Accessory Quartz Research Articles

Analytical Characterization of an Ottoman Opus Sectile Mosaic Floor from the Delta Region, Egypt

This study reports a recent analytical study of the main components of an opus sectile mosaic floor from an Ottoman bath (hammam) in the delta region of Egypt, that dates back to 1748 CE. Samples of the decorative elements of the mosaic floor including white stone, pottery mosaic pieces, black stone and setting bed mortar from the mosaic substrate were collected and analyzed. The microscopic features of the samples were examined by an optical USB digital microscope and their petrographic characteristics were evaluated by a polarized light microscope. The morphological and chemical composition of the samples were determined by a scanning electron microscope provided with an X-ray microanalyzer (SEM-EDX). The crystalline phases contained in the samples were measured by X-ray powder diffraction analysis (XRD). The results have shown that the mosaic floor was constructed using a white marble which consists of multi-sized calcite crystals with minor amounts of coesite. The pottery mosaic pieces are composed of quartz, hematite and albite. The black stone was identified as a volcanic rock of basalt which consists of andesine, anorthite and accessory quartz. A traditional lime mortar composed of quartz and calcite was used as a setting bed mortar for the mosaic pieces. Furthermore, the results showed the detection of some salt minerals which reflects the salt weathering that occurs in the site. The obtained results provided useful information on the technical structure of the studied mosaic and will be used to select appropriate materials for its restoration.

Geochemistry and mineralogy of the Jebel Aja Igneous Intrusion and the associated exotic pegmatites, Arabian Shield, Saudi Arabia

We describe the Jebel Aja igneous intrusion (JAII), which sits at the farthest northeastern edge of the exposed Arabian Shield in Saudi Arabia. The JAII consists of late Neoproterozoic post-collisional A-type granites, and was emplaced during tectonic extension. It is a composite pluton consisting of a granitic core (monzogranite, syenogranite and alkali feldspar granite) surrounded by a later phase of alkaline and peralkaline granites. Pegmatites occur as pockets and dykes at the contact between the alkaline/peralkaline granites and syenogranite. The pegmatites have sharp contacts with their host granites and show variable colors (principally pink and white), grain sizes, textures, and mineral compositions. We report here that the white pegmatites host pockets with a unique mineralogy that includes the first observation of pyroxmangite, thortveitite and thalénite-(Y) in the whole Arabian Shield as well as a novel (Sc, Y, vacancy)-rich pyroxenoid and a vacancy-rich bustamite with excess octahedral Si. The pyroxmangite-dominated lithology also features accessory quartz, albite, spessartine, pyrochroite, muscovite, and fluorite. The presence of pyroxmangite rather than rhodonite, together with vacancy-rich and Si-excess pyroxenes, suggests an elevated pressure of emplacement. The granites of the JAII are highly evolved and have undergone extreme fractional crystallization of feldspars, leading to increases in the concentration of volatile and incompatible elements in the residual melt, which resulted eventually in injection of coeval pegmatites into the mostly solidified host granite. The syenogranite and alkaline/peralkaline granite of the JAII contain sodic amphibole and have geochemical characteristics common among within-plate rare-metal bearing A-type rocks. They have positive Nb-Ta anomalies that increase from syenogranite to alkaline/peralkaline granite. The pink pegmatite is highly mineralized and contains high concentrations of Nb (1540–1769 μg/g), Ta (103–136 μg/g), Y (1116–1616 μg/g), Zr (6362–9707 μg/g), Hf (215–264 μg/g), Th (278–384 μg/g), U (110–147 μg/g), and ∑REE (2334–3251 μg/g).

