Silica Alteration Research Articles

Ore-formation at the Halasheng Ag-Pb-Zn deposit, northeast Inner Mongolia as revealed by trace-element and sulfur isotope compositions of ore-related sulfides

The Early Cretaceous Halasheng Ag-Pb-Zn deposit in NE China, which is mainly hosted in Jurassic rhyolitic tuff, andesite, and volcanoclastic rocks, comprises both proximal skarns and more distal hydrothermal discordant ore veins and stockworks with associated Fe-Mn-carbonate, illite, and silica alteration in NNW-trending extensional faults. Suggestions of a magmatic-hydrothermal relationship are tested by LA-ICP-MS trace-element and sulfur isotope studies of ore minerals, particularly sphalerite which is a major ore mineral.Sphalerite has at least three types: SpS, anhedral, mainly dark red sphalerite in skarns; Sp1, euhedral, normally pale yellow coarse-grained, and rarely oscillatory zoned sphalerite formed in Stage 2 of hydrothermal vein mineralization; and Sp2, fine-grained, red, non-zoned sphalerite formed in Stage 3 of hydrothermal mineralization. Sphalerite with different textures has As, Ag, Cd, Sb, and Pb contents that increase from proximal skarn to distal hydrothermal mineralization consistent with the increase in sulfosalt minerals in the late hydrothermal stage. In situ time-resolved depth profiles, together with tangential profiles, and LA-ICP-MS element mapping of sphalerite indicate that trace elements are largely in solid solution in Halasheng sphalerite with substitution mechanisms including: Zn2+ ↔ Fe2+, Cd2+, Mn2+, and 3Zn2+↔ (As3+, Sb3+) + Ag++Pb2+. Enrichment of In and Sn, and depletion of Ga and Ge, together with In/Sn ratios indicate the high-temperature ore-forming fluids at Halasheng that are most likely closely related to intrusions. In situ mean δ34S values of SpS, Sp1, and Sp2 sphalerites are 5.3‰, 7.6‰, and 8.3‰, respectively, indicating that sulfur was derived both from magmatic-hydrothermal fluids and from wall rocks with higher (>10‰) values.These data help confirm that the Halasheng Ag-Pb-Zn deposit is an intrusion-related deposit with local proximal skarn mineralization at the contact of Tamulangou Formation andesite with Ergunahe Formation crystalline limestone, and more distal vein-type volcanic-hosted hydrothermal mineralization, with both ore and gangue mineral assemblages and available fluid inclusion data suggesting an intermediate-sulfidation epithermal association. As ore-forming hydrothermal fluids cooled and evolved they became enriched in silver, with silver-bearing sulfosalt minerals and silver-rich sphalerite precipitated in the latter stages of deposition.In terms of future exploration, there is potential for the concealed Lower Cretaceous Pb-Zn skarns and intermediate sulfidation Ag-Pb-Zn epithermal deposits at Halasheng to be part of a larger porphyry-related mineral system with porphyry Cu-Au-Mo deposits, similar to those of Middle Triassic to Early Jurassic age in the same metallogenic belt, at deeper crustal levels than the known deposits.

Smooth rims in Queen Alexandra Range (QUE) 99177: Fluid–chondrule interactions and clues on the geochemical conditions of the primordial fluid that altered CR carbonaceous chondrites

Queen Alexandra Range (QUE) 99177 is one of the least altered CR carbonaceous chondrite known, with mineralogical and isotopic characteristics that indicate a high level of pristinity. In this study, we have examined the so-called smooth rims that surround many type I chondrules in QUE 99177, using SEM, EPMA, and FIB-TEM techniques. We have characterized their constituent phases to unravel the precursor material(s) of smooth rims, assess their formation mechanisms, and constrain the conditions of the altering fluid. Smooth rims are the most common type of rims around type I chondrules and exclusively occur around chondrules with Silica-rich Igneous Rims (SIRs). Smooth rims consist of an Fe-rich, hydrous silicate material that is Si- and Fe-rich, with minor Mg, Al, Ca, and Mn, and gives low analytical totals measured by EPMA. TEM observations reveal that the Fe-rich silicate phase is an amorphous gel that contains unaltered crystalline phases and igneous glass. Crystalline phases consist of igneous, unaltered, zoned pyroxenes with compositions consistent with pyroxenes in SIRs, as well as albite and chromite. The amorphous gel preserves previous crystal outlines with morphologies consistent with silica (cristobalite) grains in SIRs and has a composition identical to pseudomorphic silica replacements in SIRs. Based on these observations, we conclude that smooth rims derive from low-temperature aqueous alteration of silica in SIRs by an Fe-rich fluid. We suggest that the Fe was derived by leaching of amorphous silicates in the matrix, which reacted rapidly with melted water ice, although alteration of Fe,Ni metal blebs in SIRs could potentially be an additional source of Fe. Silica underwent dissolution and replacement whereas feldspar and glass remained unaltered because (1) the fluid was slightly alkaline, (2) cristobalite has a reaction rate much higher than quartz and feldspar, and (3) the alteration was very limited and fast, indicating that it was due solely to melting of accreted water ice and there was no introduction of additional fluid from external sources.

