CATHODOLUMINESCENE ZONATION IN HYDROTHERMAL VEIN-QUARTZ LINKED TO ENRICHEMENT IN ALUMINIUM

This study has focused on a series of quartz ± carbonate ± base metal mineralized veins from Cornwall, southwest England. These extensional veins are characterized by multiple phases of incremental quartz growth and, broadly, there are two major quartz textures present. The dominant texture is of prismatic, growth-zoned crystals (<2cm in length) containing abundant fluid inclusions. The second texture consists of very fine-grained, cloudy quartz, sometimes with an unusual lath-like morphology, which may be related to micro-fracture controlled quartz growth or recrystallization. Thin bands of chalcedony are present in some samples and in one sample, spherulites of cryptocrystalline silica can be seen. Analysis of fluid inclusions -dominantly from the prismatic quartz crystals- indicate that these veins were formed from saline (27 wt % NaCl + CaCl₂), low temperature (<130°C) fluids and precipitated at shallow levels in the crust (<500m). There is no fluid inclusion evidence of boiling in these samples.

Previous Fourier Transform Infra-Red studies of these quartz veins have suggested that the prismatic quartz contains appreciable quantities of Al and Li balanced hydroxyl groups as structural defects in the crystal lattice. To quantify Al concentrations, a trace element study was carried out using a Jeol 8900 electron microprobe equipped with a cathodoluminescence (CL) detector. The vein-quartz, in particular the fine-grained quartz crystals, have a strong light-to-dark CL zonation on a very fine scale (<20 microns). The luminosity variations, which in some cases appear to be growth banding, directly correlate with Al content. CL-light bands have Al contents ranging from <50 ppm to 200 ppm, whereas CL-dark bands have between 1000 ppm to over 4500 ppm. These data suggest that rapid and/or small-scale changes in fluid chemistry or quartz growth can be identified by CL imaging, and that careful electron microprobe analysis may be able to elucidate these changes.