Noble metals segregation and fractionation in magmatic ores from Ronda and Beni Bousera Lherzolite Massifs (Spain, Morocco)
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The Gogołów-Jordanów Serpentinite Massif (GJSM) and the Braszowice-Brzeźnica Massif (BBM) are the largest serpentinite outcrops in the Fore-Sudetic Block (NE part of the Bohemian Massif, Central Europe). The GJSM is a peridotitic member of the Variscan Ślęża Ophiolite (SW Poland). Podiform bodies (veins and pockets) of chromitite are found on the Czernica Hill (GJSM) and on the Grochowiec Hill (BBM) within strongly serpentinized harzburgites which occur several hundred metres below Paleo-Moho. Chromitites consist of rounded chromite grains up to 3 cm across, and of chlorite filling the interstices. The veins are embedded in serpentine-olivine-chlorite aggregates. Relics of Mg-rich olivine (Fo 95-96 ) occur in massive chromitite in the BBM. The bulk-rock total PGEs content is very low (42-166 ppm) and the PGE pattern is negatively sloped towards Pt and Pd and depleted relative to chondrite. The primary chromite I is aluminous (Cr# 0.50-0.52, Mg# 0.60-0.70). The highly aluminous and magnesian (Cr# 0.38, Mg# 0.80) chromite Ia occurs locally in the BBM. The secondary chromite II is enriched in Cr and impoverished in Al (Cr# 0.57-0.69), it replaces chromite I. Both chromite I and II contain small amounts of Ti (<0.14 wt% TiO 2 ). Silicate inclusions in chromite are scarce. The composition and mode of occurrence of both the GJSM and the BBM chromitites are similar, thus they were formed probably under the same conditions. Textures of the chromitites suggest their magmatic origin. Their current geological position indicates their emplacement and crystallization in the uppermost mantle harzburgites occurring below the Moho Transition Zone (MTZ). The chromitites and hosting harzburgites were subjected to the greenschist-facies metamorphic overprint. The moderate Cr# and low PGEs contents suggest that the chromitites originated in the arc setting, thus their host ophiolite is of supra-subduction type.
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During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ 18 O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ 18 O magmatic fluids caused δ 18 O depletions in the gneisses within the migmatite zone. These low δ 18 O fluids were derived from the young (<4 Ma), relatively low δ 18 O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40 Ar/ 39 Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.
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<p>The Pados-Tundra massif is located in the western Kola Peninsula and included in the Notozero ultrabasic rock complex (Vinogradov, 1971). The intrusion occurs as a body of ca. 13 km<sup>2</sup> stretched out to the north-east. Enclosing rocks are Archaean granite- and granodiorite-gneisses. There are three major areas in the massif structure (Mamontov, Dokuchaeva, 2005): endocontact area, rhythmically layered series, and upper area. The endocontact area with thickness of 10-20 m occurs as schistose amphibole rocks formed during the metamorphism of main rocks. The rhythmically layered series occurs as a number of rocks from dunites to orthopyroxenites and composes most of the massif. There are 7 rhythms in total, each of which starts with dunites and ends with orthopyroxenites. Dykes of mezo- and leucocratic gabbro, diorites, and hornblendites are developed in the series rocks. The upper gabbronorite area can be partially observed in the north-eastern massif. Presumably, its major volume has been overlapped by enclosing rocks as a result of the overthrust. In the massif, there are 4 horizons of disseminated stratiform chromite ores, which are confined to dunites and serpentinites, as well as to a number of lens- and column-like bodies (podiform type) of chromite ores (Mamontov, Dokuchaeva, 2005; Barkov et al., 2017). Previous isotope-geochronological studies have determined the massif rock age of 2.15 Ga (Shapkin et al., 2008). However, further geological field observations and analysis of the obtained data assume that the intrusive is much older.</p><p>New Sm-Nd geochronological data indicate that the massif rocks and its rhythmically layered series are of Paleoproterozoic age, which is similar to the age of the Cu-Ni-Co-Cr-PGE ore-magmatic system of the Fennoscandian Shield (Amelin et al., 1995; Bayanova et al., 2014, 2017, 2019; Hanski et al., 2001; Huhma et al., 1990, 1996; Layered intrusions ...; 2004; Maier, Hanski, 2017; Mitrofanov et al., 2019; Peltonen, Brugmann, 2006; Puchtel et al., 2001; Serov, 2008; Serov et al., 2014; Sharkov, 2006; Sharkov, Smolkin, 1997). Complex Sm-Nd and U-Pb isotope-geochronological studies have allowed determining the major formation and alteration stages of the Pados-Tundra complex rocks:</p><p>&#8211;&#160; formation of the rhythmically layered series rocks of the intrusive 2485&#177;77 Ma, harzburgites of the layered series &#8211; 2475&#177;38 Ma;</p><p>&#8211; metamorphism of the massif rocks at the turn of 1.95 - 1.9 Ga;</p><p>&#8211; postmetamorphic cooling of the complex rocks t&#1086; 650&#176;-600&#176;&#1057; at the turn of 1872&#177;76 Ma (Sm-Nd for metamorphic minerals) and then to 450&#176;-400&#176;&#1057; (U-Pb for rutile, 1804&#177;10 Ma).</p><p>Therefore, the study results expand geography the East-Scandinavian large Palaeoproterozoic igneous province and are prospective for further study of analogous ultramafite-mafite complexes.</p><p>All investigations and were supported by the RFBR 18-05-70082, 18-35-00246, Presidium RAS Program #48 and are in frame of the Theme of Scientific Research 0226-2019-0053.</p>
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