Silica Alteration Research Articles

Supercritical CO2 and Ionic Strength Effects on Wettability of Silica Surfaces: Equilibrium Contact Angle Measurements

Wettability of reservoir mineral surfaces is a critical factor controlling CO2 mobility, trapping, and safe-storage in geological carbon sequestration. Although recent studies have begun to show that wettability of some minerals can change in the presence of supercritical CO2 (scCO2), different laboratories have reported significantly different wetting behavior. We studied wettability alteration of silica in CO2–brine systems through measuring equilibrium water contact angles under wide ranges of pressures (0.1 to 25 MPa) and ionic strengths (0 to 5.0 M NaCl), at 45 °C. Using two independent approaches for each of the experiments, we found the following: (1) Equilibrium water contact angles on silica increased up to 17.6° ± 2.0° as a result of reactions with scCO2. This increase occurred primarily within the pressure range 7–10 MPa, and the contact angles remain nearly constant at pressure greater than 10 MPa. (2) The contact angle increased with ionic strength nearly linearly, with a net increase of 19.6° ± 2.1° at 5.0 M NaCl. These changes in contact angle induced by changes in scCO2 pressure and aqueous solution ionic strength are approximately additive over the range of tested conditions. These findings can be used to estimate the wetting behavior of silica surfaces in reservoirs containing supercritical CO2.

Early Pleistocene lacustrine sedimentation and diatom stratigraphy at Munya wa Gicheru, southern Kenya Rift Valley

This research develops a detailed palaeoenvironmental history for sediments at Munya wa Gicheru (southern Kenya Rift) and explores the roles of tectonics and climate in the formation of palaeolakes and their use in regional climate correlation. The lacustrine facies of the Munya wa Gicheru deposits were formed from ~1.9 to 1.65Ma and consist of massive and laminated diatomites and diatomaceous silts. Fresh water diatoms include Stephanodiscus transilvanicus, Stephanodiscus niagarae, Aulacoseira agassizi, Aulacoseira ambigua, Aulacoseira goetzeana, Aulacoseira granulata var. valida, A. granulata var. angustissima and Pseudostaurosira zeilleri. One moderately saline phase is marked by the dominance of Cyclotella meneghiniana and by the presence of rare Thalassiosira rudolfii. Twenty-five diatom-based zones and subzones can be distinguished, representing eight high-lake periods and eight shallow-water phases. Erosion surfaces and desiccation cracks indicate two periods of emergence. Lake waters ranged in pH from 7.2 to 9, with conductivities of ~200–11,000μScm−1, but mostly <1000μScm−1. In contrast, the uppermost sediments are comprised of fluvial silts, sands and gravels formed in fluvial, sheetwash and terrestrial settings and represent termination of lacustrine conditions. REE data suggest that the Limuru Trachytes are the dominant source of siliciclastics materials. Geochemical information indicates that catchment weathering was moderate and that CaO concentrations in the sediments are very low. Carbonate rhizoliths are absent. SEM images show minimal corrosion or alteration of the diatom silica.

Comparison of Dimensional Accuracy for Different Investment Casting Shells and Binders Based on Selective Laser Sintering

The dimensional accuracy of shells and binders of investment casting which incorporation with selective laser sintering is investigated. The results show that the dimensional accuracy of colloidal silica is higher than that of ethyl silicate, and the dimensional variation rate of investment casting shells produced with colloidal silica is much lower than ethyl silicate shells. Moreover, colloidal silica possesses better performance on environmental protection and production cost control. These indicate that the comprehensive properties of colloidal silica are better than that of ethyl silicate. Meanwhile, the average dimensional variation rate of the single colloidal silica shell and the ethyl silicate-colloidal silica alteration shell was almost identical and it was much lower than that of the other shells which were produced in this study. This means two kinds of shells are optimized in all five types of shells studied in the aspect of dimensional accuracy. The unique properties of two shells show clearly direction to choose the type of shell.

