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The Meikle mine: Elko County, Nevada.

INTRODUCTION

In a paper on the mineralogy of the Gold Quarry mine, Eureka County, Nevada (Jensen et al., 1995), one of the authors (MCJ), noted, "It is the authors' opinion that this specific region of Nevada probably contains the most complex and noteworthy mineralogical occurrences in the entire state." With the official opening of the Meikle (pronounced Meekull) mine little more than one year later, this statement has proven to be true in a remarkable way. Underground development and mining at this new operation have exposed a number of huge vugs containing outstanding crystals of yellow-orange barite that are in a class with the best barite specimens found anywhere in the world. Museum-size clusters of lustrous crystals to more than 15 cm have been collected, and unexplored portions of some vugs may yield even larger examples in the future. Barrick Goldstrike Mines, Inc. (BGMI), the owner/operator of the property, conducts mining on a two-shifts-per-day, seven-days-a-week schedule for high-grade gold ore, and the mineralized vugs, at least up through the middle of 1997, have been encountered on a fairly regular basis.

The site of this new occurrence is situated on the famous Carlin Trend gold belt in northeast Nevada. The "Trend" is a northwest-southeast oriented zone about 55 km long consisting of several low-grade, large-tonnage disseminated gold deposits, all of which are, at present, being actively mined. The two major operating companies are Barrick Goldstrike Mines and Newmont Gold Company. All of the deposits, including the Post/Betze, Blue Star, Genesis, Old Carlin, Gold Quarry, and Rain (from north to south) have been mined primarily by open-pit methods to date. Deep drilling from the surface, however, has discovered localized zones of high-grade gold mineralization, necessitating the development of underground mines. In fact, so large has been the push recently toward underground mining that experienced miners have been hired from all over the United States in order to build the necessary staff.

The fourth of these new underground operations to be placed into full-scale production (after Rain, Carlin East and Deepstar) was the BGMI Meikle mine. Named after a retired senior vice president in the company (Dr. Brian Meikle), this beautifully modern and efficient example of state-of-the-art shaft and underground mining technology is situated at the northern extremity of the Carlin Trend. From the nearest town, Carlin, on Interstate 80, a paved county road (Route 776) leads northward to the mines. The Meikle complex is located about 46 km from Carlin. The operation was officially opened on 21 September 1996 with a visit and underground tour by former United States President George Bush, who is a member of the BGMI advisory board.

GEOLOGY and DEVELOPMENT

The Meikle (formerly Purple Vein) gold deposit was discovered in 1989 (Elko Daily Free Press, 21 Sept. 1996) by conventional drilling techniques from the surface. The orebody lies approximately 2 km north of the large Post/Betze open-pit gold mine along the northward extension of the Post Fault system. According to Volk et al. (1995), the deposit is hosted by a complex series of collapse, tectonic, and hydrothermal breccias which are confined to the Devonian Popovich and Silurian-Devonian Roberts Mountains Formations. These lower-plate carbonate rocks are widespread in this region of Nevada, and consist for the most part of calcareous mudstones to dolomitic limestones. The breccia bodies range in thickness from about 120 to 250 meters and host approximately 85% of the ore reserves at Post/Betze and Meikle (Volk et al., 1995). Highly altered Jurassic monzonite and lamprophyre dikes of varying thickness locally intrude the sediments.

Economic gold mineralization, extending from 250 meters (800 feet) to 580 meters (1900 feet) below the surface at the Meikle mine, is localized for the most part along north-northwest-striking fault systems and at structural intersections (Volk et al., 1995). These high-angle structures also focused the pre-mineral acidic fluids which initiated formation of large-scale collapse breccias (Volk et al., 1995), subsequently providing immense open spaces for the growth of late-stage crystallized barite and calcite. In fact, so large were some of these open spaces that Quaternary surface alluvium has been intersected as far down as the 373-meter level, having slumped or filled voids which could not support themselves and which had collapsed up to the surface. Gold ore zones are commonly characterized by decarbonization, silicification, and the presence of fine-grained sulfides, chiefly pyrite and marcasite. Gold is microscopic, as is typical of all Carlin Trend deposits. The orebody is composed of three zones: the Main Zone, the East Zone, and the South Meikle Zone (J. Yolk, pers. comm., 1999). The age of mineralization is controversial; some say about 8.5 to 9.5 million years (Arehart et al., 1993), but Mid-Tertiary is now considered more likely.

Two circular vertical shafts (one production and one service/ventilation) provide access to the deposit. Shaft stations have been developed at 282 meters (925 feet), 328 meters (1075 feet), 373 meters (1225 feet), 396 meters (1300 feet), and 442 meters (1450 feet). Mining levels, connected via spiral ramps from the shaft stations, are situated 15 meters (50 feet) apart, beginning at 259 meters (850 feet) and continuing on down to 488 meters (1600 feet). Mining, utilizing rubber-tired equipment, produces at a rate of more than 2000 tons of ore per day (Volk et al., 1995).

