The Apex Mine

LOCATION AND SETTING

The San Carlos replacement deposit of argentiferous galena lies in desert country in east-central Chihuahua, in the Municipio of Manuel Benavides, about 20 air-kilometers southwest of the international border at me Rio Grande and 60 km southeast of the point where the Conchos River joins the Rio Grande at me twin border towns of Ojinaga (Chihuahua) and Presidio (Texas). Because the orebody lies "like a saddle blanket" (Hewitt, 1970) across the top of an anticlinal ridge rising 300 meters above the surrounding plain, the lead mine which once operated there was called the Apex mine; however, in print sources as well as in collectors' casual discourse, it is much more frequently referred to as the San Carlos mine, after the nearby village of San Carlos.

Both along the ridge and below it, the much-faulted limestone terrain is very rough: a steep-walled barranca (ravine), raging with floodwater after rains, runs generally southeast from a precipitous drop-off near the mine site. Outcrops of magnetite skarn appear over a distance of more than 1 km along the barranca's walls, but lead and zinc sulfides are visible only at the northwestern end, near the mine. The village of San Carlos lies about 5 km from the base of the ridge, but otherwise the region today is largely uninhabited, quite as it was (except for mines) when the mine was last active, between ca. 1930 and 1952.

HISTORY

Newberry (1883) and Stetefeldt (1889) noted that by the late 19th century three shafts had already been sunk (by unspecified entities) along a line crossing the San Carlos orebody, all three lying within a lateral distance of about 100 meters. The Purissima shaft, 20 meters deep, had penetrated about a meter of ore; the other two shafts, the San Cosme and Nicolas, had encountered the ore pods at depths of 13 and 20 meters respectively (Newberry, 1883). At the same time a few opencuts and at least two more shafts existed as well, and one of the shafts, the Valencia, penetrated a large outcropping ore mass to a depth of about 7 meters. Newberry (1883) enthused that "the quantity of ore on the San Carlos property is without parallel among all the argentiferous [galena] deposits in the country."

The report of Stetefeldt (1889), while considerably less celebratory, argued nevertheless for a program of large-scale, mostly opencut mining and offered detailed suggestions for the building of infrastructure, securing of labor and transporting of ore. Stetefeldt (1889) noted that three loads of ore-about 20,000 pounds altogether-had recently been shipped by the (unnamed) Mexican mine managers, to be treated by lixiviation at the Ontario mill at Park City, Utah. The argentiferous galena in these three lots had assayed 5.5, 2.1 and 2.1 ounces of silver per ton, and some "selected specimens" had assayed as high as 11 ounces of silver per ton and 25% lead.

Despite these promising early signs, no large-scale mining of the San Carlos deposit appears to have taken place before the early 1930s, when the United Verde Extension Company undertook commercial development. A few years later the Apex mine was being worked by a subsidiary of the American Smelting and Refining Company (ASARCO), whose activities at the mine lasted until 1952. During this short period the deposit was exploited by opencut excavations in an area about 500 meters long (along a northwest axis) by 300 meters maximum width, to a depth of 3 to 15 meters, producing about 1 million tons of lead ore (Hewitt, 1970). Since 1952 the site has stood abandoned, visited only (and that very infrequently) by mineral collectors searching for vanadinite and wulfenite crystal specimens.

In 2003, about 400 acres of the abandoned mine property were acquired in a government lottery by Top Gem Minerals of Tucson, a wholesale specimen dealership (Mike New, personal communication, 2007); however, the site still lies idle as of 2008.

COLLECTING HISTORY

The fame of the Apex mine among mineral collectors rests on two species, vanadinite and wulfenite, which have been found as beautifully crystallized specimens. The natural assumption is that these lead species (as well as the mediocre San Carlos mimetite specimens which have occasionally appeared) came from secondary mineralized zones in or around the argentiferous galena pods (see below). But despite considerable searching, no written accounts of crystal pockets as encountered in the mine, or any preserved stories of in situ collecting, have been found; these may be lacking because most collecting was done by local residents unconcerned with preserving "history."

