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THE DODO DEPOSIT SUBPOLAR URALS, RUSSIA.

INTRODUCTION

Fine specimens of quartz, titanite, brookite and other minerals from the Dodo mine have been reaching the Western mineral market more or less since the end of the Cold War period; the Fersman Mineralogical Museum in Moscow brought a selection of specimens to the Munich Show in 1989. But the occurrence has been known in Russia since the 1920's. The locality itself is situated in a remote area of Tyumen Oblast, near the crest of the Ural Mountains in a portion of that range known as the Subpolar Urals (Pripolyarnyy Ural). The Subpolar Urals are a high section of the mountains dominated by Mount Narodnaya (1894 meters) and Mount Neroika (1646 meters), and connecting with the Polar Urals to the north and the Northern Urals to the south.

The Dodo mine site is about 3 km northwest of the village of Neroika and 6 km northeast of Mount Neroika; the town of Saranpaul lies about 100 km to the east-southeast, and is sometimes loosely cited as the locality name (or as "near Saranpaul"). Neroika village is the headquarters of the Neroisk Exploration and Development Company, the most important sponsor of Polar and Subpolar Urals exploration and the agency in charge of mining at the Dodo site.

The Dodo deposit extends across a series of ridgetops ranging from 600 to 850 meters in elevation. The region is known for its severe polar climate, where the winter snows often do not begin melting until June.

HISTORY

The first discovery of large quartz crystals in the Subpolar Urals took place in 1927, along the upper reaches of the Lyapni River. The find was made by an expedition sponsored by the USSR Academy of Science, and headed by A. N. Aleshkov. Shortly thereafter, in that same year, Aleshkov discovered the Dodo deposit. During the following seven years the expedition studied the geology of the Subpolar Urals.

In 1935 the Polar Urals Expedition, also under the direction of Aleshkov, was sponsored by the Russian Gemstone Trust for the purpose of exploiting the quartz crystals. The Saranpaul settlement was chosen as the base of operations where mining supplies, food and manpower were organized. At that time the transportation of workers, equipment and supplies to the mine site was by horsedrawn (and deer-drawn) vehicles along trails through the taiga and along the rivers. In some cases boats were pulled upriver by ropes and poles almost the entire distance to the mine. Each boat could carry six to ten people and 500 to 1000 kg of cargo. The trip from Saranpaul took 10 to 15 days.

Mining, geological work and exploration were carried out at the mine site every summer from 1935 to 1941. During this time, all major placer deposits of quartz crystals and some quartz veins with crystal-lined pockets were worked out from the surface.

Up until 1958 the production from the Dodo deposit was relatively undistinguished, but in that year a new stage of development was begun. This revitalization was inspired by a sharp increase in the demand for industrial-grade quartz, and was facilitated by the modernization and mechanization of the workings. Year-round underground mining and exploratory drilling soon encountered large, crystal-lined cavities, and the Dodo mine rose in prominence to become one of the largest quartz-mining operations in Russia.

In 1973 the recovery of industrial-grade vein quartz began to replace pocket quartz. Detailed exploration has identified reserves (approved by the State Reserve Commission in 1983 and 1990) sufficient to sustain mining operations for several dozen years.

The Dodo deposit has now been studied to a depth of over 200 meters, with no diminution of crystals at depth. based on the author's research on the structure of mineralized zones in cleft deposits of the Polar Urals, exploitable mineralization should extend at least 150 to 250 meters deeper, which is deeper than any currently operating mines in the region (Burlakov, 1987, 1989, 1990). Each year the Dodo deposit yields dozens of tons of quartz crystals, with reserves estimated at several hundred tons more.

GEOLOGY

The Dodo deposit is located in the central part of the crystal-bearing Neroika Belt, situated near the axis of the Lyapni anticlinorium (a second-order structure related to the Urals mega-anticlinorium). Rocks of the area consist of volcanogenic-sedimentary deposits of Middle Riphean age (the Puiva Series) which have been subjected to repeated faulting and greenschist-facies metamorphism. The predominant rock types include quartz-sericite schist, quartz-sericite-chlorite phyllite-like schist, and pure quartzite. The schist terrane is intruded by numerous dikes ranging from felsic to mafic composition.

The major structure related to the deposit is the Dodo anticline, restricted to the footwall of the Neroika-Patok granitoid massif. In plan view the anticline forms a horseshoe-like fold dipping northward; the flanks are schist of the upper schist sequence of the Puiva Series, surrounding a core of phyllite-like schist of the lower schist sequence.

