Borate deposits in the Muddy Mountains include the Anniversary mine in Lovell Wash 25 miles east of Las Vegas and deposits in White Basin 35 miles northeast of Las Vegas. Borate in both areas is in colemanite beds in the upper part of the Horse Spring Formation, a widespread 11-16 Ma nonmarine sedimentary unit in southern Nevada and northwestern Arizona. Although these two areas contain the largest known reserves of borate in Nevada, present activity consists only of minor exploration and assessment work. Cessation of colemanite mining in California, and subsequent reliance on Turkey for colemanite used in textile fiberglass, may make the Muddy Mountain deposits more attractive in the future.
Although ulexite occurs in minor amounts in White Basin, and was reported from the Anniversary mine, borate in the Muddy Mountains is predominantly colemanite. Unlike colemanite/probertite/ulexite deposits in Death Valley, California, which reach thicknesses of 300 feet, Muddy Mountain colemanite deposits are relatively thin (up to 22 feet at the Anniversary mine). Grade of the Muddy Mountain deposits is 20 to 26 percent B203, which compares favorably with 11 to 31 percent B203 in Death Valley deposits.
The Anniversary mine was discovered in 1921 and produced colemanite until 1928. Total production has been estimated at 200,000 tons averaging 20 percent B203. The ore consists of interbedded borate, marl, and limestone with minor ruff and gypsum. Fine-grained to coarsely crystalline colemanite occurs in lens-shaped pods and in beds that pinch to thicknesses of less than an inch and swell to more than a foot. The ore is exposed along strike for 3,000 feet and dips moderately south in the north limb of an east-west syncline. Drilling downdip in the 1970s indicated a reserve of 3.5 million tons of ore averaging 26 per cent B203.
The borate unit is in a limestone sequence characterized by abundant finely laminated algal stromatolites. Features indicating mud-flat deposition (fossil animal tracks and ripple marks) occur in some limestone horizons. Analyses of 30 samples collected from Lovell Wash indicate that magnesium, strontium, and lithium contents of limestone increase as the borate beds are approached stratigraphically. Celestite occurs in mar] and algal limestone within and just above the borate beds, and montmorillonite is relatively abundant in marl interbedded with colemanite.
Colemanite was discovered in White Basin in 1920 and mined until 1924. Old borate diggings occur in a 2-square-mile area in White Basin, but activity was concentrated in an area of less than 100 acres in the western part of the district.
Two borate units occur in White Basin. The upper unit consists of up to 11 feet of colemanite, marl, and algal limestone. The lower unit is similar, but only 2 feet thick. Associated marl is calcitic to dolomitic with montmorillonite and local celestite. The colemanite is fine-grained to coarsely crystalline and generally occurs as nodules with radial internal structure. Locally, nodules are cut by colemanite veins up to 3 inches thick. Borate beds in White Basin dip gently, and the resource is generally closer to the surface than at the Anniversary mine. However, White Basin is cut by normal faults that segmented the borate into several different structural blocks, and much of the resource was subsequently removed by erosion. Although it is only half as thick, a composite White Basin section correlates well with the Lovell Wash section. Colemanite beds occupy the same stratigraphic position in both sections. Based on stratigraphic correlation, hypotheses regarding the origin of the Muddy Mountain borate deposits must take into account apparently synchronous deposition of similar borate beds 12 miles apart (unless there has been unrecognized lateral movement along north-south faults in the Muddy Mountains). The paucity of sandy detritus in Muddy Mountains borate beds indicates deposition during low runoff, and relatively high clay content in interbed marl suggests that volcanic ash falls were a significant depositional component. Borate deposition probably took place in a shallow perennial or ephemeral lake.
---Stephen B. Castor, Industrial-Minerals Geologist