Estimation of authigenic alteration of biogenic and reactive silica in Pearl River estuarine sediments using wet-chemical digestion methods

The extent of authigenic alteration of biogenic and reactive silica in Pearl River estuarine sediments has been estimated using wet-chemical digestion methods. Results show relatively constant distributions of biogenic and reactive Si horizontally and vertically. Based on three core measurements, the biogenic and total reactive Si average 77.91 and 264.77 μmol Si g−1, respectively. Their extents of authigenic alteration are correspondingly estimated as ~55.6 and ~70.6%. The average biogenic Si accumulation rate is calculated as 1.91 × 109 mol Si year−1, which translates into storage of ~7.15% of the annual riverine dissolved silica input. By contrast, the total reactive Si accumulation rate is as high as 6.49 × 109 mol Si year−1, improving annual riverine silicic acid storage to ~24.19%. Detailed investigation is required for a good understanding of early diagenetic process of biogenic and reactive silica in this subtropical area.

Spectral properties of carbonatized ultramafic mantle xenoliths and their host olivine basalts, Jabal Al Maqtal basin, South Eastern Desert, Egypt, using ASD FieldSpec spectroradiometer

Understanding the spectral properties of the carbonatized ultramafic mantle xenoliths and their host olivine basalts exposed at Jabal Al Maqtal strike-slip basin, south Eastern Desert, Egypt using portable ASD FieldSpec is the main task of this article. Field data revealed the presence of ultramafic mantle xenoliths at different stratigraphic levels within Jurassic olivine basalts. Ultramafic mantle xenoliths are recorded within highly altered olivine basalt topped by hard, compact brown conglomerate (erosional surface) of 1.5 m thick. They have different sizes (5–25 cm) and shapes (rounded, sub-rounded and elliptical). The spectral data are collected using ASD FieldSpec in sunny, free-cloud day. Data measurements were resampled as reference-target scan. Nearly all the mineralogical constituents of the studied ultramafic mantle xenoliths were replaced by Mg-Free (<0.5 wt%) calcium carbonate (calcite). Spectrally the ultramafic mantle xenoliths can be categorized into two main groups based on their Fe and Mn content which produce absorption features at shorter wavelength regions. The spectral profile of the first group shows a pronounced absorption features around 0.50 μm, 0.65 μm and 0.90 μm wavelength regions. These features are attributed to the electronic processes within the unfilled d-shells of transition Mn 2+ and Fe 2+ metal cations (MnO and Fe 2O 3 content are 2.11 wt% and 12.6 wt% respectively as indicated by XRF analyses). The spectral profile of the second group shows a very week absorption features in shorter wavelength regions which is attributed to the low MnO (0.7 wt%) and Fe 2O 3 (2.17 wt%) content. Both ultramafic mantle xenoliths groups show pronounced absorption features in longer wavelength regions (1.8–2.55 μm). Inspection of the spectral profiles of the mantle xenoliths revealed the presence of six absorption features around 1.80 μm, 1.95 μm, 2.00 μm, 2.35 μm, 2.4 μm. and 2.5 μm wavelength regions which are attributed to the vibrational processes of the carbonate ion. Absorption feature around 1.40 μm wavelength region is probably due to microcrystalline silica alteration. The spectral profile of fresh olivine basalt shows low flat reflectance curve whereas the reflectance curve of altered basalt shows three little absorption features around 1.40 μm, 1.95 μm and 2.35 μm wavelength regions. These absorption features are mainly due to calcite alterations.

Two episodes of regional-scale Precambrian hydrothermal alteration in the eastern Pilbara, Western Australia