Spectral Analysis of Vertical Temperature Profile Time‐Series Data in Yellowstone Lake Sediments

AbstractWe use yearlong vertical temperature profile time‐series (seven thermistors at evenly spaced depth intervals from 10 to 70 cm) from five sites in and around the Deep Hole thermal area, southeast of Stevenson Island, Yellowstone Lake, to investigate heat and mass fluxes across the lake floor. The records demonstrate that thermal gradients in surficial sediments are modulated by a rich spectrum of bottom water temperature variations generated by hydrodynamic processes, and that sites inside the thermal area also respond to hydrothermal variations. We develop and implement a new method for estimating the sediment effective thermal diffusivity and pore fluid vertical flow rate that exploits the full spectrum of observed temperature variations to generate the parameter estimates, uncertainties, and metrics to assess statistical significance. Sediments at sites outside thermal areas have gradients of ∼7.5°C/m, in situ thermal diffusivities of ∼1.6 × 10−7 m2/s consistent with highly porous (80–90%) siliceous sediments, and experience hypolentic flow in the upper ∼20 cm. Sites inside the Deep Hole thermal area exhibit considerable spatial and temporal variability, with gradients of 1–32°C/m, and higher thermal diffusivities of ∼2–12 × 10−7 m2/s, consistent with hydrothermal alteration of biogenic silica to clays, quartz, and pyrite. Upward pore fluid flow at these sites is observed across multiple depth intervals, with maximum values of ∼3 cm/day. The observed spatial and temporal variability within the thermal area is consistent with upward finger flow combined with short wavelength convection within the porous sediments above a steam reservoir.

Alteration Mapping for Lead-Zinc-Barium Mineralization in Parts of the Southern Benue Trough, Nigeria, Using ASTER Multispectral Data

The lead-zinc-barium deposits of the southern Benue Trough, Nigeria belong to a suite of clastic dominated fracture filling hydrothermal vein deposits. The alteration types and spread are poorly known yet required to aid exploration. Band ratio composites (BRC), Principal Component Analysis (PCA), and Minimum Noise Fraction (MNF) were applied to a full scene Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery covering the study area. Spectral analysis of sulphide minerals known in the area led to the development of the (B1+B3)/2 ratio, which provided a highly effective sulphide discriminant. PCA and MNF bands with high eigenvectors in the absorption features of target minerals qualified as colour composite candidates for alteration mapping. This study demonstrated the effectiveness of combining the BRC, PCA and MNF techniques in the discrimination of ferric-ferrous/sulphide and silica alteration zones in the Southern Benue Trough.

Superhydrophobic Coating Derived from Geothermal Silica to Enhance Material Durability of Bamboo Using Hexadimethylsilazane (HMDS) and Trimethylchlorosilane (TMCS).

Bamboo, a fast-growing plant from Asia, is used as building material with unique properties, while exhibiting fast degradation due to its hydrophobicity. Therefore, many attempts have been implemented using several technologies for bamboo modification to alter the hydrophobicity. Most previous studies producing superhydrophobic properties are conducted by using tetraethoxysilane (TEOS) as a precursor agent. However, this method, using TEOS with harmful properties and unaffordable compounds, requires many steps to accomplish the experimental method. Therefore, this paper employed geothermal solid waste as a silica source of the precursor. Thus, an effective and efficient method was applied to prepare superhydrophobic coating by using a precursor of geothermal silica and further modification using hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS). The research was executed by the full factorial statistical method using two numerical variables (HMDS/TMCS concentration and silica concentration) and one categorical variable (solvent types). The uncoated material revealed higher weight gain in mass and moisture content than that of the coated bamboo after the soil burial test to assess the durability of the bamboo. However, the durability of superhydrophobic coating realized hydrophobic performance for both agents during sand abrasion for a total of 120 s at an angle of 45°. Statistical results showed the optimum contact angle (CA) achieved in superhydrophobic performance with lower silica concentration for HMDS concentration and the appropriate solvent of n-hexane for HMDS and iso-octane for TMCS. All results were supported using many instruments of analysis to confirm the step-by-step alteration of geothermal silica to be used as a superhydrophobic coating, such as XRF, XRD, FTIR, SEM, and SEM EDX.

