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

摘要

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. Inorder to analyze fluid inclusions in the quartz crystal, it was cut into 4 pieces across and along the c-axis anddoubly-polished sections were prepared. Fluid inclusions trapped in this quartz supply information about thetemporal evolution of paleofluids depositing ore minerals.More than a hundred inclusions from different assemblages were observed microscopically; about half of themwere selected for further investigation. Most of the inclusions show rather similar homogenization temperatureT_h = 333-348 ℃, however, those in the peripheral (late) zones of the crystal reveal lower T_h: 312-336. The totalsalinity is between 6.0 and 9.1 wt. % in most of the inclusions, reducing to 4.0-5.7 wt. % NaCl equivalent closer tothe crystal walls.Initially, LA-ICP-MS signals for 30 elements were evaluated. Eleven elements, which concentrations were abovethe limit of detection, were used for more detailed interpretation. The molar ratios X/Na (X = K, Ca, Mg) show nosignificant changes during the crystal growth. A small increase in pH seems to be responsible for significantdecrease of PbS and ZnS solubility. Thus, the fluids become oversaturated in these substances and galena andsphalerite precipitate.The molar ratios Cu/Na, Zn/Na and Pb/Na in the fluid inclusions from the early to the late fluid inclusionsassemblages suggest a decrease in ore-metal concentrations in the fluids during the last stages of mineraldeposition.