Abundant Magnetite Research Articles

Mineralogy of carbonaceous chondrite clasts in HED achondrites and the Moon

Abstract The majority of the carbonaceous chondrite clasts found in howardites, eucrites and diogenites are CM2 material, a lesser proportion is CR2 material, and other rare types are present. A single clast that was found on the Moon and called the Bench Crater meteorite is apparently shocked CM1 material. The CM2 clasts are matrix supported mixtures of olivine‐pyroxene‐phyllosilicate‐sulfide bearing aggregates, loose olivines and pyroxenes, sulfides, carbonates, and sinuous spinel‐phyllosilicate‐diopside calcium‐aluminum‐rich inclusions (CAIs). Magnetite and metal are rare. Some aggregates have fine‐grained rims of material resembling matrix. The opaque, fine‐grained matrix consists predominantly of serpentine of extremely variable composition and sulfides; tochilinite is occasionally present. The trace element data for one Jodzie clast from this study and the average of similar clasts from Kapoeta support a CM classification; volatiles are depleted relative to CI and enriched relative to CR material.The CR2 clasts are found (in small numbers) in only four howardites: Bholghati, Jodzie, Kapoeta and Y793497. Petrographically, they are matrix‐supported mixtures of olivine aggregates (sometimes containing sulfides), loose olivines, pyrrhotite, pentlandite, low‐Ca pyroxene (minor), hedenbergite (rare), kamacite (rare and only found within olivine), Ca‐carbonates and abundant magnetite framboids and plaquets. Phyllosilicates are fine‐grained and largely confined to matrix; they are mixtures of serpentine and saponite. The matrix of CR2 clasts also contains pyrrhotite, pentlandite, chromite and a significant fraction of poorly‐crystalline material with the same bulk composition as matrix phyllosilicate.There is evidence of heating in a substantial number of clasts, both CM2 and CR2, including: (1) corrugated serpentine flakes, (2) pseudomorphs of anhydrous ferromagnesian material after flaky phyllosilicates, and (3) hedenbergite rims on calcite. While the timing of the hedenbergite rims is debatable, the destruction of phyllosilicates clearly occurred at a late stage, plausibly during impact onto the HED asteroid(s) and Moon, and required peak heating temperatures on the order of 400 °C.We note that in general, CM2 material was the most common carbonaceous chondrite lithology impacting the HED asteroids (with howardites and eucrites taken together), as it is for the Earth today. A total of 61 out of 75 carbonaceous chondrite clasts from HED meteorites belong to the CM clan, petrologic grade 2. This is also supported by published siderophile and volatile element data on howardites, eucrites and diogenites that are taken to indicate that CM‐like materials were the most common impactors on the HED asteroid(s). The ratio of CR/CM clasts in HED asteroids is essentially the same as for modern falls at Earth. This may indicate that the ratio of disaggregated CM2 to CR2 asteroidal material has been approximately constant through the history of the solar system. Finally, our results are also compatible with type‐2 carbonaceous chondrites being equivalent to or from the same source as the material that originally accreted to form the HED asteroid.

Early magnetite-amphibole-plagioclase alteration-mineralization in the Island copper porphyry copper-gold-molybdenum deposit, British Columbia

