Brochantite and Other Minerals
From the Paoli, Oklahoma Area
By Joe Lobell
(Published in The Mineralogical Record, November-December,1986)
INTRODUCTION
The first and only documented occurrence of well-crystallized brochantite in Oklahoma was discovered in 1981 in northern Garvin County near the community of Paoli. The outcrop containing the brochantite is located along the extreme eastern edge of the NE 1/4 of Sec. 24, T4N, R1W, about 3 kilometers south-southeast of Paoli. In the vicinity are numerous other exposures of copper-bearing sandstones, but none has produced specimen-quality brochantite.
The deposit was mined for brochantite by the author in 1981 and has been worked intermittently since that time. Almost all of the brochantite was removed by trenching during the initial mining period.
The brochantite deposit and most of the other deposits described in this report are on private property owned by various area farmers and ranchers. The deposits are generally not open to collecting, although special permission can sometimes be obtained from the landowners. Several of the deposits outcrop on county road rights-of-way southeast and east of Paoli.
Solid circles indicate significant copper mineral occurrences; open circles = minor copper mineral occurrences; half-black circles = significant iron mineral outcrops; dotted circle = significant barite outcrops.
HISTORY
Early settlers in Oklahoma noticed greenish and bluish stains in the sandstone outcrops east and southeast of Paoli, and several small open-pit mines were opened for short periods. Merritt (1940) reported that a few tons of copper ore were shipped from the Teepee Queen Copper Company mine in Sec. 18, T4N, RlE, but no data on the quantity of copper produced is given.
At least two other small pits were dug in the area, but no records of their production are known.
In 1971, Teton Exploration Drilling Company, Inc., along with Wolf Ridge Minerals Company, acquired mineral leases for a prospect located in section 7 (Shockey, Renfro and Peterson, 1974). An open pit was dug and core samples obtained. Analyses of mineralization showed up to 4.1% copper and 239 ounces of silver per ton in core samples of one foot in length.
Analyses of the copper deposits in the region (Shead, 1929) indicated a copper content of up to 23%.
A similar deposit several kilometers southwest of Byars along the Garvin-McClain County line was mined for silver in 1897 and 1898 (Merritt, 1940). These efforts apparently were unprofitable.
The Byars deposit was reworked in 1913 through 1916, producing more than 8,000 ounces of silver during that period. Dunlop (1916) describes the deposit (known as the Criswell mine) as follows:
The ore deposits lie on the side of a canyon running north and south, and there is evidence that there was prospecting here before the present owner of the property purchased it. The ore shipped in 1914, amounting to 190 tons, was obtained from an open cut eight feet deep and was hand-sorted before shipment. This ore, which is a light-grey decomposed sandstone containing silver chloride and a small quantity of copper, has averaged about 33 to 37 ounces of silver a ton. The production from this mine is notable for the reason that it is the only mine in the Central States in which silver is the predominant value in the ore.
There has been no active metallic ore mining in the Paoli or Byars areas in recent years.
GEOLOGY
The host rocks for the copper and silver mineralization in the Paoli district are sandstones and a few shales belonging to the Garber and Wellington formations of Permian age (Dott, 1927), and are typical of the "red bed" formations of the southwestern United States. The rocks are frequently cross-bedded and fractured and are difficult to trace for extended distances at the surface (Merritt, 1940).
Malachite cements the sandstone in some outcrops while barite is the dominant cementing agent in others. The sandstone frequently contains abundant hematite and shows manganese oxide staining.
Photomicrograph of brochantite crystal on barite matrix, near Paoli, Garvin County.
Brochantite with cuprite (?), near Paoli, Garvin County.
Malachite-sand "skull," near Paoli, Garvin County.
Malachite with included sand, near Paoli, Garvin County.
Brochantite (dark green) with malachite, near Paoli, Garvin County.
Brochantite, malachite and chrysocolla, near Paoli, Garvin County.
Photomicrograph of goethite on calcite in claystone geode, crystals several mm in length, near Byars, along the Garvin-McClain county line near Byars.
Photomicrograph of goethite on calcite in claystone geode, crystals several mm in length, near Byars, along the Garvin-McClain county line near Byars.
Photomicrograph of goethite on calcite in claystone geode, crystals several mm in length, near Byars, along the Garvin-McClain county line near Byars.
Brochantite outcrop was discovered in the road cut at right. Washita River Valley is in the background.
Photomicrograph of brochantite crystals, near Paoli, Garvin County.
MINERALS
Brochantite
Brochantite occurs as druses of small (less than 1 mm) stubby emerald-green crystals only at the deposit in the NE 1/4 of section 24.
The brochantite was restricted to an intermittent vein varying in width from 25 to 50 cm and roughly 7 meters in length. The thickness of the brochantite layer at any given point was only a few millimeters, although several thin beds extended over a vertical distance of several centimeters.
Crystals were most abundant at a contact between an overhanging sandstone bed and a clay layer below. A few crystals of brochantite were found coating chalcocite nodules embedded within the clay layer.
The overhanging sandstone layer, which varied from 50 cm to 1 meter in thickness, contained numerous nodules of iron-bearing and copper-bearing minerals, increasing in frequency with depth. Near the bottom of the sandstone bed were long, thin stringers of a dark material primarily composed of chalcocite, with some cuprite. The brochantite was intimately associated with the chalcocite-bearing nodules and stringers.
Matrix coverage by the brochantite ranges from less than 10% on specimens obtained from the edge of the occurrence to greater than 90% on specimens from the center of the deposit. Matrix specimens of up to 15 by 45 cm have been removed.
