Nickel-rich chromian muscovite from the Indus suture ophiolite, NW Pakistan: Implications for emerald genesis and exploration

Abstract

Ubiquitous veins and stockworks of quartz traverse the ophiolitic emerald-hosting, carbonate-altered ultramafic rocks in the Swat Valley. Some of the emerald-bearing quartz veins contain chromian muscovite and tourmaline. In addition, veins and clusters consisting of chromian muscovite and/or tourmaline occur in zones of carbonate-altered rocks where the quartz veins are most abundant. The chromian muscovite is characterized by high Mg/Fe ratios (4-9) and contains variable and in some cases anomalously high concentration of Ni (ranging up to 9 wt% NiO). A detailed investigation reveals that the Ni and Mg entered the chromian muscovite structure as a part of a complex coupled substitution: (Fe^VI, Mn^VI, Mg^VI, Ni^VI)²⁺ + [Si^IV]⁴⁺ ↔ (Al^VI, Cr^VI)³⁺ + [Al^IV]³⁺. The stable coexistence of quartz, chromian muscovite, tourmaline and emerald suggests that all these phases are cogenetic and precipitated from Si-rich, Al-, Be-, B- and K-bearing fluids related to a single episode of hydrothermal activity. The Mg, Cr and Ni contents in chromian muscovite were most probably extracted by the percolating hydrothermal solutions from the host carbonate-altered ultramafic rocks through wall rock reaction. The observed high variability in the Mg, Cr and Ni contents of chromian muscovite probably reflects low mobility of these elements during the hydrothermal process or a result of local equilibrium under relatively low T conditions.