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The Lost Arch Mine


The Turtle Mountains are geologically diverse. The southern half of the Turtle Mountains consists of an uplifted core of older Precambrian igneous and metamorphic rocks. The northern portion is overlain by Tertiary volcanic rocks (mostly basalt) intruded by small plugs of hypabyssal Tertiary rhyolite. Some small exposures of Precambrian granite occur in the northern half of the Turtle Mountains. These granites are found in the rugged, broken country just north and east of Mohawk Spring and are host to several mines and prospect pits. Although the southern half of the Turtle Mountains is comprised mostly of Precambrian igneous and metamorphic basement rock, a small exposure of Mesozoic granite occurs just to the north of Horn Peak.

The range is cut by several fault zones that trend northwest-southeast. The great majority of mines in the Turtle Mountains are hosted in Precambrian igneous and metamorphic rock.

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This is also true in the Whipple Mountains, located just east of the Turtles. Indeed, the two ranges are geologically identical. Both consist of older Precambrian basement rock and younger Tertiary volcanics cut by northwest-southeast trending fault zones. The western portion of the Whipple Mountains is highly mineralized and contains many mines and prospect pits. Again, the great majority of mines in both ranges occur within the older Precambrian country rock. The fault zones are the result of the classic block-faulting that is so characteristic of the Basin and Range Province of North America. The contact between Precambrian basement rock and the younger Tertiary volcanics is sometimes fault-bounded.


The southern half of the Turtle Mountains is underlain by the same Precambrian basement rock as that of the Whipple Mountains, which lie only 15 miles to the east. The southern edge of the Turtle Mountains does show mineralization but not to the extent of the Whipple Mountains. Nevertheless, the area below Horn Spring probably deserves a detailed search with a metal-detector. Prospectors

may also want to concentrate on the rugged canyons near the Tertiary rhyolite plugs in the northern portion of the range. A natural arch is rumored to exist somewhere northeast of Martin's Well, near the center of the range. A search of the arroyos, canyon floors, and gravels in this area may also be productive.