Two regional-scale hydrothermal events have been identified and mapped in the Precambrian rocks of the eastern Pilbara, Western Australia. The first post-dates lithification and tilting of the 2630 ± 6 Ma Carawine Dolomite and pre-dates the 1317 ± 11 Ma Manganese Group. It involved epithermal, mostly silica alteration with subordinate carbonate and minor chlorite and hematite. The most significant result of this event was the development of a regional-scale silica carapace (known as the Pinjian Chert Breccia) on the Carawine Dolomite, ranging from a few metres (?) to about 100 m thick. The second event comprised two phases. The first phase involved regional-scale hematite (±quartz) mineralisation that was broadly synchronous with deposition of the Manganese Group. The second involved regional-scale dissolution of Carawine Dolomite and local precipitation of barite and manganese oxide. It post-dates lithification of the lower part of the Manganese Group and was either synchronous with, or post-dated, deposition of the upper part of the Group. This study highlights the important roles of bedding-parallel aquicludes and unconformities in Precambrian hydrothermal systems, the role of physical and chemical variations in lithologies in controlling the locations of alteration and mineralisation, and shows that ancient crustal fractures acted, repeatedly, as fluid conduits. It also shows that Precambrian hydrothermal events in non-magmatic rocks in non-orogenic settings can be intense and widespread, affecting thousands of square kilometres. Further, by dating such events, links can be made between relatively undeformed sedimentary successions and seemingly unrelated Precambrian orogens. The study also challenges a long-standing, supergene-based model for the formation of silica and manganese in the eastern Pilbara and adds a new component to the Pilbara iron ore story.

Coupled silicon–oxygen isotope fractionation traces Archaean silicification

Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ30Si and δ18O values of bulk rock increase with increasing amount of silicification from unsilicified basalts (−0.64‰<δ30Si<−0.01‰ and +8.6‰<δ18O<+11.9‰) to silicified basalts (δ30Si and δ18O values as high as +0.81‰ and +15.6‰, respectively). Cherts generally have positive isotope ratios (+0.21‰<δ30Si<+1.05‰ and +10.9<δ18O<+17.1), except two cherts, which have negative δ30Si values, but high δ18O (up to +19.5‰).The pronounced positive correlations between δ30Si, δ18O and SiO2 imply that the isotope variation is driven by the silicification process which coevally introduced both 18O and 30Si into the basalts. The oxygen isotope variation in the basalts from about 8.6‰ to 15.6‰ is likely to represent temperature-dependent isotope fractionation during alteration. Our proposed model for the observed silicon isotope variation relies on a temperature-controlled basalt dissolution vs. silica deposition process.

Testing Hypotheses for the Mount Isa Copper Mineralisation with Numerical Simulations

We have applied reactive transport simulations to evaluate conceptual models of hydrothermal fluid flow related to the Mesoproterozoic Mount Isa copper mineralisation. Numerical experiments have been performed specifically to investigate whether fluid flow was driven by mechanical deformation, higher than hydrostatic fluid pressure gradients, or thermal buoyancy, and what the mechanism of ore deposition was. One distinct feature of the Mount Isa mineralising system is a region of massive silica-rich alteration that surrounds the copper ore bodies within the Urquhart shale, indicating upward flow of a cooling fluid. Hydromechanical modelling revealed that contraction and horizontal shear can produce a dilation pattern that favours upward fluid flow, whereas strike slip movement causes dilation of pre-existing vertical structures. Reactive transport models show that hydraulic head driven flow is more likely to produce a more realistic silica alteration pattern than free thermal convection, but neither process generates a flow pattern capable of precipitating copper at the appropriate location. Instead we propose that gravity driven flow of a dense oxidised basin brine led to chalcopyrite mineralisation by fluid-rock reaction.

Silica alteration zones in the Barberton greenstone belt: A window into subseafloor processes 3.5–3.3 Ga ago

Silicification of volcanic rocks, ranging from komatiitic to dacitic in composition, is a common phenomenon of the 3.5 to 3.2 Ga old volcano-sedimentary succession of the Barberton greenstone belt. Silica enrichment occurs in zones several tens of metres thick at the top of volcanic sequences that are capped by sedimentary chert horizons. SiO 2 contents increase upsection, from the original igneous value up to 92%. Silicification is associated with a depletion of most elements commonly mobile during water–rock interaction, while K 2O, Rb and Ba are enriched in these zones. Some transition metals (Sc, V, and Cr) were immobile, whereas Ni and Co and some other metals (Cu, Zn) were mobile and are depleted. The δ 18O values of volcanic rocks show a positive linear relationship with silica content and range from 9.0 to 17.3‰. The element depletion-enrichment patterns and oxygen isotope data indicate low-temperature (c. 100–150 °C) hydrothermal processes for the origin of the alteration zones. The loss of most elements coupled with the addition of SiO 2 as well as REE systematics indicate high water–rock ratios and a fluid REE composition typical of Archaean seawater that had a δ 18O value of ~ 0‰. Hydrothermal activity is attributed to heat derived from cooling of volcanic rocks and a high regional heat flow in an oceanic plateau-like setting and resulted in the establishment of shallow subseafloor convection cells. Diffuse venting of hydrothermal fluids over broad areas of the ancient ocean floor provided an ideal environment for both seafloor and subsurface biota 3.5 Ga ago.

