luminescent geochemical anomalies in the lithosphere and...

Per. Mineral. (2001), 70, 3, 85-100

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Luminescent geochemical anomaliesin the lithosphere and haloes of ore bodies

ALEXANDER PORTNOY I , BORIS GOROBETS2, ALEXANDER ROGOZHIN3*,

ALEXANDER BUSHEy3 and TATIANA KYITKo3

I MoscowState Geological Exploration Academy. Miklukho-Maklaya, 23, 117873, Moscow, Russia2 MoscowState University of Environmental Engineering Ecology.St. Basmannaya, 2114, 107884, Moscow,Russia

3 All-RussiaResearch Instituteof MineralResources(VIMS). Staromonetnyi per., 31, 109017, Moscow,Russia

Submitted, June 2001 - Accepted, January 2002

ABSTRACT. - Haloes of luminescent mineralsaround kimberlite pipes, emerald-bearing bodies inmetasomatites, gold-bearing bodies, mica pegmatiteveins and mountain crystal veins were first revealedand studied. They are special cases of luminescentgeochemical anomalies which occur in those parts ofthe Earth's crust where considerable interaction ofrocks and abyssal fluids took place. The fluidsleached trace elements, including the luminogenoustransition metals of 3[, 41 and some other groupsfrom the country rocks, and then accumulated anddeposited them. Accumulations of Mn, REE, U andother luminogenous elements form glow centers insecondary minerals. Outcrops of photoluminescentcalcite, apatite, zircon, fluorite, cerussite,chlorargyrite and uranyl minerals in altered rocksmay serve as indicators of mineral deposits.Photoluminescent grains of apatite having a violetblue glow (Ce 3+, Eu2+) detected by sand analysis,allow mapping of secondary mechanical haloes(scattering), helpful in searching for kimberlitepipes.

Luminescence spectra of pathfinder minerals fromthe haloes are also presented, with the luminescenttracer elements identified in them.

* Corresponding author, E-mail: [email protected]

RIASSUNTO. - Sono stati registrati e studiati glialoni di minerali luminescenti presenti nei pipekimberlitici, nei corpi contenenti smeraldi nellemetasomatiti, in quelle delle masse aurifere e nellevene pegmatitiche micacee. Questi rappresentanocasi particoli di anomalie geochimiche luminescentipresenti nelle porzioni della Crosta terreste nelelquali si attuano interazioni tra rocce e fluidi diorigine profonda. Tali fuidi lisciviano, accumulano edepositano, prelevandoli dalle rocce ospiti, alcunielementi in tracci inclusi i metalli a transizioneluminescente 3d-, 4f e alcuni di altri gruppi.

L'accumulo di Mn, Terre rare, U e di altrielementi luminescenti forma centri di emissione neiminerali secondari. Gli affioramenti di calcite,apatite, zircone, fluorite, cerussite, clorargirite eminerali uraniferi nelle rocce alterare funziona daindicatore di depositi minerari.

I grani fotoluminescenti di apatite, che emettonobande blu-violetto (Ce3+, Eu 2+) rilevate sulle analisidi sabbie, consentono la mappatura di alonisecondari meccanici (dispersione) utili nella ricercadei pipe kimberlitici.

Sono anche illustrati spettri di luminescenza dialcuni minerali guida precisando il tipo di elementoche funge da tracciante.

KEY WORDS: luminescence, mineral, geochemicalanomaly.

86 A. PORTNOY, B. GOROBETS, A. ROGOZHIN, A. BUSHEV and T. KVITKO

INTRODUCTION

The emission of light excited in minerals byincidental ultra-violet (UV) radiation, known asphotoluminescence (PL), is a resonance opticprocess different from non-resonancerecombination processes such as cathodo-, Xray- or thermo-luminescence. In the history ofscience, the luminescence effect was firstdescribed in 1602 by the Italian alchemist,Vincencio Casciarolla, from Bologna. Henoticed an enigmatic red glow when barite wasannealed with carbon after having beenexposed to sunlight. Such mineral substanceswere called phosphors (i.e., carrying light,although phosphorus as a chemical elementwas unknown at that time).

Only 250 years later, in 1852, Stokes provedthat the blue emission of fluorite excited by UVlight had a longer wavelength and was notdiffused light; he called it fluorescence, afterfluorite (quoted in Pringsheim and Vogel,1946, Pringsheim, 1949, and other works). In1889, Wiedemann introduced the termluminescence, from the Latin «lumen = light +escence» (cf. ibidem). The first luminescentspectra of minerals were registered andcorrectly interpreted as early as in the secondhalf of the 19th century. In 1859, Becquerelrealized that the orange-red glow of calcite wasrelated to a Mn activator (Becquerel, 1867, alsoquoted in Nichols et al., 1928). Thecathodoluminescent spectral lines of REEimpurities in fluorite (Sm, Eu, Dy, Gd) weredescribed by Urbain (1909). In 1918-1919, thePL spectra of uranium minerals(schroekingerite, autunite, and some others)were deciphered by Nichols and Howes, andthe equidistant spectral lines were attributed tothe uranyl ion (quoted in Nichols et al., 1928).

