Genesis of metastable marcasite phase and associated pyrite in the Proterozoic lamprophyre dyke, Nellore Schist Belt, Southern India

Marcasite is a metastable phase in iron-sulphide solution.If, preserved, it can be used as a significant indicator phase to understand the physio-chemical condition of its formation and associated sulphide system within ore deposits.The present study reports marcasite within the Proterozoic lamprophyre dyke, intruded into the amphibolite of Nellore Schist Belt, Southern India.Marcasite commonly occurs in association with pyrite, chalcopyrite and pyrrhotite in different textural associations.It shows development along the rim and fracture as an alteration of pyrrhotite at places forming characteristic bird eye structure.Pyrite forms later than marcasite, typically occurring along the boundary between marcasite and pyrrhotite.The formations of both marcasites followed by pyrite in the lamprophyre, suggest >l00 C and <240 C temperature, at low pH during the replacement of pyrrhotite through dissolutionreprecipitation-process under the influence of sulphur-rich hydrothermal solutions.However, the presence of both phases in different textural associations, reveals that the formation conditions of both the Fe sulphide phases i.e. pyrite and marcasite polymorphs are of neutral to alkaline solutions and have an influence on both hydrothermal as well as alkaline parental rock signatures.