LA-ICP-MS analysis of single fluid inclusions in a quartz crystal (Madan ore district, Bulgaria)

Abstract

A “long-living” crystal of barren quartz from Kroushev Dol Pb-Zn deposit (Madan district, Rhodope Mountains, Bulgaria) was studied. The semitransparent base part (the “root”) of the crystal contains abundant inclusions, predominantly along healed cracks, while the upper half or third of the crystal is clear and poor in inclusions. In order to analyze fluid inclusions in the quartz crystal, it was cut into 4 pieces across and along the c-axis and doubly-polished sections were prepared. Fluid inclusions trapped in this quartz supply information about the temporal evolution of paleofluids depositing ore minerals. More than a hundred inclusions from different assemblages were observed microscopically; about half of them were selected for further investigation. Most of the inclusions show rather similar homogenization temperature T h = 333-348 °C, however, those in the peripheral (late) zones of the crystal reveal lower T h: 312-336. The total salinity is between 6.0 and 9.1 wt. % in most of the inclusions, reducing to 4.0-5.7 wt. % NaCl equivalent closer to the crystal walls. Initially, LA-ICP-MS signals for 30 elements were evaluated. Eleven elements, which concentrations were above the limit of detection, were used for more detailed interpretation. The molar ratios X/Na (X = K, Ca, Mg) show no significant changes during the crystal growth. A small increase in pH seems to be responsible for significant decrease of PbS and ZnS solubility. Thus, the fluids become oversaturated in these substances and galena and sphalerite precipitate. The molar ratios Cu/Na, Zn/Na and Pb/Na in the fluid inclusions from the early to the late fluid inclusions assemblages suggest a decrease in ore-metal concentrations in the fluids during the last stages of mineral deposition.