Or ganic and in or ganic geo chem i cal study of the Lower Perm ian Walchia shale of the Intrasudetic Ba sin (SW Po land)
El¿bieta BILKIEWICZ1, *, Jadwiga PIECZONKA1 and Adam PIESTRZYÑSKI1
1 Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro tec tion, AGH Uni ver sity of Sci ence and Tech nol ogy, al. A. Mickiewicza 30, 30-059 Kraków, Po land
Bilkiewicz, E., Pieczonka, J., Piestrzyñski, A., 2018. Or ganic and in or ganic geo chem i cal study of the Lower Perm ian Walchia shale of the Intrasudetic Ba sin (SW Po land). Geo log i cal Quar terly, 62 (3): 631–643, doi: 10.7306/gq.1427
As so ci ate ed i tor: Stanis³aw Wo³kowicz
The Lower Perm ian Walchia shale of the Intrasudetic Ba sin is lac us trine sed i ment en riched in or ganic mat ter and base met - als. Rock-Eval geo chem i cal anal y sis was con ducted on 110 drill-core rock sam ples of the Walchia shale in or der to char ac - ter ize the quan tity, ge netic type and ma tu rity of dis persed or ganic mat ter. Con cen tra tions of U, Th, Cu, Ni, V, Pb, Zn, Co, Mo, Cr, P, S and Fe were de ter mined and mi cro scopic stud ies to rec og nize min eral com po si tion were per formed on se lected sam ples. Or ganic mat ter of the Walchia shale is im ma ture and dom i nated by ter res trial type III kerogen with an ad mix ture of plank tonic type II kerogen. The Walchia shale is gen er ally poor source rock, but some parts of the pro file ex hibit good and even ex cel lent hy dro car bon po ten tial. The av er age con cen tra tions of an a lysed met als are higher than the av er age re ported for black shales. The con cen tra tions of base met als do not cor re late with TOC con tents, sug gest ing that their pres ence is as - so ci ated with in or ganic fea tures of the Walchia shale or they could have been re lated pri mar ily to or ganic mat ter which was sub se quently ox i dized and al tered. Ura nium may be partly as so ci ated with phos phates. Re sults of mi cro scopic in ves ti ga - tions re vealed the lack of de tect able ra dio ac tive min er als, abun dance of framboidal py rite, and prev a lent ga lena, sphalerite and chal co py rite.
Key words: Intrasudetic Ba sin, Walchia shale, or ganic mat ter, ura nium, base metal mineralisation.
INTRODUCTION
Some shales, es pe cially black shales, are known for their el e vated con cen tra tions of mi nor el e ments, which can ex ceed more than a hun dred and even thou sand times their av er age crustal abun dance (e.g., Krauskopf, 1955; Vine and Tourtelot, 1970). Met als oc cur ring in shales can be as so ci ated with sulphides, or ganic mat ter and/or clay min er als and are ac cu mu - lated syngenetically, dur ing diagenesis, or can be mo bi lized epigenetically and moved to new places (Leventhal, 1993).
Metal en rich ment in sed i men tary rocks re sults from me chan i cal en rich ment, ad sorp tion, chem i cal pre cip i ta tion and pro cesses in volv ing or ganic mat ter (Krauskopf, 1955).
The Walchia shale is a note wor thy rock unit com posed mainly of dark mudstones and shales, lo cally car bo na ceous, known for its ura nium and or ganic mat ter en rich ment (Wo³kowicz, 1990;
Yawanrajah et al., 1993). The lac us trine dark sed i ments of the Walchia shale were de pos ited in vari able sed i men tary en vi ron -
ments and un der chang ing con di tions dur ing the Early Perm ian within the Intrasudetic Ba sin (Wo³kowicz, 1988).
The main ob jec tive of this study was to de fine the or ganic and in or ganic char ac ter is tics of the Walchia shale by de ter min - ing the quan tity, ge netic type, ther mal ma tu rity and hy dro car - bon po ten tial of dis persed or ganic mat ter based on re sults of Rock-Eval geo chem i cal anal y sis, con cen tra tions of se lected el - e ments (U, Th, Cu, Ni, V, Pb, Zn, Co, Mo, Cr, P, S and Fe) and their ex pres sion in min eral com po si tion. More over, sev eral in - or ganic and or ganic re dox prox ies have been im ple mented to in ter pret the con di tions dur ing de po si tion of the Walchia shale.
Pre vi ous stud ies con cern ing Walchia shale geo chem is try (Wo³kowicz, 1990) in di cated a close re la tion ship be tween or - ganic mat ter con tent and ura nium con cen tra tions, sug gest ing their syngenetic oc cur rence. This pa per is an at tempt to re-eval u ate pre vi ous in ves ti ga tions on the or ganic mat ter role in the for ma tion of ura nium min er ali sa tion in the Walchia shale, as well as to search for other re la tion ships be tween or ganic-in or - ganic fea tures of this strata and me tal lic min er ali sa tion.
GEOLOGICAL SETTING
The study area is lo cated in the SE part of the Intrasudetic Ba sin (ISB) be tween the town of Radków and the vil lage of Wambierzyce, cov er ing ~2.3 km2 (Fig. 1A). The ISB is a
* Corresponding author, e-mail: ebil@agh.edu.pl
Received: January 23, 2018; accepted: April 16,2018, first published online: September 12, 2018
NW–SE-trending oval intramontane trough, 70 km long and 35 km wide, formed in the Early Mis sis sip pian on the pe riph ery of the Bo he mian Mas sif as a re sult of Variscan evo lu tion of the Sudetes. The synclinorial struc ture of ISB is sep a rated from sur round ing, up lifted, mostly crys tal line, tec tonic units by fault zones, which were ac tive dur ing its de vel op ment (Speczik et al., 1995). The ISB is filled with a few km thick Lower Mis sis sip - pian to Lower Perm ian vol cano-sed i men tary suc ces sion and over lain by Lower Tri as sic and Up per Cre ta ceous de pos its of con ti nen tal and ma rine or i gin, re spec tively (Awdankiewicz et al., 2003; Bossowski and Ihnatowicz, 2006). Ma te rial de pos ited within the ISB was fed from meta mor phic com plexes of the Kaczawa and Rudawy Janowickie ranges and the hy po thet i cal
“South ern Mas sif”, which were rap idly up lifted dur ing Variscan move ments and then in tensely eroded dur ing the Early Mis sis - sip pian subperiod (Teisseyre, 1975; Awdankiewicz et al., 2003).
