The Flambeau mine, Ladysmith, Wisconsin.
The Flambeau mine site is located in Rusk County, Wisconsin, about 1 mile southwest of the town of Ladysmith. The terrain in the area is generally flat, with forests, lakes and wetlands interspersed among dairy farms and small communities. The Flambeau River (from which the mine takes its name) passed within 140 feet of the southwestern end of the open pit, before the pit was completely backfilled.
The Flambeau deposit was a high-grade copper orebody which, over the 4.5-year life of the mine, yielded about 1.8 million tons of ore averaging 10% copper and 0.18 ounce of gold per ton. About 330,000 ounces of gold and 3 million ounces of silver were produced. Mine construction began in early 1992; the operation is the first and only sulfide metal mine to open under Wisconsin's new mining regulations. Ore production began in May of 1993, maximum pit depth of 220 feet was reached in March of 1997, and mining of the orebody was completed in July 1997. The pit, which reached a maximum size of 2,500 by 500 feet, has since been totally filled, and the site reclaimed and revegetated.
During the course of mining some extraordinary specimens of chalcocite began to show up; contract specimen recovery by two of the authors (CJ and JKJ) began in June 1994 and continued until the closing of the mine in 1997. From the 1.8 million tons of ore mined, around 1,000 pounds of crystallized chalcocite specimens were ultimately recovered, including many large, extremely well-formed and (surprisingly) beautifully colored specimens.
When news of the Flambeau chalcocites first began to circulate, many people were surprised by the location, considering that Wisconsin is generally not thought of as a mining state today. However, in the previous century its lead and zinc resources were well-known and helped the territory to attain statehood. Wisconsin's nickname of the "Badger State" is a reference to early lead miners who lived in abandoned mine tunnels and hillside burrows. The Wisconsin State Seal depicts a miner and his tools, a badger, and a stack of lead ingots. Consequently, in addition to its instant fame among mineral collectors as a new "classic locality" for chalcocite, the Flambeau mine has stimulated renewed interest in the mining history of the state. The Flambeau Mining Company, Kennecott Minerals, and the various individuals who helped to facilitate specimen recovery are owed a debt of gratitude for working to preserve and share the unique mineralogical aspects of the site with the scientific and collecting communities. This applies especially to mine manager Greg Fauquier, who made the decision in favor of specimen recovery and never waivered in his support. He is the person most responsible for the preservation of the Flambeau specimens.
Like many mining stories, the history of the Flambeau orebody discovery and subsequent mining project is colorful and begins long before ground was broken. Early in 1915 George W. O. Hotchkiss, working for the Wisconsin Geological and Natural History Survey, was prospecting and mapping much of north-central Wisconsin. His notes and rock samples were eventually stored at the offices of the Wisconsin Geological Survey in Madison. Amazingly, his original specimen of volcanoclastic rock with chalcopyrite and malachite mineralization is still on file in Madison (May and Dinkowitz, 1996).
Many years elapsed between this early work and Kennecott's interest in the Lake Superior region starting in the 1950's. This interest was based in part on the supposition that mineralization in the Canadian Precambrian Shield (which to the north in Manitoba and Ontario includes many massive sulfide deposits) might continue south into Minnesota, Michigan and Wisconsin. In 1954, George Moerlien began prospecting in Wisconsin and Michigan for Kennecott's wholly owned subsidiary, Bear Creek Mining. While in Madison that same year, Moerlien was shown Hotchkiss's rock specimens and information. One specimen was particularly exciting in that it assayed 0.7% copper. The report by Hotchkiss on this specimen stated that it came from a well dug near an old schoolhouse south of Ladysmith.
Moerlien traveled to the schoolhouse and searched the area; although no outcrop was visible, azurite and malachite-stained fragments of volcanic rock from the digging of the old well were found. It was decided that, since airborne electromagnetic (AEM) surveys were being flown over parts of Minnesota and Wisconsin anyway, some smaller targets should also be flown. The schoolhouse prospect was selected as one of these. No anomalies were detected by the airborne survey, but enthusiasm persisted about the possibility of deep massive sulfide mineralization.
W. F. Read and Russell C. Babcock of Lawrence University continued geological mapping through the late 1950's, generating data on which the next stages of exploration were based. The period between 1960 and 1965 was quiet while Kennecott went west looking for copper porphyry deposits.
Kennecott re-entered Wisconsin in the mid-1960's, at about the same time that Carl E. Dutton of the United States Geological Survey began conducting a library research program whereby all of the notes, rocks and mineral specimens collected since 1915 were re-examined and compiled. Dutton and Reta Bradley eventually organized all of this early data into a comprehensive geological map of Wisconsin, published in 1970.(1)
Great Lakes Exploration, a wholly owned Kennecott subsidiary, with Jack Phillips as exploration geologist, came to Wisconsin in 1965 to continue the massive sulfide exploration program. This new interest was due in part to developments in airborne electromagnetic survey technology allowing deeper penetration. Jack Phillips spotted the same sample Moerlien had seen, sitting on Dutton's desk. based on this sample, plus other weakly mineralized outcrops found in western Rusk County, and a review of the extensive data generated by Read and Babcock in the 1950's, Phillips convinced Great Lakes Exploration to fly an airborne survey over a large area of the county. This area included the schoolhouse site, the Flambeau River, and the Blue Hills. E. H. Eisenbrey supervised the AEM program. This time a strong anomaly was discovered on the east bank of the Flambeau River, just south of Ladysmith. In 1967, Eisenbrey authorized two additional short AEM lines to be flown from the river east to highway 27, a distance of 1 mile. This survey confirmed the anomaly.
In the spring of 1968, an extensive exploratory core-drilling program began in Rusk County and on November 6th of that year the Flambeau deposit was discovered. Core pulled from that hole showed 48 feet of massive supergene enriched chalcocite ore averaging 9.25% Cu. Environmental studies, public hearings and drilling to define the deposit continued through 1976.