The water table has been pumped down to facilitate access to the ore and the mine workings. Very hot and humid conditions are present locally. Hot, moisture-laden air commonly issues from small fracture zones as well as from the mineralized vugs. Rock temperatures in excess of 71 [degrees] C (160 [degrees] F) have been recorded, and selected small fracture zones continually issue forth minute scorching plumes of steam. All of the crystal-bearing vugs, when first encountered, are excessively hot, humid, and often gassy, with ambient temperatures of about 40 [degrees] C (105 [degrees] F). Factoring in the "beat index" due to the 100% humidity, the pockets feel as if they are actually about 60 [degrees] C (140 [degrees] F). For safety reasons, BGMI has implemented stringent policies regarding access to these caverns. The largest mine refrigeration system in North America, with a cooling plant capacity of 10 megawatts (Barrick Goldstrike, Annual Report to Stockholders, 1996), has been installed to provide adequate ventilation throughout the workings.

VUGS and MINERALS

During early development of haulage drifts and ramps, vugs were encountered essentially from the top of the orebody on down. The first few such vugs were opened on the 297-meter level and were small (for the mine), being about 3 to 4 meters in diameter and lined only with thick (20-cm) crusts of pale green, drusy, crystalline calcite. During subsequent work on the 328-meter level, a huge vug (more accurately, a crystal-lined cavern) was discovered. This unsupported natural opening, although somewhat irregular in shape, has maximum dimensions of at least 49 meters (160 feet) horizontally by 49 meters in vertical extent and 27 meters (90 feet) in width. A plywood viewing platform has been erected at the entrance to the vug, and lights (always on) have been installed throughout to illuminate this amazing geological feature. The pocket has been informally named the George Bush Vug, since the former President visited it on his underground tour at the time of the official mine opening. The floor, walls, and ceiling of this "crystal cave" are entirely covered with thick, semi-smooth crusts of bladed, pale green, crystallized calcite. On some surfaces of the calcite, multitudes of sparkling tabular barite crystals averaging 1.5 cm have formed. At the end opposite the platform, the rug pinches down and plunges into a near-vertical structure (a "natural shaft"). In this area, possibly due to increased fluid flow during formation (R. Pye, personal communication, 1996), the barite crystals dramatically increase in size and abundance, covering most of the walls and ceiling. It is difficult to comprehend and to describe the magnitude of the crystallization in this area; on virtually every surface visible in the light of the miner's lamp, large, deep orange-yellow, lustrous, undamaged barite crystals stand out from the matrix of pale green calcite. The opportunity to personally examine such an occurrence surely ranks as one of the most magnificent and memorable experiences in the life of a student of mineralogy.

As if this were not enough, the vug twists slightly and continues downward (containing local superb barite clusters) and then drops off sharply for about 10 meters, opening once again into another large chamber. At the limit of the miner's lamp light, statuesque knobs and bulbous masses of bladed calcite decorated with clusters of crystallized barite can barely be seen on the walls and floor of the room. To date, no one has entered or examined this area. Somewhere a connection with drifts on the 373-meter level below has been made, as attested to by the air flow coming up through the rug, but exactly where the connection is remains a mystery.

Barite crystals from these large, calcite-coated vugs [ILLUSTRATION FOR FIGURE 10 OMITTED] exhibit a unique and distinctive morphology, tapering slightly along the b axis toward the termination, which is in itself stepped like the top of a castle (castellated). The deep, caramel orange-yellow color is nearly identical to that of historic barite specimens from the former Sherman Tunnel locality in Colorado. The combination of unique morphology and distinctive color will serve, in the future, to instantly characterize this style of barite as being from the Meikle mine.

Another tremendous rug similar to the above was encountered along the rib of the 373-meter level, South Meikle access drift. Though this pocket is similar in dimensions and in the size, quality and abundance of barite, there are also noticeable differences. In addition to wall coatings and bulbous knobs of calcite/barite combinations, well-developed stalactitic and sheet-like structures were also locally present. These protuberances are exceptionally aesthetic and make beautiful specimens [ILLUSTRATION FOR FIGURES 7 AND 15 OMITTED]. When continued pumping lowered the water level in the main open chamber of this vug, it became possible to examine even deeper clefts and chambers. All proved to be lined with lavish crystals. Undulating sheets of crystallized pale green calcite dangling from the ceiling were coated with lustrous orange-yellow barite coalescing into areas several meters square of hummocky, undamaged crystal clusters.

