MONTANA SAPPHIRES - THE VALUE OF COLOR* by
ABSTRACT A model for magma genesis provides insight into the origin of high quality “cornflower blue” sapphire in the Yogo dike near Utica, Montana. Ultramafic lamprophyre magma rose through the lithospheric mantle in the Eocene (48 million years ago) and “pooled” at or near the base of the abnormally thick (50-55 km) and abnormally cool (550°-600°C) crust, where it assimilated high alumina rocks in the eclogite metamorphic facies. Corundum (sapphire) formed by replacing kyanite in a desilication reaction that occurred during metamorphism of the wall rock. Corundum did not form in the magma. Rare inclusions in the sapphire include primary igneous analcime that has a stability range of 8-14 kb water pressure and 600-640°C. Ferrous iron and titanium were incorporated in the corundum under reducing conditions, creating the violet-blue hue. The reducing conditions also limited the solubility of ferric iron in corundum. Although corundum is stable over a wide range of conditions, gem-quality corundum apparently forms in the temperature range of 600°-750°C (and pressure range of 6-14 kb). The lithosphere underlying the three principal placer deposits of sapphire in western Montana—Missouri River near Helena, Dry Cottonwood Creek, and Rock Creek—is much thinner than under central and eastern Montana. The geothermal gradient is much higher; and the 600°-750° temperature “window” is at mid-crustal depths of 25-35 km. where conditions are predominately oxidizing and ferric iron is abundant. The ferrous iron and titanium content in these sapphires is much lower than at Yogo. Heat-treating can enhance the color, but can not increase the titanium content. The resulting “steel blue” is commercially viable, but not as attractive as cornflower blue. Mining the placer deposits involves relatively low cost, high volume surface operations. The normally high costs of underground mining at Yogo are increased due to the complexity caused by pre-intrusion and post-intrusion karst development. Thus, heat-treated steel blue sapphire from Montana placer deposits can supply a high volume market at competitive prices, whereas the much more expensive cornflower blue sapphire from Yogo Gulch can supply a smaller up-scale market. ---------------------------------------------------------- * - adapted and updated from an article by D.W. Baker that appeared in Northwest Geology (1994, volume 23, pages 61-75), published by the Tobacco Root Geological Society. The article was written for professional geologists and assumes that the reader has a university-level understanding of earth science and is familiar with Montana geology. However, the lay reader will find the 16 photos and drawings informative. Copyright © 1994-2007 David W. Baker. All rights reserved. Republication or redistribution of website content, including by framing or similar means, is prohibited without the prior written consent of David W. Baker.
INTRODUCTION
COLORED CORUNDUM The Value of Color and the Structure of the Retina. The value of colored, precious gem stones is a function of hue, saturation or chroma of the color, clarity, cut, weight in carats, rarity, and location of the deposit (Hughes, 1990; Newman, 1994). In suites of gemstones showing a range of hues but with the other parameters the same, certain hues are most highly valued. Value versus wavelength curves correlate with the spectral response of the retina, suggesting a scientific basis for the adage, “Beauty is in the eye of the beholder.” The electrical outputs of the red-, green-, and blue-sensitive cones of the retina are “processed” so that “red minus green” and “blue minus yellow” signals are sent to the brain via the optic nerve (Boynton, 1979, p. 207-250). The (absolute value of the) spectral output of these two “color channels” (Boynton, 1979, Fig. 7.4; Wyszecki and Stiles, 1982, p. 648-653) is a good approximation to a plot of dollars versus wavelength on color grading charts for sapphires and emeralds, and to a lesser extent, for rubies (Fig. 2). Both curves peak on the violet side of blue (sapphire), the yellow side of green (emerald) and the orange side of red (ruby).