Within the mo not o nous, thick, coarse-grained con ti nen tal Lower Perm ian (Rotliegend) sed i ments, con tain ing vol ca nic and pyroclastic rocks, three dis tinct lac us trine se quences con - sist ing of or ganic-rich mudstones and claystones with some lime stone in ter ca la tions oc cur as end-mem bers of sed i men tary cyclothems. Con glom er ates and sand stones de pos ited in oxic en vi ron ments pass pro gres sively into the cal car e ous, sandy claystones and mudstones of lac us trine or i gin, which are rec og - nized as the Lower and Up per Anthracosia and Walchia shales (Speczik et al., 1995; Fig. 1B). The Lower and Up per Anthracosia shales rep re sent the up per most Penn syl va - nian/Autunian and Autunian, re spec tively (Jerzykiewicz, 1987;
Górecka-Nowak, 2008; Górecka-Nowak and Nowak, 2008),
while the Walchia shale was de pos ited on top of the third cyclothem at the end of the Autunian (Jerzykiewicz, 1987;
Mastalerz and Wojewoda, 1988; Speczik et al., 1995; Fig. 1B).
The Anthracosia shales are dis tin guished by their or ganic mat - ter and Cu en rich ment (Speczik et al., 1995 and ref er ences therein), whereas the Walchia shale sed i ments have been stud - ied ow ing to their U en rich ment (Wo³kowicz, 1990, 1992). The is sues of ura nium min er ali sa tion in the Walchia shale were dis - cussed by Miecznik and Strzelecki (1979), Bareja et al. (1982), Miecznik (1989) and Wo³kowicz (1990, 1992). The Walchia shale oc curs in the Up per Autunian strata (Fig. 1B) and be longs to the S³upiec For ma tion, Ratno Dolne Mem ber (Wo³kowicz, 1990), which is equiv a lent to the Broumov For ma tion, Olivìtin Mem ber, in the Czech part of ISB (Tásler et al., 1979). It reaches a thick ness of 300–350 m dip ping ~15° SW to wards the ba sin cen tre (Radwañski, 1955; Wo³kowicz, 1992; Speczik et al., 1995). The Walchia shale is rich in or ganic mat ter, es pe - cially the plant re mains of Walchia piniformis from which their name is de rived. Wo³kowicz (1988, 1990, 1992) dis tin guished five com plexes vary ing in li thol ogy (Fig. 1B), rec og niz ing ho ri - zons en riched in ura nium within four of them. Their fur ther de - scrip tion is af ter that au thor. At the base of the Walchia shale suc ces sion there is a subtuffite com plex com posed of mudstones and sand stones con tain ing two car bon ate lay ers.
These rocks were de pos ited mostly in ox i dized pe lagic zone of the lake. The subtuffite com plex is cov ered by a 25–40 m thick ho ri zon of rhyolitic tuffs. The ox i dized brown-red sand stones of com plex I were de pos ited in the lit to ral zone and dem on strate bioturbation. Sed i ments of com plex I over lie rhyolitic tuffs through out the area, reach ing a thick ness of 35 m. Within this Fig. 1A – geo log i cal sketch-map of the study area (mod i fied af ter Augustyniak and Grocholski, 1968; Nemec et al., 1982; Speczik et al., 1995); B – the Walchia shale po si tion in the lithostratigraphic pro file of the Lower Perm ian of the Intrasudetic Ba sin (mod i fied af - ter Mastalerz and Wojewoda, 1988; Nowak, 2007), with the Walchia shale lithological di vi sion and pre vail ing depositional en vi ron - ment (af ter Wo³kowicz, 1992)
W-8 – Wambierzyce IG 8 bore hole; W-11 – Wambierzyce IG 11 bore hole; W-12 – Wambierzyce IG 12 bore hole; W-14 – Wambierzyce IG 14 bore hole; W-15 – Wambierzyce IG 15 bore hole; 1 – Lower Anthracosia shale, 2 – Up per Anthracosia shale, 3 – Walchia shale, 4 – volcanites and volcanoclastics; Penn. – Penn syl va nian; Step. – Stephanian (stage de fined re gion ally in Eu ro pean stra tig ra phy, cor re spond ing to the Gzhelian); lith. – lithological; sed. – sed i men tary
com plex, no ura nium ho ri zons have been re ported. Sed i ments of com plex II form a 40–65 m thick se ries of black mudstones with in ter ca la tions of bi tu mi nous lime stones, de pos ited in an anoxic pe lagic en vi ron ment. Within this com plex, asphaltite lenses have been found. Com plex III is com posed of vari able red sand stones to siltstones and grey-black claystones pass ing into mudstones which are usu ally cal car e ous. Rocks of this com plex con tain high amounts of or ganic mat ter (up to 10%).
The vari a tions in li thol ogy re sulted from rapid en vi ron men tal changes from a lit to ral through a coastal plain to a pe lagic zone, where grey to black claystone-mudstone sed i ments were de - pos ited. Com plex IV is the up per most se ries of the Walchia shale and con sists of black claystones and mudstones in ter ca - lated by sand stone-mudstone lay ers and car bon ate laminae oc cur ring at its basal part. This com plex reaches a thick ness of up to 120 m and the sed i ments were de pos ited in both oxic and anoxic zones of the ba sin.
ROCK SAMPLING AND PREPARATION PROCEDURE
Due to strong weath er ing, con firmed lat terly by Rock-Eval (RE) re sults on sam ples taken from out crops at the T³umaczów and Ratno Dolne lo cal i ties (Fig. 1A), the stud ies were lim ited to sam ples de rived from bore holes. A to tal of 110 drill-core rock sam ples of the Walchia shale have been col lected from the Wambierzyce IG 8, Wambierzyce IG 11, Wambierzyce IG 12, Wambierzyce IG 14 and Wambierzyce IG 15 bore holes, cov er - ing an area of ~2.3 km2 (Fig. 1A). The col lected rock sam ples rep re sent all lithological com plexes iden ti fied within the Walchia shale se quence: com plex IV (43 sam ples), com plex III (15 sam - ples), com plex II (31 sam ples), com plex I (2 sam ples) and subtuffite com plex (19 sam ples). The sub di vi sion of the Walchia shale into com plexes, pro posed by Wo³kowicz (1988), has been im ple mented for the pres ent study and the rock sam - ples have been di vided and iden ti fied ac cord ing to the above-men tioned lithological cri te ria.
The sam pling was re stricted to rel a tively dark and fine-grained parts of the cores. Upon ar rival to the lab o ra tory, rock sam ples were crushed to the frac tion <2 cm, then ho mog e - nized and a rep re sen ta tive por tion of ~100 g was pul ver ized to the frac tion <0.2 mm for RE pyrolytic anal y sis. For multi-el e - ment con cen tra tion anal y sis a rep re sen ta tive por tion of a crushed sam ple was pul ver ized to the frac tion <0.1 mm and a por tion of ~4 g was poured off and sent for anal y sis. Mi cros copy anal y sis was per formed on pol ished sec tions of gravel-size sam ples.