In the 1920s, Lenard and colleaguessynthesized solid luminescent materials dopedwith various impurities, and introduced theterms «center of luminescence» or «glowcenter» and «crystallophosphor» or «phosphor»(Lenard et al., 1928). The most importantgroup of phosphors was ZnS activated by Cu,Ag, AI, and other impurities.

Comparisons with synthetic pure and dopedphosphors showed that W042- complexes wereresponsible for blue luminescence in scheeliteand Mo042- complexes for yellowluminescence in powellite (Tiede and Schleede,1923; Schleede and Tsao, 1929, quoted inPringsheim and Vogel, 1946).

R-lines of Cr3+ were identified inluminescent spectra of ruby, spinel, alexandriteand topaz (Deutschbein, 1932; Tiede andLueder, 1933, quoted in Pringsheim and Vogel,1946).

Mn2+ was found to show green luminescencein willemite by Leverenz and Seitz, 1939(quoted in Leverenz, 1950).

Several REE2+ were first found to beresponsible for broad bands in the PL spectraof some minerals. The blue glow of Eu2+, redglow of Sm2+ and green glow of Yb 2+ wereobserved in fluorite, and the blue glow of Eu2+in plagioclase by Haberland et al. (1934, 1939).We quote these pioneering studies because theyhave often been ignored in recent literature.Detailed references and exhaustive historicalinformation may be found in Pringsheim andVogel (1946) and in the more modern work«Cathodoluminescence in Geosciences» (eds.Pagel et al., 2000).

Starting from the 1960s, systematicspectroscopic studies of luminescence resultedin the identification of glow centers in as manyas 280 minerals (Tarashchan, 1978; Gorobets,1981; Gotze, 2000; Gorobets and Rogozhin,2001). In particular, centers of violet and blue(Ce 3+, Eu2+), yellow (Mn 2+, Dy3+) and pink(Sm3+, Sm2+) luminescence were identified in.apatite (Gorobets, 1968; Portnov and Gorobets,1969; Tarashchan, 1978; Marfunin, 1979). Theviolet glow of Eu 2+ in microcline wasdescribed (Moroshkin et al., 1987), the redglow of Fe 3+ was described in microc1ine,orthoclase (adular) and p1agioc1ases (albite,oligoclase) (Geake et al., 1973; Tarashchan,1978), and R-lines of Cr3+ were established inthe PL spectra of some plagioclases (Bakhtinand Moroshkin, 1986).

The PL spectra of uranyl, U022+, whichforms many secondary uranium minerals, were

Luminescent geochemical anomalies in the lithosphere and haloes ofore bodies 87

studied in detail (Gorobets and Sidorenko,1974). It was found that some classes of uranylminerals (hydroxides, molybdates, silicates,vanadates) showed bright green to orangeluminescence only after cooling. The PLspectra of chalcedony, opal, allophane, calcite,barite and zircon, which adsorb uranylimpurities, were obtained and interpreted(Sidorenko et al., 1986).

The bluish-green PL of chlorargyrite andbromargyrite was discovered, their maincenters presumably being donor-acceptor pairsof silver and halide vacancies (Gaft, 1993).

Luminescent elements are not yet reliablyidentified in zircon and cerussite. The broadband in the PL spectrum of zircon is notelementary (Gaft, 1993). Some lines belong toDy3+ (Trofimov, 1962; Gotze, 2000) and thediffused component is presumably due toradiation defects (Votiakov et al., 1993). Thereare no adequate data concerning the origin ofthe broad yellow band in the PL spectra ofcerussite.

The general picture of the luminescentmineral set is as follows.

The PL of a few minerals is defined by theirintrinsic elements: Mn2+ ions, oxy-complexesof uranium, tungsten, molybdenum andtitanium form luminescence centers in many«title» minerals of their corresponding metals.In such complexes, the optic electrontransitions between oxygen and metal orbitalsare not quenched by electron-phononinteraction with the crystal lattice. Much moreoften, the PL of minerals is caused by thepresence of transition metal cations, in minoror trace amounts, substituting for lattice cationsof similar ionic size. The main luminescentelements are: 3d ions of the iron group: Mn2+,Fe3+, Cr3+, Ti4+, and Agt of the noble metalgroup; 4/ rare-earth elements (REE): Ce 3+,Sm2+, Sm3+, Eu2+, Eu3+, Dy3+, Yb2+, Yb3+, etc.They form luminescence centers which canabsorb and emit light owing to electrontransitions: 3d-3d in Mn2+, Cr3+, Fe3+, etc.; 4/4/ in REE3+; 4f-5d in Ce3+, Eu2+, Sm2+, Yb2+

(Tarashchan, 1978; Marfunin, 1979; Walker,1985; Waychunas, 1988; Gorobets and Walker,