Cornflower Blue Sapphire Formed under Reducing Conditions. Studies of aluminum oxide ceramics show that the high temperature solubility of iron in corundum increases with oxygen fugacity (Meyers et al., 1980). The relatively high concentration of titanium and ferrous iron in Yogo sapphire and the lack of strong peaks for ferric iron in the absorption spectra in Figure 4 indicate that the Yogo corundum formed under reducing conditions. Sapphire crystals coated with small hercynite grains are abundant—another indication of reduction rather than oxidation. ORIGIN OF The Yogo Sapphire Deposit Sapphires at Yogo Gulch occur in
a 5 km long dike, located on the northeast flank of the Little Belt Mountains,
one of the foreland uplifts in
The igneous host rock consists primarily of clinopyroxene, phlogopite mica, and analcime (Clabaugh; 1952; Dahy, 1988, 1991). It also contains about 4% titaniferous magnetite (Meyer and Mitchell, 1988), providing a magnetic signature which is used for geophysical prospecting. It is an ultramafic lamprophyre called an ouachitite (Clabaugh, 1952, p. 14; Meyer and Mitchell, 1988; Brownlow and Komorowski, 1988; Dahy, 1988, 1991). The dike contains abundant globules or ocelli of carbonate that Dahy (1988, 1991) interpreted as evidence of immiscibility of a separate carbonatite liquid. The Yogo dike is part of the Central Montana Alkalic Province (CMAP), an igneous province characterized by mafic and ultramafic igneous rocks with unusually high alkali contents (dominantly potassium) that formed in the mantle far below the 45 to 55 km thick crust (Baker and Berg, 1991, Baker, 1992). The Yogo Dike has been dated at 48.7 million years by Harlan (1996) using the 40Ar/39Ar radiometric age determination method. Plate Tectonic Model for the CMAP. The onset of igneous activity
during the Laramide orogeny, which
formed the Magma Genesis. As the Farallon
plate sank under central Magma Density and Buoyancy. Mafic and ultramafic magmas rise to the base of the crust where they tend to “pool” because the density contrast between magma and country rock is less in the crust than in the mantle (Philpotts, 1990, p. 466). The magma density estimated from the chemical analysis given by Clabaugh (1952), using the method of Bottinga and Weill (1970), was 2.6 g/cm compared with 2.9 - 3.0 for lower crustal rocks. “Pooling” at or near the base of
the crust allowed the magma to cool by heating and locally melting crustal
rocks. Dahy (1988) mapped a rhyolite sill and a
rhyolite-cored laccolithic dome only 1.3 and 2.3 km from the Yogo dike,
respectively. These granitic magmas were generated from the heat supplied by
mantle-derived mafic and ultramafic magmas “pooling” in the lower crust. Embry
(1987) described how a mantle-derived shonkinite magma
formed the Sapphire Genesis. Pirsson (1897; 1900, p. 554) and Clabaugh (1952, p. 57) surmised that Yogo sapphires formed when the ultramafic lamprophyre magma assimilated some metamorphosed shale. However, they did not envision the great depth at which this occurred. Kyanite-Bearing Xenoliths from the Lower
Crust. The kyanite-quartz xenolith in the Yogo dike described by Clabaugh (1952, p. 16) and the kyanite-garnet-quartz
xenolith with accessory rutile described by Dahy
(1991) are the kind of mineral assemblages expected for a
highly-metamorphosed shale near the base of the crust where the
conditions were 550-600°C (Eggler et al., 1988, Fig. 5), 50-55 km depth (Prodehl
and Lipman, 1989, Figs. 2 and 24; Braile
et al., 1989, Fig. 3), and
approximately 12 kilobars or 1.2 gigapascals
of pressure. At these temperatures, pressures above 10 kilobars
correspond to the eclogite metamorphic facies,
characterized by absence of hydrous phases such as muscovite (Philpotts, 1990, p. 328). A long history of burial and
metamorphism allowed the water to escape. A plausible original mineralogy is a
relatively common rock type—shale composed of kaolinite and quartz ± iron
oxides. HaIl (1987, p. 271-275) described the process
of assimilation of metapelites by basic magmas.
Silica is removed from aluminosilicates (in this case
kyanite) to form corundum (cf. Helmley et al.,
1980) and from garnet to form spinel. I suspect that a comprehensive search for
xenoliths in the Yogo dike will find some with corundum replacing kyanite,
similar to the description of Altherr et al. (1982) of gem-quality corundum
replacing kyanite in gneiss undergoing anatexis in Sapphire-Bearing Cognate Xenoliths. Heating country rock to generate granitic magmas and assimilating some pelitic rock lowered the magma temperature, resulting in crystallization of clinopyroxene. Dahy (1988, 1991) described the bright green clots in the Yogo dike consisting mostly of clinopyroxene, but also with phlogopite and sapphires, which were (presumably formed as cumulates and) subsequently brought up with the magma as cognate xenoliths according to Meyer and Mitchell (1988). Aluminum coordination in the clinopyroxene phenocrysts in the Yogo dike indicates low pressure (i.e. lower crust) rather than high pressure (i.e. mantle) crystallization. They suggest a crystallization temperature for phlogopite in the magma of 900°C. Analcime Inclusions in Sapphire.
The rare small inclusions in Yogo sapphires noted by Gübelin
and Koivula (1986) contain analcime, pyrite, calcite,
rutile, zircon, and a dark mica that is most likely phlogopite.