Mapping hydrothermal alteration zone through aster data in Gadag Schist Belt of Western Dharwar Craton of Karnataka, India

Alteration zones are developed in a wide range of geological milieus such as volcano-plutonic rocks. Hydrothermal alteration zones are considered an important guide for mineral exploration and can act as a potential zone of mineralization. The minerals in alteration zones have very diagnostic absorption features in visible–near infrared (VNIR) and shortwave infrared (SWIR) regions of the electromagnetic spectrum. The Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) data have the potential to identify mineral assemblages in different hydrothermal alteration zones because of its 14 spectral bands which cover a wide spectral range. In the present article, FCC, band ratio, and SPCA techniques were applied to map the hydrothermal alteration zones in Gadag Schist Belt (GSB) of Western Dharwar Craton (WDC) of Karnataka. FCC has highlighted lithology, alteration zones, and given a fair idea about the lithological control and alteration occurrences in the area. Band ratio technique for muscovite, kaolinite, chlorite, iron oxide gossan, and silica alteration have yielded promising results. SPCA analysis successfully mapped hydrothermal alteration and delineated phyllic, argillic, and propylitic zones in the area. These results were verified through the existing geological map and by geological fieldwork in the area and these results are satisfactory. The total area, altered by different alteration process, was also calculated. In the GSB, iron alteration zone, phyllic, argillic, propylitic, and silica alteration zones were effectively mapped with the help of ASTER data.

Biostratinomy and Diagenetic Impact on Exceptional Preservation of Coccospheres from Lower Oligocene Coccolith Limestones

Lower Oligocene coccolith limestones, known as Tylawa Limestones, in the Polish part of the Outer Carpathians have been analyzed using back-scattered electron (BSE) and charge contrast imaging (CCI) techniques and X-ray diffraction. The aim of the study was to reconstruct the fossilization history of the excellently preserved coccospheres, commonly observed in these sediments, deposited in the Paratethys basin. Multiple petrographic and geochemical analyses indicate that the exceptional preservation of coccospheres has been caused by the special coincidence of biostratinomic processes: pre- and post-depositional processes within the water–sediment interface and possible early cementation. We discuss the origin of the calcite and silica and provide some hypotheses regarding the environment and possible factors responsible for the spectacular preservation process. Based on the observed intermediate steps of calcite and silica alteration, we propose an extended model of biostratinomic processes and diagenesis. The possibility of finding exquisitely preserved coccospheres not only in soft clay-rich deposits but also in heavily lithified rocks extends the possibilities of research on the development and evolution of this group of organisms.

Understanding the transfer of contemporary temperature signals into lake sediments via paired oxygen isotope ratios in carbonates and diatom silica: Problems and potential

