During the Miocene (mid-Tertiary), extensive brine lakes covered most of southern Arizona. The Colorado River basin had not formed yet and the water pooled in what is now Arizona in deep rift zones (below diagram). This resulted in salt deposits 10,000 feet thick at Luke AFB near Phoenix, and the largest gypsum deposit in the world which is found at nearby Picacho, AZ.
Occurring concurrently with the brine lakes was the active period of the Superstition volcanic field, which was comprised of many rhyolitic volcanoes, calderas, and fissure eruptions. Some of the lakes engulfed a portion of the Superstition volcanic field, and eroded several of the volcanoes to their bases.
The brine from the lakes leached metals from volcanic debris and coarse arkosic sandstone totaling up to 2500 feet thick, once found above and near the Molly Marie Prospect. The metal-laden water boiled off above the magma chamber, above which the Molly Marie Prospect is situated.
The boiling water destroyed phreatic arkosic breccias that are in abundance above the magma chamber, and the metals precipitated as sulfides replacing the breccias.
The magma chamber is indicated by a collapse caldera over one mile in diameter, and within and on its perimeter are found abundant phreatic arkosic breccias, gossan breccias, silica replacement, copper and gold mineralization, tufa deposits, intense propylitic alteration, and ring dikes.
Near the center of the caldera is a rhyolitic porphyry stock complex, the neck of a strata volcano that has been eroded to its base. Surrounding the stock are many diatremes and gossan breccias with copper and gold mineralization.
Banded Jasper, Jaspillite