The initial hydrothermal event in the evolution of the ca. 377 Mt, 0.41 percent Cu, Island Copper porphyry Cu-Au-Mo deposit, northern Vancouver Island, was the formation of an extensive alteration-mineralization facies dominated by magnetite, calcic amphibole, and intermediate to sodic plagioclase. Early-stage quasipervasive magnetite-rich alteration and associated magnetite-rich veinlets in this Middle Jurassic, island arc-hosted, hydrothermal system developed in both an axial dacitic porphyry dike and contiguous basaltic flows and pyroclastic strata. The magnetite-rich alteration zone, 500 to 700 m wide and originally extending over at least 450 m vertically, exhibits an outward zonation from quartz-magnetite-albite (An (sub < or =6) )-amphibole (trace) - apatite (trace), through quartz-magnetite-amphibole-albite (An (sub 6-10) ) or oligoclase-andesine (An (sub 15-36) )-apatite (trace) + or - scapolite (trace), to amphibole-magnetite-oligoclase or sodic andesinc (An (sub 15-39) ) + or - quartz + or - apatite (trace) assemblages. Definition of early-stage mineral assemblages, impeded by unusually intense later intermediate argillic alteration and more local phyllic and advanced argillic alteration, was facilitated by the use of novel petrographic techniques, including incident-light Nomarski differential interference contrast microscopy. Early-stage mineralization comprises several well-defined vein-veinlet types which were demonstrably emplaced prior to the main-stage chalcopyrite-pyrite stockwork and associated K silicate alteration. Minor chalco-pyrite and pyrite occur in some early-stage veins, and magnetite is a constituent of many later main-stage sulfide-rich veins, but the distinction between these two major vein systems is clear.Mass-exchange calculations demonstrate that early alteration involved intense iron metasomatism of both felsic and mafic country rocks, with an Fe enrichment approaching 450 percent (20 g/100 cc) in the dacite dike, and lesser and more variable Na enrichment, with both Ti and Al behaving as mobile constituents. Paragenetically early fluid inclusions in the quartz phenocrysts of the dike are tentatively correlated with the magnetite-rich alteration-mineralization and imply that this event was initiated above 650 degrees C and at pressures of at least 1 kbar and was generated by FeCl 2 -rich brines, at first single phase and moderately saline (avg 15 wt % NaCl equiv), but latterly boiling at ca. 560 degrees to 645 degrees C and < or = ca. 0.55 kbars. The main-stage potassic alteration zone has the configuration of an annulus, ca. 100 to 150 m in width, which was entirely superimposed on the more extensive early alteration-mineralization zone at ca. 430 degrees to 575 degrees C and below 450 bars. Gold, although correlated overall with Cu and potassic alteration, was probably extensively introduced in the early stage.It is inferred that the scarcity of sulfides in the early alteration assemblages reflects the highly oxidized nature of the initial fluids, in which SO (super -) 2 exceeded both H 2 S and SO (super 2-) 4 . The deposition of abundant magnetite and the widespread and intense Fe metasomatism which define the early-stage record the high solubility of Fe as FeCl 2 in high-temperature brines in equilibrium with magnetite-bearing quartzofeldspathic rocks, as well as the retrograde solubility of magnetite between ca. 550 degrees and 750 degrees C. Whereas the high SO 2 /H 2 O + SO 4 ratios of the initial hydrothermal fluids at Island Copper, and in other similar deposits, may have resulted from the high inherent oxygen fugacity of the parental dacitic magmas, they may have directly reflected the composition of supercritical fluids expelled on quenching of underplating, S-rich, mafic melts. The mineral assemblages of the early-stage veins and associated quasi-pervasive alteration cannot be assigned to the accepted alteration-mineralization facies of porphyry systems, such as A veins and potassic alteration, and record specific and distinct conditions of fluid-rock interaction.

Differentiation of Ferro-Basaltic Magmas under Conditions Open and Closed to Oxygen: Implications for the Skaergaard Intrusion and Other Natural Systems

Because processes such as fractional crystallization and crystallization under conditions closed to oxygen are difficult to simulate in the laboratory there is a need for quantitative models of magma crystallization behaviour. Comparison of experimental data on an iron-rich basaltic composition with predictions of the MELTS free energy minimization algorithm shows that although liquidus temperatures and silicate mineral equilibria are predicted relatively well, the saturation of Fe-Ti oxides is not We have used the same experimental data to construct an alternative crystallization model based on known equilibrium phase relations, mineral-melt partitioning of major elements, and mass balance constraints. The model is used to explore the consequences of equilibrium and fractional crystallization in systems open and closed to oxygen. Liquid lines of descent for perfect equilibrium and perfect fractional crystallization are predicted to be very similar. In a system open to oxygen the model predicts that magnetite saturation leads to strongly decreasing iron and increasing silica contents of residual liquids, whereas systems closed to oxygen crystallize less abundant magnetite, leading to a less pronounced iron depletion in the liquid. Predicted bulk solid compositions and variations offo, with falling temperature agree well with those observed or inferred from the cumulates of the Skaergaard intrusion, but none of the predicted liquid lines of descent are consistent with the extreme iron enrichment proposed for this intrusion based on mass balance calculations. Compositional factors such as water and phosphorus are not thought to be the source of the discrepancy as the cumulates of the Basistoppen sill (which closely resemble those of Skaergaard) may be used to calculate a liquid line of descent in agreement with that predicted by the model for fractional crystallization closed to oxygen. A comparison of the predicted T-io, paths and liquid lines of descent with those inferred from natural systems suggests that volcanic centres such as Iceland and Hawaii evolve under conditions open to oxygen, whereas evidence from plutonic environments (e.g. Skaergaard and Kiglapait layered intrusions) suggests that they evolved under conditions more closed to oxygen. The compositional evolution of the melt phase in volcanic and plutonic systems may therefore be different, although the results of this study suggest that magnetite saturation will limit Fe enrichment in all environments to <20wt% FeO*, consistent with enrichments reported for volcanic glasses.

Sulphur sources for epithermal and mesothermal veins in Cretaceous–Tertiary magmatic-arc rocks, Livingston Island, South Shetland Islands