Although the oxygen isotope composition (δ18O) of calcite (δ18Ocalcite) and, to a lesser extent, diatom silica (δ18Odiatom) are widely used tracers of past hydroclimates (especially temperature and surface water hydrology), the degree to which these two hosts simultaneously acquire their isotope signals in modern lacustrine environments, or how these are altered during initial sedimentation, is poorly understood. Here, we present a unique dataset from a natural limnological laboratory to explore these issues. This study compares oxygen and hydrogen isotope data (δ18O, δ2H) of contemporary lake water samples at ~2-weekly intervals over a 2-year period (2010–12) with matching collections of diatoms (δ18Odiatom) and calcite (δ18Ocalcite) from sediment traps (at 10 m and 25 m) at Rostherne Mere (maximum depth 30 m), a well-monitored, eutrophic, seasonally stratified monomictic lake in the UK. The epilimnion shows a seasonal pattern of rising temperature and summer evaporative enrichment in 18O, and while there is a temperature imprint in both δ18Odiatom and δ18Ocalcite, there is significant inter-annual variability in both of these signals. The interpretation of δ18Odiatom and δ18Ocalcite values is complicated due to in-lake processes (e.g. non-equilibrium calcite precipitation, especially in spring, leading to significant 18Ocalcite depletion), and for δ18Odiatom, by post-mortem, depositional and possibly dissolution or diagenetic effects. For 2010 and 2011 respectively, there is a strong temperature dependence of δ18Ocalcite and δ18Odiatom in fresh trap material, with the fractionation slope for δ18Odiatom of ca. −0.2‰/°C, in agreement with several other studies. The δ18Odiatom data indicate the initiation of rapid post-mortem secondary alteration of fresh diatom silica (within ~6 months), with some trap material undergoing partial maturation in situ. Diatom δ18O of the trap material is also influenced by resuspension of diatom frustules from surface sediments (notably in summer 2011), with the net effect seen as an enrichment of deep-trap 18Odiatom by about +0.7‰ relative to shallow-trap values. Contact with anoxic water and anaerobic bacteria are potentially key to initiating this silica maturation process, as deep-trap samples that were removed prior to anoxia developing do not show enrichment. Dissolution (perhaps enhanced by anaerobic bacterial communities) may also be responsible for changes to δ18Odiatom that lead to increasing, but potentially predictable, error in inferred temperatures using this proxy. High resolution, multi-year monitoring can shed light on the complex dynamics affecting δ18Odiatom and δ18Ocalcite and supports the careful use of sedimentary δ18Odiatom and δ18Ocalcite as containing valuable hydroclimatic signals especially at a multi-annual resolution, although there remain substantial challenges to developing a reliable geothermometer on paired δ18Odiatom and δ18Ocalcite. In particular, δ18Odiatom needs cautious interpretation where silica post-mortem secondary alteration is incomplete and diatom preservation is not perfect, and we recommend dissolution be routinely assessed on diatom samples used for isotopic analyses.

A Remote Sensing-Based Application of Bayesian Networks for Epithermal Gold Potential Mapping in Ahar-Arasbaran Area, NW Iran

Mapping hydrothermal alteration minerals using multispectral remote sensing satellite imagery provides vital information for the exploration of porphyry and epithermal ore mineralizations. The Ahar-Arasbaran region, NW Iran, contains a variety of porphyry, skarn and epithermal ore deposits. Gold mineralization occurs in the form of epithermal veins and veinlets, which is associated with hydrothermal alteration zones. Thus, the identification of hydrothermal alteration zones is one of the key indicators for targeting new prospective zones of epithermal gold mineralization in the Ahar-Arasbaran region. In this study, Landsat Enhanced Thematic Mapper+ (Landsat-7 ETM+), Landsat-8 and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) multispectral remote sensing datasets were processed to detect hydrothermal alteration zones associated with epithermal gold mineralization in the Ahar-Arasbaran region. Band ratio techniques and principal component analysis (PCA) were applied on Landsat-7 ETM+ and Landsat-8 data to map hydrothermal alteration zones. Advanced argillic, argillic-phyllic, propylitic and hydrous silica alteration zones were detected and discriminated by implementing band ratio, relative absorption band depth (RBD) and selective PCA to ASTER data. Subsequently, the Bayesian network classifier was used to synthesize the thematic layers of hydrothermal alteration zones. A mineral potential map was generated by the Bayesian network classifier, which shows several new prospective zones of epithermal gold mineralization in the Ahar-Arasbaran region. Besides, comprehensive field surveying and laboratory analysis were conducted to verify the remote sensing results and mineral potential map produced by the Bayesian network classifier. A good rate of agreement with field and laboratory data is achieved for remote sensing results and consequential mineral potential map. It is recommended that the Bayesian network classifier can be broadly used as a valuable model for fusing multi-sensor remote sensing results to generate mineral potential map for reconnaissance stages of epithermal gold exploration in the Ahar-Arasbaran region and other analogous metallogenic provinces around the world.