Geological setting of Earth's oldest fossils in the ca. 3.5 Ga Dresser Formation, Pilbara Craton, Western Australia

Detailed stratigraphic, petrographic, and zircon U–Pb geochronological data are provided for surface outcrops and newly obtained, unweathered drillcore intersections of Earth's oldest fossiliferous sedimentary rocks, the lowermost chert–barite unit of the ca. 3.5 Ga Dresser Formation, Warrawoona Group, Pilbara Craton. Results show that the ∼8 m thick unit at the drilling locality consists of thinly bedded, originally micritic carbonates deposited under quiet water conditions that are interbedded with volcaniclastic conglomerates and coarse polymict conglomerates (diamictites) deposited during periods of tectonic instability. The presence of rapid vertical and lateral facies changes, tilted bedding in some members, and internal erosional unconformities, combined with analysis of fault offsets, indicate that tectonically unstable periods were caused by growth faulting. Intense hydrothermal fluid flow accompanied episodes of growth faulting and resulted in pulsed, repeated precipitation of silica ± barite ± sphalerite that alternated with precipitation of pyrite. Clasts of hydrothermal minerals in sandstone and coarse, polymict conglomerate beds at several levels within the unit highlight the repeated nature of hydrothermal fluid circulation during sediment accumulation. Hydrothermal fluid circulation caused widespread acid–sulfate alteration of the footwall, extensive replacement of the newly deposited carbonate sediments by hydrothermal precipitates, and crystallization of hydrothermal chert–barite–pyrite in veins perpendicular to bedding in the footwall and parallel to bedding in the sedimentary unit. The combined evidence points to deposition of the chert–barite unit within an active volcanic caldera. A 10 cm thick bed of felsic volcaniclastic tuff within finely bedded carbonates near the top of the unit has yielded a maximum age of deposition of 3481.0 ± 3.6 Ma (2 σ uncertainty), confirming earlier Pb–Pb age data for the antiquity of these rocks. Putative signs of life are present as stratiform, columnar, domical, and coniform stromatolitic laminates at various levels throughout the unit. Petrographic observations show that red- and black-weathering stromatolitic laminates on the surface consist of pyrite in unweathered drillcore material. Observation of local relics of carbonate between pyrite crystals in these laminates indicates a carbonate protolith prior to replacement by hydrothermal pyrite, which provides support for a biological origin of stromatolitic laminates. Further support is provided by clasts of laminated carbonaceous material in thinly bedded, primary micritic carbonates. Textural analysis of jaspilitic “cherts” near the top of the unit reveal haematite as tiny crystals within recrystallized siderite/dolomite rhombs in carbonate beds affected by hydrothermal silica alteration. The presence of unaltered diagenetic pyrite crystals in the haematite-altered siderite indicates that alteration did not result from oxidizing fluids. Rather, haematite alteration is interpreted as the result of an increase in pH during diagenetic alteration by mildly reducing, silica-rich fluids associated with eruption of overlying basalts, possibly with the influence of microbial activity. This has important implications for the origin of jaspilitic cherts throughout the early Archean record and for atmospheric conditions of early Earth.

