Complex Mineral Assemblages Research Articles

Kinetic factors in geothermometry

The application of geothermometer/geobarometer mineral assemblages as markers of temperature and pressure in geologic formations has become highly sophisticated by the inclusion of kinetic factors in analytic procedures. That a chemically complex mineral assemblage has equilibrated during its geologic history under intense conditions is a major premise in geothermometry. That the equilibrium conditions have been quenched into the phases so that composition and crystal structure may be used to reveal the temperature‐pressure‐fugacity that characterized a point—the most intense point—in an assemblage's geological history is another premise. Rarely does either premise prove entirely true, but kinetic factors, if understood, could assist in their interpretation.

Copper, lead, zinc and cobalt mineralization in the English Lake District: classification, conditions of formation and genesis

The copper, lead, zinc, and minor cobalt mineralization of the English Lake District has been classified on the basis of mineralogy, spatial distribution and geological setting of the veins, and age of mineralization. The two major types of mineralization are: (1) ‘chalcopyrite-pyrite-arsenopyrite’, of Lower Devonian (Caledonian) age, and (2) ‘galena sphalerite’ of early Carboniferous age. When the former is closely associated with granite intrusions, more complex mineral assemblages are found involving tungsten or molybdenum bearing phases. Also distinguished from these two major types are later assemblages which probably result from their alteration. A later (Upper Carboniferous–Permian) baryte mineralization is also present. Minor graphite and stibnite deposits, and the major haematite mineralization found in the area, are incorporated into the classification but are not discussed, since they have been dealt with by other authors. Using data from fluid inclusions and experimental data on particular mineral assemblages occurring in the major types of mineralization, estimates have been made of the temperatures and compositions of the ore-forming fluids. The Lower Devonian veins formed at temperatures in the range of 350–200°C with typical a S 2 , values of 10 −10 –l0 −16 atm and a O 2 of ~10 −25 –10 −45 atm or even less. The solutions were probably brines of low to moderate salinity (~5–10 equiv. wt% NaCl). The Lower Carboniferous veins formed at temperatures in the region of 110–130°C from highly saline brines (~23 equiv. wt% NaCl). Theories regarding the origins of the mineralization are also discussed. The distribution of Lower Devonian veins shows a clear relationship to the underlying composite granite batholith; the Lower Carboniferous galena-sphalerite veins show no such relationship, but tensional tectonic activity in the early Carboniferous may have provided a mechanism for the opening of fissures. The source of the mineralizing fluids forming the Lower Devonian veins may have been the Borrowdale Volcanic Group, with contributions from fluids associated with the granites, which are also likely to have provided a source of heat. The high salinities of the fluids producing the Lower Carboniferous veins suggest that seawater may have been involved in leaching the metals from basement rocks.

Hydrothermal behaviour of simulated waste glass—And waste—Rock interactions under repository conditions