Dynamic Wettability Alteration of Calcite, Silica and Illite Surfaces in Subsurface Environments: A Case Study of Asphaltene Self-Assembly at Solid Interfaces

A fundamental understanding of dynamic wettability alteration at complex solid interfaces in varying fluidic environments representative of the subsurface environments is essential for engineering field-scale strategies to enhance energy recovery while reducing environmental impacts. In this study, we characterize dynamic wettability alteration at solid surfaces (e.g., calcite, silica and illite) resulting from the self-assembly and adsorption of polyaromatic macromolecules known as asphaltenes in heptane and toluene solvents at 200 bar and 313 K using classical molecular dynamics simulations. These conditions are chosen to be representative of the subsurface environments. On adding asphaltenes, the equilibrium contact angles on calcite, silica and illite surfaces which are in the range of 25–34° increased to 69–79°. The adsorption and self-assembly of asphaltene molecules on the solid surfaces and the accumulation on water/hydrocarbon interfaces reduces the spreading distances of water molecules and enhances the hydrophobicity of the solid surfaces. Further, the accumulation of asphaltene molecules at the water/liquid hydrocarbon interfaces results in a reduction of the interfacial tension of water/heptane and water/toluene, thus stabilizing the water droplet. The enhanced surface hydrophobicity contributes to the formation of layered structures of heptane and toluene close to the solid surfaces and reduces the self-diffusivities of these molecules close to the surface.

Prediction Potential Acid Mine Drainage of Epithermal High Sulfidation Deposits using Static Test

Acid mine drainage can be predicted by testing the geochemical characteristics of rocks, to be knowing acid-forming rocks and acid neutralizing. Prevention and mitigation efforts from acid mine drainage can be predicted by knowing the characteristics of rocks and looking for the direct contact of acid rocks with air and water. The static test aims to determine the potential for acid formation that occurs in rocks as a first step to characterize rocks. Tests were carried out on argillic silica alteration rock samples from the Lanut site, PT J-Resources Bolaang Mongondow. The calculation of total sulfur shows a smaller percentage of the value of the advance argillic than the silisic sample. The results of advance argillic samples, NAG pH> 4.5 were obtained and the value of Net Acid Producing Potential (NAPP) in the range of 110-153 kg H2SO4 / ton rocks which means that the potential nature of rocks cannot be ascertained PAF or NAF. Whereas for silisic alteration rock samples the pH value of NAG <4.5 and NAPP values in the range of 146 - 355 kg H2SO4/ton rocks means silisic alteration has the potential acid forming.

What is Down Plunge of the Dobroyde Hill High-Sulphidation Epithermal Deposit, near Junee, NSW? An Emerging Carbonate-Base-Metal-Gold Epithermal System

Discovered by Getty Oil in 1983, the outcropping Dobroyde Hill 77k Oz gold high-sulphidation epithermal prospect is now known to transition into a carbonate base-metal epithermal gold system down plunge. The project is 50km north of Wagga Wagga and 10km north of Junee which is the far southern toe of the Junee-Narromine Volcanic Belt. It is hosted within a 4km long package of middle Ordovician andesitic to basaltic sub-marine volcanic rocks and early Silurian plagioclase and hornblende phyric shoshonitic andesite sub-marine breccias (Crawford 2014, 2017). Revival of the prospect was driven by field mapping armed with a TerraSpec Halo, ground magnetics and 3D IP and magnetic inversions (by Chris Moore, Moore Geophysics). This lead to the recognition of a 3m x 2km surficial dickite ± pyrophyllite ± kaolinite ± illite ± silica ± pyrite alteration in shoshonitic andesite breccias. Follow up drilling which targeted a zone 700m down plunge of the Dobroyde Hill resource was completed in partnership with the NSW Government through the New Frontiers cooperative drill funding scheme. This hole, NDD005, intersected a far larger system than expected, with 280m of intense dickite + pyrophyllite + kaolinite + pyrite ± illite ± silica alteration then 200m of unmistakable carbonate-base-metal-gold epithermal style alteration as presented in Corbett & Leach (1998). Evolution Mining’s giant Cowal Gold Mine (~8M Oz Resource) 120km north of Dobroyde is also characterised as carbonate-base-metal-gold epithermal. Both mineralising styles show a genetic link by overprinting relationships and both having associated broad low-grade gold and silver mineralisation with individual 1m intervals to 9.5g/t gold. NDD005 is interpreted to have stepped over the top of the main epithermal gold zone, hence the target remains open to further exploration. Additionally, this 700m down plunge gap between the new and historic drilling is completely untested and if mineralised could extend the strike length of the deposit to one kilometre.

