iron ore of the lahn-dill type formed by diagenetic seeping of pyroclastic sequences — a case...

401

Geologische Rundschau 79/2 1401-415 ]Stuttgart 1990

Iron ore of the Lahn-Dill type formed by diagenetic seeping of pyroclastic sequences - a case study on the Schalstein section at G~insberg (Weilburg)

By HEINER FLICK, H. DIETER NESBOR & ROMAN BEHNISCH, Heidelberg*)

With 11 figures and 1 tabie

Zusammenfassung

Untersuchungen zur Rekonstruktion der Abl~ufe an und in den submarinen Vulkangeb~iuden des Devons (Gi- vet/Adorf-Phase) der Lahnmulde (stidliches Rheinisches Schiefergebirge) schlief~en die sekund~re Alteration mit ein urld werden an dem ausschlief~lich aus basischen Pyroklasti- ten (Schalstein) aufgebauten Profil am Giinsberg bei Well- burg exemplarisch vorgestellt. Diese anhand petrographi- scher und geochemischer Kriterien erkennbaren sekund~ren Prozesse zeigen eine mehrphasige Entwicklung auf, die dutch Kathodenlumineszenz-Untersuchungen der Karbo- natzemente best~tigt und zeitlich geordnet werden kann. Die an die pyroklastische Abfolge gebundene Roteisensteinverer- zung vom Lahn-Dill-Typ l~it~t sich mit der bei diesen Vorg~n- gen erfolgten Wan&rung verschiedener Elemente korrelie- ren. Damit wird die Vererzung zu einem Produkt der diage- netischen Alteration und nicht der magmatischen Differen- tiation, wodurch sie mit rezent im ozeanischen Raum beob- achteten Prozessen vergleichbar ist. Diese Vorstellungen fii- gen die Lahn-Dill-Erze in aktualistisch begriindete Modelle zur Genese yon Erzen ein. Damit erscheint die Besonderheit und fast nur auf den rhenoherzynischen Raum beschr~nkte Verbreitung dieses Typus hinf~llig.

Abstract

Investigations on the reconstruction of processes and facies relationships from submarine volcanics of Devonian age in the Lahn syncline (Rhenish Mountains, Western Ger- many) reveal a complex development of secondary alteration. This is well illustrated by a mafic pyroclastic sequence (~,Schalstein, 0 at the G~insberg near Weilburg where alteration processes are visible by petrographic and geochemical

*) Authors' addresses: H. FLICK, Geologisch-Pai~tontologi- sches Institut, Universit~it Heidelberg, Im Neuenheimer Feld 234, D-6900 Heidelberg, H. D. NESI3OR, Geologisch-P. alSon- tologisches Institut, Universk~it Heidelberg, Im Neuenhei- mer Feld 234, D-6900 Heidelberg - present address: Hessi- sches Landesamt ftir Bodenforschung, Leberberg 9, D-6200 Wiesbaden, and R. BEHNISCH, Geologisch-Pal~iontolo- gisches Institut, Universit~it Heidelberg, Im Neuenheimer Feld 234, D-6900 Heidelberg

Manuscript received: 25. 5. 89; accepted: 28.3.90

means and can be further classified by cathodoluminescence. Iron ore formation of Lahn-Dill type is recognized as part of this alteration process, resulting from diagenetic seeping. Until recently a direct magmatic source for the Lahn-Dili type iron ore has been the generally accepted model. These bodies have therefore been viewed as a rather unique stratiform deposit, whose occurrence was virtually confined to this type area. However, it is here considered that the formation of the iron ore corresponds well with existing models of alteration processes within recent oceanic environments which are of a more universal occurrence.

