Phosphate mineral associations in the Cema pegmatite (San Luis Province, Argentina) paragenesis , chemistry and significance in the pegmatite evolution

Résumé

Fe-Mn phosphates nodules are common in the border core unit of the Cema pegmatite (San Luis range, Argentina). Two different complex phosphates associations have been distinguished in these nodules. One of them is lithiophilite-sicklerite-rich (LS), with varulite and hureaulite as the main secondary products, Fe/(Fe+Mn) values in the range 0.37-0.39, and MgO contents always below 1 wt%. The other association is sicklerite-beusite-qingheiite-rich (SBQ), with varulite and joosteite as the main replacement products, higher Fe/(Fe+Mn) values, between 0.46-0.48, and also higher MgO values, up to 2.14 wt%. According to the mineralogical and chemical data, a higher temperature of crystallization is proposed for the SBQ association in relation to the LS one. On the other hand, the important Mn enrichment observed in these phosphates, in a pegmatite that is not extremely fractionated, could be explained as a result of the previous crystallization of other Fe-(Mg-Mn)-rich minerals, such as almandine and schorl, in the border and external zones of this pegmatite. The crystallization of these silicates would have strongly depleted the activity of Fe in the remaining pegmatite melt.