Chondrule porosity in the L4 chondrite Saratov: Dissolution, chemical transport, and fluid flow

Porosity is an important physical property of meteorites and asteroids that affects density, material strength, and thermal conductivity. Porosity can also promote chemical exchange by facilitating the transport of fluids and dissolved ions. We measured the porosity of individual chondrules from the L4 ordinary chondrite Saratov, using X-ray microtomography (μCT) and scanning electron microscopy, to examine the abundance and distribution of porosity in chondrules, and to understand how porosity relates to chemical exchange during parent body processes. Porosity was 1–2% by volume in the chondrules we measured and maximum pore sizes were ∼300 μm. Porosity distribution and morphology indicate that porosity is a secondary feature and most pores >1 μm were formed from the dissolution of chondrule mesostasis glass. Fe and K are preferentially enriched in phases adjacent to the most porous regions: Fe is enriched in pyroxene, and K is enriched in mesostasis where it is observed as either the silica alteration phase merrihueite, or fine-scale, K-feldspar exsolution in albitic feldspar. Some pores can be described as vugs, as they contain euhedral olivine and chromite, with textures indicating vapor deposition. Knowing the chondrule porosity, we estimate the matrix porosity in Saratov to be very high, 40–60%. We suggest that during prograde metamorphism, an aqueous fluid originating from the matrix dissolved chondrule mesostasis glass, producing the observed porosity, and introducing Fe into the pyroxene phenocrysts. Fluids were less abundant through peak metamorphism, and chondrule mesostasis glass crystallized to fine-grained albite. During retrograde metamorphism, high temperature, short duration bursts of a dry, alkali-bearing fluid from the asteroid interior infiltrated the pore network, formed the vug phases, altered silica to merrihueite, and introduced K to the secondary albite. Fine-scale K-feldspar then exsolved from albite during rapid cooling to the ambient temperature. Overall, development of porosity during metamorphism on the L chondrite parent body contributed to the chemical evolution of ordinary chondrite material, as well as affecting physical properties of the parent asteroid.

Multistage alteration, mineralization and ore–forming fluid properties at the Viper (Sappes) Au–Cu–Ag–Te ore body, W. Thrace, Greece.

The mineralogy of ore and hydrothermal alteration of the high-sulfidation enargite–Au–Ag–Te Viper (Thrace) orebody, and fluid inclusions, were studied in drillcore samples. The hydrothermal system has evolved through several stages from pre-ore advanced argillic I+vuggy silica alteration, ore-stage advanced argillic II+vuggy silica alteration and silicification that has developed to argillic alteration (sericite)+silicification through pH increase, and a return to acid conditions as crosscutting post-ore advanced argillic alteration III+silicification. Ore is characterized by early barren pyrite I corroded by: (i) enargite–Au± complex Pb–Bi–Cu sulfosalts, tellurides and selenides, coexisting with euhedral quartz, and (ii) zoned pyrite II distinguished by anomalous concentrations of Au, Cu, As, Te, Bi, Pb, Se, within vuggy quartz. High-grade gold ore is also intergrown with late brecciacementing and vein-type epithermal-like banded quartz+ pyrite. These alteration and mineralization observations are consistent with the changing composition, water fugacity, and density of an expanding column of metal-laden magmatic vapor, combined with changes in structural permeability. Part of the enargite─Au─ quartz assemblages have been probably quenched from sulfosalt melt at high─temperatures (&gt;575°C). End product of the enargite–sulfhide–silica crystallization sequence is the formation of high-grade epithermal quartz-gold colloformbanded ore during cooling and/or dilution/mixing down to ~200°C.

Identification of Multi-Style Hydrothermal Alteration Using Integrated Compositional and Topographic Remote Sensing Datasets

The western part of the island of Milos, Greece has undergone widespread, intense alteration associated with a range of mineralization, including seafloor Mn-Fe-Ba, sub seafloor Pb-Zn-Ag, and epithermal Au-Ag. The surrounding country rocks are a mixture of submarine and subaerial calc-alkaline volcanic rocks ranging from basaltic andesite to rhyolite in composition, but are predominantly andesites and dacites. The current surface spatial distribution of the alteration mineralogy is a function not only of the original hydrothermal, but also subsequent tectonic and erosional processes. The high relief and the excellent rock exposure provide ideal conditions to evaluate the potential of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite remote sensing data to identify and differentiate the different styles of alteration mineralisation. Laboratory spectral reflectance and calculated emittance measurements of field samples, supported by XRD analysis and field mapping, were used to support the analysis. Band ratio and spectral matching techniques were applied to the shortwave-infrared (SWIR) reflectance and thermal-infrared (TIR) emissivity imagery separately and were then integrated with topographic data. The band ratio and spectral matching approaches produced similar results in both the SWIR and TIR imagery. In the SWIR imagery, the advanced argillic, argillic and hydrous silica alteration zones were clearly identifiable, while in the TIR imagery, the silicic and advanced argillic alteration zones, along with the country rock, were differentiable. The integrated mineralogical–topographic datasets provided an enhanced understanding of the spatial and altitude distribution of the alteration zones when combined with conceptual models of their genesis, which provides a methodology for the differentiation of the multiple styles of alteration.