From 1977 to 1985 the project was dormant, but in 1986 Kennecott reopened the Flambeau project when copper metal price forecasts turned optimistic. Progress toward actual construction and mining involved numerous public hearings, reviews, and permit applications from 1987 to 1991. Permits were finally granted in June of 1991. Ground was broken on the first sulfide metal mine to operate under Wisconsin's new mining laws in 1992 and on June 28, 1993, the first crushed copper ore was shipped via rail for processing.
From 1993 until its closure in 1997, the mine successfully operated without any violations, citations or complaints. This is a truly outstanding accomplishment considering that the mine is within 150 feet of the Flambeau River, 500 feet of Highway 27, within 1000 feet of private residences, and 2500 feet from a liberal arts college, the Rusk County hospital, and a nursing home. Ore shipments during the four years of operation totaled 1,896,000 tons averaging about 10% copper (including 266,000 tons of especially rich ore averaging 17% copper!), 0.18 ounce of gold per ton, and also some silver. Although comparatively small, the Flambeau orebody was one of the richest copper deposits ever mined, so rich that no beneficiation of the ore was required at the mine site! The copper ore was shipped 800 miles by rail to the smelter in Timmins, Ontario, and the gold-rich gossan was shipped about 1,000 miles to the smelter at Rouyn, Quebec.
By July of 1998 the pit had been completely backfilled (in the reverse order of initial rock and soil removal) and the surface revegetated. Plans are being made for construction of a series of hiking trails and other recreational facilities on the site.
The Flambeau mine is located in a broad belt of Precambrian volcanic and associated sedimentary rocks known as the Wisconsin magmatic terranes. which lies on the southern margin of the Canadian Shield. These rocks are the remnants of an extensive volcanic island arc that collided with the margin of an ancient continent in the Lake Superior region about 1,860 million years ago. The collision zone is shown on modern geologic maps [ILLUSTRATION FOR FIGURE 4 OMITTED] as the Niagara Fault Zone (Sims, 1992). The collision resulted in intense deformation of the volcanic and sedimentary rocks and produced a mountain range known as the Penokean Mountains. This once-lofty mountain range eroded to a landscape of low rolling hills, the only remnants of which are found in northern Wisconsin. The original extent of the Penokean Mountains is not known because the eroded roots of the mountains have been buried beneath younger sedimentary rocks from the Michigan border eastward, and westward from Ladysmith, Wisconsin.
The orebody at the Flambeau mine is a volcanogenic massive sulfide deposit, one of ten similar sulfide bodies found along the length of the volcanic belt in northern Wisconsin in recent years (DeMatties, 1996). Several of the sulfide bodies were evidently deposited on the flanks of rhyolitic volcanic islands where hot hydrothermal brines containing iron, copper, zinc and lesser amounts of lead, silver and gold issued forth onto the ocean floor. At Ladysmith these metal-rich brines produced a broad, thin layer about 50 feet thick composed of sulfide mud with some admixed volcanic materials coveting perhaps a square mile. The Flambeau sulfide deposit is underlain by a thick sequence of felsic volcanic lava flows and tuffs, with lesser amounts of volcanogenic sediments. A thick pile of basaltic volcanic rocks, some of which are pillowed, as well as some felsic volcanic rocks, overlie the ore horizon (May and Dinkowitz, 1996).
The primary ore consisted of vaguely layered, fine-grained pyrite, chalcopyrite, sphalerite, pyrrhotite and minor galena (which probably contained some silver) (May and Dinkowitz, 1996). Minor gold was disseminated in the sulfide-rich lens. Abundant cherty horizons within the sulfide ore indicate that the hot springs also carried considerable silica. A possible siliceous mound, or "chimney," was encountered during mining operations. The sulfide orebody had a chert-rich "cap" approximately 5 feet thick, which was overlain by a 10-foot-thick cherty quartz-sericite schist containing substantial amounts of very fine-grained gold (May and Dinkowitz, 1996).
The felsic volcanic rocks underlying the orebody were extensively altered to sericite and kaolinite over a broad zone by the hydrothermal solutions that deposited the ore. Unlike some volcanogenic massive sulfide orebodies, the altered rocks beneath the ore zone at the Flambeau mine contain copper mineralization. In fact, no well-defined alteration pipe has been identified beneath the Flambeau orebody (May and Dinkowitz, 1996).
Folding and faulting associated with the formation of the Penokean Mountains about 1,850 million years ago tilted the volcanic rocks in the Ladysmith area more than 90 [degrees], so that the originally horizontal layers are now nearly vertical [ILLUSTRATION FOR FIGURE 5 OMITTED]. Metamorphism resulted in recrystallization of the sulfide minerals to a coarser grain size (about the size of coarse sand), and converted the felsic volcanic rocks to quartz-sericite schists and the basaltic rocks to actinolite-rich schists (May and Dinkowitz, 1996). Widespread andalusite and spessartine garnet developed in the aluminum-rich alteration zone beneath the orebody.
The Penokean Mountains were then subjected to a long period of weathering and erosion. The mountain range was reduced to low rolling hills by about 1,200 million years ago - perhaps much earlier. The Barron Quartzite, exposed about 12 miles west of the Flambeau mine, and the Flambeau Quartzite exposed about 15 miles in the south, were deposited on the eroded edges of volcanic rocks that formed the Penokean Mountains. The age of these quartzite units is not well established. They may be as young as about 1,200 million years (LaBerge, 1994), or as old as about 1,700 million years (Dott, 1983). (As discussed later, the age of the quartzite may be significant with regard to the timing of the enrichment of the ores.) Regardless of its age, the quartzite was heavily eroded during later Precambrian time, leaving only widely scattered remnants in northern Wisconsin.
The Upper Cambrian sea which covered Wisconsin some 520 million years ago, deposited a layer of sandstone over most of Wisconsin, including the Flambeau deposit. The sandstone was, in turn, mostly eroded from the area; but, fortuitously, a small remnant (an outlier) directly above the Flambeau orebody persisted. Finally, continental glaciers moved across northern Wisconsin about 20,000 years ago and removed most of the weathered rock that had accumulated over the eons of weathering. The small outlier of Cambrian sandstone protected the Flambeau orebody from glacial erosion, and from erosion by streams following the retreat of the glaciers.
