Quartz-biotite Schist Research Articles

Magma Mixing and Mingling for Xiangjiananshan Granitic batholith at eastern area of the East Kunlun Orogenic Belt

Magma Mixing and Mingling for Xiangjiananshan Granitic batholith at eastern area of the East Kunlun Orogenic Belt

Geological Features of Iron Formations and Associate Rock in Bulunkuole Group, West Kunlun, Xinjiang, China

West Kunlun is located at northwest part of the Tibetan plateau, and it’s divided into North Kunlun terrane, South Kunlun terrane and Tianshuihai terrane. Bulunkuole Group is located in Tianshuihai terrane, and Taaxi, Yelike, Laobing, Zankan, Mokaer and Jiertiekegou iron deposit was found in this terrane. Those iron deposits are set to the important iron ore mineralization belt of Xinjiang and even China. By investigation the geological features, ore distributions, and mineralization characters; and analyze the electron microprobe of magnetite and pyrite, stable isotope of pyrite, magnetite and anhydrite to discussion the tectonic setting and its genesis analysis iron ore deposit in Blunkuole Group, and establish the metallogenic model in this area. By contrastive analyzing of Taaxi, Yelike Zankan-Mokar iron ore deposits in Bulunkuole Group, it was found that all the iron ore deposit share the similar sedimentary environment. Those deposits have 3 ∼ 4 ore bodies, each get a meter to dozens of meters in width, extend to thousands of meters. The wall rocks are biotite quartz schist and plagioclase amphibole schist, and the two rocks interbed usually. It should be distinguish in the stratigraphic correlation of Bulunkuole Group for the influenced by the magma intrusion, such as granite and felsite porphyry.

S, O and Pb isotopic evidence on the origin of the İnkaya (Simav-Kütahya) Cu–Pb–Zn–(Ag) Prospect, NW Turkey

The İnkaya Cu–Pb–Zn–(Ag) prospect is a typical example of the hydrothermal mineralization occurring in the Menderes Massif, which crop out in Western Anatolia. The prospect located approximately 20km west of Simav (Kütahya-Turkey) in northern part of the Menderes Massif have been characterized through the detailed examinations involving geological, mineralogical, whole-rock geochemistry, fluid inclusion, stable isotope and lead isotope.The İnkaya Cu–Pb–Zn–(Ag) prospect is located along an E–W-trending fault in the Cambrian Simav Metamorphics, which consist of quartz–muscovite schist, quartz–biotite schist, muscovite schist, biotite schist and the Arıkayası Formation, which is composed of marbles. Galena, sphalerite, chalcopyrite, pyrite and fahlore are the main minerals, and they are accompanied by small amounts of cerussite, anglesite, digenite, enargite, chalcocite, covellite, bornite, and Fe-oxides with gangue quartz. In addition to Pb, Zn, Cu, Ag, the ore samples contain substantial quantities of As, Cd and Bi and small amount of Au. Average contents of Cu, Pb, Zn and Ag are 77,400ppm, 102,600ppm, 6843ppm and 203ppm, respectively.The δ34S values for galena, chalcopyrite and pyrite formed in the same stage vary in the range from −1.7 to −2.1‰ (average −2.0), 0.1 to 0.3‰ (average 0.2) and −1.5 to 2.6‰ (average +1.5), respectively.δ34S values for H2S, representing the composition of the fluids responsible for the sulfide mineral formations and calculated from the δ34S value are between −2.77 and 1.33‰; it is consistent with the sulfur in sulfide minerals. δ18Oquartz values range from 11.3 to 16.4‰ and estimated δ18Ofluid values range from 5.4 to 10.6‰.Pyrite–galena and pyrite–chalcopyrite pairs calculated to determine equilibrium isotope temperatures based on δ34S values are between 254.6 and 277.4°C for pyrite–galena and 274.7°C for pyrite–chalcopyrite. Sulfur and oxygen isotope values similar to the values for fluid equilibrated with an felsic magmatic source.Fluid inclusion studies on quartz of the same silicification stage coexisting with galena, sphalerite and chalcopyrite collected from the mineralized vein indicate that the temperature range of the fluids is 235°C to 340°C and that the salinities are 0.7 to 4.49wt.% NaCl equivalent. The wide range of homogenization temperatures and relatively lower salinities of the fluid inclusions indicate that at least two different fluid generations were trapped in the quartz from only one fluid type. Also, lower salinities of fluid inclusion probably indicate mixing of meteoric water and magmatic fluid.The galena has 206Pb/204Pb values of 18.862–18.865, 207Pb/204Pb values of 15.707–15.711, and 208Pb/204Pb values of 39.033–39.042. The lead isotope values show a similarity with upper crustal values.