ANALYTICAL PROCEDURE
ROCK EVAL PYROLYSIS
The RE py rol y sis is a stan dard tech nique used in pe tro leum ex plo ra tion for char ac ter iz ing or ganic mat ter in sed i men tary rocks. The RE method al lows for pre lim i nary de ter mi na tion of hy dro car bon po ten tial and iden ti fi ca tion of the amount, type and ma tu rity of or ganic mat ter. The RE py rol y sis was com pleted with a Vinci Tech nol o gies Rock-Eval 6 Turbo ap pa ra tus on all col lected rock sam ples. The de tails of the anal y sis are de -
scribed in Lafargue et al. (1998). For pre lim i nary char ac ter iza - tion of or ganic mat ter of the Walchia shale the fol low ing pa ram - e ters and in di ces have been used: to tal or ganic car bon con tent (TOC, wt.%), S1 and S2 re ferred to as free and re sid ual hy dro - car bon con tents, re spec tively (mg HC/g rock), hy dro gen in dex – HI (mg HC/g rock), ox y gen in dex – OI (mg HC/g rock), and Tmax (°C) ob tained at max i mum of S2 peak which cor re sponds to the ther mal ma tu rity of or ganic mat ter.
MULTI-ELEMENT ANALYSIS
In to tal, 49 rock sam ples have been ana lysed for el e men tal con cen tra tions (Ap pen dix 1*). The sam ples were se lected by quan ti ta tive or ganic mat ter con tent (high TOC con tents) and sta tis ti cal cri te ria ac cord ing to li thol ogy. The con cen tra tions of U, Th, Cu, Ni, V, Pb, Zn, Co, Mo, Cr, P, S and Fe were de ter - mined by in duc tively cou pled plasma-mass spec trom e try (ICP-MS) tech nique. The anal y sis is per formed by di ges tion of pow dered sam ples to com plete dry ness with an acid so lu tion of (2:2:1:1) H2O-HF-HClO4-HNO3. Then, the res i due is dis solved in 50% HCl and heated us ing a mix ing hot block. Af ter cool ing, the so lu tions are trans ferred to test-tubes, brought to vol ume us ing di lute HCl, and ana lysed by ICP-MS. The de tec tion lim its are 0.1 ppm for U, Th, Cu, Pb, Ni and Mo, 0.2 ppm for Co, 1.0 ppm for Zn, V and Cr, 0.1% for S, 0.01% for Fe, and 0.001%
for P. For quan ti fi ca tion of the re sults, the fol low ing ref er ence ma te ri als were used: DS11and OREAS 45Ea in ter nal stan - dards for aqua regia di ges tion, SO-19 in ter nal stan dard for whole-rock anal y sis, and GS 910-4 and GS 311-1 cer ti fied stan dards for sul phur and car bon as says. The el e men tal anal y - sis was per formed by Bu reau Veri tas Min eral Lab o ra to ries in Van cou ver, Can ada.
MICROSCOPY STUDIES
Mi cros copy stud ies were made on Walchia shale sam ples se lected based on re sults of pre vi ous anal y ses. The stud ies were per formed on pol ished sec tions of rock sam ples with the high est TOC con tents and U and Th con cen tra tions (Ap pen - dix 1). In to tal, 15 rock sam ples were in ves ti gated in re flected light us ing a NIKON op ti cal mi cro scope. Ad di tion ally, two sam - ples with the high est U con cen tra tions were ana lysed us ing an EDS (en ergy-dispersive spec tros copy) sys tem com bined with an elec tron scan ning mi cro scope FEI Quanta-200 FEG (20 kV ac cel er a tion volt age).
RESULTS AND DISCUSSION
GEOCHEMICAL CHARACTERISTICS OF ORGANIC MATTER
Re sults of RE py rol y sis were used to de ter mine the quan - tity, ge netic type and ther mal ma tu rity of or ganic mat ter dis - persed in the Walchia shale. The min i mum, max i mum and av - er age val ues of RE TOC con tent (wt.%), S2 (mg HC/g rock), hy - dro gen in dex (HI, mg HC/g rock) and min eral car bon (MinC, wt.%) for each sam pled com plex are pre sented in Ta ble 1.
Rock sam ples with low TOC val ues, <0.2 wt.%, are gen er ally con sid ered as not pro vid ing re li able RE data, there fore the RE pa ram e ters and in di ces from only 56 sam ples have been used for in ter pre ta tion and were as sessed as valid (Figs. 2 and 3).
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1427
Higher con tents of MinC, up to 6.1 wt.% in com plex II (Ta ble 1), rep re sent cal car e ous interbeds and ce ments within the mudstone-claystone se quence of the Walchia shale, in di cat ing lo cal in flu ence of car bon ate sed i men tary con di tions.
The RE py rol y sis re vealed that the or ganic mat ter dis persed in the Walchia shale shows nei ther dis tinct ver ti cal zonation of or ganic geo chem i cal pa ram e ters nor sig nif i cant geo chem i cal dif fer ences be tween par tic u lar lithological com plexes (Ta ble 1, Ap pen dix 1 and Figs. 2–4A) with one ex cep tion of com plex I, which is coarse-grained and poor in or ganic mat ter and was sam pled only for com par i son pur poses. How ever, geo chem i cal char ac ter is tics of the Walchia shale or ganic mat ter are not uni - el baT1 lavE-kcoRla c imeh coegsci ts ire tc arahcdnasnoi tar tne cnocfostne m elenihtiwla udi vi dnicigo l ohtilsexel pmocfoehtaihclaWelahs S2– laudisermuelorteplaitnetop ;IH– negordyhxedni ;CniM– larenimnobrac ;n.ppa– .tonelbacilppa ;n.a– .tondesylana ;lavE-kcoRataddnasnoitartnecnoc fostnemeleeranevigsasrotaremunfomuminimdnamumixamseulav ,nirotanimoned :egarevaeulav
Fig. 2A – Rock-Eval free hydrocarbon S1 parameter; B – S2
residual petroleum potential versus total organic carbon content (TOC) of organic matter of the Walchia shale Petroleum source quality according to criteria of Peters and Cassa
(1994); genetic boundary after Hunt (1996); key for sample codes see Appendix 1
form, as be ing the re sult of vari able con di tions and depositional en vi ron ments within the lac us trine ba sin.