The Flambeau mine is basically like many other volcanogenic massive sulfide deposits in the Canadian shield. Indeed its origin is similar to at least ten other sulfide deposits in northern Wisconsin (DeMatties, 1996). However, it differs fundamentally from other massive sulfide deposits in its mineralogy (mainly chalcocite) and in the high grade of the ore. This major difference at the Flambeau mine is the result of the orebody having been extensively altered and enriched by chemical weathering. In oxygen-rich ground waters the sulfide minerals became unstable. Above the water table the sulfur in the sulfides was oxidized to sulfate [(S[O.sub.4[).sup.2] which produced sulfuric acid. The iron in the original pyrite (Fe[S.sub.2]), pyrrhotite ([Fe.sub.1-x]S) and chalcopyrite (CuFe[S.sub.2]) was oxidized to goethite (FeO(OH)) or hematite ([Fe.sub.2][O.sub.3]). These iron minerals remained at the surface due to their extreme insolubility, and formed an iron-rich cap (gossan) on the deposit. Gold was also concentrated in the surface environment, with the result that the gossan was also rich in gold. Near the surface the acid-rich groundwater dissolved nearly all of the copper, zinc and silver and carried them downward to the water table. Below the water table, where oxygen was much less abundant, the copper which had been dissolved in the groundwater replaced iron in the sulfide minerals. Primary pyrite and pyrrhotite were convened to chalcopyrite, and primary chalcopyrite was subsequently convened to bornite ([Cu.sub.5]FE[S.sub.4]), with the iron going off in solution. Continued percolation of copper-bearing water, and replacement of the remaining iron in these copper minerals, eventually converted the chalcopyrite and bornite to chalcocite ([Cu.sub.2]S); this greatly enriched the original ore in copper in a zone just below the water table. This enrichment process resulted in a crude three-fold zonation of the deposit with an upper zone rich in chalcocite, underlain by a zone rich in secondary bornite and a lower zone rich in secondary chalcopyrite. The boundaries between these zones are highly irregular. Because zinc is much more soluble than copper, it does not form an enriched zone below the water table. The zone of copper enrichment at the Flambeau mine was approximately 200 feet thick. Below the enriched zone the primary ore minerals occupy the ore horizon.
As shown in Figure 7, many weathered deposits contain a variety of secondary minerals in the leached zone above the water table. Azurite, malachite, chrysocolla, cuprite, native copper and native silver were present at the Flambeau mine, but they occurred in very subordinate quantities. Grade and tonnage calculations (May and Dinkowitz, 1996) indicate that a volume of primary ore approximately six times the volume that was mined had to be concentrated by supergene enrichment to produce the rich copper ore. The relatively thin covering of gossan and leached ore above the rich chalcocite zone suggests that most of the leached zone was removed by erosion during or after the sulfide enrichment.
A vast majority of the chalcocite at the Flambeau mine consisted of massive to granular material; this constituted most of the rich ore. Grain size and texture of the ore suggest that the chalcocite was produced as a result of copper replacement of iron in the earlier recrystallized pyrite. Relatively open cavities were developed in about ten sites within the enriched ore zone. Three large cavities ("pockets") with dimensions of several feet in size (discussed later), were encountered during mining operations. These pockets were evidently formed where the groundwater flow was channeled along confined pathways. Most of the collectible crystals occurred in these three large pockets.
A consistent and distinctive pattern of crystal development occurred in the major pockets. The massive to granular chalcocite on the pocket walls was coated with a meshwork of chalcocite with a distinctive cellular or ropy texture [ILLUSTRATION FOR FIGURE 6 OMITTED]. The cellular chalcocite is succeeded inward by peculiar curved or twisted chalcocite crystals up to at least 5 cm long [ILLUSTRATION FOR FIGURE 7 OMITTED]. Euhedral crystals and crystal groups usually developed only in a limited portion of a pocket. The crystals typically show orthorhombic morphology, indicating that they grew originally as chalcocite, and thus, they are not replacements of earlier bornite or some other crystal phase. Like other orthorhombic minerals, they exhibit a wide variety of crystal habits. Additionally, X-ray studies show that the crystals are essentially pure chalcocite, not a combination of several mineral phases. Therefore, both the field relations and the crystal morphology suggest that the chalcocite crystals at the Flambeau mine developed late in the enrichment process. Curiously, and perhaps significantly, there was almost no development of euhedral crystals of bornite or chalcopyrite associated with the enrichment. However, botryoidal masses of chalcopyrite did develop, which were partially to completely replaced by bornite, and this, in turn, was replaced by chalcocite.
Perhaps the most distinctive feature of Flambeau mine chalcocite is the attractive brassy yellow, bronze, purple or blue patina on the crystals. Nearly all of the crystals recovered have a patina of one or more colors. Lead-gray crystals, of the kind typical of most deposits, are decidedly uncommon at the Flambeau mine. EDAX analyses performed by Lance Kearns first determined that the coating is not chalcocite. Microprobe studies by Joseph Mandarino and Malcolm Back show that the various colors are produced by a very thin coating of bornite.
Various lines of evidence indicate that the chalcocite crystals at the Flambeau mine are very old. First, the observed geology and the grade and tonnage calculations (May and Dinkowitz, 1996) demonstrate convincingly that the supergene enrichment occurred before the invasion of the Cambrian sea which deposited a layer of sandstone almost directly on the enriched ore some 520 million years ago. Thus, the enrichment must have been older than Cambrian in age. The volcanic rocks that host the deposit were deformed about 1,860 million years ago (Sims and others, 1989) during the Penokean Orogeny. Therefore, the enrichment might have occurred at any time between 1,860 million and 520 million years ago.
