Fluid inclusions in sphalerite as negative crystals: a case study

The crystal morphology of vacuoles of fluid inclusions in some transparent, low-iron and relatively low-temperature sphalerites from the vein and replacement Pb-Zn ore deposits of the Madan ore district, Bulgaria, was studied both by optical microscopy in oriented thin sections and by SEM on open cleavage surfaces. The vacuoles represent negative crystals bounded by faces of d{110}, of {111} and -o{(1) over bar 11) forms together with small faces of a {100}, e{210}, n{211}, p{221}, -n{(2) over bar 11} and -p{(2) over bar 21} forms. Their corners and edges are rounded. The primary inclusions, formed mainly at 200-220 degreesC, have an isometric, elongated, tubular, spindle-like or acute-angled habit depending on their position and the influence of the growth defects (growth zones, subgrain and sector boundaries, twin planes, etc.). The secondary fluid inclusions, healing fractures in sphalerite crystals mainly at 160-185 degreesC, are strongly controlled by the thickness and local configuration of the crack surfaces. They are flat, irregular or isometric in shape. It is suggested that the fluid inclusions in the low-T sphalerite usually preserve their initial high-energy and non-equilibrium crystal morphology formed during periods of rapid growth.