1994). Mn2+ substitutes for Mg2+ and Ca2+ inmany magnesian and calcian minerals, andREE3+ and REE2+ substitute for Ca2+ in manycalcian minerals. Cr3+ substitutes for AP+ inoxygen octahedra (e.g. in ruby and spinel), Fe3+substitutes for AP+ and Si4+ in tetrahedra inaluminosilicates, and so on. Thesystematization of luminescent minerals basedon their crystal chemistry has been carried out(Gorobets et al., 1995). The «Luminescencespectra of minerals» Reference Book (Gorobetsand Rogozhin, 2001) includes the PL, X-rayand cathodoluminescence spectra of more than280 minerals, and the majority of spectral linesare ascribed to certain luminescent elements.

A number of practical applications have beendeveloped on the basis of luminescence centeridentification in minerals. Exploration targetswere uranium (de Neufville et al., 1981;Sidorenko et al., 1986), molybdenum, tungsten,lead, zinc (Seigal and Robbins, 1985), tin (Gaftet al., 1988), lithium, beryllium, etc. (Bakhtinand Gorobets, 1992). Some of these methodsinvolve remote-sensing techniques based onlidars with powerful UV lasers. In mineralprocessing, techniques using PL and X-rayluminescence are being developed (Gorobets etal., 1997).

In the present work, haloes ofphotoluminescent minerals around varioustypes of ore deposits were first revealed andstudied. The aims of this paper were: (1) torepresent the PL spectra of key mineralsforming luminescent geochemical anomalies(LGA), especially luminescent haloes (LH)around ore bodies; (2) to summarize the maintypes of LGA; (3) to distinguish specific LHtypes in the deposits of diamond, gold, ruby,emerald, mountain crystal, etc. and to provide afirst approximation of their monomeric models.

EXPERIMENTAL TECHNIQUES

First, reference specimens of all keyluminescent minerals were selected from othergeologic objects in order to register their PLspectra and identify the main spectral lines


corresponding to certain luminescent elements.The minerals in question are apatite, zircon,calcite, fluorite, microcline, plagioclase,scheelite, ruby, opal, chalcedony, allophane,chlorbromargyrite, autunite and cerussite.Monomineral grains ranged from 0.1 to a fewmillimeters in size. PL was excited by UV light(220 - 400 nm) from a mercury lamp or a UVlaser (337 nm), usually, at 300K. Emissionspectra (Figs. 1-2) were registered over aspectral range of 400-750 nm with the help ofgrating monochromators (types MDR-2 andMDR-23, Leningrad Optical MechanicalFactory, LOMO), combined with a FEU-I06photomultiplier.

Luminescent haloes were sought by means ofexamination of luminescent minerals in piecesof cores from exploratory holes, rock samples,and direct observation of tunnel walls. Thesampling interval was usually 10 meters.Occasionally, a more detailed check wascarried out, reducing the interval to 3-5 meters.Cores from nearly 200 holes of depths of about300 meters were examined during mapping ofhaloes around kimberlite pipes. The boringnetwork was 1OOx 100 meters and the totallength of cores was nearly 60 kilometers. Thus,about 6 thousand points were sampled withinthe kimberlite field.

The number of sampling points in the haloesof other mineral deposits described in thispaper totalled nearly 4000: about 2000 pointsin cores, 1000 points as rock pieces, and 1000points in tunnel walls of deposits.

Thus, the halo models mapped below arebased on nearly 10 thousand points. About30% of them showed very low PL intensity,approaching the background level. About 50%of all samples surpassed the background levelby a factor of 2-3, and about 20%, by 10 oreven more. The latter samples represented thepeak zones of the LH of ore deposits.

The PL level was evaluated from the areaconcentration calculated as an average of thenumber of luminescent grains per 1 em- of rocksamples examined by UV. The great diversityand heterogeneity of substances and themorphology of geologic objects depending on

type and individual features of ore deposits,and of rock and mineral samples from thesedeposits, severely complicated data processing.Unlike chemical fluorescence analysis, themineralogical luminescence analysis used inthis work deals with material not chemicallyprepared for experiments. Moreover, the rockspecimens observed under UV light wererough, broken and irregular. Theircharacteristic size varied from 2-3 to 8-12centimeters. The luminescent grains on thesurface of rock pieces, which served as identityelements and units of measurement (of PLlevel), were of different sizes and often hadvarious hues of glow (for a single mineral).Luminescent grains in cores from the contactzone of kimberlite pipes were tiny, - 0.1 mm insize. Mineral grains in samples from pegmatiteand hydrothermal veins were larger than 1 mm.Thus, results were based on observationscarried out at the level of approximate massanalysis.

