The Twin Creeks Mine, Humboldt County, Nevada

    


 

 

 

 

 

 

 

Collector's Edge Minerals, Inc. recognizes and thanks the Mineralogical Record for permission to use portions of, or all of, their original article from the "Mineralogical Record", Volume 31, Number 4, pp. 311-322 and pp. 331. Collector's Edge has made some additions and changes to the original article, which are incorporated below.

 

INTRODUCTION:

In early 1999 an extraordinary occurrence of beautifully crystallized orpiment was discovered during ongoing gold mining activities at Newmont Mining Corporation’s Twin Creeks mine in Winnemucca, Nevada.

Prior to early 1999, Twin Creeks mine geologists seldom saw specimen-quality minerals; microscopic gold was the only thing of interest to them. Then, in April, 1999 a fresh blast revealed a zone of yellow and orange smears. Upon close inspection, the zone was found to be full of small pockets lined with brightly crystallized orpiment. The orpiment was well crystallized and showed a range of bright colors, from reddish orange to orange to honey-yellow. Specimens were mostly large miniature to small cabinet in size. The color, high luster and pristine condition of the Twin Creeks’ orpiment specimens render these among the best of species.

Newmont geologists contacted Collector’s Edge Minerals, Inc., and invited the company to come out and see the discovery. Soon after, an agreement was reached to recover and preserve the orpiment specimens. Between April and October, 1999, the entire orpiment zone was worked out and thousands of specimens were collected. Through this partnership, these magnificent specimens were saved from the crusher.

LOCATION:

The Twin Creeks mine lies approximately 40 miles northeast of Winnemucca, Nevada.

 

Gold mine locations in north-central Nevada and their relationship to four major Carlin-type deposit trends. The Twin Creeks mine is at the upper left, in the Getchell Trend, Humboldt County, just a few miles from the famous Getchell mine.

HISTORY:

The Twin Creeks orpiment occurrence, lying in the Getchell Trend, is one of the many Carlin-type gold deposits in west-central Nevada. Gold was first discovered in the Twin Creeks area in 1984 by the Goldfields Mining Company, followed by an additional discovery in the same area by the Santa Fe Pacific Gold Corporation in 1987. In 1993, the two companies merged and consolidated their two mines to form the Twin Creeks mine.

In 1997, Newmont Mining Corporation merged with Santa Fe Pacific, creating a relationship that exists today.

The Twin Creeks mine, currently the third largest gold producing mine in North America, is divided into two open pits: the Vista and Mega pits. Annual gold production of nearly a million ounces is extracted from over 15 million tons of ore. Average production is over 40,000 tons per day, split between oxide and

sulfide milling and oxide leaching operations. Over 700 people work at the facility on a 24-hour-per-day, 7-day-per-week schedule. The Twin Creeks mine accounts for over 22% of Newmont’s worldwide gold production of over 4 million ounces per year.

 

Aerial photo of the Twin Creeks mine complex looking north-northeast (courtesy Newmont Mining Corporation).

 

View of the Mega pit looking southwest.

Geologists observed signs of crystallized orpiment in Cut 62 of the Mega pit in late 1998. The quality was superior to any seen in the past, and prompted Newmont geologist Leroy Schutz to contact Collector’s Edge. A decision was made to keep close track of the orpiment production areas for signs of better specimens.

In January, 1999, mining in Cut 62 continued downward through increasingly better orpiment zones. Then, in March, incredible orange-colored orpiment was discovered by Newmont geologist Pat Donovan. Newmont regional mine geologist Ron Thoreson contacted Collector’s Edge to set up an on-site evaluation. Ken Roberts, from the Collector’s Edge, traveled to the mine for a 1-day inspection and ended up spending an entire week collecting the newly discovered pocket zone on the 4220 Bench of cut 62.

Unfortunately, the zone had severe blast damage and few top-quality specimens were collected, but the potential was clear; the orpiment was world-class.

 

Nice vug of newly exposed orpiment crystals.

