Fossilization of Precambrian microfossils in the Volyn pegmatite, Ukraine

We report on Precambrian soft-tissue microfossils from igneous rocks of the Volyn pegmatite district, associated with the Paleoproterozoic Korosten Pluton, north-western Ukraine. The fossils were recovered from m-sized miarolitic cavities and show a well-preserved 3D morphology, mostly fibrous, but with a large variety of fiber types, and also in irregular, flaky shapes reminiscent of former biofilms, and rare spherical objects. Based on literature data, own pyrolysis experiments and reflected light microscopy results, the organic matter (OM) is characterized as (oxy)kerite. Further investigations with microscopic techniques, including scanning and transmission electron microscopy, and electron microprobe analysis show that fossilization likely occurred during a hydrothermal, post-pegmatitic event, by silicification dominantly in the outermost 1–2 µm of the microfossils. The hydrothermal fluid, derived from the pegmatitic environment, was enriched in SiF4, Al, Ca, Na, K, Cl, and S. The OM shows O enrichment where N and S content is low, indicating simultaneous N and S loss during anaerobic oxidation. Mineralization with Al-silicates starts at the rim of the microfossils, continues in its outer parts into identifiable encrustations and intergrowths of clay minerals, feldspar, Ca-sulfate, Ca-phosphate, Fe-sulfide, and fluorite. Breccias, formed during collapse of some the miarolitic cavities, contain also decaying OM, which released high concentrations of dissolved NH4+, responsible for the late-stage formation of buddingtonite and tobelite-rich muscovite. The age of the fossils can be restricted to the time between the pegmatite formation, at ~1.760 Ga, and the breccia formation at ~ 1.49 Ga. As geological environment for growth of the microorganisms and fossilization we assume a geyser system, in which the essential biological components C, N, S, and P for growth of the orgabisms n the miarolitic caves were derived from microorganisms at the surface. Fossilization was induced by magmatic SiF4-rich fluids. The Volyn occurrence is a prime example of Precambrian fossils and the results underline the importance of cavities in granitic rocks as a possible habitat for microorganisms of the deep biosphere.

Tourmalines of the Velence Granite Formation and the surrounding contact slate, Velence Mountains, Hungary

Three distinct paragenetic and compositional types of tourmaline were described from the Velence Granite and the surrounding contact slate. Rare, pitch-black, disseminated tourmaline I (intragranitic tourmaline) occurs in granite, pegmatite, and aplite; very rare, black to greenish-gray, euhedral tourmaline II (miarolitic tourmaline) occurs in miarolitic cavities of the pegmatites; abundant, black to gray, brown to yellow or even colorless, acicular tourmaline III (metasomatic tourmaline) occurs in the contact slate and its quartz-tourmaline veins. Tourmaline from a variety of environments exhibits considerable variation in composition, which is controlled by the nature of the host rock and the formation processes. However, in similar geologic situations, the composition of tourmaline can be rather uniform, even between relatively distant localities. Tourmaline I is represented by an Al-deficient, Fe3+-bearing schorl, which crystallized in a closed melt-aqueous fluid system. Tourmaline II is a schorl-elbaite mixed crystal, which precipitated from Li- and F-enriched solutions in the cavities of pegmatites. Tourmaline III shows an oscillatory zoning; its composition corresponds to schorl, dravite, and foitite species. It formed from metasomatizing fluids derived from the granite. This is the most abundant tourmaline type, which can be found in the contact slate around the granite.

Crystal mush dykes as conduits for mineralising fluids in the Yerington porphyry copper district, Nevada

Porphyry-type deposits are the world’s main source of copper and molybdenum and provide a large proportion of gold and other metals. However, the mechanism by which mineralising fluids are extracted from source magmas and transported upwards into the ore-forming environment is not clearly understood. Here we use field, micro-textural and geochemical techniques to investigate field relationships and samples from a circa 8 km deep cross-section through the archetypal Yerington porphyry district, Nevada. We identify an interconnected network of relatively low-temperature hydrothermal quartz that is connected to mineralised miarolitic cavities within aplite dykes. We propose that porphyry-deposit-forming fluids migrated from evolved, more water-rich internal regions of the underlying Luhr Hill granite via these aplite dykes which contained a permeable magmatic crystal mush of feldspar and quartz. The textures we describe provide petrographic evidence for the transport of fluids through crystal mush dykes. We suggest that this process should be considered in future models for the formation of porphyry- and similar-type deposits.