The signal-to-noise ratio of the areaconcentration of luminescent grains in LH andin background country rocks was evaluated foreach mineral observed in the halo. Thenumerator of the ratio is the value of areaconcentration of PL mineral grains taken at acertain distance from the border of the orebody, this value being calculated as an averagefor a narrow interval (up to 10 meters). Thedenominator of the ratio is related to thebackground value, measured sufficiently farfrom the ore body and characteristic ofunaltered country rocks. Isolines were drawnon the basis of these mass measurements andshow the PL level for each luminescentelement in each mineral. Smoothed curvesrepresenting the profile models of the haloeswere also plotted (see below; Figs. 3,4).

RESULTS AND DISCUSSION

PL spectra ofminerals

The typical PL spectra of minerals formingluminescent haloes of various types are shownin Figures 1-2. Identification of luminescent


10

11

A, 100 nm54

I

4

4 5 A, 100 nrn

2+ 3 4Eu

94 5 A, 100 nrn3+

5 Cr

4 5 A, 100 nrn 6 A, 100 nmFig. 1 - PL spectra of main luminescent minerals in the haloes: apatite with (1) bluish-violet PL kimberlite, (2) yellow and(3) pinkish glow, Emerald Mines, the Urals and (4) blue glow, mountain crystal vein Dodo, the Polar Urals; zircon (5) withyellow glow (from kimberlite pipe and from its luminescent halo) Yakutia, Russia; calcite with (6) orange, (7) pinkish and(8) bluish-violet glow, Snejnoye, skarn, the East Pamirs, Tadjikistan; ruby (9) with red glow from the Snejnoye the EastPamirs, Tadjikistan; fluorite (10) with violet-blue glow from a gold-quartz vein, Darasun, Transbaikal region (Asian part ofRussia); plagioclase (11) with violet-blue glow and microcline (12) with violet glow, both from a mica pegmatite vein,Mama, Eastern Siberia, Russia.


4

1

4

4

5 A, 100 nm 5

exe 337 nrn

A, 100nrn

4 5 A, 100nrn

5

5 5,5 A, 100nrn 4 5 6 A, 100nrn

4 5 6 A, 100nrn 5 5,5 A,100 nrn

Fig. 2 - Scheelite: (1) with blue glow from greisen, Boyovka, Urals, Russia (0,01% Mo03); (2) molybdoscheelite withyellow-white glow from skarn-greisen type deposits, Vostok-II, Far Eastern Russia (0,2% Mo03); (3) scheelite withreddish hue of glow from a gold-bearing quartz vein, Berezovskoyie, Urals, Russia (up to 0,01% EU203); quartz (4) withwhitish-blue glow of organic impurities from a montain crystal vein, Dodo, Northern Urals, Russia; autunite (5) with greenPL from an oxidation zone, Streltsovka, Eastern Siberia, Russia; chalcedony, opal, allophane (6) with green glow: PLspectra of UOi+ are essentially identical in all specimens of these minerals; chlorbromargyrite (7) with green glow underUV-laser light, Kasmanachi, Kizil-Kum, Uzbekistan (all specimens from oxidation zones of silver and silver-and-golddeposits showed similar PL); cerussite (8) with whitish-yellow PL from oxidation zone of a gold-bearing quartz vein, KokBulak, Uzbekistan.


centers was carried out with the help ofprevious data obtained by our group and worksby the authors mentioned in the Introduction.

Luminescent geochemical anomalies (LGA)

For a mineral to become luminescent (in thiscontext, PL), the following three conditionsmust be met simultaneously: (l) theappropriate type of crystal lattice must favourthe formation of intrinsic or impurity emissioncenters (this condition is commonly satisfied indielectrics and more rarely in semiconductors);(2) a sufficient percentage of PL centers(usually> 0.01%) must be available; (3) smallnumbers of quenching centers, primarily ofiron (less than 0.5 - 1.0%) must be available.

The bulk of the lithosphere is nonluminescent, because the petrogenic cations ofSi, AI, Ca, Mg, K and Na are not transitionelements. Their ions have no levels for intrinsicoptical electron transitions in the crystal latticeof minerals. Fe is the only transition petrogenicelement. Fe2+ is the most important quenchingelement because of the peculiar structure of itselectron levels. Fe3+ can luminesce in silicatesand aluminosilicates only if its content is lowerthan ~1%; concentration quenching starts if itis higher. The average abundance of otherluminescent elements in the Earth's crust isabout 10-3 - 10-6%, which is much lower thantheir limiting sensitivities (~ 10-3 - 10-1%). Sothe key rock-forming minerals, feldspars,quartz, calcite and dolomite, show no PL inbackground rocks such as basalt, granite andsedimentary «layers» (they may showrecombination luminescence, which is not dealtwith here).

The situation changes dramatically in somelocalized areas in the lithosphere where stronginteraction of abyssal fluids and hydrothermalsolutions with background rocks took place.The fluids leached rare elements, includingluminogenous metals, from country rocks,accumulated and deposited them.