San Carlos mineral specimens were first seen on the collector market in the early 1950s: presumably these were specimens dug by the miners and stockpiled until around the time when mining ceased. The most commonly marketed San Carlos species by far has always been vanadinite: a typical notice, placed in Rocks & Minerals by the dealer Hugh Ford in 1952, offers "Vanadinite, San Carlos, Mexico. Scarlet & brown xld. mass. 2.5 × 2-$2.50." Specimens showing red-orange vanadinite crystals associated with calcite are distinctive enough that when individual pieces were offered for sale, even in the absence of precise locality attribution, collectors would surely have recognized them - e.g. "Vanadinite, Chihuahua, Mexico: excellent miniature of subparallel groups of orange to reddish curved crystals to 1.5 cm, with partial coating of small calcite crystals, no matrix; 5.2 × 4.2 × 2.7-$25.00" (Dr. David Garske, Elmhurst, IL, specimen price list, 1978).

Former mineral dealer Jack R. Young, now of El Paso, Texas, recalls that at some time during the 1970s, he and an assistant collected fairly intensively in the old mine workings and took out a small number of fine vanadinite/calcite specimens, with vanadinite crystals to 2 cm. But Young decided that there was insufficient potential for the recovery of fine specimens to justify longer-term work there, so he called a halt to the project (Jack R. Young, personal communication, 2008).

Field trips to the site by casual collectors have largely gone unrecorded-but in the February 1967 issue of Lapidary Journal, Harry F. Miller and Ronald L. Olson wrote of their visit in autumn 1966 to nearby Santa Eulalia, during which they acquired some fine San Carlos vanadinites from a "local." Their account is probably typical of the mode by which specimens of San Carlos material began their journeys to collectors' cabinets during the 1950s, 1960s and 1970s:

This year, strange to say, we found very little Santa Eulalia material that we wanted . . . However, we did find something even better. We were told that there was a young man at the company office who had an outstanding specimen for which he was asking a stiff precio fijo (fixed price). The man proudly brought out a vanadinite, and what a vanadinite! The matrix, about four inches wide, was covered with big red hollow crystals . . . The cluster came from the San Carlos mine over near the Rio Grande, a famous location. The proud possessor told us that various gringos had bid on it and that one of them had come within three dollars of meeting his price; however, he had made up his mind to get such and such a price and that was that. We didn't argue; we just paid up and were glad to do so, for the vanadinite was as nice a one as we had ever seen. We would be willing to wager that the lad who stopped three dollars short is still kicking himself. Only when the transaction was finished did he tell us that a couple of gambustinos had gone over to San Carlos and had brought back not only the specimen we had just acquired but several more. So we moved next door, where a young man was waiting for us. He brought out a big box and began to lay out vanadinite specimens on the floor. None of them came up to our first purchase in quality but they were very good indeed. They varied in size from one to four inches; some had scattered crystals that were a transparent ruby red. Others were a brownish yellow and others were brown. In general, only the brownish yellow ones had hollow crystals. What wonderful trading material! So we picked out several nice specimens and started bargaining for them. The young man knew that we had paid a fat price for the prize specimen and that marked us as gringos ricos; his original asking price on the lot was about four times what they were actually worth. After a spirited bargaining session enjoyed by the participants plus a half-dozen spectators, we shook hands on a satisfactory figure. Then the young man told us he had a personal collection for sale. There were more than fifty pieces but only six of them were outstanding. The rest were on the junky side, even including such things as tumbled stones. But it was all or nothing, so we passed it up.

Wulfenite specimens from the Apex mine are much rarer than vanadinite specimens. Wulfenite from San Carlos first appeared on the market in the early 1970s (White, 1972), at which time Richard Bideaux noted that the wulfenite had first emerged "several years ago" and that "This district used to provide rich vanadinite specimens, but without evidence of associated wulfenite" (Bideaux, 1972).