Hydrothermal experiments have been performed investigating granodiorite-water, waste glass-water and granodiorite-waste glass-water systems in the temperature range 100–350°C at pressures of 50 or 60 MPa. Experimental equipment consisted of closed-system large-volume gas-pressurised reaction vessels. Run durations lasted from less than one day to one hundred days at fluid/solid mass ratios between 0.4 and 6. Fluids generated in the granodiorite-water experiments were alkaline (pH = 7.4–8.9) with low total dissolved solids (< 500 ppm) and low chloride (< 40 ppm) and sulphate (< 30 ppm) contents. Silica concentrations in solution approximate to theoretical values in the quartz-water system at 100° and 150°C, although 200°C fluids show evidence of re-equilibration during the quenching process. From available thermodynamic data, the chemistries of the 100° and 150°C fluids appear to be governed by feldspar-water equilibria, whilst at 200°C, feldspar-montmorillonite reactions dominate the fluids. Montmorillonite is an identified secondary phase at 200°C. From these preliminary data, the “evolved groundwaters” produced through reaction of granodiorite and water at high temperatures seem suitable for canister preservation, actinide immobilisation and compatibility with bentonite as a backfill material. Two sodium borosilicate glasses containing simulated waste components have been used in a kinetic study of dissolution at 100° and 150°C, 60 MPa. The dissolution of both glasses is governed by parabolic kinetics with run-lengths up to 14 days, and this is interpreted as being controlled by diffusion through a surface layer. This surface layer develops through incongruent dissolution of the glass and is enhanced by saturation of major components in solution and precipitation of secondary-mineral forms. The crystallinity of this alteration layer is increased by increasing time and temperature of the reaction. SEM and XRD analysis have indicated that at 100° and 150°C the layer is largely amorphous with traces of a poorly crystalline dioctahedral smectite. At 200°C the layer consists of fairly well crystallised smectite and at 350°C degradation of the glass is rapid and the solid reaction products are composed of a complex mineral assemblage including smectite, a lithium-sodium borosilicate hydrate, aegirine, riebeckite, albite, stillwellite, zektzerite and two barium molybdates. Some of these mineral phases incorporate and concentrate waste components, e.g., the zektzerite may contain up to 20 wt.% zirconium oxide, 2.5 wt.% caesium oxide and 0.7 wt.% uranium oxide. The rates of release of certain radionuclides have been investigated in granodiorite-waste glass-water systems. This study has revealed that the rate of release of a particular nuclide is a function of temperature and time. Caesium leach rates obtained in this manner are at least an order of magnitude lower than leach rates derived from refluxing dynamic leach tests. This is because incongruent dissolution of the glass, saturation of major components in solution and precipitation of mineral phases incorporating waste components in these closed-system experiments all serve to lower rates of radionuclide release. The work was carried out by N.E.R.C.-I.G.S. under contract to the U.K. Department of the Environment and the Commission of the European Communities.

Zeolite Diagenesis Below Pahute Mesa, Nevada Test Site

AbstractThe Tertiary silicic volcanic rocks in the Silent Canyon Caldera beneath Pahute Mesa, of the Department of Energy's Nevada Test Site have been divided into three vertical mineralogical zones that vary in thickness and transgress stratigraphic boundaries. Zonal contacts are generally sharp. Zone 1, the uppermost zone, includes unaltered or incipiently altered rhyolitic glass. Zone 2 is characterized by a predominance of clinoptilolite and subordinate amounts of smectite, cristobalite, and mordenite. Zone 3 is a complex mineral assemblage that includes analcime, quartz, calcite, authigenic K-feldspar and albite, kaolinite, chlorite, and mixed-layer illite/smectite. The mixed-layer clay shows an increase in ordering and a decrease in expandability with depth.Shortly after deposition and after shallow burial, hydration of relatively impermeable, highly porous vitric rocks resulted in the rapid formation of the Zone 2 assemblage (except mordenite). This stage of alteration resulted in a net porosity loss and negligible mass transfer. Continued burial and rise in temperature led to a dehydration stage in which the Zone 2 assemblage was replaced by the Zone 3 minerals. The dehydration stage resulted in a porosity increase and an increase in permeability of several orders of magnitude. This process, like the earlier reactions, also conserved mass. Precipitation of mordenite followed the formation of this zonal configuration. The diagenetic zones below Pahute Mesa were caused by: (1) changing pore-water chemistry in an essentially closed hydrologic system; (2) disequilibrium or kinetic precipitation of metastable phases; and (3) a higher thermal gradient than that now present.

Hibonite [Ca 2(Al, Ti) 24O 38] from the Leoville and Allende chondritic meteorites

Hibonite [Ca 2(Al, Ti) 24O 38] was discovered in light-colored, Ca Al Ti-rich and Si Fe-poor, achondritic inclusions of the Leoville and Allende HL-group chondrites. Two varieties of hibonite occur: One emits a bright red-orange luminescence under electron bombardment and has high amounts of Al 2O 3 (87.7; 87.9) and low amounts of MgO (0.65; 0.8) and TiO 2 (0.68; 0.8). The other emits a bright blue luminescence and is low in Al 2O 3 (78.7; 79.2) and high in MgO (3.3; 3.7) and TiO 2 (6.5; 7.9) (in wt. %). The oxide CaO is about the same in both varieties. It is suggested that the change in the color of the visible luminescence results from changes in composition. The origin of hibonite which occurs in complex mineral assemblages together with anorthite, gelhenite, wollastonite, aluminous diopside, andradite, Ca-pyroxene, perovskite, spinel, taenite, chromite, and pentlandite, and in close proximity to nodules containing calcite, whewellite, forsterite and many of the aforementioned phases, is discussed. The proposition that hibonite and associated phases originated by contact metamorphism and metasomatism of calcite-dolomite bearing assemblages cannot, at this time, be completely ruled out.

