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Mineralogical Survey of the Mount General Area, Barstow CA

Last Updated: 18th Sep 2009

Mineralogical Survey of the Mount General Area, Barstow CA
Robert M. Housley
[email protected]

As part of a continuing program to document notable mineral occurrences in Southern California a group of Southern California Friends of Mineralogy volunteers including Walter Margerum, Garth Bricker, Paul Malone, Jennifer Rohl, Bob Reynolds, and myself spent some time during 2001 surveying the Mount General area, which is located about 6 miles northwest of Barstow CA, or about 5 miles north of the Lenwood exit off of I-15. The most up-to-date and complete reference to the mines and geology of this region is given Bezore and Shumway (1994).

Geographically Mount General is also located about 5 miles west of Mount Waterman and about 11 miles west of the famous Calico mining district. Although it contains no large or famous mines it was probably initially prospected at about the same time as these regions and is covered with numerous prospect pits and small workings. The history is hard to come by, but one mine, the Pedry, is briefly discussed in “Mines and Minerals of San Bernardino County” (Wright et al, 1953) and “Geology of the Barstow Quadrangle” Bowen (1954). Bezore and Shumway (1994) have assigned numbers to many of the prospects, which we will also use for consistency, for example referring to the Pedry as location BS-28.

The mountain itself seems to consist largely of volcanic rocks, considered by Bezore and Shumway (1994) to be of Miocene age. In many places these volcanics contain short discontinuous veins of barite with silica and limonite, ranging in thickness from a fraction of an inch up to about three feet. A short adit on the lower northwest slope explores a two foot thick barite, silica, limonite vein. Two shafts and an adit in the southern part of the mountain also explore brecciated barite. Nothing of mineralogical interest was found at these locations.

A very short adit on the western edge of the mountain explores a green tuff layer.

Walter Margerum sparked our current interest in the Mount General area when he stopped to have a look at the Pedry Mine and came up with some nice micro specimens of dipyramidal orange wulfenite associated with malachite and chrysocolla from the north shaft.

The Pedry Mine itself consists of six patented claims that encompass a brecciated quartz vein about one mile long extending northwestward from the north side of Mount General. This vein seems to form the backbone of a very low ridge. Other gangue minerals in the vein are principally barite, calcite, and limonite. The ore minerals mentioned by Wright include the sulfides galena, sphalerite, pyrite, and chalcopyrite, along with anglesite and cerussite.

The main workings of the Pedry consist of two vertical shafts near opposite ends of the vein and series of shallow pits and trenches along its length. The north shaft is about 200 feet deep and has short drifts at several levels. With further exploration here we found nice micros of pale blue hemimorphite and more of the prismatic wulfenite with malachite and chrysocolla. In other samples the wulfenite was on a gray coating in turn on brown earthy material. Under the SEM the earthy material proved to consist of sub-micrometer grains of plumbojarosite and the gray druzy material to be mimetite/pyromorphite solid solution. Dipyramidal wulfenite with pyromorphite and plumbojarosite are illustrated in the SEM images below.

Wulfenite with pyromorphite FOV 140 micrometers

Plumbojarosite FOV 14 micrometers

Left. Scanning backscattered electron image of Pedry Mine wulfenite. Smaller poorly formed crystals are mimetite/pyromorphite. The field of view is 140 micrometers.

Right. Poorly crystallized Pedry Mine plumbojarosite. Field of view 14 micrometers.

The Pedry Mine was under lease beginning in 1949 with work concentrated on the southern shaft, listed by Wright (1953) as being 270 feet deep in early 1952 with short drifts at approximately 50 foot intervals and a 40 foot crosscut to a parallel vein at 100 feet. A shipment of concentrates later in 1952 returned 26% lead and almost 9 ounces per ton of silver. At that time a head frame, hoist, and loading bin were in place (Bowen, 1954). No further history has been found. This south shaft is now filled to the surface with trash. Much of the original dump has been bulldozed. In the remains of the dump here we found crystals of cerussite and barite to about 5 millimeters, and more earthy plumbojarosite.

On the east side of Mount General about a mile southeast of the Pedry a small prospect pit on a low white hill, first called to our attention by Professor David R. Jessey of California Polytechnic University, Pomona, yielded nice barite crystals up to about one inch on a matrix of greenish druzy quartz. Good specimens can be obtained from the surface along a vein exposed for about 150 feet and in the tailings from a couple of small pits. The barite generally shows zoning and the flat faces are frequently coated with druzy quartz. In some cases the barite has been totally replaced with quartz producing attractive pseudomorphs. There is also galena inside some massive barite chunks at this site.