The two main physico-chemical factors ofthis process are the acid-base properties andredox potential of fluids percolating throughcountry rocks. Unlike rocks, fluids can easily

change their physico-chemical parameterswhile ascending to the surface. Thetemperature and pressure of fluids decreases,together with their basicity-acidity and redoxpotential. On the way from the Earth's mantleto the surface, polyvalent luminescent elementsreach the peaks of their activity in fluidsroughly in the following sequence: N, Cr 3+,Ce3+, Eu2+, Yb2+, Mn2+, Fe3+, Dy3+, W6+, M06+,

U6+ and Agt, At the same time, hydrothermaland metasomatic processes cause ironseparation and formation of its own mineralphases: sulfides, oxides, and hydroxides. Thesetwo factors result in crystallization andrecrystallization of many secondary and veinluminescent minerals: calcite, apatite, fluorite,plagioclase, zircon, etc .. Ascending throughfractures and faults in the Earth's crust, fluidsdeposit rare elements in zones of lowertemperature and pressure, forming ore depositsof well-known types: kimberlite, carbonatite,nepheline syenite, pegmatite, skarn, greisen,hydrothermal veins and their derivatives. Insuch away, LGA accompanying ore depositsare formed. For instance, N (in diamond), Cr3+

(in plagioclase, ruby, spinel), Eu2+ and Ce3+ (inplagioclase, apatite, barite, etc.) areluminescent elements accompanying deepderi ved rocks related to mantle sources,whereas D y3+, Eu3+, Fe3+, and W6+ tracegranite magma sources. Near the Earth'ssurface, the environment is strongly acidic andoxidizing owing to the nitrogen and oxygen inthe atmosphere. Hence, luminescent mineralscomposed of ions of the highest valencies, U6+,Ag', M06+, vs+ and Cr6+, occur in hypergeneand sedimentary minerals: uranyl minerals,opal and chalcedony, chlor- and bromargyrite,powellite, vanadinite, crockoite, etc .. Carbonyl,carboxyI and cyclic organic molecules take anactive part in the formation of luminescentcenters in calcite, dolomite and rare carbonateminerals, cerussite and hydrozincite.

Systematization of the LGA is given in theTable. It was obtained from experimental data,which included some thousand samples ofdozens of PL minerals from different types ofLGA.


TABLE 1

Systematization of luminescent geochemical anomalies (LGA) in the lithosphere with the help ofspecific sets of the luminescent elements (in brackets) in minerals.

1 2 3 4

Type of Subtype of Geological objects Key PL minerals andLGA LGA luminescent elements

SUPER- Biogenous Bogs and other Inanimate bio-organic substancesFICIAL superficial (ST)*

bodies of bio-organicChemogenous substances

Main lumi- Bituminous limestone, shale Oils, bitumen (ST)*; carbonates,nescent sulphates (ST)moleculesand elements Salt marsh Chemogenous organic

substances, halides, carbonates,singlet- sulphatestripletorganic Zones of oxidation of Uranium deposits Uranyl and vanadyl mineralsmolecules ore deposits: U6+, (UOl+, Y043-); chalcedony,

Mo6+, y5+, Cr6+, opal, allophane, kaolin, calcite,carbonyl- Ag". barite (UOl+- adsorp.)containingorganic Polymetallic deposits Chlorargyrite (Ag+, Cl" -molecules vacancies); hydrozincite,

cerussite, anglesite, smithsonite,cations: calcite, barite (<<CO»); willemiteU6+,Mo6+, (Mn2+)

v», Cr6+,

Ag" Eluvium placer Source material Greisens.high- Powellite, wulfenite (MoOi-);subtype: W6+,Mo6+, temperature crocoite (CrOi-); opal (UOl-)U6+, Mn 2+, Ce3+, hydrothermalEu 2+, N. deposits

Greisen, albite Zircon (radiation defects RD)**,scheelite (W042-, Mo042-) ,

cassiterite (?), apatite (Mn2+).

Carbonatite Calcite (Mn2+), apatite (Ce3+,

Eu2+), zircon, baddeleyite(RD**, Ti4+, REE3+)

Kimberlite Diamond (N), apatite (Ce3+),

zircon (RD)**.

Luminescent geochemical anomalies in the lithosphere and haloes ofore bodies

Table 1: CONTINUED

93

1 2 3 4

ABYSSAL Crust subtype Au-hydrothermal, poor Calcite (Mn 2+), scheeliteMain W6+,Mo6+, Eu3+, sulphide deposits (W042-, MoOi-, Eu 3+)

luminescent Eu2+, Dy3+, Sm3+,

elements: Sm2+, Yb2+, Mn 2+. Hydrothermal, polymetallic, Calcite (Mn 2+), fluorite (Eu 2+,W6+, M06+, Au-Ag, rare metal deposits Yb2+, Sm2+), bariteDy3+, Yb2+,Sn2+, Mn 2+, Skarns Calcite (Mn 2+), scheeliteEu2+, Eu 3+, (WOi-, M0042-), scapoliteSm3+, Sm2+, (Mn 2+, S2-) datolite andCe3+, Ti4+, danburite (Ce3+, Eu2+, Yb2+),

Cr3+, S2-' N wollastonite (Mn 2+), fluorite(Eu 2+, Yb2+)

Greisens, albitites Fluorite (Eu2+), scheelite(W042-, REE3+), apatite(Mn 2+, Dy3+), cassiterite (?)