Surface prospecting and exploratory mining have revealed over 800 quartz veins in the Dodo mine area. These veins can be categorized into six major structural-morphological types:

(1) Quartz veins consisting of lenticular to plate-like bodies filling steeply dipping (50[degrees]-80 [degrees]) rupture joints. Such veins are generally not crystal-bearing but are an important source of industrial-grade vein quartz.

(2) Quartz veins emplaced along gently dipping benches of shear-thrust faults and shear-overthrust (0 [degrees] to 5 [degrees]) faults. In shape the veins are plate-like with abundant apophyses and with many xenoliths from the host rock. Such veins are generally not crystal-bearing.

(3) Quartz veins filling shear fissures which feather out from the overthrusts and thrust faults at a dip of 20 [degrees] to 60 [degrees]. In form they are lenticular to wedge-shaped. Veins of this type are known to contain crystal-bearing open pockets.

(4) Quartz veins in fracture zones within the diabase dikes; generally not crystal-bearing.

(5) Quartz veins of tubular shape invading diabase and albitophyre dikes; generally not crystal-bearing.

(6) Quartz veins of complex and combined forms incorporating features of vein types 1 and 3, or vein types 2 and 3. The veins occupy rupture joints and feathering fissures from shear joints. As a rule, such veins contain crystal-bearing pockets. Larger veins of this type have been known to yield several tens of thousands of tons of vein quartz and several dozen tons of quartz crystals.

Six crystal-beating tectonic zones with distinguishing fissure structures have been identified. In plan view and in cross-section they can be seen to intersect, resulting in the lenticular conformation of the cleft zones. The largest of the crystal-bearing clefts are associated with thrust faults. The crystal-bearing tectonic zones are accompanied by numerous diabase dikes which themselves are often controlling structures for the crystal clefts. The largest and most productive of the crystal-beating zones in the region has been designated Zone 1-70A, and has been under exploitation for several decades.

The tectonic fractures in the area have been subdivided into three groups with respect to quartz vein and crystal formation:

(1) Pre-quartz fractures, represented by local crumpling zones and zones of intense schistosity.

(2) Quartz-controlling fractures, represented by faults, thrust faults and overthrusts. The largest crystal-bearing clefts are located in the overthrusts.

(3) Post-quartz fractures, represented by secondary faults and diagonal shears.

The quartz crystal cavities in the Dodo deposit have been subdivided into three types as well:

(1) Cavities connected with quartz lenses. These include not only the cavities within quartz lenses but also those which have formed along the contact with country rock. These cavities are of a piece with the quartz lenses and are controlled by the same fissures. The crystal-bearing cavities are most common in the lowermost parts of the quartz lenses.

(2) Mineralized clefts in country rock. These are characterized by the absence of quartz lenses, with quartz crystals growing directly on the cleft walls. As a rule, these clefts are associated with well-developed aureoles (leached zones) of hydrothermally altered rock.

(3) Mineralized fault zones. In this case the quartz crystal cavities have formed within brecciated fault zones, localized directly on fault planes and step-dislocations. They have irregular contours and show inclusions of country rock.

The shapes that can be taken by the crystal-bearing cavities range from wedge-shaped to lenticular, tubular, and blocky or equidimensional. The sizes can be huge, ranging from I to 40 meters long, up to 16 meters high, and 30 cm to 5 meters wide. Cavities are considered to be "small" if they yield less than 5 tons of quartz crystals! Large pockets produce 20 to 50 tons, and very large cavities can produce well over 50 tons of quartz crystals. The single largest crystal-bearing cleft encountered so far (designated no. 6/28) was found in connection with adit no. 28, and yielded no less than 300 tons of quartz crystals!

A typical crystal cleft of the first type (one connected with a quartz lens) carries first-generation calcite, usually massive; 5% to 30% of the cavity is filled by chlorite sand containing fragments of country rock and of the quartz lens, and also containing detached quartz crystals and crystal clusters. Other quartz crystals remain attached to the cavity walls. The remaining open space is filled with ice or water.

PARAGENESIS

Many years of geological, geochemical and mineralogical study have led to a general understanding of the special geological/structural and mineralogical/geochemical factors unique to the Dodo deposit. Three essential factors have been revealed:

(1) The Dodo deposit formed at greater depths than other deposits of the Subpolar Urals. This great depth resulted in a particular geochemical environment (fractionation of heavy and light rare-earth elements, of elements differing in ionic radius, and of isotopes) during mineral deposition.

(2) The peculiar geological and structural setting resulted in the formation of a fluid regime which influenced the multi-stage, polygenic, polycyclic processes of mineral formation.