Epithermal veins in early Triassic turbidites on Hurd Peninsula are isotopically homogeneous over an area of 13 km × 2 km (mean δ 34 S = +2.2‰, 1 σ = 1.7, n = 65) suggesting derivation from a deep circulating, neutral-chloride hydrothermal plume containing magmatic sulphur. The sense of fractionation (pyrite = +3.5, arsenopyrite = +3.2, sphalerite = +2.9, chalcopyrite = +2.5, galena = +0.9‰) and isotopic temperatures between 250 and 345°C suggest partial isotopic equilibration. Sulphides in massive dark carbonate/peperitic dyke breccias, with abundant magnetite, hematite and titanite and traces of barite, garnet, zircon and monazite are isotopically heavy (+7 to +14.9‰, n = 12) due either to dyke intrusion into the hydrothermal system resulting in degassing of H 2 S, or to a second, isotopically heavy and sulphate-bearing solution in fault zones along which dykes were subsequently intruded. Vein sulphides in nearby Cretaceous volcanic rocks are similar to the epithermal system (mean = + 1.8‰, 1 σ = 0.9, n = 6, pyrite = +2.0, chalcopyrite = +1.5), suggesting sulphur-derivation by degassing of sub-volcanic magma, or remobilization of disseminated sulphides during plutonism. Sulphide in an Eocene tonalite pluton is slightly 32 S-enriched (mean = 0‰, 1 σ = 1.9, n =7, molybdenite = +0.7, pyrite = +1.5, chalcopyrite = –2.6) and was exsolved directly from the cooling tonalitic magma.

Aqueous alteration and brecciation in Bells, an unusual, saponite-bearing, CM chondrite

The petrological and mineralogical characteristics of the unusual CM2 chondrite, Bells, have been investigated in detail by scanning electron microscopy (SEM), electron microprobe analysis (EPMA), and transmission electron microscopy (TEM). Bells is a highly brecciated chondrite which contains few intact chondrules, a very low abundance of refractory inclusions, and is notable in having an unusually high abundance of magnetite, which is disseminated throughout the fine-grained matrix. Fragmental olivines and pyroxenes are common and, based on compositional data, appear to have been derived from chondrules as a result of extensive brecciation. The fine-grained mineralogy of matrix in Bells differs considerably from other CM chondrites and has closer affinities to matrix in CI chondrites. The dominant phases are fine-grained saponite interlayered with serpentine, and phases such as tochilinite and cronstedtite, which are typical of CM chondrite matrices, are entirely absent. Pentlandite, pyrrhotite, magnetite, anhydrite, calcite, and rare Ti-oxides also occur as accessory phases.Based on its oxygen and noble gas isotopic compositions (Zadnik, 1985; Rowe et al., 1994), Bells can be considered to be a CM2 chondrite, although its bulk composition shows some departures from the typical range exhibited by this group. However, these variations in bulk chemistry are entirely consistent with the observed mineralogy of Bells. The unusual fine-grained mineralogy of Bells matrix can be reasonably attributed to the combined effects of aqueous alteration and advanced brecciation in a parent body environment. Extensive brecciation has assisted aqueous alteration by reducing chondrules and mineral grains into progressively smaller grains with high surface areas, which are more susceptible to dissolution reactions involving aqueous fluids. This has resulted in the preferential dissolution of Fe-rich chondrule olivines, which are now completely absent in Bells although present in other CM chondrites. The formation of saponite in Bells probably resulted from the dissolution of relatively silica-rich phases, such as pyroxene and olivine, that were derived from chondrules. The result of such dissolution reactions would be to increase the activity of silica in the fluid phase, at least on a localized scale, stabilizing saponite in preference to serpentine. An increase in aSiO2 would also have destabilized preexisting cronstedtite which may have reacted to form magnetite and Mg-Fe serpentine under conditions of constant ƒO2 .

Oxygen isotopes in separated components of CI and CM meteorites

Oxygen isotopic compositions have been determined for whole-rock and separated components from the CI chondrites Alais, Ivuna, and Orgueil, and from the atypical CM chondrites Bells and Essebi. The composition of CI matrix is in agreement with previous measurements and is systematically different from that of CM matrix, probably due to warmer and wetter conditions of aqueous alteration for CI matrix. Although Bells matrix is like that of other CM chondrites, the matrix of Essebi falls in the CI field. Essebi also contains abundant magnetite, like CIs and unlike other CMs. The isotopic compositions of magnetite in Alais, Ivuna, Orgueil and Essebi define a trend with Δ 17O of +1.3 to +1.8, distinctly out of isotopic equilibrium with the phyllosilicates in each meteorite. The magnetite serves as an isotopic tracer for the fluid which brought about aqueous alteration and oxidation, and thus provides additional evidence for a gaseous reservoir in the solar nebula which was more 16O-depleted than the earth.

The compositional classification of chondrites: VI. The CR carbonaceous chondrite group