Other lines of evidence suggest that the enrichment may have occurred well back in Precambrian time. During mining operations several blocks of quartzite were encountered in the gossan and altered volcanic rocks beneath the Cambrian sandstone cover on the deposit. These quartzite blocks may be remnants of the Barron or Flambeau Quartzite, which may, at one time, have covered the orebody. If so, their presence may indicate that the enrichment occurred during erosion of the Penokean Mountains which occurred prior to deposition of the Barron and Flambeau Quartzites. As stated earlier, the age of these quartzite bodies is not well established.
Mineralogical evidence also suggests that enrichment probably occurred well before the invasion of the Cambrian sea. Fresh, tin-white arsenopyrite and vein quartz [ILLUSTRATION FOR FIGURE 8 OMITTED] cut the enriched chalcocite and bornite ore at several locations in the mine. It seems highly unlikely that the arsenopyrite would have survived unaltered through the entire enrichment process. It is far more likely that arsenopyrite-quartz veins were emplaced after the enrichment was completed. The question, then, is to identify a potential hydrothermal event that might have added the arsenopyrite. LaBerge (1997) suggests that the arsenopyrite may have been emplaced during igneous activity associated with the 1,100-million-year-old Keweenawan mid-continent rift system. Arsenic is locally abundant in the copper deposits of northern Michigan as domeykite ([Cu.sub.3]As) (Nicholson et al., 1992). Basaltic dikes of Keweenawan age are widespread throughout northern Wisconsin, including the Ladysmith area. These observations suggest that the enrichment, and therefore the chalcocite crystals may have formed more than 1,100 million years ago.
During the period of Earth history when the enrichment of the Flambeau mine occurred, the area destined to become Wisconsin (and most of North America) was located near the earth's equator. In fact, many geology textbooks show that the Great Lakes region straddled the equator during the latter part of the Precambrian and much of the Paleozoic era. During much of this time, the climate may have been similar to that in present day Australia, where the water table is a hundred meters or more below the surface. Since enrichment at the Flambeau mine extends down over 60 meters below the present-day surface, the water table must have been from 60 meters to as much as 360 meters below the surface.
When plate movements carried North America into more temperate climate zones the water table would have moved closer to the surface. (The present water table in the vicinity of the mine is less than 6 meters below the surface.) As the water table rose closer to the surface the chalcocite crystals, which had formed near the former deep water table, were subjected to groundwater that presumably contained significant amounts of dissolved iron from weathering of sulfide minerals. The chemical environment of the alteration system may be seen on the Eh/pH diagram in Figure 5. A very thin layer of bornite was precipitated on the chalcocite crystals from the iron-rich groundwater. However, with the exception of a few scattered bornite replacements of small chalcocite crystals, no large-scale formation of bornite related to a higher water table has been recognized at the Flambeau mine.
Locations within the open pit are specified according to the two parameters used by the mining company: (1) the horizontal hundred-foot markers, beginning on the extreme western edge of the pit with marker 400 and counting off due eastward 2,600 feet to marker 426 at the eastern margin of the pit, and (2) the vertical levels, in feet above sea level, ranging from the 1100 level at the top rim of the pit down to level 970 at the floor of the pit.
Important pockets have been given names for easy reference;. these are, from west to east: the First Pocket (marker 401.2, 1030 level, found June 30, 1994), the Watercourse Pocket (marker 401.2, 1030 level, found June 30, 1994), the Drill Pocket (marker 401.5, 1010 level, found January 31, 1995), the Lucky Friday Pocket (marker 402.0, 1000 level, found October 13, 1995), the Rocket Pocket (marker 402.3, 970 level, found August 15, 1996), the Sunrise Pocket (marker 406.2, 1010 level, found October 10, 1994), the Fog Pocket (marker 409.6, 1000 level, found March 30, 1995), and the Quick Pocket (marker 415.2, 980 level, found May 20, 1996). All are in the "A" (northern) orebody [ILLUSTRATION FOR FIGURE 6 OMITTED]. During the three years of specimen recovery only these eight significant pockets were found, several of which were quite small. The vast majority of specimens collected came from the Drill, Lucky Friday and Rocket Pockets.
Table 1 lists all of the species thus far identified at the Flambeau mine, including sulfates found as a result of ore studies conducted by Kennecott.
Small crystals of ankerite up to 1 cm were encountered on the east end of the orebody in an area confined within markers 420.0 and 422.5 on levels 1020, 1010, 1000, and 990. The ankerite was found lining small pockets and vugs in massive chalcocite. Microcrystals of chalcocite, chalcopyrite, marcasite, pyrite and siderite occur on the ankerite. Ankerite on the outer edges of these small vugs fluoresces a pale green under shortwave ultraviolet light. The identification as ankerite (instead of ferroan dolomite) is tentative; EDS analysis reveals considerable variation in Ca, Fe and Mg, even within single crystals.
Arsenopyrite occurred as microcrystals through the deposit, but was more common on the eastern end of the orebody, at markers 419.0 to 423.0, on the 1020 level. Several specimens with crystals up to 2 cm were recovered at marker 406.2, 1010 level, in a massive quartz pod within enclosing massive bornite. One crystal, however, was found in a pocket in massive secondary bornite.
Crystallized and massive azurite was encountered in the oxide zone beneath the gossan cap, particularly in the fault zones at markers 406.9 and 413.0. Crystals do not exceed 4 mm. Only a few specimens were recovered because most of the material was shipped as ore in 1994 before specimen recovery efforts began. Specimens saved include deep blue druses of microcrystals lining vugs in digenite, malachite and limonite gossan.
Massive bornite was quite common along the strike of the orebody. Most bornite formed by copper enrichment of the earlier chalcopyrite, which in turn had formed by enrichment of primary pyrite. No macroscopic crystals of bornite have been observed or recovered.
The Sunrise Pocket, at marker 406.2, 1010 level, on the footwall contact contained several fine specimens of bornite after chalcocite with crystals up to 3.5 cm. Some of these were found as floaters in a bornite sand and others were found attached to chalcocite crystals. At marker 415.0, 990 level, on top of a blast pile, half a dozen more pseudomorphs of bornite after chalcocite were discovered. The largest of these is a group with one crystal of bornite-after-chalcocite measuring 5.5 cm by 3.8 cm. Bornite is also present as a blue, purple, brown or reddish iridescent coating on chalcocite crystals.
