Secondary Silicate Research Articles

Complex multistage genesis for the Ni–Cu–PGE mineralisation in the southern region of the Platreef, Bushveld Complex, South Africa

In the southern portion of the Platreef, Ni–Cu–Pt-group element (PGE) mineralisation results from a complex interplay of pre-, syn- and post-magmatic processes. Sulphide minerals throughout the successio comprise < 1% to > 25% (mode) and occasionally up to 45% over short intersections of core. Generally, these occur as centimetre to millimetre-sized fractionated blebs and interstitial grains of pentlandite, pyrrhotite and chalcopyrite. Where present, zones of massive sulphides rich in chalcopyrite are found close to the footwall contact. More compositionally complex sulphides are associated with felsic melt phases that pervasively infiltrated the package soon after its partial or complete crystallisation. There is a close correlation between Cu and Ni concentrations but a poor correlation between PGE and base metal contents. Thus high Cu+Ni values do not necessarily indicate high PGE grade, although the highest PGE grades are located within sulphide-rich zones towards the floor. Platinum-group minerals (PGMs) are dominated by Pd-bearing tellurides, antimonides, bismuthides, bismutho-antimonides and complex bismuthotellurides. Pt-bearing phases mostly occur as Pt-arsenides and antimonides. PGMs are found in rims around orthopyroxenes, as discrete grains within secondary silicates and as grains adjacent to, or along the margins of, composite sulphides. Rarely are they found as inclusions within sulphide minerals. The present-day distribution of sulphide minerals and PGEs results from a continuum of processes, beginning with scavenging of PGEs by immiscible sulphide droplets. Subsequently, these were modified by the incorporation of meta-sedimentary-derived S, As, Te, Bi and Sb from the devolatilisation of floor rocks and xenoliths. Felsic melts redistributed some of the sulphides and PGMs as they percolated through the Platreef. Finally, late-stage metasomatic fluids altered the primary silicates and led to further remobilisation of the PGEs to form PGMs disseminated within tremolite, talc and serpentine. This final stage may account for the observed decoupling of Cu and Ni from Pt and Pd.

Melonite group minerals and other tellurides from three Cu–Ni–PGE prospects, Eastern Desert, Egypt

Melonite group minerals and other tellurides are described from three Cu–Ni–PGE prospects in the Eastern Desert of Egypt: Gabbro Akarem, Genina Gharbia and Abu Swayel. The prospects are hosted in late Precambrian mafic–ultramafic rocks and have different geologic histories. The Gabbro Akarem prospect is hosted in dunite pipes where net-textured and massive sulfides are associated with spinel and Cr-magnetite. Michenerite, merenskyite, Pd–Bi melonite and hessite occur mainly as inclusions in sulfides. Typical magmatic textures indicate a limited role of late- and post-magmatic hydrothermal processes. At Genina Gharbia, ore forms either disseminations in peridotite or massive patches in hornblende-gabbro in the vicinity of metasedimentary rocks. Actinolitic hornblende, epidote, chlorite and quartz are common secondary silicates. Sulfide textures and host rock petrography suggest a prolonged late-magmatic hydrothermal event. Michenerite, merenskyite, Pd–Bi melonite, altaite, hessite, tsumoite, sylvanite and native Te are mainly present in secondary silicates. The Abu Swayel prospect occurs in conformable, lens-like mafic–ultramafic rocks in metasedimentary rocks and along syn-metamorphic shear zone. The sulfide ore and host rocks are metamorphosed (amphibolite facies; 550 to 650 °C, 4 to 5 kbar) and syn-metamorphically sheared. Melonite group minerals are represented by merenskyite and Pd–Bi melonite. Other tellurides comprise hessite, altaite and joséite-B. Melonite group minerals and tellurides occur as inclusions in mobilized sulfides and along cracks in metamorphic garnet and plagioclase. The different geological history of the three prospects permits an examination of the role played by magmatic, late-magmatic and metamorphic processes on the mineralogy of melonite group minerals and diversity of tellurides. The contents of PGE and Te in the ore and temperature of crystallization control the mineralogy and compositional trends of the melonite group minerals. Crystallization of the melonite group minerals over a wide range of temperatures in a Te-rich environment enhances the elemental substitutions. Merenskyite dominates the mineralogy of the group at low Te activity, while Pd–Bi melonite is the common phase at high Te activity.

