CRYSTAL MORPHOLOGY OF QUARTZ, CALCITE, PYRITE AND RUTILE: POTENTIAL TOOL FOR EMERALD EXPLORATION IN THE HIDDENITE AREA (NORTH CAROLINA)

The Hiddenite district, located in the Inner Piedmont Belt of western North Carolina, contains an intricate network of Alpine-type quartz veins that sometime host gem-quality beryl (variety emerald) and spodumene (variety hiddenite). Both gem minerals are found in cavities typically 1 to 10 cm wide and are associated with quartz, feldspars, carbonates and sulfides. Historically, the search for hiddenite and emerald in the Hiddenite area has largely been carried out using non-systematic approaches (e.g., random prospecting of plowed fields, wooded areas and stream beds). Recent studies of emerald- and hiddenite-bearing quartz veins from the North American Emerald Mine and the Adams properties have revealed that the crystal morphology of quartz, calcite, pyrite and rutile (QCPR) may serve as potential exploration guides for the discovery of hidden deposits.

Emerald-bearing veins are characterized by: (1) quartz with multiple generations that include fine-grained doubly terminated crystals and very coarse-grained prismatic crystals, (2) calcite with largely rhombohedral habit, often accompanied by Fe-rich carbonates (dolomite and siderite), (3) pyrite crystals dominated by octahedral faces and (4) rutile that varies from single untwined crystals to highly reticulated aggregates. On the other hand, cavities which host hiddenite generally contain “Tessin”-habit quartz or sceptered amethystine quartz, calcite with bihexagonal prism faces, pyrite with cubic morphology and simple untwined rutile crystals.

In addition to distinct differences in the crystal morphology of QCPR, associated carbonate and sulfide mineral assemblages provide additional support for the discrimination of the two gem-bearing quartz vein types.