MICROTEXTURAL CHARACTERISTICS OF BLADED CALCITE AND QUARTZ PSEUDOMORPHS IN EPITHERMAL VEINS: IMPLICATIONS TO THE PROCESSES OF ORE MINERAL FORMATION IN SHALLOW HYDROTHERMAL SYSTEMS

Bladed calcite is common in low-sulfidation epithermal veins and is thought to form as a result of gentle boiling of the thermal liquids. Bladed calcite is typically only present over a restricted vertical interval of a few hundred meters within epithermal veins. Using optical microscopy, the microtextures of bladed calcite and pseudobladed quartz formed by replacement of the calcite has been studied in representative samples from nine epithermal deposits to characterize textural variations and to test whether the presence of bladed calcite or pseudobladed quartz can be used as a predictor for ore grade.

Calcite crystals exhibit splintery, bladed, or platy shapes. These blades are approximately 0.025‒0.5 mm in widths and can range up to several centimeters in length. Pseudobladed quartz shows similar dimensions to the bladed calcite. The pseudobladed quartz consists of parallel to subparallel bands of mosaic to blocky quartz that formed as a result of replacement. The bladed calcite and pseudobladed quartz form lattice, acicular, and parallel aggregates depending on the orientation of the blades and pseudoblades. The degree of replacement of calcite by quartz is variable.

In the samples investigated, the occurrence of bladed calcite and pseudobladed quartz correlates with the presence of ore minerals. Electrum, acanthite, naumannite, chalcopyrite, and pyrite occur as disseminated grains within the open spaces between the blades and pseudoblades or are located within the calcite and quartz. This is consistent with previous models suggesting that gentle boiling of the thermal liquids represents an effective mechanism for metal precipitation. However, many bonanza grade samples lack bladed calcite or pseudobladed quartz and ore minerals occur in distinct colloform bands containing relic silica microspheres. These colloform bands are interpreted to have formed as a result of vigorous boiling or flashing. The observations of this study suggest that the intensity of phase separation represents a key control on vein texture and ore grade.