Surface charges and Np(V) sorption on amorphous Al and Fe silicates

Sorption onto Si-rich alteration layers of crystalline minerals and nuclear glasses, and onto amorphous secondary silicates of rocks and soils, are expected to retard the migration of actinides in the near- and far-field of high-level waste repositories. In this work, we present experimental and modeling studies on the effects of silicate structure and bulk chemistry, and of solution chemistry, on charges and adsorption of neptunyl ions at surfaces of synthetic, amorphous or poorly ordered silica, Al silicates and Fe silicates. The Al silicates display similar pH-dependent surface charges characterized by predominant Si–O − sites, and similar surface affinities for neptunyl ions, irrespective to their Si/Al molar ratio (varying from 10 to 4.3). Such experimental features are explained by incorporation of Al atoms in tetrahedral position in the silicate lattice, leading to only trace amounts of high-affinity Al–OH surface groups due to octahedral Al. By contrast, the structure of the Fe silicates ensures the occurrence of high-affinity Fe–OH surface groups, whose concentration is shown by proton adsorption measurements to increase with decreasing of the silicate Si/Fe molar ratio (from 10 to 2.3). Nevertheless, experimental data of the adsorption of neptunyl and electrolyte ions show unexpectedly weak effect of the Si/Fe ratio, and suggest predominant Si–OH surface groups. A possible explanation is that aqueous silicate anions, released by dissolution, adsorb at Fe–OH surface groups and/or precipitate as silica gel coatings, because experimental solutions were found at near-equilibrium with respect to amorphous silica. Therefore, the environmental sorption of Np(V) onto Si-rich, amorphous or poorly ordered Al silicates may primarily depend on pH and silicate-specific surface areas, given the low overall chemical affinity of such phases for dissolved metals. By contrast, the sorption of Np(V) on natural, amorphous or poorly ordered Fe silicates may be a complex function of silicate bulk chemistry and solution chemistry, i.e., of pH and aqueous Si concentrations. Simple conceptual models of the surface chemistry of the Al and Fe silicates are developed here, based on the wealth of experimental data of silicate surface charges. The surface complexation models predict reasonably the effect of solution chemistry on the sorption of neptunyl ions on poorly ordered silicates of various compositions, and can thus be useful in extrapolating neptunyl mobility in many geochemical systems.

PH neutralization and zonation in alkaline-saline tank waste plumes.

At the Hanford Site in Washington State, the pH values of contaminant plumes resulting from leaking of initially highly alkaline-saline radioactive waste solutions into the subsurface are now found to be substantially neutralized. However, the nature of plume pH neutralization has not previously been understood. As a master geochemical variable, pH needs to be understood in order to predict the fate and transport of contaminants carried by the waste plumes. Through this laboratory study, we found that the plume pH values spanned a broad range from 14 (within the near-source region) down to the value of 7 (lower than the pH value of the initial soil solution) while the plume was still connected to an actively leaking source. We defined two zones within a plume: the silicate dissolution zone (SDZ, pH 14-10) and the neutralized zone (NZ, pH 10-7). Quartz dissolution at elevated temperature and precipitation of secondary silicates (including sodium metasilicate, cancrinite, and zeolites) are the key reactions responsible for the pH neutralization within the SDZ. The rapid and thorough cation exchange of Na+ replacing Ca2+/Mg2+, combined with transport, resulted in a dynamic Ca2+/Mg2+-enriched plume front. Subsequent precipitation of calcite, sodium silicate, and possibly talc led to dramatically reduced pH within the plume front and the neutralized zone. During aging (after the plume source became inactive), continued quartz dissolution and the secondary silicate precipitation drove the pH value lower, toward pH 11 at equilibrium within the SDZ, whereas the pH values in the NZ remained relatively unchanged with time. A pH profile of 11 from the plume source to pH 7 at the plume front is expected for a historical plume. This laboratory-based study provided realistic plume pH profiles (consistent with that measured from borehole samples) and identified underlying mechanisms responsible for pH evolution.