R&um~

L'6tude de [a succession des ph~nom~nes et des relations facielles dans les volcanites sou>marines d'~ge d6vonien du synclinal de la Lahn (Massif schisteux rh&an) fait apparakre des processus complexes d'alt&ation secondaire. Ceci est par- ticuli&ement bien illustr6 par la s6quence pyroclastique (,,Schalstein,,) du G~.nsberg pr6s de Weilburg: les processus secondaires, identifiables par des crit&es p&rographiques et g6ochimiques y pr&entent un d&eloppement polyphas~ dont l'histoire peut &re reconstitu& par application de la cathodo-luminescence au ciment carbonat& Les min&alisa- tions en fer associ&s ~. la s&ie pyroclastique peuvent &re cor- r6i&s avec Ies migrations de divers 616ments impliqu& dans ces processus. Ces min&afisations apparaissent ainsi comme le produit d'une alt&ation diag6n&ique et ne correspondent donc pas au module g&&alement admis d'une diff&encia- tion magmatique. Cette gen~se par alt&ation est d'ailleurs conforme ~ ce qu'on observe dans les domaines oc&niques r&ents de sorte que les gisements de la r~gion Lahn-Dill doi- vent&re consid&& non pas comme une singularit6 du massif schisteux rh&an, mais comme l'expression d'un ph&lo- m~ne plus g~n&al ~ caract6re actualiste.

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402 H. FLICK et al.

npol~ecebi nMeJm Mnoroqba3oBytO rlCTOprItO pa3BrlTrl~i; ee nO~TBep~HaH H HCCJIeRoBaHrL~ Kap6orIaTrloro uescenTa MeTO~aMH KaTO~HO~ 3tOMHtleCKelqIBIFI; y~aaoc~, TavoKe ycTanOmlTb n BpeMermyto nocaeRoBaTea~,HOCTJ, 3Tmr npolIeCCOB. Py~oo6pa3oBanrm Kpacnoro )Ke.rle3ILqKa TtaqIa Lahn-Dill , cB~3anrIoe c nocae~oBaTem, HO npoTeKaBma- M~ nnpoKnaCTriqecKrrMH npoKeccaMn, MO)g~qO Koppean- pOBaT~ C Mr~rpa~neI~ pa3Jirrqrtb~ 3JIeMeHTOB, B/~I3BaHI-IO~ aTrrMrI npoKeccaMa. T.o. opy~rmerme B ~armoM peraoHe tt~o B pe3yaJ, TaTe ~aarerIeTnqecKgx npot~eccoB, a He BbI3blBa3IOCb ~mqb@epeni~natmefi MarM~I, n ero MOXHO cpaBrmT~ c ilpolleccaMn, IIpOTeKaIOralrlMa B Hame BpeMn B oKeaHax. TaKoe npeBcTaBJIeH[Ie o IIpOHCXOFK~eHHI/I py~ Tmla Lahn-Dill MomeT IlpeACTaBaaT~ CO60fi O6OCHOBaH- ItylO io~e3ab o6pa3oBam~ py~ BOO6me. FIo3ToMy He cne- ~yeT roBop~ITb 00co6eHHOeT~X 3TOFO T~IHa opy~gaHeHn.q, orpanrlqemloro IlpaKTrmeclcn TO~I~KO pefiHcgo-rept~nr~c- K~IMH o65IaeT~IMg.

1. Introduction

The Lahn-Dill type iron ores of the southern Rhenish Mountains are named after the Lahn and Dill synclines within which they occur. The area was characterized by widespread bimodal volcanism during Devonian and Carboniferous times, along with elastic and carbonaceous sedimentation. The volcanic products include lava flows, dykes, sills (at several stratigraphic levels), and pyroclastic rocks of various origins. The mafic rocks, often termed diabases in the local literature, have yielded a spilitic assemblage. In- termediate to felsic volcanics occur as keratophyres and quartz keratophyres. These mineral assemblages can be attributed to secondary alteration mainly during diagenesis, with only minor alteration during a weak Hercynian metamorphism (WEDEPOHL et al., 1983; FLICK & NESBOR, 1988).

Formations of generally poor quality sedimentary iron ores have been found associated with the mafic volcanics. The more profitable Lahn-Dill type ores, associated with abundant pyroclastic rocks, have been mined at many sites within the type area. Equivalent ore deposits are also known from other parts of the Rhenohercynian belt (QuADE, 1970, 1976; ROSLER & WERNER, 1979; BOTTKE, 1981).