Early diagenetic alterations of biogenic and reactive silica in the surface sediment of the Yangtze Estuary

Sedimentary biogenic silica (BSi) is an important parameter for understanding biogeochemical processes in estuarine ecosystems. In this study, a two-step mild acid–mild alkaline extraction procedure was used to leach BSi and its early diagenetic products from the sediments of the Yangtze Estuary. A Si/Al correction of the mild alkaline leachable silica (Si-Alk) was applied to estimate the contents of BSi in the sediments. The BSi contents varied from 18.90 to 120.10μmolSi/g in the sediments, whereas mild acid leachable silica (Si-HCl) and Si-Alk levels ranged from 17.43 to 73.56 and from 19.56 to 185.63μmolSi/g, respectively. Furthermore, the degrees of diagenetic alteration of biogenic and reactive silica were also calculated and discussed. The diagenetic alteration ratios of biogenic and reactive silica increased seaward during May, August and November 2012, whereas an opposite trend was observed in March 2013. The diagenetic alteration of the biogenic and reactive silica was mainly controlled by the redox conditions in benthic sediments. Additionally, the deposition of fresh diatoms and authigenic products could temporarily affect the distribution of silica pools in the sediments and ultimately affect the diagenetic alteration ratios of biogenic and reactive silica. Detailed investigations are still necessary to understand the early diagenetic processes of biogenic and reactive silica in this warm temperate area.

Cooperative constrained inversion of multiple electromagnetic data sets

We evaluated a method for cooperatively inverting multiple electromagnetic (EM) data sets with bound constraints to produce a consistent 3D resistivity model with improved resolution. Field data from the Antonio gold deposit in Peru and synthetic data were used to demonstrate this technique. We first separately inverted field airborne time-domain EM (AEM), controlled-source audio-frequency magnetotellurics (CSAMT), and direct current resistivity measurements. Each individual inversion recovered a resistor related to gold-hosted silica alteration within a relatively conductive background. The outline of the resistor in each inversion was in reasonable agreement with the mapped extent of known near-surface silica alteration. Variations between resistor recoveries in each 3D inversion model motivated a subsequent cooperative method, in which AEM data were inverted sequentially with a combined CSAMT and DC data set. This cooperative approach was first applied to a synthetic inversion over an Antonio-like simulated resistivity model, and the inversion result was both qualitatively and quantitatively closer to the true synthetic model compared to individual inversions. Using the same cooperative method, field data were inverted to produce a model that defined the target resistor while agreeing with all data sets. To test the benefit of borehole constraints, synthetic boreholes were added to the inversion as upper and lower bounds at locations of existing boreholes. The ensuing cooperative constrained synthetic inversion model had the closest match to the true simulated resistivity distribution. Bound constraints from field boreholes were then calculated by a regression relationship among the total sulfur content, alteration type, and resistivity measurements from rock samples and incorporated into the inversion. The resulting cooperative constrained field inversion model clearly imaged the resistive silica zone, extended the area of interpreted alteration, and also highlighted conductive zones within the resistive region potentially linked to sulfide and gold mineralization.

The Massawa gold deposit, Eastern Senegal, West Africa: an orogenic gold deposit sourced from magmatically derived fluids?