An area on the footwall side of the orebody at marker 418.0 produced about 100 specimens of translucent white calcite crystals. This occurrence is in the country rock, 12 meters from the orebody.
Chalcanthite [Cu.sup.2+]S[O.sub.4] [multiplied by] 5[H.sub.2]O
George Robinson (personal communication) has identified chalcanthite (by EDS) from the Flambeau mine, as post-mining coatings on sericite schist.
The mineral that has made this locality a world-class specimen producer is chalcocite. It was the dominant ore mineral at the Flambeau mine, occurring as massive and granular material as well as fine crystals. The crystals were found on fracture surfaces and within cavities and pockets, primarily near the contact of the orebody with the quartz-sericite schist, on both the hanging and the footwall. The identity of the chalcocite has been confirmed by X-ray diffraction (Lance Kearns, pers. comm.).
The multi-colored patina or coating present on a vast majority of the chalcocite crystals from the Flambeau mine has been confirmed to be Bornite (J. A. Mandarino and Malcolm Back, Royal Ontario Museum, pers. comm.). These Bornite coatings are typical of the species in that almost every color of the rainbow is present, and the specimens are notably iridescent.
The best of the Flambeau mine chalcocite specimens are stunning and set the standard for contemporary occurrences of this species. The habits vary from poorly developed "ropy" reniform groupings to sharp cyclic and penetration-twin crystals. Classic pseudohexagonal twin crystals to over 5 cm have been round. One amazing penetration twin, an "X-twin" crystal, is just over 7 cm in size.
Botryoidal chalcocite is decidedly rare, occurring at only two locations in the orebody (at marker 423 and marker 408 at the footwall contact). About a dozen high-quality pieces were recovered. Some poorly crystallized specimens have a habit resembling ropy lava or pahoehoe, with well-developed crystals extruding out along the ridges of the "flow."
Massive chalcopyrite was less common than either bornite or chalcocite at the Flambeau mine. Microcrystals up to 3 mm (even less common) were present throughout the deposit. A few very fine, iridescent, botryoidal specimens were collected at the footwall contact at marker 407.5.
Chrysocolla [([Cu.sup.2+],Al).sub.2][H.sub.2][Si.sub.2][O.sub.5][(OH).sub.4][multiplied by] n[H.sub.2]O
Chrysocolla was seen only rarely, as crusts and films coating rock. It is present in a range of colors, from pale to bright blue, green and brownish green, in intimate association with azurite in the gossan and the oxide zone.
Native copper occurred primarily as fracture-filling sheets and foil. These occurrences were within the ore horizon, as far as 15 to 18 meters from the footwall. One notable location was in the maker 402.5 to 403.5 area on the hanging wall side. Another was at marker 418.0, 1010 level, on the footwall side of the deposit. One other location, marker 423.00, 1000 level, produced four specimens of crystallized native copper. Flambeau copper is chemically unusual in that it is high in mercury (0.11 weight %) and low in arsenic and selenium, unlike the Lake Superior copper deposits (Motti et al., 1998).
Cuprite [Mathematical Expression Omitted]
Microcrystals of cuprite were found with other copper minerals. Acicular cuprite occurred entirely as microcrystalline material with varying thickness and coverage, not exceeding 3 mm in thickness and 2 x 2 cm in area of coverage.
Dzhalindite, a rare indium hydroxide, was found at the far northeastern end of the orebody, at marker 422.0, 1000 level. The Flambeau mine appears to be the fourth known world occurrence. It was discovered while identifying other material, (Robert M. Housley, pers. comm.). Originally described from the type locality as pale yellow and semi-transparent, the mineral at Flambeau is only visible at 500X, with an Autoscan scanning electron microscope; crystals are 400 micrometers is size. Associations include chalcocite, spheroidal siderite and hessite.
Subhedral green grains and blebs of gahnite in granular sulfides have been identified by EDS and optical analysis (George Robinson, pers. comm.).
Galena is very rare at the Flambeau mine; only four specimens have been collected, at marker 423.5, 980 level.
Glaucodot was identified by Excalibur Mineral Company (on behalf of Dan Behnke) during routine analytical work. It was collected from an undocumented location during the gathering of educational ore samples intended for distribution to schools.
All native gold collected at the Flambeau mine came from one occurrence at marker 402.0, 1000 level, in vein quartz, shattered by blasting, between the orebody and the surrounding quartz-sericite schist. The gold occurred as microcrystalline sponge-like masses and coatings, intimately associated with veinlets of bornite and chalcocite running through the shattered quartz. One specimen, in which the gold area measures about 1.2 x 7.6 cm on quartz, is probably the largest mass of gold ever found in Wisconsin; it is currently preserved in the Kennecott Minerals collection.
Goethite forms a significant proportion of the gossan at the Flambeau mine.
Gypsum CaS[O.sub.4] [multiplied by] 2[H.sub.2]O
Microcrystals of gypsum have been reported (Dan Behnke, pers. comm.). Some take on the shape of flames, especially those with the bright violet-red bornite patina. These occurred most notably in the Drill Pocket, the Rocket Pocket and the Lucky Friday Pocket.
Kolbeckite ScP[O.sub.4] [multiplied by] 2[H.sub.2]O
Another rare mineral discovered quite by chance at the Flambeau mine is kolbeckite. Had it not been a sunny day, the reflection off the tiny crystal might have gone unnoticed. Only a dozen specimens have been recovered thus far. They were collected at the hanging-wall contact in the quartz-sericite schist near marker 412.0, 1010 level. The largest crystal, 1 mm in size and a vivid lemon-lime green, is associated with chalcopyrite. The identification was verified by Robert M. Housley (pets. comm.).
