PGE AND Ag MINERALIZATION IN A BRECCIA ZONE OF THE PRECAMBRIAN NUASAHI ULTRAMAFIC-MAFIC COMPLEX, ORISSA, INDIA

Abstract

The Nuasahi ultramafic-mafic complex, in Orissa, India, contains high concentrations of platinum-group elements (Pt up to 9.6 ppm, Pd up to 17.9 ppm) and Ag (up to 6.6 ppm) in a chromiferous sulfide-rich breccia zone. The plutonic complex occurs within an Archean belt of supracrustal rocks. The complex consists of an ultramafic unit in fault contact with and overlain by gabbros whose stratigraphic top is characterized by a zone of brecciation. Lode chromite deposits occur in the ultramafic suite and are currently being mined. Magnesiochromite compositions in the lodes are refractory and show little variation within the ultramafic suite (Cr 2 O 3 ~60%, Al 2 O 3 ~12%, MgO ~15%). Sudburyite, (Pd,Bi)Sb, members of the michenerite-testibiopalladite series, and irarsite-hollingworthite (IrAsS-RhAsS) are the common platinum-group minerals (PGM) in the chromite-sulfide assemblage of the breccia zone. Sudburyite is preferentially included within coarse-grained ferrian chromite, along with numerous inclusions of chalcopyrite, pyrrhotite, rutile, ilmenite and oriented lamellae of clinochlore. The distinctive noble-metal composition (high Pd, Pt and Ag with high Cu) and dominant Pd-Bi-Te-Sb-bearing PGM in the mineralized portion indicate its evolved nature. Mineralization developed by magmatic processes; a hybrid magma resulting from partial incorporation of serpentinized chromite-bearing ultramafic rocks and chromitite in the younger gabbroic magma was emplaced within the preexisting breccia zone. The breccia zone developed during brittle deformation along a NNW-trending shear zone.Keywords: ultramafic complex, breccia zone, chromite, sulfides, platinum-group element mineralization, inclusions, platinumgroup minerals, Nuasahi, Orissa, India. SOMMAIRELe complexe ultramafique-mafique de Nuasahi, à Orissa, en Inde, contient des concentrations élevées des éléments du groupe du platine (juqu'à 9.6 ppm de Pt, 17.9 ppm de Pd) et d'argent (jusqu'à 6.6 ppm) dans une zone brécifiée enrichie en spinelle chromifère et en sulfures. Le complexe plutonique est situé dans une ceinture de roches archéennes d'origine supracrustale. Il contient une unité ultramafique en contact tectonique avec une unité supérieure gabbroïque dont le sommet est bréchifié. Les gisements in situ de spinelle chromifère se trouvent dans la suite ultramafique, et sont présentement exploités. La composition de la magnésiochromite est réfractaire et varie peu dans la suite ultramafique (Cr 2 O 3 ~60%, Al 2 O 3 ~12%, MgO ~15%). Parmi les minéraux du groupe du platine, on trouve sudburyite (Pd,Bi)Sb, des membres de la série michenerite-testibiopalladite, et irarsitehollingworthite (IrAsS-RhAsS) associés au spinelle chromifère et aux sulfures de la zone bréchifiée. La sudburyite se présente surtout en inclusions dans la chromite ferrique à grains grossiers, de même que chalcopyrite, pyrrhotite, rutile, ilménite, et clinochlore en lamelles orientées. L'enrichissement distinctif en métaux nobles (Pd, Pd, Pt et Ag avec Cu élevé) et la dominance de minéraux du grou...

Chromian-spinel compositions from the Bo Xinh ultramafics, Northern Vietnam: Implications on tectonic evolution of the Indochina block

“…4. Mg/(Mg + Fe 2+ ) vs. Cr/(Cr + Al) variation diagram: field I represents Crspinels in primitive basalt, mantle peridotites and chromitites, field II represents magnetite from metamorphic rocks, field III is magnetites from un-metamorphosed igneous rocks (fields are from Roeder (1994) and Mondal et al (2001)). Dick and Bullen (1984), Bloomer et al (1995) and Ohara et al (2002).…”

Dispersion Pattern of PGE and Ag in Chromiferous Ultramafic Suite of Rocks in Sukinda Valley, India

“…Unlike the Baula-Nuasahi chromiferous maficultramafic complex, located in Keonjhar district, 70 km NNE of Sukinda, no other mafic variant like gabbro, etc., has been encountered in this area. Further, enrichment of PGE has been reported in the gabbrobreccia zone of the Mesoarchean Baula-Nuasahi massif (Mondal et al, 2001;Auge et al, 2002). On the contrary, in the Sukinda ultramafic complex Das Sharma (1965) first reported incidence of PGE followed by Page et al (1985) and Pattanaik (1990).…”

The 3.1 Ga Nuggihalli chromite deposits, Western Dharwar craton (India): Geochemical and isotopic constraints on mantle sources, crustal evolution and implications for supercontinent formation and ore mineralization

“…PGE mineralization has been reported from the mid Archean (3.2 Ga) Nuasahi ultramafic-mafic complex in the Singhbhum craton (eastern India), where they are hosted within pegmatitic gabbroic matrix in a breccia zone (Mondal and Zhou, 2010;Mondal et al, 2001); this is an important PGE mineralized zone. PGE mineralization has also been reported from the Barberton greenstone belt (3.5 Ga Stolzburg Complex, Kaapvaal craton, South Africa; e.g., DeWit and Tredoux, 1987), Abitibi greenstone belt (2.7 Ga Alexo deposit, Superior craton, Canada; e.g., Houlé et al, 2012), Kambalda (2.7 Ga; Norseman-Wiluna greenstone belt, Yilgarn craton, Said et al, 2011), and also from the 2.7 Ga Model illustrates the close spatial association of the TTG and the komatiitic rocks in the craton.…”