New classification of magmatic sulphide deposits in China and metallogenesis related to small intrusions
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Prospecting
Flood basalt
Felsic
Ultramafic rock
Serpentinization of ultramafic rocks in ophiolites is key to understanding the global cycle of elements and changes in the physical properties of lithospheric mantle. Mongolia, a central part of the Central Asian Orogenic Belt (CAOB), contains numerous ophiolite complexes, but the metamorphism of ultramafic rocks in these ophiolites has been little studied. Here we present the results of our study of the serpentinization of an ultramafic body in the Manlay Ophiolite, southern Mongolia. The ultramafic rocks were completely serpentinized, and no relics of olivine or orthopyroxene were found. The composition of Cr-spinels [Mg# = Mg/(Mg + Fe2+) = 0.54 and Cr# = Cr/(Cr + Al) = 0.56] and the bulk rock chemistry (Mg/Si = 1.21–1.24 and Al/Si < 0.018) of the serpentinites indicate their origin from a fore-arc setting. Lizardite occurs in the cores and rims of mesh texture (Mg# = 0.97) and chrysotile is found in various occurrences, including in bastite (Mg# = 0.95), mesh cores (Mg# = 0.92), mesh rims (Mg# = 0.96), and later-stage large veins (Mg# = 0.94). The presence of lizardite and chrysotile and the absence of antigorite suggests low-temperature serpentinization (<300 °C). The lack of brucite in the serpentinites implies infiltration of the ultramafic rocks of the Manlay Ophiolite by Si-rich fluids. Based on microtextures and mineral chemistry, the serpentinization of the ultramafic rocks in the Manlay Ophiolite took place in three stages: (1) replacement of olivine by lizardite, (2) chrysotile formation (bastite) after orthopyroxene and as a replacement of relics of olivine, and (3) the development of veins of chrysotile that cut across all previous textures. The complex texture of the serpentinites in the Manlay Ophiolite indicates multiple stages of fluid infiltration into the ultramafic parts of these ophiolites in southern Mongolia and the CAOB.
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The Wuwamen ophiolite is located on the southern margin of Middle Tianshan Mountains, Xinjiang. In this paper,petrology, mineralogy and geochemistry of the ultramafic rocks from Wuwamen ophiolite were studied to constrain their origin and tectonic setting. The ultramafic rocks from Wuwamen ophiolite are serpentinized lherzolites composed of olivine(Fo=89.1- 90.6),orthopyroxene(Wo0.4- 2.4En87.2~90.7Fs8.9- 10.9; Mg#=89.0-91.0), clinopyroxene(Wo49.1- 51.3En16.0~48.4Fs0.9- 4.3; Mg#=90.2-92.1) and spinel(Mg#=71.8- 77.5; Cr#=9.3- 13.4). They are characterized by relatively lower content of Mg O(37.74%- 41.34%), and higher content of Al2O3(2.58%-3.39%), Ca O(2.23%-3.68%) and Ti O2(0.05%-0.11%). The ultramafic rocks are also depleted in rare earth elements(REE; total REE = 1.73×10-6-4.63×10-6) and incompatible elements(e.g., Rb=0.4×10-6-1.39×10-6, Zr=0.73×10-6-3.28×10-6, Hf=0.04×10-6-0.11×10-6), and enriched in compatible elements(e.g., Cr=2516×10-6-2793×10-6, Co=84.6×10-6-119×10-6, Ni=1641×10-6-2261×10-6). These data indicate that the ultramafic rocks from Wuwamen ophiolite are residues of mantle rocks after low degrees(ca. 5%-10%) of partial melting in a Mid-Ocean-Ridge(MOR) environment.