Concepts as to the formation of these ores have varied with time, the most commonly quoted origin been that of a direct descent from the mafic volcanism (QUADE, 1970, 1976; BOTTKE, 1981). The idea of diagenetic mobilization was put forward by HENT- SCHEL (1960) and ROSLER (1964), but was not established. This origin was again considered more recently from a study of sulfides genetically related to Lahn-Dill type iron ores in the northern Rhenish

Mountains (MEILLER et al., 1986). Results from a detailed investigation of the type area presented here are in agreement with this view, and are comparable with similar relationships which have been described for recent ocean floor environments (HERZIG et al., 1988).

2. Geological setting

Volcanic activity accompanied the extensional tec- tonics (crustal thinning) which influenced the pelagic shelf to the south of the Old Red continent in the Up- per Palaeozoic. In the Lahn-Dill region volcanism occurred in four episodes: (A) Emsian/Eifelian, (B) Givetian/Adoffian, (C) Nehdenian-Wocklumian, and (D) Dinantian II. The mafic volcanics (during episodes B, C, and D) are the most widespread, whereas the intermediate to felsic volcanics (during episodes A, B, and D) are less extensive in distribution. Minor element chemistry as well as REE analyses of the mafic volcanics point to an intraplate environment with tholeiitic as well as alkali basaltic magmas (WEDEPOHL et al., 1983; NESBOR & FLICK, 1988a; SCHMINCKE, 1988a). The felsic volcanics probably originated within the continental crust (FLICK & NESBOR, 1988).

Mafic volcanism was most widespread during the Givetian/Adorfian episode (Fig. 1). It resulted in submarine volcanic edifices several to many hundred metres high with basal extensions exceeding 10 kilometers. Palinspastic reconstruction of such submarine volcanoes within the central and southwestern Lahn syncline (NESBOR & FLICK, 1988b, 1989) has led to the following characteristic threefold facies development (Fig. 2): The central facies is defined by a concen- tration of submarine lava flows, often with pillows, sub-effusive sills, and dykes. Pyroclastic rocks define the outer two facies: a proximal facies with an inter- calation of a considerable amount of lava flows, sills, and dykes, succeeded by a distal facies in which they are lacking. The facies development is reflected by changes of the structure of the pyroclastic particles as well.

Petrographical differences in the tephra reflect the varying processes of fragmentation, eruption, and depos i t ion (FISHER & SCHMINCKE, 1984; SCHMIN- CKE, 1988b). During the Givetian/Adorfian episode tephra was deposited (in decreasing quantities) as submarine pyroclastic flows, turbidites, and air fall tufts (FLICK & NESBOR, 1988). The scarceness of the latter within thick carbonate reef successions (STAPF & NIEMANN, in FLICK et al., 1988) indicates the sparseness of sub-aerial volcanism during that episode.

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404 H. FLICK et al.

Altogether the pyroclastic rocks constitute the bulk of the volcanic edifices, thereby forming the most characteristic and important series in the whole Lahn- Dill area, known as ,,Schalstein,<.

3. The pyroclastic sequence (Schalstein) at G~insberg/Weilburg

3.1 S e t t i n g

The G~insberg near Weilburg (arrow in Fig. i; NESBOR & FLICK, 1988b) provides especially good conditions for investigating the &positional as well as the secondary processes of the Lahn-Dill volcanics and is therefore presented here. The section is situated at the northern margin of the central tectonostrati- graphic unit of the Lahn syncline which forms the main volcanic area known as the ,~Schalstein-Hauptsat- tel~ by AHLBURG (KEGEL, 1922). The section at G~insberg has attracted the interest of geologists for a long time because of the good exposure and relative abundance of otherwise rare, well bedded pyroclastics (HENTSCHEL, 1951; HENTSCHEL & MICHELS, 1953, MICHELS, 1962; RIETSCHEL, 1966; MEISL et al., 1982). Similar sections can be found elsewhere in the Lahn- Dill area, but are generally less accessable.