Diffusion of HTO and cement pore fluids through host rock

Summary In the post-closure period of a deep disposal facility for low- to intermediate-level radioactive waste, highly alkaline pore-fluids chemically equilibrated with cementious components of the engineered barriers will migrate into the surroundings. Primary minerals in the host rock could dissolve and secondary calcium silicate hydrate (CSH) phases might be formed in the micro pore system of the rock and change the available porosity for radionuclides and hence the permeability. Since the retardation of radionuclides in the host rock depends on sorption and matrix diffusion, the study of the mineral reactions is of importance. Experiments with diffusion of HTO and synthetic cementitious pore-waters through 1 cm thick discs of Äspö diorite have been made in a nitrogen-flushed glovebox facility. Synthetic cementitious pore-waters representative of fresh and leached concrete were used. For the fresh pore-water the results show that hydroxide ion diffusion is retarded relative to HTO tracer, indicating that reactions between hydroxide and host rock take place. The result can be interpreted as hydroxide sorption in the rock. Al and Si did also accumulate on the sampling side which was attributed to mineral dissolution. For the evolved pore-water, no through-diffusion of hydroxide ions or accumulation of other elements in the sampling cell was observed during the sampling period, indicating that the through-diffusion of hydroxide is hindered by this type of pore-water.

Metamorphism and PGE-Au content of chromitite from the Ipanema mafic/ultramafic Complex, Minas Gerais, Brazil

The Santa Cruz massif, which forms part of the Ipanema mafic/ultramafic Complex, Minas Gerais, Brazil, has an exposed upward sequence of metadunite, metaharzburgite (including three separate chromitite layers), metapyroxenite, metagabbro, and meta-anorthosite. Primary igneous chromite grains in the main chromitite layer are poikiloblastic and tectonically fragmented, and have a narrow (10–20 μm) margin of chromian spinel. Cataclased chromite fragments are extensively replaced and mantled by chromian spinel; they have a composite margin comprised of an inner zone of more aluminous spinel and an euhedral outer zone of more Cr-rich spinel, representing granulite and amphibolite facies metamorphic events, respectively. The contents of platinum-group elements (PGE) and Au in chromite separates are relatively high (Os 45, Ir 23, Ru 136, Rh 19, Pt 98, Pd 63, and Au 83 ppb), and significantly enriched (∼ 4x) over whole rock values. Platinum-group minerals are not observed and micrometre-sized inclusions of sulfide minerals (chalcopyrite and pentlandite) in relict chromite are rare. However, comparison of mineral proportions in the separated chromite and whole rock shows that the precious metals are hosted predominantly in the relict igneous chromite grains, rather than the secondary chromian spinel and primary and secondary Mg-rich silicates. The major element composition and average chondrite-normalized PGE pattern of the separated chromite correspond to S-poor stratiform chromitite. We suggest that the precious metals accumulated with chromite during crystallization of a S-poor magma, and were not remobilized in the relict chromite during the subsequent high grade metamorphism.

Mineralogy of granulated wood ash from a heating plant in Kalmar, Sweden

The central heating plant of Kalmar, Sweden produces 200–300 tons wood ash every year. A stabilised material for nutrient recycling is produced by adding water and dolomite to the wood ash and granulating the mixture. Combined mineralogy and chemistry can be used to interpret the transformation processes that occur during hardening and weathering of the granules, thus leading to a possibility to refine the production process and final characteristics of the granules. Mineralogy was separately studied in the wood ash, dolomite, self-hardened wood ash and granules by X-ray diffraction. Magnesium- and calcium-containing minerals are most common in the ash materials in the present study. The amounts of portlandite and calcite present in self-hardened and granulated ash samples are clearly higher than those in the untreated ash, showing that these minerals are formed during the treatments. Additionally, one potassium-containing secondary mineral, syngenite, is formed during the self-hardening of wood ash. Quartz, dolomite and the Fe–K–Mg-silicate in the granules originate from dolomite. The secondary minerals gypsum and calcium silicate hydrate are present in the granules. Portlandite occurs only in control granules in the field study. This suggests that hardening of granules continues in the field and portlandite is transformed into calcite. After up to 3 years on forest soil, the crystalline compounds dolomite, calcite, quartz, ankerite, albite and alumohydrocalcite are present in granules, of which alumohydrocalcite is formed as a secondary mineral in the field. These results suggest that the dissolution of granulated wood ash is strongly delayed compared with untreated wood ash and self-hardened wood ash because of the formation of less soluble compounds during the granulation process.