During the 1970s, a number of fine Apex mine wulfenite specimens were handled by two Tucson dealer-collectors, Dan Caudle and Clayton Gibson (Mike New, personal communication, 2007). Later market appearances of Apex mine wulfenite may represent later discoveries, but were most likely just a recycling of older pieces. Scovil (1997) records an offering at the 1997 Spring Denver Show of a few San Carlos specimens with sharp, gemmy, bright orange wulfenite crystals to about 1 cm, but the year in which these were collected is not recorded.

GEOLOGY AND MINERALIZATION

The San Carlos orebody is a stratiform replacement deposit of magnetite, later fragmented by widespread faulting and then infused, probably in distinct hydrothermal pulses, by sulfides, most notably the argentiferous galena which was of ore interest during the Apex mine's brief lifetime. This is one of the many lead-zinc replacement deposits which occur in the 2,200-km belt of folded Mesozoic-age carbonate rocks in northeastern Mexico called the Mexican Fold Belt (Megaw, 1996). "Replacement" deposits are so called because they are characterized by in situ replacements of limestone or dolomitic rocks by sulfides and/or silicates introduced by the action of hydrothermal fluids which emanate, in most cases, from intrusive igneous stocks. When the replacement bodies are roughly horizontal they are called "blankets" or, in the Spanish term of accustomed use, mantos. (When the replacing bodies are steeply inclined or near-vertical they are called "chimneys"-but the San Carlos replacement bodies are of the manto type).

In the San Carlos area, calcareous sedimentary rocks of Upper Cretaceous to Lower Cretaceous age (Hewitt, 1970) are conformably underlain by siliceous sedimentary rocks ranging from very fine-grained quartzites to conglomerates with pea-size grains. Intrusions of felsic igneous rocks, variously described as syenite, granite, diorite and monzonite, intermittently penetrate the sedimentary sequences; some contacts of these intrusions with the quartzites are exposed in the southeast-striking barranca.

Shaly limestones and marls, chiefly of the Glen Rose Limestone formation, are exposed at the surface around the Apex mine, but this is the weathered crest of an anticline and the calcareous rocks are only 12 to 20 meters thick above the contact with underlying quartzites. Along this contact lie the mantos of magnetite mineralization. At an undetermined time after the mantos were formed, block faulting accompanying the formation of the anticline cut the quartzite-limestone contact, and with it the mantos, into many segments. At some still later time, pyrite and sphalerite were hydromermally deposited in countless fractures in the faulted rocks. Hewitt (1970) argues that still later fracturing and hydrothermal activity are responsible for emplacement of the fine-grained sulfide pods, up to 25 meters thick, which are found near the top of the deposit, i.e. at the site of the Apex mine, at the anticlinal crest.

The major replacement bodies consist largely of massive magnetite, pyrite and hematite and other iron oxides, with concentrations of sulfides, chiefly galena and sphalerite, present locally. In the topmost and richest part of the deposit, galena and magnetite concentrations occur in overlapping lenses which are broad and low-grade in their basal (eastern) segments and narrow and highgrade in their (western) segments.

Dense, bluish green to black masses of metamorphic silicate minerals (chiefly staurolite, actinolite and garnets) underlie the richest magnetite concentrations. Hewitt (1970) argues that the magnetite/silicate mineralization along the quartzite-limestone boundary resulted from a combination of contact metamorphism and simultaneous hydrothermal activity, and that in the most highly altered zones the original magnesium-rich limestone was wholly replaced by magnetite, staurolite, actinolite, garnet and recrystallized dolomite.

MINERALS

No available source lists the species found in the magnetite bodies, much less in the country rocks, of the mine area. The following species descriptions are based on material which has reached the specimen market, probably from the sulfide pods which were mined as ore. Aside from galena, the sulfides occurring in those pods-bornite, chalcopyrite, pyrite, sphalerite-seem to have been found exclusively in massive form. Details regarding the collectorquality species are as follows:

Calcite CaCO^sub 3^

The most distinctive-looking and most widely distributed mineral specimens from the Apex mine show orange-red to yellow-brown vanadinite crystals intergrown with lustrous, translucent white crystals of calcite, the latter rarely exceeding 1 cm. The vanadinite crystals mix intimately with the calcite crystals and in some cases penetrate or impale them, resulting in attractive red-and-white specimens which may reach cabinet size. The few rare wulfenite crystals known occur typically on druses of small white calcite rhombs.