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Ultramafic rocks, mainly serpentinized peridotites of mantle origin, are mostly associated with the ophiolites of Mesozoic age that occur in belts along three of the margins of the Caribbean plate. The most extensive exposures are in Cuba. The ultramafic-mafic association (ophiolites) were formed and emplaced in several different tectonic environments. Mineralogical studies of the ultramafic rocks and the chemistry of the associated mafic rocks indicate that most of the ultramafic-mafic associations in both the northern and southern margins of the plate were formed in arc-related environments. There is little mantle peridotite exposed in the ophiolitic associations of the west coast of Central America, in the south Caribbean in Curacao and in the Andean belts in Colombia. In these occurrences the chemistry and age of the mafic rocks indicates that this association is mainly part of the 89 Ma Caribbean plateau province. The age of the mantle peridotites and associated ophiolites is probably mainly late Jurassic or Early Cretaceous. Emplacement of the ophiolites possibly began in the Early Cretaceous in Hispaniola and Puerto Rico, but most emplacement took place in the Late Cretaceous to Eocene (e.g. Cuba). Along the northern South America plate margin, in the Caribbean mountain belt, emplacement was by major thrusting and probably was not completed until the Oligocene or even the early Miocene. Caribbean mantle peridotites, before serpentinization, were mainly harzburgites, but dunites and lherzolites are also present. In detail, the mineralogical and chemical composition varies even within one ultramafic body, reflecting melting processes and peridotite/melt interaction in the upper mantle. At least for the northern Caribbean, uplift (postemplacement tectonics) exposed the ultramafic massifs as a land surface to effective laterization in the beginning of the Miocene. Tectonic factors, determining the uplift, exposing the peridotites to weathering varied. In the northern Caribbean, in Guatemala, Jamaica, and Hispaniola, uplift occurred as a result of transpresional movement along pre-existing major faults. In Cuba, uplift occurred on a regional scale, determined by isostatic adjustment. In the south Caribbean, uplift of the Cordillera de la Costa and Serrania del Interior exposing the peridotites, also appears to be related to strike-slip movement along the El Pilar fault system. In the Caribbean, Ni-laterite deposits are currently being mined in the central Dominican Republic, eastern Cuba, northern Venezuela and northwest Colombia. Although apparently formed over ultramafic rocks of similar composition and under similar climatic conditions, the composition of the lateritic soils varies. Factors that probably determined these differences in laterite composition are geomorphology, topography, drainage and tectonics. According to the mineralogy of principal ore-bearing phases, Dominican Ni-laterite deposits are classified as the hydrous silicate-type. The main Ni-bearing minerals are hydrated Mg-Ni silicates (serpentine and “garnierite”) occurring deeper in the profile (saprolite horizon). In contrast, in the deposits of eastern Cuba, the Ni and Co occurs mainly in the limonite zone composed of Fe hydroxides and oxides as the dominant mineralogy in the upper part of the profile, and are classified as the oxide-type.
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Thermal and other physical properties of rocks and minerals are of considerable significance for deriving mineralogical and compositional models of the Earth's mantle. We have determined these properties for the mafic rock such as gabbro and ultramafic rock like harzburgite of the Oman ophiolite suite by utilizing the Debye characteristic property ,Θ-
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Cr-spinels from ultramafic rocks from Lokris (Megaplatanos and Tragana), and Beotia (Ypato and Alyki) ophiolitic occurrences were studied. These rocks comprise principally harzburgite with minor dunite. Small amounts of clinopyroxene-rich harzburgite and lherzolite have been observed along with the harzburgite in Alyki. The Cr# in the studied spinels displays a wide variability. The spinels hosted in harzburgite and cpx-rich harzburgite display low Cr# (<0.6), typical for oceanic (including back-arc basins) ophiolites, whereas the spinels hosted in dunite with Cr# (>0.6) characterize arc-related ophiolitic sequences. Cr-spinels from Alyki indicate a moderate fertile character and are analogous to those from abyssal peridotites. The dunitic and harzburgitic spinel–olivine pairs are consistent with a Supra-Subduction Zone origin. The relatively large range in spinel Cr# and Mg# may have been resulted from a wide range of degrees of mantle melting during the evolution of the host peridotites.
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In the Alpine nappe stack, the Antrona Ophiolite (Italian side of the Western Central Alps) is sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic domain) and the underlying Camughera-Moncucco continental Unit (middle Penninic).The ophiolite sequence includes ultramafic rocks, metagabbros and mafic rocks covered by calcschists.Ultramafites constitute a huge body of serpentised peridotites including interbedded layers of gabbros, clinopyroxene-rich and amphibole-rich rocks, and chloriteschist.In spite of the Alpine tectonic and metamorphic reworking, the ultramafic portion of the Antrona Ophiolite still preserves relict textures and minerals that can be referred to the pre-Alpine or early Alpine evolution.A detailed microstructural analysis performed at polarised microscope and SEM on less serpentinised, olivine-rich samples is here presented.It is integrated with a quantitative textural analysis of Lattice Preferred Orientation (LPO) by neutron diffraction acquired on selected samples of olivine-rich samples.The results allow to infer a mantle origin for the ultramafic rocks, suggesting T conditions > 800°C for the activation of slip systems in olivine.
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