Phosphate nodules containing two distinct assemblages in the Cema granitic pegmatite, San Luis province, Argentina: Paragenesis, composition and significance

Abstract

Nodules of Fe–Mn phosphates are common in the outer core zone of the Cema pegmatite, San Luis Range, Argentina. Two different complex phosphates associations have been distinguished in these nodules. One of them is beusite–wyllieite–sicklerite-rich (BWS), with varulite and staněkite among the main products of replacement, Fe/(Fe + Mn) values between 0.46 and 0.48 for sicklerite, and MgO values up to 2.14 wt%. The other association is lithiophilite–sicklerite-rich (LS), with varulite and hureaulite among the main secondary products, lower Fe/(Fe + Mn) values, in the range 0.37–0.39 for lithiophilite, and lower MgO contents, invariably below 1 wt%. According to the mineral-chemistry data of these Fe–Mn phosphate nodules, a higher temperature of crystallization is proposed for the BWS association compared to the LS one. The Mn enrichment observed in these phosphates, in a pegmatite that is not highly fractionated, could be explained as a result of the early crystallization of other Fe–(Mg–Mn)-rich minerals, such as schorl, in the border and wall zones of this pegmatite. The crystallization of this silicate would have strongly depleted the activity of Fe in the remaining pegmatite-forming melt.