The G~insberg exposes the upper half of a mafic pyroclastic sequence with a true thickness of more than 110 m (Fig. 3). It consists of at least 11 units, made up of four petrographic types, which are as follows (in order importance): non-bedded lapilli tufts; well bedded lapilli tufts; well bedded phenocrystic lapilli to ash tufts; and ash tufts (for details see NESBOR & FLICK, 1988b). Differences in comparison with the section through the northern part of the ,Schalstein-Hauptsat- tel,, along the Well river (FLICK & NESBOR, 1988: Fig. 10) have led to the recognition of the G~insberg sequence as representing a distal volcanic facies.

3.2 G e o c h e m i s t r y

Geochemistry of the spilitic rocks in the Lahn-Dill area clearly show that they originated from primarily basaltic material. Trace elements and REE indicate that both tholeiitic and alkali basaltic compositions are present in the lava flows, sills, and dykes (FLICK & NESBOR 1988; NESBOR, 1988). In the pyroclastic rocks, however, it has not been possible to demonstrate a similar bimodal distribution. Instead geochemical analyses reveal a dependence on the degree of alteration (indicated by the colouration of the rock) which in-" fluences even the incompatible elements often used in discriminant diagrams (Fig. 4).

The pyroclastic sequence in the Giinsberg area shows a change in colour from generally green in the main part of the section to violet below the overlying iron ore horizon (in the uppermost 20 m, Fig. 5). Two bleached horizons have been located within this upper violet zone. Such violet staining of the top parts of pyroclastic sequences is rather frequent and can be related to the distribution of iron ore (so called ,,Edler Schalstein,, used as key horizon during mining ac- tivities).

Bleaching and staining are the result of alteration processes in the pyroclastic sequences. There appears to be a strong dependence of texture and rock colour with the contents and distribution of the elements. In the coarse grained rocks (lapilli tufts) CaO is enriched through alteration processes which increase toward the top parts. This is accompanied by an increase in violet staining and bleaching in the course of which SiO2 and Fe203 are successively depleted (Fig. 5), whilst the trace elements show patterns of both enrichment and depletion within the same samples, e.g. Nb and Y (Fig. 4). However in contrast, fine grained ashes exhibit enrichment of Fe203.

3.3 S e c o n d a r y a l t e r a t i o n

The pyroclastic rocks at G~nsberg yield a spilitic mineral assemblage, as do the other mafic Devonian volcanics from the Lahn-Dill area. In recent years it has been generally agreed that this assemblage is the result of secondary alteration processes (cf. MEISL, 1970; WEDEPOHL et al., 1983; TANUMIHARDJA et al., 1986; SCHMINCKE & SUNKEL, 1987; FLICK & NESBOR, 1988; SCHMINCKE, 1988a) although the timing of this event is uncertain. The pyroclasts consist of chlorite and leucoxene, resulting from recrystallization of a mafic glass, with phenocrysts of low albite replacing Ca-rich plagioclase. The primary magmatic iron content is still present in the chlorite and leukoxen but at a lower level. Relic pyroxenes, as found at other localities, were not observed. Calcite is present as a blocky cement and exhibits a varying degree of replacement between the pyroclasts (Fig. 6) as well as infilling chlorite rimmed vesicles.

Secondary alteration of the Lahn-Dill volcanics occurred at two different stages, firstly during diagenesis directly following deposition and secondly during slight metamorphism of the Hercynian orogeny. Cer- tain mineral phases such as pumpellyite are clearly of metamorphic origin and are found both in the basic lavas and pyroclastic rocks (MEISL et al., 1982). The age of the main constituents calcite and chlorite can be uncertain. The filling of late cracks or fissures can be attributed to the metamorphic event. However, the

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The pyroclastic sequence at G~insberg can be divided into two parts according to the colour of the rock: mainly green (chlorite) and subordinate violet (haematite) at the top (Fig. 5). These colours are due to secondary alteration and result from different processes.