(3) Concurrently with the formation of typical alpine-type clefts in the deposit, there was an influx of chemical components that had originated from deep-seated sources: potassium, fluorine, chlorine, boron, mercury, arsenic, antimony, uranium and thorium. Consequently the quartz crystal cavities in the deformation zone contain as many as 30 additional accessory species. In addition, there are commonly two or three generations of minerals such as titanite, rutile, monazite, anatase, and brookite. These multiple generations reflect repeated leaching and solution processes taking place at intervals between paragenetic phases.

In those clefts located outside of the deformation zone, only a simple mineral assemblage is typically present, reflecting the general composition of the local country rock. Usually only 5 to 8 mineral species comprise the assemblage, and as a rule are all of one depositional generation.

It can be concluded, therefore, that the large number of crystal clefts in the Dodo deposit are intermediate in genesis between hydrothermal quartz veins and true alpine-type clefts mineralized solely by lateral secretion from the local country rock.

MINERALOGY

Almost since their first discovery, the Dodo clefts and their mineralogy have been investigated by many researchers: Lemlein (1936, 1937, 1954 etc.), Schafranowsky (1937, 1944), Bukanov (1961-1974), Malyshev et al. (1974), Kolbin (1977), and Juchtanov (1981). All of the more recent data on the mineralogy of the deposit - from 1984 to the present - are to be found in the scientific papers and production reports of the author of the present article.

The Dodo deposit exceeds all other deposits of the Polar Urals in the richness and variety of its mineralogy. At this writing, 62 different mineral species directly associated with alpine-type cleft formation are known from Dodo. These species have been identified by various analytical methods, including chemical analysis and X-ray and infrared spectroscopy. Below, only minerals of the crystal-bearing parageneses - i.e., true cleft minerals - are described; of the minerals of the country rock, only those which recrystallized within the cavities (e.g., garnets, zircon) are listed.

The paragenetic status of corundum and moissanite is questionable; these were discovered by different authors as xenoliths in the country rock, but later also listed among the cleft minerals (e.g., by Burlakov, 1989). Corundum is rare, being found intergrown with the cleft minerals as 0.1 to 0.2-mm grains, pink to blue or green. Hexagonal moissanite - a high temperature/high pressure silicon carbide known primarily as synthetic "carborundum," a grinding abrasive - was first found in 1974 by the mineralogist Soochoovoy in dressed, pulverized quartz crystal samples (grain size 0.1-0.5 mm) and identified by X-ray analysis. It appears as green to pale blue tabular crystals 0.1 to 0.3 mm in diameter. However, the author as of this writing has identified moissanite neither in the country rock nor as crystals, nor with quartz crystals. Thus it is not to be ruled out that the 1974 moissanite formed or was introduced during sample preparation.

A series of secondary minerals (hemimorphite, cerussite, malachite and others) formed either during the last phase of cleft mineralization or as weathering/oxidation products (Burlakov, 1989).

Below, all cleft minerals of the Dodo deposit are described in alphabetical order.

Actinolite [Ca.sub.2][(Mg,[Fe.sup.2+]).sub.5][Si.sub.8][O.sub.22]([OH.sub.2])

Actinolite has not often been found in the Dodo deposit. It is seen as pale green acicular crystals included in quartz, and as aggregates of tangled filaments in birds-nest shapes (see also tremolite).

Albite NaAl[Si.sub.3][O.sub.8]

Albite forms water-clear crystals twinned on the Albite Law, in sizes to 7 mm. These crystals are often found on the walls of mineralized fissures, in association with apatite, anatase, chlorite and adularia.

Allanite-(Ce) [(Ce,Ca,Y).sub.2][(Al,[Fe.sup.2+],[Fe.sup.3+]).sub.3][(Si[O.sub.4]).sub.3](OH)

Allanite-(Ce) is rare in the deposit. It is found in cleft fillings as dark brown, tabular crystals between 0.5 and 1 mm.

Almandine [([Fe.sup.2+],Ca,[Mn.sup.2+]).sub.3][(Al,[Fe.sup.3+]).sub.2][(Si[O.sub.4]).sub.3]

Garnet from the alpine-type clefts of the Dodo deposit is compositionally a mixture of almandine and grossular: 58.3% almandine, 37.4% grossular, 4.3% spessartine, corresponding to the general formula [Mathematical Expression Omitted]. In the schists and quartzites of the deposit one finds chloritized garnets as 0.5-2 mm crystals in the fissures. Garnets which have crystallized in the clefts are much larger and fresher. Here are found well-developed, transparent crystals in sizes to 7 mm, with smooth, lustrous faces showing growth figures.