Cu-Ni-PGE Mineralization within the Copper Cliff Offset Dike, Copper Cliff North Mine, Sudbury, Ontario: Evidence for Multiple Stages of Emplacement

Detailed petrographic and quantitative (electron microprobe, SEM-EDS) mineral-chemical analyses of sulfide ore from the 100 orebody at the Copper Cliff North mine, Copper Cliff, Ontario, reveal numerous stages of ore development. Sharp contacts between weakly mineralized quartz diorite and blebby to massive sulfides indicate multiple intrusive events within the Copper Cliff offset dike. A spatial association of sulfides and hydrous minerals, such as amphibole, biotite, chlorite and epidote, suggests that most primary ore has been affected by later hydrothermal fluids. Sulfides are often enclosed or bordered by these secondary silicate phases. The ore consists primarily of pyrrhotite, pentlandite, chalcopyrite, with trace michenerite, cobaltite/gersdorffite, irarsite, hessite, tsumoite, and bismuth tellurides. Geological and petrological comparison of the 100 orebody and the adjacent 900 and 890 ore-bodies has revealed distinct similarities in ore distribution. Data show that the 100 orebody is depleted in platinum-group elements (PGE) relative to the 900 orebody. Platinum-group elements from the 100 orebody may have been hydrothermally remobilized and partly emplaced in the 900 orebody, in adjacent veins, or in magmatic breccia. Analysis of Cu, Ni, and PGE assay data of ore samples has characterized their distribution among the eight different ore textures in the orebody. There is a strong association between Pt and interstitial sulfide, and to a lesser extent massive sulfide, but little correlation between chalcopyrite-rich ore and PGE in the 100 orebody, unlike many other deposits in the Sudbury region.

A Gold- and Platinum-Mineralized Layer in Gabbros of The Kap Edvard Holm Complex: Field, Petrologic, and Geochemical Relations

The Lower Layered Series a cumulates of the Kap Edvard Holm Complex contain a stratiform layer, 1 to 20 m thick and > 6 km long, that is mineralized with gold and platinum-group elements (PGEs). The Lower Layered Series a cumulates (>650 m thick) are dominated by modally layered gabbro with lesser volumes of granular anorthosite and troctolite that together form 2- to 30-m-thick layers that are most abundant above the mineralized layer. Minor volumes of poikilitic wehrlite form a few 1- to 2-m-thick layers throughout the section. Field and petrographic relations as well as cryptic variations in cumulus mineral compositions indicate that the Lower Layered Series a formed in an open system magma chamber from repeated magma injections. Within the mineralized layer, average concentrations over 3 m thickness are 250 ppb Pt, 40 ppb Pd, and 50 ppb Au, with individual samples containing up to 5 ppm Pt and 6 ppm Au. The concentrations of Ir, Os, Ru, and Rh are uniformly low (<20 ppb). Stratigraphic zoning of Pt, Pd, and Au is consistent along strike with peak Au concentrations overlying Pt and Pd peaks. Platinum minerals are primary Pt-Fe alloy, sperrylite, and moncheite, with secondary Pt sulfide overgrowths. Gold occurs as alloys with Ag and Cu. These minerals are found in contact with primary and secondary silicates and oxides and with intercumulus base metal sulfides that consist of the assemblage bornite + chalcopyrite + digenite. Deposition of the mineralized layer is associated in space and time with injections of plagioclase-saturated, primitive magma that mixed with more evolved, resident magma. These events caused efficient partitioning of Au and PGE into sulfides derived from a large volume of magma, deposition of these sulfides to form the mineralized layer, and subsequent deposition of the overlying anorthosite layers. The wehrlite layers, however, were formed by postcumulus reactions between gabbro cumulates and hydrous, intercumulus melts, probably derived from the compacting cumulus pile. Subsolidus reactions between primary minerals and meteoric hydrothermal solutions caused extensive alteration of the mineralized gabbro, oxidation and desulfurization of Cu-Fe sulfides and local recrystallization of PGE minerals.

Volcanic lake systematics II. Chemical constraints

A database of 373 lake water analyses from the published literature was compiled and used to explore the geochemical systematics of volcanic lakes. Binary correlations and principal component analysis indicate strong internal coherence among most chemical parameters. Compositional variations are influenced by the flux of magmatic volatiles and/or deep hydrothermal fluids. The chemistry of the fluid entering a lake may be dominated by a high-temperature volcanic gas component or by a lower-temperature fluid that has interacted extensively with volcanic rocks. Precipitation of minerals like gypsum and silica can strongly affect the concentrations of Ca and Si in some lakes. A much less concentrated geothermal input fluid provides the mineralized components of some more dilute lakes. Temporal variations in dilution and evaporation rates ultimately control absolute concentrations of dissolved constituents, but not conservative element ratios.Most volcanic lake waters, and presumably their deep hydrothermal fluid inputs, classify as immature acid fluids that have not equilibrated with common secondary silicates such as clays or zeolites. Many such fluids may have equilibrated with secondary minerals earlier in their history but were re-acidified by mixing with fresh volcanic fluids. We use the concept of ‘degree of neutralization’ as a new parameter to characterize these acid fluids. This leads to a classification of gas-dominated versus rock-dominated lake waters. A further classification is based on a cluster analysis and a hydrothermal speedometer concept which uses the degree of silica equilibration of a fluid during cooling and dilution to evaluate the rate of fluid equilibration in volcano-hydrothermal systems.