By April, 1999, everyone was excited about the new orpiment and finding a means of preserving the specimens became a high priority. Collector’s Edge presented the Twin Creeks mine staff with a collecting and marketing proposal that suggested ways two companies might work together. Newmont staff responded with an excellent plan to identify orpiment zones and protect them from harm during blasting work.

Ideas about collecting schedules, techniques and requirements were discussed in order to create a project model. The primary project goal was to design a mineral collecting procedure that did not cause ore production slowdowns. After all, it was unheard of to consider altering the production schedule of a major mine in order to collect mineral specimens.

A collecting contract was completed by May. A mining crew, supervised by Graham Sutton, was borrowed from the Sweet Home mine, in Colorado, and sent to the Twin Creeks mine. Meanwhile, Randy Ford, Production General Foreman, and Lyle

Avey, Drill and Blast General Foreman, designed ways to continue gold mining activities in and around the collecting crews without production disruptions.

By late May, 1999, the Collector’s Edge crew was busy collecting orpiment on the 4200 Bench of Cut 62. During the next 5 months, the zone was worked to a depth of 30 meters and thousands of specimens were collected.

 

View of Cut 62 and 4220 bench. Orpiment zone is seen behind the compressor.

 

 

Equipment provided by Newmont to help excavate the orpiment zone.

 

Orpiment zone – note exposed pockets in upper portion.

 

Using pneumatic tools to expose orpiment pockets.


 

Exposed orpiment pocket.

 

Close-up of orpiment specimen just removed from the pocket.

 

Ken Roberts next to exposed recently exposed vug of orpiment crystals.

 

Orpiment zone with several exposed pockets.

 

Outstanding specimen extracted from the site.

 

In-situ vug of orpiment crystals waiting to be extracted.

 

Another vug of exposed orpiment crystals.

 

View of newly exposed pocket containing orpiment crystals.


 

Close-up photo of outstanding orpiment crystals.

 

Excellent cabinet specimen of orpiment crystals just removed from pocket.

 

Standing next to exposed orpiment pocket.

 

Extracting orpiment specimens from orpiment zone.


 

Wrapping specimens prior to shipment to the Collector’s Edge laboratory in Golden, Colorado.

Weather conditions in the Mega pit vary from blowing snow in March to 100+ degree days in August. Wide temperature fluctuations created challenges to specimen collecting. In springtime, cold fingers and stiff bodies made collecting difficult. During the still, 100+ degree days in August, the desert sun beat down relentlessly care had to be taken to avoid heat exhaustion. Adding to the experience was the dust. After about 20 minutes of collecting, the finely disseminated black pyrite and other sulfide minerals coated everything, and everybody. Sunburn was a constant companion.

Collecting crews rotated in and out of the project on a monthly basis. The crews consisted of three to four people borrowed from the Sweet Home mine in Colorado. Modern mining codes require MSHA-trained staff, therefore each miner had to have a current MSHA (Mine Safety and Health Administration) certificate in order to gain access to the site. Additional on-site training was provided by Newmont safety staff prior to any collecting activity.

Between March and September a total of 6 trips, lasting one to two weeks each, took place. The excitement level reached a peak in mid-summer when, on each of two collecting trips; over a thousand specimens were collected.

During the first trips, Newmont engineers and Collector’s Edge staff devised a means to save the orpiment zones while not impeding mine production. The plan began with logging and mapping orpiment zones during the drilling stage of the blasting schedule. While drilling each blast hole, care was taken to log the areas where orpiment was seen in the drill cuttings. Then, all holes, except those showing orpiment, were shot. After the blast rock was removed, a long orpiment-bearing column was left standing in the middle of the pit floor. This structure often measured about 7.6 meters wide by 30 to 60 meters long by 6 meters high. The entire structure was removed down to the pit bottom during collecting activities and then the drilling cycle would begin again to drop the floor another 6 meters.

Collecting orpiment required a variety of tools. Newman provided   Collector’s Edge with a large excavator and front-end loader to expedite the removal of thousands of tons of rock separating pocket zones. Once inside pocket zones, everything slowed down and collecting consisted of breaking open pockets with air chisels and hand tools. Several excellent orpiment specimens were extracted with the diamond chain saw.

