Hydrothermal mobilization of high field strength elements in alkaline igneous systems: Evidence from the Tamazeght complex (Morocco)

The Tamazeght alkaline complex (High Atlas Mountains of Morocco) is a middl e Eocene volcanic to plutonic mass, ranging from silica-saturated to silica -undersaturated rocks and carbonatites. Some of the most evolved units are nepheline syenites and pegmatites, which show evidence of hydrothermal alte ration. These rocks also contain the highest concentrations of Zr in the co mplex, although these concentrations are not sufficient to form exploitable reserves. High field strength element (HFSE) minerals (predominantly the z irconosilicates) commonly occur as secondary phases that are richer in Ca a nd F (cancrinite Na6Ca(Co-3)) (AlSiO4)(6). 2H(2)O), calcic catapleiite ((Ca ,Na)ZrSi3O9. 2H(2)O), rinkite ((Ca,Ce)(4)Na(Na,Ca)(2)Ti(Si2O7)(2)F-2(0,F)(2 )) Furthermore, they partly or completely replace primary HFSE phases (e.g. , zircon, eudialyte (Na-4(Ca,Ce)(2)(Fe2+,Mn,Y)ZrSi8O22(OH,Cl)(2))) Altered nepheline crystals adjacent to these replaced minerals host fluid inclusion s that. based on textural evidence, are interpreted to have formed during a lteration. The inclusions have high salinity (25 wt % NaCl equiv), are Ca r ich, and were trapped at similar to 300 degrees C. They contain several sol ids in addition to a liquid phase and a vapor bubble. One of the most commo n daughter minerals is a Ca-La-Ce phase (parisite?). Other daughter or trap ped minerals include zircon, a Ti (+/-Nb) silicate or oxide (rutile?), an M n hydroxide(?), and less commonly: sphalerite and galena. The occurrence of zircon, parisite, and rutile are evidence for mobilization of Zr and other HFSE during hydrothermal alteration. Based on data available for the solubility of ZrO2 in H2O-HF solutions and estimates for the concentration of Zr in the fluids at the Tamazeght comple x (possibly as high as similar to 0.05 m), we suggest that HFSE transport o ccurred by mixed metal OH-F complexing. This transport requires a fluid wit h an elevated F concentration and a temperature of +/-300 degrees C. In add ition, it can only take place in the near absence of Ca. We propose that a residual F-rich brine evolving from crystallization of pegmatites at <500 d egrees C leached and transported HFSE. Upon interaction with surrounding li mestone, the fluid cooled rapidly and gained Ca. This caused deposition of fluorite (fluorite is very sparingly soluble), which buffered a(F) to low l evels in the fluid causing precipitation of the HFSE in the form of seconda ry minerals. Because the hydrothermal event was short-lived and relatively re stricted in space, HFSE were not concentrated to ore grades.