Ultramafic Clasts Research Articles

Age of the Baula PGE mineralization (Orissa, India) and its implications concerning the Singhbhum Archaean nucleus

Abstract Two types of platinum group element (PGE) mineralization are identified in the mafic facies of the Baula Complex intruding the Gabbro-Anorthosite Unit of the Singhbhum Archaean nucleus. The first, Pt-dominated, is magmatic—the PGM are included in the magmatic silicates of a gabbro-norite intrusion (the Bangur Gabbro) cutting the Complex’s ultramafic facies and are found preferentially where the gabbro incorporates clasts of dunite and chromitite near the gabbro/ultramafics contact. The second, Pd-dominated, is hydrothermal—the PGM are associated with hydrous silicates located in the gabbro matrix of a magmatic breccia in which the blocks are from the Baula ultramafic facies. The originating fluids for the second type of mineralization were exsolved from the magma and concentrated PGE and volatile elements; we see no evidence of remobilization or external fluid contamination. A structural study has demonstrated that (a) the breccia was provoked by the intrusion of the upper part of the Bangur Gabbro, and (b) the breccia matrix corresponds to the Bangur Gabbro. Consequently, both types of PGE mineralization are associated with the same magmatic event. A zircon age of 3122±5 Ma has been obtained for the main Bangur Gabbro intrusion, and similar ages (3123±7 and 3119±6 Ma) have been obtained on the PGE-mineralized gabbro matrix of the breccia. This supports the hypothesis of a genetic link between the Bangur Gabbro with its Pt-dominated PGE and the breccia with its Pd-dominated PGE, giving a mean weighted age of 3121±3 Ma for the mineralization. Sm–Nd data on eight gabbro samples of the Baula Complex yielded a similar age of 3205±280 Ma. The dating of these events places them close to the broad-scale stabilization of the Singhbhum Archaean nucleus, estimated at 3.1 Ga, and defines one of the oldest PGE mineralizing events ever recorded. The Bangur Gabbro shows many affinities with the Gabbro-Anorthosite Unit of the Singhbhum Archaean nucleus in terms of age of emplacement and isotopic composition. It may be, therefore, that the Bangur Gabbro actually forms part of the Gabbro-Anorthosite Unit and that a fundamental control of its two types of PGE mineralization is the presence of ultramafic clasts.

High resolution measurement of bedload transport

A bedload movement detector of novel design was installed in a gravel-bed stream as a component of the ongoing research on sediment transport dynamics in the Stuart–Takla Experimental Watersheds in north-central British Columbia, Canada. The device is designed to collect information on the patterns and timing of bedload transport during a flood event. The device is based on a passive magnetic sensor that produces signals of 10−2 to 10−5 V as its magnetic field is disturbed by passing clasts. A series of 82 sensors is housed in an aluminum beam placed across the stream, inserted such that its surface is flush with the gravel bed. The device can be raised or lowered to compensate for bed aggradation and scour. A data acquisition system gathers voltage signals from the sensors at rates of 30 to 100 Hz. This device is sensitive enough to record the movement of most volcanic, metamorphic, granitic and ultramafic clasts larger than a few millimetres. O'Ne-ell Creek watershed is a 68 km2 tributary basin of the Middle River drainage system in the northern headwaters of the Fraser River. Bedload transport generally occurs twice a year in the Stuart–Takla streams: once during spring floods and again during salmon spawning activity. Bedload moved only during two days in 1998, at the peak of the relatively small nival flood in May. Nevertheless, the device detected at least 3×106 passing clasts. A continuous record of bedload transport was obtained, showing: a pulsating pattern of activity seemingly independent of stage, lateral movement of the transport zone, and a sudden onset of bed movement with a tapered cessation. We anticipate that more sophisticated calibration of the sensors and accelerated sampling rates will provide detailed information on the size and/or velocity and magnetic permeability of particles moving over the device, and will contribute to a better understanding of bedload transport. Copyright © 2000 John Wiley & Sons, Ltd.