 

Collecting orpiment in the floor of the Mega pit; the orpiment zone here is about 7.6 meters wide and dips near-vertically.

The Mega pit is large enough that, at any one time. It was possible for three or four areas to be mined at once. Since orpiment collecting occurred in only a single area, the mine planners allowed collecting while shifting gold mining activity to elsewhere in the pit. The flexibility of this schedule promoted collecting without negatively impacting gold production schedules.

Once or twice a day, all mining and collecting activities would cease as the entire Mega pit was evacuated for blasting. Each blast broke up over 350,000 tons of rock and consumed 61 tons of explosives. After the blast, work resumed as dozens of large trucks carried the blast rock out of the pit. Much of this material, about

85% went to the waste rock piles while the balance, about 40,000 tons, went to the processing plant for gold recovery. Each truck can haul between 180 and 240 tons of rock (that is, up to half a million pounds!).

 

An afternoon blast at the Mega pit. Twice each day the pit is cleared for blasting.

After blasting, work resumed in the specimen collecting zones to complete a 10-hour workday. Then it was a 1-hour trip back to Winnemucca for a shower, dinner and sleep. Six or seven hours later, the cycle started over again. Time was of the essence; since Newmont staff went to extremes to work around the collecting effort, there was much pressure to finish collecting the orpiment zones, so that gold mining activities could resume. By October, 1999, 4000 individual specimens had been collected.

 

Graham Sutton, CEMI, working on an orpiment pocket in the Mega pit. Although the orpiment zone is 7.6 meters wide, individual pockets are generally less than 15 cm wide and 30 cm long. Great care is required to avoid bruising the soft crystals.

Ideal conditions in any collecting scenario require large open spaces unencumbered by pocket bridging. Large pockets also increase the odds of recovering plates with minimal margin contact Cut 62 contained a few large orpiment pockets that realized the collector’s dream of pulling out large, clean, outstanding specimens. Unfortunately, most of the largest orpiment plates contained minor damage.

Perhaps the most critical factor to clean specimen recovery in the Mega pit was the orientation of the pocket zone to the nearest blast hole. While Newmont engineers made extraordinary efforts to blast around the pocket zones, some shots telegraphed into neighboring open spaces anyway. Since the largest open spaces are most susceptible to such collateral damage, they suffered the most. Quite often, large pockets (those measuring up to 1.2 meters long) were partially collapsed due to blast damage. It was rare to pull a completely undamaged, un-contacted orpiment specimen from anywhere in the Mega pit.

As the pit was deepened, more and more crystallized orpiment was discovered. Beginning with the 4220 Bench (at the top of the fold structure), the pit was deepened down through the 4120 Bench. The 30 meters of orpiment-rich ground was removed in 6-meter intervals. At about the 4120 Bench, the bottom of the fold structure pinched out and the orpiment specimen collecting ended. The entire zone was removed over a 6-month period. The orpiment zone measured approximately 30 meters deep by 7.6 meters wide by 91 meters long, blocking out a volume of 28,000 cubic yards of rock.

Orpiment pockets were usually small, averaging 25 to 35 cm in diameter and 2.5 to 7.6 cm wide. Occasionally, a pocket measured up to 1.2 meters deep with an opening up to 15 cm wide. Good pocket zones contained pockets clustered tightly together,

one atop the other. Some isolated minor pockets occurred up to 30 meters away from the major pocket zones and did not contain significant quantities of orpiment specimens. Collecting activity was boringly slow to impossibly frantic depending on where and when good zones were discovered.

Most pockets produced less than 10 specimens, and many produced only one good specimen. Because most pockets measured less than 35 cm, many of the orpiment specimens contain contact damage from stalactitic growths bridging the open space (a phenomenon dubbed “pocket bridging” by the collecting crew). Damage also occurred when specimen plates were pulled apart during the collecting process, as interlocking crystals broke away from around the pocket margin.

