Palaeomagnetic and petromagnetic study of ura nium-bear ing
polymetallic-flu o rite min er al iza tion in the Orlík-Kladsko crys tal line com plex (near Kletno, Lower Silesia, Po land)
Katarzyna SOBIEŃ and Jerzy NAWROCKI
Sobień K. and Nawrocki J. (2010) – Palaeomagnetic and petromagnetic study of ura nium-bear ing polymetallic-flu o rite min er al iza tion in the Orlík-Kladsko crys tal line com plex (near Kletno, Lower Silesia, Po land). Geol. Quart., 54 (3): 325–336. Warszawa.
Palaeomagnetic mea sure ments of polymetallic-ura nium ore in the Old Ura nium Kletno Mine were car ried out. Ther mal and al ter nat ing field (AF) demagnetizations of the rocks stud ied (flu o rite and quartz veins, cataclased gneiss es, cal car e ous-sil i cate rocks with epidote/grossular) en abled iso la tion of two well-de fined mag ne ti za tion com po nents. A nor mal po lar ity palaeomagnetic di rec tion was pre served in mag ne tite and coarse he ma tite, whereas re versed po lar ity is linked with fine he ma tite grains. Both sta tis ti cally well-de fined com po nents do not dif fer within lim its of er ror. The cal cu lated mean palaeomagnetic pole was com pared with the Eu ro pean ap par ent po - lar wan der path. This com par i son points un am big u ously, within lim its of sta tis ti cal er ror, for an Early Cre ta ceous to Paleogene age of char ac ter is tic com po nents of mag ne ti za tion. Con se quently this age limit con strains the time of ura nium-bear ing polymetallic-flu o rite min er al isa tion.
Katarzyna Sobień and Jerzy Nawrocki, Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute, Rakowiecka 4, PL-00-975 Warszawa, Po land, e-mails: Katarzyna.Sobien@pgi.gov.pl, Jerzy.Nawrocki@pgi.gov.pl (re ceived: April 27, 2009; ac cepted: June 18, 2010).
Key words: Po land, palaeomagnetism, Ura nium, Sudetes.
INTRODUCTION
It has been dem on strated world wide that palaeomagnetism can con trib ute greatly to es ti ma tion of the tim ing of min er al iza - tion, mainly for Mis sis sippi-Val ley Type Zn-Pb ores (e.g., Symons et al., 1998; Symons and Arne, 2005; Boni et al., 2005; Pannalal et al., 2008) as well as for bar ite (Symons and Sangster, 1991) or sul phide ores (Gose and Kyle, 1993). In Po - land, a few ore ac cu mu la tions have been ex am ined: Symons et al. (1995) es ti mated the time for a Zn-Pb pre cip i ta tion event, while the age of the Lower Silesian cop per min er al iza tion, the so-called “Kupferschiefer”, was ana lysed by Jowett et al.
(1987) and re cal cu lated by Nawrocki (2000).
The palaeomagnetic method of age es ti ma tion of min eral de pos its is, in short, based on the com par i son of the palaeomagnetic pole po si tion ob tained with the ref er ence ap - par ent po lar wan der path (APWP) for the re spec tive con ti nent or terrane. Cor rect in ter pre ta tion of the re sults de pends on sev - eral im por tant fac tors (Trench et al., 1992) such as:
– re li abil ity of the APWP for the con ti nent;
– a dis tinct min er al iza tion event last ing long enough to av er age the palaeosecular vari a tion of the Earth’s mag - netic field;
– a rel a tively iso tro pic mag netic fab ric of the sam ples stud ied to avoid a bi ased ac qui si tion of the palaeomagnetic vec tor;
– lack of any tec tonic move ment af ter ac qui si tion of the palaeomagnetic re cord (or aware ness of it so that the orig i nal po si tion can be re con structed);
– lack of post-ac qui si tion al ter ation of mag netic phases lead ing to palaeomagnetic over print ing.
In the case when the last is ev i dent, the es ti mated age of the remagnetization could be still used as the min i mum age es ti ma - tion for the min er al iza tion stud ied.
Among hy dro ther mal parageneses, he ma tite, es pe cially fine he ma tite ag gre gates dis persed in var i ous hy dro ther mal min er als (e.g., flu o rite, cal cite, quartz), was found to be the most re li able ma te rial for the age de ter mi na tion of ore de pos - its (Hanuš and Krs, 1963; Krs, 1964; Ev ans et al., 2001).
Since he ma tite oc cur rence is in sep a ra bly con nected with ura - nium ores in the Sudety Mts. (Bareja et al., 1982) and since this type of min er al iza tion has been suc cess fully dated ei ther
in Aus tra lia (Indurm and Hein rich, 1993) or the Czech Re - pub lic (Krs and Stovichova, 1966), it led us to un der take pre - lim i nary palaeomagnetic stud ies of the Kletno polymetallic- ura nium de pos its.