Far down in the widening chamber a truly unbelievable barite occurrence could be seen. Hanging from the ceiling was a perfect mushroom-shaped knob about 1 meter across and 1 meter long, totally overgrown with radiating barite crystals. This eerie "orb" probably weighed on the order of 225 kg and would have required scaffolding and a major effort to collect and preserve. Shortly after its discovery, however, this exceptional pocket had to be backfilled with waste rock when the Griffin ramp access drift was driven over its roof.

Along a fault-controlled zone bordering ore on the 328-meter level, another style of spectacular vug occurs. These rugs are characterized by a general lack of calcite, a simple, tabular morphology of the barite, and abundant, loose, pocket contents. Frustratingly hot geothermal air rising forcefully out of the openings, however, prohibits examination for longer than about five to ten minutes, since longer exposures become dangerous. Although not as large as the calcite-lined caverns, these vugs are nonetheless similarly breathtaking. The largest of these pockets yet discovered is an opening about 3 meters wide by 15 meters tall and 20 meters deep with parallel, near-vertical walls totally covered with drusy crystallized barite. Cubic meters of crystal-bearing breccia fragments ranging in size up to slabs weighing several tons loosely plug the lower half of the vug. Individual barite crystals are mirror-lustrous, razor sharp, glass-clear, lemon-yellow crystals to about 6 cm (!). Absolutely magnificent specimens have been collected from these vugs [ILLUSTRATION FOR FIGURES 5, 8 AND 14 OMITTED].

From the 328-meter level on down, large rugs are still relatively abundant, but the majority contains only the thick, drusy, pale green calcite. Although the calcite is crystallized and lustrous, the large size (about 10 cm) of individual crystals and the thickness of drusy wall coatings all but preclude the collection of true specimens, since a typical knob or plate with any aesthetic appeal would easily weigh in excess of 200 kg.

Calcite crystals are also widespread throughout the mine in smaller rugs (1 to 2 meters across) apparently paragenetically unrelated to the larger "caves." A wide variation in morphology has been noted for calcite from these pockets, and some truly superb clusters have been seen. Two rather distinct types of crystals occur. Most common are druses of equant, glassy, colorless crystals to 1.5 cm forming plates and knobs on thin selvages of limestone and jasperoid. Far rarer are pockets lined with brilliant, smoky gray to pale yellow to colorless, elongated scalenohedrons ranging up to 24 cm in length. Close inspection of these latter rugs has revealed the presence of stubby contact twins (on (0001)) to 2 cm, elongated "football" contact twins to 5 cm, and very uncommon fishtail twins (on (012)) to 5 mm. Calcite crystals from these rugs easily furnish the finest examples of the species from Nevada, and possibly anywhere else in the United States with the exception of Mississippi Valley-Type deposits.

A relatively small number of other mineral species has been verified and reported from the Meikle mine. In general, however, these occur either as thin coatings, inclusions, or microcrystals. Following is a brief list of these additional minerals.

Clays

The common alteration clay minerals of kaolinite, illite and fraipontite occur throughout the deposit.

Dolomite CaMg[(C[O.sub.3]).sub.2]

Dolomitization, developed around zones of extensive collapse brecciation (Volk et al., 1995), has produced "zebra dolomite" (rhythmically banded) textures containing local pearly-white, euhedral, rhombic crystals to 2 min.

Goethite FeO(OH)

Goethite, as an extensive oxidation product of altered pyrite, occurs predominantly above the 373-meter level.

Other Sulfides

Predominant among other sulfide minerals is sphalerite, in fine-grained, brownish red masses and clearable grains to 2 mm. Galena has been found on the 328-meter level as silvery grains to 2 mm in crystallized seams of calcite. Tetrahedrite, as lustrous crystals no larger than 0.5 mm, occurs with massive pyrite in samples from the 328-meter level. Massive stibnite fans to 5 cm enclosed in siliceous rock and cementing altered monzonite occur with rarer, red coatings of metastibinite in stopes on the 282-meter level. Very uncommon drusy quartz casts (epimorphs) after euhedral single stibnite crystals to 5 cm were also discovered in this area. The most common silver-bearing species, a selenian miargyrite, has been verified by both electron microprobe and X-ray diffraction techniques on samples from the 282-meter level. Pyrargyrite, in small (1 mm) crystals similar to the selenian miargyrite, has also been found at this location. Chalcopyrite and boulangerite have also been verified (P. Emsbo, pets. comm., 1998).

Tellurobismuthite (Volk et al., 1995), has been reported, but has not been verified (following rather extensive inquiry) by this author.

Phosphates

Microcrystalline phosphate minerals, including carbonate-fluorapatite and englishite, occur very rarely as late-stage products grown upon barite crystals from the 282-meter level.

Post-Mining Minerals

Mineralogical reaction products where mine wall-rock has been exposed to humidity and air include gypsum, halotrichite, ilsemannite, jarosite, and szmolnokite.

Pyrite and Marcasite Fe[S.sub.2]

Two of the polymorphs of iron sulfide, pyrite and marcasite, are also relatively common, occurring for the most part as fine-grained veinlets and masses in ore zones. Interesting examples of golden, drusy, crystallized pyrite perfectly coating tabular barite crystals to 2 cm have been seen on the 442-meter level.

Quartz Si[O.sub.2]

Quartz is the most widespread of the other species, and is present as a massive replacement (silica flooding) of limestone, as (jasperoid) stockwork veinlets, and as euhedral hexagonal crystals up to 6 cm in length (locally overgrown with lustrous, yellow, tabular barite crystals as on the 282-meter level).

CONCLUSION

These are, indeed, good days for mining in Nevada. They are the times of economic boom in the rural towns of the northeast part of the state, the days of the big red and white Al Park Petroleum tankers crossing Interstate 80 carrying diesel fuel to the mines, and the momentary windows of exceptional mineralogical opportunities. The Meikle mine crystallized barite occurrence will be remembered as one of the most significant events during these times.

I have collected in and toured a large number of rich, specimen-producing mines around the world, from localities including Elmwood to Cave-in-Rock to the Viburnum Trend to Panasqueira to Touissit to Dalnegorsk, and it is my opinion that the Meikle mine possesses the greatest concentration of vugs I have ever seen. Throughout the mine workings, large, open holes locally abound. As Watson (1904) said in reference to his examination of the fluorite occurrences in Weardale, England: "The student cannot fail to observe, in the mine as on the surface, how largely the idea of beauty enters into the plan of Creation."

As mining has progressed from September, 1996 to the present, the upper, flat-lying zone is becoming depleted and the deeper, steep zone containing the bulk of the ore is rapidly being developed. Concomitant with this progress has been a dramatic decrease in the number and size of vugs, together with a lack of barite. It appears that the upper portions of the deposit were more favorable to the formation of large rugs and superb barite crystals, because the deeper workings are now encountering only crystallized calcite. The old adage that "the best crystals come out first" may very well apply to the Meikle mine. Then again, who knows what further mining will reveal?

Barrick Goldstrike Mines has recognized the high value that the public places on the preservation of important crystallized specimens. For this reason, in 1997 BGMI entered into a specimen-recovery agreement with Geoprime Minerals Company (Casey and Jane Jones). The purpose of this contract has been to preserve crystallized barite and calcite specimens and to make them available to educators, museums, collectors and researchers.

[Note: Regretfully, the author has no specimens available for sale or trade.]

ACKNOWLEDGMENTS

I deeply appreciate and respect the perspectives, discussions, and insights I have gained from interaction with numerous BGMI personnel. Selected staff of Connors Drilling, including K. Brown, Iceman, J. D, J. R, Lash, Uncle Tom, and especially The Captain, showed me some of the most grueling, intense, and wholeheartedly fun work I have ever known. To them all, I owe a salute. I cannot overemphasize my sincere gratitude to A. Soregaroli and Dr. W. E. Wilson for their excellent editorial assistance, skills, support and friendship. Analytical work was conducted by the author, with supplemental confirmation being kindly provided by D. Brosnahan (formerly Barrick Goldstrike, Nevada) and A. C. Roberts (Geological Survey of Canada, Ottawa). H. Gordon, Reno, Nevada, generously made his selection of specimens available for examination and also shared details of his knowledge of the deposit. T. Cordova of Reno, Nevada, and Beavis (M.D.) Leising are thanked for their enthusiasm and interest in this project, and for providing helpful suggestions. Joanne Newton performed the typing of the numerous revisions of, the manuscript. And, to all those individuals who took the time to save a specimen out of the hundreds of tons of crystals exposed during mining, a similar note of thanks is due.

REFERENCES

AREHART, G. B., FOLAND, K. A., NASER, C. W., and KESLER, S. E. (1993) K/Ar and fission track geochronology of sediment-hosted disseminated gold deposits at Post-Betze, Carlin Trend, northeastern Nevada. Economic Geology, 88, 622-646.

JENSEN, M. C., ROTA, J. C., and FOORD, E. E. (1995) The Gold Quarry mine, Eureka County, Nevada. Mineralogical Record, 26, 449-469.

VOLK, J. A, LAUHA, E., LEONARDSON, R. W., and RAHN, J.E. (1995) Structural Geology of the Betze-Post and Meikle deposits, Elko and Eureka Counties, Nevada. Geology and Ore Deposits of the American Cordillera Symposium Proceedings, 10-13 April, 1995, Sparks, Nevada; Trip B, Structural Geology of the Carlin Trend, 142-156.

WATSON, S. (1904) The Boltsburn Flats - their interest to the student of nature. Transactions of the Weardale Naturalists' Field Club, 1, 146-150.
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Author:Jensen, Martin
Publication:The Mineralogical Record
Date:May 1, 1999
Words:3341
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