Leucophosphite [Mathematical Expression Omitted]
Leucophosphite has been identified from the Flambeau mine as minute clusters and bundles of glassy, transparent, pale brown crystals less than 1 mm in size (Dan Behnke and Tom Buchholz, pers. comm.).
Interesting habits of malachite were found throughout the orebody in the upper levels, the 1080 level through the 1010 level, and particularly at markers 402.0, 406.0 and 413.0. One of the more unusual forms consist of filaments or hairs of malchite. Masses of these fine filaments were sometimes exposed when breaking open host rock known to contain malachite. Some of these filaments are associated with azurite microcrystals. Analyses by George Robinson (pers. comm.) have revealed many of these filaments to be poorly crystallized chrysocolla, perhaps altered from malachite.
Marcasite microcrystals have been found rarely at the deposit.
Minute, gemmy, colorless prisms of orthoclase have been identified from the Flambeau mine, in association with rounded yellow crystals of siderite (Dan Behnke, pets. comm.).
Pyrite is almost exclusively massive at the Flambeau mine. Only a few well-formed pyrite crystals are known, and most of those are microcrystals. Pyritohedral crystals from marker 422.5, 990 level, however, reach 1.5 cm; some are coated with sooty chalcocite.
Motti et al. (1998) report pyrrhotite as an accessory mineral in the pyrite/chalcopyrite ore.
Dark red, prismatic microcrystals of rutile have been identified by EDS (George Robinson, pers. comm.).
Microcrystals of siderite form coatings on ankerite and bornite from the marker 422.0 area, level 1010 to the 980 level.
Silver has been found very rarely at the Flambeau mine as half-breeds with native copper, occurring as fracture-filling sheets and foil with native copper, and as distorted crystals. Flambeau silver carries an unusually high mercury content of 1.13 weight % (Motti et al.. 1998).
Sphalerite has been found as medium-grained massive ore restricted to a small zone at the far east end of the pit, at marker 424.5 on the 980 level. Microcrystals are also present sparingly throughout the orebody.
Steely black tetrahedral microcrystals of tennantite have been identified by EDS. some of which have a thin coating of chalcopyrite (George Robinson, pers. comm.).
Vivianite (or Metavivianite) [Mathematical Expression Omitted]
Yellow-green, bladed microcrystals of vivianite or metavivianite have been identified by EDS (George Robinson, pers. comm.).
As with so many operating mines in the past, collecting at the Flambeau mine began with geologists and miners on site picking up a pretty rock. The mine visitors' center needed specimens from the pit to put on display for the interested public, so mine geologists collected specimens of azurite, malachite, native copper and chalcocite. Seeing these specimens encouraged the authors (CJ and JKJ) to seek a specimen recovery contract with the Flambeau Mining Company in June of 1994. By that time most of the azurite-malachite-containing oxide zone had been mined out and shipped as ore. Evidently some fine specimens of azurite and malachite were found during the first year of mining, but only a few have been preserved, in administration offices and in the Visitors' Center.
Our first day of collecting at the Flambeau mine (in June 1994) was very much different from the underground environments we had been accustomed to. We went to work immediately in an area where the orebody contacted the siliceous chert, and were quickly rewarded with a few reniform to sub-euhedral crystallized chalcocite specimens. All of these first few pieces were of the typical black to steely-gray color so characteristic of chalcocite at other localities.
First Pocket and Watercourse Pocket
Two days of sporadic finds of poorly formed gray chalcocite only served to whet our appetite. The much better specimens we had seen at the visitors' center and administrative offices convinced us that the good ones were just inches away. On June 30, 1994, our intuition proved right. A small pocket located beneath a ledge had been opened when a large block of chalcocite ore was removed; it contained five chalcocite specimens that rivaled anything we could have realistically imagined. The excitement of the discovery of what are surely the finest chalcocites ever found in the Western Hemisphere encouraged us to continue working.
This led us to name this void the First Pocket, located at marker 401.2, 1030 level. Thankfully, this pocket led us later that day to the Watercourse Pocket, which was a few inches into the face of the work area where the First Pocket was located. The Watercourse Pocket, as the name implies, had water running through it. The chalcocite from this void all had a gold-colored patina when fresh; some remained gold-colored, whereas other specimens tarnished to a violet or purple color over time. The crystallization was not quite up to the quality of the crystals of the First Pocket, but this confirmation that good crystallization was present bolstered our confidence.
The next pocket was found on our return to Ladysmith in October of 1994; we named it the Sunrise Pocket. It had been a long interval between this discovery and the previous success earlier in July. This was due in part to heavy rains in September, which caused local flooding of such severity as to be expected only once every 500 years.
On October 10, Joe Celski, the excavator operator, cut into a pocket area. He was able to remove muck from our promising zone without impeding production elsewhere by taking a couple of cleanup swipes with his excavator shovel on his way to another mucking spot. Of course, starting time for collecting is as soon as there is just enough light to see. Joe expertly maneuvered the shovel of the excavator to get as much of the muck away from the corner that looked the most promising. Still, we had to hand-muck approximately three or four cubic yards of sulfide ore. Just as the sun was peeking over the rim of the pit, the outline of an in situ pocket was revealing itself in a corner formed by the 1010 level bench and the footwall rocks.
Slowly brushing away the loose, damp bornite sand revealed gleaming bright, metallic golden chalcocite crystals. They were tightly packed with just enough space between specimens to allow removal one by one. Laying on my stomach, I slowly worked the pieces out using the sand as a cushion for the crystal specimens when needed. Small broken shards of quartz-sericite schist and small chalcopyrite casts after calcite (looking like hundreds of tiny ice-cream cones) made up the pocket debris. The chalcocites that came from this pocket are unusual in that they are associated with pseudomorphs of bornite after chalcocite. These specimens are spectacular in size, generally from 1.2 to 3 cm. One crystal group, which we named the "Flambeau Chief," has a prominent crystal almost 4 cm across.