Moving on about two miles further southeast another unnamed working (the western part of BS-34) first called to our attention by Al Ordway has yielded quite a lot of nice vanadinite and descloizite as micros. Actually Bezore and Shumway (1994) also noted vanadinite at this site. Since we were unable to find a recorded name for this location we have been calling it the AO Prospect. This is in an area of low rolling hills and the exposed rocks here seem to be largely metamorphic, although there is an exposure of light colored volcanic material, possibly a tuff, in direct contact with the marble immediately south of the working. Bezore and Shumway (1994) correlate some of the exposed metamorphics with the Waterman gneiss. Seams of dolomitic marble sometimes containing graphite flakes to several millimeters are found in the vicinity of many of the prospects. In some cases mineralization seems to follow the marble/gneiss contacts.

The AO Prospect itself appears to have been initially explored by a shallow adit, although subsequent work with a bulldozer and backhoe make it hard to tell much about the early workings. A fairly large west facing excavation below the adit level now exposes the main vanadinite/descloizite area. These minerals mainly occur in dark porous silica below the adit and in both walls, but particularly in the south wall. The mineralized zone is about 4 feet thick with a total depth below the original surface of probably about 30 feet.

The mineralized silica is described as a mylonitic gneiss by Bezore and Shumway (1994). The original adit appears to have followed a weakly mineralized, nearly vertical east west shear zone. In the trench a nearly horizontal brecciated zone just above the mineralization is evident. It appears to me that the cavities later mineralized resulted from the dissolution of a carbonate component near the contact. No sulfides have been observed, but the silica contains numerous iron hydroxide/silica remains. No cerussite or anglesite has been observed either. All the lead has been converted to vanadate minerals.

This vanadinite varies in aspect from stubby prisms to long needles and in color from white or yellow to black through various gray and bluish shades. Oddly it is never orange or red. The descloizite is largely druzy yellow or greenish yellow rosettes. Especially attractive are vanadinite crystals on balls of yellow descloizite. Also very nice are specimens of vanadinite on clear druzy quartz and druzy quartz pseudomorphs after hemimorphite. Images of typical descloizite as well as less common chlorargyrite are shown below.

Descloizite FOV 350 micrometers

Chlorargyrite FOV 70 micrometers


Left. Backscattered electron image of descloizite from the AO Prospect. Field of view 350 micrometers.

Right. Bromian chlorargyrite from the AO Prospect dump. Field of view 70 micrometers.

The vanadate mineralization here appears to have repeated in more than one cycle. Specimens showing descloizite on fresh vanadinite in turn deposited on descloizite pseudomorphs after vanadinite are common. Copper minerals are almost absent from the main pit and no mottramite has been found here. All zinc here has also been incorporated in the vanadates, but silica shells attest to the former presence of hemimorphite.

Apparently the material removed in generating the pit where these minerals occur was shipped to a smelter since no vanadinite has been found on the dump. However some nice specimens of descloizite on calcite can be found on the dump. Occasional pieces on the dump contain chrysocolla along with mottramite, bromian chlorargyrite, and rhombic pseudomorphs, probably after a carbonate. Nice specimens of mottramite on calcite have also been found along the trench to the east along with possible chalcophanite. A bromian chlorargyrite crystal is shown above. A single specimen from the dump yielded fresh hemimorphite and a trace of cerussite.

About a quarter of a mile further east still another small working (BS-38) yielded mottramite, possibly after vanadinite, on calcite and celestine with malachite. The overprint of exploration during the 1950s is also evident, as at the AO Prospect, in these more eastern localities. The original dumps and surrounding areas have been bulldozed and backhoed. In a wash south of this locality bulldozer trenches apparently unrelated to earlier work expose some copper stained silica that contains malachite and mottramite, and traces of barite.

Generally since the workings in The Mount General area are small and the dumps are correspondingly small, the mineral collecting potential here appears limited even though the variety and quality of the minerals themselves are good. However, we have collectively probably only explored about half the accessible workings so far.

The contrast in mineralization of these areas, only separated by about 4 miles, seems to be worth noting. The Pedry is in a massive quartz vein with lots of barite, which also is abundant in the mountain itself. There is no vanadium in the secondary minerals. Around sites BS-34 and BS-38 the mineralized gneiss contains no quartz veins and very little barite. All the lead present has been converted to vanadates!

Without the encouragement of Bob Reynolds this would not have been written. In addition the field and library research of Walter Margerum has contributed greatly and helpful comments were made by Jennifer Rohl and Paul Malone.

References

S. P. Bezore and D. O. Shumway (1994) Mineral Land Classification of a part of Southwestern San Bernardino County: The Barstow-Victorville Area, California. CDMG OFR 94-04.

O. E. Bowen, Jr. (1954) Geology of the Barstow Quadrangle, California. CDMG Bulletin 165.

L. A. Wright, R. M Stewart, T. E. Gay, and G. C. Hazenbush (1953) Mines and Mineral Deposits of San Bernardino County, California. Annual Report of the State Mineralogist 49.






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