GEOLOGY:

The Twin Creeks mine is situated in a valley southeast of the Dry Hills in northern Humboldt County, Nevada. The area is underlain by Paleozoic sedimentary rocks containing gold-mineralized rock characteristic of Carlin-type deposits. The mine is located at the intersection of the Getchell Trend and the northern end of the Valmy Trend, along which lie many other Carlin-type gold deposits.

 

Geologic and paragenetic history of the Mega pit area.

The host rock for the orpiment is Ordovician in age, and consists of a thick sequence of carbonates and siltstones, intrusive sills, basalt flows and tuffs. The Comus Formation, making up much of these Ordovician rocks, contains most of the sulfide gold reserves and is an important mining target in the Mega pit. Emplaced by the Robert’s Mountain Thrust, an Ordovician sequence known as the Valmy Formation overlays the Comus Formation.

Lying unconformably over the Valmy Formation is the Permian- Pennsylvanian Etchart Formation. This unit is the primary source of gold in the Vista pit. To date, no crystallized orpiment has been discovered in this pit and none is expected as this formation contains oxide ore and few if any sulfides. The Etchart sequence, over 914 meters thick, consists of limestone, siltstone and sandstone.

Pushed over the Etchart Formation by the Golconda Thrust is the Mississippian-Permian Havallah Formation. It consists of intensely sheared mudstones, siltstones and sandstones. These rocks do not contain gold, and no orpiment has been discovered in them.

Cretaceous granodiorite dikes and sills can be found throughout the Paleozoic rock sequences at the Twin Creeks mine. They are pre-ore-stage systems and do not contain gold mineralization. Tertiary volcanic deposits and Quaternary/Tertiary alluvium overlay the Paleozoic rocks and fill the valley floor.

 

Geology of the Mega pit and West pit, Twin Creeks mine.

Qtg = Quarternary/Tertiary gravel. QTt = Quaternary/Tertiary tuff. PPe = Permian-Pennsylvanian Etchart Formation. Ov = Ordovician Valmy Formation. Oc = Ordovician Comus Formation: igneous (blue-violet) and sediments (red-violet). Fold axes shown in red. Green outline (below) = orpiment collection site. Cross-hatched = >0.08 oz/st Gold.

 

Geological cross-section through the Mega pit. Twin Creeks mine (looking north).

Both gold and orpiment mineralization are contained very complex, northwest-trending fold called the Conelea anticline, developed during the Antler Orogeny. This system lies within the Ordovician sequence and plunges to the northwest. The mineralization postdates the anticline and is contained within the overturned nose of the fold. The fold is broken up by a series of northeast-trending faults. In its northern portion, classic “Z”-shaped chevron fold structures can be seen in the walls of the Mega pit (Fig. 6).

 

View of the west wall of the Mega pit, showing the dark-colored Z-shaped fold structure. Orpiment occurs within the overturned nose of the anticline in the pit floor. (Note the collecting operation under way at the pit bottom).

Within the anticline, the best-crystallized orpiment is found near the upper hinge area, where the most stress, and therefore the largest openings, occurred. These open areas were conduits for mineralized solutions which created replacement ore bodies in and around the hinge zones. These ore bodies contain some of the highest gold values found in the Mega pit.

The age of mineralization has been determined by Simon et al. (1999) to be 41.9 million years, based on argon isotopic ratio analysis.

The rock throughout the orpiment zone is heavily decalcified, argillized and silicified. Hand samples in orpiment pocket zones show chert-like inclusions indicative of heavy silicification. Besides arsenic and silica, the matrix contains large proportions of aluminum, at 35,600 ppm, and iron, at 33,500 ppm. Zinc. Calcium and magnesium constitute a total of 1.5% of the matrix material.

Decalcification, silicification and folding created ideal conditions for orpiment deposition. Intense folding prepared the ground, allowing for later decalcification and silicification. The two latter events enlarged and then strengthened the open spaces. The voids were later filled with orpiment crystals. Latent solutions, perhaps meteoric in origin, remobilized small amounts of silica from the surrounding bedrock and deposited it as light-colored dustings on some of the orpiment. Analysis of the coatings shows the quartz content over 41% while carbonate is only 0.1%. Other important constituents of the coatings include aluminum at 64,500 ppm and iron at 38,200 ppm. One other item of interest is the presence of carbon: 0.96%. The silica-rich rock surrounding the crystal pockets includes sulfide mineralization containing the highest gold values in the Mega pit.