Geochemistry of eclogite xenoliths from the Udachnaya Kimberlite Pipe: Section of ancient oceanic crust sampled

A suite of seventeen unique, large, and fresh eclogite xenoliths from the Udachnaya pipe have been studied for their whole-rock and mineral major- and trace-element compositions. Based on their major-element compositions, the Udachnaya eclogites can be subdivided in two groups: high magnesian (Mg# 68.8–81.9) and low magnesian (Mg# 56.8–59). The two eclogite groups are clearly different in the style of correlation between major elements. Positive correlations of FeO and CaO with MgO are observed in the low-magnesian group, whereas these correlations are negative in the high-magnesian group. In terms of trace element composition, the Udachnaya eclogites are enriched over Primitive Mantle, but comparable to mid-ocean-ridge basalt composition, except for significant enrichment in large-ion lithophile elements (LILE; Rb, Ba, K, Sr). Most of the samples show a positive Eu anomaly, irrespective of group. Reconstructed whole-rock composition from clinopyroxene and garnet modal abundances contains much less incompatible elements (LILE, light rare earth elements, high field strength elements) than measured composition. Approximately 60 to 100% of the middle rare earth elements, Zr, and Hf, and nearly 100% of the heavy rare earth elements, Co, V, and Sc of the whole-rock budget are concentrated in Gar and Cpx. Variations in major element compositions cover a full section of the modern and Archaean oceanic crust, from troctolite, through gabbroic rocks, to basalts. The low-Mg# eclogites could have formed from upper oceanic crust protoliths, being a mixture of basalts and gabbro, whereas the high-Mg# eclogites are originated from gabbro-troctolite section of the lower oceanic crust. Concordant variations of Eu anomaly with the Lu/Sr ratio and the V and Ni contents in the eclogite compositions are in agreement with the fractionation of plagioclase, clinopyroxene, and olivine in their low-pressure precursor rocks. Negative correlations of SiO2 and MgO, and a low Nd/YbNMORB ratio, in the low-Mg# eclogites are in agreement with partial melt loss, but the presence of accessory quartz limits the degree of melting to 13%. Major and trace element compositions suggest that the high-Mg# eclogites, and, consequently, the lower oceanic crust, could not have experienced significant melt loss, and subduction in the Archaean may have been essentially dry, compared to the present day.

The hydrothermal alteration of carbonatite in the Fen Complex, Norway: mineralogy, geochemistry, and implications for rare-earth element resource formation

ABSTRACTThe Fen Complex in Norway consists of a ~583 Ma composite carbonatite-ijolite-pyroxenite diatreme intrusion. Locally, high grades (up to 1.6 wt.% total REE) of rare-earth elements (REE) are found in a hydrothermally altered, hematite-rich carbonatite known as rødbergite. The progressive transformation of primary igneous carbonatite to rødbergite was studied here using scanning electron microscopy and inductively coupled plasma-mass spectrometry trace-element analysis of 23 bulk samples taken along a key geological transect. A primary mineral assemblage of calcite, dolomite, apatite, pyrite, magnetite and columbite with accessory quartz, baryte, pyrochlore, fluorite and REE fluorocarbonates was found to have transformed progressively into a secondary assemblage of dolomite, Fe-dolomite, baryte, Ba-bearing phlogopite, hematite with accessory apatite, calcite, monazite-(Ce) and quartz. Textural evidence is presented for REE fluorocarbonates and apatite breaking down in igneous carbonatite, and monazite-(Ce) precipitating in rødbergite. The importance of micro-veins, interpreted as feeder fractures, containing secondary monazite and allanite, is highlighted. Textural evidence for included relics of primary apatite-rich carbonatite are also presented. These acted as a trap for monazite-(Ce) precipitation, a mechanism predicted by physical-chemical experiments. The transformation of carbonatite to rødbergite is accompanied by a 10-fold increase in REE concentrations. The highest light REE (LREE) concentrations are found in transitional vein-rich rødbergite, whereas the highest heavy REE (HREE) and Th concentrations are found within the rødbergites, suggesting partial decoupling of LREE and HREE due to the lower stability of HREE complexes in the aqueous hydrothermal fluid. The hydrothermal fluid involved in the formation of rødbergite was oxidizing and had probably interacted with country-rock gneisses. An ore deposit model for the REE-rich rødbergites is presented here which will better inform exploration strategies in the complex, and has implications for carbonatite-hosted REE resources around the world.