The green section is replaced by carbonate in a few discrete horizons only. The individual pyroclasts can be identified with certainty and show just partial replacement (Fig. 6). In the violet section a few pale white horizons can be observed besides vents and streaks of iron oxide enrichment. In both, pyroclasts

Iron ore of the Lahn-Diil type formed by diagenetic seeping of pyroclastic sequences 407

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Fig. 5. Rock column of the upper part of the section and corresponding contents of Fe203, CaO and SiO2, correlating with the colours of the rock. Key as in Fig. 3.

are more or less completely replaced by carbonate within coarse layers, where they can be recognized as phantoms only (Fig. 7). The fine grained ash tuff and fossiliferous limestones on top have been haematized. The bleaching has affected, among others, the concen- tration of Fe203, SiO2 and CaO (cf. section 3.2).

For the first time cathodoluminescence of the carbonate cement was applied to pyroclastic rocks in this study. It allows interpretation of the alteration processes and reconstruction of their temporal development. Investigations were carried out with a cold- cathode luminescence model (Technosyn 8200 MK2),

Fig. 6. Thin section of a green non-bedded lapilli tuff. Strongly vesicular pyroclasts are embedded within a calcite cement.

408 H. FLICK et al.

Fig. 7. Thin section of a pale white lapilli tuff. Pyroclasts are more or less completely replaced by calcite, almost obliterating the original texture.

accelerating voltage of 15 kV, current of 400-500 #A. The Fe and Mn content was measured with a Microprobe Camebax from Cameca, with an accelerating voltage of 15 kV, specimen current of 0.15 #A and counting time of 10 sec.

Ordinary illumination reveals just a blocky calcite cement in thin section, almost without recognizing

any different stages of cement generation (Fig. 8; which corresponds to Fig. 9 D).

However, cathodoluminescence allows the recognition of varying conditions of diagenesis (Fig. 9), despite the long time interval since alteration as well as subse- quently undergoing very low grade metamorphism. The luminescence reflects the Fe/Mn ratio in the car-

Fig. 8. Thin section of a violet stained lapilli tuff (sample D in Fig. 3; identical with Fig. 9D). Ordinary illumination reveals just a blocky calcite cement between the lapilli.


bonate, changes in which result in zoning, whereas the absolute content of Fe and Mn substituting for Ca in the structure is indicative of the redox potential (cf. FRANK et al., 1982).

Six stages of calcite generation could be distinguished within the violet stained upper part of the section, but not all are developed in the lower, main part of rock which is the green stained portion:

1) In a strongly reducing environment pyroclasts were replaced, especially in the violet stained parts, by a weakly luminescing carbonate (Fig. 9 B-D).

2a) Spheroidal calcite surrounded the pyroclastic particles, except for the uppermost metres of a depositional unit. Within the green section a strongly reducing environment gave rise to a strong substitution of Fe

and Mn for Ca (Fig. 10, Tab. 1). A high Fe/Mn ratio thereby produced a weak luminescence (Fig. 9A). A stronger luminescence which corresponds to a less reducing environment, occurs within the violet section (Fig. 9B). Weak zoning in the calcite spheroids is due to changes in the redox potential.

2b) A less reducing environment in the uppermost metres of the violet section coincides with a small in- corporation of Fe (Fig. 11, Tab. 1). A low Fe/Mn ratio gives rise to strong luminescing skaIenoeders instead of the spheroides, the strong zoning indicating frequent changes in the redox potential (Fig. 9D). Altogether Mn is considerably lower than in the green section (2a) as well.

3) Conspicuous clear carbonate cements without

Fig. 9. Cathodoluminescence of thin section from the pyroclastic sequence at GSinsberg (samples A to D in Fig. 3). A = green non-bedded lapilli tuff (lapilli = black); B = violet non-bedded lapilli tuff (lapilli replaced by calcite); C = haematized detritic fossiliferous limestone; D= violet stained well-bedded lapilli tuff (cf. Fig. 8). Numbers refer to the different stages of cement generation; for details cf. text.