Rare earth elements and Sr and Nd isotopic compositions of dissolved and suspended loads from small river systems in the Vosges mountains (France), the river Rhine and groundwater

The aim of this study is to characterize the evolution of the rare earth elements (REE) in non-mature streams from small catchment areas in the Vosges mountains downstream to more mature plain rivers including the river Rhine. The dissolved load REE distribution patterns of the low-pH Vosges streams are very different from those of high-pH plain rivers indicating that different physico-chemical parameters control REE transport in these different water systems. The plain rivers and groundwater show similar REE distribution patterns with a strong negative Ce anomaly and heavy rare earth element (HREE) enrichment. In addition, the river Rhine has a positive Gd anomaly which is of anthropogenic origin. Similar to the world's major rivers the light rare earth element (LREE) enrichment in the plain rivers is mainly pH controlled. Their Sm/Nd and 143 Nd/ 144 Nd isotope ratios are close to average continental crust values. This is not the case for the less evolved, non-mature and low pH Vosges streams. Their high Sm/Nd and 143 Nd/ 144 Nd ratios but low 87 Sr/ 86 Sr ratios suggest that chemical alteration of accessory middle rare earth element (MREE) enriched minerals such as apatites from rocks in the catchment area control the REE abundances of these waters. A comparison of the dissolved load REE distribution patterns with those of the principal lithologies in the corresponding drainage basins illustrates that especially the Eu anomalies of the Vosges streamlets are strongly lithology dependent. Leaching experiments indicate that the suspended load is isotopically very inhomogeneous. The REE distribution patterns of the suspended load leachates have no similarities with those of the corresponding dissolved load indicating that the leachable reservoir not only contains adsorbed REE but also REE from leachable mineral phases. Their 143 Nd/ 144 Nd , Sm/Nd and 87 Sr/ 86 Sr ratios support this suggestion being always higher and lower, respectively, than those of their corresponding dissolved loads. They rather point to the presence of relic primary apatite in the leachable portion of the suspended load. The flat PAAS normalized REE distribution patterns, the high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd isotopic ratios of the corresponding residues suggest secondary silicate phases such as clay minerals in the residual phase of the suspended load. Leachates and corresponding residual phases define alignments in the Sm/Nd isochron diagram whose slopes define ages ranging between 200 and 390 Ma indicating that the mineral phases in the suspended load retain some memory of their primary precursor minerals in the Hercynian granitic source rocks.

Mineralogy and Geochemistry of Profiles Through Lateritic Nickel Deposits at Kansa, Sukinda, Orissa

Abstract In the Sukinda ultramafic belt of Orissa, significant deposits of nickeliferous laterite have formed by weathering of large chromiferous dunite-peridotite suite of rocks. Two typical laterite profiles obtained by drilling (BH-1 = 50 m and BH-2 = 46 m) from Kansa sector of Sukinda belt were studied in order to establish petrochemical relationships between supergene Ni-laterites and the parent rock. In BH-1, ferruginous layers comprise the profile where goethite is intimately admixed with quartz, kaolinite and gibbsite at various depths. In BH-2, a complete laterite profile from bed rock (altered dunite) to top pisolitic laterite with transitional clay-serpentinite zone is encountered. The altered dunite is essentially composed of olivine (FO93.8 ? 35% vol.) and serpentine. The clay-serpentinite zone consists of varying proportions of smectite clay (nontronite) and antigorite with minor amount of chromite, goethite and quartz. This zone is enriched in Ni with a maximum value of 1.09% Ni. In the ochreous ferruginous zone overlying clay-serpentinite zone, serpentine and nontronite disappear completely and goethite constitutes the principal mineral with varying proportions of secondary quartz. This zone is rich in Fe, Ni, Co, Mn, Al and Cr and is formed by residual concentration of stable oxides. Mass-balance equation assuming Cr2O3 as constant chemical constituent reveals that nickel is strongly accumulated in the clay-serpentinite zone. A second maximum in nickel concentration is found in ochreous ferruginous zone with almost complete removal of magnesium. Normalised major and trace element data generated on the bore hole samples have been processed, by using R-mode factor analysis of correlation matrix to understand the natural process of weathering. Leaching studies of the samples suggest that nickel occurs in adsorbed state within amorphous hydrated iron oxide and in weakly bonded and lattice bound states within gaethite and secondary silicates.