The crystals of brochantite are occasionally intergrown to form a lustrous botryoidal surface.
Malachite
Malachite is widespread in the Paoli district as alterations of chalcocite nodules and as cement and intergrowths with sandstone.
Malachite from the region near the brochantite occurrence tends to be earthy with very little enclosed sand. In other areas it is found predominantly as cement and as a surface alteration of chalcocite nodules up to 10 cm in diameter. Sometimes the malachite cements sandstone concretions.
A sand-malachite shell resembling the copper "skulls" of Michigan in size and shape was found in section 7. The skull, approximately 5 by 10 by 15 cm in size, includes a slightly altered core of chalcocite and sand.
Azurite
Azurite is comparatively rare in the district, occurring sparsely as films or thin intermittent layers with malachite and sand.
Chalcanthite
A thin layer of crudely crystallized chalcanthite was found associated with brochantite on the exposed portion of the outcrop. The occurrence was restricted to a portion of the sandstone ledge shielded from contact with rainwater.
A similar chalcanthite occurrence is found beneath a sandstone ledge a few hundred meters east of the Byars silver-copper deposit.
Chrysocolla
Chrysocolla, light blue to bluish white in color, occurred sparingly along tiny fracture surfaces in the sandstone, associated with the richest portion of the brochantite deposit. It is probably an alteration product of the brochantite.
Barite
Barite is widespread in the area and assumes many forms.
In sections 18 and 19 southeast of Paoli, large barite-claystone concretions cover the floors of several gullies. The barite is crystalline, assumes several pale tints, but does not form distinct crystals. These concretions range up to 25cm in diameter.
Almost pure barite balls of a radiating crystal structure occur in scattered locations throughout the Paoli district. Known locally as "raisin rocks," these barite nodules are most concentrated where removed from enclosing clay by weathering. The nodules range from a few millimeters to several centimeters in diameter. A few exhibit yellow to orange fluorescence in concentric bands under shortwave ultraviolet radiation.
Elsewhere barite is a cementing agent of the sandstone, and occasionally forms the well-known "rose rocks" consisting of rounded, intergrown plates with much included sand, in the rough shape of a flower. The Paoli "roses" are poorly formed and paler in color than those found in Cleveland County to the north.
Bedded crystalline barite occurs in several layers embedded in clay a few kilometers north of Paoli in McClain County and at the Byars deposit. Barite also occurs at the Byars deposit as incomplete crystals or fibrous aggregates in claystone concretions and geodes.
Yellowish white parallel barite crystals to about 1 millimeter occur at the brochantite location in association with the contact between the sandstone and the clay beds.
Hematite
Hematite is common in the sandstone, sometimes to the extent that the hematite content is greater than the sand content. In section 18 near one of the pits is a large outcrop of black hematite-impregnated sandstone. A few specimens exhibit pseudomorphs, probably after pyrite, to several millimeters in size.
Hematite is also common at the Byars deposit as relatively pure, irregular fragments to several centimeters.
Silver minerals
Native silver was reported as the primary silver ore at the Wolf Ridge deposit (Shockey, Renfro and Peterson, 1974). The silver mineralization at Byars was reported to be silver chloride (Merritt, 1940).
The author found no visual evidence of silver mineralization at either locality.
Aragonite and calcite
Aragonite and calcite coat the interiors of claystone geodes at the Byars deposit. The geodes range in size to 6 cm; the aragonite and calcite crystals seldom exceed 2 mm in maximum dimension.
Goethite
Goethite (visual identification) occurs as thin, acicular blades to several millimeters in a few of the claystone geodes from the Byars deposit. The crystals are black with dark brownish red translucent edges.
Cuprite
Cuprite (visual identification) occurs as very small (less than 0.2 mm), brilliant, red, irregular crystals associated with barite and brochantite. It has also been found sparingly as the acicular variety (chalcotrichite), associated with malachite and disseminated within the chalcocite at one locality.
Pyrite
Crystals of pyrite partially altered to "limonite" occur in the sandstone at the brochantite locality. The crystals are cuboctahedral in form and range in size to 3 mm.
Unknowns
Microscopic examination of several of the brochantite specimens reveals a brilliant blue crystallized material associated with cuprite, brochantite and chalcocite; crystals are generally about 0.1 mm in maximum dimension. Also, a bluish green fibrous material was found on a few brochantite specimens. These may be copper sulfates.
ACKNOWLEDGMENTS
The author would like to thank Robert Fay, Oklahoma Geological Survey, for his assistance in identifying specimens and providing background information on the deposits; and Robert North and Robert Eveleth of the New Mexico Bureau of Mines and Mineral Resources for their assistance with manuscript preparation and specimen photography.
REFERENCES
- DOTT, R.H. (1927) Geology of Garvin County. Oklahoma Geological Survey Bulletin 40-K.
- DUNLOP, J.P. (1916) Mineral Resources of the United States, 1914, Part I - Metals (Oklahoma mineral production summary). U.S. Geological Survey, 114.
- MERRITT, C.A. (1940) Copper in the Red Beds of Oklahoma. Oklahoma Geological Survey, Mineral Report No. 8.
- SHEAD, A.C. (1929) Chemical analysis of Oklahoma raw materials. Oklahoma Geological Survey Bulletin 14.
- SHOCKEY, T. F., RENFRO, A. R., and PETERSON, R. J. (1974) Copper-silver solution fronts at Paoli, Oklahoma. Economic Geology, 69, 266-268.