Geochemical and Sm–Nd isotopic constraints on the petrogenesis and tectonic setting of the Proterozoic mafic magmatism of the Gwalior Basin, central India: the influence of Large Igneous Provinces on Proterozoic crustal evolution

Abstract Palaeoproterozoic mafic magma intruded the Gwalior Group of sediments in the form of gabbroic sills. Huge sills extending more than 120 m in depths are exposed in quarries. They are coarse to medium grained, massive and have chilled margins. They predominantly consist of plagioclase feldspar, pyroxene with accessory quartz, iron oxides and apatite. Ophitic to sub-ophitic, porphyritic and intergrowth textures are common. These are tholeiitic, sub-alkaline magma types which have been generated by varying degrees of partial melting and have experienced fractionation of dominantly pyroxene + olivine ± plagioclase. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and exhibit negative Nb and P anomalies and positive Pb anomalies. A mineral–whole-rock Sm–Nd isochron corresponds to an age of 2104 ± 23 Ma with an initial 143 Nd/ 144 Nd = 0.509938 ± 0.000023 and ϵNd t = −0.9 ( t = 2000). Model ages (2.6–1.7 Ga) indicate that their mantle sources had a protracted evolution. Mantle source melting may have been triggered by mantle plume in an extensional intra-continental rift tectonic setting, facilitating emplacement of a Large Igneous Province (LIP) in the northern Indian Shield. Thus, Gwalior mafic magmatism represented by the studied sills is the manifestation of one such LIP in the northern Indian Shield. These magmatic activities are contemporaneous with the widespread mafic magmatic activities concomitant with the development of supra-crustal basins on Archaean cratons worldwide and the Indian Shield in particular, where upwelling plumes triggered large-scale crustal extension, breaking-up of supercontinents and emplacement of LIPs.

Ultrapotassic dykes in the Moldanubian Zone and their significance for understanding of the post-collisional mantle dynamics during Variscan orogeny in the Bohemian Massif

We report mineral textures, geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the Western part of the Moldanubian Zone at contact to the Teplá-Barrandian Block. The dykes have orientation almost perpendicular to the NNE–SSW trending Central Bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites. Based on mineral compositions and geochemistry, the dykes show close relations to Mg-K syenite plutons in the Moldanubian Zone. The two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine-grained matrix that consists of K-feldspar, phlogopite with small amounts of clinopyroxene and accessory quartz, apatite, titanite and sulphides of Fe, Cu, Ni. The syenite porphyry dyke cross cuts the vaugnerite. It contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs.The vaugnerite and syenite porphyry have high K2O (6 to 7wt.%) and mg-number (0.66–0.74), but low CaO and Na2O contents. The vaugnerite is markedly rich in P2O5 (>2wt.%) and comparing to syenite porphyry has higher amount of mantle-incompatible elements (e.g. Rb, Cs, Ba, Pb, Th, U), V and Cr. ID-TIMS analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338.59±0.68 and 337.87±0.21Ma, respectively. Mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent. Formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post-collisional mantle dynamics initiated by slab break-off and asthenosphere upwelling into the Moldanubian accretionary complex during the Variscan Orogeny.

English

Geological mapping of the Gulani area revealed that the area consists of Precambrian Basement Complex rocks, Cretaceous sediments and the Tertiary/Quaternary basalts of the Biu plateau. The Basement Complex rocks are represented by diorite and granites while the sediments include the Bima, Yolde, and Pindiga Formations. Epigenetic baryte-copper mineralizations occur with the baryte veins restricted to the Bima and Yolde sandstones while the Copper mineralizations occur in granites. Petrographic study indicates that the diorite is largely composed of andesine plus hornblende and accessory quartz and iron oxide. The granites, on the other hand, consist essentially of quartz, microcline and orthoclase with minor biotite and iron oxide. The Bima and Yolde sandstones are composed of quartz, microcline, plagioclase and accessory biotite and opaque crystals. The large quartz and microcline crystals in both granites and sandstones are fractured. The Kanawa member of the Pindiga Formation is made up of shales interbedded with Limestones. The limestone interbeds are both crystalline and fossiliferous types. The basalts consist of labradorite laths, olivine, augite and opaques. Transmitted light petrography of the hydrothermally altered Bima and Yolde sandstones show the presence of quartz, baryte and microcline. The barite occurs as pore space in-filling mineral between the quartz and microcline and within the fractures of these brittle minerals. The Cu (chalcopyrite/malachite) mineralizations occur within highly fractured zones of the granites. Reflected light microscopy indicates that only relicts of the primary chalcopyrite exist while the veins/veinlets are now almost entirely occupied by the secondary mineral of malachite and its associated limonite which are the alteration products of the chalcopyrite. Key words: Petrography, fractures, baryte-copper, mineralization, Gulani, Nigeria.