Ac cord ing to Pe ters and Cassa’s cri te ria (1994) for source qual ity of or ganic mat ter the re sults of RE py rol y sis in di cate that the Walchia shale is gen er ally a poor source rock as de scribed by a TOC me dian value of 0.22 wt.% and an av er age value of 0.61 wt.% for the whole ana lysed se quence. The TOC con tents change in a wide range from 0.02 wt.% in com plex II to 5.2 wt.%
in a cal car e ous shale sam ple rep re sent ing com plex III (Ta ble 1 and Ap pen dix 1). Re sid ual hy dro car bons con tents vary from 0.00 mg HC/g rock in com plex IV to 21 mg HC/g rock in com - plex II (Ta ble 1 and Fig. 2A). The high est S2 con tent of 21 mg/g rock and the high est HI value (re sult ing from high S2 con tent) of 638 mg/g rock have been mea sured in the claystone sam ple W-15/23 (Ap pen dix 1). The high est av er age S2 con tent of 5.8 mg HC/g rock is ob served in the sam ples rep re sent ing com - plex III (Ta ble 1). Lo cally high TOC val ues, >1 wt.%, ac com pa - nied by el e vated re sid ual hy dro car bons con tents, >2.5 mg HC/g rock, in di cate fair to even ex cel lent pe tro leum po ten tial (Fig. 2A). These sam ples rep re sent sed i ments de pos ited likely in pe lagic, anoxic parts of the ba sin. Rocks of com plexes III and II re spec tively show the best hy dro car bon po ten tial, as in di - cated by S2 and HI val ues (Ta ble 1, Figs. 2 and 3). Tak ing into ac count cri te ria pro posed by Hunt (1996; Fig. 2B) and Espitalié et al. (1977; 1985; PI > 0.5 for epigenetic HC) for iden ti fi ca tion of mi grat ing or con tam i nat ing hy dro car bons, all hy dro car bons ac - cu mu lated within the Walchia shale are only syngenetic, and there fore were gen er ated ex clu sively from or ganic mat ter of the Walchia shale. En riched in or ganic mat ter, sed i ments of the Walchia shale were de pos ited likely due to fa vour able anoxic depositional con di tions and low sed i men ta tion rates. Oc ca sion - ally, the sed i ments were de pos ited in high-en ergy en vi ron - ments which might be re lated to flood ep i sodes in the ba sin, re - sult ing in de po si tion of thicker, coarser and ox i dized lay ers found in com plex I. Rapid sed i men ta tion also re sulted in di lut ing or ganic mat ter.
Plot ting HI against OI and Tmax val ues (Fig. 3) on the di ag - nos tic di a grams (Espitalié et al., 1985) in di cates the pres ence
of at least two ge netic groups of or ganic mat ter. Ac cord ing to these di a grams (Fig. 3) most of the ana lysed sam ples con tain or ganic mat ter with dom i nant ter res trial type III kerogen be ing de rived from vas cu lar plants, whose rem nants (e.g., Walchia piniformis, Callipteris confereta) have been pre vi ously re ported in the sed i ments of the Walchia shale (Kurowski, 2004 and ref - er ences therein). How ever, 10 rock sam ples, rep re sent ing com plexes II, III and IV, show high HI val ues, ex ceed ing 200 mg HC/g rock, which ev i dences an ad mix ture of plank tonic lac us trine type II kerogen, but some con tri bu tion of al gal type I kerogen can not be ex cluded as well (Fig. 3). Petrographic and palynological stud ies of or ganic mat ter in the Walchia shale (Górecka-Nowak and Nowak, 2008) in di cated that fine-grained sed i ments rep re sent ing deeper parts of the ba sin are dom i - nated by sapropelic or ganic mat ter, whereas humic or ganic mat ter, com posed mainly of vitrinite, pre vails in coarser sed i - ments, most likely rep re sent ing shal lower zones of the lake.
These ob ser va tions agree with con clu sions re ported by Wo³kowicz (1990), ob tained from n-alkane dis tri bu tion show ing dom i nance of C16-C18 and C25 hy dro car bons which are de rived from al gae-bac te rial and vas cu lar plant or ganic mat ter, re spec - tively. The Tmax val ues within the range of 411–438°C (Ap pen - dix 1) and the PI value (not ex ceed ing 0.4) in di cate that or ganic mat ter is im ma ture. This is also in agree ment with CPI data pub lished by Wo³kowicz (1990) and Ro data re ported by Górecka-Nowak and Nowak (2008).
Hy dro car bon type in dex ex pressed as S2/S3 ra tio of most sam ples ex hib its val ues be low 2 in di cat ing pre dom i nance of gas-prone or ganic mat ter, how ever in nine sam ples this ra tio ex ceeds 5 cor re spond ing to the oil-type or ganic mat ter (Ap pen - dix 1; Hunt, 1996). High OI val ues, ex ceed ing 150 mg/g TOC, may ev i dence ox i da tion of or ganic mat ter. Low HI ac com pa nied with high OI val ues may sug gest the in flu ence of sec ond ary ox i - da tion pro cesses re lated to radiolytic deg ra da tion of or ganic mat ter, which is a fre quent phe nom e non in ura nium de pos its (Landais, 1996), but such in ter pre ta tion for the strata is rather un likely due to low U con cen tra tions. More over, high OI val ues are ac com pa nied by low TOC con tents, which may ev i dence Fig. 3. HI versus (A) OI and (B) Tmax temperature based on Rock-Eval pyrolysis
of the Walchia shale
Genetic pathways after Espitalié et al. (1985); key for sample codes see Appendix 1
par tial ox i da tion of rocks as ob served by e.g., Pieczonka et al.
(2008) in Up per Perm ian Kupferschiefer strata and Wiêc³aw (2016) in Kimmeridgian strata of the Pol ish Low lands. Low HI val ues are as so ci ated with low TOC con tents, which may re sult from or ganic mat ter dis per sion within the sed i ment, known as
“min eral ma trix ef fect” (e.g., Wiêc³aw, 2016).