Rare metal pegmatites Apatite (Dy3+, Mn 2+),

spodumene (Mn 2+)

Muscovite pegmatites Plagioclase and microcline(Eu2+)

«Mantle» subtype Nepheline syenites Apatite (Ce3+, Eu 2+, Sm3+,Cr3+, Ce3+, Eu2+, Mn2+), sodalite (S2-)' (Ti, Zr)-Ti3+,Ti4+, S2-' N silicates (Ti4+)

Carbonatites Calcite (Mn 2+), apatite (Ce3+,

Eu 2+), baddeleyite and zircon(Ti4+, RD**)

Kimberlites Diamond (N), apatite (Ce3+),

zircon (RD**)

* ST means singlet-triplet molecules.** RD means radiation defects.

Although LGA represent potential oreobjects, most of them have no commercialvalue.

Luminescent effects around ore bodies wereobserved in their contact zones in the form ofhaloes.

Haloes of luminescent minerals in and aroundore bodies

Luminescent haloes were discovered andmapped in the following mineral deposits(Figs. 3,4).

Haloes of kimberlite pipes (Fig. 3 (1). Sharp


o 100 200 300 400WI:

~

500 m 0 10 20 30 40 50 m[4]

5 10 20 m 25 50m

~ Kimberlite (1) ~ Ruby-bearing zone in

~Sedimentary rocks (1)

marble (3)o 0 Marble PR3(3) and marmorizedo 0 0

Mica-bearing pegmatite limestone (4)~ vein (2)

~ Scheelite-bearing altered

~Gneisses and crystal limestone (skarn) (4);shales (2)

Fig. 3 - Schematics of photoluminescent haloes in:1 - contact zone of a kimberlite pipe; 2 - contact zone of mica-bearing pegmatite vein; 3 - ruby-bearing skarn; 4 - scheelitebearing skarn. (Ca - calcite, Zr - zircon; Ap - apatite; Rub ruby; Sch - scheelite; PI - plagioclase; FI -fluorite; Cer cerussite).

discontinuous contact zones of pipes alwayssurprised explorers. The PL characteristics ofcalcite, apatite, zircon and scarcer barite andfluorite show the considerable influence of

kimberlite on their country rocks: LH stretchup to some hundred meters and even to 1-1.5km outside kimberlite borders. The kimberlitepipes of the Arkhangelsk (Northern European


n2+ 3+

(Mn ,Sm )Ap

95

o

5

1 2

15

34m

25 m

5 15 25 m

Emeralds (1)

Margarite-muscovitephlogopite metasomatite (1)

Oligoclasite (1)

Gold-bearing vein (2)

Volcanic-sedimentaryrock (2)

Sandy shale (3)

Mountain crystal vein (3)

Fig. 4 - Outlines of photoluminescent haloes in: I - emerald-bearing metasomatic bodies; 2 - gold-bearing ore bodies; 3 mountain crystal vein. (Ca - calcite, Zr zircon; Ap apatite; Rub - ruby; Sch scheelite; PI plagioclase; FI -fluorite;Cer - cerussite).

part of Russia) and Mirniy (Yakutia, Russia)ore fields have been studied. The LH in bothregions consist of: (1) PL mineral zone, 20-50m thick; (2) zone of concentration of PLminerals, over an interval of 50-400 m; (3)zone of decreasing contents of PL minerals,several hundred meters thick, passing into thebackground rock. The ratios of PL grain

concentrations in zones (2) and (3), whichexpress the signal-to-noise ratio, are on average110/3 for calcite, with generally red PL ofMn 2+ showing hues from orange-red topinkish-purple (Fig. 1 (6, 7)), 280/4 for calcitewith bluish-violet PL (Fig. 1 (8)), 0.7/0.06 forapatite with yellow PL of Mn2+ (Fig. 1 (2)),1.1/0.02 for apatite with bluish-violet PL of

96 A. PORTNOY, B. GOROBETS, A. ROGOZHIN, A. BUSHEY and T. KYITKO

Ce3+ and Eu 2+ (Fig. 1 u», and 2.5/0.02 forzircon with yellow PL (Fig. 1 (5)). Obviously,the enormous masses of country rocks wereaffected by mantle fluids and recrystallized. Atthe same time, abyssal elements Zr and P andREE were fixed in a form of zircon, apatite,etc.. Heating favoured Mn2+ incorporation intocalcite and apatite. Taking into account theseLH, it is recommended that the fine-grainedsand fraction «0.25 mm) should be analysedseparately from the coarse fraction in contrastwith current practice. The peculiar PL ofmetasomatic minerals in the fine non-magneticfraction, apatite (with impurities of Ce3+, Eu2+),barite and zircon, reveal the mechanicaldispersion haloes of these minerals aroundkimberlite pipes.

