THE GENESIS OF TURQUOISE IN THE SOUTHWESTERN UNITED STATES

Turquoise from ancient Mesoamerica (and SW USA/northern Mexico) has been mined for over a thousand years; beginning with Aztecs and Mayans, followed by the Native Americans and pioneers near the turn of the 20th century, and is still being mined today. However, the formation of this highly sought after gem mineral has remained controversial since its discovery. Two hypotheses have been proposed: (1) the hydrothermal or hypogene hypothesis suggests that turquoise was formed from hot fluids flowing upwards from depth; and (2) supergene hypothesis suggests that turquoise formed from cool meteoric (rain) water flowing downwards through the rocks that host the gem mineral. Here we present a new model for the formation of turquoise.

Turquoise mines in the Cerrillos area, New Mexico are classic examples of turquoise deposits throughout the Southwestern United States. These are generally located within 100 meters of the surface and are associated with the most altered areas of the porphyritic quartz monzonite plutons. These plutons have several distinctive zones of alteration which include propylitic and argillic zones. Samples were collected from each zone to better understand the fluid-rock interaction. Based on detailed petrography, electron microprobe, and x-ray diffraction data, these deposits formed shortly after the emplacement of these plutons. Hydrothermal convection of the ground water interacting with the plutons leached P, Ca, Cu, Al, and Fe. These fluids moved to shallower depths depositing turquoise along fractures. The data also show that supergene processes are slowly altering the turquoise deposits of the Cerrillos area and eventually gem quality turquoise from the Southwestern United States will become more rare.