U–Pb zircon chronology, geochemistry and isotopes of the Changyi banded iron formation in the eastern Shandong Province: Constraints on BIF genesis and implications for Paleoproterozoic tectonic evolution of the North China Craton

The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. The BIF shows alternating quartz-rich light and magnetite-rich dark bands with magnetite (15–65vol.%), quartz (25–65vol.%) and amphibole (15–30vol.%) constituting the major minerals. Minor garnet, epidote, chlorite, calcite, biotite and pyrite occur locally. The BIF bands are interlayered with amphibolite, hornblende gneiss, biotite quartz schist, garnet biotite schist, biotite gneiss and leptynite, and are intruded by granites. LA-ICP-MS U–Pb dating on zircons separated from the BIF bands and the wallrocks constrains the depositional age as 2240–2193Ma and metamorphic age as ~1864Ma. The dominant composition of SiO2+Fe2O3T (average value of 92.3wt.%) of the BIF bands suggests their formation mainly through chemical precipitation. However, the widely varying contents of major elements such as Al2O3 (0.58–6.99wt.%), MgO (1.00–3.86wt.%), CaO (0.22–4.19wt.%) and trace elements such as Rb (2.06–40.4ppm), Sr (9.36–42.5ppm), Zr (0.91–23.6ppm), Hf (0.04–0.75ppm), Cr (89.1–341ppm), Co (2.94–30.4ppm), and Ni (1.43–52.0ppm) clearly indicate the incorporation of clastics, especially continental felsic clastics, as also confirmed by the presence of ancient detrital zircons in the BIF bands. When normalized against Post Archean Average Shale (PAAS), the seawater-like signatures of REE distribution patterns, such as LREE depletion, positive La and Y anomalies, and superchondritic Y/Ho ratios (average value of 36.3), support the deposition in seawater. Strong positive Eu anomalies (Eu/Eu*PAAS=1.14–2.86) also suggest the participation of hydrothermal fluids. In addition, the sympathetic correlation between Cr, Co and Ni as well as the Co+Ni+Cu vs. ∑REE and the Al2O3 vs. SiO2 relations further indicates that the iron and silica mainly originated from hydrothermal fluids. Combined with regional geological investigation and protolith restoration of the wallrocks, a continental rift environment is suggested for the Changyi BIF deposition. The appearance of negative CePAAS anomalies might suggest the influence of the Great Oxidation Event at the time of deposition. The Changyi BIF witnessed the major Paleoproterozoic rifting–collision events in the NCC and their unique distribution in the NCC contrasts with other examples elsewhere in the world.

Fluid inclusion and sulfur isotope thermometry of the Inkaya (Simav-Kütahya) Cu-Pb-Zn-(Ag) mineralization, NW TURKEY

