Basalt – There are two layers of basalt and their thickness in total is about 130 feet thick at Goldfield. As described by Wilburn, there is a +/-1 meter bed of welded tuff between the layers of basalt, the Saddle Rock Tuff.
Whitetail Assemblage – Wilburn describes the Whitetail as lying unconformably on the granite up to a thickness of 500 feet. The upper Whitetail is approximately 250 feet thick, and contains more conglomerate than the lower Whitetail, varying from thin conglomerate beds in the sandstone to dense cobbles at the top. A large percentage of the cobbles are limestone and marble. The Whitetail on west side of the Prospect has been brecciated, and forms the large ridges of phreatic breccia on the caldera margin.
The lower portion of the Whitetail (approximately 250 feet thick) is nearly conglomerate-free. Snotnicki & Ferguson (Geologic Map of the Goldfield Quadrangle) label the lower portion of the Whitetail as Tertiary Sandstone(Ts). This arkosic sandstone is composed of fragments of eroded granite.
Pre – Cambrian Granite – This is the basement rock and is called the Ruin granite. It varies in texture from a medium equi-granular granite to that of pegmatite granite with large phenocrysts of feldspar approximately 2 inches in width. The granite is highly magnetic in a zone approximately 400 to 500 yards wide surrounding the caldera.
Some of the Ruin granite surrounding the Molly Marie and other calderas in the area has been altered to a Syenite with an amphibole matrix. It closely resembles that original Syenite found in Syene, Eqypt which also has large phenocrysts. One dike of this Syenite is found within the Molly Marie caldera.
The arkosic breccias on the west side of the Molly Marie Caldera post-date the granite, Whitetail assemblage, and basalt. Rhyolite dikes (and the volcanic neck) postdate the granite, Whitetail assemblage, and basalt.
Below is a geologic map of the claim group. This is a portion of the Skotnicki, S.J. and Ferguson, C.A., 1995, Geologic map of the Goldfield Quadrangle and the northern part of the Superstition Mountains. SW Quadrangle, Maricopa and Pinal Counties, Arizona. Arizona Geological Survey Open File Report, OFR-95-09, 2 map sheets, map scale 1:24,000, . As shown on the pages within, the large volcanic neck of Rhyolite porphyry was not identified or mapped on the east side of the claim group.
The area inside the claim group has dropped over 700 feet respective to the granite on the outside of the claim group.
Rhyolite and Rhyolite Porphyry
Short dikes of high-silica Rhyolite and Rhyolite Porphyry cut all formations described above.
Below is a link to the detailed geologic maps, cross-sections, and reports of the Goldfield quadrangle and surrounding area: