Photo by Jane JonesMEIKLE MINE INFORMATION
The Meikle mine entered production in September 1996, and now averages a mining rate of 2,200 tons per day.
Mining is taking place
in the upper, relatively flat zone of the deposit and the upper
portion of the underlying steep zone, which contains the
bulk of the ore.
Occasionally mining operations encounter large voids containing for the most part sub-euhedral calcite. Small areas within these large cavities sometimes contain astounding barite crystals, some up to 7 or 8 inches .
Barite also occurs in voids or pockets alone. The habit of these barites are tabular as opposed to the the semi-prismatic morphology of the barites with calcite.
Color of the barites range from an almost lemon yellow to a grayish-yellow to honey-golden orange. Not to be out done calcite from one particular chamber exhibits a distinct lime green hue.
The Meikle Mine
Barrick Goldstrike Mines is wholly owned and operated by Barrick Gold Corporation which entered the gold business in 1983 and currently operates five mining properties in North and South America, with plans to soon begin mining in Tanzania. Despite the low price of gold, Barrick Gold has profited by focusing aggressively on increasing production, cutting production costs, and using innovative financial strategies. The Goldstrike property exploits four orebodies: the Betze-Post, Meikle, Griffin, and Rodeo deposits, and a fifth in development, the Storm deposit, all located in the North Carlin Trend about 27 miles north of Carlin, Nevada. The Meikle deposit, discovered in 1989, and formerly known as the Purple vein, is located about 1.5 miles north-northwest of the Post-Betze deposit. The regional geology is characterized by the siliciclastic (clastic noncarbonate rocks) assemblage of the Ordovician Vinini Formation and the carbonate assemblage of the Devonian Popovich Formation and Silurian-Devonian Roberts Mountains Formation. One of a series of faults, the Post Fault Zone (PFZ), is intruded by Jurassic monzonite and lamprophyre dikes and sills that control local mineralization. (Volk, 1999 pers. comm.) Meikle is bounded to the east and the north-northwest by this system and its associated dikes. Complex hydrothermal, tectonic and collapse breccias in the Roberts Mountains Formation and the Popovich Formation host submicron gold (mostly in microscopic pyrite) at Meikle (Volk et al. 1995). The two end member types of breccia are a dolomite breccia and a monzonite breccia. After several trips underground to new working faces with geologists and samplers, it became quite clear that the monzonite porphyry in the hanging wall was low in gold, whereas the high-grade mineralization occurred in the dolomite breccias near the limestone footwall. Gangue minerals include barite, quartz, calcite, stibnite, and minor sphalerite. The location of the gold appears to have little bearing on the presence or absence of crystallization, although voids and caverns which bear the richest crystal pockets are typically in the foot wall of the ore body. The upper zone of the main ore body, between a depth of 875 feet and 1,225 feet, has so far contained the best occurrences of barite. The lower, steeply dipping zone of the ore body, which extends to more than 1,900 feet, has so far produced only calcite specimens.
Collecting History.
Specimens were collected at the Meikle mine by employees, contractors,
and thieves as early as 1995. In
early July 1997, we responded to an invitation to tour the Meikle mine. The
physical conditions on this first trip characterized the many that followed. A cold cage ride down the shaft with several layers of
clothing, the beginning of unlayering as we stepped into the drift, and
the continual progression of heat as we approached a new face or working
area finally led to removal of as many layers as propriety allowed. The few
specimens we obtained on this first trip were quickly absorbed by the collecting
community at the September 1997 Denver Show.
The
first area we visited was the 975 level at
stope 3525. A large pocket had been
exposed on the right lower corner of the stope.
This pocket was completely lined with gemmy transparent yellow to yellow
gold colored square tab-shaped crystals up to 6 cm on a friable limonite matrix
, which made collecting difficult. The
pocket continued downward at a steep angle to the right, with crystals as far as
the light would penetrate. We
collected the edges, but it was impossible to enter due to the tight space, high
temperatures, and sulfur dioxide gas. This
pocket was one of the most beautiful underground scenes we had personally
witnessed, but there was yet more to see. It was also our first experience with the intense heat.
As we put our hands on the pocket wall, the limonite was still hot, and a
short distance away hot thermal steam was escaping from another open void.
Our
next stop on our first brief collecting tour was on the 1075 level but away from
the orebody near the batch plant where they mix concrete with the waste rock to
backfill mined-out stopes. During
the course of mine development when constructing this batch plant, a large
cavern was opened that Barrick has maintained intact.
This cavern is completely lined with calcite and, in select areas,
beautiful orange barite crystals to 20 cm in a variety of habits, but
predominantly flat, spear-shaped crystals.
Many of these were collected by the contractors during mine development.
What remained would require specialized equipment to recover.
On subsequent trips we did use some equipment to recover barites from
accessible parts of this cavern.
After another trip or two, we signed a contract in early February 1998, at which time our specimen recovery began. The mine superintendent, even went so far as to give us samples from his office. Since there was little time to collect, these were among the specimens which appeared at the February 1998 Tucson show. The first few pockets that we recovered were mainly from the 1075 level, some from stope 3475. This stope encounters the same structure found a level above on 975; the structure appears to be a breccia pipe. Most of the specimens again were on a friable limonite matrix, (fig.) with a few collected in 1999 on a hard, black silicified limestone. A collecting trip to the 1075 level at the 3475 area in January 1999 exemplifies some of the difficulties encountered by the crew trying to help us throughout 1998 and 1999. January 3 had been scheduled for two months as the date to shoot a round in this known barite-producing area, exposing new ground. When we arrived on a Monday, the holes had not been drilled due to mining delays. So for a few days we worked in calcite areas. The holes were finally drilled by Thursday and were to be loaded and shot in between day and night shift, allowing us to work during night shift. Ground temperatures indicated the rock was too hot to be loaded (measuring more than 164° F). When we visited the area a couple hours later, the ground had cooled into an acceptable average of 127°, but it was too late to blast. The round was loaded by the night crew and we returned on Friday with the day shift after the blast had occurred. Mine crews performed support work to allow us safe access to the area. We dug that day and part of Saturday. We planned to come back again on Sunday, but an incident at the mine prevented our returning that week; subsequently, the area was designated as unsafe to work in. The plan had been carefully worked out and scheduled, and the mine put considerable labor hours and machinery into accessing the area, yet unpredictable circumstances prevented collection. This kind of situation is frustrating not only to us but to the many Meikle people who have tried to see this project work.
We returned to the 1075 throughout 1998 waiting for mining to extend the
stope. On this same level, we also
debated how to access and collect in what we called the batch plant cave (also
known as the George Bush cavern) mentioned above.
The specimens from the batch plant cave as described above are large
orange-gold spear-shaped crystals, some on calcite matrix.
In the fall of 1998 we recovered some nice specimens from a small
tunneling void that leads into a deeper cavern beyond using a diamond chainsaw.
It took careful planning and strong backs to carry the 250lb. power pack
for the saw to a position near enough to the specimen location to prevent water
and hydraulic power from being diminished by distance.
We proceeded collecting with the saw as far as safety would allow at the
time. Future mining may make the larger area accessible to recovery of
exceptional specimens. Although in a known barite area, it waits to be seen if the drifting
being planned will encounter voids containing barite or only calcite.
A small cavern encountered on the 1125 level measured about 16 feet deep by 18 feet wide and 5 feet high. Crystals lined the back and ribs but were curiously absent on the floor except in local areas where large crystals periodically occurred. This was one of the many spots where an underground crew helped significantly in providing ground, electrical, and air support, making it safe for us to enter the void. The crystals recovered are large, thick and tabular. Clear sections of broken crystals have yielded large clean faceted stones.
There are quite a number of smaller areas that have produced slightly varying forms of barite and calcite. One occurrence is that of yellow barite crystals on quartz crystals that were collected in the spring of 1997 on the 925 level at stope 3675. This habit has not been found anywhere else in the mine. We also recovered some fine barites from the 1225 level behind the mechanics bay. There were light lemon yellow, large crystals arranged in an attractive rosette habit. The elevated voids required ladder access and the voids were barely large enough to kneel in. The prone position was the most comfortable for digging -- only our boots sticking out into the drift signaled our presence.
Photo by Jane Jones
Casey Jones
Another
of a number of elevated voids occurred on the South Meikle Drift, again only
accessible with a ten foot ladder. One
void had already been wired in for wall support, and the wire netting had to be
pulled back during collecting. This
meant the collector was pressed slightly between the wire and the opening of the
void with constricted arm movement. Several nice pieces and a large plate came
from this small upper chamber. Due
to the nature of the ore, mining of the South Meikle drift was put on hold until
completion of the new roaster which will more efficiently process the ore.
Until January 1999, the calcite we encountered was mainly potato-chip-habit crystals and cloudy or smoky dogtooth crystals. One large spectacular cave just off of the Griffin drift produced large pieces of lime-green, blocky and bladed calcite, some with black tips, but no smaller pieces. The Griffin drift connects the Meikle deposit and the Rodeo deposit, a new adjacent mine still in development stages. As mining continued, large calcite voids were encountered at lower depth, many directly in the path of the ramp, making recovery difficult. However both management and crew made efforts to allow access before ramp development had to continue. Gemmy calcite twins were found on the 1450 and 1650 levels in January 1999. The January void on the 1650 level was literally a crystal cathedral, with a secondary elevated crystalline calcite room nestled inside. Whereas lighting for photos was usually difficult, the mine lamp reflecting off the myriads of crystals in this upper chamber created more than adequate lighting for a photo or two. Unfortunately, safety necessitated that the void be filled with concrete even as we scurried to collect. This did not permit much camera time as we could collect only as long as it took for the concrete to dry (thankfully the size of the cavern required a longer drying period). Sitting in this upper sanctuary suspended from the ceiling of the larger crystallized chamber, almost like a chandelier in the middle of the larger room, was a remarkable and memorable experience. Clear calcite crystals were found in a cavern encountered on a decline off the 1750 level in March of 1999. Further calcite pockets are sure to be encountered during the course of mining, and wherever possible we will continue to document and preserve them.
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© 2000 GEOPRIME MINERALS