New analytical data combined with recent studies by other researchers allow the definition of a Renazzo (CR) group of carbonaceous chondrites. We analyzed nine CR chondrites (Acfer 187, Acfer 209, El Djouf 001, Elephant Moraine 87747, Elephant Moraine 87770, Elephant Moraine 87847, MacAlpine Hills 87320, PCA91082, and Yamato 793495) constituting at least five independent fall events by instrumental neutron activation analysis for twenty-seven elements. Along with previously analyzed Renazzo, six or more closely related fall events are represented. Key CR properties include refractory lithophile abundances ~1.0 × CI levels, Zn/Mn ratios ~0.3 × CI levels, metal contents of 100–160 mg/g, (unusually high for a carbonaceous chondrite group), relatively large chondrules (mean size ~ 700 μm), and the presence of magnetite framboids. Al Rais is a close relative but too different in chemical and isotopic composition to be considered a normal member of the CR group; we suggest that it be treated as an anomalous member (CR-an), but that its properties not be included in CR ranges and means. MAC87320, PCA91082, EET87770, and Acfer 187 were studied petrographically along with Renazzo and Al Rais. Renazzo has a lineation possibly caused by fluid-lubricated, impact-induced shearing. The CR chondrites experienced some reduction during weak thermal metamorphism; the heating must have taken place prior to hydrothermal alteration. We suggest that formation of magnetite (and framboidal magnetite in particular) in CI and CR chondrites is due to hydrothermal alteration of metal-rich (or opaque-rich) precursors, and that the low abundance of magnetite in CM chondrites relative to CI indicates that the CM precursors were metal poor. Some carbonates in CI and CR chondrites may have formed by H 2O reaction with cohenite or poorly crystallized graphite.

Manganese carbonate mineralization in the Molango District, Mexico

The Molango manganese deposit in Mexico is by far the largest known Mn deposit in North America. Mn carbonate mineralization is hosted by a finely laminated Upper Jurassic marine sedimentary sequence that forms the base of the Chipoco facies of the Taman Formation (Kimmeridgian). Manganese enrichment occurs over a total stratigraphic thickness of approximately 50 m, and a length of over 50 km along strike and serves as the protore for supergene Mn oxide ore. The carbonate ore zone comprises the basal 1 to 10 m of the Chipoco facies.The carbonate ore bed consists of fine-grained rhodochrosite and dispersed organic matter, magnetite, and maghemite but generally only trace quantities of pyrite. Fine laminations and clotted textures suggest deposition in a restricted marine environment. Manganese content is highest at the base of the mineralized interval, drops abruptly to subore grade, Mn-rich carbonates, and then gradually decreases upward to background levels (i.e., trace Mn in calcite). Chemical and mineralogical studies indicate that the predominant mineralogy changes upward from rhodochrosite in the ore zone, to mixed carbonates (Mn calcite, kutnohorite, + or - rhodochrosite), to Mn calcite.Several geologic and geochemical processes resulted in the formation of Mn carbonate by the early diagenetic reduction of Mn oxides through the oxidation of organic matter and iron sulfide. Mn oxides were concentrated at the margins of a dysaerobic to mildly anoxic stratified basin and became an important oxidizing agent of organic matter in the sediment pile. Effects of organic matter oxidation reactions included production of manganous ion (Mn (super +2) ), MnCO 3 precipitation, oxidation of iron monosulfide (precursor to pyrite), and formation of Fe oxide. The latter two processes explain the anomalously low pyrite and abundant magnetite content in the ore zone. The observed vertical variation of Mn carbonate mineralogy and the mineral compositions reflect variations in the concentrations of reduced manganese in the pore waters. Thus, Mn carbonate precipitated from pore water and did not form as a chemical sediment in the water column. The ultimate source of the manganese is uncertain but may have been fluvial-sediment loads or hydrothermal activity associated with the rifting of the Gulf of Mexico.

Maralinga, a metamorphosed carbonaceous chondrite found in Australia

Abstract— The Maralinga meteorite was found near the village of Maralinga, South Australia (30°18′S, 131°16′E) in 1974, but was not recognized as a meteorite until 1989. One weathered individual was recovered with a total mass of 3.38 kg. The bulk composition and petrography of Maralinga indicate that it is a metamorphosed (petrographic type 4) carbonaceous chondrite with major similarities to the Vigarano‐subtype. However, recent trace element data from the literature suggest that Maralinga should be included with the CK (Karoonda‐type) carbonaceous chondrites. We classify Maralinga as an anomalous CK4 chondrite because of its abundant chondrules and refractory inclusions relative to other known members of the CK group. Maralinga contains homogeneous silicates, including Ni‐bearing olivines (Fa34), high‐Ca clinopyroxene, and rare orthopyroxene. Plagioclase is chemically heterogeneous and falls into two distinct compositional groups one of ∼ An20 and an ∼ An80 group. Highly oxidizing conditions during metamorphism of Maralinga are indicated by the abundance of magnetite and the paucity of Fe‐Ni metal.

Magnetic susceptibility and relation to initial 87Sr/86Sr for granitoids of the central Sierra Nevada, California

Measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the Mariposa 1° by 2° quadrangle, which crosses the central part of the Sierra Nevada batholith between 37° and 38°N latitude, shows that magnetic susceptibility values are above 10−2 SI units in the east and central parts of the batholith and drop abruptly to less than 10−3 SI units in the western foothills. In a narrow transitional zone, intermediate values (10−3 to 10−2) prevail. Magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values. Magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite, which depends, in turn, on the total iron content of the rocks and their oxidation ratio. Lower magnetic susceptibilities of felsic members of Sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content, but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks. The relatively unaltered condition of the samples and restriction of δ18O to the range of +7 to +10.3‰ indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios. Correlations of magnetic susceptibility with initial 87Sr/86Sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized. Reduction of Fe3+ to Fe2+ by organic carbon or other reducing substances may also have affected magnetic susceptibility.