GELOGICAL AND MINERALOGICAL SETTING
The re search area is sit u ated in the Orlík-Kladsko crys tal - line com plex, in the Pol ish part of the Sudety Moun tains, which de fines the NE mar gin of the Bo he mian Mas sif (Fig. 1A), and
be longs to the Eu ro pean Variscan orogen com posed of sev eral microterranes amal gam ated and fi nally in te grated dur ing the Car bon if er ous (Mazur et al., 2006). The mo saic tec tonic struc - ture is ad di tion ally com pli cated by late Me so zoic and Neo gene block move ments. Nu mer ous ore min er al iza tion events took place dur ing the com plex geo log i cal his tory of the Bo he mian Mas sif (Ber nard, 1991). The Kletno ore de pos its are lo cated very near the bor der zone be tween the East and West Sudetes (Don et al., 2003), along the NNW–SSE ex tended Kletno Fault, a part of the Staré Město–Kletno–Marcinków–Waliszów Zone (Fig. 1B), where gneiss is folded over crys tal line schists and skarns and dips at 40–70° to the ENE. The main ore body
Fig. 1A – Lo ca tion map of the Kletno mine within a tec tonic sketch map of Po land (af ter Winchester and the PACE TMR Net work Team 2002, mod i fied); B – geo log i cal map of the Kletno fold area (af ter Don, 2001, mod i fied)
TESZ – Trans-Eu ro pean Su ture Zone, CDF – Cal edo nian De for ma tion Front; Stronie var ied for ma tion: 21 – light quartzites, 22 – mica schists, 23 – graph ite schists, 24 – lime stones, 25 – erlanes; SM-K-M-W FZ – Staré Město–Kletno–Marcinków–Waliszów Fault Zone
(thick ness up to 5 metres) oc curs above the mar ble footwall un - der the hangingwall of the Śnieżnik gneiss es. The de tailed de - scrip tion of ore de posit struc ture and its gen e sis is given in Banaś (e.g., 1965, 1991). He dis tin guished three min eral as so - ci a tions: mag ne tite, polymetallic and quartz-flu o rite-sul phide.
Ac cord ing to Banaś (op. cit.) the mag ne tite vein min er al iza tion is the old est one, con nected with re gional meta mor phism of Fe-de pos its to gether with con tact meta mor phism of the Śnieżnik gneiss in tru sion. On the other hand, the polymetallic and flu o rite stages are thought to be a re sult of sev eral low-tem - per a ture hy dro ther mal phases re lated to a deep-seated, post-mag matic or meta mor phic source. Polymetallic-flu o rite min er al iza tion is spa tially and ge net i cally con nected with the ox ide ura nium min er al iza tion. Ore bod ies oc cur as nests and small veins with pitch blende, sulphides, flu o rite and quartz.
The main ura nium ore min eral is pitch blende, form ing with flu - o rite the larg est ura nium ac cu mu la tions. Down the dip the flu o - rite amount drops with in creas ing quartz and mag ne tite con - tent. There are sev eral he ma tite gen er a tions within the ore zone. Dis tinct parts of the mag ne tite ore were af fected by martitization of vari able in ten sity, up to com plete pseudomorphoses of Fe2O3 af ter Fe3O4. As the pro cess signs can be traced to a depth of ca. 50 m and some hy drated Fe ox - ides are as so ci ated with other sec ond ary U, Cu, Pb min er als, it is thought to be mainly a supergene phe nom e non re lated to tec - tonic up lift (Banaś, op. cit.). On the other hand, only a mi nor part of the min eral sub stance trans fer has been as cribed to sur - face wa ter cir cu la tion. The ac tiv ity of hy dro ther mal so lu tions was prob a bly of small im por tance in the mag ne tite ox i da tion pro cess but could have re sulted in martite for ma tion dur ing pitch blende pre cip i ta tion. This view is in agree ment with Bareja et al. (1982), who linked ura nium-bear ing zones with sur round ing rock hematitization. Authomorphic crys tals of specularite sur round pitch blende in the form of nee dles in quartz, cal cite, flu o rite and also ra dial and rose ag gre gates as well as pow dered he ma tite con cen tra tions. Specularite is ge - net i cally con nected with the hy dro ther mal ox ide re gime (UO2, Fe2O3) pres ent just be fore the main polymetallic crys tal li za tion stage. Sig nif i cant dis persed cryptocrystalline he ma tite abun - dance has been no ticed in polymetallic-flu o rite as sem blages and also as a thin layer be tween quartz and flu o rite, there fore hematitization is thought to be ge net i cally linked with quartz-flu o rite-cal cite bear ing pro cesses. Przeniosło and Sylwestrzak (1971) de scribed large oc cur rences of a tiny he - ma tite pow der in a quartz rock lens and cal cu lated that dur ing quartz rock for ma tion sil ica was sup plied as well as some Fe2O3. Since he ma tite oc cur rence is strictly and in dis put ably con nected with ore min er al iza tion we as sume that age es ti ma - tions based on iron ox ides is di rectly re lated to the time of min - er al iza tion.