Estimating the contribution of the authigenic mineral component to the long-term reactive silica accumulation on the western shelf of the Mississippi River Delta

Previous studies have shown how biogenic silica particles undergo conversion to aluminosilicate phases in large tropical deltaic systems, thus affecting the world ocean budget of major seawater cations. This study tackles the important question of the silica budget in the coastal zone of the Mississippi River Delta, providing evidence for the role of biogenic silica diagenesis in this subtropical system from direct examination of individual diatom particles, sediment leachates and pore-water composition. The estimated reactive silica stored in the study area (5990 km 2) is based on operational leaches that account for altered biogenic silica particles and other authigenic aluminosilicate phases in addition to fresh biogenic silica. Early diagenesis of silica in the delta front occurs mainly where more siliceous material is deposited. An inner-shelf area, where hypoxic conditions are found, significantly contributes to the formation of authigenic products of Si alteration. Data suggest that the limiting factor of silica alteration processes is the availability of detrital phases such as Al and Fe. The estimated total reactive silica accumulation in the study area is 1.45×10 10 mol Si year −1, representing ∼2.2% of the long-term bulk sediment accumulation. On the basis of a conservative appraisal, the authigenic mineral components account for ∼40% of the long-term reactive silica storage. This study shows that non-tropical deltaic systems are significantly more important sinks of silica than previously thought and that, where conditions are favourable, a consistent portion of reactive silica not leaving the shelf is stored within the delta in the form of authigenic components.

Outokumpu revisited: New mineral deposit model for the mantle peridotite-associated Cu–Co–Zn–Ni–Ag–Au sulphide deposits

The metaturbidites of the Palaeoproterozoic Jormua–Outokumpu thrust belt in eastern Finland enclose m- to km-scale ultramafic massifs that are distributed over an area of more than 5000 km 2. These bodies, which almost entirely consist of highly depleted mantle peridotites (now metaserpentinites and metaperidotites), are intimately associated with massive to semimassive, polymetallic Cu–Co–Zn–Ni–Ag–Au sulphide deposits that sustained mining in the region between 1913 and 1988. Currently, one deposit (Kylylahti) is proceeding into a definitive feasibility study emphasising the renewed economic interest for Outokumpu-type deposits. The origin of these Outokumpu-type Cu–Co–Zn–Ni–Ag–Au deposits is now re-interpreted to be polygenetic. First, their formation requires deposition of a Cu-rich proto-ore within peridotitic sea floor at ∼ 1950 Ma. Close modern analogues to the proto-ore setting include, for example, the Logatchev and Rainbow fields at the Mid-Atlantic Ridge, where venting of high-T–low-pH hydrothermal fluid resulted in accumulations of Cu–Zn–Co–Ag–Au sulphides on serpentinised ultramafic seafloor. Second, the Ni-rich composition of Outokumpu sulphide ores calls for a separate source for nickel: Some 40 Ma after the deposition of the Cu-rich proto-ore – concomitant with the obduction of the ultramafic massifs – disseminated Ni sulphides formed through chemical interaction between obducting peridotite massifs and adjacent black schists. This process was related to listwaenite–birbirite type carbonate–silica alteration at margins of the ultramafic massifs. Due to this alteration, silicate nickel was released from the primary Fe–Mg silicates and redeposited as Ni sulphides in the alteration fringes of the massifs. We propose that syntectonic mixing of these two “end-member” sulphides, i.e., the primary Cu-rich proto-ore and the secondary Ni-sulphide disseminations, resulted in the uncommon metal combination of the Outokumpu-type sulphides. Late tectonic solid-state re-mobilisation, related to the duplexing of the ore by isoclinal folding, upgraded the sulphides into economic deposits.

Carbonaceous Cherts in the Barberton Greenstone Belt and Their Significance for the Study of Early Life in the Archean Record

