Paymaster Mining District, Chocolate Mountains, Imperial County, California, USAi

Regional Level Types
Paymaster Mining District	Mining District
Chocolate Mountains	Mountain Range
Imperial County	County
California	State
USA	Country

This district was a Ag-Mn-Cu mining area located in the central Chocolate Mountains between Quartz Peak and Midway Well.

Data extracted from: Hadley, Jarvis Bardwell (1942):

Location & History: The Paymaster manganese district is in the northeast corner of Imperial County, California, 6 miles west of the Colorado River and about halfway between Blythe, Calif., and Yuma, Arizona. The manganese deposits are in sections 16, 18, and 19, T11S, R21E, SBM. By road they are 42 miles from Blythe and 54 miles from Yuma. Paved road extends 17 miles S from Blythe, and 10 miles NW from Yuma. The remainder is graded earth road, with the exception of 6 miles of unimproved road extending from a point on the Blythe-Yuma road 2+ miles N of Midway Well to the district. Manganese deposits were discovered in the western part of the district in 1916 and 1917, and were worked intensively during 1917 and 1918. Production records from published sources show that manganese ore produced in Imperial County amounted to 1,907 tons valued at $38,140 (period values) in 1917, and 1,241 tons valued at $46,900 (period values) in 1918. Shipments from the Paymaster district account for most of this. trlct account for most of this. Hewett reports that 3,500 tons of 42-percent ore were shipped. Shipments ceased late in 1918, and very little work has been done since. The properties have recently been consolidated under the ownership of Mr. V. B. Whedon and associates of Beverly Hills, California. No work was being done in February and March 1941, but 15 tons of high-grade ore was mined by lessees in 1940. Small deposits occur on what are known as the Turtle claims, in the eastern part of the district, but no ore is known to have been shipped from them.

Geology: The rocks of the Paymaster manganese district consist of a basement complex of schist, gneiss, and intrusive rocks, but these are overlain by volcanic rocks, which in turn are overlain by 1,000 feet or more of coarse fanglomerate. These rocks have been broken by many normal faults, at least "three of which have displacements of several" hundred feet.

Others, of smaller displacement, contain the manganese deposits. Slightly consolidated boulder-bearing gravel, younger than theore deposits, is widespread south and southeast of the area mapped, and extends into the area as fill in old erosion channels in the fanglomerate.

Rocks: The basement complex consists of intensely folded schist and gneiss cut by numerous dikes of fine-grained granite and by dikes and sills of pegmatite. These rocks make up the southern part of the range bordering the district on the W and appear also at the E side of the district.

Dark reddish-gray andesite, 600 to 1,000 feet thick, rests with pronounced angular unconformity on the basement rocks in the western part of the district. Much of the andesite is flow breccia, and all of it is intensely fractured.

Coarse, indistinctly bedded, red fanglomerate occupies most of the district. It forms steep, irregular slopes, and massive rounded summits distinct from the jagged peaks and ridges typical of the region. The fanglomerate is composed largely of subangular fragments from a few inches to as much as 3 feet in diameter, with a minimum of coarse, pebbly matrix. Volcanic fragments predominate, and many are of the same type as the volcanic rocks now exposed. Other fragments, derived from the basement complex, are also abundant. Lenses of coarse, crossbedded sandstone, a few inches to a foot thick, are common throughout the formation but amount to less than 1 percent of the whole. The fanglomerate is well indurated and stands well in the mine workings.

The fanglomerate lies on an irregular eroded surface of the andesite. The thickness of the fanglomerate in the district is at least 1,000 feet and may be considerably greater. Its age, like that of the andesite, is not definitely known; both are probably Tertiary. On the southern and southeastern borders of the area mapped, the eroded surface of the fanglomerate and older rocks is largely buried under slightly consolidated, bouldery gravel. The gravel forms smooth conical hills as much as 100 feet high, among which isolated outcrops of the more resistant fanglomerate appear. Similar gravel is found in old erosion channels in the vicinity of the Tolbard mine, where it is seen to be younger than the manganese veins. Still younger material, unconsolidated, forms alluvial fans and the bottoms of the major valleys in the district.

Structures: There are three principal faults, all of them normal, and numerous subsidiary faults in the district. Two parallel major faults, trending north-northwest, cut off the fanglomerate on the west. The more westerly of these dips 50°-60° E. Along it, the fanglomerate and underlying volcanics have been thrown down at least 500 feet, bringing the fanglomerate locally against the basement complex. To the south, rocks of the complex lie on both sides of this fault, which is marked by coarse fault breccia from 15 to 50 feet wide. The stratigraphic displacement along the eastern fault is at least 100 feet at the north, and increases southward to at least 300 feet at Black Hill. The third major fault trends approximately at right angles to the other two and dips 50° SSE. It cuts off the basement complex on the south and has a displacement of several hundred feet.