The occurrence of white “snowballs” of analcime is unique among Ascent Through the Crust. The buoyancy
imparted by the volatiles to the Yogo magma caused it to rise through the
crust. As the magma ascended through the crust, it decompressed and cooled,
generating many changes in the chemistry of crystallizing phases.
Clinopyroxenes became progressively enriched in iron, titanium, aluminum and
some acquired a rim of acmite (Meyer and Mitchell, 1988). Titaniferous
magnetite grains and sapphire crystals were partially resorbed by the magma,
leaving in the latter case reaction rims of dark-green spinel (=hercynite) (Clabaugh, 1952, p. 18; Dahy,
1988; Meyer and Mitchell, 1988; Brownlow and Komoroski, 1988). Many of the sapphire crystals had grown
as thin plates (basal pinacoids) (cf . Berezhkova,
1980; Hughes, 1990, p. 149), but developed rhombohedral
faces (and etch pits) by dissolution during ascent of the magma (cf. Berezhkova,
1980; Pratt, 1897; Clabaugh, 1952, p. 18-21).
Nepheline formed hexagonal prisms about 1½ cm long and 1 cm in diameter (Weed,
1900, p. 457; Clabaugh, 1952, p. 16; Dahy, 1991), but these are now pseudomorphs
consisting of fine grained calcite and quartz (L.G. Zeihen,
personal communication, 1994). The size of the nepheline crystals appears to
rule out extremely rapid ascent of the magma, such as occurred in The other large sapphire
deposits in Missouri River near Helena. Although no kyanite-bearing metamorphic
rocks crop out in the Dry Cottonwood Creek. The placer deposit at Dry Cottonwood Creek is
underlain by Tertiary-age rhyolitic tuff and the Rock Creek. Clabaugh (1952)
discussed the occurrence of sapphire-bearing pebbles of andesite in the Rock
Creek deposit. There are also pebbles of rhyolite tuff and basalt (American
Gem, 1994). Wallace et al. (1986)
mapped volcanics with andesites, latites and rhyolites in the area around the
placer deposit—the presumed source rock for the sapphires. These Eocene (?)
volcanics postdate the nearby Source Rock. All three deposits are adjacent to a
batholith and to large volumes of volcanic deposits. Thus they occur where
large volumes of crustal rocks were undergoing anatexis. A cross section of
crust, lithospheric mantle and asthenosphere shows systematic changes from Yogo
to Rock Creek (Fig. 6). The geothermal gradient is much higher in the west than
in the east. The lithosphere thins abruptly at the Sapphire Genesis. Although
corundum is stable under a wide range of conditions (cf.
Berman, 1988), one can make a good case for gem-quality corundum forming in
the temperature range of 600° to 750°C. Suggested pressures are in the range of
6 to 14 kilobars. These conditions are close to or somewhat above the melting curve
for granite. Altheer et al.
(1981) found that gem-quality corundum formed in When the temperature “window” of 600° - 700°C for sapphire genesis is plotted on the cross section of the lithosphere (diagonal hatching in Fig. 6), the difference in source regions between Yogo and the other deposits is obvious. Under the three western deposits this temperature interval occurs in the middle third of the crust at depths of approximately 25 to 35 km where rocks with ferric rather than ferrous iron are abundant and the rocks are more oxidized. Higher oxygen fugacity meant far greater amounts of iron were dissolved in the sapphire. Under Yogo the temperature “window” extends from the base of the crust down into the mantle. However, the sapphire was generated in the crust, not the mantle, because the kyanite-bearing xenoliths—the presumed source of aluminum for the sapphire—are rich in quartz, a mineral absent in the mantle. For quartz-rich rocks to exist at such depths in the lower crust (the stippled region in Fig. 6 without melting required an abnormally low geothermal gradient. The low geothermal gradient was a consequence of the abnormally thick lithospheric mantle. Mantle-derived ultramafic magma “pooling” in the lower crust supplied the necessary heat to create the sapphire.