ORE PLACEMENT STATE OF KNOWLEDGE
The tim ing of the ore events in the Kletno re gion has pre vi - ously been es ti mated on the ba sis of gen eral geo log i cal, struc - tural and min er al og i cal ev i dence. Banaś (1965) pos tu lated an Archaeon?/Cal edo nian? time of mag ne tite place ment. On the
other hand, this au thor pro posed a late Variscan age for ura - nium-bear ing polymetallic-flu o rite min er al iza tion, but also al - lowed a late Al pine age. Don et al. (2003) sug gested a late Ter - tiary age for the ura nium-flu o rite min er al iza tion in Kletno. He ob served that one of the larg est veins cuts the Staré Město–Kletno–Marcinków–Waliszów Falut Zone al most per - pen dic u larly and shows no in di ca tions of tec tonic ac tiv ity or dis place ment on this fault.
Since the ura nium-bear ing polymetallic ore and also the flu o rite as so ci a tion were dis tin guished in the Kletno mine, var i - ous Bo he mian Mas sif min eral age es ti ma tions should be taken into ac count in our con sid er ations. Tens of min er al iza tion events were in ter act ing in the same ore/place in the Bo he mian Mas sif area (Ber nard, 1991), there fore a huge age range is pres - ent in the lit er a ture. Flu o rite min er al iza tion in the Sudetes fol - lows NW-ex tend ing zones of steep dip. Flu o rite-bar ite as so ci a - tions’ stud ies made in the Bo he mian Mas sif area (Ber nard et al., 1976) link (e.g., on the ba sis of pub lished fluid in clu sion anal y ses) to the Tri as sic? re newal of tec tonic ac tiv ity along the sys tem of NW–SE fis sures with low-tem per a ture hy dro ther mal fluid as cents, bear ing el e ments, which un der suit able con di - tions formed flu o rite-bar ite-quartz fis sure-fill ings. Sudetic polymetallic (sul phide-ura nium) min er al iza tion to gether with flu o rite and bar ite min er al iza tion are thought to be long to one Perm ian-Tri as sic min er al iza tion cy cle, due to deeper crustal struc ture re newal (Jerzmański, 1976). In ad di tion, Muszer (2004) sug gested that the pro cess which ini ti ated metal mi gra - tion was not re lated to re gional meta mor phic flu ids and that old Variscan fault-dis lo ca tions and mi gra tion paths were used by youn ger metal-bear ing flu ids. Mikulski (2007) con nected ox i - da tive low tem per a ture hy dro ther mal pro cesses in the West ern Sudetes with post-Variscan ba sin for ma tions in up per Permian–Triassic or even Cre ta ceous time. Sim i lar con clu sions were ex pressed by Slobodník et al. (2008), who on the ba sis on Pb iso tope data, in di cated (Perm ian) Tri as sic–Ju ras sic and also Ju ras sic-Cre ta ceous min er al iz ing events in Moravia, while an Early Cre ta ceous age for the Mis sis sippi-Val ley Type (MVT) de pos its in the Up per Silesia Ba sin is sug gested in a model of Heijlen et al. (2003).
In the Kowary min er al iza tion area (Mochnacka, 1966, 1967, 1982, 1991), where the mag ne tite and polymetallic-ura - nium ore struc ture and gen e sis is sim i lar to that at Kletno, Pb/Pb and Pb/U pitch blende dat ing (Lis et al., 1971) sug gest 265 and 70 Ma ab so lute age es ti ma tion. There fore Variscan Oro gen esis as well as the Lara mide Phase of Al pine Oro gen - esis was pro posed for pitch blende for ma tion and mod i fi ca tion.