G. Groppelli and L. Viereck-Goette (eds.): Stratigraphy and geology of volcanic areas. The Geological Society of America Special Paper, 2010, 464 pages, ISBN 978-0-8137-2464-5 (pbk)

This 291-page long volume consists of a collection of 14 articles that are an outcome of a scientific session “devoted to the methodology in field mapping of volcanic areas” held at the 32nd International Geological Congress (IGC Florence, 2004). It also includes an accompanying CD-ROM not available to the reviewer. Field-work based “hard-rock geologists” mapping in volcanic areas are invariably faced with the problem of choosing how to establish map units and their contacts and then display them. In the early 1970s of the last century, when I was a young geologist myself, enrolled in mineral exploration and geological mapping in the Neogene volcanic areas in the East Carpathians (Romania), the major issue was to establish effective rock map divisions in a petrographically quite monotonous volcanic field, so that amphibole–pyroxene andesites were distinguished from pyroxene–amphibole andesites, and hypersthene-dominant andesites from augitedominant ones within the larger class of pyroxene andesites. Moreover, rocks containing opacitized and/or resorbed amphiboles were further distinguished, and the presence of accessory quartz crystals was a sufficient feature to map “quartz andesites”, even though the quartz was xenocrystic in origin. Textural criteria (e.g., separating “aphanitic” rocks from the “normal” porphyritic ones) completed the scheme. As whole-rock chemical analyses became accessible, petrochemical criteria were introduced to supplement the petrographical ones. The resulting maps, looking like mosaics of variously colored patches representing “petrotypes” of volcanic rocks, were frustratingly irrelevant from a volcanological point of view; volcanic structures and edifices were hardly seen in such maps. Later, in the 1980s, we introduced genetic criteria among the mapping tools, so that belonging to a certain volcanic structure and the genetic type of the volcanic products were both considered higher-order map entities than those based on precise petrographic features of the mappable rock units. The maps became more volcanologically relevant. Interestingly, the possible Cover used with permission; © 2010 Geological Society of America.

Petrographic investigation of melt and matrix relationships in Chicxulub crater Yaxcopoil-1 brecciated melt rock and melt rock-bearing suevite (846–885 m, units 4 and 5)

The drill core from the International Continental Drill Program’s Chicxulub Scientific Drilling Project Yaxcopoil-1 (Yax-1) borehole, in the annular trough of the Chicxulub crater, exhibits from 794 to 895m a continuous sequence of impactites consisting of reworked fallout, fallout suevite, and brecciated impact melt rock. These impact breccias exhibit a complex history of deposition, fracturing, matrix emplacement and hydrothermal alteration. Detailed investigation of the mineralogy and chemistry of these breccias has led to a better understanding of the complex events involved in their formation. We find that the paragenesis of the brecciated impact melt rock (unit 5, 861–885m) involved fracturing of melt rock and early K-metasomatism during a hydrothermal alteration episode as suggested by earlier work. However, the present work has identified the role of multiple episodes of precipitation of Mg-rich phyllosilicates and formation and dissolution of accessory minerals in a relatively high temperature (>300°C) hydrothermal event. The earliest matrix formation event involved precipitation of Mg-rich phyllosilicate, accessory quartz, calcite, apatite, and andradite garnet from a hydrothermal fluid with a brine or seawater component. The fluid could have partly incorporated elements and shock-metamorphosed mineral phases derived from sedimentary lithologies, including calcite and dolomite that underwent complex phase transformations such as melting, decomposition and possible back-reactions. The discovery of andradite garnet in the matrix confirms the presence of an early high temperature hydrothermal event previously identified by mineralogical, stable isotope and fluid inclusion studies. The mineral assemblage, including Mg-rich saponite, suggests the involvement of seawater by comparison with similar alteration assemblages in hydrothermal systems involving seawater around the world. The presence of a later, low-temperature phase of the hydrothermal system, with different fluid chemistry is indicated by the partial dissolution of andradite garnet, and continued precipitation of matrix phyllosilicate minerals, but without the accessory quartz, abundant calcite or andradite, and without an accompanying K-metasomatism event.