Figure 6. Cross section A-A’ in Fig. 1, showing crust and lithospheric mantle, and temperature profiles at Rock Creek, Missouri River by Helena and Yogo. DCC - Dry Cottonwood Creek. Crustal thickness from Prodehl and Lipman (1989). Lithosphere thickness from Eggler and Furlong (1991) and Iyer and Hitchcock (1989). Base of lithosphere is assumed to be 1200°C after Eggler and Furlong (1991). Diagonal hatching - suggested thermal window of 600°-750° for sapphire generation. Stippling - suggested source for Yogo sapphire. Thus, it appears that the massive production of heat-treated sapphire by American Gem Corporation in Helena can produce very substantial quantities of “pure” blue or steel blue sapphire geared to mass marketing at lower prices, where as the cornflower blue of the natural, untreated Yogo sapphire will preserve its up-scale market for those preferring the violetish-blue that gives a greater single-channel response in the human visual system. THE IMPACT OF MINING COSTS Mining the three placer deposits at Rock Creek, Dry Cottonwood Creek, and near Helena involves surface operations with relatively low mining costs and high volume. In contrast, underground mining at Yogo (Figures 7 and 8) is much more expensive. A number of mining ventures at Yogo have failed because of the inability to contain mining costs (Barron 1982; Voynick, 1987).
Figure 7. Sketch map showing mining activity on the Yogo Dike. The main Yogo Dike produced most of the sapphires and is located by the American Mine, the Middle Mine, the English Mine, and the Intergem Cut. The wide blue lines indicate where mining or exploration created a trench or cut. The dike rock contains approximately 4% magnetite, which allows subsurface dikes to be traced by magnetic anomaly surveys. The dashed brown line is the "barren" dike on the north side of the main Yogo Dike. This dike is a minette, not an ouachitite, and is apparently devoid of sapphire. Several dikes, shown as dashed cyan-colored lines, on the south side of the main dike have not been well studied. The 1,000 foot grid shown in the sketch map is based on the Montana State 10,000 Foot Grid. Black dots are drill holes. The gray pattern is patented land or other private land. The location of 3 bulk samples collect by Amax is indicated as A, B, and C. North is the top of the map and south, the bottom of the map. (after Clabaugh, 1952; Dahy, 1988; Pacific Sapphire, 2000)
Figure 8. Vertical section along the length of the main Yogo Dike. Mined out sections of the dike shown in green. Shafts, adits, and tunnels indicated by black lines. Amax Mining Company collected 3 bulk samples (shown in purple) in 1993-1995. As reported by Pacific Sapphire (2000) Sample A consisted of 3000 tons that averaged 14 carats per ton, Sample B - 800 tons @ 9 to 12 carats per ton, and Sample C - 3000 tons @ 9 carats per ton). Mining was limited to portions of the dike above 5000 feet where the dike had been hydrothermally altered. Horizontal lines show elevation in feet above sea level. Horizontal scale in feet. Note the 5X vertical exaggeration. One very significant geological factor impacting mining costs has been the myth that the Yogo dike is a simple tabular body. The magma intruded into a mature karst system developed in limestone of the Madison Group. Hydrothermal alteration in the uppermost part of the dike transformed phlogopite and clinopyroxene into chlorite and clay minerals, cemented the cave collapse breccia matrix with carbonates, quartz, and pyrite, and formed pyrite cubes (Dahy, 1988). Circulating ground water continued karst development after the intrusion and chemically weathered the lamprophyre. The problems associated with karst (Figure 9) are best illustrated at the west end of the dike where the American Mine and the Kunisaki Tunnel are located.
The Kunisaki Tunnel
In 1972 the owner of the Yogo Mine, Chikara Kunisaki, invested $5 million to drive a 1 km long tunnel the length of the western-most of the three segments that form the Yogo dike (Figures 7, 8, and 10). His intent was to mine a vertical tabular dike; how ever, he found geological complexity (Voynick, 1987). Aerial photos show that where the limestone forming the top of the Madison Group is exposed on the surface, the ground is “pockmarked” with collapsed sinkholes (Fig. 11), developed shortly after the limestone was deposited (cf. Sando and Dutro, 1979; Sando, 1988). The Yogo magma intruded into an extensive, but collapsed cave system, filled with break-down breccia (cf. Figure 9). As in other segments of the dike, the limestone breccia acted as a filter, trapping sapphire xenocrysts as the magma filled all cavities. The sapphire content of some pre-dike breccia exceeded the richest dike rock (Voynick, 1987, p. 158). Throughout the American-Kunisaki Mine much of the igneous rock has been altered hydrothermally and chemically weathered to a soft, crumbly mass, easily mined and washed for sapphires. Continued karst activity since the Eocene created more caverns that eventually collapsed, and collapsed the deeply weathered lamprophyre. Dahy (1988) described pieces from the overlying Kibbey and Otter formations, which had fallen down 100 m from above, and are now in the eastern end of the Kunisaki Tunnel. Considerable money and effort was spent trying to follow the main dike in areas where it simply did not exist. It had been transformed into an irregular mass of angular limestone blocks and some weathered igneous clasts embedded in a matrix of silt (washed down into the cave system from above) and clay (altered from the intrusion). In some places the only evidence of the intrusion was the presence of montmorillonite in the matrix of the breccia. Amax Mining, later Cyprus-Amax, drilled several holes and in 1994-1995 constructed two "declines" on the middle and eastern segments of the Yogo Dike. They extracted an 8000 ton "sample" which they processed. Pacific Sapphire of Vancouver, B.C. reported the results (Figures 7 and 8). Cyprus-Amax did not work on the west segment of the dike where karst is a major feature.