Anatase Ti[O.sub.2]

Anatase is widespread in the deposit. It is found as nest-shaped aggregates on the walls of mineralized fissures, as inclusions in quartz crystals, and as overgrowths on quartz faces. The average crystal size is 2.5 mm; the maximum size is 1.2 cm. Most anatase crystals are of typical dipyramidal habit, although pinacoidal crystals are also found. The color ranges from yellow-green to pale blue and black; the luster is adamantine to metallic.

Ancylite-(Ce) SrCe[(C[O.sub.3]).sub.2](OH)[multiplied by][H.sub.2]O

Ancylite-(Ce) was first discovered and described from Dodo in 1974 by V. V. Bukanov. This ancylite always contains some Ca substituting for Sr, and La and Nd substituting for Ce. It forms dipyramidal crystals to 2 mm. The color is pale pink, pale yellow or cream, sometimes with a waxy white overgrowth on the faces. Ancylite-(Ce) is found as crystal aggregates and as sprinklings on the faces of quartz crystals; associated species include anatase, fluorapatite and chlorite. A thus-far unnamed lanthanum and neodymium-containing cerium calcium carbonate occurs intergrown with the ancylite; this may be the Ca analog of ancylite-(Ce).

Anglesite PbS[O.sub.4]

Anglesite overlies galena as rough crystal aggregates and crusts.

Ankerite Ca([Fe.sup.2+],Mg,Mn)[(C[O.sub.3]).sub.2]

Ankerite is rare, occurring as small (to 5 mm) crystals grown on the faces of quartz crystals.

Arsenopyrite FeAsS

Arsenopyrite is rare at Dodo, forming 0.5 to 0.8-mm crystals intergrown with chalcopyrite.

Azurite [Mathematical Expression Omitted]

Azurite is found as microcrystal aggregates and radial spherules. It forms inclusions in massive quartz near the clefts, and in quartz crystals, apparently as an alteration product of tetrahedrite and chalcopyrite.

Biotite K[(Mg,Fe).sub.3]Al[Si.sub.3][O.sub.10][(OH).sub.2]

Biotite is found in the clefts as 0.5 to 1-mm inclusions in quartz crystals.

Boulangerite [Pb.sub.5][Sb.sub.4][S.sub.11]

Boulangerite forms inclusions in the outer zones of quartz crystals, as aggregates of tangled, threadlike crystals and as oriented bundles of needles. The needles average 0.01 to 1 mm thick, and may be 5 cm long. The color is steel-gray to blue-gray, with a metallic luster. Associations include rutile, anatase, ancylite-(Ce), chlorite and brookite.

Brookite Ti[O.sub.2]

Brookite is found as inclusions in quartz crystals, as overgrowths on their faces, and as fillings in the clefts, with calcite and anatase. The transparent crystals show a typical "alpine" hourglass-shaped dark zone, the colors varying from dark brown to honey-yellow; the luster is brilliant and adamantine to metallic. These very brittle crystals have an average size of 0.5 mm thick and 1 x 2 cm across, but the largest one known to the author is 12 cm along the c axis! Such giant crystals are usually found already broken in the clefts, as a result of recent tectonic movements, frost action, or blasting by the miners. Brookite also occurs, not uncommonly, as a secondary overgrowth on rutile.

Calcite CaC[O.sub.3]

Calcite is found in practically every cleft. In the Dodo deposit, four calcite generations have been distinguished. The first generation, forming before the beginning of quartz crystal growth, appears as massive grains or pinacoidal crystals; it is opaque, and very dark brown to medium brown and yellowish.

The second calcite generation forms simultaneously with quartz crystals and grows in parallel with them. After later partial dissolution of the calcite, "sawtooth" crystals remain on the quartz faces. The second calcite generation can also grow on the first without essential changes in crystal form; however, contamination by chemical impurities here is less by an order of magnitude. The color of these calcites is generally white.

The third calcite generation crystallizes during and after the final stages of quartz crystallization. These calcite crystals are rhombohedrons sometimes weighing 100-200 kg, of white, lilac or lemon-yellow color, and to varying degrees transparent. Weakly colored or wholly colorless, clean "Iceland-spar" crystals of optical quality are not uncommon.

The fourth calcite generation crystallizes during the last phase of cleft formation. These crystals are of scalenohedral habit and are at most 5 cm in size. They are transparent and colorless, sometimes weakly amethyst-colored, and contain goethite inclusions.

Cerussite PbC[O.sub.3]

Cerussite forms red-orange needles to 2 mm long, resting on crystal faces of galena, calcite, quartz and other minerals.