AbstractThe Inkaya Cu-Pb-Zn-(Ag) mineralization, located about 20 km west of the Simav (Kütahya-Turkey), is situated in the northern part of the Menderes Massif Metamorphics. The mineralization is located along an E-W trending fault in the Cambrian Simav metamorphics consisting of quartz-muscovite schist, quartz-biotite schist, muscovite schist, biotite schist and the Arıkayası formation composed of marbles. Mineralized veins are 30–35 cm in width. The primary mineralization is represented by abundant galena, sphalerite, chalcopyrite, pyrite, fahlore and minor amounts of cerussite, anglesite, digenite, enargite, chalcocite, covellite, bornite, limonite, hematite and goethite with gangue quartz.Fluid inclusion studies on the quartz samples collected from the mineralized veins indicate that the temperature range of the fluids is 235°C to 340°C and the salinities are 0.7 to 4.49 wt. % NaCl equivalent. The wide range of homogenization temperatures indicates that two different fluid generations were trapped in quartz.Sulfur isotope studies of the sulfide minerals showed that all of the δ 34S values are between −2.1 and 2.6 per mil. These values are a typical range for hydrothermal sulfide minerals that have sulfur derived from a magmatic source.Pyrite-galena and pyrite-chalcopyrite sulfur isotope fractionation is consistent with an approach to isotopic equilibrium, and calculated temperatures are 254.6 and 277.4°C for pyrite-galena and 274.7°C for pyrite-chalcopyrite. The microthermometric data and sulfur isotope thermometry indicate the existence of a hydrothermal fluid that circulated along the fault crossing the Simav metamorphics and Arıkayası formation.Fluid inclusion and sulfur isotope thermometry can be used in combination with ore petrographical and geological information to provide site-specific targets for meso-hypothermal metal concentrations.

Sulfur- and oxygen-isotope constraints on the sedimentary history of apparent conglomerates from the Nuvvuagittuq Greenstone Belt (Nunavik, Québec)

Mafic igneous rocks of the Nuvvuagittuq Greenstone Belt (NGB) crystallized before 3.8Ga and possibly as early as 4.3Ga, potentially making the belt the oldest known supracrustal sequence on Earth. However, detrital zircons from a rare quartz-biotite schist in the NGB yield significantly younger ages of ≈3.77Ga or less. These appear to be inconsistent with the ages of the mafic igneous rocks, as the quartz-biotite schist has been interpreted as a metaconglomerate, formed by the dismantling of preexisting lithologies. In order to assess this genetic interpretation, we performed a sulfur and oxygen isotope study of the quartz-biotite schist. Sulfide grains found in quartz clasts and the matrix show significant mass-independent fractionation of sulfur isotopes (+0.2‰≤Δ33S‰≤+1.0‰; mean Δ33S=+0.5±0.1‰). Secondary sulfides from crosscutting veins do not show mass-independent fractionation of sulfur isotopes (−0.1‰≤Δ33S≤+0.3‰; mean Δ33S=+0.1±0.1‰). Oxygen isotope compositions of quartz from clasts, matrix and a fine-grained lens are highly enriched in 18O (16.9‰≤δ18O≤26.7‰). Non-zero Δ33S values indicate a surficial origin for sulfur, probably the Eoarchean atmosphere, while high δ18O values suggest a low-temperature (65±18°C) origin for the quartz, likely as chemical precipitation of a chert precursor from Si-saturated seawater. Therefore, the coupled S- and O-isotope measurements show that primary isotopic signatures characteristic of surficial environments survived the protracted metamorphic history of the NGB, and suggest that the quartz-biotite schist contains material that originated as chemical metasediments. The near mono-mineralic compositions of the clasts (quartz) and their shared 18O-enrichment suggest that they had a common protolith, which was deposited prior to the formation of the schist, and subsequently reworked. Whether the quartz-biotite schist represents a metaconglomerate or a structural melange, it preserves remnants of some of the oldest chemical sediments on Earth.

Geology, age and field relations of Hadean zircon-bearing supracrustal rocks from Quad Creek, eastern Beartooth Mountains (Montana and Wyoming, USA)