Vortex
The small Vortex Mine has features similar to those in the nearby American-Kunisaki Mine, but is perhaps even more chaotic. In 1992 a 60 m shaft and mine workings exposed solid limestone, limestone with (sapphire-bearing) clay-filled joints, marble, shattered marble, open cavities (caves), breccia with clasts of limestone (and locally marble fragments) in a matrix of calcite and montmorillonite clay, and a small body of reddish-purple altered and chemically-weathered igneous rock rich in sapphire (Figures 12 and 13). The breccia matrix typically contains only 3 to 5 carats per ton, whereas one “pod” of the weathered lamprophyre had a concentration of 70 cts/ton (Mychaluk, 1992). The reddish-purple ore is composed primarily of montmorillonite and calcite, with lesser amounts of phlogopite, chlorite, and iron oxides (Mychaluk, 1992).
“Exotic” clasts in the breccia include soft, bright red shale from the overlying Kibbey formation containing kaolinite, illite, and quartz in the fine grained fraction (R.B. Berg, personal communication, 1994) and porphyry (Mychaluk, 1992). The Kibbey shale presumably fell down a sink hole from above, whereas the porphyry was presumably carried up from an underlying intrusion. The latter may have been the distal portion of the nearby Sawmill Gulch laccolith, which was emplaced in the Cambrian-age Flathead sandstone (Dahy, 1988). Slickensides in clay- rich material are common. Some of the clay-rich zones show deformation textures that the material has been sheared. The transitions from one kind of rock to the next are abrupt, consistent with a mechanical juxtaposition of very different rock types.
Loss of water to the Madison limestone, determined during spring run-off in 1964 by measuring stream flow upstream and downstream from the Vortex Mine, was 1.2 m per second (Feltis, 1980). Some features in the mine can be explained by collapse of a post-intrusion, dominantly vertical cave system. However, Dahy (1988) interpreted a small structure in Kelly Coulee, 200 m north of the Vortex Mine, as a diatreme and more recently suggested that the Vortex Mine is a breccia pipe (Dahy, personal communication, 1993). Mining down to the 450 foot depth confirms Dahy's interpretation. Hydrothermal alteration is limited to the upper parts of the Vortex Mine and other mines on the main Yogo Dike (Figures 8 and 14). As shown in Figure 14, mining extended down to approximately 5000 feet (above sea level)--the lower limit of the altered zone. Production, Reserves and Values Some estimates of past
production and remaining reserves are listed in Table 1. Brown (1982) did not take into account the lower limit of hydrothermal alteration in the Yogo dike. His estimate of Yogo reserves that can be economically mined was far too high. The wholesale price in
Despite marketing claims to the contrary, the sapphire in the engagement ring of Diana, Princess of Wales came from Sri Lanka. It is not a gemstone from the Yogo deposit. ACKNOWLEDGEMENTS The manuscript was improved by
comments of R.B. Berg. Research for this article was greatly aided by the
On-Line Search services of the Montana State Library and the Lewis and Clark
Library in Table 1.
Estimates of production and reserves of
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Precious
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Color in Blue Sapphire.
Cornflower-blue sapphire owes its color to an intervalence charge transition (IVCT) in which light absorbed
by the crystal causes electrons to jump from iron (Fe+2) atoms to titanium (Ti+4)
atoms (Townsend, 1968; Nassau, 1983, p. 140-145). Trivalent iron Fe+3
easily substitutes for trivalent aluminum Al+3 in the corundum
lattice. Divalent iron Fe+2 has a different charge than
aluminum and thus must
substitute pairwise with titanium Ti+4 for two aluminum Al+3
atoms in order to preserve electrical neutrality. Fe and Ti are in trace concentrations in Yogo sapphire—0.14 and 0.013
atomic percent, respectively (Meyer and Mitchell, 1988). Although most of the
iron is Fe+3; it is the Fe+2/Ti+4 pairs that
give the sapphire its vivid blue color due to the IVCT. Absorption
spectroscopy shows that this vivid, 
