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NEW MEXICO

The Lost Dupont Mine

GEOLOGY OF THE AREA


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The San Pedro Mountains of north-central New Mexico consist of an uplifted core of ancient Precambrian basement rock bounded by Pennsylvanian and Permian sedimentary rocks on its northern, eastern, and southern edges. This wedge of Precambrian crystalline rocks and Paleozoic sediments abuts the younger Cretaceous and Tertiary sedimentary rocks of the San Juan Basin on its western margin. The north-south trending Nacimiento Fault forms the western boundary of the San Pedro Mountain block and indeed that of the Sierra Nacimiento further south. To the north and northwest, the uplifted wedge of Precambrian and Paleozoic rock is bounded by the Triassic Chinle Formation. Again, most of the contact is fault-bounded. To the east and southeast, the older Paleozoic rocks are overlain by the Quaternary-age Bandelier Tuff, a product of the Jemez volcanic event. The Bandelier Tuff consists of a layer of volcanic ash and glass with occasional agglomerates. The Precambrian core of the range

consists of gneisses with minor amounts of metamorphosed volcanics and sediments, all intruded by slightly younger granitic rocks. This exposure of ancient Precambrian basement rock has an outcrop area of nearly 80 square miles. Surrounding this core of Precambrian crystalline rock, and comprising most of the remainder of the San Pedro Mountains fault block, are Pennsylvanian and Permian limestones, sandstones, and shales. The limestones in particular would serve as excellent host rocks for mineralized veins or replacement deposits. Several small exposures of Tertiary-age rhyolitic tuff occur in the San Pedro Mountains. These older Tertiary-age volcanics probably hold greater promise for mineralization than the younger Quaternary volcanics that blanket much of the country east of the San Pedros. These Tertiary volcanics occur in the heart of the range and along a portion of its eastern flank.

PROSPECTING POTENTIAL

The San Pedro and Nacimiento mountain block is an uplifted dome-like structure consisting of a core of ancient Precambrian basement rock surrounded by fault-bounded wedges of late Paleozoic sedimentary rock. The surrounding sediments, especially the limestones, would serve as excellent host rocks for small ore-bearing deposits. The fault-bounded western slope of the San Pedros may also harbor some small metal-bearing veins. The Nacimiento Fault system would certainly provide an adequate conduit for mineral-rich fluids. Most accounts of the Lost Dupont Mine place it on the western flank of the range, near San Pedro Parks.


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Prospectors may want to concentrate on the rugged, heavily-wooded slopes of the mountains, extending from the town of Gallina southward to San Senorito Canyon. As mentioned previously, a number of small Tertiary rhyolites occur in the San Pedro Mountains. The contact between these volcanic plugs and the adjoining sedimentary rocks should certainly be investigated. In addition, the Precambrian basement rock itself may be host to certain gold-bearing deposits. Gold-bearing sulfide replacement bodies or gold-bearing quartz veins like those of the Hopewell District may also exist in the San Pedros.