Modification of high-Mg and high-Cr chromite within the Katpal breccia zone of the Mesoarchean Sukinda Complex, Singhbhum Craton, India: implications for platinum-group element–Ni–Cu–Co mineralization

The Mesoarchean Sukinda Complex in the Singhbhum Craton hosts the largest chromite deposits in India. In the Katpal area, chromitites and host ultramafic rocks of the Sukinda Complex are extensively fragmented, forming a tectonic breccia that is mantled by pegmatitic gabbro and granodiorite. The chromite grains in various parts of the chromitite clasts show different textural and compositional variations. The cumulus chromites in the interior part of large chromitite clasts show textural and chemical character (Cr-ratio (Cr/Cr + Al) and Mg-ratio (Mg/Mg + Fe 2+ )) resembling cumulus chromites of massive chromitite bands from the main ultramafic unit. The chromite grains of the large chromitite clasts at the contact of the gabbroic matrix show an alteration rim of ferrianchromite and/or chrome-magnetite surrounding the chromite core. The altered chromite grains contain numerous inclusions of chrome-chlorite and lamellae of ilmenite–rutile and porosities along the grain boundary. The core regions of the altered chromites show a high and constant Cr-ratio (0.79–0.82) with a variable Mg-ratio (0.32–0.54), and chrome-magnetite rims have a Cr-ratio >0.82 with Mg-ratio = 0.03–0.06. The chromite grains within the smaller chromitite clasts are completely altered to chrome-magnetite. These altered chromites are characterized by a higher Fe 3+ /(Cr + Fe 3+ + Al) ratio (0.80–0.90) than chromites from the larger chromitite fragments of the breccia zone. This compositional modification of the chromitite breccia initially took place due to interaction with fluid-rich gabbroic to granodioritic magma that formed the matrix of the breccia zone. The breccias were compositionally metasomatized as a result of this interaction event, where primary chromites were modified to ferrianchromite (enriched in iron and titanium). The whole breccia assemblages were further modified during a later hydrothermal event when Fe–Ti-bearing phases were exsolved from the metasomatized chromite grains. Chrome-chlorite, amphibole and other low-temperature silicate phases were formed during this hydrothermal event. In Katpal, the major matrix material is granodioritic rather than pegmatitic gabbro, and is the main explanation for the lower prospects for any primary magmatic Ni–Cu–Co–platinum-group element deposits.