The 3.5-3.2 Ga old volcano-sedimentary succession of the Barberton greenstone belt (South Africa) is characterized by lithological units that are repeated in a regular manner. Komatiitic, basaltic, and dacitic volcanic and volcaniclastic sequences are capped by zones of silica enrichment, followed by bedded carbonaceous cherts. Stratiform and crosscutting carbonaceous chert veins are common in silica alteration zones and bedded cherts. A detailed field study of several chert horizons and chert veins that range in age from 3.47 to 3.30 Ga revealed the importance of syndepositional hydrothermal activity for their origin. Bedded cherts consist of silicified detrital and tuffaceous sediments that were deposited on the seafloor. Silicification took place at the sediment-water interface as a result of diffuse upflow of low-temperature hydrothermal fluids, which gave rise to the formation of impermeable chert caps. Fluid overpressure resulted in the breaching of the cap rocks at times. Chert veins contain angular host rock fragments, replace wall rocks, and show evidence of multiple vein fillings and in situ brecciation of earlier generations of vein fillings. They represent hydraulic fractures that were initiated by overpressuring of the hydrothermal system. The vein networks were infilled, partly by hydrothermal chert precipitates, and partly by still unconsolidated (not yet silicified) sedimentary material derived from overlying sedimentary horizons. Field, petrographic, isotopic, and trace element evidence indicate that most carbonaceous matter represents sedimentary material that originated by biogenic processes in the Archean oceans and not by hydrothermal processes in the subsurface.

Metallogeny of the Mont-de-l’Aigle IOCG deposit, Gaspé Peninsula, Québec, Canada

The Mont-de-l’Aigle deposit is located in the northern part of Dome Lemieux, in the Connecticut Valley-Gaspe Synclinorium, Gaspe Peninsula, Quebec. The Dome Lemieux is a subcircular antiform of Siluro–Devonian sedimentary rocks that is cut by numerous mafic and felsic sills and dikes of Silurian to Late Devonian age. Plutonism occurred in a continental within-plate extensional setting typical of orogenic collapse. The Cu−Fe (± Au) mineralization of Mont-de-l’Aigle occurs in veins, stockworks, and breccias. Mineralization is located near or within N−S and NW−SE faults cutting sedimentary rocks. IOCG mineralization postdates intrusions, skarns, hornfels, and epithermal mineralization typical of the southern part of the Dome Lemieux. The paragenetic sequence comprises: (1) pervasive sodic, potassic, chlorite, and silica alteration, (2) hematite, quartz, pyrite, magnetite, and chalcopyrite veins, stockworks and breccias and, (3) dolomite ± hematite veins and veinlets cutting the earlier mineralization. Intrusions display proximal sodic and potassic alteration, whereas sedimentary rocks have proximal decalcification, silicification, and potassic alteration. Both intrusive and sedimentary rocks are affected by a pervasive distal chlorite (± silica) alteration. The sulfur isotope composition of pyrite and chalcopyrite (δ34S=−1.5 to 4.8‰) suggests that sulfur was derived mainly from igneous rocks. Fluid δ18O (−0.4 to 2.65‰) indicates meteoric or seawater that reacted with the country rocks. Mixing of hot magmatic fluids with a cooler fluid, perhaps meteoric or seawater is suggested for mineral deposition and alteration of the Mont-de-l’Aigle deposit. The mineralogy, alteration, and sulfur isotope composition of the Mont-de-l’Aigle deposit compare well with IOCG deposits worldwide, making the Mont-de-l’Aigle deposit a rare example of Paleozoic IOCG mineralization, formed at shallow depth, within a low metamorphic grade sedimentary rock sequence.

Silica alteration zones and cherts as a record of hydrothermal processes on the Archaean seafloor

Silica alteration zones and cherts as a record of hydrothermal processes on the Archaean seafloor

Pukala intrusion, its age and connection to hydrothermal alteration in Orivesi, southwestern Finland

The Pukala intrusion is situated in the Paleoproterozoic Svecofennian domain of the Fennoscandian Shield in the contact region between the Central Finland Granitoid Complex and the Tampere Belt. The acid subvolcanic intrusion, which is in contact or close to several altered domains, mainly consists of porphyritic granodiorite and trondhjemite. The Pukala intrusion was emplaced into volcanic sequence in an island-arc or fore-arc setting before or during the early stages of the main regional deformation phase of the Svecofennian orogeny. On the basis of the geochemical data, the Pukala intrusion is a peraluminous volcanic-arc granitoid. After crystallisation at 1896±3 Ma, multiphase deformation and metamorphism caused alteration, recrystallisation, and orientation of the minerals, and tilted the intrusion steeply towards south. The 1851±5 Ma U-Pb age for titanite is connected to the late stages of the Svecofennian tectonometamorphic evolution of the region. Several hydrothermally altered domains are located in the felsic and intermediate metavolcanic rocks of the Tampere Belt within less than one kilometre south of the Pukala intrusion. Alteration is divided into three basic types: partial silica alteration, chloritesericite±silica alteration, and sericite alteration in shear zones. The fi rst two types probably formed during the emplacement and crystallisation of the Pukala intrusion, and the third is linked to late shearing. Intense sericitisation and comb quartz bands in the contact of the intrusion and the altered domain at Kutemajarvi suggest that the hydrothermal system was driven by the Pukala intrusion.