It was a long, cold winter between the Sunrise Pocket and the next significant pocket to be discovered. While we were attending the Tucson Show in February, 1995, the drill rig operator, Randy Ralston. broke into what could be considered a small chamber measuring approximately 2 meters long by I meter wide and extending to an unknown depth. The location was documented as marker 401.5, 1000 level, on the hanging-wall contact. We named it the Drill Pocket, because the drill rig had punched into the top of the void. The Drill Pocket contained what we still believe were the finest chalcocites in the world. Unfortunately, because we were not there to collect the specimens and mining needed to proceed, the area was blasted away and many of these specimens were either heavily damaged or lost forever. Fortunately, mine personnel were able to salvage and sack specimens while mucking proceeded. But we still dream of what this void could have yielded if we had been there to collect it properly.
Typically the Drill Pocket specimens have a deep blue patina which appears to be very stable. The choicest specimens are tabular, 6 to 10-cm crystals in distinctive X-shaped twins. These came only from the Drill Pocket. Clusters and matrix pieces were also recovered in some abundance. A number of plates of tabular, deep blue chalcocite crystals were presumably removed from the pocket wall. The Drill Pocket also yielded cyclic twins and untwinned chalcocite; some of the cyclic twins have a brilliant yellow patina which tends to turn purple or brown with time.
The next significant pocket was the Fog Pocket, at marker 409.6, 1000 level, found on March 30, 1995. This small pocket was encountered one cold morning while scouting the exposed 1000-level bench on the footwall contact. While probing for loose ground a small avalanche of rock and ore debris fell away at chest level. Quickly recovering my balance and homing in on the new muck pile at my feet, I (CJ) saw beautiful, delicate, small, perfectly shaped six-sided crystals of golden/violet chalcocite scattered atop the broken muck. The two dozen or so thumbnail-size specimens were quickly wrapped.
Tracing the route of the rock-fall upward revealed a small opening densely packed with crystallized chalcocite, trending up to the crest of the bench, just overhead. Due to the cold weather, recovery was particularly difficult, wearing fingerless gloves and using only small screwdrivers and bare fingers to extract the beauties. I was crouching as much for warmth as for access to the pocket opening, and my breathing was creating a transient but none-the-less annoying fog bank, thus the Fog Pocket. This pocket produced a dozen outstanding specimens. The sharp edges and delicate habit of these crystals surpassed those of any other pocket.
Lucky Friday Pocket
Next came the Lucky Friday Pocket, found on Friday, October 13, 1995. This pocket was also a bitter-sweet experience. By this time we were living full time in Ladysmith and were on site every day watching every load of ore and waste rock lifted out of the broken and shattered blast pile. We were also checking almost all of the drill holes that were being drilled by Randy Ralston and crew. On-this day there was guarded optimism.
The day before, while drilling the blast pattern for this shot, one hole penetrated a good size void at 7.5 feet. This hole was left unloaded with explosives, and we stuffed as many burlap bags as we could down the drill hole for cushioning. Credit must be given here to the blasting crew for their expertise, and in particular to Ames Construction project manager, Bob Donaldson, for the creative blasting techniques used to help minimize destruction of specimens while still "pulling the round." Late that day (the 12th), scouting this blasted zone, we found broken loose crystals. We had Joe Celski position the excavator so he could direct the trucks in such a way as to be able to dig perpendicular to the strike of the ore-body, taking small slices from the area where we expected to find the burlap bags. With heavy clouds threatening snow, the bucket swept past where the hanging-wall contact was exposed, and big crystals started sliding out and down the soft pocket muck. The feeling was one of elation yet concern that nothing would survive. No burlap was yet visible. There were 3 meters of heavy broken ore on top of this exposure. Joe expertly worked the cap off the area and the burlap bags became visible. With just an hour of light left, expedient removal of the best specimens was of the utmost concern. Incredibly many specimens survived the harsh treatment of blasting, mucking and rushed recovery.
Once again the nature of these chalcocites is completely different from any we had previously seen. The crystals range from thick, disc-like, subparallel stacks to rarer heavy, thick, penetration X-twins and the classic, perfect, sixling twins. The predominant color was golden, darkening to iridescent brass, violet, purple, blue and gray, depending on the angle of light and viewing. There are a very few specimens that have a non-iridescent druse of matte-blue bornite coating the chalcocite.
The Lucky Friday pocket produced many excellent crystal specimens ranging from thumbnail to large cabinet size. Some are sharp bronze-colored singles and groups; others are matrix pieces with thick discoidal crystals to 4 cm perched on massive chalcocite. The largest specimens are spectacular, three-dimensional clusters 17 to 20 cm in diameter and up to 15 cm tall, with numerous thick discoidal, bronze-colored, pseudohexagonal, 1 to 3-cm crystals radiating from a central core. These appear to be "floaters," having no obvious points of attachment, which grew in black granular chalcocite.
The Quick Pocket needs to be mentioned because of its unusual location. It was found on the hanging-wall contact but far from the more productive west end. This pocket was encountered on May 20, 1996. Digging while drilling was taking place just above the pocket area necessitated especially vigilant monitoring of loose ground. The drilled pattern was to be shot regardless of mineral specimen concerns, because the main ramp into the pit was at stake. Therefore, quick recovery methods had to be used (thus, the name "Quick Pocket"). The chalcocite from here is generally less well developed and tends to be more or less replaced by Bornite. However, a few specimens of chalcocite with well-developed crystals were recovered; many have a violet to reddish plum-colored patina. Crystals range up to 6 cm.
The Rocket Pocket was found at marker 402.3, 970 level, in August of 1996. This was to be the last significant pocket encountered. It did not come quickly, but rather was drawn out over almost a week. The process began with the drilling of the pre-split holes on the 970 level at marker 402.3, on the far west end of the pit. These holes were drilled for the 6-foot step outs which were planned for every 30 feet (or three levels). Consequently these holes were drilled to a depth of 30 feet, versus the normal blast hole of 10 feet. Starting on Friday, August 9, drilling progressed smoothly beginning on the far south footwall side of the pit. But trouble began when the first drill hole closest to the hanging-wall contact zone collapsed, and the drill steel became hopelessly stuck. A new hole was begun, near this bad hole, and at a depth of about 2 feet the drill steel dropped through 4 feet of void before biting into hard ground again. Whenever drill-steel dropped, our hopes soared!
