Sisir's research statement

Research Statement: Sisir K. Mondal 

All of my current research activities are under the broader subject area of ore geology of igneous systems with special emphasis on petrology and geochemistry of magmatic ore deposits such as chromite, platinum-group elements (PGE) and Ni-Cu-sulfide mineralization. Additional aspect of this research is to understand early crustal evolution and mantle heterogeneity of Earth. Supervision of students for M.Sc. dissertation and PhD thesis are significant component of my research activities. In addition, my research projects involve direct interaction and collaboration with the geological survey, mining and exploration groups.

1. Geochemical evolution of mafic layered intrusions and origin of the world-class chromite, Ni-sulfide plus PGE deposits

  • Large mafic layered intrusions commonly host world-class platinum group element (PGE) deposits such as the Merensky Reef of the Bushveld Complex of South Africa. However, massive chromitite, either in layered intrusions or from ophiolitic complexes are always found to be enriched in PGE concentrations than the host rocks and one of which, such as the UG2 chromitite layer of the Bushveld Complex in South Africa hosts largest Pt-Pd resource in the world. Although better known PGE deposits have been well studied, there is no consensus with respect to their origin and the mechanism of PGE concentration in chromitite is also not clear. In this context, petrogenetic study of the massive chromitites and their host rocks are important to understand the fundamental process involved in chromitite formation as well as chromitite-hosted PGE ore genesis.
  • In the Bushveld Complex, it has been shown that the rocks enriched in the platinum-group elements (e.g., Merensky Reef and UG2 chromitite) are also those that show evidence of metasomatism. Based on petrologic and geochemical studies Mondal and Mathez (2007, Journal of Petrology 48, 495-510) interpreted that the evolved petrological characters of the UG2 footwall pyroxenite is either due to metasomatism by infiltration of an evolved, interstitial melt during the waning stages of crystallization or in situ crystallization of the trapped interstitial melt generated from the parental magma of the footwall pyroxenite. The evidences bear the relevance to the evolution of partially molten rocks in a large magma chamber. In the partially molten rocks interstitial melts that derived out due to compaction may become enriched in incompatible elements (including water) and thus become reactive with the crystal assemblages through which they pass. Therefore, it is necessary to understand the post-accumulation history of the rocks in which the PGE deposits are contained.

2. Origin and geochemical significance of mineral inclusions in chromite

  • Chromite in massive chromitites or in PGE mineralized reef often contains mineral inclusions which are highly variable in mineralogy. The basic consideration for interpretation of these inclusions is that they have trapped at magmatic stage and evolved in a closed system. However, in layered intrusions it has been shown that the post accumulation chemical and textural re-equilibration of the cumulus crystals is vital in understanding the evolution of the complex. In this context, the study of mineral inclusions in chromite with petrologic studies of the host rocks is important to further our knowledge on evolution of the ultramafic-mafic complex.
3. Osmium (Os) isotopic and PGE geochemical constraints on mantle heterogeneity
  • Geochemistry of noble metals (Os, Ir, Rh, Ru, Pt, Pd and Re) provides unique clues to the early origins of our planet. How noble metals are distributed within the Earth is the subject of intense debate. The fundamental differentiation of the Earth was into a Fe-Ni-rich metallic core and a surrounding silicate mantle. During this process, which is thought from isotopic evidence to have occurred soon after accretion, the siderophile elements (6 PGEs and Re) were preferentially partitioned into the core-forming metal, leaving the mantle depleted in these elements relative to their original solar or chondritic abundances. However, if the Earth’s mantle was in equilibrium with its core, the mantle would contain three orders of magnitude less of the noble metals than observed. The most common explanation put forward to account for this disparity has been that the last 1% of the Earth’s accretion occurred after the iron-rich core had separated from the mantle (late veneer hypothesis). If noble metals in the mantle are derived from the accretion of a late veneer, then the upper mantle should have noble metal abundances and ratios that reflect the composition of the late-accreted material. Or, may be in a well-mixed upper mantle, material from the ‘late veneer’ would have to combine with, pre-existing, PGE-poor mantle depleted by core formation. Another idea, that outer core material can be transported back into the mantle as trace elements in plumes rooted at the core-mantle boundary has received a great deal of attention. The study of mantle-derived rocks (e.g., mantle xenoliths) or the melts (e.g., basalts or high-Mg komatiitic magma) in terms of their noble metal geochemistry sheds light on these topics. The advantage of using the Os isotope system is that this system develops large parent-daughter contrasts among magma sources and it helps to identify effect of any secondary events over primary magmatic events.
  • Study of volcanic and plutonic rocks of greenstone belts provides great insight in understanding the formation and stabilization of cratons and evolution of crust and mantle. Dismembered ultramafic bodies with deformed chromitite depositions are important component in Archaean greenstone belts. Experimental work on komatiitic rocks and geochemical studies of high-Mg low-Ti siliceous volcanic rocks in greenstone belts suggest that some komatiites and komatiitic basalts of greenstone belts formed via hydrous mantle melting processes in subduction zone settings during the Archean. In this context, it is important to understand the geochemical evolution of the ultramafic fragments including the chromitites within the Archean greenstone belts and their bearing in crustal evolution particularly in terms of material addition and material recycling in early history of the Earth.