Experimental and major element constraints on the evolution of lavas from Lihir Island, Papua New Guinea

The major element chemistry of SiO2-undersaturated arc lavas from Lihir Island, Papua New Guinea, and 1 atmosphere experiments on an alkali basalt from this island show complex polybaric fractionation affected this suite of lavas. Low Ni and MgO are typical of these arc lavas and result from olivine fractionation, probably at high pressure. Fractionation at low pressure (<5 kb) produces two evolutionary trends. Separation of clinopyroxene, plagioclase and minor olivine from the primitive lavas results in increasing normative nepheline contents and major element trends similar to those of the experiments. In contrast, addition of magnetite and amphibole to the fractionating assemblage in the evolved lavas results in decreasing normative nepheline and major element trends which are markedly different from those of the experiments. The composition of experimental glasses and 1 atmosphere liquid lines of descent, derived from anhydrous melting experiments run at the fayalite-magnetite-quartz (FMQ) buffer and at higher oxygen fugacities, are displaced from the lavas on oxide-oxide plots. HighfO2 produces high Fe3+/Fe2+ and the early crystallization of abundant magnetite, and high H2O contents are responsible for crystallization of amphibole. Crystal fractionation of these phases and the high Fe3+/Fe2+ are responsible for the displacement of the lavas and experimental glasses in mineral projection schemes from the 1 atmosphere olivine-clinopyroxene-plagioclase saturation boundary of Sack et al. (1987).

Tetrataenite in carbonaceous chondrites.

Presence of tetrataenite grains has been recently confirmed in Yamato 791717 carbonaceous (CO3) chondrite. The observed coexistence of tetrataenite fine grains with more abundant magnetite and kamacite in this CO3 chondrite suggests that the tetrataenite grains were directly formed by a coalescence process of Fe and Ni fine particle smokes in the solar nebula.

Source of Anomalous Magnetization in Area of Hydrocarbon Potential: Petrologic Evidence from Jurassic Preuss Sandstone, Wyoming-Idaho Thrust Belt

The Jurassic Preuss Sandstone, which crops out in the central part of the Wyoming-Idaho thrust belt on trend with a hydrocarbon-producing region to the south, has been previously identified as the source of anomalous magnetization in the area. Elsewhere, anomalous magnetization in sedimentary rocks near hydrocarbon accumulations has been attributed to hydrocarbon-engendered magnetic minerals, but magnetization of the Preuss is controlled by detrital magnetite. Evidence of a detrital origin for magnetite includes (1) concentration of magnetite grains along laminations containing other heavy minerals, (2) the presence of exsolved ilmenite, hematite, and spinel in the magnetite grains, and (3) titanium contents typical of igneous-derived magnetite. That detrital magnetite is responsible for the anomalous magnetization in the Preuss is further indicated by the systematic eastward decrease in magnetite abundance corresponding to a similar eastward decrease in magnetic susceptibility and remanent magnetization of the unit. Petrologic and vitrinite reflectance studies indicate a complex low-temperature (<150°C or 302°F) diagenetic history for the Preuss. Nevertheless, preservation of detrital magnetite, the presence of diagenetically early ferric oxide minerals, and the absence of sulfide minerals all indicate that the Preuss has not experienced sulfidic-reducing conditions common in areas of hydrocarbon seepage. The marine carbon isotopic composition of calcite that cements most Preuss sandstones (^dgr13C values ranging from -2.47 to 1.48^pmil is evidence that carbonate diagenesis also was not influenced by hydrocarbons. The results of this multidisciplinary study of the Preuss underscore the importance of similar studies when evaluating the sources of aeromagnetic anomalies in areas of hydrocarbon potential.

Late Paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east Tennessee: Evidence for a post‐ore chemical event

Thermal demagnetization studies on samples from mineralized and unmineralized rocks in the Lower Ordovician Knox Group of east Tennessee reveal a mean direction of D/I = 157°/+18° (α95 = 7.7°, k = 52.9) after 60% unfolding, combining stable endpoint and remagnetization circle data. Although not significant at the 95% confidence level, the fold test is suggestive of a synfolding (Alleghenian) age of the magnetization. The magnetization cannot be postfolding, i.e., Permian or younger, because the in situ directions do not resemble those for post‐Carboniferous times. The magnetization is unlikely to be much older than the folding because the directions with respect to bedding yield a paleopole characteristic of the Early Permian, but unlike poles for pre‐Late Carboniferous times. Characteristic magnetizations from mineralized and unmineralized collapse breccias are in good agreement and fail the conglomerate test. The best estimate for the paleopole resulting from this study falls at 40°N, 126°E, within the Late Carboniferous segment of the North American Paleozoic apparent polar wander path. Thus the magnetization is interpreted as synfolding and secondary due to widespread Kiaman remagnetization. Blocking temperatures of up to 520°C and the acquisition of isothermal remanent magnetization suggest magnetite as the carrier of the magnetization. Scanning electron microscopy and energy dispersive analysis of magnetic extracts from limestone samples reveal abundant magnetite spheres. Secondary dolomites have the same magnetic characteristics as the primary limestones, and it is inferred that similar magnetite spheres are the carriers of their magnetization. Because this magnetization, and therefore its carrier, occurs in sphalerite and secondary dolomite, it must be coeval with or younger than the ore, which itself is thought to be of middle to late Paleozoic age. It is argued that the magnetization was acquired during growth of magnetite, associated with a late Paleozoic chemical event.