Based on fluid in clu sion and sta ble iso tope data, the brine gen - er a tion of the Zálesí ura nium de posit (Czech Re pub lic – Dolniček et al., 2009), 56 km NE from Kletno, has been linked to the Perm ian-Tri as sic tectono-sed i men tary evo lu tion of the Pol ish Ba sin (see Schmidt Mumm and Wolfgramm, 2004), while 160–200 Ma was pro posed for the nearby Javorník min - er al iza tion by Legierski (1973). The Rožná ura nium de pos its, 150 km south from Kletno, are be lieved to be of post-Variscan age (late Stephanian/early Perm ian) on the base of K-Ar dat ing, and an Early to Mid Tri as sic hy dro ther mal event is thought to be re spon si ble for ura nium remobilization and quartz-car bon - ate-sul phide min er al iza tion (Kříbek et al., 2009). On the other hand, 110–160 Ma was pro posed as a Pb model age (Legierski,
op. cit.). How ever, the most west ward placed polymetallic-ura - nium ore in the Bo he mian Mas sif (Joachimstahl) has been palaeomagnetically dated by Krs and Stovichova (1966), in di - cat ing a Me so zoic-Ter tiary min er al iza tion age, while Legierski (op. cit.) sug gested 210–150 Ma. Among the Bo he mian Mas sif met al lo gen ic prov inces, sev eral ore-vein (quartz-he ma tite and he ma tite) have been ana lysed palaeomagnetically. The cal cu - lated poles cor re sponded to a late Paleozoic age (Cen tral Bo he - mian pluton con tact zone and Krusné hory). On the other hand, a Paleogene min er al iza tion age for flu o rite-bar ite and he ma tite has been sug gested in west ern Bo he mia (Hanuš and Krs, 1963;
Krs, 1964 and lit er a ture therein).
SAMPLING AND LABORATORY METHODS
Fif teen in de pend ently ori ented hand sam ples were col - lected from the Old Ura nium Mine in Kletno, in the aban doned part of the ura nium-bare, flu o rite tun nel no. 18, that has been ren dered ac ces si ble. The gal lery is lo cated in the high est, north - west ern part of the mine, 773 m above sea level. Among the strongly tectonized rocks, with mac ro scop i cally vis i ble he ma - tite pow der, we could dis tin guish: brecciated quartz veins with flu o rite and cal cite (K3–K5, K8, K14), as well as leucocratic schists (K1, K2, K12), cal car e ous-sil i cate rocks with epidote (K15), grossular and mag ne tite ag gre gates (K7, K13), partly cataclased gneiss es (K6, K10) and sil i cate mar bles (K9, K11).
From each hand spec i men sev eral cu bic spec i mens (vol - ume 8 cc) were ob tained, giv ing a to tal num ber of 70 spec i - mens. All palaeomagnetic and a part of the rock mag netic anal - y ses were car ried out in the palaeomagnetic lab o ra tory of the Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute in War saw, in a mag net i cally shielded space, re duc ing the am bi - ent geo mag netic field by about 95% (man u fac tured by Mag - netic Mea sure ments, UK). In or der to char ac ter ize the mag - netic min er al ogy, iso ther mal remanent mag ne ti za tion (IRM) ex per i ments were car ried out. IRM ac qui si tion curves were ob - tained us ing a MMP1 pulse mag ne tizer (Mag netic Mea sure - ments, UK). IRM was im parted along three per pen dic u lar axes in dif fer ent DC fields (1.5 T, 0.4 T, and 0.1 T along z, y, and x axes, re spec tively) and was ther mally de mag ne tized (in 14 steps, ev ery 50°C up to 700°C) fol low ing the method of Lowrie (1990). Ad di tional stud ies of mag netic prop er ties, such as tem per a ture (20–700°C) de pend ence of the iso ther mal remanence (TUS de vice), and hys ter esis loops (MicroMag 2900 Al ter nat ing Gra di ent Mag ne tom e ter, Prince ton Mea sure - ments Corp.) were per formed by T. Werner at the In sti tute of Geo phys ics of the Pol ish Acad emy of Sci ences.
In ad di tion to the rock mag netic ex per i ments, sev eral pol - ished sec tions were pre pared for re flected light mi cro scope ob - ser va tions.
Dur ing the palaeomagnetic anal y ses, spec i mens were sub - jected to both ther mal (us ing the non-mag netic oven MMTD, Mag netic Mea sure ments, UK) and al ter nat ing field (up to 100 mT; AF, Molspin Ltd., UK) de mag ne ti za tion ex per i ments.
Af ter each de mag ne ti za tion step, an in ten sity and a di rec tion of remanent mag ne ti za tion were mea sured us ing an Agico JR-6A
spin ner mag ne tom e ter (noise level 10–5A/m). Char ac ter is tic com po nents of a remanent mag ne ti za tion were cal cu lated by prin ci pal com po nent anal y sis (Kirschvink, 1980) us ing the PALMAG pack age of Lewandowski et al. (1997) and, in a few sam ples, us ing a remagnetization cir cle method (McFadden and McElhinny, 1988). Dur ing ther mal de mag ne ti za tion ex per - i ments, mag netic sus cep ti bil ity was mea sured with an Agico KLY-2 Kappabridge to mon i tor any min eral al ter ation due to in creased tem per a ture.
RESULTS
MAGNETIC MINERALOGY
Re flected light mi cro scope anal y ses (Fig. 2) re vealed ir - reg u lar as sem blages of crys tal line he ma tite (specularite – Fig.