410 H. FLICK et al.

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Fig. 10. Fe and Mn contents with ratios, generations 2a, 4, and 6 of the carbonate cement of sample A (Fig. 9A). Densely stippled = very fiant luminescence, intermediately stippled = dull luminescence, not stippled = bright luminescence.

luminescence was developed in the higher parts of each depositional unit (Fig. 9B). Low to non-tracable Fe and Mn contents (Fig. 11, Tab. 1) demonstrate an oxidizing environment as coexisting sulfides are absent (FRANK et al., 1982). In the uppermost metres of the section they grew as idiomorphic skalenoeders in solution

cavities, interrupted by strong luminescing zones (Fig. 9C and D).

4) Cavities left open were mostly filled by a more or less equally strong luminescing cement, the low Fe content being formed in a weakly reducing environment (Fig. 9A and C). The Mn contents are con-


spicuously higher than in calcite generations 2b and the strong luminescing parts of generation 3 (Fig. 10 and 11, Tab. 1).

5) Remaining open space in the violet stained upper part of the section was filled by a weakly luminescing cement formed in a strong reducing environment (Fig. 9B).

6) Later fractures were sealed by a strongly luminescing cement which grew during reducing conditions (Fig. 9A and C).

The first five of these carbonate generations repre- sent diagenetic processes, ranging from near surface

conditions to deep burial. The last generation of cement may be correlated with the Hercynian metamorphic event.

3.4 I n t e r p r e t a t i o n

The diagenetic processes relate to the deposition of the pyroclastic sequence reflecting their development and local conditions. The sequence consists of several units which were deposited preferentially as (non- welded) pyroclastic flows, each up to several tens of metres in thickness, Porosity was high in these beds

SECTION 1

No. F e ( p p m ) Mn(ppm) Fe/Mn

1 11230 14820 0,76 2 3040 8010 0,38 3 3260 11480 0~28 4 3030 7290 0,42 5 7890 4830 1,63 6 4540 9250 0,49 7 7590 5180 1,47 8 9260 5730 1,62 9 12740 9280 1,37

I0 11870 8890 1,34 II 6240 8290 0,75 12 11250 6670 1,69 13 3100 6370 0,49 14 3150 7530 0,42 15 3410 8990 0,38 16 3320 9190 0,36 17 10050 5270 1,90 18 6490 7370 0,88 19 7070 7780 0,91

SECTION 3

No. Fe (pprn ) Mn(ppm)

1 0 180 2 60 0 3 20 0 4 470 90 5 0 770 6 280 0 7 280 2890 8 450 790 9 320 150

I0 0 1640 ii 0 6020 12 700 0 13 700 1230 14 II0 40

SECTION 2

No. F e ( p p m ) Mn(ppm) F~Mn

1 7830 5510 1,42 2 3830 8510 0,45 3 1790 7180 0,25 4 5140 5580 0,92 5 4000 5490 0,73 6 3440 4400 0,78 7 10720 4420 2,43 8 9130 11860 0,77 9 7910 4730 1,67

I0 5050 6190 0,82 II 6210 4660 1,33 12 7600 4990 1,52 13 7500 4090 1,83 14 10450 8500 1,23 15 6990 4530 1,54 16 8370 3350 2,50 17 11390 6420 1,77 18 9870 10270 0,96 19 4210 8690 0:48 20 8890 12080 0,74 21 4110 4990 0,82 22 5510 4440 1,24 23 2280 7330 0,31 24 15410 3280 4,70 25 12870 4130 3,12 26 17640 3930 4,49 27 18310 4820 3,80 28 14800 4720 3,14 29 15490 5180 2,99 30 15690 5700 2,75 31 830 1230 0,67 32 760 2150 0,35 33 740 180 4,11 34 2580 3610 0,71

Tab. 1. Fe and Mn contents with ratios of the carbonate cements (cf. Fig. 10 and 11).