Low-grade orpiment is abundant throughout the northern portion of the Mega pit. Mapping orpiment mineralization is helpful to mine geologists because orpiment occurrences correlate well with areas of high gold values, and serve as a valuable exploration tool. It was not until late 1998, however, that quantities of well- crystallized orpiment were observed. The first good orpiment outcrop came on the 4220 Bench of Cut 62 in the northern portion of the Mega pit. This discovery coincides with the deepening of the pit over the Conelea anticline. As the open spaces inside the nose of the anticline were exposed, beautifully crystallized orpiment specimens were uncovered.

SPECIMEN PREPARATION

At the Collector’s Edge cleaning lab in Golden, Colorado, cleaning experiments started in May to determine the best approach for specimen preparation. After dozens of tests, it became apparent that heavy coatings would not come off without damaging the specimen luster. Specimens with any degree of etching could not be improved.

An unattractive gray-brown material was sometimes found coating and/or included in specimens, rendering them unaesthetic. Specimens whose quality was un-improvable were returned to the Mega pit for ore processing. Of the entire specimen production (over 4000 pieces), about one third had to be returned. During the entire collecting effort, hundreds of pockets were uncovered that contained heavily coated, or off—color specimens. These pockets were dug out, but the specimens ultimately had to be sent to the crusher. If not for such coatings, many tens of thousands of specimens could have been saved.

Specimens with only slight coatings could usually be cleaned display an excellent to perfect luster. A sodium citrate bath worked best to remove such coatings. The specimens were then neutralized in a solution of sodium bicarbonate and water. Almost any acid or strong base reacts with orpiment and should be avoided. Hundreds of specimens were recovered with glassy luster, and needed no sodium citrate cleaning at all. Many specimens have a resinous luster, the product of complex crystal overgrowths, and also required little cleaning. Un-etched crystals are translucent to semi-transparent.

 

Nice cabinet specimen just received at Collector’s Edge Laboratory.

 

Sorting orpiment specimens just received at Collector’s Edge Laboratory.

 

View of orpiment specimens at Collector’s Edge Laboratory ready for further cleaning and preparation.

 

Another view of orpiment specimens being readied for cleaning and preparation.

 

Some cleaned and prepped orpiment specimens being readied for sale.

ORPIMENT: As2S3

Orpiment, originally named auripigmentum (Latin for “golden paint”) in 1747, is a common mineral in low-temperature hydrothermal veins and hot spring deposits. Good crystals, however, are rather rare; until the Nevada discovery came to light, the highest quality crystals known were those from the Quiruvilca mine, Peru, and Shimen, Hunan, China. Twin Creeks orpiment is at least their equal.

Morphologically the crystals resemble those from other prominent localities, consisting primarily of {I01} and {lll} on the terminations and {1l0}, {l20} and (rarely) {100} comprising the other faces in the [001] zone. Faces are usually striated, probably through oscillation of {I01} and {lll} across the terminal faces and {1l0} and {120} parallel to [001]. The {010} form, parallel to the very easy cleavage, is very rare or absent. Crystals have a rather stubby, chisel-shaped aspect, measuring not much more along the c direction than the a, perhaps to a maximum ratio of 2:1.

 

Orpiment crystal drawings showing habits typical of Twin Creeks mine specimens (based on crystals from Tajov, Czech Republic; Goldschmidt, 1913). O{101}, p{111}, i{100}, m{110}, a{010} (rare or absent), u{120}.

The Twin Creeks crystals are fairly consistent in size throughout the deposit, ranging from 5 mm to 2 cm along the c axis and 2.5 mm to 1 cm across. A few crystals reach 4 cm. but their quality is generally poor.

Crystal clusters and matrix specimens are generally less than 15 cm across; most consist of plates of interlocking crystals covering a dark-colored matrix, but a few specimens were recovered in which isolated crystals and small clusters perch aesthetically on a larger matrix.