Successive zoning of amphiboles during progressive oxidation in the Daito-Yokota granitic complex, San-in belt, southwest Japan

Clinoamphibole present in shallow intrusive masses of quartz diorite in the Daito-Yokota area of the San-in belt, southwest Japan, is strongly zoned successively from a magnesio-homblende core to an actinolite rim, systematically decreasing in Fe 2+ (Fe 2+ + Mg) towards the rim. The features of compositional variation and cation substitution are similar to those of amphiboles which have been altered by fluid separated during secondary boiling in porphyry copper-bearing granitoids. Mineralogie data and textural relations of amphibole and the accompanying minerals indicate that the zoned amphibole has crystallized or recrystallized from a subsolidus fluid phase under progressively oxidizing conditions. Truly magmatic amphibole is restricted to small patches in the core of strongly zoned amphibole grains. Progressive oxidation is responsible for reequilibration of early-formed Fe-Ti oxide to yield extensive development of secondary magnetite and sphene. The decreasing Fe trend of amphibole has resulted from progressive Fe depletion of the fluid phase by abundant magnetite precipitation. The amphibole grains usually show an alternation of chemically distinct zones with unconformable boundaries, indicating repeated growth and corrosion by multiple circulation of fluids.

The Mount Grace Carbonatite; an Nb and light rare earth element-enriched marble of probable pyroclastic origin in the Shuswap Complex, southeastern British Columbia

A thin carbonatite marble unit, referred to here as the Mount Grace carbonatite, has been traced discontinuously for approximately 25 km along the northwestern margin of Frenchman Cap dome, southeastern British Columbia, Canada. It forms essentially one conformable layer that averages 3 m in thickness. It contains abundant apatite, phlogopite, and sodic amphibole and contains high concentrations of Mn, Sr, Ba, Nb, and the light rare earth elements, La, Ce, and Nd. Based on detailed stratigraphy and structural setting, the Mount Grace carbonatite is correlative and assumed to be continuous, with a similar carbonatite in the Perry River area, 10 to 15 km to the south. Here, nepheline syenite gneiss and discontinuous carbonatite lenses intrude the stratigraphic succession.The Mount Grace carbonatite is interpreted to be a pyroclastic carbonatite that was deposited on shallow marine tidal flats. Evidence includes its great lateral extent, its distinctly layered nature, its occurrence at a specific stratigraphic horizon, and its lack of contact alteration. It is distinguished mineralogically and chemically from the Perry River intrusive carbonatites, which are discontinuous lenses, have fenitized margins, and contain abundant magnetite, ilmenite, and columbite, minerals which are rare or absent in the Mount Grace carbonatite. The intrusive carbonatites are also distinguished by a higher average concentration of Nb but lower average concentrations of the light rare earth elements.Although the analyzed Mount Grace carbonatite samples are generally low grade, with total Nb, La, Ce, and Nd ranging from approximately 0.04 to 0.8 percent, the large lateral extent makes it a tremendous potential source for these elements.

Macrocryst Fe - Ti oxides in olivine melilitites from Namaqualand-Bushmanland South Africa

Ilmenite macrocrysts in olivine melilitites from Namaqualand-Bushmanland, South Africa, have decomposed by subsolidus reduction to form oriented Mg-titanomagnetite along {0001} ilmenite planes. Residual ilmenite contains 10–11 wt% MgO, ∼1 wt% MnO, and ∼0.1 wt% Cr2O3. This macrocryst assemblage is mantled by an annulus of Mg-titanomagnetite, followed by an overgrowth of radiating magnetite + perovskite. Terminal compositions of these magnetites are similar to groundmass spinels, and to the outermost margins of magnetite macrocrysts that have very high Fe3+ core contents. The assemblages are remarkably similar to oxide intergrowths in kimberlites and an upper mantle derivation is proposed for ilmenite macrocrysts in these melilitites. Oxidation states in the source regions are also very similar, whether on-or off-craton, being slightly above FMQ (∼NNO), but reduced to FMQWM with the onset of decompression, volatile loss, and carbonate immiscibility. In the case of the melilitites, late stage, low pressure crystallization above NNO precipated abundant magnetite + perovskite. The oxide fO2 data are consistent with, and refine the fO2 estimates obtained previously for the behavior of Fe/Mg and Ni contents in olivine from the same suite of samples.