In Situ Observations of the Transition Between Beryl and Phenakite in Aqueous Solutions Using a Hydrothermal Diamond-Anvil Cell

Beryl and phenakite are important industrial beryllium minerals. In the hydrous melt of the BeO–Al2O3–SiO2–H2O (BASH) system, experiments using quench-type high-temperature and high-pressure equipment have revealed that the different activities of Al2O3 and SiO2 (αAl2O3 and αSiO2) are the main factors that lead to different beryllium mineral assemblages. In this study, we attempted in situ observation of the crystallization process of phenakite and beryl in an aqueous solution of the BASH system using a hydrothermal diamond-anvil cell. Experimental results indicate that phenakite and beryl can crystallize faster in this regime (i.e., 2.93–0.58 × 10−5 cm/s in length and 22.39–3.23 μm3/s in volume) than from a hydrous melt. In addition, in the phenakite and beryl crystallization, pressure–temperature conditions were greater than 467 °C and 220 MPa and 495 °C and 221 MPa, respectively, and their upper temperatures and pressures attained 845–870 °C and 500–1300 MPa. These features indicate that temperature is not the main factor that controls the stability of phenakite and beryl in the BASH system. This stability can be attributed to the diffusion of components in aqueous solution that change αSiO2 and αAl2O3 during the heating and cooling processes. During heating, αSiO2 increases while beryl is dissolving, which leads to phenakite crystallization; during cooling, αSiO2 and αAl2O3 are sufficient for the remaining beryl to recrystallize. Therefore, the transition between phenakite and beryl in the aqueous solution in the BASH system may be different during heating and cooling processes. This reasoning can explain the abundance of phenakite in miarolitic cavities and the occurrence of phenakite, rather than beryl, in hydrothermally altered pegmatites, volcanic rocks, and other beryllium-rich rocks.

Episodes of fast crystal growth in pegmatites

Pegmatites are shallow, coarse-grained magmatic intrusions with crystals occasionally approaching meters in length. Compared to their plutonic hosts, pegmatites are thought to have cooled rapidly, suggesting that these large crystals must have grown fast. Growth rates and conditions, however, remain poorly constrained. Here we investigate quartz crystals and their trace element compositions from miarolitic cavities in the Stewart pegmatite in southern California, USA, to quantify crystal growth rates. Trace element concentrations deviate considerably from equilibrium and are best explained by kinetic effects associated with rapid crystal growth. Kinetic crystal growth theory is used to show that crystals accelerated from an initial growth rate of 10−6–10−7 m s−1 to 10−5–10−4 m s−1 (10-100 mm day−1 to 1–10 m day−1), indicating meter sized crystals could have formed within days, if these rates are sustained throughout pegmatite formation. The rapid growth rates require that quartz crystals grew from thin (micron scale) chemical boundary layers at the fluid-crystal interfaces. A strong advective component is required to sustain such thin boundary layers. Turbulent conditions (high Reynolds number) in these miarolitic cavities are shown to exist during crystallization, suggesting that volatile exsolution, crystallization, and cavity generation occur together.

Helvine-group minerals from Norwegian granitic pegmatites and some other granitic rocks: Cases of significant Sc and Sn contents

ABSTRACT Helvine-group minerals from two granitic pegmatites have disparate compositions, from nearly pure helvine (Ågskardet, northern Norway; Devonian) to helvine close to ternary compositions (Heftetjern, southern Norway; Precambrian). Metagranite from Høgtuva (northern Norway; Precambrian with Caledonian metamorphic overprint) contains Zn-rich danalite. The Ågskardet helvine contains up to 0.46 wt.% SnO2, and the Heftetjern ternary helvine shows a maximum of 1.74 wt.% Sc2O3. Helvine minerals were also analyzed from three occurrences connected to peralkaline granite (ekerite) of the Permian Oslo Rift. Nearly pure genthelvite (99.19 mol.%) occurs in miarolitic cavities at Gjerdingselva. Two mineralogically different granitic pegmatites derived from the same ekerite pluton in the southern part of the Oslo Rift show quite distinct helvine compositions, from nearly continuous solid solution between helvine and genthelvite in crystals with oscillatory zonation (Rundemyr) to solid solutions midway between danalite and genthelvite (Bakstevalåsen). The Rundemyr crystals have a maximum SnO2 content of 1.28 wt.%. The incorporation of minor elements (Ca, Mg, Al, Sn, Sc) in helvine-group minerals is discussed with emphasis on their chalcophilicity characteristics. For stereochemical reasons, Sn in helvine minerals must be tetravalent, even if Sn2+ is more chalcophile than Sn4+.