The color of the crystals varies from reddish orange-to-orange, yellow-orange and honey-yellow, with many subtle shades in between.

The general quality of the specimens collected varies dramatically. Although most specimens have a bright luster, others are dull (the result of coatings or etchings).

Although orpiment specimens from other localities have graced the world’s mineral collections for decades without suffering noticeable change, some collectors have expressed concern about the long-term stability of Nevada specimens. Will exposure to direct sunlight, for example, cause any degree of alteration over time? Twin Creeks orpiment has not been known for long enough to answer that question definitely, but, as with any mineral species, it is always advisable to protect them from long-term exposure to direct sunlight.

Perhaps the fact that realgar alters, under exposure to light, to a powdery yellow orpiment-like mineral named pararealgar has inspired worries about the stability of orpiment, which in fact is not known to undergo any such transition.

It might be speculated that impurities or inclusions of other species might affect stability. Electron microprobe analyses of orpiment specimens have determined them to be essentially pure As2S3, with no realgar and with traces of Sb and Fe too small to have any conceivable effect on stability. The purest and least pure

points analyzed were (respectively) (As1.84Sb0.005), and (As1.8Sb0.04), (points Pl-5-4 and P2-1-7).

 

Electron microprobe analyses of two orpiment crystals (P1 and P2) from the Mega pit (atomic %). Analyses courtesy of Karen Wenrich; Comeco electron microprobe; beam current 17 nanoamps. Standqrds used; orpiment for As, pyrite for Fe, stibnite for Sb.

  • Below detection limit (<0.02)

**   Sums adjusted to 100% - Weight % totals ran between 98 to 101%

The difference between the crystals of clean, bright color and those having a darker, murky appearance was found to be the presence of microscopic pyrite inclusions. Orpiment surrounding such inclusions carries a higher iron content which may also cause a darkening of the color. Brightly colored orpiment lacks such inclusions and has an extremely low iron content.

 

Photomicrograph showing orpiment crystals in thin section; the pale crystal is essentially pure orpiment whereas the murky crystal is discolored by trace amounts (~ 500 ppm) of iron. Field of view: 1.5 mm.

Because realgar is unstable it was specifically sought for as inclusions, but none was found, either during the microprobe analyses or the study of eight polished thin sections.

Considering the above studies, it is our conclusion that Twin Creeks orpiment is as stable as pure orpiment from any other locality.

 

Orpiment, 4.3 cm, from the mega pit. Newmont Mining Corporation specimen.

 

Orpiment with barite, 3.4 cm, from the mega pit. Newmont Mining Corporation specimen.

 

Orpiment, 7.1 cm, from the mega pit. Newmont Mining Corporation specimen.

 

Orpiment, 7 cm, from the mega pit. Newmont Mining Corporation specimen.

 

Orpiment, 13 cm, from the mega pit. Newmont Mining Corporation specimen.

 

Orpiment, 10.2 cm, from the mega pit. Azurite Corporation specimen.

 

Orpiment, 9.5 cm, from the mega pit. Newmont Mining Corporation specimen.

 

Orpiment, 12 cm, from the mega pit. Carnegie Museum of Natural History specimen.

 

Outstanding orpiment specimen, 5 x7.6 cm

 

Another outstanding orpiment specimen, 5 x7.6 cm

 

Beautiful orpiment specimen, 10 cm x 13 cm.

 

Orpiment crystals on matrix, 9.5 cm wide.

OTHER MINERALS:

The only significant associated species is barite, occurring rarely as elongated, thin blades to 7 cm on orpiment: these crystals vary from opaque gray to golden yellow in color. Microcrystalline arsenian pyrite, realgar and stibnite have been found in or near the ore zones. The pyrite is commonly seen as a dusty, sooty-looking dark layer on rock. Of course there is also finely disseminated gold accompanied by arsenic enrichment in the zone between the TC and DZ faults wherein the orpiment occurrence is located.

CURRENT STATUS

Orpiment mining activities at the Twin Creeks mine preserved many fine mineral specimens and created no slowdowns in ore production. Constant communication between mine staff and collecting crews ensured that high levels of safety were maintained and the best possible collecting procedures were followed.