Interactions of Cement Pore Fluids with Host Rock and the Effects on HTO, Na and Cs Diffusion

In the post-closure period of a deep disposal facility for low- and intermediate-level radioactive waste, pore-fluids chemically equilibrated with cementious components of the engineered barriers will migrate into the surroundings. Secondary calcium silicate hydrate (CSH) phases might be formed in the micro pore system of the rocks and change the available porosity for radionuclides and hence permeability. Since the retardation of radionuclides in host rock is dependent on the sorption and matrix diffusion, these chemical reactions are of concern. Experiments have been undertaken to react discs of Aspo diorite with alkaline cement porewaters. These experiments provided aged material for through diffusion experiments and samples for studies of chemical reactions. 1 cm thick discs of Aspo diorite were reacted with waters representative for fresh and leached concrete at 70C in nitrogen filled vessels for six months. Analysis of the waters indicates dissolution of primary minerals and precipitation of secondary CSH phases. HTO, Na and Cs diffusion experiments were then undertaken with both reacted and unreacted discs in a nitrogen-flushed glovebox facility. The effects of alkaline degradation of Aspo diorite on tracer through-diffusion was found to be small and of the same order of magnitude as natural variations in rock heterogenity between undegraded samples.

Soil Surface Coatings at Costa Rican Recently Active Volcanoes

AbstractUnvegetated slopes of recently active volcanoes in perudic Costa Rica have locally thin, hard, laminated coatings at the soil surface. These coatings, which cover both ash deposits and boulders, are actively forming in some locations and eroding in others. We investigated the chemistry and formation of these coatings. Optical analysis of the coatings shows that they consist of anisotropic ash particles embedded in isotropic, translucent, fine‐textured amorphous material. Scanning electron microscope analysis clearly shows secondary coatings covering and cementing primary mineral grains and rock fragments. In situ microchemical analysis shows that the coatings consist predominantly of silica. Primary mineral grains in well‐developed coatings may also be replaced by silica. The internal fabric of the coatings suggests that accumulation of secondary silica and deposition of (reworked) ash are simultaneous processes and that all coatings were formed in various episodes. Precipitation of secondary silica from solution may occur where rapid weathering of primary minerals (volcanic ash) combined with high deposition of strong acids (HCl, H2SO4) from fumarolic and solfataric activity results in high dissolved Si concentrations. This process is further enhanced by evaporative concentration at the soil surface. Low pH precludes formation of secondary aluminium silicates and gibbsite. Erosion of the coatings seems to be largely due to episodically transported water.

The Role of Convective Geothermal Systems in the Generation, Migration, and Entrapment of Oil: ABSTRACT

Modern convective geothermal systems (for example, at Yellowstone National Park) and their fossil equivalents, epithermal mineral deposits (like McLaughlin, California) have traditionally been considered poor petroleum prospects. The concentrated heat which drives these systems is commonly viewed as a negative influence: in other words, any oil initially present in or generated by these systems is quickly degraded to a useless carbonaceous residue. This is true in extreme cases, but numerous examples from the Great Basin, the northern California Coast Ranges, and elsewhere suggest that under certain circumstances, geothermal systems can efficiently generate, transport, and entrap significant quantities of producible petroleum. B.R.T. Simoneit has shown that oil can be hydrothermally generated in an {open_quotes} instant{close_quotes} of geologic time. It follows, then, that away from geothermal systems` high-temperature centers (or in wholly moderate-temperature systems), the encompassing shallow thermal anomalies can distill large volumes of oil from otherwise immature hydrocarbon source rocks. Transport of this newly-generated oil is enhanced by the buoyant upwelling of heated aqueous fluid. Porosity for oil transport and storage is created or increased by hydrothermal dissolution of rock-forming or secondary silicates and carbonates. Finally, geothermal {open_quotes}self-sealing{close_quotes} -- the deposition of secondary minerals (especially silica and clay) at the marginsmore » of a system -- can provide a very effective hydrocarbon seal. Lee Allison has noted the coincidence of igneous intrusions and oil reservoirs in Nevada. It is suggested that here and elsewhere, both igneous-related and amagmatic geothermal systems in otherwise favorable settings should be viewed as prime petroleum exploration targets.« less