Haloes around kimberlite-like pipes due tomechanical scattering of minerals were alsodetected with the aid of sand analysis. Searcheswere carried out in the Pechora-Timan region(Northern European part of Russia). The aimwas to select the most promising localmagnetic anomalies from dozens detectedduring an aerial survey. The key mineralindicator in the sand samples was apatite,showing a weak bluish-violet glow of Ce3+ andEu 2+. More than 400 sand samples wereexamined under a mercury UV lamp. About10% of them had one to four grains of apatite,of a kind called «mantle» type. In addition,almost all samples contained a great number ofzircon grains with bright yellow PL. Somesamples also contained apatite grains withyellow PL of «granitoid» type, which wereuseless for kimberlite exploration. The mostinformative was the non-magnetic fractionsmaller than 0.25 mm. Occurrences of apatiteof «mantle» type within haloes surroundingsome magnetic anomalies (Fig. 5) significantlyincrease the probability that a source of mantlefluids is located under them. Since apatite issofter and more susceptible to chemicaldecomposition than the classic mineralindicators of kimberlites, such as pyrope,picroilmenite and chromspinelides, it may betransported no more than 1-2 kilometers fromits original location. Therefore, apatite with

bluish-violet PL may be considered as a newindicator of kimberlites (and other «mantle»rocks) and even more reliable than othermineral indicators.

Boring of anomaly points identified by theaerial survey and fitted to the haloes mappedwith the help of sand PL analysis showed thatthese anomalies were related to pipes anddykes or kimberlite-like rocks ofpicroporphyrite type. Apatite grains with violetPL were also found in these bodies. This factconfirmed the «mantle» origin of apatite grainsin the sand samples.

The new modification of sand analysisdescribed above was successfully applied by usin Yakutia and the Yenisei (Asian part ofRussia) ridge during searches for «mantle»rocks.

Haloes of mica pegmatite veins (Fig. 3 (2)).LH up to 20 m thick were mapped in gneissesand micaschists in the deposits of the MamaTchuya and Biriussa regions of Eastern Siberia(Asian part of Russia). The main PL mineralsin the LH are albite-oligoclase with violet-blueglow and microcline with violet glow of Eu2+

(Fig. 5 (2, 3)). This cation is indicative of (K,Na) metasomatism, which promotes countryrock recrystallization in a reducingenvironment.

Haloes in skarn (Fig. 3 (3-4)). LH up to 20 mthick were discovered in the ruby-bearing skarnof the Snejnoye deposit in the East Pamirs,Tadjikistan. The skarn is situated in carbonateterrigenous rock intruded by granitoids andpegmatite. The main PL mineral is calcite. Itsglow is red (Mn 2+) inside the ore zone andbluish-violet or purple outside it. Ruby showsred PL (Cr3+) (Fig. 4), allowing the detection oftiny ruby grains in sands. The highest level ofPL is observed inside ore bodies.

Likewise, scheelite-bearing skarn ischaracterized by the most intense PL inside orebodies, owing to the blue glow of W042- inscheelite (Fig. 2 (l)) or the yellow glow ofM0042- in molybdoscheelite (Fig. 2 (2)). Thered PL of calcite (Mn2+) was observed insideand outside ore bodies forming rather thin LH.

Haloes of emerald-bearing bodies in


1I-----+--------\------b.Lf------II 640

M 1:500000

97

Fig. 5 Scheme of magnetic anomalies (dots) mapped during aerial survey for kimberlite-bearing deposits in PechoraTiman region (European part of Russia). Areas of dots overlapping occurences of apatite (violet glow) in sands wererecommended for subsequent boring.

metasomatites (Emerald Mines, Urals, Russia)(Fig. 4 (1». Beryllium ore bodies may besubdivided as follows: (1) muscovite-quartzplagioclase; (2) margarite-plagioclase veinswithout emerald; vein-disseminatedmetasomatic zones: (3) poor in emerald, (4)rich in emerald. Apatite outcrops show yellow(Mn 2+) (Fig. 1 (2», pinkish (Mn 2+, Ce3+, Eu2+)(Fig. 1 (3» and bluish-violet (Ce3+, Eu2+) PL;its spectrum is similar to the PL spectrum ofapati te from kimberlite (Fig. 1 (1». Theaverage frequency ratios of the occurrence ofgrains with yellow-pinkish PL to that withbluish-violet PL are 5.3, 1.8, 1.8, and 1.0 inzones (1)-( 4), respectively (Kuprianova and

Moroshkin, 1987). An additional indicator ofemerald is plagioclase with violet-blue PL ofEu 2+ (Fig. 1 (11». In zones (1) - (3), itsoutcrops range from a few percent up to 40%of the total surface and in zone (4) it is up to100%. Since Cr3+ forming green centers inberyl is related to mantle fluids, a similarnature may be presumed for Eu 2+ and Ce 3+

centers in apatite and plagioclase. Thus, thelatter may serve as pathfinder minerals inemerald exploration.