Chalcopyrite CuFe[S.sub.2]

Chalcopyrite is precipitated onto quartz crystals in irregular habits to a few centimeters, and as small, complex crystals to a maximum size of 1.2 mm. It is associated with sphalerite, galena and adularia.

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 is found as bluish green spherules of radiating, rough crystals encrusting chalcopyrite and tetrahedrite.

Cobaltite CoAsS

Cobaltite occurs rarely in the cleft fillings with pyrite and chlorite, as crystals of octahedral, cuboctahedral and tetrahedral habits. Crystal sizes range between 0.1 and 0.5 mm; the color varies from black to medium-gray, sometimes with a pink patina.

Cosalite [Pb.sub.2][Bi.sub.2][S.sub.5]

Cosalite, like boulangerite, is found as inclusions in quartz crystals, in tangled bundles of acicular crystals commonly intergrown with small, equant crystals of bismuthian galena, as well as with anatase, apatite and chlorite. The cosalite needles are between 0.005 and 1.5 mm thick, and reach 7 cm long. The luster is brightly metallic, and the color is steel-gray with a bluish sheen.

Chlorite Group

Chlorite from Dodo varies chemically from an iron-rich clinochlore to chamosite, and is one of the most widespread mineral groups in the alpine-type Dodo clefts. Its crystallization began before and ended after the phase of quartz crystal growth; the late chlorite is found both as inclusions within quartz crystals and as encrustations on their faces. Granular chlorite can fill up to 50% of the volume of the crystal clefts; this "sand" is an agglomeration of wormlike, elongated chlorite crystals from 2 to 15 mm long, which break easily along cleavage planes to form loose hexagonal platelets.

Clinozoisite [Ca.sub.2][Al.sub.3][(Si[O.sub.4]).sub.3](OH)

Clinozoisite occurs as inclusions in quartz crystals. The length of the transparent prisms can reach 1.5 cm; the color is gray-green to swamp-green. In some cases clinozoisite is partly or wholly replaced by sericite.

Epidote [Ca.sub.2][([Fe.sup.3+],Al).sub.3][(Si[O.sub.4]).sub.3](OH)

Epidote is included in quartz crystals. It forms dull, long-columnar gray-green crystals. Small crystals of 1-5 mm are more or less transparent; larger ones are usually strongly fractured and opaque.

Fluorapatite [Ca.sub.5][(P[O.sub.4]).sub.3]F

Fluorapatite is found in almost every crystal cleft. Most commonly it forms single crystals in chlorite sand, or crystals growing in the fissures, in crystal-bearing vugs. Associations include quartz crystals, titanite, adularia, anatase and calcite. The tabular or thin-platy crystals are greenish to pale blue. To be sure, fluorapatite does not occur in the Dodo deposit in such large crystals as are found at Puiva, where it may reach 10 cm in size. The average size of Dodo apatite crystals is 1-2 cm, the maximum size being around 4 cm.

Fluorite Ca[F.sub.2]

Fluorite is rare in the deposit. It is found in the chlorite sand as irregular grains to 2 cm, usually colorless, sometimes with a weak violet tinge.

Galena PbS

Galena is commonly found in association with bismuthian galena in the crystal clefts. Normal galena usually crystallizes in a cubic habit, as a first generation, on quartz, essentially before the beginning stages of the growth of quartz crystals. Bismuthian galena, with an octahedral habit, forms in the final stages of quartz crystal growth. It makes equant skeletal crystals, often hoppered, to 5 cm, and is often encrusted with anglesite.

Goethite FeO(OH)

Goethite is an alteration product of sphalerite, pyrite, pyrrhotite or chlorite, and forms partial or total pseudomorphs after these.

Gold Au

Gold is seen very rarely as flat to equant grains to 1 mm in the outer zones of quartz crystals.

Gypsum CaS[O.sub.4][multiplied by]2[H.sub.2]O

Gypsum is found in clefts having abundant sulfide mineralization, as transparent, pale blue, curved, filiform crystals to 1 cm long.

Graphite C

Graphite forms finely foliated crusts on quartz and calcite crystal aggregates in clefts in carbonaceous phyllites.

Hematite [Fe.sub.2][O.sub.3]

Hematite occurs in peripheral zones of the deposit, as thin, hexagonal tablets to 5 mm.

Hemimorphite [Zn.sub.4][Si.sub.2][O.sub.7][(OH).sub.2][multiplied by][H.sub.2]O

Hemimorphite forms white to yellow-white radiating aggregates and mammillary crusts occurring as inclusions in the quartz lenses near the clefts and in quartz crystals accompanied by sphalerite.