Secondary ZnPbCu minerals, an aid to mineral exploration in the Aggeneys area, Bushmanland, South Africa

A close spatial association exists between secondary and primary CuPbCu minerals in the Aggeneys area, Bushmanland, South Africa. A wide variety of metamorphic and non-metamorphic indicator mineral assemblages occur within this area which may successfully be utilized in mineral exploration programmes. The mode of occurrence, associations and parageneses of secondary silicates, oxides, sulphates, phosphates, carbonates, carbonate-sulphates and chlorides are described, while their importance as indicators of mineralization is stressed.

酸化変質した黒雲母と新鮮な黒雲母を含むデイサイト溶岩 二上層群寺山デイサイトマグマの酸素フュガシテイ

The Terayama dacite, which is middle Miocene in age, has a lava dome/flow about 3 kilometers in diameter in the Setouchi volcanic zone, Japan. The dacite contains quartz, plagio-clase, magnesium rich orthopyroxene, hornblende and biotite phenocrysts. The dacite is divided into three rock types by degrees of oxidation of biotite phenocryst. The first type, named F-type, includes fresh biotite. The second, named M-type, includes fine grain Fe-Ti oxide minerals concentrated along its cleavage and a margin of biotite. The third, named O-type includes dark red biotite which has been completely replaced by Fe-Ti oxide minerals and secondary silicate minerals. These Fe-Ti oxide minerals are composed of ilmenite, magnetite, hematite and rutile. In the primary stages, though they were composed of magnetite+ilmenite, sometimes secondary hematite has replaced the margin of magnetite. Temperature and oxygen fugacity of both M and 0-types of magma are about 700°C and fo2=10-14 on the basis of chemical data of magnetite+ilmenite. This temperature and ƒo2 fall between the FMQ and MH buffer curve, which is a stable location for biotite and/or amphibole phenocryst. Therefore Fe-Ti oxide minerals have been formed from an alteration of biotite by dehydration with degassed dacite magma, not by oxida-tion. F-type lava erupted at the beginning and was followed by the extension of M and O-type lava. Dehydration in vent and/or magma chamber was started by an eruption of F-type magma.

Application of normative calculations in quantitative comparative mineralogical studies of bauxite

A CIPW-derived petrochemical calculation procedure is proposed in this paper for the conversion of whole-rock chemical analyses of bauxites and related materials into normative mineralogical suites. This method is based on the use of normative chemical formulas on a weight percentage basis using eight oxides and hydroxides, five residual and secondary silicates, amorphous silica and quartz.

Oxide zone geochemistry

Part 1 General reactions and processes: redox reactions in oxidising sulfides reactions of sulfur species and simple sulfides lower valence sulfur oxyanions in oxide zones redox behaviour of other elements weathering of silicate gangues groundwater geochemistry. Part 2 Secondary mineral formation in oxide zones: elements and oxides carbonate minerals sulfate minerals halides and halogen-containing species phosphates, arsenates and other oxyanionic minerals secondary silicates and silica.

Source-Term Constraints for the Proposed Repository at Yucca Mountain, Nevaida, Derived from the Natural Analog at PeÑa Blanca, Mexico

ABSTRACTThe source term for nuclear waste repository performance assessments can be constrained by the solubilities of radioelement-bearing solids and/or the rates of release of radioelements from nuclear waste forms. Both solubility and rate limits for the proposed repository at Yucca Mountain, Nevada, can be assessed using information from the natural analog at PeÑa Blanca, Mexico. Petrographic and field relations indicate that uraninite oxidation and transformation to secondary uranyl silicate minerals have been rapid relative to mass transport of uranium out of the PeÑa Blanca system. The rate limiting process for uranium removal is likely to be advective transport in groundwaters with uranium contents controlled by interactions with uranyl silicate minerals such as uranophane. A maximum limit on the rate of uraninite oxidation at PeÑa Blanca is calculated to be 0.032 tons of UO2per year using geologic constraints on the amount of oxidation and the available time.