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The Lost Mine of Bronco Canyon


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The Agua Fria River and its tributaries, including Bronco Creek, drain the central portion of the uplifted belt of rugged mountains and mesas that comprise the Central Highlands Province. The Central Highlands physiographic province forms 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. This tectonically-active belt has undergone several episodes of folding, faulting, and uplift. Uplift has been quite dramatic. In fact,

so dramatic that the thick blanket of Paleozoic and Mesozoic sedimentary rocks that once draped the ancient Precambrian core has been completely stripped away by erosion. Consequently, many mountain ranges in the Central Highlands Province consist of an exposed core of Precambrian igneous and metamorphic rocks. The Bradshaw Mountains, which rise up west of Black Mesa, are such a range. The Bradshaws consist almost entirely of Precambrian schist intruded by slightly younger Precambrian granitic rocks. East of the Bradshaws, in the Bronco Canyon country, an extensive field of Tertiary-Quaternary basalt covers much of the area. These basalt flows rest unconformably on deeply eroded Precambrian basement rock. They overlie Black Mesa and much of the rugged country east of the Agua Fria River. About 8 miles east of Black Mesa and the Agua Fria River, ancient Precambrian basement rock is again exposed. This belt of Precambrian crystalline rock trends roughly north-south and is about 3 to 4 miles wide. These ancient schists and granites occasionally harbor mines and mineralized veins.


Much of the Bronco Canyon country is overlain by Tertiary-Quaternary basalt flows. These younger basalts are almost invariably devoid of mineral deposits. On the other hand, the same Precambrian basement rock that forms the bulk of the Bradshaw Mountains extends southeastward beneath the younger basalt flows of the Bronco Canyon area and then crops out again in a narrow belt about 8 miles east of Black Mesa. Prospectors should probably focus on these Precambrian rocks

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rather than the Tertiary-Quaternary basalts that overlie them in this part of Arizona. It is possible that a small "window" of mineralized Precambrian rock is exposed in some deep canyon where the overlying basalt has been eroded away.