Quad Creek Paleoarchean (≤3250Ma) quartzites in southern Montana host Hadean (pre-3850Ma) detrital zircons. Although an accessible resource for investigating early Earth processes distinct from other better known ancient zircon localities, the outcrop-scale geological and geochemical context of these rocks has not previously been well documented. New (1:250) mapping reveals a varied suite of isoclinally folded, sheared and variably deformed chromite-bearing banded and massive quartzites, garnetiferous siliceous (migmatitic) paragneisses, amphibolite, quartz–biotite schists and quartz+magnetite rocks (banded iron-formation; BIF). Conventional ion microprobe U–Pb zircon ages of populations from different quartzites and a paragneiss show outgrowth rim ages on older inherited detrital igneous zircon cores that match documented regional metamorphic events evidenced elsewhere in the Wyoming Craton. Weighted mean 207Pb/206Pb ages for the youngest concordant zircon cores of igneous derivation indicate the Quad Creek sediments were deposited by about 3250Ma. Coupled with a large zircon 207Pb/206Pb age survey (n=1274), and an extended U–Th–Pb depth-profile of the oldest grain in our sample set, these data support the notion that the oldest crust tapped by these sediments was comparable in age to the ca. 4000Ma Acasta Gneiss Complex. This similarity is suggestive of both of a linkage between the Wyoming Craton and the Western Slave Province, and the lingering influence of Hadean crust well into the Archean.

In situ radiation measurement and estimation of U/Th ratio to reflect on the uranium bearing potential zone in metamorphic rocks of Mahendragarh district, Haryana

Gross gamma radiation survey was carried out using Nal(Tl) scintillator based portable gamma ray spectrometer (PGRS) around areas of Gaonri, Dholera, Pachnota and Meghot in Mahendragarh district, Haryana. Geologically the area forms part of north Delhi fold belt comprising calc-silicate, quartz biotite schist, impure marble, quartzite and pegmatite rocks. Equivalent uranium (eU3O8) concentration in ppm was estimated in situ on a regular grid pattern of 500 m (E-W) × 1000 m (N-S) and grab samples were collected at grid locations for analyzing in the laboratory for estimating the contents of eU3Os Raeq, ThO2 and %K. A comparison with the laboratory analysed grab samples for eU308 data and in situ radiation measurements shows a good match of the two sets of data. The in situ measurements indicate higher concentration of eU3O8 in Chapra Bibipur in northeastern most corners, Maghot area in central part, Gaonri in western part and Pachnota in southwestern part of the study area.

Geological and geotechnical investigations of Loharinag-Pala Hydroelectric Project, Garhwal Himalaya, Uttarakhand

The project area, forming a part of Bhagirathi valley, exposed rocks classed as central Himalayan crystallines and are medium to high grade metamorphics. The rock types exposed are feldspathic gneisses, quartz-biotite schists, garnet-biotite schist, biotite gneisses, migmatites and amphibolites. To design the rock support for the underground structures of desilting chambers, HRT, surge shaft, pressure shafts, power house, TRT and for the foundations of barrage and intake of desilting chambers, rock mass classifications was attempted following the methods of Bieniawski Rock Mass Rating (RMR) Classification and Tunnelling Quality Index (Q) of Barton et al. RMR technique involves collection of data on rock strength, RQD (%), spacing of discontinuities, condition of discontinuities and groundwater condition, while the ‘Q’ involves collection of data on RQD (%), joint set numbers (Jn), joint roughness number (Jr), joint alternation number (Ja), joint water reduction factor (Jw) and stress reduction factors (SRF). The permeability test in the overburden was done by the constant head method, while in the bed rock portion conducted by packer test. The result indicates that the rock masses of the area fall under the good, fair and poor rock quality. Augen gneiss of power house area is coming under the category of moderately strong rock as proved by deformability characteristics and strength parameter. On the basis of above study recommendations have been made for the proper and safe construction of the project components.

Pre-3750 Ma supracrustal rocks from the Nuvvuagittuq supracrustal belt, northern Québec

