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NEVADA

The Lost Diamond Mine of the McCullough Mountains

GEOLOGY OF THE AREA

The McCullough Mountains of southern Nevada are part of the vast Basin and Range Province of North America. Like most of the mountain chains in this province, the McCulloughs are fairly long, relatively narrow, and trend roughly north-south. The range stretches some 40 miles from historic Crescent Peak in the south to Henderson and Boulder City in the north.

The McCullough Mountains are comprised of two basic rock "provinces", ancient Precambrian crystalline rock and much younger Tertiary-age volcanics. The northern half of the range consists of Tertiary andesite and basalt flows with minor andesite breccias while the southern half is comprised of a core of uplifted Precambrian metamorphic and igneous rocks. This Precambrian basement rock is mostly gneiss, schist, and older granite (1.7 billion years old), but a small, slightly younger granitic intrusion occurs in the area. This younger intrusion exhibits striking rapakivi texture and was emplaced 250 million years after the older granite.

Faulting occurs along the northeastern edge of the range and in the central portion of the range. The central cluster consists of seven faults that apparently radiate from the Hidden Valley area. All except one are 2 to 3 miles long. The longest fault runs lengthwise through the center of the range. It is nearly 13 miles long and bifurcates just north of McCullough Pass. The northeastern fringe of the range consists of a series of highly faulted wedges of volcanic rock. Faulting is particularly complex near Dutchman Pass.

Mineralization in southern Clark County is many times associated with or emplaced within ancient Precambrian basement rock. In the Searchlight District, the ore bodies are found in Precambrian gneiss near the contact with a Tertiary quartz monzonite intrusion. In the Crescent District, the precious and base-metal deposits are emplaced in Precambrian gneiss and granite. The same is true in the Sunset District.

PROSPECTING POTENTIAL

The McCullough Mountains are literally surrounded by mining districts. The Alunite District lies at the northern end of the range, about 4 miles west of Boulder City. The historic Crescent District lies at the opposite end of the range, just north of the California/Nevada state line. The rich ore bodies of the Searchlight District are located just 15 miles east of the southern end of the McCulloughs while the mines near Goodsprings are only about 16 miles west of the range.

Although the McCullough Mountains have been heavily prospected, it is entirely possible that an experienced gold-seeker would overlook a small kimberlite plug. Except for the bluish-gray color of the weathered kimberlite, the outcrop itself would be quite unremarkable and nondescript. Many times the outcrop will be nothing more than a small, eroded depression covered by decomposed and rotten fragments of kimberlite mixed with bluish clay.

Prospectors must confine their search to kimberlites as they are the only known source of diamonds in the world today. Kimberlites are vertical, carrot-shaped bodies of ultramafic rock derived from the melting of extremely deep-seated rocks. Indeed, when one looks at a kimberlite, they are looking at a sample of the lower crust and upper mantle. Kimberlites are the volcanic equivalent of a peridotite. They are composed of olivine, phlogopite, pyrope garnet, pyroxene, picotite, melilite, and ilmenite. Sometimes they contain fragments of partially melted country rock. Rarely they contain diamonds.