Supercritical carbon dioxide and sulfur in the Madison Limestone: A natural analog in southwest Wyoming for geologic carbon–sulfur co-sequestration

The Madison Limestone on the Moxa Arch, southwest Wyoming, USA contains large volumes (65–95%) of supercritical CO 2 that it has stored naturally for 50 million years. This reservoir also contains supercritical H 2S, aqueous sulfur complexes (SO 4 2− and HS −), and sulfur-bearing minerals (anhydrite and pyrite). Although SO 2 is not present, these sulfur-bearing phases are known products of SO 2 disproportionation in other water–rock systems. The natural co-occurrence of SO 4 2−, S 2−, supercritical CO 2 and brine affords the opportunity to evaluate the fate of a carbon–sulfur co-sequestration scenario. Mineralogic data was obtained from drill core and aqueous geochemical data from wells outside and within the current supercritical CO 2–sulfur–brine–rock system. In addition to dolomite, calcite, and accessory sulfur-bearing minerals, the Madison Limestone contains accessory quartz and the aluminum-bearing minerals feldspar, illite, and analcime. Dawsonite (NaAlCO 3(OH) 2), predicted as an important carbon sink in sequestration modeling studies, is not present. After confirming equilibrium conditions for the Madison Limestone system, reaction path models were constructed with initial conditions based on data from outside the reservoir. Addition of supercritical CO 2 to the Madison Limestone was simulated and the results compared to data from inside the reservoir. The model accurately predicts the observed mineralogy and captures the fundamental changes expected in a Madison Limestone-brine system into which CO 2 is added. pH decreases from 5.7 to 4.5 at 90 °C and to 4.0 at 110 °C, as expected from dissolution of supercritical CO 2, creation of carbonic acid, and buffering by the carbonate rock. The calculated redox potential increases by 0.1 V at 90 °C and 0.15 V at 110 °C due to equilibrium among CO 2, anhydrite, and pyrite. Final calculated Eh and pH match conditions for the co-existing sulfur phases present in produced waters and core from within the reservoir. Total dissolved solids increase with reaction progress, mostly due to dissolution of calcite with an accompanying increase in dissolved bicarbonate. The Madison Limestone is a natural example of the thermodynamic end point that similar fluid–rock systems will develop following emplacement of a supercritical CO 2–sulfur mixture and is a natural analog for geologic carbon–sulfur co-sequestration.

The petrology and geochemistry of Miller Range 05035: A new lunar gabbroic meteorite

Miller Range (MIL) 05035 is a lunar gabbroic meteorite. The mineralogy, Fe/Mn ratios in olivine and pyroxene, bulk-rock chemical composition and the bulk oxygen isotope values (δ 17O = 2.86–2.97‰ and δ 18O = 5.47–5.71‰) are similar to those of other mare basalts, and are taken as supporting evidence for a lunar origin for this meteorite. The sample is dominated by pyroxene grains (54–61% by area mode of thin section) along with large plagioclase feldspar (25–36% by mode) and accessory quartz, ilmenite, spinel, apatite and troilite. The bulk-rock major element composition of MIL 05035 indicates that the sample has a very low-Ti (VLT) to low-Ti lunar heritage (we measure bulk TiO 2 to be 0.9 Wt.%) and has low bulk incompatible trace element (ITE) concentrations, akin to samples from the VLT mare basalt suite. To account for these geochemical characteristics we hypothesize that MIL 05035’s parental melt was derived from a mantle region dominated by early cumulates of the magma ocean (comprised principally of olivine and orthopyroxene). MIL 05035 is likely launch paired with the Asuka-881757 and Yamato-793169 basaltic lunar meteorites and the basaltic regolith breccia MET 01210. This group of meteorites (Y/A/M/M) therefore may be a part of a stratigraphic column consisting of an upper regolith environment underlain by a coarsening downwards basalt lava flow.

