4·4wt %. The pre-eruptive Toconao magma was cooler (730–750°C)and its melt more water rich (6·3–6·8 wt% H₂O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (⁸⁷Sr/⁸⁶Sr_i 0·7094–0·7131;¹⁴³Nd/¹⁴⁴Nd 0·51222–0·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H₂O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber     

Plagioclase dissolution related to biodegradation of oil in Brent Group sandstones (Middle Jurassic) of Gullfaks Field, northern North Sea

Brent Group sandstones from the north side of the Gullfaks Oilfield contain mostly 5–8% albitic plagioclase, whereas plagioclase is almost absent in the same strata in the southern part of the field. Absence of plagioclase throughout the entire vertical extent of the Brent Group in the southern wells seems to rule out provenance as the principal explanation for differing plagioclase content, which is therefore interpreted as the result of diagenesis. Hypotheses for the nature of the inferred leaching event include epigenetic meteoric diagenesis and mesogenetic release of acid components from clay minerals or kerogen, but these explanations are unable to account for the observed spatial distribution of the plagioclase-bearing and plagioclase-free sandstone intervals. However, overall correspondence between the area lacking plagioclase and oil compositions having both anomalously high CO₂ and organic geochemical indications of advanced biodegradation suggest a link between plagioclase dissolution and biodegradation of the present oil column. It is, therefore, proposed that acid components from biodegradation selectively reacted with albitic plagioclase to form kaolin, releasing sodium bicarbonate into the residual water. The plagioclase-free sandstones contain more kaolin than the plagioclase-bearing sandstones, as would be expected due to aluminium conservation. However, the wide and overlapping ranges of kaolin content in both groups suggest that most of the kaolin originated from processes other than biodegradation-driven plagioclase alteration, potentially including both epigenetic and mesogenetic dissolution, as well as deposition of detrital kaolin and kaolin precursors.     

Geochemical and electron paramagnetic characteristics of quartz from a multi-stage vein environment, Cowarra gold deposit, New South Wales

The characterisation of quartz using trace element geochemistry and electron paramagnetic resonance measurements has been tested for different generations of quartz veining in the multi-stage hydrothermal vein system at Cowarra. Quartz closely associated with gold mineralisation can be distinguished from earlier and later quartz generations in the mineralised environment, and both types can be distinguished from regional, barren quartz veins. These separate stages of quartz veins are also recognised from their fluid inclusion characteristics. Geochemical differences in the quartz partly reflect the presence of minor mineral inclusions, particularly sulfides and arsenopyrite.The study highlights the importance of understanding the paragenetic sequence of quartz vein development in a mineralised environment and the need for careful, thorough sampling before attempting geochemical and EPR characterisation of quartz as an exploration technique.     

Application of the photoacoustic method for characterization of natural galena (PbS)

Electron transport properties of single crystal and polycrystalline natural mineral galena (PbS) samples from the Trep?a mine, Yugoslavia, were determined using the photoacoustic frequency transmission technique. Their thermal diffusivity (D _T≈0.16 × 10^?5 m² s^?1), the coefficient of diffusion (D between 0.15×10^?2 0.16×10^?2 m² s^?1) and lifetime of the excess carrier (τ≈35 μs and the front and rear recombination velocity (s _g≈65.5 m s^?1 and s _b≈66.4 m s^?1, respectively), were calculated by comparing the experimental results and the theoretical photoacoustic amplitude and phase signals. The lattice parameter obtained by X-ray work was a?=5.936?Å. The free carrier concentration of these single-crystal samples was measured using the Hall method (N?=?3×10¹⁸ cm^?3). Measurements of the optical reflectivity of the same samples, as a function of wavelength, in the infrared and far infrared ranges, were performed. In the far infrared range a free electron plasma frequency was observed and numerically analyzed, using the least-squares fitting procedure. The values of optical parameters were calculated and the value of the free carrier concentration obtained by the Hall method was confirmed.