2C) or ir reg u lar patches and small veins of crys tal line he ma - tite in ter spersed with cryptocrystalline as sem blages (Fig. 2A) as well as Ti-mag ne tite grains (only in sam ples K7, K13) re - placed by he ma tite (martite) along frac tures and grain bound - aries (Fig. 2B).
Rock mag netic anal y ses dis tin guished two spe cific mag - netic min eral as sem blages. The first group re vealed rel a tively low nat u ral remanent mag ne ti za tion (NRM) in ten si ties, of be - tween 2.5 and 27 mA/m, and a mag netic sus cep ti bil ity range from 40 to 90 ´ 10–6 SI. A grad ual rise of the iso ther mal remanent mag ne ti za tion (IRM) ac qui si tion curve with no sat u - ra tion reached at 1.5 T in di cates the dom i nance of a high coercivity min eral. This trend is typ i cal for 85% of the sam ples (ex clud ing cal car e ous-sil i cate rocks with grossular). Ther mal de mag ne ti za tion curves of three-axis IRM show a high coercivity com po nent (blue tri an gles on Fig. 3A, C) with un - block ing tem per a tures from 650 to 700°C, which are char ac ter - is tic of he ma tite. Thermomagnetic curves for iso ther mal remanent mag ne ti za tion (Fig. 4) and hys ter esis loops (Fig. 5) show some dif fer ences among dif fer ent lithologies. Cataclased gneiss (K6) con tains abun dant he ma tite but a hys ter esis loop of typ i cal “wasp-waisted” shape is ob served. This is due to a mix - ture of SD and SP grains (Tauxe et al., 1996) or more likely to a mix ture of two mag netic phases of a high and a low coercivity (Dunlop and Özdemir, 1997). The hys ter esis curve as well as ther mal treat ment of SIRM from a het er o ge neous quartz vein with flu o rite and cal cite (K14) re veal the pres ence of goethite?, mag ne tite and he ma tite. An epidote-bear ing cal car e ous-sil i cate sam ple (K15) is strongly para mag netic, but con tains typ i cal he - ma tite with wider tem per a ture spec tra af ter heat ing. These anal y ses are in agree ment with re flected light thin sec tion ob - ser va tions (Fig. 2A, C), that con firm the pres ence of crys tal line as well as of cryptocrystalline he ma tite as sem blages.
The sec ond set of spec i mens, com posed of cal car e ous-sil i - cate rocks with grossular (K7, K13), is char ac ter ized by higher NRM in ten si ties from 40 to 500 mA/m and by very high mag - netic sus cep ti bil ity val ues from 1320 to 163 900 × 10–6 SI. IRM ac qui si tion ex per i ments are char ac ter ized by a rapid rise of the IRM curve in ap plied fields up to 0.2 T, which is in dic a tive of a low coercivity min eral, most prob a bly mag ne tite (Fig. 3B). The in ten sity of IRM grad u ally in creased even above ca. 0.2 T
Fig. 3. Rock mag netic prop er ties: (I) – stepwise ac qui si tion of the iso ther mal remanent mag ne ti za tion – IRM (II), ther mal de mag ne ti za tion of the three axes IRM, ac quired in the fields of 1.5 T, 0.4 T and 0.1 T, (III) – mag netic sus cep ti bil ity (k) changes
dur ing the ther mal treat ment
A – cataclased gneiss, B – cal car e ous sil i cate rock with grossular, C – cal car e ous sil i cate rock with epidote Fig. 2. Mi cro pho to graphs: A – ir reg u lar marks and small veins of crys tal line he ma tite in ter - spersed with cryptocrystalline as sem blages; re - flected light, with out an a lyser, sam ple K6; B – mag ne tite with Ti (grey) re placed by he ma - tite-martite (pale blue) along frac tures and grain bound aries; re flected light, sam ple K7; C – ir reg - u lar as sem blages of crys tal line he ma tite – specularite (pale grey); re flected light, with out an a lyser, sam ple K15
Hem – he ma tite, Mag – mag ne tite
Fig. 4. Thermomagnetic curves for sat u ra tion iso ther mal remanent mag ne ti za tion (SIRM; ac quired in a 9 T field) Black curve – fresh sam ple, grey – af ter one step of heat ing in air; curves nor mal ized to its ini tial in ten sity and to the ini tial in ten sity of SIRM (at the room tem per a ture) in creased af ter heat ing to 700°C by 0.79 to 2.00 times; K6 – cataclased gneiss, K7 – cal car e ous-sil i -
cate rock with grossular, K14 – quartz vein with flu o rite and cal cite, K15 – cal car e ous-sil i cate rock with epidote
Fig. 5. Hys ter esis loops from MicroMag 2900 mag ne tom e ter af ter cor rec tion of para mag netic con tri bu tion K6 – cataclased gneiss, K7 – cal car e ous-sil i cate rock with grossular, K14 – quartz vein with flu o rite and cal cite,
K15 – cal car e ous-sil i cate rock with epidote
which in di cates the pres ence of a high coercivity min eral as well. The triaxial IRM de mag ne ti za tion pat tern dis closed a dom i nance of hard (0.4–1.5 T, blue tri an gles in Fig. 3B) and me dium (0.1–0.4 T, or ange squares) com po nents. Fi nal de - mag ne ti za tion at a tem per a ture of over 700°C im plies the pres - ence of he ma tite, but a rapid drop of in ten sity (me dium com po - nent) in tem per a tures be tween 550–600°C is typ i cal of mag ne - tite (pres ent also as a soft – black squares-com po nent).