Geochemistry and U-Pb ion microprobe zircon geochronology guided by high-resolution mapping (1:50 scale) was used to define a minimum age of ca. 3750 Ma for supracrustal rocks from the Nuvvuagittuq supracrustal belt (NSB) in the northern Superior Province, Québec (Canada). Mineralogy and geochemistry of critical field relationships preserved at the Porpoise Cove locality describe a supracrustal succession of (mafic) amphibolites and ultramafic rocks, finely banded quartz-magnetite units with intermixed coarse-grained ferruginous quartz-pyroxene rocks and quartz-biotite schists that superficially resemble polymict meta-conglomerates with large (up to 10 cm) deformed polycrystalline quartz and mafic clasts. All units in the mapped outcrop have sharp lithological contacts. Narrow (dominantly trondhjemitic) orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals. The total extent of the supracrustal enclave is ∼ 8 km 2; it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions. The quartz-biotite schists record complex zircon growth at ∼ 3500 and ∼ 2800 Ma, interpreted to reflect metamorphic events. Zircons separated from orthogneisses in the enclave – including one sheet that transects a banded quartz-pyroxene (± magnetite) unit – yield magmatic 207Pb/ 206Pb ages of ca. 3750 Ma. These ages are slightly younger than earlier provisional reports for an NSB orthogneiss from Porpoise Cove. The Nuvvuagittuq supracrustals are the oldest rocks thus far reported for the Minto Block, they overlap in age with the ca. 3.70–3.83 Ga Isua supracrustal belt and Akilia association rocks in West Greenland, and they represent an important new area for exploration of the early Earth.

Chemical and isotopic evidence for widespread Eoarchean metasedimentary enclaves in southern West Greenland

New mapping, geochemistry and zircon U–Pb ion microprobe geochronology of pre-3750 Ma rocks from West Greenland was used to identify sedimentary protoliths in a problematic high-grade metamorphic terrane. Samples were collected from southernmost part of the Itsaq Gneiss Complex where Akilia association supracrustal rocks have previously been noted. Supracrustal lithologies include laterally continuous and variably deformed units of amphibolite, ultramafics and ferruginous quartz–pyroxene rocks. Oxygen isotope and mass-independently fractionated sulfur isotopes, immobile trace elements and rare earth element patterns are consistent with origin of quartz–pyroxene rocks as chemical sediments deposited in a marine hydrothermal setting. We describe a further supracrustal lithology: Garnet-bearing quartz–biotite schists with elevated oxygen isotope values (δ 18O SMOW ⩾ +16‰) and mass-independently fractionated S isotopes consistent with a low-temperature aqueous sedimentary origin. In several enclaves, granitoid gneisses within low-strain limbs transect lithologic contacts and contain inclusions of surrounding rocks. This supports the interpretation that some orthogneisses were originally emplaced as igneous veins that cut supracrustal lithologies. Zircon geochronology on orthogneisses that preserve intrusive relationships confirms minimum ages of ca. 3750 Ma for the supracrustals and pooled [Th/U] zircon and δ 18O zircon values of older zircon populations are consonant with igneous growth in the bulk composition of the host rocks. Low [Zr] WR and high Zr saturation temperatures further minimize the possibility of zircon inheritance. A >3750 Ma age and chemical sedimentary origin for various Akilia association lithologies underscores the widespread occurrence of rocks of this kind beyond the type locality on Akilia (island) at the southern limit of the Itsaq Gneiss Complex.

Geology and geochemistry of the Changba SEDEX Pb-Zn deposit, Qinling orogenic belt, China

The Changba Pb-Zn SEDEX deposit occurs in the Middle Devonian sequence of the Anjiaca Formation of the Western Qinling Hercynian Orogen in the Gansu Province, China. The Changba-II orebody is hosted in biotite quartz schist and is the largest of 143 stratiform orebodies that are hosted either in biotite quartz schist or marble. The Changba-II comprises two types of mineralization: a bedded facies and an underlying breccia lens. The bedded section exhibits three sulfide sub-facies zoned from bottom to top: 1) banded sphalerite intercalated with quartz albitite; 2) interbedded massive pyrite and sphalerite ore; and 3) banded sphalerite ore intercalated with banded baritite. Major metallic minerals are sphalerite, pyrite, galena, with minor arsenopyrite, pyrrhotite, boulangerite, and rare chalcopyrite. The bedded sulfides are underlain by a lens of brecciated and albitized biotite-quartz schists cemented by sulfides and tourmaline.

Permian volcanism in the Mongolian orogenic zone, northeast China: geochemistry, magma sources and petrogenesis

Lower Permian volcanism was the first magmatic activity to occur after the collision events in the Mongolian orogenic zone, east China. The Permian volcanic rocks are therefore a key to understanding the dynamics of the unified continental lithosphere. The volcanic rocks consist of basic and intermediate rocks. The intermediate rocks with high initial 87Sr/86Sr ratios (0.7051 to 0.7052) and low εNd values (−0.73 to −3.57) generally overlie the basic rocks in the field. The basic rocks have relatively low initial 87Sr/86Sr ratios (0.7034 to 0.7051) and high εNd values (2.72 to −0.10). Two parallel Rb–Sr isochrons give almost the same age, about 270 Ma. One consists of the basic rocks giving an initial isochron 87Sr/86Sr ratio of 0.7035. The other consists of the intermediate rocks and one sample of basalt, which give an initial isochron 87Sr/86Sr value of 0.7051. The strong correlations between SiO2 and other major elements suggest that fractional crystallization played an important role in the magmatic processes. However, fractional crystallization cannot explain the geochemistry of most incompatible trace elements and Sr–Nd isotope characteristics. The positive correlation between Th/Nb and (La/Sm)N ratios demonstrates the direct relation between the enrichment of the light rare earth elements and the contamination of continental sediments. The high contents of large ion lithosphere elements (LILE) in the Permian volcanic rocks may suggest an additional ‘crust + fluid’ component, especially in the intermediate rocks, which are highly enriched in Ba (> 400 ppm) relative to the basic rocks (> 200 ppm). We propose that the subduction slab dropped into depleted mantle and released fluid, which induced the mantle metasomatism and LILE enrichment. The metasomatized mantle partially melted and formed the ‘primary’ magma. This primary magma assimilated with the Proterozoic biotite–quartz schist during its rise, and finally formed the Permian volcanic rocks. Magma assimilated with the Proterozoic biotite–quartz schist in small amounts could have produced the basic rocks, while assimilation of larger amounts of magma (because of longer assimilation time) would generate intermediate rocks.

An investigation into strength and porous properties of metamorphic rocks in the Himalayas: A case study

If a rock is porous in nature, the genetic complexity associated with its petrofabric makes it difficult to predict its behavior. Here, a comprehensive study of the porous nature, physical and geotechnical responses of three varieties of schist, i.e. quartz mica schist, quartz mica schist with quartz veins and biotite schist of low grade metamorphism obtained from Nathapa-Jhakri hydroelectric project site in the foot hills of Himalayas, India, has been presented. The porous nature of the schists has been brought out through the Mercury Intrusion Porosimetry (MIP) testing and strength behavior through uniaxial testing. Through an experimental invesigation, the porous nature and pore size distribution parameters from MIP, which are closely related to the strength and performance of metamorphic schistose rocks, have been studied. The significance of the porous nature and its response for consideration in the design is emphasized.

Geochemistry and origin of cordierite-orthoamphibole gneiss and associated rocks at an Archaean volcanogenic massive sulphide camp: Manitouwadge, Ontario, Canada

Cordierite-orthoamphibole gneiss, amphibolites, quartz-biotite schist and calc-silicate rocks occur in spatial association with Archaean metamorphosed volcanogenic massive sulphide (VMS) deposits at Manitouwadge, Ontario. Trace-element characteristics, particularly those of the high field strength elements (HFSE), support field relationships that most of the cordierite-orthoamphibole gneisses represent metamorphosed equivalents of chlorite-rich rocks that were derived hydrothermally from tholeiitic basalts as an integral part of the syngenetic ore-forming processes on the Archaean seafloor. However, a garnet-rich cordierite-orthoamphibole dike, that is characterized by low TiO 2 content and high concentrations of Zr, Y, Nb and REE, and has single-zircon Pb-evaporation 207 Pb 206 Pb ages of 2715–2743 Ma, may have been derived from a synvolcanic felsic dike. Evidently, the seafloor hydrothermal alteration at Manitouwadge resulted in similar alteration products from contrasting protoliths and, therefore, was a fluid-dominant system. The amphibolite lenses within the cordierite-orthoamphibole gneiss also have been affected by the seafloor hydrothermal alteration, and some of them most likely represent metamorphosed equivalents of calcsilicate alteration products formed on the seafloor. Mass-balance calculations indicate that HFSE and REE were mobilized during the seafloor hydrothermal alteration. The mobility of HFSE and REE was controlled by both their host minerals and the hydrothermal fluids. In particular, a significant portion of igneous zircon in the garnet-rich dike has survived the seafloor hydrothermal alteration and the subsequent high-grade regional metamorphism. The regional metamorphism did not change the bulk-rock REE characteristics but was important in redefining the mineralogy of REE. Trace-element geochemistry of mafic metavolcanic rocks at Manitouwadge are similar to those of the adjacent Schreiber-Hemlo greenstone belt and are consistent with a volcanic island arc setting. However, further work is required to determine whether the Manitouwadge and adjacent greenstone belts of the Wawa Subprovince were originally a larger, continuous terrane.

