ARIZONA

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The Superstition Mountains are part of the Central Highlands physiographic province, a transition zone between the stable platform deposits of the Colorado Plateau to the north and the desert mountains of the Basin and Range Province to the south. The northwest-southeast trending belt of mountains and valleys that comprise the Central Highlands Province has undergone several episodes of folding, faulting, and uplift. Uplift has been quite dramatic. Indeed, so dramatic that many mountain ranges in the Central Highlands Province consist of an exposed core of ancient Precambrian basement rock that has been pushed up to the surface. (The overlying mantle of Paleozoic and Mesozoic sedimentary rock was completely stripped away by erosion.) This tectonically-active belt witnessed some incredibly violent eruptions during the mid-Tertiary, about 20 to 30 million years ago. This mid-Tertiary volcanism produced the Superstition Mountains.

The Superstition Mountains consist almost entirely of silicic dacite lava flows and welded ash-flow tuffs. Silicic lavas traditionally produce the most violent eruptions. The volcanic complex that became the Superstition Mountains was no exception. Vast amounts of silicic lavas and pyroclastics were emitted during several pulses of volcanism during the mid-Tertiary. A number of calderas have been identified, the largest being the Superstition Caldera. This collapsed crater has an outcrop area of about 100 square miles and is overlapped by at least three other smaller calderas. Many lost mine tales involving native gold in rose quartz are told of the mountains in this part of Arizona. The nearest gold deposits to the Superstition volcanic field occur in the Goldfield District, located just northwest of the range. These gold-bearing veins are hosted in Precambrian granites which comprise most of the Usery Mountains and part of the Goldfield Mountains. Precambrian basement rock also crops out in a 1 to 2 mile thick belt along the southern margin of the Superstition Caldera. This belt or band of Precambrian crystalline rocks marks the southern boundary of our area of interest.

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Canyon. Prospectors may want to limit their search to areas overlain by non-pyroclastic rocks. Mineral deposits are rarely associated with pyroclastic volcanic rocks. Prospectors may want to search for erosional "windows" in the overlying dacites through which older mineralized rock is exposed.