Hematite [Fe.sup.2+]Ti[O.sub.3]

Hematite is one of the most widely distributed minerals of the deposit. It is found both in the chlorite sand and as inclusions in quartz, calcite and apatite crystals. It is deposited as thin tabular crystals to 4 cm, black and lustrous, with faint striations on the faces. Hematite commonly replaces brookite, anatase and rutile.

Kainosite-(Y) [Ca.sub.2][(Y,Ce).sub.2][Si.sub.4][O.sub.12](C[O.sub.3]) [multiplied by] [H.sub.2]O

Kainosite-(Y) occurs in leached-out cavities once filled with first or second-generation calcite; it is associated with albite, titanite and chlorite. The crystal habit shows a combination of rhombic prisms and dipyramids. Color is yellowish brown; the average crystal size is around 0.5 mm, to a maximum of around 2 mm.

Magnetite [Fe.sub.3][O.sub.4]

Magnetite occurs in the cleft fillings with anatase, albite, titanite and chalcopyrite. It forms brilliant octahedral crystals to 3 mm, whose faces show sharp growth figures.

Malachite [Mathematical Expression Omitted]

Malachite is seen as radial aggregates and mammillary crusts to 5 mm thick. It is an alteration product of chalcopyrite and tetrahedrite, but occurs also as inclusions in quartz crystals and in massive quartz near the clefts.

Melanterite [Fe.sup.2]+S[O.sub.4].7[H.sub.2]O

Melanterite is uncommon. It is always of the magnesian variety, forming opaque white mammillary crusts and radial aggregates to 1.5 cm, associated with gypsum, calcite and pyrite.

Meneghinite [Pb.sub.13]Cu[Sb.sub.7][S.sub.24]

Meneghinite occurs in the galena-quartz crystal paragenesis. It forms six-sided, stepped, parallel-growth aggregates of thin acicular to filiform crystals; the longest individual needles reach 2 cm, with a thickness of up to 1 mm. The color is dull blue-gray.

Monazite-(Ce) (Ce,La,Nd,Th)P[O.sub.4]

Monazite-(Ce) is a characteristic mineral in the neighborhood of clefts in metasomatically altered rocks. In the quartz crystal zones at Dodo it is found growing on quartz faces, and in the chlorite sand. Associated species include brookite, anatase, fluorapatite, titanite and boulangerite. The equant crystals, up to 1 mm, are bounded by pinacoids; their color varies from pale yellow through orange.

Montmorillonite [(Na,Ca).sub.0.3][(Al,Mg).sub.2][Si.sub.4][O.sub.10][(OH).sub.2][multiplied by]n[H.sub.2]O

Montmorillonite is found in the cleft filling in the form of compact gray-white masses. The clay mineral is also found as oriented inclusions in growth zones in many quartz crystals.

Muscovite K[Al.sub.2]Al[Si.sub.3][O.sub.10][(OH).sub.2]

Muscovite rarely forms leafy crystals 1 to 1.5 cm across. The dominant variety in the deposit is the finely divided sericite, which is found included in many growth zones of quartz crystals.

Orthoclase KAl[Si.sub.3][O.sub.8]

Orthoclase is common in the Dodo deposit as the transparent variety adularia, especially in the crystal clefts inside diabase bodies. The crystal habit is recognizable as a combination of the rhombic prism and the pinacoid. Average crystal size is about 3 cm. Translucent through milky white adularia crystals are associated with titanite, fluorapatite, chlorite, sphalerite and quartz.

Parisite, Sr-analog

In the Dodo deposit a type of parisite occurs as a cleft mineral, but it may well correspond to a hitherto unnamed strontium analog of parisite-(Ce), formula ([Sr.sub.0.7][Ca.sub.0.3])([Ce.sub.1.0][La.sub.0.6][Nd.sub.0.3][Pr.sub.0.1]).sub.2][[F.sub.2][where][(C[O.sub.3]).sub.3]]. The parisite analog forms tabular hexagonal prisms, frequently with corroded edges, having a strong cleavage along {0001}, and varying in color from pale brown to yellow-brown. The mineral crystallized during the final growth phase of the quartz crystals, and is to be found on their outer faces. It is also found in fissures in hydrothermally decomposed schist, associated with ancylite, anatase, fluorapatite and boulangerite. The crystal sizes reach only 0.5-1 mm.

Piemontite [Ca.sub.2][(Al,[Mn.sup.3+],[Fe.sup.3+]).sub.3][(Si[O.sub.4]).sub.3](OH)

Piemontite is rare. In the filling of one crystal cleft, tiny prisms to 0.6 mm were observed.