The decision was made to pull out of this,hole before the steel became stuck, because the condition of this hole was rapidly deteriorating. After pulling the steel from this hole it was evident that it could not be used to set explosives. A 3-foot section of 2-inch PVC pipe was pounded as far as possible into the broken and collapsing hole. The next hole to the north was barely competent enough to load. The rest of the holes to the north held up well and were not a problem. This drilling had taken Friday, Monday and Tuesday. That was one long weekend. The regular blast pattern was drilled out and loaded with explosives along with the pre-split holes on Wednesday. Just as the words "fire in the hole" finished leaving Bob's mouth, the ground erupted in the familiar staccato of timed charges going off. The PVC pipe had become airborne, shooting almost straight up and trailing smoke and dust like a rocket. There was an easy name for a pocket. We were on site and ready to go at first light, tools at hand; this had to be a good one. A void was there, it was up against hard face, and we were due. Again, Joe set up to mine away the hanging-wall contact area in the most productive and merciful (to the specimens) way.
At about 10:00 am the outline of a large cavity was exposed at the hard face of the far west end of the pit, right below where the pipe had taken flight. Recovery began as soon as the excavator's bucket was out of reach of possible contact with body parts. The pocket was full of blasted ore so crystals could not easily be seen. Careful probing and removal of this waste material began to show more and more specimens. The crystals once again were completely different in habit from any of the other chalcocites we had found. We believe that as much as 20% of this pocket was completely destroyed by the blast. Among what remained were the largest specimens recovered, four weighing over 20 pounds, with up to 90% crystal coverage. Habits include sharp single crystals, single sixling twins, stacked sixling twins and unusual feather-like or flame-like crystals. All the specimens have a distinctive multicolored iridescent sheen. It took a total of 16 hours to remove all of the specimen material from this pocket. The void was big enough to sit in with room to spare, approximately 3 feet wide by 4 feet high lay 3.5 feet deep. Although we continued to monitor mining during the days that followed, the Rocket Pocket was to be the last crystal pocket encountered at the Flambeau mine.
The beautiful, uniquely colored chalcocite specimens will remain as a tangible reminder of this small, short-lived mine which now qualifies as a new "classic" locality for the species. But the discovery of the Flambeau orebody has yielded other benefits as well. Geological study helped to define distinctions and anomalies within the Canadian Shield which may lead to the discovery of new orebodies in the area. As a case study of industry and community relationships, the Flambeau operation set benchmarks for mineral [TABULAR DATA FOR TABLE 1 OMITTED] and resource partnerships built on mutual respect, unlike the often paternalistic mining company relationships of the past. The Flambeau Mining Company, Kennecott, and Kennecott's parent company, Rio Tinto, believed that as many parts of the community should be involved as possible, including local businesses, educators, museums and mineral collectors. As a business venture, the small size and high grade of the ore allowed extraction of a substantial amount of copper in a short period of time, benefiting company, community and consumer. From an ecological and community standpoint, Flambeau exemplified the goals and standards of organizations such as the United Nations World Commission on Environment and Development, which revolve around a long-term view of sustainable development. How we manage and acquire resources today affects future generations in the long term, and community relations in the short term. Mine management was willing to accept the idea that the collectible specimens had more social and scientific value as specimens than as ore, an enlightened viewpoint for which we can all be grateful.
We would like to express our appreciation to the following people who helped this project succeed. Special thanks to Greg Fauquier, who believes that the mineral industry has nothing to fear from challenge, new ideas, and even criticism, and much to gain from new opportunity; to Edward and Maryann May whose support and commitment to this project went far beyond geology; to John Barlow for his ear, his advice and support, and his intense love of minerals which probably willed there to be world-class specimens in Wisconsin; and to George Burnham, for his inspiration as an explorer and recoverer of new mineral localities. There are many more whose help on the project and this article we deeply appreciate: Tom Baker, John Barnes, Dan Behnke, Tom Bucholz, Laura Childs, Jack Christman, David Cline, Bill Cordua, Stephen Dinkowitz, Stanley Dyl, Jeff Earnshaw, Bob Housley, Jane Huckstorff, Anthony R. Kampf, Sally LaBerge, Mark and Lynn Langenfeld, Jana Murphy, Tom Myatt, Neil Prenn, George Robinson, Tom Rosemyer, Karl Smith, Holly Wiesman, Wendell Wilson, Ron Vick, and Marty Zinn. The mineral collectors from the Rusk County area and elsewhere around Wisconsin were generous with their help and information. We wish to thank especially Russ Babcock and David Litvin of Kennecott Minerals, who provided insightful comments and corrections to the manuscript. We hope that all of the personnel of Kennecott Minerals, the Flambeau Mining Company and the Ames Construction Company know the gratitude we feel toward them for each and every individual act beyond their job requirements, which helped these beautiful specimens survive.
1 The source for much of this history is Ed May, who over a long period of time guided the development of the Flambeau mine and held various positions from chief geologist to project manager to consultant at various periods of the project. Mr. May now lives in Littleton, Colorado. Other information is selected from M. G. Mudry, Jr. et al. (1991), and Babcock (1991).
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SIMS, P. K. (Compiler) (1992) Geologic map of Precambrian rocks, southern Lake Superior Region, Wisconsin and Northern Michigan. USGS Miscellaneous Investigation Series, MAP I-2185, 2 sheets.
SIMS, P. K, VAN SCHMUS, W. R., SCHULTZ, K. J., and PETERMAN, Z. E. (1989) Tectonostratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen. Canadian Journal of Earth Science, 26, 2145-2158.
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|Author:||Jones, Casey; Jones, Jane Koepp; LaBerge, Gene|
|Publication:||The Mineralogical Record|
|Date:||Mar 1, 1999|
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