An Overview of the ULU Gold Deposit, High Lake Volcanic Belt, Nunavut, Canada

Abstract BHP Minerals discovered the Archean lode gold ULU deposit on the western margin of the High Lake Volcanic Belt in 1989. The greenschist to amphibolite facies mafic volcanic and sedimentary rocks on the property are folded into a 5 km-long anticline. There is a close spatial association of Au-As zones to the trace of this F2 ULU anticline. The largest of these auriferous zones, the Flood zone, is localized at the core of the fold. The southeast-trending Flood zone consists of several anastomosing lenses that have been traced for 435 m on surface. Overall, it dips steeply (70° to 80°) to the southwest and has been intersected to depths below 600 m. The hosting high-iron tholeitic basalt displays a lower amphibolite mineral assemblage of ferrohornblende + plagioclase + ilmenite with accessory quartz and epidote. Alteration minerals include biotite, chlorite, hornblende, actinolite-tremolite, and potassium feldspar (microcline) with minor calcite, epidote, tourmaline, and titanite. The highest gold values occur where brecciated basaltic wall-rock clasts are replaced by acicular arsenopyrite + quartz + K-feldspar. BHP Minerals completed 54,783 m of both exploration and resource development drilling before selling the ULU property to Echo Bay Mines in 1995. Development of ULU as a satellite deposit to Echo Bay’s Lupin mine began in 1996. A -15% ramp was extended to the 155-meter level, and 16,011 m of underground drilling were completed before the decline in gold prices forced a suspension of activities at ULU in August 1997. The current owners, Wolfden Resources Inc., drilled 18,569 m in 2004 to increase confidence in the resource blocks. Following this drill program, a revised resource calculated by an independent consultant determined that the ULU deposit contains an inferred/indicated mineral resource of 1,130,000 t grading 11.34 g/t gold (373,748 oz) to the 360-m level. The portal was re-opened in 2005 and a prefeasibility study is underway.

Petrology and Petrogenesis of Syenites from the Cuddapah Basin, Andhra Pradesh

Abstract Rare bodies of syenites of alkaline or subalkaline character occur as intrusions in the rocks of the Cuddapah Supergroup within the Cuddapah basin. The alkali syenites (found at Racherla, Idamakallu and Giddalur) are mainly composed of riebeckite (arfvedsonite), ilmenite, orthoclase and the subalkaline syenites (located at Gundlapalle) comprise hedenbergite, and microcline-perthite. Accessory quartz is invariably present in both the syenites. These intrusive bodies display contrasting mineralogical and geochemical characteristics. The quartz- and hypersthenenormative subalkaline syenite has the attributes of a ferrosyenite and has been subjected to hydrothermal alteration which not only resulted in the formation of secondary minerals like nontronite, amphibole, biotite and magnetite- all at the expense of hedenbcrgite, but also partly converted the Fe+2 into Fe +3. These subalkaline syenites are characterized by an extreme paucity of Mg, has low to moderate contents of Ba, Sr, Rb, Zr and REE. The quartz, hypersthene and acmite- normative alkali syenites (lusitanite) on the other hand are abnormally enriched in Ti; Rb is low but Ba, Sr, Ce, Nb, Nd, Ta, Sm, La, Zr am also the ratios K/Rb, Ti/Zr, Ce/Y and HREE/LREE are conspicuously high. The syenites of the Cuddapah basin, seem to have crystallized under contrasting conditions. While the alkali syenites crystallized under high PH2O, the crystallization of subalkaline syenite took place under dry arid reducing conditions signifying a closed system. It is envisaged that the subalkaline ferrosyenite of Gundlapalle was formed by low-pressure crystal fractionation of a continental tholeiitic basaltic magma. The alkali syenites, on the other hand, are postulated to have crystallized from a mantle-derived, hydrous, alkaline magma enriched in Large Ion Lithophile Elements and Light Rare Earth Elements.

