DISTINCTION BETWEEN MAGMATIC AND HYDROTHE RMAL SIGNATURES IN APATITESFROM NORTH PORTUGAL - CATHODOLUMINESCENCE, SEM AND ELECTRON-MICROPROBE STUDY

Scanning electron microprobe (SEM), electron probe micro analysis (EPM A) and cathodoluminescence (CL) were performed for the study of fluora patite crystals from samples collected in northwestern Portugal: in sy n- to late-tectonic Hercynian granites, in one Sn-bearing aplite-pegma tite vein, and in two Sb-Au bearing quartz veins. Two types of CL spec tra are distinguished: (1) apatite with a strong yellow luminescence e xhibits a Mn broad band spectrum at 575 nm: (2) apatite with a weak pi nk-yellow to pale violet luminescence shows a spectrum with narrow pea ks characteristic of rare earth elements (REE). EPMA reveal two differ ent chemical signatures: apatites characterized by high grades in Mn ( MnO less-than-or-equal-to 7,3%), Fe and Sr, and by the presence of La and Ce, corresponding to type 1 spectra; apatites with very low Mn and Fe concentrations, weak Sr-concentrations, higher Y and Si contents, and the presence of Nd, Sm and Gd. Some peaks of type 2 spectra are du e to Dy, Eu, Sm, Tb and Gd.So, the study of apatite by CL optical obse rvation and by CL spectrometry results in the distinction of differenc es that were not detectable by the use of optical microscopy or by SEM , and these data were a useful guide for microprobe analysis. The corr elation between CL and chemical composition is a cheap and efficient t ool for identification of apatite geochemical signature. That tool cou ld be improved by a better knowledge of trace elements activators, esp ecially REE, by mean of synthetic doped crystals. Much less than Magma tic much greater than signature is mainly characterized by high concen trations of Mn, Fe and Sr in apatite of type 1, and much less than hyd rothermal much greater than signature by high contents in Y and Si in apatite of type 2. The apatites from Ribeiro da Igreja Sb-Au bearing q uartz vein show the transition between these two signatures. These res ults support the hypothesis of a genetic relation between Sb-Au quartz veins, Sn-W bearing veins, and a tardi-plutonic hydrothermal activity .