Petromagnetic inhomogeneity in the grossular-bear ing cal car e - ous-sil i cate sam ple (K7 in Figs. 4 and 5) was cor rob o rated in ad di tional stud ies, where for a larger vol ume sam ple, the tem - per a ture de pend ence of SIRM points to he ma tite, while the hys ter esis loop is typ i cal of a low coercivity min eral. Mi cro - scope ob ser va tions (Fig. 2B) re veal the pres ence of he ma tite along frac tures and mag ne tite grain bound aries. A rapid sus - cep ti bil ity de crease dur ing the ther mal treat ment at a tem per a - ture of about 400°C might be in dic a tive of maghemite in some of the sam ples, whereas a sus cep ti bil ity in crease at tem per a - tures above 500°C doc u ments an ox i da tion pro cess in para - mag netic min er als.
RESULTS OF DEMAGNETIZATION
From the to tal num ber of 51 spec i mens (from 11 hand sam - ples) 65% were ther mally de mag ne tized and the rest were se - lected for the AF pro ce dure. Fifty five per cent of sam ples pre - served de mag ne ti za tion in fields of up to 100 mT, there fore they were sub jected to ther mal treat ment as well.
Con sid er ing the de mag ne ti za tion be hav ior, sam ples can be di vided into four groups (Fig. 6). To the first group be long two hand spec i mens (com pris ing 8 spec i mens) from cataclased gneiss (K6) and a brecciated quartz vein with flu o rite and cal - cite (K8). They re veal a mod er ately steep neg a tive in cli na tion com po nent, lo cated be tween the sec ond and third quar ter of the hemi sphere. An un block ing tem per a ture of about 700°C (Fig. 6A) as well as AF de mag ne ti za tion in abil ity in a field of 100 mT, points to tiny he ma tite grains (which agrees with the ob ser va tions of Kletetschka et al., 2000; Ev ans et al., 2001) as the main remanence car ri ers (see Figs. 3A and 5). A mag netic sus cep ti bil ity drop (Fig. 3A) in a tem per a ture of about 350°C in di cates maghemite, that is ox i dized to less sus cep ti ble he ma -
Fig. 6. Rep re sen ta tive ex am ples of or thogo nal pro jec tions (Zijderveld di a grams) of de mag ne ti za tion paths typ i cal for A – cataclased gneiss and quartz vein; B, C – cal car e ous-sil i cate rock with grossular, D – brecciated quartz vein
and epidote-bear ing cal car e ous-sil i cate rock In ten si ties of NRM: ´ 10–4 A/m
tite. How ever, there is no cor re spond ing mag ne ti za tion com po nent iso lated at this tem per a ture.
Twelve spec i mens of a grossular-bear ing cal car e - ous-sil i cate rock with mag ne tite (K7, K13) be long to the sec ond and third type of de mag ne ti za tion pat tern.
Fig ure 6B shows a dis tinct pos i tive in cli na tion com po - nent di rected to the NNW. A grad ual drop in remanent mag ne ti za tion in ten sity up to a tem per a ture of 580°C is con sis tent with mag ne tite as a prin ci pal mag netic car - rier (see also Figs. 3B and 5). The ef fi cient al ter nat ing field method (Fig. 6C) con firms the dom i nance of a low coercivity min eral as a car rier of the pos i tive in cli - na tion com po nent. How ever, some of the sam ples (55%) from this group were not fully de mag ne tized in the field of 100 mT and re vealed also a sec ond, neg a - tive in cli na tion com po nent di rected to the S. He ma tite is sus pected to be the mag ne ti za tion car rier in this case (com pare Figs. 2B, 3B and 4).
The last set of spec i mens con tains a brecciated quartz vein (K14) and an epidote-bear ing cal car e - ous-sil i cate rock (K15), that showed a nor mal po lar ity com po nent (Fig. 6D), grad u ally de mag ne tized up to the block ing tem per a tures of just above 600°C. It is pre sum ably a coarse grained (Ev ans et al., 2001) he - ma tite com po nent, since there is no proof of maghemite oc cur rence (see Figs. 2C and 3C–5). With its shal low in cli na tion it is subparallel to the nor mal po - lar ity mag ne tite com po nents (Fig. 6B). Leucocratic schists (K1, K2, K12) as well as sil i cate mar bles (K9, K11) did not re veal any in ter pret able com po nent.