Tonsteins and Clay-Rich Layers in Coal-Bearing Intervals of the Eocene Gibbons Creek Member, Manning Formation, East-Central Texas: ABSTRACT

Samples from five clay-rich layers collected from the coal-bearing interval in the upper part of the Eocene Gibbons Creek member of the Manning Formation were mounted in epoxy, polished, and analyzed by scanning electron microscopy and energy-dispersive x-ray fluorescence to determine their origin. Two layers were from surface-mine exposures of the 3500 bed near Bryan, Texas, and the other layers were from an exposure of a correlative interval at the Lake Somerville Spillway about 60 km southwest of Bryan. Preliminary data suggest that both a 2-cm-thick claystone from the upper part of the 3500 bed and the upper part of an 11-cm-thick mudstone from the floor of the lower coal bed at the spillway may be derived from volcanic ash falls. Both possible tonsteins are composed of kaolinite and accessory quartz, euhedral to subhedral zircon, feldspars, and Al phosphates (crandallite?). Only K-feldspars were observed in the parting from the 3500 bed, whereas both alkali and plagioclase feldspars, as well as high-Ti biotites, were observed in the sample from the spillway. These compositional differences are suggestive of two separate volcanic sources. Because waterborne, recycled volcanic minerals could have been brought into the paleopeat swamps, we must examine the associated minerals frommore » these sites to verify ash-fall origins. The other layers contain textures that are more suggestive of detrital rather than ash-fall origin; rounded to subrounded zircons and feldspars are present as accessory minerals.« less

Significance of epidote in orbicular diorite from the Grenville Front zone, eastern Labrador

AbstractOrbicules in diorite from the Grenville Front zone of eastern Labrador are defined by shell structures alternately enriched and depleted in biotite, epidote and magnetite. Hornblende occurs locally in orbicule cores and the matrix, but not in the shells. The shells enclose plagioclase-rich (An40–45), leucodioritic cores containing biotite, epidote, magnetite and/or hornblende-bearing mafic clots. The matrix of the orbicules is mineralogically-similar to the orbicule cores, but is mesocratic, and contains relatively sodic plagioclase and accessory quartz and K-feldspar. In places, hornblende contains quartz oikocrysts, implying the resorption of early-formed clinopyroxene, and is rimmed by biotite and epidote. The latter phases also occur as inclusions in quartz-free hornblende interpreted to have crystallized directly from the magma. Epidote has a pistacite content of 21 to 26 and occurs as (1) tiny, idiomorphic crystals (‘epidote I’) enclosed by plagioclase or hornblende, and (2) relatively large (to 1 mm) grains with vermicular textures (‘epidote II’), particularly where in mutual contact with biotite (or hornblende) and plagioclase. These microstructures suggest that epidote is a magmatic phase which formed by direct crystallization from the magma, and by reaction of previously-formed minerals with the magma.The following approximate paragenetic sequence has been inferred for orbicule cores and the matrix: clinopyroxene (clinopyroxene resorbed [→ poikilitic hornblende]), epidote I, Ca-Na plagioclase, biotite, hornblende (biotite and/or hornblende ± plagioclase resorbed [→ epidote II]), quartz + K-feldspar. Biotite compositions became progressively more Fe-rich during crystallization (XMg ⋍ 0.6 → 0.4), and the first-formed plagioclase (inclusions in quartz-free hornblende in orbicule cores) is more calcic (An51) than the last (matrix grains: An35).The appearance of epidote early in the crystallization history of the diorite testifies to elevated PH2O and PTotal (PT). The most aluminous hornblende indicates maximum PT of 5 to 6 kbar. Orbicule shell structures are interpreted to have crystallized from supercooled boundary layers enclosing water-saturated globules within the dioritic magma. Although sufficient to suppress the formation of hornblende in the shell structures, the extent of magma supercooling did not permit the development of comb layering in the orbicules. Supercooling is attributed to an influx into the magma of water from an unidentified source.

Proposed geotechnical classification of carbonate rocks based on Portuguese and Algerian examples

Carbonate rocks are common and widely dispersed materials. They include pure limestones and dolostones as well as a large variety of composites differentiated by the relative occurrence of accessory quartz, clay minerals and other components. Geotechnical properties reflect this wide variation of composition and a classification scheme would be helpful for consistent communication. References and data obtained for this study substantiated the proposed geotechnical classification based on porosity and swelling strain of the rocks. The relevance of these parameters for the explanation of the geotechnical behaviour of carbonate rocks as well as their simplicity render them very suitable for this purpose. The field behaviour of some Portuguese rocks is employed to demonstrate correlation with the proposed classification.