MULTI-ELEMENT ANALYSIS OF ROCK SAMPLES
The con cen tra tions of ana lysed el e ments in the Walchia shale sam ples dif fer con sid er ably (Ta ble 1, Ap pen dix 1 and Figs. 4B–9), but mostly show high amounts, greater than those re ported for av er age shale (Turekian and Wedepohl, 1961) and even av er age black shale (Vine and Tourtelot, 1970; Quinby et al., 1989), be ing sim i lar to data re ported for the Antrim Shale from the Mich i gan Ba sin (Leventhal, 1993), and gen er ally sig - nif i cantly higher than data re ported by Speczik et al. (1995) for the Anthracosia shales. The max i mum, min i mum and av er age con cen tra tions of these el e ments are shown in Ta ble 1, and the vari a tions of their con cen tra tions and ra tios with depth are shown in Fig ure 4B–J. The high est av er age con tents of U, Th, P and Cu have been mea sured in rock sam ples rep re sent ing com plex III, reach ing 44 ppm, 22 ppm, 0.17% and 88 ppm, re - spec tively (Ta ble 1, Ap pen dix 1, Figs. 4B, C and 5A–C). The high est U con cen tra tions reach 128 ppm in com plex III (sam ple W-12/19a, Ta ble 1, Ap pen dix 1, Figs. 4B and 5A). This en rich - ment is as so ci ated with the high est TOC con tent, which sug - gests that or ganic mat ter may hold U, while high HI and low OI val ues (Ap pen dix 1 and Fig. 3) prove re duc ing con di tions con - du cive to uraninite pre cip i ta tion from rel a tively ox i dized U-bear - ing so lu tions. Wo³kowicz (1990) also re ported max i mum U con - cen tra tions in rocks of com plex III, how ever much higher (up to 500 ppm), link ing the min er ali sa tion with lam i nated black cal - car e ous shales con tain ing laminae and streaks of or ganic mat - ter, dis persed ga lena and py rite, and asphaltite lenses. The low est av er age con cen tra tion of U (12.9 ppm) was ob served in rocks rep re sent ing com plex IV, which is also con sis tent with re - sults re ported by Wo³kowicz (1990). Ura nium and tho rium con - cen tra tions for or di nary shales re ported by Turekian and Wedepohl (1961) are 3.5 pm and 12 ppm, re spec tively. Data ob tained for the Walchia shale are re verse, higher con tents have been re corded for U than for Th, as with data re ported for Kupferschiefer strata where this dif fer ence is as sumed to be caused by sec ond ary en rich ment in U de rived from the Rotliegend (Piestrzyñski, 1990). So far, the U en rich ment in the Walchia shale, like in the Kupferschiefer, have been con sid ered as so ci ated with or ganic mat ter (Wo³kowicz, 1990, 1992;
Piestrzyñski, 1990 and ref er ences therein). Av er age U con cen - tra tions ob tained for in di vid ual com plexes of the Walchia shale are higher than the av er age 15 ppm re ported for black shales (Quinby et al., 1989) ex cept for the av er age for com plex IV (Ta - ble 1). Plot ting the U con cen tra tion with the TOC con tent (Fig. 5A) does not ex actly sup port this the sis; how ever, the high est U con cen tra tion of 128 ppm was re corded in the sam ple with the high est TOC con tent (5.2 wt.%; Ap pen dix 1). More -
Fig. 4. Depth vari a tion of (A) to tal or ganic car bon con tent (TOC), U con cen tra tion (B), Th con cen tra tion (C), Ni con cen tra tion (D), V con cen tra tion (E), Pb con cen tra tion (F), Zn con cen tra tion (G), V/(V+Ni) ra tio (H), V/Cr ra tio (I) and U/Th ra tio (J) of the Walchia shale sam ples
Depositional set tings af ter Hatch and Leventhal (1992) and Jonnes and Man ning, (1994); b.g.l. – be low ground level; key for sam ple codes see Ap pen dix 1
over, no cor re la tion oc curs be tween the U con cen tra tions and HI for all the rock sam ples, but it does ex ist for the subtuffite com plex (Ap pen dix 1). These ob ser va tions sug gest that U min - er ali sa tion is as so ci ated with or ganic mat ter dom i nated by a ge - netic com po nent of type III kerogen that pre vails in the subtuffite com plex (Fig. 3). On the other hand, the third high est U con tent of 61 ppm was re corded in a sam ple rep re sent ing the subtuffite com plex with a TOC con tent of only 0.74 wt.% (Ap - pen dix 1 and Fig. 5A). The cor re la tion be tween the TOC and U con tents is not clearly ob serv able, nei ther in the whole rock pop u la tion (R2 = 0.23) nor in the in di vid ual com plexes, and may be lim ited only to some parts within the Walchia shale with out in di cat ing any par tic u lar com plex (Fig. 5A). This sug gests that the U oc cur rence within the Walchia shale is re lated to and con - trolled by more fac tors than only as so ci ated with or ganic mat - ter. This the sis is sup ported by the cor re la tions be tween the U and P con cen tra tions (Fig. 6A) and be tween the Th and P con - cen tra tions (Fig. 6B), which in di cate that the pres ence of U and Th in the Walchia shale is likely re lated to phos phate min er als, e.g., ap a tite (de scribed in the next chap ter). Ura nium of ten ac - com pa nies phos phate rocks, es pe cially those of sed i men tary or i gin (e.g., Schnug and Haneklaus, 2014). Tetravalent ura - nium (U4+) can sub sti tute for Ca in the ap a tite lat tice and ac cu - mu late in car bo na ceous ma te rial ac com pa ny ing phos phates (Baturin and Kochenov, 2001; Dill, 2011 and ref er ences therein). Ma rine phosphorite de pos its con tain U usu ally be low 100 ppm, whereas or ganic phosphorites can con tain U up to 600 ppm (Dill and Kantor, 1997; Dill, 2011). The most im por tant rep re sen ta tive of U de pos its in ma rine phosphorites oc curs in Khouribga, Mo rocco (Oulad Abdoun Ba sin), and in or ganic phosphorites at Melovoe, Kazakhstan (Dill, 2011). The pos i tive cor re la tion be tween U and P in the Walchia shale is also an in - di ca tor of re duc ing con di tions dur ing U ac cu mu la tion, since U is able to in cor po rate phos phates in a reductive me dium (Baturin and Kochenow, 2011 and ref er ences therein) as con firmed here by high HI val ues (Fig. 3 and Ap pen dix 1).
The U and Th con cen tra tions in the Walchia shale do not cor re late with each other (R2 = 0.29; Ap pen dix 1), which sug - gests that they have been de rived from in de pend ent sources.