Haloes of gold-bearing ore bodies (Fig. 4(2». These were observed in gold-quartz veins(Darasun) and skarn (Andriushkinskoye) inTransbaikalia (Asian part of Russia); black

98 A. PORTNOY, B. GOROBETS, A. ROGOZHIN, A. BUSHEY and T. KYITKO

shales of the Central Kizil-Kum; volcanic rocksof the Kuramin ridge (Kotch-bulak) and theOkhotsk-Chuckchee mountain belt (Karamken,Khakandjia, Dukat, Asian part of Russia); andlower Palaeozoic terrigenous rocks of CentralMongolia (Boro) (Bushev and Portnov, 2000).The LH are related to outcrops of apatite(bluish-violet PL of Ce3+ and Eu2+, yellow PLof Mn 2+) (Fig. 1 (1, 2», fluorite (blue PL ofEu2+) (Fig. 1 (10», calcite (orange-red PL ofMn2+) (Fig. 1 (6», scheelite (two intense redlines of Eu3+ in the background of the blue glowof WOi-) (Fig. 2 (1» and molybdoscheelite(yellow PL of MoOi-) (Fig. 2 (2».

The luminescence of Eu3+ in scheelite mustbe especially mentioned. Narrow red lines at612 and 616 nm were first observed in the PLspectra of scheelite from quartz veins(Gorobets and Nauczyciel, 1975; Gorobets andKudina, 1976). They were particularly intensein the PL and XL spectra of scheelite fromgold-bearing quartz veins (Fig. 1 (12»(Gorobets, 1986; Uspenski et al., 1989). Eventhe visually reddish hue of luminescence ofeuropium impurity in scheelite may beobserved. Geochemical studies show that Eu3+

in scheelite may serve as an ore guide (e.g.,Uspenski et al., 1998).

Sometimes quartz, especially its latervarieties, shows whitish-blue PL due to organicimpurities. Hypergene minerals such aschlorbromargyrite, uranyl minerals, and opalwith uranyl impurities display green PL(sometimes, only at low temperature); cerussiteshows bright yellow PL related to unknownimpurities. All these minerals are not easilydetected without the luminescence method, andcontribute much to LH detection.

Haloes of mountain crystal veins (Dodo,Puyva, Northern Urals and Kyshtym, CentralUrals, Russia) (Fig. 4 (3». These LH weretraced up to 100 meters along vein directions atthicknesses of up to 40 meters. The main PLminerals are calcite (red glow of Mn2+) , apatite(pinkish-yellow PL of Mn 2+, Dy3+, Sm 3+ orbluish-violet PL of Eu2+; the latter spectrum isgiven in Fig. 1 (4», adularia, microcline andalbite (dull-red PL of Fe3+) . The country rocks

are completely free of such PL minerals,whereas their average contents in the contactzone are up to 10-40 grains per 1 cm-. Quartz,zeolite and kaolinite show bluish-white PL dueto organic cyclic impurities. The association ofPL minerals in the contact zone depends on thecountry rocks: calcite prevails in amphibolites,and apatite and fluorite occur in gneisses. Thezonal structure of LH was observed: the intenseglow of Mn2+ in calcite and apatite of the innerzone of LH indicates heating and the relativelymore oxidizing and acidic character ofhydrothermal solutions forming the veins.Alteration processes in the country rocks weremore reducing and less acidic.

CONCLUSIONS

PL centers in minerals are formed oftransition elements of 3d (metals of the irongroup), 4f (REE), Sf (uranium), and 4d and 6dgroups (molybdenum and tungsten). Ingeology, they are considered as ore elements.The distribution of luminescent elements in thelithosphere is frequently controlled by oreforming fluid-hydrothermal processes, mostlyby their acid-base properties and redoxpotential. This is why metasomatichydrothermal bodies are usually accompaniedby luminescent geochemical anomalies (LGA)consisting of PL calcite, apatite, fluorite,plagioclase, zircon and other vein and oreminerals. Classification of such anomalies hasbeen carried out according to the PL mineralassociations OCCUlTing in various altered rocks.LGA may sometimes coincide with ore objects,although they frequently have no ore depositsof commercial value.

Haloes of luminescent mineral outcrops havebeen discovered around ore bodies stretching upto tens and hundreds of meters in hydrothermaland metasomatic deposits of gold, diamond,ruby, emerald, mountain crystal and mica. Thehaloes are caused by the accumulation oftransition luminescent elements (Mn, TR, U,etc.) in calcite, apatite, zircon and otherminerals crystallizing in the hydrothermal


process of host rock alteration. They may beused as indicators of diamond, emerald, ruby,mountain crystal, silver, tungsten, molybdenum,uranium, and other ores.

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