Breccias of the Norilsk ore fields and their significance for interpretation of mineralization

Magmatic breccia lays down strong constraints on interpretations of an order of geological events, reveals physical-rheological properties of both matrix and fragment’s protholith, and may indicate ways of magma emplacement, stress and deformation. The primary classification of endogenous breccias is based on the inferred role of magma and/or fluids in breccia formation with magmatic, phreatomagmatic, phreatic and hydrothermal breccias recognised. Here we consider ore-related breccias of a magmatic origin that mark the different stages of the magmatic sulphide ore formation in the Norilsk ore fields. Lyulko et al. (1972) subdivided the Norilsk ore-related breccias into: i) intrusion breccia at the top of intrusions, ii) magmatic breccia at the base of intrusions, iii) breccias of the front of emplacement. We follow this classification albeit revising some connotations. Intrusion breccia (known as “eruptive” breccias in Russian literature) occurs as lense-like bodies up to 30 m thick at the top of the Norilsk 1, Norilsk 2, Chernogorka, Talnakh and Mt. Pegmatitovaya intrusions. The intrusion breccias are composed of fragments of basalts and Tunguska argillites, sandstone and coals cemented by magmatic while heavily metasomatized matrix. The Tunguska coal in breccias that are emplaced into trap basalt over 1 km in stratigraphy above the Tunguska source rocks suggests significant lateral transport of the fragments for more than several kilometres. According to Lyulko et al. (1972), magmatic breccia is represented by a breccia-textured variety of taxitic gabbrodolerite that includes fragments of metamorphosed sedimentary rocks (hornfelses, anhydrite and carbonate marbles) and neighbouring igneous rocks metamorphosed into granular aggregates. Our XRF mapping and chemical data suggest that taxitic gabbrodolerite from both upper and lower marginal series should be considered as a true magmatic breccia where most clasts have been dissolved with a small proportion of semi-dissolved xenoliths present. Some varieties of picritic gabbrodolerite with fragments of granular olivine and clots of chromite likely belong to magmatic breccias as well. Fragments of leucogabbro and anorthosite among gabbrodolerite in the central parts of magmatic conduits also indicate intra-magmatic brecciation of the earliest cumulates and floats. The fragments of leucogabbro are sporadic in the differentiated series of the Norilsk-type orebearing intrusions, but plagioclase-rich clasts locally accumulate in thick clast-rich layers in the Morongo-type intrusions. As a whole, the diversity of the fragments indicates a pulsatile character of magmatic influxes with earlier cumulates underwent periodic fragmentation and further separation in the extended (tens of kilometres) plumbing systems. Frontal breccias are typical for the frontal aureole of the intrusions emplaced into evaporitic-haloic rocks within the Kharaelakh ore field. The igneous body there splits into smaller apophyses (tens of meters) and offshoots (up to half meter) surrounded by peperite-like rocks that are formed by spraying of low-viscosity magma into evaporites with a scale of mingling in the cm scale and smaller. These fluidal dolerite clasts are not, therefore, true fragments but frozen injections of liquid as evidenced by common quenched glass margins. The “peperitic” aureole was formed at a depth of at least 1 km so the ductile regime of brecciation was partly supported by elevated pressure and partly by melt depolymerization due to excess of halogens and other volatiles from evaporite. Brecciation is typically induced and accompanied by a release of volatiles that were derived from consumption of xenoliths by magma along the pathways. The bulk composition of an intrusion, therefore, cannot be used to infer a parental melt composition because of an undefinable degree of assimilation in the marginal series.