Patterns of Scheelite Mineralization in the Muruntau Gold Deposit, Uzbekistan

Spatial and structural patterns associated with scheelite mineralization in a major gold deposit in Uzbekistan are investigated. Examination of the morphology of tungsten-bearing zones, their internal structure, and relation with gold orebodies reveals that the overwhelming majority of tungsten-bearing zones in the Muruntau gold ore deposit are stratified, with gold orebodies usually lying separate from tungsten-bearing orebodies. The two occasionally merge near thick cross-cutting quartz veins. Topomineralogic mapping, sampling of mineral bodies, and laboratory research (e.g., thermo- barogeochemistry) provided the rationale for a three-stage model of mineral genesis. The three generations of scheelite that can be distinguished at Muruntau are: scheelite-1, formed as a result of pyroxene- amphibole metasomatism of biotite-quartz schist; scheelite-2, which exhibits close spatial and temporal association with gold; and scheelite-3, which is post-ore and formed in the course of redeposition of earlier schee...

Systematic variation in the tin content of pegmatites in western central Namibia

Abstract More than 900 pegmatites were sampled during a tin exploration programme conducted along the Omaruru River in western central Namibia. The pegmatites are hosted by late Precambrian metasediments of the Damara Sequence and are probably of Cambrian or somewhat younger age. The pegmatites sampled occur in a belt 60 km long by 5 km wide which represents the bulk of what is known as the central tin belt, a sub-province within a greater tin province covering some 24,000 km2. Although there are S-type granites in the vicinity, there is a closer spacial association between tin-bearing pegmatites and a stratigraphic unit which will for convenience be referred to as the ‘tin schist’. This unit consists mainly of knotted, quartz-biotite schist which is in part tourmaline-bearing. The same rock-type is also present 40 km to the northwest at the Uis tin mine, the largest producer in the region. The tin schist has a very distinctive satellite imagery signature and this makes it possible to trace its regional distribution, which matches that of tin deposits to a remarkable degree. Richards (1986) stated that the tin schist contains anomalous concentrations of tin irrespective of whether any pegmatites or granites occur in close proximity or not. The possible genetic meaning of this association and the observed distribution pattern of tin deposits in the greater tin district are discussed.

The nickel sulfide mineralization and metamorphic setting of the Forrestania area, Western Australia