Abstract The Massawa gold project is situated on the Senegalese side of the highly prospective/productive Palaeo-Proterozoic (Birimian) Kédougou–Kéniéba inlier, which hosts several world-class orogenic gold deposits/districts in western Mali (e.g. Loulo and Sadiola). The Massawa ore body has a strike length of at least 4 km and a current resource of 3.61 Moz at a grade of 2.8 g t −1 . The ore body is structurally controlled and located within a package of low-grade regionally metamorphosed volcaniclastic sediments (agglomerates, tuffs and ash-tuffs), quartz–feldspar and lithic wackes, carbonaceous shales, hydrothermal breccias, and gabbro and porphyry sills. These rocks have undergone pervasive silica alteration followed by a sericite–ankerite–chlorite alteration event related to mineralization. Two major styles of mineralization are recognized at Massawa from field and laboratory studies. The first stage of sulphide–Au mineralization is associated with disseminated arsenopyrite–pyrite, which follows shear zones in the sedimentary and volcano-sedimentary host rocks. The second stage consists of quartz–stibnite±tetrahedrite veining distinguished by coarse visible gold and represents a late stage overprint on the primary mineralization. The two stages of gold mineralization are separated by a phase of quartz–molybdenite veining. A distinctive base metal trace assemblage is linked to stibnite formation including multiple Sb phases such as chalcostibite, zinkenite, roshchinite, aurostibite, jamesonite and robinsonite. Secondary ion mass spectroscopy-based gold deportment data indicate that up to 90% of stage 1 gold is held as a solid solution within either arsenopyrite or arsenian pyrite. Stable isotope data yield δ 34 S sulphide values of between 0 and 4.1‰ and δ 18 O H2O values of 5.5–10.9‰ for all stages of mineralization, suggesting a magmatic fluid influence. This is consistent with field data that suggest that mineralization is synchronous with emplacement of a sequence of concordant felsic sheets. That mineralization occurred at shallow (&lt;6 km) depths is suggested both by the presence of stibnite and by fluid inclusion studies. Low-temperature (homogenization temperatures between 150 °C and 230 °C) H 2 O–NaCl fluids (&lt;6 wt% NaCl equiv.) and coeval CO 2 –CH 4 inclusions, observed in both phases of mineralization, indicate trapping conditions of 220–315 °C at 1–1.65 kbar. A combination of phase petrology, fluid inclusion and stable isotope data suggests deposition of gold from low-salinity, magmatic fluids, most probably released from felsic rocks similar to those emplaced into the Massawa sequence during mineralization.

Diagenetic aluminum uptake into diatom frustules and the preservation of diatom-bound organic nitrogen

The chemistry and isotopic composition of siliceous diatom frustules and the organic matter bound within them have been used to reconstruct past changes in the environment. However, the alteration of biogenic silica in the sediments and its influence on the chemical properties of the diatom opal and diatom-bound organic matter has not been broadly studied. We use inductively coupled plasma atomic emission spectrometry and energy dispersive X-ray analysis to investigate the concentrations of various cations, especially aluminum, in cleaned diatom frustules extracted from sediments at three sites from the Bering Sea, where the opal content in the sediments and preservation conditions vary both spatially and over the last glacial cycle. Relative to cultured diatoms, cleaned diatom frustules from the sediments are found to have a high Al/Si ratio, most likely by homogenous Al incorporation but possibly also by formation of an aluminosilicate coating of significant thickness relative to the diatom walls. Variations in the Al/Si ratio of the cleaned diatoms among samples are strongly correlated with the aluminosilicate to opal ratio of the bulk sediment, suggesting that sedimentary clays are the primary source of the aluminum taken up by the diatoms. The aluminum and nitrogen contents of the diatom frustules are negatively correlated, which is best explained by the loss of intrinsic N during the chemical changes associated with Al incorporation. The lack of a consistent relationship between nitrogen isotopic composition and the aluminum content of the cleaned opal suggests that the diagenetic organic N loss does not fractionate N isotopes, which is as expected given that this loss process relates to the geochemistry of the opal, not the organic matter. Our findings help to explain the previous observations that opal from clay-rich sediments (1) requires harsher chemical cleaning to remove diagenetic N and (2) has diatom-native N that is better protected from alteration by excessively harsh chemical cleaning. The diagenetic alteration of the opal may have led to partial protection of the external N from the surrounding sediments. At the same time, the diatom frustules have been chemically ‘hardened’ by the diagenesis associated with Al uptake, reducing the chemical accessibility of the diatom-native N that remains.

SiO&lt;sub&gt;2&lt;/sub&gt; Content Retrieving Based on ASTER and its Application in Metallogenic Prediction in Abra, Australian

The remote sensing measurement of the surface SiO2content has important geological significance .An empirical formula for estimate SiO2content was established based on the correlation between ASTER thermal infrared data and tested Si element from NITON sites in Abra, Australia. The average error of estimated SiO2content is about 15%. ASTER silica alteration derived from SiO2content has good indicative function to find galena. Combined with other prospecting factors as bouger gravity, transient electromagnetic and so on, three vision prospecting targets were optimized, which need more exploration works.