Ura nium is most likely de rived from un der ly ing ox i dized sed i - ments of the Rotliegend strata, while Th is pre sum ably re lated to clay min er als and tuffs (Piestrzyñski, 1989). The geo chem i - cal S2/S3 ra tio plot ted against the U and Th con tent re veals in - verse cor re la tion in di cat ing a de crease in or ganic mat ter con - tent with in creas ing con cen tra tion of ra dio ac tive el e ments (Fig. 7). Sim i lar phe nom ena have been ob served in the Dictionema Shale by Kosakowski et al. (2017) who ex plained this by de cay of ra dio ac tive el e ments caus ing poly meri sa tion and ox i da tion by or ganic struc tures. The trend ob served in the pres ent study is sim i lar to that seen in the Dictionema Shale;
how ever, the TOC and U con tents re ported by Kosakowski et al. (2017) are on av er age sev eral times higher. This sug gests that or ganic mat ter in the Walchia shale might have been al - tered to some ex tent by de cay of radiolytic el e ments. More over, the U con cen tra tions cor re late with MinC con tents (Ap pen - dix 1), which sug gests that U is ac cu mu lated in car bo na ceous mat ter which may be as so ci ated with phos phates. No cor re la - tions be tween TOC con tent and Th and Cu con cen tra tions, nei -
Fig. 5. Total organic carbon content versus concentrations of U (A), Th (B), Cu (C), Ni (D), V (E), Pb (F) and Zn (G) measured
in the Walchia shale samples Key for sample codes see Appendix 1
ther for the whole rock pop u la tion nor for in di vid ual lithological com plexes, have been ob served (Fig. 5B, C).
The abun dance of Cu, Ni, V, Pb and Zn in the Walchia shale is ob served within the whole pro file; how ever, their con - cen tra tions cor re late with nei ther TOC (Fig. 5C–G), HI and OI val ues nor U and Th con tents (Ap pen dix 1). The nickel and V con cen tra tions vary from 17.7 to 83 ppm and from 31 to 255 ppm (Ta ble 1, Ap pen dix 1, Figs. 4D, E, 5D, E), re spec - tively, and do not dem on strate cor re la tions with to tal S (Ap pen - dix 1). Both met als show pos i tive cor re la tion with Fe (Ap pen - dix 1) sug gest ing their as so ci a tion with Fe min er als. Nickel and V com monly oc cur in or ganic mat ter as Ni and V por phy rins.
Nickel, af ter or ganic mat ter deg ra da tion re lated to ox i da tive al - ter ation, can be re tained within a rock in as so ci a tion with py rite (e.g., Tribovillard et al., 2006). Va na dium and Ni can also be in - cor po rated into clay min er als, as ob served in the Kupferschiefer (Bechtel et al., 2002). The nickel con cen tra tions mea sured in the Walchia shale sam ples ex ceed more than twice the val ues pub lished by Mastalerz (1996) for Rotliegend lac us trine de pos - its of the ISB.
The re sults ob tained for Pb and Zn are sig nif i cantly higher than those re ported by Mastalerz (1996) for lac us trine de pos its of ISB, or by Wo³kowicz (1992) for the Walchia shale, reach ing a max i mum of 5166 and 7162 ppm, re spec tively (Ta ble 1, Ap - pen dix 1, Figs. 4F, G, 5F, G). Their oc cur rence cor re lates very well with each other in di cat ing their pres ence in sul phide min - eral paragenesis.
Fig. 6. Total P versus U (A) and Th (B) concentrations Key for sample codes see Appendix 1
Fig. 7. S2/S3 index versus U and Th contents Key for sample codes see Appendix 1
Fig. 8. V/(V + Ni) ver sus V/Cr plot for the Walchia shale sam ples Ranges of depositional set tings from Hatch and Leventhal (1992) and Jones and Man ning (1994); di a gram af ter Lecomte et al. (2017);
key for sam ple codes see Ap pen dix 1
The co balt con cen tra tions are up to 47 ppm and cor re spond to Ni and V con tents, and also poorly to Cu and Fe (Ap pen - dix 1). The Co, sim i larly to Ni, can be as so ci ated with py rite (Bechtel et al., 2002). The Co con tents are higher than the av er - age for black shale (10 ppm; Vine and Tourtelot, 1970); how - ever, they are lower than those re ported in the Anthracosia shales (Speczik et al., 1995), Kupferschiefer (Sun and Püttmann, 1997) and Alum Shale (Lecomte et al., 2017).
Some trace el e ments, such as U, Th, Ni, V, Mo, Co, Cr and their ra tios, have been widely used for iden ti fi ca tion of diagenetic re dox en vi ron ments due to their lower sol u bil ity un - der re duc ing con di tions, re sult ing in pre cip i ta tion un der ox y - gen-de pleted sed i men tary con di tions (e.g., Hatch and Leventhal, 1992; Jones and Man ning, 1994; Zhou and Jiang, 2009; Zhou et al., 2012; Wójcik-Tabol, 2015; Lecomte et al., 2017; Mouro et al., 2017). The Mo con cen tra tions dem on strate a gen eral in crease with in creas ing TOC con tent, and range from 0.4 to 59 ppm, at tain ing even 609 ppm in one sam ple rep - re sent ing com plex II (Ap pen dix 1). Mo con cen tra tions in di cate vari able re dox con di tions dur ing de po si tion of the Walchia shale (e.g., Mouro et al., 2017). The V/(V + Ni), V/Cr, U/Th, and Ni/Co ra tios have been ap plied to eval u ate the pres ence or ab sence of dis solved O2 or H2S in the wa ter col umn dur ing de po si tion of the Walchia shale. The V/(V + Ni) val ues are within the range of 0.62–0.85 (Ap pen dix 1, Figs. 4H and 8) in di cat ing anoxic to euxinic sed i men tary con di tions, while the V/Cr val ues vary from 0.76 to 2.55 (Ap pen dix 1, Figs. 4I and 8) sug gest ing oxic and par tially dysoxic bot tom con di tions, which agree with the U/Th and Ni/Co ra tios rang ing from 0.13 to 6.0 and 1.63 to 5.3, re - spec tively (Ap pen dix 1, Fig. 4J; Jones and Man ning, 1994). The dis crep ancy in eval u a tion of depositional con di tions may sug - gest that not all used re dox prox ies are re li able and can be em - ployed in the dis cussed case, as in di cated also by the lack of cor re la tion be tween them (Ap pen dix 1).
The plots of to tal or ganic car bon con tent and to tal sul phur con tent have been com monly used to dis crim i nate be tween oxic, anoxic and euxinic en vi ron ments (e.g., Leventhal, 1993, 1995; Jones and Man ning, 1994; Sun, 1998; Lecomte et al., 2017). The TOC/S ra tios ob tained for Walchia shale sam ples vary to a large ex tent from 0.18 to 15 (Fig. 9). Pos i tive cor re la - tion, mostly along euxinic sed i ments, in di cates syngenetic and
diagenetic sul phide for ma tion by mi cro bial sul phate re duc tion and re ac tion of sul phide with re ac tive Fe (Leventhal, 1995). A sim i lar pat tern has been re vealed for sam ples from the Pol ish and Ger man Kupferschiefer by Lewan et al. (2008) and Bechtel et al. (2001), re spec tively. The pos i tive in ter cept with the to tal S axis may ev i dence the for ma tion of py rite in the wa ter col umn and at the wa ter-sed i ment in ter face (Lecomte et al., 2017).