Sulfide and platinum mineralization in the Merensky Reef: evidence from hydrous silicates and fluid inclusions

The base metal sulfides of the Merensky Reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates. Biotite and phlogopite crystallized in the vicinity of sulfides from a volatile-enriched highly fractionated intercumulus melt. Amphibole, chlorite, and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase. Biotite is often accompanied by zircon, rutile, and quartz. Accessory quartz hosts a complex suite of H2O-NaCl-(CaCl2)-CO2-CH4 fluid inclusions which have thus far not been described from the Merensky Reef. The earliest fluid inclusion compositions are NaCl-(H2O) with less than 10 vol.% water; CO2 coexisting with a halite daughter crystal and brine; and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations. The maximum trapping temperature is around 730° C at 4 to 5 kb pressure. Later inclusion generations are H2O-NaCl, CO2-H2O, and pure CO2 and CH4. The presence of Cl-rich fluids during the intercumulus stage of the crystallizing Merensky Reef is directly related to the mode of sulfide precipitation. Prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as H2S. Sulfur saturation causes reaction of H2S with oxides of the silicate melt to form a sulfide melt plus water. During reaction the magma is enriched in water until a separate fluid unmixes. It carries all compounds with high fluid/melt partition coefficients, as well as metals capable of forming OH- and Cl-complexes. Precious metals are assumed to have fractionated into the Cl-rich fluid as Cl-complexes rather than being dissolved in the sulfide melt. During the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets. The model proposed explains the distribution pattern of platinum-group minerals in the Merensky Reef better than any orthomagmatic mineralization concept offered so far.

ANORTHOSITE-ADAMELLITE COMPLEX OF NAIN, LABRADOR

The anorthosite-adamellite complex has invaded basement-complex gneisses which have a low Na 2 O/K 2 O ratio and show many features of the granulite facies. Ultramafic rocks of the Alpine type occur. Assignment of the metamorphic rocks to the Grenville Province is at least open to question. The anorthositic rocks are subdivided into three facies. A dark facies is characterized by dark plagioclase and olivine. A pale facies is characterized by pale plagioclase and a low content of hypersthene with thin exsolution lamellae. A buff-weathering facies is characterized by antiperthitic inclusions in plagioclase, more abundant pyroxenes with exsolution lamellae indicating inversion from pigeonite, and accessory quartz. There is an imperfect progression from the dark facies in the north and east through a central area of pale facies to the buff-weathering facies in the south and west. Adamellite margins are variable in character near anorthosite and in places show clearly intrusive relations. A short distance within the margin a fayalite facies appears. In it plagioclase and pyroxenes resemble those minerals in buff-weathering anorthosite. It is greenish gray when fresh but rusts readily. The fayalite facies is succeeded westward by a hornblende facies in which much of the hornblende is poikilitic. There is also a biotite facies with intrusive relations toward the fayalite facies. Possibly it is more abundant toward the west margin of the complex. Small intrusive bodies occur, in many cases in gneiss zones involved with the anorthosite-adamellite complex. Many of them show affinities with the adamellitic rocks. Some olivine gabbroic rocks may be related to the anorthosite. Variations in composition of rocks and minerals of the complex could result from repeated intrusion of magmatic material evolving through differentiation by partial crystallization, with the zone of intrusion migrating westward. Faulting occurred in the region, but field data are insufficient for evaluation.

The paragenesis of kyanite-eclogites

In Haüy's original definition of eclogite, the presence of kyanite as an accessory constituent was recognized, and it is apparent that he was familiar with examples from the Sau Alps and Styria. The best known of eclogites—those of the Bavarian Fichtelgebirge (e.g. Silberbach, Weissenstein, and Eppenreuth)—contain significant kyanite interspersed among the omphacite grains or associated with accessory quartz and muscovite in the rocks. Of these rocks rather extended descriptions have been published. Analyses of the minerals and the rocks themselves belong to an early date and do not reach the standard of modern analyses. Later, Drill (1902) published a systematic account of these Bavarian eclogites and has added an analysis of an eelogite with kyanite from Unterpferdt near Silberbach and an analysis of a garnet from a Silberbach eclogite.