Tetrahedrite mineral chemistry and metal zoning; a thermodynamic assessment from the Rajpura-Dariba polymetallic deposit, India

Abstract

The polymetallic ore deposit at Rajpura-Dariba, comprising a metamorphosed synsedimentary banded ore facies and a younger unmetamorphosed pegmatoidal vein ore facies of minor intensity, displays a conspicuous base metal zoning in the banded ore from the footwall to the hanging wall in the classic sequence of Cu-(Pb,Zn)- (Zn,Fe) that, notationally, registers the fluid flow direction. Tetrahedrite is of widespread occurrence. Samples of the banded ore, collected systematically from footwall to hanging wall at various mine levels, register extensive substitution of Ag for Cu, Zn for Fe, and As for Sb in the mineral. In contrast, tetrahedrite from the vein ores exhibits much restricted compositional variability; in particular its silver content is too low to allow any correlation of variation in the Ag and Sb contents. In general, the footwall samples have high amounts of As and low amounts of Ag, hanging-wall samples have low amounts of As and high amounts of Ag, whereas samples from the interveining region record intermediate values of Ag and As. These features confirm the existence and preservation of a footwall to hanging-wall tetrahedrite compositional zoning which indicates that the fluid flow direction is consistent with that shown by the base metal zonation trend. Anomalous samples whose spatial positions do not match those prescribed by their Ag and As contents are interpreted to be the result of metamorphic reequilibration and/or localized interaction with younger vein ore fluid which partially modified the pristine chemical signature.The tetrahedrite zoning patterns compare favorably with those derived from curves of constant Ag(Cu) (sub -1) and As(Sb) (sub -1) exchange potentials (mu (sub Ag(Cu) (sub -1) ) and mu (sub As(Sb) (sub -1) ) , respectively) passing through the appropriate compositional coordinates in terms of As/As + Sb (X ₃ ) and Ag/Ag + Cu (X ₄ ). Most of the footwall samples plot on higher (mu (sub As(Sb) (sub -1) ) and lower mu (sub Ag(Cu) (sub -1) ) curves of constant exchange potentials. From footwall (proximal) to hanging wall (distal), through the intermediate zone samples, there is an initial decreasing trend in mu (sub As(Sb) (sub -1) ) and an increasing trend in mu (sub Ag(Cu) (sub -1) ) values. The anomalous samples constitute exceptions to this pattern.The study indicates that in Rajpura-Dariba a relict tetrahedrite compositional zoning trend persists, parallels the base metal zonation trend, and is only partially modified due to the later metamorphism and/or vein-forming episodes.