Some of the sam ples plot a ver ti cal trend along the S-axis, in di - cat ing S-rich TOC-poor sed i ments. Such phe nom e non has been ob served by Sun (1998) in Kupferschiefer sam ples.
The base metal dis tri bu tion within the Walchia shale is sim i - larly patchy within the whole pro file. The metal oc cur rence seems to be lim ited to the fine-grained sed i ments within com - plexes with out sand stone and coarse-grained lay ers. This ob - ser va tion sug gests post-depositional mech a nisms of metal ar - range ment re lated to the move ment of metal-bear ing ox i dized flu ids leach ing met als from the Rotliegend and pos si bly also ad - ja cent vol ca nic rocks, which pen e trated the whole Walchia shale prob a bly us ing lo cal fault zones and pro duc ing min er ali - sa tion in or ganic and sul phide-rich sed i ments. The ev i dences of fluid mi gra tion in the study area have been pre vi ously re - ported by Lewandowski (1990), who iden ti fied sig nif i cant trans - for ma tions of rhyolitic tuffs, re sult ing in analcimisation caused by low-tem per a ture hy dro ther mal ac tiv ity. The metal en rich - ment in the Walchia shale was in duced by (al tered dur ing diagenesis) or ganic mat ter, and formed a re dox bar rier that re - tained met als. Epigenetic char ac ter of min er ali sa tion might be dif fi cult to no tice due to lam i nated struc ture of the Walchia shale. The struc ture pro motes a lin ear flow, be cause hor i zon tal per me abil ity is higher than ver ti cal. Sim i lar min er ali sa tion pro - cesses have been pro posed by Speczik et al. (1995) for the Anthracosia shale and partly for Kupferschiefer strata (sum ma - rized by Alderton et al., 2016). Met als in the Walchia shale could have been as so ci ated with or ganic mat ter; how ever, weak or lack of cor re la tions be tween ana lysed metal con cen tra - tions and TOC con tents may re sult from sub se quent ox i da tion and al ter ation of the lat ter. Pyroclastics un der ly ing the Walchia shale are char ac ter ized by sim i lar val ues of dose rate as the Walchia shale (Bilkiewicz, 2014), which may in di cate these rocks as an other po ten tial source of Th.
1 2 3 4 5
1 2 3
EUXINIC
NORMAL MARINE
NON-MARINE
Total organic carbon [wt.%]
W-12/19b W-15/4
W-11/5
W-12/27 W-8/12
W-12/17
W-12/19a complex II
complex III complex IV
sub-tuff complex
Fig. 9. Plot of total sulphur content versus total organic carbon of the Walchia shale samples, showing environmental conditions
(Leventhal, 1993)
Key for sample codes see Appendix 1
MINERALOGICAL CHARACTERISTICS OF THE WALCHIA SHALE
Mi cro scopic in ves ti ga tions showed that most of the ana - lysed sam ples were mudstones, with mi nor claystones, con - tain ing var i ous amounts of clay min er als and car bon ates, mainly in the form of micrite. Pre vi ous stud ies on the Walchia shale (Wo³kowicz, 1988, 1990) re vealed the pres ence of framboidal py rite and oc ca sional hematized and dis persed ga - lena within these sed i ments.
Mi cros copy stud ies in re flected light re vealed no de tect able oc cur rence of typ i cal ra dio ac tive min er als. Or ganic mat ter oc - curs as ir reg u lar thin laminae up to sev eral tens of mm. Among the ore min er als the most com mon is py rite which oc curs in all ana lysed sam ples ex cept in sam ple W-12/33, bar ren in de tect - able ore min er als. Py rite forms both euhedral crys tals and framboids, which may reach 150 mm in size. Framboids are of - ten dis persed through out the ana lysed sec tion and com monly scat tered form ing col o nies of var i ous shapes (sam ple W-8/12, Fig. 10A). The py rite con tent in the sec tions var ies from <0.1 to 4% and, in as so ci a tion with marcasite, up to 10.8% (sam ple W-15/20). Py rite ag gre ga tions com monly re place or ganic mat - ter clus ters (Fig. 10B, C), and fre quently oc cur in clay min er - als-rich zones (e.g., sam ple W-15/23) and grow within frac tures in quartz (e.g., sam ple W-15/28). Py rite also oc curs as inter - growths within chal co py rite. Other com mon ore min er als are sphalerite and ga lena, which fre quently form mu tual inter - growths, lo cally show ing the so-called “pock-marked” struc - tures, in which they re place car bon ates and or ganic mat ter clus ters (sam ples W-11/15, Fig. 10D). Their quan ti ties within the sec tions are up to 0.4% (sphalerite) and 0.2% (ga lena). The high Zn and Pb con cen tra tions within the Walchia shale sam - ples are there fore re lated to the com mon pres ence of their sul - phide min er als. Cu is as so ci ated with chal co py rite that occured as in de pend ent crys tals or ag gre gates up to 75 mm (sam ple
W-11/6, Fig 10E), oc ca sion ally form ing inter growths within sphalerite (sam ple W-15/23). Ga lena, sphalerite and chal co py - rite may also re place or ganic forms (Fig. 10F).
Com bined EDS and elec tron scan ning mi cro scope in ves ti - ga tions were con ducted on pol ished sec tions of two sam ples (W-11/5 and W-12/19a), in which high U con cen tra tions have been ob served. The anal y sis also con firmed the lack of de tect - able ra dio ac tive min er als; how ever, they con firmed the pres - ence of, not de tected in re flected light, ap a tite (Fig. 11A) oc cur - ring also with py rite inter growths (Fig. 11B). Ap a tite shows struc tural sim i lar ity to francolite, a flu o rine-bear ing va ri ety of ap - a tite, which was re ported by Mayer and Piestrzyñski (1986) in Weissliegend strata of the Lower Zechstein in the Rudna Mine.
Both these sam ples con tain laminae of diagenetic bi tu mi - nous-type or ganic mat ter sim i lar to thucholite ob served in the Lubin polymetallic de posit by Piestrzyñski (1988). How ever, or - ganic mat ter in sam ple W-11/5 is com pletely dif fer ent. Or ganic mat ter in sam ple W-12/19a is typ i cal sed i men tary or ganic mat - ter (Fig. 11C), whereas that in sam ple W-11/5 (Fig. 11D) oc curs in spaces be tween min eral grains, be ing the re sult of al ter ation pro cesses re lated pos si bly to mi grat ing flu ids that were re spon - si ble for metal min er ali sa tion of the Walchia shale. Such al ter - ation pro cesses might have en hanced en rich ment with met als, es pe cially Pb and Zn (Figs. 4F, G and 5F, G).