Galena PbS

Argentiferous galena was the chief ore species found in the sulfide pods. Rarely, it occurred as dull gray, somewhat crude, hoppered cubic crystals in parallel aggregates to several centimeters (Peter Megaw, personal communication, 2008).

Hedyphane Pb^sub 3^Ca^sub 2^(AsO^sub 4^)^sub 3^Cl

Hedyphane, a rare member of the apatite group, was occasionally found at San Carlos as yellow microcrystals in clusters to a few millimeters (Panczner, 1987).

Hematite (?) Fe^sub 2^O^sub 3^

Rob Lavinsky's The Arkenstone dealership once handled a lustrous, jet-black, thumbnail-size cluster of distorted rhombohedral hematite crystals purportedly from San Carlos. Massive hematite is a minor constituent of the magnetite bodies which occurred in promixity to the San Carlos sulfide pods (Hewitt, 1970). The occurrence of good crystals, however, requires confirmation.

Hydrozincite Zn^sub 5^(C0^sub 3^)^sub 2^(OH)^sub 6^

Hydrozincite, probably as small encrustations, has been noted from San Carlos (Panczner, 1987).

Mimetite Pb^sub 5^(As0^sub 4^)^sub 3^Cl

According to Panczner (1987), mimetite occurs at San Carlos as botryoidal masses associated with vanadinite and calcite; Cook (2001) writes that "fine botryoidal masses of lemon-yellow mimetite from San Carlos . . . were once relatively plentiful." Mimetite has also been found as yellow, translucent, lustrous microcrystals, with individuals reaching about 7 mm, in masses to 7 cm across.

Vanadinite Pb^sub 5^(VO^sub 4^)^sub 3^Cl

Vanadinite is the best known well-crystallized species from the Apex mine. It occurs there as simple hexagonal prisms, in some cases modified by pyramid faces and commonly hoppered, to 6 cm (Panczner, 1987). The crystals are commonly intergrown with small, translucent white calcite crystals. The vanadinite crystals range from orange-red with medium luster to red-brown or brown. A dark brown skeletal habit may represent a high-arsenic variety similar to the well-known so-called "endlichite" specimens from Los Lamentos, Chihuahua. The crystals can occur scattered individually over matrix, and also as dense clusters, in some cases showing parallel growth.

Wulfenite PbMoO^sub 4^

Lustrous, orange to red-orange, simple tabular wulfenite crystals modified along the edges by a dipyramid perch on, or are partly embedded in, white to pale gray calcite, without vanadinite or other associated species (Bideaux, 1990). The wulfenite crystals reach 3 cm on edge (Mike New, personal communication, 2007), and are reminiscent of the famous specimens from the Red Cloud mine, but for the general lack of a second-order dipyramid which often renders the Red Cloud tablets octagonal rather than square.

As already noted, San Carlos wulfenite specimens are very much rarer than San Carlos vanadinite specimens. They reached the mineral market 20 years later, and did not remain available for long. The wulfenite occurrence in the mine was extremely localized and very limited in extent, and has probably been mucke in.

ACKNOWLEDGMENTS

I am grateful to Peter Megaw for directing me to (the few) extant print references on San Carlos, for permitting me to examine several San Carlos specimens in his superb collection of Mexican minerals, and for helpful discussions. For further helpful discussions I am indebted to Gene Schlepp, Mike New and Jack R. Young. Various photographs of San Carlos wulfenite specimens were located by Wendell Wilson, and Peter Megaw kindly provided snapshots of the Apex mine site and adjacent desert vistas.