The fanglomerate and volcanics are broken by hundreds of minor faults, many of which are subsidiary to the major, normal faults. The minor faults strike northeastward and dip 60° or more either northwest or southeast. On most of them the displacement cannot be determined, but probably only a few have displacements exceeding a few tens of feet. Most of them are too small to appear on the geologic map, but many are shown on the detailed map. The more persistent fissures contain several inches to a foot of fault breccia. Many of them, moreover, have been opened as a result of tension for considerable distances along their length, and contain pebbles and sand fallen from the walls. The impression was gained that the faulting occurred at rather shallow depth, possibly before the fanglomerate became as well indurated as it is now. Many of the minor fissures might be expected therefore to die out at no great distance below the present surface.

Manganese Veins: Extent and structure: The manganese deposits are veins and stringers in minor fault fissures and associated fractures in both fanglomerate and andesite. The major faults are not mineralized, and no manganese deposits were found in the basement complex. The veins are most abundant in an area of about one-fourth square mile in the western part of the district, and in a much smaller area at the Turtle claims in the unmapped eastern part. The western veins are largely in fanglomerate the principal eastern veins are in andesite. Individual veins generally range in width from 1 inch to 2+ feet, although at a few places veins of greater width were seen. Only a few veins extend more than two or three hundred feet but zones of branching or overlapping fissures, in part mineralized, can be traced 2,500 feet. A good example is the Tres Amigos vein. Narrow, parallel veins 5 to 15 feet apart are common, but are usually not spaced closely enough to permit the mining of several veins as a single ore body.

In the fanglomerate, veins that are nearly vertical have fairly well defined walls. Veins dipping about 60° commonly have well-defined footwalls and gradational hanging walls, the latter of which are considerably mineralized. Post-ore faulting has taken place on nearly all the larger veins and follows closely the preexisting fault fissures. The fault surfaces are smooth and locally striated, more or less parallel with the dip of the fault. They are surfaces of easy separation, and have been extensively used by the miners in
breaking the ore.

Mineralogy and Types of Ore: The ore minerals are, in order of abundance: Psllomelane (manganese oxide with barium, potassium, and water), pyrolusite (MnOg), and manganite (MiigO, with water). Coarse-grained white calcite, finely divided carbonates, and chalcedony are the principal gangue minerals. The manganese oxides fill open spaces in fault fissures, impregnate and replace fault breccia, and impregnate and replace the fissure walls. Open-space fillings are composed principally of psilomelane and subordinate manganite in the form of mammillary layers 1/8 inch to 2 or 3 inches thick. Some of the psilomelane layers are porous and contain coarse-grained, white calcite. This type of psilomelane is distributed in the veins in well-defined lenses, which range from 1 to 12 inches in width and from a few feet to 50 feet or more in length. Such lenses are common, and where wide or numerous enough they constitute good ore. Many of them are too narrow to be mined singly.

Much of the psilomelane, especially that adjacent to post-ore faults, is brecciated and cemented with pulverized oxides and carbonates, and commonly contains more or less rock material. Lenses of "psilomelane-breccia", many feet long, contain a few percent to 25 percent of psilomelane fragments an inch or more in diameter, mixed with rubble. Most of the manganese deposits consist of mineralized fault breccia, which contains a considerable amount of rock material calcite. Much of the mineralized fault breccia from veins in the fanglomerate resembles the "pudding ore" of manganese deposits in fanglomerate in New Mexico. In the andesite, fragments in mineralized fault breccia are more completely replaced, and soft manganese oxides are generally more abundant than psilomelane.

The walls of fissures in the fanglomerate are commonly impregnated with manganese oxides, but generally the amount of manganese is small. In andesite; however, the fissure walls are more thoroughly fractured and contain many thin seams of manganite and pyrolusite. In the main working on the southeast vein on Black Hill, a fissure zone 12 feet wide has been extensively replaced by manganese oxides.

Origin: Good evidence of the downward migration of manganese is found in the Paymaster district in the form of stalagmitic crusts of psilomelane which coat vein walls and irregular surfaces within the veins. Such surfaces are commonly covered with vertical, upward-pointing cones of psilomelane as much as 2 inches high, and 1 inch thick. The upper surfaces of these cones are smooth and the lower surfaces are rough and botryoidal. They were formed presumably by solutions moving downward over surfaces exposed to the air at a late stage in the history of the veins.

Apparently the veins have been enriched by supergene processes. Although no definitely hypogene manganese mineral or other vein mineral was found, supergene enrichment is characteristic of this type of deposit. The presence of hypogene black calcite, as well as white calcite, bearing manganese and iron in similar deposits in the Little Florida Mountains suggests that such material may also have been the original material in the Paymaster district. A lower limit to the manganese-oxide ore is therefore to be expected not far below the surface, although there is no evidence at present to indicate at what depth.

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