Re sults of mi cro scope in ves ti ga tions, in clud ing the ab sence of ra dio ac tive min er als, in di cate that U and Th in the Walchia shale may be as so ci ated with or ganic mat ter and phos phate min er als, e.g., francolite-type ap a tite that is very com mon in an en vi ron ment such as rep re sented by the Kupferschiefer (Mayer and Piestrzyñski, 1986). The source of phos pho rous could be or ganic mat ter re lated to plank tonic and al gae or gan isms, which re leased P dur ing ox i da tion-re duc tion re ac tions pro vid ing it for the pre cip i ta tion of phos phate min er als. The prob a ble source of F might be vol ca nic ex ha la tions. The oc cur rence of Fig. 10. Microscopic photographs in reflected light
Py rite (py) framboids, sam ple W-8/12 (A) clus ters of or ganic mat ter (OM), one is be ing re placed by py rite (py), sam ple W-15/28 (B), clus ters of or ganic mat ter (OM), one is be ing re placed by py rite (py), graph ite (gr) crys tal in the cen tre, sam ple W-15/28 (C), “pock-marked” struc tures of ga lena (ga) and sphalerite (sp), sam ple W-11/15 (D), chal co py rite (cp) crys tal and or ganic mat ter (OM), sam ple W-11/6 (E), sphalerite (sp), py rite (py) and chal co py rite (cp) re plac ing or ganic forms, sam ple W-11/5 (F)
francolite is char ac ter is tic for early diagenetic low-tem per a ture pro cesses (e.g., Mayer and Pie strzyñ ski, 1986).
Sta ble sul phur iso to pic data for framboidal py rite from Kupferschiefer strata (Pieczonka, 2011) show en rich ment in light sul phur iso topes ev i denc ing its or ganic or i gin from mi cro - bial ac tiv ity, most likely re lated to mi cro bial sul phate re duc tion (MSR). The pres ence of framboid py rite and “pock-marked”
struc tures of sphalerite and ga lena may con firm MSR pro - cesses within the Walchia shale, re sult ing in sul phate re duc tion, or ganic mat ter ox i da tion, and for ma tion of sulphides un der a re - duc ing en vi ron ment. The en rich ment of sulphides in the Walchia shale as so ci ated with a rel a tively high amount of or - ganic mat ter in di cates euxinic and anoxic con di tions dur ing sed i men ta tion, which could fa cil i tate pre cip i ta tion of sul phide min er als from avail able met als in the wa ter col umn; how ever, the def i nitely higher av er age con cen tra tions of base met als than the av er age for black shales in di cate post-depositional min er ali sa tion pro cesses re lated to mi gra tion of rel a tively ox i - dized flu ids from Rotliegend strata.
CONCLUSIONS
The Walchia shale is gen er ally a poor source rock; how - ever, some parts of the pro file, re lated to deeper, re duc ing zones of the ba sin, re veal better source rock pa ram e ters, in di - cat ing good and even ex cel lent hy dro car bon po ten tial. Re sults of Rock-Eval py rol y sis in di cate pre dom i nance of type III kerogen with sub stan tial ad mix ture of plank tonic or ganic mat ter re lated to type II kerogen. Tmax val ues cor re spond to ther mally im ma ture or ganic mat ter.
The Walchia shale sed i ments were de pos ited in a wide lake un der vari able sed i men tary con di tions. Most of them were ac - cu mu lated in a pe lagic zone un der both anoxic and oxic con di - tions. The en rich ment in or ganic mat ter of mixed ge netic type in di cate that the lake de vel oped in a warm cli mate and was eutrophic, as also pre vi ously con cluded by Wo³kowicz (1988).
The pres er va tion of or ganic mat ter within the Walchia shale was most prob a bly caused by the over ly ing anoxic-euxinic wa - ter col umn that pre vented ox i da tion and fa voured synsedi - mentary con cen tra tion of met als.
Fig. 11. BSE image of apatite (ap), sample W-12/19a (A), apatite (ap) with pyrite (py) intergrowths, sample W-12/19a (B), bituminous-type organic matter (OM), sample W-12/19a and W-11/5, respectively (C and D)
Mi cros copy in ves ti ga tions re vealed the lack of ra dio ac tive min er als and the com mon pres ence of py rite that oc curs fre - quently in the framboidal form sug gest ing its or ganic or i gin from MSR pro cesses or S-rich or ganic mat ter. Ga lena and sphalerite are wide spread within the Walchia shale se quence. No cor re la - tion of metal con cen tra tions with TOC con tent sug gests that the metal en rich ment could be re lated to in or ganic fea tures of the Walchia shale or had been as so ci ated with or ganic mat ter which was sub se quently ox i dized and de stroyed, pre sum ably by ox i - dized so lu tions pen e trat ing sed i ments af ter their de po si tion.
The Walchia shale sys tem is priv i leged for metal en trap ping due to its re duc ing char ac ter. The el e vated con cen tra tions of base met als in di cate their epigenetic ac cu mu la tion. Al tered dur ing diagenesis, or ganic mat ter acted as a re dox bar rier for ox i dized flu ids leach ing met als from the Rotliegend and pos si - bly from ad ja cent vol ca nic rocks, pro duc ing min er al isa tion in or - ganic and sul phide-rich sed i ment. Ura nium is most likely as so -
ci ated with or ganic mat ter and phos phate min er als, and was de liv ered by flu ids from the un der ly ing Rotliegend strata, while Th is pre sum ably re lated to clay min er als and/or it mi grated from the un der ly ing tuffs.
Ac knowl edge ments. The study has been fi nan cially sup - ported by the stat u tory re search of the Fac ulty of Ge ol ogy, Geo - phys ics and En vi ron men tal Pro tec tion at the AGH Uni ver sity of Sci ence and Tech nol ogy in Kraków, pro jects Nos.
11.11.140.626 and 11.11.140.320. We thank Prof.
S. Wo³kowicz for pro vid ing geo log i cal data of the Walchia shale, help ful com ments and ed i to rial work, Prof. M.J. Kotarba for help in re search, Mr. H. Zych for an a lyt i cal work, and Dr.
P. Kosakowski for as sis tance in rock sam pling. We grate fully ac knowl edge the re view ers: M. Fabiañska, B. Køíbek and G.J.
Nowak for valu able re marks that greatly im proved the qual ity of the pa per.
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