PETROGENESA BATUAN INTRUSIF GUNUNG BERJO - BUTAK, DAERAH GODEAN BERDASARKAN DATA PETROGRAFINYA

Daerah Godean merupakan bagian dari vulkanisme Miosen, yang tersisa pada daerah Yogyakarta bagian tengah. Vukanisme pada daerah Godean, diperkirakan sebagai produk dari busur gunungapi kepulauan dengan diferensiasi yang terbentuk mulai dari andesit basaltik, andesit dasitik, dasit, riodasit, dan kembali menjadi andesit basaltik. Penelitian dilakukan pada batuan beku pada gunung Berjo, gunung Butak, batuan intrusi subvulkanik dengan orientasi sebaran selatan- timurlaut. Analisis data menggunakan petrografi sebanyak 17 sayatan tipis untuk melihat mineralogi dan tekstur khusus yang terdapat dalam batuan. Morfologi daerah penelitian berupa bukit terisolir dengan kelerengan 51 – 56 %, merupakan produk denudasional yang dikelilingi endapan kuarter dari fluvio-vulkanik gunung Merapi. Litologi pada gunung Berjo dan gunung Buthak terdiri dari andesit piroksen, dasit, andesit, basalt piroksen, yang merupakan batuan intrusi dangkal (subvolcanic intrusion) dengan tekstur utama berupa intersitial, mortar, porfiritik yang diikuti tekstur khusus berupa oscilating zoning, sieve, dan miarolitic cavities terisi epidot-feldspar-kuarsa sebagai indikasi proses magmatic-hydrothermal pada suhu ± 200 - 400ºC diikuti alterasi hidrotermal berupa phyllic dan propylitic alteration yang terbentuk pada pH 4-5 dengan temperatur 200-3000C. Petrogenesa batuan gunung Berjo dan Butak diinterpretasi terbentuk sebagai batuan intrusi dangkal pada kedalaman 500 m dari permukaan purba, afinitas magma kapur alkali busur gunung api kepulauan dengan konten air tinggi, yang diikuti proses fraksinasi kristalisasi dengan pergerakan magma intrusi yang relatif melambat dan menerobos batuan sedimen karbonat. Petrogenesa daerah Godean masih perlu diperkuat dengan riset mineralogi lebih detil dan data lain yang mendukung.

The geology, petrology, and geochronology of the Archean Côté Gold large-tonnage, low-grade intrusion-related Au(–Cu) deposit, Swayze greenstone belt, Ontario, Canada

The Archean Côté Gold Au(–Cu) deposit is the first large gold deposit discovered in the Swayze greenstone belt of the Abitibi Subprovince. The deposit is a low-grade, large-tonnage type with a combined indicated and inferred resource of 8.65 M oz Au (245.2 t Au). The deposit is hosted by the Chester intrusive complex (CIC), a multiphase, subvolcanic intrusion composed of low-Al tonalite, diorite, and quartz diorite, plus magmatic and hydrothermal breccia bodies. The age of the tonalite and dioritic phases is constrained at 2741–2739 ± 1 Ma using high-precision isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon geochronology. Although these phases are co-temporal and co-spatial, they appear to be petrogenetically unrelated. The CIC was emplaced into mafic metavolcanic rocks of the Arbutus Formation whose geochemistry reflects a back-arc environment. The tonalite of the CIC is coeval and co-genetic with the felsic to intermediate metavolcanic rocks of the Yeo Formation. Emplacement of the CIC into a shallow crustal level is inferred based on the incorporation of screens and inclusions of the Yeo Formation and is supported by the presence of textures in tonalite and dioritic rocks (e.g., granophyres, miarolitic cavities, and pegmatites), as well as Al-in-hornblende geobarometry results of ≤1.3 ± 0.6 kbars (1 kbar = 100 MPa). The CIC is petrologically similar to other subvolcanic, low-Al tonalite–trondhjemite–diorite intrusions that underlie volcanogenic massive sulphide (VMS)-type deposits and which themselves may contain syn-intrusion mineralization. Several geochemically unrelated dykes and deformation events crosscut and postdate the CIC.

Micro-structural and compositional variations of hydrothermal epidote-group minerals from a peralkaline granite, Corupá Pluton, Graciosa Province, South Brazil, and their petrological implications

Epidote-group minerals, together with albite, quartz, fluorite, Al-poor and Fe-rich phyllosilicates, zircon, and minor oxides and sulphides, are typical hydrothermal phases in peralkaline alkali-feldspar granites from the Corupá Pluton, Graciosa Province, South Brazil. The epidote-group minerals occur as single crystals and as aggregates filling in rock interstices and miarolitic cavities. They display complex recurrent zoning patterns with an internal zone of ferriallanite-(Ce), followed by allanite-(Ce), then epidote-ferriepidote, and an external zone with allanite-(Ce), with sharp limits, as shown in BSE and X-ray images. REE patterns show decreasing fractionation degrees of LREE over HREE from ferriallanite to epidote. The most external allanite is enriched in MREE. LA-ICP-MS data indicate that ferriallanite is enriched (>10-fold) in Ti, Sr and Ga, and depleted in Mg, Rb, Th and Zr relative to the host granite. Allanite has lower Ga and Mn and higher Zr, Nb and U contents as compared to ferriallanite, while epidote is enriched in Sr, U and depleted in Pb, Zr, Hf, Ti and Ga. The formation of these minerals is related to the variable concentrations of HFSE, Ca, Al, Fe and F in fluids remaining from magmatic crystallization, in an oxidizing environment, close to the HM buffer. L-MREE were in part released by the alteration of chevkinite, their main primary repository in the host rocks.