Prehnite [Ca.sub.2][Al.sub.2][Si.sub.3][O.sub.10][(OH).sub.2]

Prehnite was found in a cleft as transparent, pale green, rounded crystal aggregates to 5 mm, in association with calcite, sphalerite and fluorapatite.

Psilomelane

Psilomelane, a mixture of manganese oxides, forms black dendrites on the faces of quartz and calcite crystals. Not uncommonly it appears as compact earthy masses in mineralized fissures.

Pyrrhotite [Fe.sub.1-x]S

Pyrrhotite is widespread in the deposit. In the crystal clefts, two types have been identified. The first type is columnar, barrel-shaped crystals measuring up to 6 x 10 x 15 cm; these are usually in part limonitized. The second type is thin, hexagonal, platy crystals to 1 cm, as inclusions in or encrustations on calcite crystals. Typical associations are chalcopyrite, sphalerite, galena, chlorite and fluorapatite.

Pyrite Fe[S.sub.2]

Pyrite is also widespread. It is likewise found as inclusions in or overgrowths on quartz and calcite crystals. The dominant form is the cube, sometimes in combination with the octahedron. The average crystal size barely exceeds 2 cm; but in one cleft were found fractured, rounded crystals to more than 10 cm across.

Quartz Si[O.sub.2]

The average weight of the clear quartz crystals of the deposit is between 2 and 5 kg. Of course, in any middle-sized cleft, crystals weighing a few dozen kilograms may also be found, and in large clefts, crystals up to a few hundred kilograms. Even crystals weighing one or two tons are not unusual in this deposit! The habit, for the most part, is pseudohexagonal, with equal development of prism faces and a dominance of prism over rhombohedral faces, as in the "normal" Alpine cleft habit. The crystals are mostly short prismatic, with an average length/width ratio of 1.7 to 3.

The coloring of the crystals is preponderantly pale smoky, but [TABULAR DATA FOR TABLE 1 OMITTED] absolutely colorless "rock crystal" is not uncommon. Colorless crystals are mostly found in clefts associated with the quartz lenses and diabase intrusions. Crystals from clefts in the schistose country rock show a smoky coloring. Dark smoky quartz is not characteristic of this deposit and is only rarely encountered.

Still rarer is amethyst, in scepter growths on columnar quartz crystals. The size of the amethyst crystals averages only 5-8 mm, and never exceeds 2 or 3 cm. Commonly, this amethyst contains inclusions of platy or acicular goethite crystals.

Multiple phases of growth, dissolution and regeneration are typically seen in the tension fissures of the deposit. Flattened, distorted, doubly terminated crystals, crystals with solution pits on their faces and edges, and skeletal crystals are all characteristic of the Dodo deposit. Fairly common also are large twisted crystals (gwindels) and crystals with "white stripes" (fadens). The gwindels are most often found in small pockets and mineralized fissures along the flanks and upper horizon of the Dodo deposit. The Dodo gwindels are somewhat smaller in size than those from the nearby Puiva deposit. But only the Dodo mine has produced clusters of three to six quartz gwindels on matrix.

The quartz crystals commonly contain impurities to a varying extent. Included fragments of sericite schist are most common, along with inclusions of other cleft minerals. Aesthetic "compositions" can result, which resemble landscapes. The minerals which most commonly form inclusions within the quartz crystals or overgrowths on their faces are chlorite, muscovite (sericitic), rutile, brookite, anatase, calcite, titanite, epidote, ilmenite, pyrite, cosalite, boulangerite and ancylite-(Ce).

Rectorite

Rectorite clay forms leafy pale green, 1-2 cm aggregates on the faces of quartz crystals.

Ruffle Ti[O.sub.2]

Ruffle is not widely distributed as a primary mineral in the Dodo deposit; only on the western and southern flanks are found isolated crystal clefts containing primary rutile. But this ruffle differs strongly from that in other deposits. Its color is white through grayish white (!), and the luster is silky. Fibers less than 5 microns thick may be up to 10 cm long. Primary ruffle forms asbestiform or cottony aggregates either filling spaces in quartz or calcite druses or as inclusions in the crystals. Not uncommonly rutile occurs as parallel groups of fibers which, as inclusions, give quartz crystals a shimmering milky turbidity resembling a cat's-eye effect. Thus mineralogists refer to rutile from Dodo as "asbestiform," while collectors call it "cottony" rutile.

Secondary, yellow to red-brown ruffle is commonly found in the deposit, associated with anatase, brookite, ilmenite and titanite.