The Chemical Composition of Serpentine/Chlorite in the Tuscaloosa Formation, United States Gulf Coast: EDX vs. XRD Determinations, Implications for Mineralogic Reactions and the Origin of Anatase

AbstractThe chemical composition of mixed-layer serpentine/chlorite (Sp/Ch) in Tuscaloosa Formation sandstone was analyzed by energy dispersive X-ray spectroscopy (EDX) in the scanning electron miscroscope (SEM) and by X-ray diffraction (XRD). EDX results indicate little depth-controlled variation in composition, whereas XRD results suggest distinct decreases in octahedral Fe and tetrahedral Al. XRD-determined compositions appear to be erroneous and actually reflect progressive changes in Sp/Ch unit-cell dimensions caused by polytype transformations of Ibb layers to Iaa layers in a mixed-layer Ibb/Iaa polytype. The relative lack of variation in Sp/Ch composition, especially when compared to other studies of chlorite minerals over similar temperature ranges, is attributed to a reaction mechanism whereby mineralogic transformations (serpentine layers to chlorite layers and Ibb layers to Iaa layers) occur on a layer-by-layer basis within coherent crystallites, rather than by dissolution-precipitation crystal growth.The lack of titanium in chlorite minerals is attributed to high levels of octahedral Al3+ that prohibit inclusion of the highly charged Ti4+ in the octahedral sheet. Anatase (TiO2) in the Tuscaloosa Formation apparently formed when Ti was liberated during crystallization of Sp/Ch following the breakdown of a Ti-bearing precursor (detrital ultramafic clasts and/or odinite). Odinite, an Fe-rich 7-Å phyllosilicate that forms in some shallow marine sands, apparently existed as a short-lived, poorly crystallized intermediary between dissolution of the ultramafic clasts and formation of Sp/Ch.

Arc-Related Origin of Volcaniclastic Sequences in the Misis Complex, Southern Turkey

The Miocene ophiolitic mélange-flysch association of the Misis Complex in southern Turkey contains three main units of clastic sediments that exhibit abundant acidic, mafic, and ultramafic clasts of magmatic origin. These are: (a) mixed volcaniclastics (vitric, lithic, and crystal tuffs) from the "matrix" of the mélange; (b) volcaniclastics (vitric tuffs) from large bedded sedimentary olistoliths; and (c) turbiditic flysch sediments with an admixture of magmatic, metamorphic and sedimentary clasts. Chemical and petrographie data suggest that magmatic clasts within the volcaniclastic sediments (units a and b) were derived from two main sources: an acidic calc-alkali volcanic arc and a back-arc ophiolite complex, now represented by basalt pillow lavas and ultramafic blocks within the mélange. The turbiditic flysch sediments (unit c) represent the accumulated debris of the denuded acidic arc, together with ophiolitic material derived via dismemberment and tectonic erosion of the back-arc basin crust. Chemical data indicate that the ophiolitic source, however, did not come into prominence until the late Middle Miocene.

Calcium Carbonate Cementation of Tills Rich in Ultramafic Rocks, Northern Puget Sound Region, Washington: ABSTRACT

Late Pleistocene tills containing appreciable amounts of ultramafic clasts and matrix occur on and adjacent to ultramafic bodies (commonly harzburgite) in the northern Puget Sound region. Many exposures of such tills are strongly cemented, typically by aragonite, although Mg-calcite and calcite also occur; cement comprises 1 to 6% of the till. Cementation appears to be controlled by ground water enriched in calcium derived from calcium-bearing pyroxene in the ultramafic rocks. The role of incompletely serpentinized rocks and ground water is indicated by the following observations. (1) Cementation occurs only in association with incompletely serpentinized rocks; other calcium sources (e.g., limestone) are absent. (2) Calcium-rich cemented-till halos surround fresh ultramafic clasts that were glacially transported from source outcrops; completely serpentinized clasts have no cemented halo. (3) Massively cemented till forms preferentially at topographic lows on ultramafic bodies where ground-water discharge is concentrated. At these sites the till is strongly cemented in the wetted zone at and near the bed-rock contact, and th degree of cementation decreases a few meters above the contact. These tills are unusual in that they contain the first reported CaCO3 cement in till that is not derived from preexisting carbonate materials, although the process of carbonate (ophicalcite) generation from unserpentinized ultramafic rocks has been increasingly recognized in recent years. End_of_Article - Last_Page 970------------