Geology and Genesis of the Multistage High-Sulfidation Epithermal Pascua Au-Ag-Cu Deposit, Chile and Argentina

The giant Pascua epithermal Au-Ag-Cu deposit is located in the El Indio belt of north-central Chile and Argentina and was the product of a high-sulfidation hydrothermal system. The host rocks consist mainly of Triassic granite and heterolithic Miocene breccia pipes. Granitic rocks host ~60 percent of the mineralization (or >80% if granitic breccia fragments are included) but were not the cause of hydrothermal activity. The economic mineralization forms a large orebody centered on Brecha Central, the largest of the breccia pipes, and several smaller satellite bodies that are separated by zones of subeconomic mineralization. Hydrothermal activity produced two main stages of advanced argillic and vuggy silica alteration. These were separated by an intermediate stage of argillic alteration, silicification and hypogene jarosite, which occurred penecontemporaneously with the emplacement of Brecha Central. Main-stage Au-Ag-Cu mineralization occurred toward the end of the second stage of alteration and involved precipitation of native gold with pyrite and enargite and incorporation of Au in the structure of these minerals. Four types of mineralization are recognized based on the occurrence of economic concentrations of Au in rocks containing significant amounts of the following minerals: (1) alunite, pyrite, and enargite; (2) pyrite; (3) pyrite and szomolnokite; and (4) native gold. Main-stage gold mineralization was followed by a sulfate stage represented by barite and anglesite at lower elevations and barite, anglesite, plus primary szomolnokite, the first reported occurrence of this phase as a hydrothermal ore mineral, at higher elevations. Late-stage silver mineralization, characterized by the occurrence of microscopic Cl-, I-, and Hg-bearing phases in voids and fractures, enriched the upper parts of the deposit, forming an extensive subhorizontal zone that overprints previous alteration and mineralization. Advanced argillic alteration and high-sulfidation mineralization at Pascua are interpreted to have resulted from a high-level hydrothermal system developed above a porphyry stock located in Argentina. Extensive vuggy silica and advanced argillic alteration reflect interaction of the wall rocks with acidic magmatic vapors. Gold, copper, and arsenic are interpreted to have been transported by these vapors and to have deposited as a result of their cooling and subsequent condensation. Ore-forming hydrothermal activity terminated with silver enrichment in response to the condensation of residual magmatic vapors during the waning stages of the system.

Early diagenesis of biogenic silica in the Amazon delta: alteration, authigenic clay formation, and storage

Deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques. Rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets. A variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating Amazon delta sediments indicates that both of these general assumptions are unjustified. Apparent lack of significant biogenic silica storage in deltaic environments, particularly in the tropics, may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica. Biogenic silica particles buried in suboxic Amazon delta deposits can be unaltered, partially dissolved, covered with aluminosilicate or metal-rich coatings, or completely reconstituted into authigenic K-Fe-rich aluminosilicate minerals. Pore water (K, Mg, F, Si) and solid-phase distributions, direct observations of particles, laboratory experiments, and depositional context indicate that authigenic clays form rapidly (<1 yr) in the seasonally reworked surface layer (∼ 0.5–2 m) of the delta topset and are disseminated during sediment remobilization. Fe, Al-oxide rich debris derived from the tropical drainage basin is an abundant reactant, and thus the supply of biogenic silica is a major control on the amount of clay formed. The mild 1% Na 2CO 3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0.1N HCl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution. Well-crystallized clays are not significantly affected by this modification nor is bulk Amazon River bed sediment. The two-step procedure indicates that ∼90% of the biogenic silica originally present in deposits is converted to clay or otherwise altered, raising the effective quantity of biogenic silica stored from ∼33 to ∼296 μmol Si g −1 (∼1.8% SiO 2). Biogenic Si stored in the delta increases away from the river mouth, across shelf and along the dispersal system where primary production is highest. The K/Si ratio of labile authigenic material is ∼0.19 mol mol −1, far higher than Amazon River suspended matter (∼0.07 mol mol −1). Diagenetic models indicate formation rates in the mobile sediment layer of ∼2.8 μmol K g −1 yr −1 (∼16 μmol Si g −1 yr −1). Inclusion of authigenic alteration products of biogenic silica in estimates of reactive Si burial increases the deltaic storage of riverine Si to ∼22% of the Amazon River input. The rapid formation of aluminosilicates from biogenic SiO 2, seawater solutes, and remobilized Fe, Al-oxides represents a form of reverse weathering. Rapid reverse weathering reactions in tropical muds and deltaic deposits, the largest sediment depocenters on Earth, confirms the general importance of these processes in oceanic elemental cycles.

ALACRANITE, As4S4: A NEW OCCURRENCE, NEW FORMULA, AND DETERMINATION OF THE CRYSTAL STRUCTURE

Between July 20 and August 3, 1998, cruise SO–133 aboard the R/V Sonne completed detailed mapping and sampling of the seafloor around Lihir Island in Papua New Guinea. Sampling of Conical Seamount resulted in the discovery of an unusual occurrence of alacranite and realgar associated with polymetallic vein mineralization consisting of pyrite, sphalerite, galena, minor chalcopyrite and sulfosalts, along with intense clay and silica (amorphous) alteration. Alacranite occurs as subhedral to euhedral tabular orange crystals that are transparent to translucent with a vitreous to resinous luster. Crystals occur both is olated and as groupings within vugs and on matrix, and have a maximum length of 0.1 mm. Single-crystal diffraction data were collected

Mineral Paragenesis of the Lepanto Copper and Gold and the Victoria Gold Deposits, Mankayan Mineral District, Philippines

Abstract: Mineral paragenesis of the alteration, ore and gangue minerals of the Lepanto epithermal copper‐gold deposit and the Victoria gold deposit, Mankayan Mineral District, Northern Luzon, Philippines, is discussed.The principal ore minerals of the Lepanto copper‐gold deposit are enargite and luzonite, with significant presence of tennantite‐tetrahedrite, chalcopyrite, sphalerite, galena, native gold/electrum and gold‐silver tellurides. Pervasive alteration zonations are commonly observed from silicification outward to advanced argillic then to propylitic zone. The ore mineralogy of the Lepanto copper‐gold deposit suggests high fS2 in the early stages of mineralization corresponding to the deposition of the enargite‐luzonite‐pyrite assemblage. Subsequent decrease in the fS2 formed the chalcopyrite‐tennantite‐pyrite assemblage. An increase in the fS2 of the fluids with the formation of the covellite‐digenite‐telluride assemblage caused the deposition of native gold/electrum and gold‐silver tellurides.The principal ore minerals of the Victoria gold deposit are sphalerite, galena, chalcopyrite, tetrahedrite and native gold/electrum. The alteration halos are relatively narrow and in an outward sequence from the ore, silica alteration grades to illitic‐argillic alteration, which in turn grades to propylitic alteration. The Victoria gold mineralization has undergone early stages of silica supersaturation leading to quartz deposition. Vigorous boiling increased the pH of the fluids that led to the deposition of sulfides and carbonates. The consequent decrease in H2S precipitated the gold. Gypsum and anhydrite mainly occur as overprints that cut the carbonate‐silica stages.The crosscutting and overprinting relationships of the Victoria quartz‐gold‐base metal veins on the Lepanto copper‐gold veins manifest the late introduction of near neutral pH hydrothermal fluids.