Smithsonite ZnC[O.sub.3]

Smithsonite has developed as an alteration product of sphalerite, as white through whitish gray, opaque crystalline aggregates and crusts.

Sphalerite (Zn,Fe)S

Sphalerite, like rutile, is found more commonly on the southern and western flanks of the deposit, where it makes lustrous black tetrahedrons to 1.5 cm (exceptionally to 5 cm). It occurs intergrown with calcite, chlorite, chalcopyrite, pyrrhotite and quartz.

Tetrahedrite [(Cu,Fe,Ag,Zn).sub.12][Sb.sub.4][S.sub.13]

Tetrahedrite occurs in the cleft fillings and as inclusions in quartz, in small tetrahedrons, and as massive grains to 10 cm. Associations include chalcopyrite, galena and quartz.

Thomsonite [Ca.sub.2]Na[[Al.sub.5][Si.sub.5][O.sub.20]][multiplied by]6[H.sub.2]O

Thomsonite occurs in the cleft fillings as transparent, platy crystals to 2 mm.

Titanite CaTiSi[O.sub.5]

Titanite is common, especially in clefts in diabase and greenschist. The average crystal size is about 2 cm, but the largest crystal known to the author is 16 cm long (!). Titanite forms complex, sharp, platy crystals in V-twins, and also in the typical alpine "sphene" habit. V-twins making distinctive little "shoes" are typical of Dodo. The color ranges from pale to dark brown, reddish brown, brown, yellow-green, blue and colorless. Blue titanite contains an order of magnitude more yttrium than does titanite of other colors.

Commonly one sees color zoning in which the pyramids and pinacoids are green or yellowish green, and the prisms reddish brown or dark brown (resulting in a kind of "hourglass" pattern). Titanite occurs intergrown with fluorapatite, calcite, adularia and other cleft minerals.

Tremolite [Ca.sub.2][Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2]

Tremolite is encountered only rarely. It is found in cleft fillings as cottony aggregates of threadlike crystals with a grayish white color.

Tourmaline Group

Tourmaline is observed as acicular inclusions of black schorl in quartz crystals and also loose in the cleft fillings (with asbestiform rutile).

Xenotime-(Y) YP[O.sub.4]

Xenotime-(Y) forms brownish yellow, prismatic crystals to 1 mm associated with anatase, ancylite-(Ce), brookite, cosalite, calcite and other minerals.

Zircon ZrSi[O.sub.4]

Zircon occurs in the cleft fillings as prisms to 1 mm of grayish pink color. Together with this recrystallized zircon, a clastic zircon (as gray rounded grains) is found in the country rock of the deposit, as the probable source material for the "regenerated" zircon of the clefts.

Zoisite [Ca.sub.2][Al.sub.3][(Si[O.sub.4]).sub.3](OH)

Zoisite is found in the lower part of many quartz vugs, as gray-green prisms to 2 cm long.

ACKNOWLEDGMENTS

The author expresses his heartfelt thanks to V. V. Bukanov, for his consultations upon completion of mineralogical studies in the Polar Urals, and to Mrs. N. P. Popova, for her assistance in laboratory work. Thomas P. Moore kindly produced the English translation of sections of an earlier version of this article which appeared in Lapis.

REFERENCES

BUKANOV, V. V. (1974) [Quartz crystal from the Subpolar Urals.] Nauka, 212, 2 (in Russian).

BURLAKOV, E. V. (1985) [Rare-earth minerals from the quartz crystal deposit in the Schiefer zone.] [Geologic and mineralogical raw materials reserves of the West Siberian Plateau and its fold belt.] [Proceedings of the 5th Annual Conference in Tyumen], 3, 75-76 (in Russian).

BURLAKOV, E. V. (1989) [Mineralogy of the quartz vein deposits and mineralogical-geochemical characteristics of the ore-controlling tectonic fractures. - Author's lecture on a dissertation for the awarding of the academic degree in geology-mineralogy. Science.] Syktyvkar, No. 229, 5 p. (in Russian).

BURLAKOV, E. V. (1995) Dodo: Alpine Klufte im Polar-Ural. Lapis, 20 (3), 13-26, 35.

BURLAKOV, E. V., and JAKOVLEVA, O. A. (1988) [A new mineral from the quartz clefts on the east flank of the Polar Urals.] In: [New Data on the Mineralogy of the Urals.] Sverdlovsk, UrO AN SSSR, 4, 7-11 (in Russian).
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Title Annotation:Dodo mine
Author:Burlakov, Evgeni V.
Publication:The Mineralogical Record
Geographic Code:4EXRU
Date:Nov 1, 1999
Words:6012
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