Hydrothermal Mobilization of High Field Strength Elements in Alkaline Igneous Systems: Evidence from the Tamazeght Complex (Morocco)

摘要

The Tamazeght alkaline complex (High Atlas Mountains of Morocco) is a middle Eocene volcanic to plu-tonic 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 alteration. These rocks also contain the highest concentrations of Zr in the complex, although these concentrations are not suf-ficient to form exploitable reserves. High field strength element (HFSE) minerals (predominantly the zir-conosilicates) commonly occur as secondary phases that are richer in Ca and F (cancrinite (Na_6Ca(CO-_3))(AlSiO_4)_6 centre dot 2H_2O), calcic catapleiite ((Ca,Na)ZrSi_3O_9 centre dot 2H_2O) rinkite ((Ca,Ce)_4Na(Na, Ca)_2Ti(Si_2O_7)_2Ti(Si_2O_7)_2F_2(0,F)_2). Furthermore, they partly or completely replace primary HFSE phases (e.g.. zircon eudialyte (Na_4(Ca,Ce)_2(Fe~(2+),Mn,Y)ZrSi_8O_(22)(OH,Cl)_2)). Altered nepheline crystals adjacent to these replaced minerals host fluid inclusions that, based on textural evidence, are interpreted to have formed during alteration. The inclusions have high salinity (25 wt percent NaCl equiv), are Ca rich. and were trapped at ~300 deg C. They contain several solids in addition to a liquid phase and a vapor bubble. One of the most common daughter minerals is a Ca-La-Ce phase (parisite?). Other daughter or trapped minerals include zircon, a Ti(+-Nb) silicate or oxide (rutile?), an Mn hydroxide(?), and less commonly; sphalerite and galena. The occurrence of zircon, paristie, and rutile are evidence for mobilization of Zr and other HFSE during hydrothermal alteration.Based on data available for the solubility of ZrO_2 in H_2O-HF solutions and estimates for the concentration of Zr in the fluids at the Tamazeght complex (possibly as high as ~0.05 m), we suggest that HFSE transport occurred by mixed metal OH-F complexing. This transport requires a fluid with an elevated F concentration and a temperature of +-300 deg C. In addition, it can only take place in the near absence of Ca. We propose that a resid-ual F-rich brine evolving from crystallization of pegmatites at <500 deg C leached and transported HFSE. Upon interaction with surrounding limestone, the fluid cooled rapidly and gained Ca. This caused deposition of flu-orite (fluorite is very sparingly soluble), which buffered a_F to low levels in the fluid causing precipitation of the HFSE in the form of secondary minerals. Because the hydrothermal event was short-lived and relatively re-stricted in space HFSE were not concentrated to ore grades.