Putt ing to gether all the sam ples ana lysed, we can dis tin - guish two dis tinct com po nents of char ac ter is tic remanent mag ne ti za tion (Fig. 7 and Ta ble 1). The first com po nent (de - noted as A) is a high tem per a ture, nor mal po lar ity com po nent of mod er ately steep in cli na tion (from 50 to 63°), di rected NNW, which is pre sum ably car ried by mag ne tite as well as he ma tite. The sec ond well-de fined, high tem per a ture and high coercivity com po nent (de noted as B) with mod er ately neg a tive in cli na tion (from –44 to –71) to wards the S was iso - lated in he ma tite sam ples. The pres ence of both nor mal and re versed po lar ity com po nents points to a sta ble mag ne ti za tion dif fer ing from the pres ent-day field. A sim ple re ver sal test
con firms that both nor mal and re versed po lar ity di rec tions are an tip o dal within the lim its of er ror (“C” cat e gory af ter McFadden and McElhinny, 1990).
DISCUSSION
Dual po lar ity com po nents in di cate an ex tended min er al iza - tion pro cess. For the he ma tite grains, very re sis tant to remagnetization, the dual po lar ity palaeomagnetic com po nent is prob a bly co eval with he ma tite growth, but the same nor mal po lar ity di rec tion pre served in mag ne tite might have been re - corded af ter its crystallization. Mag ne tite grains are very sus -
Fig. 7A – Ste reo graphic pro jec tion of char ac ter is tic di rec tions iso lated from the rocks stud ied with mean di rec tions (crosses) and a95 plot ted;
B – over all mean di rec tion
Cir cles – he ma tite, squares – mag ne tite, black sym bols – lower hemi sphere pro jec tion (com po nent A), white sym bols – up per hemi sphere
pro jec tion (com po nent B)
T a b l e 1 Char ac ter is tic palaeomagnetic di rec tions iso lated in the Kletno mine (lat. = 50.26°N, long. = 16.86°E)
Com po nent N n D I a95 k Plat Plong dp dm H/M
Nor mal - A 4 17 10 54 10.2 82.3 73 167 10 14 M + H
Re versed - B 4 12 174 –55 11.9 60.3 75 216 12 17 H
Over all mean (A + B) 8 29 2 55 7.1 62.1 75 189 7 10 M + H
N – num ber of hand sam ples used to cal cu late mean di rec tions; n – num ber of spec i mens; D – dec li na tion; I – in cli na tion; a95, k – Fisher’s sta tis tics pa ram e ters; Plat – palaeopole lat i tude; Plong – palaeopole lon gi tude; dp – , dm – semi-axes of con fi - dence oval: dp – in site-to-pole di rec tion, dm – in per pen dic u lar di rec tion; H – he ma tite, M – mag ne tite
cep ti ble to remagnetization and eas ily could have re set their pri mary sig nal due to a strong event con nected with tem per a - ture and stress field changes. In this par tic u lar case it might be a hy dro ther mal fluid flow event as well as mag ne tite ore crum - bling and tec tonic move ment pro cesses, which led to mag ne tite remagnetization. As sum ing that the main ura nium pre cip i ta tion event is co eval with he ma tite min er al iza tion (Banaś, 1965), our re sults could be di rectly rel e vant to age es ti ma tion of the ura - nium ore.
For this pur pose the palaeopole was cal cu lated from the mean ori en ta tion com bin ing nor mal and re versed po lar ity com po nents. The palaeopole po si tion ob tained (Plat. 75°N, Plong. 189°E) was com pared with the ap par ent po lar wan der path cal cu lated for Eu rope for the last 200 Ma (Besse and Courtillout, 2003) in or der to de ter mine an ox ide min er al iza - tion age (Fig. 8). The pro cesses might have taken place in the Early Cre ta ceous, con sid er ing only the mean ori en ta tion of the palaeopole. How ever, the sta tis tic er ror limit for this es ti - ma tion is quite sig nif i cant. The com par i son of the mean val - ues of the dec li na tion/in cli na tion data (see Ta ble 1) with the ex pected sta ble Eu ro pean dec li na tions/in cli na tions cal cu lated for the geo graph ical co or di nates of the Kletno mine (50.26°N, 16.86°E) leads to the con clu sion that even Paleogene age is pos si ble within a 95% limit of con fi dence.