The level of cooperation between Newmont and the Collector’s Edge is, perhaps, unprecedented in modem mineral collecting and proves that mineral collecting and commodity mining can be symbiotic in nature rather than detrimental to mining company profits. Newmont’s commitment to preserving mineral specimens is an exciting event in the mineral world. Too often, great discoveries are carried off to a crusher and lost forever. Specimen preservation efforts such as the orpiment project may help encourage other mining companies to re-evaluate their mineral collecting policies and become more proactive in saving mineral specimens. Mineral specimens are one of earth's treasures and saving them in today’s environment-conscious world cannot help but improve mining’s image. The positive benefits to public relations are immeasurable.

Since June 1999, several Nevada newspapers have covered Newmont‘s efforts to preserve mineral specimens. The response to these articles has been very favorable. And, as if to underscore the effort, Barrick Resources’ specimen preservation effort to save the golden barite from their Meikle mine in Nevada has received similar attention in Nevada newspapers and mining and mineral journals.

The orpiment in Cut 62 is exhausted. At the depth of 4120 Bench, much of the fold structure containing the orpiment mineralization is completely mined out. The current pit model places the 4120 level at the lowest developed part of the mine, so there will be no additional mining of the structure at depth in this area.

There are several places within the Mega pit walls where drilling intercepts show additional orpiment zones occurring along the strike of the fold axis. Unfortunately, the pit wall will not advance to uncover them because they lie in areas of uneconomical gold concentrations. Should the price of gold skyrocket; the potential exists for revising the mine plan and widening the Mega pit.

To the northwest, work is underway on Cut 20 (near Cut 62). During mining activity in this area, signs of orpiment have been seen. This is the most likely target for future specimen production. However, no crystallized orpiment has been encountered here to date. This cut will be the western-most extension of the pit.

ACKNOWLEDGMENTS:

Collector’s Edge Minerals, Inc. recognizes and thanks the Mineralogical Record for permission to use portions or all of their original article from the “Mineralogical Record”, Volume 31, Number 4, pp. 311-322 and pp. 331.

The author also wishes to thank the geology staff at the Twin Creeks mine; Ron Thoreson, Pat Donovan and Leroy Schutz provided invaluable support in promoting this project and supplying information for this article. Trent Tempel, General Manager of the Twin Creeks mine, was instrumental in clearing the way for the collecting effort to begin. Lyle Avey and Randy Ford, the engineering staff, and the entire crew at the Twin Creeks mine could not have been more helpful in making the collecting effort as efficient and successful as it could be. Doug Hock and Louis Schack, Newmont Public Relations managers, spent long hours promoting the project both inside and outside the Newmont organization.

And finally, thank you to all of the people in the Newmont organization for believing in the project and helping to make it happen.

REFERENCES:

NAKAI, 1., YOKOI, H., and NAGASHIMA, K. (1986) Crystal chemistry of the system As-Sb-S (1): Synthesis of wakabayashilite and synthetic study on the solid solutions in the As2S,-Sb2S3 system. Mineralogical Journal, 13 (4), 212-222.

NEWMONT MINING CORPORATION (1998) Annual Report.

NEWMONT MINING CORPORATION (1999) Unpublished internal report; notes on the geology of the Twin Creeks Mine, 19 p.

RADTKE, A., TAYLOR, C., DICKSON, F. W., and HEROPOLOS, C. (1974) Thallium-bearing orpiment, Carlin gold deposit, Nevada. Jour. Research U.S. Geol. Survey, 2 (3), 341-342.

RADTKE, A., TAYLOR, C., and HEROPOLOS, C. (1973) Antimony-bearing orpiment, Carlin gold deposit, Nevada. Jour. Research U.S. Geol. Survey, 1 (1), 85-87.

SIMON, G., KESLER, S. E., and CHRYSSOULIS, S. (1999) Geochemistry and textures of gold-bearing arsenian pyrite, Twin Creeks, Nevada: Implications for deposition of gold in Carlin- type deposits. Economic Geology, 94, 405-422. K