The McCullough Mountains of southern Clark County, Nevada are part of the vast Basin and Range Province of North America. Like most of the mountain chains in the Basin and Range Province, the McCullough Mountains are fairly long but relatively narrow by comparison. They extend roughly 40 miles from the site of Crescent north-northeastward to Henderson. In no place is the range more than 8 miles wide. The McCullough Mountains are bounded by Eldorado Valley to the east and Ivanpah Valley to the west. The range is cut by McCullough Pass which is located near the middle of the chain.
The McCullough Mountains consist of two basic rock "provinces" of vastly different age, composition, and origin. One consists of ancient crystalline rock while the other is made up of much younger volcanic rocks. The northern portion of the McCulloughs consists of Tertiary basalt and andesite flows with minor andesite breccias while the southern portion of the range consists of a core of uplifted Precambrian metamorphic and igneous rocks. The Precambrian basement rock is mostly gneiss, schist, and older granite (1.7 billion years old) intruded by a younger, porphyritic rapakivi granite (1.45 billion years old). The rapakivi granite was formed during North America's first great rifting event.
The central portion of the McCulloughs is cut by a large, bifurcating fault that trends roughly north-south. The fault extends for nearly 13 miles from Hidden Valley south to McCullough Mountain. A number of smaller, high-angle faults radiate southward from Hidden Valley, but these extend no more than a mile or two. The northeastern flank of the range is cut by a fairly extensive fault zone which trends northeast-southwest and consists of numerous fault blocks. Faulting is very complex along the extreme northeastern fringe of the McCullough Mountains.
The majority of the ore bodies in southern Clark County are associated with or emplaced within ancient Precambrian crystalline rock. In the Searchlight District, the ores are found in Precambrian gneiss near the contact with a younger Tertiary quartz monzonite. In the Crescent District, the precious and base metal deposits are all hosted in Precambrian gneiss and granite. The same is true for the Sunset (or Lyons) District.
The McCullough Mountains have been prospected heavily for over two centuries. It seems unlikely that any large gold deposits have been overlooked. And yet, the rich silver lodes on Crescent Peak were not discovered until 1904. Crescent Peak had been prospected by Mexican miners during the 1860's and by the Spaniards a century before that. All of them had missed the silver veins.
Prospectors may want to focus on the southern half of the McCulloughs in their search for the lost Mormon mine. The Precambrian basement rocks exposed here certainly hold some promise for the
prospector based on the fact that the nearby Crescent Peak District contains gold-bearing veins in Precambrian gneiss and granite. Similarly, the Searchlight District (located about 10 miles east of the Crescent Mine) also contains gold-bearing quartz veins in Precambrian country rock.
Prospectors may want to concentrate on the fault zones near the middle of the range. A small vein or fracture filling of gold-bearing quartz may lie hidden in some offshoot of the main vein.