Carbonates: Association with Organic Matter in Surface Seawater

Carbonate mineral particles suspended in surface seawater in tropical and subtropical areas do not interact measurably with the water. Identical complex mineral assemblages occur in waters in various states of saturation. Drastic alterations of temperature and pH in the laboratory do not affect the composition of the suspended carbonates. It appears that the mineral grains are protected from the water by resistant organic coatings.

CONTACT METAMORPHISM OF MAGNESIAN LIMESTONES AT CRESTMORE, CALIFORNIA

The contact-metamorphic rocks at Crestmore, California, occur between magnesian marbles and a plutonic mass of quartz diorite (Bonsall tonalite) and between the same marbles and a relatively small pipelike hypabyssal mass of quartz monzonite porphyry. The marbles are preserved as two crudely lenticular bodies about 400 and 500 feet thick, respectively, that occur as screens in the quartz dioritic intrusive rocks of the southern California batholith. Both bodies, which are very similar petrographically, are composed of alternating layers of predazzite and coarsely crystalline calcite marbles and are nearly free of silica, alumina, iron, and the alkalies. The quartz diorite that engulfed the marbles is contaminated locally very near the contacts where minor monzonitic and gabbroic variants are present. Its exomorphic effects on the carbonate rocks consist mainly of: (1) the formation of metasomatic silicate contact rocks that generally are less than 1 foot thick and are composed of diopside, wollastonite, and grossularite; and (2) the conversion of nearly all the magnesium-bearing carbonate beds to periclase marbles, which subsequently were altered to predazzites. In contrast, the younger quartz monzonite porphyry, which was injected into the upper or Sky Blue marble unit, is nearly all contaminated as a result of reactive assimilation of marble. Moreover, its exomorphic silicate aureole is as much as 50 feet thick and contains the numerous complex mineral assemblages for which Crestmore is famous. Several lines of evidence indicate that much of this aureole was formed prior to the final consolidation of the porphyry intrusive mass. The thicker parts of the silicate contact aureole that surrounds the quartz monzonite porphyry exhibit the following well-defined zonal distribution of mineral assemblages, as traced outward from the intrusive body: (1) a garnet zone that is composed of grossularite and lesser amounts of wollastonite and diopside; (2) a zone characterized by only one mineral, idocrase; and (3) a zone in which monticellite is the most abundant mineral, but in which there are various amounts of clinohumite, cuspidine, ellestadite, forsterite, melilite, merwinite, perovskite, spinel, spurrite, tilleyite, and xanthophyllite. This zonation of mineral assemblages reflects a corresponding zonation in the bulk chemical composition of the rocks, as shown by changes in the ratio of metasomatic to indigenous constituents (Si + Al + Fe/Ca + Mg) from 0.66 in the monticellite zone, through 1.15 in the idocrase zone, to 1.62 in the garnet zone. There also is a strong tendency toward a mineralogical zonation within the monticellite zone, such that clinohumite, forsterite, spurrite, and spinel are concentrated in the silica-poor and calcite-rich outer part, and merwinite, cuspidine, and melilite in the more silica-rich inner part. Hence, a sequential occurrence of mineral assemblages and a change in bulk chemical composition can be traced inward toward the intrusive mass from un-metasomatized marbles. Moreover, textural features reveal a corresponding paragenetic sequence in which the more highly metasomatized assemblages appear to have formed at the expense of those that were less metasomatized. Available evidence indicates that: (1) the contact-metamorphic mineral assemblages at Crestmore and their zonation are largely the compositionally controlled products of silica, alumina, and iron metasomatism of relatively pure magnesian limestones; (2) temperatures of 625°C. or higher were reached prior to the introduction of silica into the present monticellite-zone rocks; and (3) the so-called “high-temperature” assemblages, such as monticellite, spurrite, and melilite, formed directly from the magnesian marbles without the intervention of “lower-temperature” steps that involve diopside, wollastonite, and grossularite. Therefore, it is proposed that contact metamorphism at Crestmore should be viewed as progressive metasomatism with consequent decarbonation at elevated temperatures rather than as progressive decarbonation attendant simply upon rising temperature.