The ox ide min er al iza tion could have taken place no ear lier than 148 Ma and not later than 36.5 Ma (Fig. 9). The mean in -
Fig. 8. Palaeomagnetic pole ob tained from the rocks stud ied (with 95% con fi dence oval) plot ted on the top of the ap par ent po lar
wan der path of Eu rope (af ter Besse and Courtillot, 2003)
Fig. 9. Ex pected palaeodeclinations/palaeoinclinations cal cu lated for the geo graphic co or di nates of the Kletno mine (data af ter Besse and Courtillot, 2003) and dec li na tion/in cli na tion
of the char ac ter is tic com po nent iso lated from the Kletno mine rocks The shaded area in di cates er ror of age es ti ma tion re lated to the val ues of a95
cli na tion value of 55° (Ta ble 1) may also be a sign of a Late Tri as sic min er al iza tion age. On the other hand, that pre sump - tion would re quire a large coun ter clock wise ro ta tion of about 40 de grees. The es tab lished re gional tec ton ics (Don et al., 2003), as well as the avail able palaeomagnetic data from the Sudetes (Nawrocki, 1998) al low us to ex clude such sig nif i - cant Al pine tec tonic ro ta tions in this re gion.
The re sult of our work dis pels the doubts con cern ing the pre vi ously sug gested Variscan age (Banaś, 1965) of the ura - nium-flu o rite veins, though ad di tional sam pling and more pre - cise ex am i na tion is planned to im prove the sta tis tics. How ever, the palaeomagnetic data did not let us con struct a min er al iza - tion model.
The ura nium-bear ing polymetallic-flu o rite ore is be lieved to be of hy dro ther mal or i gin (Banaś, 1965). In the Kletno re - gion, fluid in clu sion mea sure ments (Zieliński, 1997) point to low-tem per a ture so lu tions (120–240°C), while the low sa lin - ity sug gests “post-ore” quartz and flu o rite crys tal li za tion.
Barbier (1974) sug gested that con ti nen tal weath er ing of granitoid rocks con tain ing ura nium might be a pos si ble cause of the French vein ura nium de pos its. In ox i diz ing con di tions (due to a lack of or ganic mat ter) ura nium-rich con cen tra tions might have been trans ported down wards along frac tures by supergene flu ids. Sec ond ary re place ment of “dis persed ura - nium” from granitoids and its pre cip i ta tion in fa vour able con - di tions was pro posed by Bareja et al. (1982) as a pos si ble source of some ura nium ore veins in the Sudetes. This sce - nario looks re al is tic, all the more as Au gust and Wojewoda (2004) sug gested that the Late Ju ras sic/Early Cre ta ceous should be con sid ered as the most prob a ble pe riod of soil cover for ma tion in the Sudetes. Deep con ti nen tal weath er ing pro - cesses in Cen tral and North ern Eu rope have been well doc u - mented by Migoń and Lidmar-Bergström (2001, 2002) since the Me so zoic up to the Ce no zoic. The im pact of in tense weath er ing in the Kletno supergene ore zone is well doc u - mented (Banaś, 1965). In the time of a block up lift re gime and
sub se quent sta bi lized land con di tions, hy dro ther mal, metal-rich as well as supergene flu ids could pos si bly have mi - grated us ing the same paths and thus gen er ated spe cific min - eral ac cu mu la tions. Flu ids of me te oric or i gin, de scend ing dur ing the Cre ta ceous ba sin in ver sion, were sug gested as a pos si ble source of the youn gest hy dro ther mal vein sys tem in the North East ern Ger man Ba sin (Schmidt Mumm and Wolfgramm, 2004).
CONCLUSIONS
– Strongly tectonized rocks (quartz and flu o rite veins, leucocratic schists, cal car e ous-sil i cate rocks with epidote or grossular, cataclased gneiss es and sil i cate mar bles) from the Old Kletno Ura nium Mine re vealed two com po nents of mag - ne ti za tion. The first one of nor mal po lar ity re corded in mag ne - tite and coarse he ma tite and the sec ond one with re versed po - lar ity di rec tion car ried by fine grained he ma tite; these di rec - tions do not dif fer from each other within the er ror lim its.
– Com par i son of the palaeomagnetic data ob tained with the ap par ent po lar wan der path for the Eu ro pean craton in di cates an Early Cre ta ceous up to Paleogene age limit for the ura nium min er al iza tion.
– In or der to con strain the age of ura nium min er al iza tion more pre cisely, ad di tional palaeomagnetic sam pling and im - prove ment of the sta tis tics of our data would be help ful.
Ac knowl edge ments. The au thors would like to thank L. Krzemiński for pol ished sec tion anal y ses, T. Werner for thermomagnetic curves and hys ter esis pa ram e ter mea sure - ments and S. Mikulski for use ful dis cus sion. The jour nal re - view ers: M. Kądziałko-Hofmokl and M. Slobodník, whose com ments im proved the first ver sion of the manu script, are greatly ac knowl edged.
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