Field relations and strontium isotope systematics of the marginal rocks of the eastern Bushveld Complex

The well-exposed basal contact of the eastern Bushveld Complex was mapped at scales of 1:10,000 and 1:20,000 along 130 km of strike and through a 9-km vertical section. Field relations, petrography, and geochemistry of the marginal rocks and associated sills have aided their subdivision into two main groups. A broadly pyroxenitic group rims the complex from its base to the top of the pyroxenitic subzone of the critical zone. A gabbroic group occurs above this level. Each group consists of a number of closely related rock types, most of which have sill analogs. Rocks of the first group (B1) include pyroxenites, norites, and peridotites, all of which may have quench textures. All contain orthopyroxene (>En 78 ), plagioclase (>An 65 ), and chromite, with subordinate clinopyroxene and biotite. Rare earth element patterns are mostly strongly fractionated (Ce N /Yb N approximately 10) and have 40X chondritic abundances at Ce. Their initial strontium isotope ratios at 2.05 b.y. fall in the range 0.7032 to 0.7057 and Rb/Sr ratios approximate 0.2. The gabbroic group consists of microcrystalline, two-pyroxene gabbros (B2N and B3N) and their coarser grained equivalents (B2 and B3) that border the upper critical and main zones, respectively. Mineral assemblages are dominated by plagioclase ( approximately An 55 ), clinopyroxene, and orthopyroxene ( approximately En 63 ), with abundant magnetite and traces of ilmenite and biotite. Their rare earth element patterns are less fractionated than B1 rocks. The B2N and B2 family have Ce N /Yb N approximately 6 and 30X chondritic abundances at Ce; the B3N and B3 rocks have Ce N /Yb N approximately 3, 6X chondritic abundances at Ce, and pronounced positive Eu anomalies. Strontium isotope initial ratios at 2.05 b.y. fall in the range 0.706 to 0.708 and the Rb/Sr ratios are approximately 0.02. Whole-rock and Sr isotope compositions of the peridotitc sills can be successfully simulated by a model involving olivine fractionation from a liquid with an initial Sr isotope ratio (R 0 ) similar to that of the Transition Zone (0.7056). A fraction of such liquid, enriched in cumulus olivine crystals, when mixed with up to 30 percent of a pyroxenitic liquid with low R 0 (0.7034), yields a product having the isotopic and compositional characteristics of the peridotite sills.A continuum exists in the initial Sr ratios through the pyroxenitic and gabbroic marginal rocks, suggesting that these represent mixtures of at least two sources with distinct initial Sr isotope ratio characteristics. It is argued that neither magma aging nor crustal contamination adequately accounts for the Sr isotope characteristics of the marginal rocks or their derived cumulates. The unusual chemistry of the pyroxenitic early liquids (SiO 2 approximately 55%, MgO approximately 13%, coupled with high concentrations of incompatible elements) should be considered primary.

The nature and origin of titaniferous magnetite-rich layers in the upper zone of the Bushveld Complex; a review and synthesis

The occurrence and geologic relationships of Ti magnetite layers indicate that their genesis is intimately related to fractional crystallization processes that were responsible for the formation of their silicate host rocks. The exact mechanism by which these oxide-rich layers form is, however, still debated. Most recent models recognize that precipitation of copious quantities of Ti magnetite is triggered by episodic increases in f (sub O 2 ) , but the process whereby this occurs is not well understood. The nature and occurrence of Ti magnetite-rich layers in the Bushveld Complex is reviewed and the requirements of any satisfactory genetic model are outlined. The formation and subsequent textural evolution of Ti magnetite layers is believed to have taken place as follows:1. Favorable conditions for the precipitation of large quantities of Ti magnetite were created by a lengthy period of fractional crystallization that resulted in the concentration of large amounts of Fe, Ti, and V in the residual magma.2. Large-scale in situ bottom crystallization of plagioclase resulted in a marked increase in the total Fe content and density of the immediately surrounding melt which collected as a dense layer on the bottom of the magma chamber. This dense Fe-Ti-(V)-enriched liquid did not mix with the overlying magma and formed a stagnant layer from which copious amounts of Ti magnetite may have crystallized.3. Crystallization of Ti magnetite is controlled by the Fe 2 O 3 /FeO ratio of the liquid and is a function of f (sub O 2 ) , temperature, and the f (sub H 2 O) /f (sub H 2 ) ratio. The Fe 2 O 3 content of the liquid will also increase during the crystallization of ilmenite and Fe (super +2) -bearing silicates (clinopy-roxene, pigeonitc, and fayalitic olivine). The H 2 O content of the residual liquid will also be increased by the fractionation of anhydrous silicate phases.4. A complex interplay of these factors resulted in the precipitation of relatively large amounts of Ti magnetite required for the development of ore-rich layers. The bulk of the Ti magnetite formed by in situ bottom crystallization. The nucleation and growth of magnetite within the stagnant layer and gravitative settling of these crystals over a short distance (on the order of meters) may have augmented the growth of the layers.5. Variations in the relative proportions of coexisting oxide and silicate phases may be related to differences in concentrations and diffusion rates of ions in the liquid immediately above the crystallizing layer. This resulted in differences in the rates of nucleation and growth of individual phases.6. Precipitation of abundant magnetite lowered the density of the stagnant layer until it reached that of the overlying magma. The two liquids then mixed, thus terminating the cycle of magnetite layer formation and marking a return to silicate-dominated fractionation.7. The ore-rich layers originally consisted of Ti magnetite crystals and variable amounts of silicate crystals which contained a certain amount of interstitial residual liquid. Densification of these layers and expulsion of significant amounts of this liquid were accomplished by annealing at high subsolidus temperatures, possibly augmented by the addition of suitable material from interprecipitate liquids.8. Variations in f (sub O 2 ) during subsolidus cooling resulted in the development of a wide range of ulvoespinel and ilmenite microintergrowths.

