THE FORMATION AND ALTERATION OF TURQUOISE DEPOSITS IN THE AMERICAN SOUTH WEST

Turquoise is a semiprecious gemstone that has been mined and coveted world wide throughout history. This mineral is prized for its striking colour and has spiritual meaning to people of different cultures. However, the genesis of turquoise deposits is not well understood and controversial. There are two main hypotheses: (1) hydrothermal precipitation where hydrothermal fluids rise along fractures through the copper-rich porphyritic host rock, and (2) supergene enrichment where meteoric water travels downwards through fractures in the host rock, leaching the elements and precipitating turquoise. Here we present tectonic and geochemical data from south western turquoise deposits that support the supergene enrichment model. In this study turquoise deposits from Nevada, California, Arizona and New Mexico were mapped. Turquoise deposits are closely associated with copper porphyry intrusive bodies that are occasionally capped by volcanic or folded sedimentary rocks. These cap rocks occasionally host turquoise however it is of poor quality and quantity. Turquoise deposits in northern Nevada occur at the intersection of faults oriented primarily at Az 000° ± 20° and Az 090° ± 30°. Whereas, turquoise deposits from California, Arizona, southern Nevada and New Mexico occur at the intersection of faults having slightly different orientations (Az 120° ± 20°> and Az 050° ± 10°). Rocks that host turquoise deposits are mechanically brecciaed and turquoise generally occurs along fractures. Turquoise at or near the surface typically alters to hydroxide and clay minerals such as gibbsite and pyrophyllite. The intense brecciation caused increased permeability, allowing meteoric water to percolate downwards precipitating turquoise at discreet horizons no more than a few thousand feet below the surface. Although turquoise formed from descending meteoric waters, it is also altered by meteoric waters because it requires a discreet set of physiochemical conditions (eg. fO₂, pH) and forms at or just above the paleo-water table. A petrographic study of the Blue Bell deposit, NM provides detailed information on the formation of turquoise deposits.