The Forrestania group of nickel sulfide occurrences of the Archean Southern Cross province is localized in intrusive dunite bodies at the base of a lower komatiite unit, which is interlayered with tholeiitic mafic rocks, para-amphibolites, quartz-biotite schists, chemical sedimentary rocks (iron-formation and sulfide cherts), and komatiites. Concordant, lenticular, metamor-phosed, sill-like, mineralized dunites have komatiitic affinities and appear to be feeder chambers to overlying thick sequences of dominantly extrusive ultramafic komatiites and rare pyroclastic komatiites. Unmineralized portions of the dunites are extremely rich in magnesium, with MgO values ranging between 45 and 51 percent. The important nickel sulfide occurrences are pentlandite-rich disseminations distributed in the basal portions of metamorphosed intrusive dunites (e.g., Cosmic Boy and Digger Rocks-South Digger Rocks). Similar mineralization also occurs in hanging-wall and internal portions of dunites but is generally of less importance than the basal mineralization because of lower nickel tenor. Massive pyrrhotite-rich sulfides occur within reaction zones between dunite and footwall country rocks and as remobilized veins and stringers in footwall metasediments. This type of mineralization forms important small tonnage-high grade deposits at New Morning and Flying Fox. Supergene alteration of primary sulfides is extensive, and massive violarite-pyrite-magnetite zones are developed in weathering profiles.Polyphase deformation and metamorphism, with peak conditions estimated at 655 degrees + or - 30 degrees C and 4.0 + or - 1.0 kb, resulted in widespread recrystallization and tectonic remobilization of ductile sulfides. Cumulate dunites were metamorphosed to bladed, granular, and granoblastic aggregates of forsterite-talc, forsterite-talc-anthophyllite, and forsterite-talc-enstatite, formed under varying f (sub CO 2 ) and f (sub H 2 O) conditions. Retrograde serpentinization and talc-carbonate alteration followed by later supergene alteration has caused textural modification and oxidation of primary sulfides, with development of pyrite-, violarite-, and magnetite-rich ores.Nickel values and Ni/Cu ratios vary for different types of mineralization. The economically important basal disseminations of pentlandite-pyrite-pyrrhotite-chalcopyrite have Ni values in the range 2 to 5 percent and constant Ni/Cu ratios in the range 14:1 to 22:1. Bulk composition of primary sulfides of this type at Cosmic Boy is S rich, with Fe = 39.4 percent, Ni + Co = 18.6 percent, and S = 42.0 percent. Massive remobilized veins of pyrrhotite-pentlandite-pyrite-chalcopyrite have high Ni (3 to 8%) and erratic Ni/Cu ratios, generally in the range from 10:1 to 200:1, with a mean of 45:1. This mineralization is Fe rich, with Fe = 50.4 percent, Ni + Co = 7.4 percent, and S = 42.2 percent. Broad, irregular sparse disseminations (less than 2%) of pentlandite-heazlewood-chalcopyrite form in the internal portions of cumulate and metamorphosed dunites. They have low Ni (0.3 to 0.5%), high Ni/Cu ratio (50:1 to 100:1), and bulk composition close to pentlandite composition, with Fe = 31.8 percent, Ni + Co = 34.2 percent, and S = 34.0 percent. The bulk compositions of sulfides generally plot on the S-rich side of the monosulfide solid solution field at 650 degrees C, and this appears to reflect nickel and sulfur mobility during retrograde metamorphism.

Age determinations of rocks from northern Victoria Land, Antarctica

Abstract The purpose of this note is to report new age determinations by the rubidium—strontium (Rb—Sr) method of whole-rock samples from northern Victoria Land in Antarctica. The rocks which have been dated include a specimen of quartz-biotite schist of the Rennick Group and a specimen of muscovite-biotite granite of the Campbell Plutonics from the mountains along the western edge of the upper Rennick Glacier. These specimens were collected by H. S. Gair who has described the geology of this region (Gair, 1967). In addition, we have dated two specimens of felsic volcanic rocks collected by Mr S. J. Carryer in the Gallipoli Heights of the Southern Freyberg Mountains.

THE ATHABASCA VALLEY ERRATICS TRAIN, ALBERTA AND PLEISTOCENE ICE MOVEMENTS ACROSS THE CONTINENTAL DIVIDE

The Athabasca Valley Erratics Train contains a variety of low- to medium- grade metamorphic rocks, the most abundant of which is talcose schist, with lesser amounts of garnet schist and biotite–quartz schist. This erratics train occurs in and west of the Athabasca Valley west of Edson, Alberta. It is probably a late stage deposit of the same glacier that carried and deposited the Erratics Train, Foothills of Alberta. The metamorphic erratics were incorporated into a glacier that originated in the northern part of the Monashee Mountains and Premier Range of British Columbia. This ice movement is also recorded by numerous U-shaped valleys, which extend across the Continental Divide. Thus, during a brief period in late(?) Wisconsin time, the Cordilleran ice in the Rocky Mountains of the Jasper National Park area was partly derived from west of the Continental Divide and the Rocky Mountain Trench. These data agree with the inferred ice movements shown on the 1958 Glacial Map of Canada.