An oxygen and sulfur isotope study of skarn-type magnetite deposits of the Cornwall type, southeastern Pennsylvania

Contact metasomatic magnetite ores of the Cornwall type in Pennsylvania occur as replacements of carbonate rocks adjacent to sheets and dikes of Triassic diabase. In addition to large (10 8 tons) magnetite deposits at Cornwall and Morgantown (Grace mine), about 45 smaller occurrences are known in a zone 150 km long. Major gangue minerals include early diopside, phlogopite, and garnet followed by actinolite, ohiorite, talc, serpentine, pyrite, chalcopyrite, and pyrrhotite associated with abundant magnetite. Based on mineral assemblages, temperatures of deposition are inferred to be less than 500 degrees C for most of the magnetite.At Cornwall, 50 samples of magnetite have delta 18 O between 5.2 and 11.0 per mil (SMOW), with the highest values (mean (x) delta 18 O = 9.3ppm, standard deviation (Sigma ) = 0.66ppm, n = 19) adjacent to the diabase and the lowest values (x = 6.6ppm, Sigma = 0.69ppm, n = 14) near replaced marble. Thirty-five magnetite samples from 19 other Cornwall-type deposits fall within a similar range (x = 7.9ppm, Sigma = 1.97ppm). Silicates from the Cornwall deposit also show unusually high delta 18 O (13 actinolites, x = 12.4ppm, Sigma = 0.47ppm; 6 phlogopites, x = 12.3ppm, Sigma = 0.73ppm; 3 diopsides, x = 13.1ppm, Sigma = 0.19ppm). At Cornwall, 36 samples of pyrite and chalcopyrite have delta 34 S (CDT) of 6.3 to 19.9 per mil (x = 10.9ppm, Sigma = 2.91ppm).Calcites from the ore (delta 13 C = -5.9ppm (PDB), Sigma = 0.93ppm, delta 18 O = 17.5ppm, Sigma = 0.45ppm, n = 4) are distinctly depleted in both 18 O and 13 C relative to the unaltered host limestones (delta 13 C = -1.3ppm, Sigma = 0.44ppm, delta 18 O = 20.6ppm, Sigma = 0.68ppm, n = 15), which are similar to other Cambrian limestones in the literature. Marbles adjacent to ore are intermediate in 13 C and 18 O between carbonates in ore and unaltered limestones. Thermally metamorphosed marbles adjacent to diabase but away from ore are depleted only in 13 C relative to unaltered limestones.The magnetite, silicate, and carbonate minerals in the skarn-type ore in all 20 deposits are calculated to have formed in fluids with delta 18 O (sub H 2 O) of 13 to 16 per mil. The very high delta 18 O of the fluid is difficult to explain by conventional ideas on a magmatic origin for skarnforming fluids. The diabase, which has a normal igneous delta 18 O of 7.4 to 7.8 per mil, would have furnished fluids with delta 18 O of 7 to 10 per mil and therefore camlot be the direct source of the ore fluids.The preferred explanation of the 18 O-rich skarn is that an 18 O- and 34 S-enriched ore solution was formed by heating and circulation of meteoric, connate, or possibly magmatic waters in sedimentary rocks near the contact of the diabase. Exchange of 18 O with shales, argillaceous sandstones, or limestones at high temperatures created the 18 O-rich fluid. This fluid, upon encountering carbonate in the contact zone of the diabase, replaced it with magnetite and silicates. The relatively high Cu and Co contents of the Cornwall ore suggest that the ore fluid either contained a magmatic component or interacted with diabase to some extent. Zoning of 18 O away from the contact appears to result from decreasing temperature of deposition of the skarn farther from the contact.An alternative explanation is that the high delta 18 O values of magnetite and silicates were determined by the high delta 18 O values of the replaced carbonate in a diffusion-controlled process. In a diffusion-controlled process, the amount of water flowing through the replacement front might be negligible compared to the flux of Fe and other dissolved species. The presence of delta 18 O-rich alteration within the diabase and the lack of correlation of delta 18 O-rich of ore with delta 18 O of replaced carbonate indicate that this process is probably not the dominant one causing the high delta 18 O, but it might contribute to the range of values observed.