412 H. FLICK et al.

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0 - -

A B

1 2 3 4 5 6 7 8 9 10 11 12 13 14

SECTION 3

Fig. 11. Fe and Mn contents with ratios, generations 2b and 3 of sample D (Fig. 9D). Dense shading = no luminescence, no shading = bright luminescence.

due to the abundance of coarse particles of lapilli size and the rapid depositional process. In addition, the heat capacity was high which elsewhere has resulted in welding of ash flow deposits even in a submarine environment (LOWMAN & BLOXAM, 1981).

As a consequence of the conditions outlined previously, diagenetic processes started early during rapid burial which were driven by a strong compaction stream. Tubes and streaks as well as the carbonate cementation allow the paths of solutions during seeping to be traced. The first three stages of diagenesis depended on near surface conditions, indicating that their formation was earlier than the deposition of the following pyroclastic or sedimentary unit. Generation four and five are indicative of conditions of deeper burial.

The diagenetic development can be reconstructed as follows:

Stage 1) Alteration of the pyroclasts led to a release of certain elements, among these Fe, Si and Ca, a pro-

cess which was accelerated by fairly high temperatures. This was especially dominant in the upper part of the section where the particles were more or less completely replaced by carbonate. Lower within the section car- bonatization occurred within discrete horizons with incomplete replacement of the particles. Reasonable explanations include: (a) the compaction stream was preferentially loaded with ions in the upper part; (b) porosity was high because of the low compaction at the top; (c) the compaction stream increased to the top; and (d) access of minor amounts of sea water was possible just in the upper part.

Depletion of Fe occurred in the coarse-grained beds of the upper part whereas it is enriched in the fine- grained ash layers. The process of replacement took place within a reducing environment because Fe is mobile only in the ferrous state. This is confirmed by cathodoluminescence which indicates a strong reducing environment for the replacing carbonate (Fig. 9B-D).


Stage 2) Spheroides (cement 2a) grew on top of the particles in a strong reducing environment (Fig. 9A + B, Fig. 10).

Calcite or Mg-calcite skalenoeders (cement 2b) grew instead of the spheroides in the highest portion of the section (Fig. 9D, Fig. 11), where the influence of sea water lowered the temperatures and raised the redox potential. Beds of detrital fossiliferous limestone in the top part of the section which were impregnated by the iron contain few pyroclastic particles. They show sack- ing and solution cavities.

Stage 3) A decrease of the compaction stream allowed oxygen rich sea water to penetrate from above and ox- idize the upper part of the section giving rise to violet staining. Oxidation is shown by the clear, non luminescing cement which filled the open cavities (Fig. 9B-D, Fig. 11).

Stage 4) Further burial initiated by ]ater sedimentation on top of the pyroclastic sequence produced a cement which filled open spaces during uniformly weak reducing conditions (Fig. 9A and C, Fig. 10).

Stage 5) Finally, the remaining few cavities were filled during deeper burial which is indicated by the strong reducing environment (Fig. 9B).

Stage 6) Fissures which developed following the diagenetic stages (probably in the course of Hercynian deformation) cut all the earlier generations of carbonate cements (Fig. 9A and C).

4. Discussion: Formation of the Lahn-Dill type iron ore

Various rather contradictory concepts on the formation of the Lahn-Dill type iron-ores have been publish- ed during the decades of investigation accompanying mining activity. As the discussion shall not be repeated here, see QUADE (1970, 1976), ROSLER & WERNER (1979) or BOTTKE (1981) for compilation. Generally, a direct magmatic descent of ore bearing hydrothermal fluids or exhalations was considered. However, investigations in the type region, especially those presented here, invoke iron ore genesis by diagenetic seeping of the pyroclastic sequence during secondary alterations. Similar ideas have been put forward earlier (HENTSCHEL, 1960; ROSLER, 1964), but these ac- counts lacked the detailed evidence to support such views.

The formation of the Lahn-Dill type iron ore (in- tegrated with the results from G~insberg) can be outlined as follows:

1) Ore formation is dependent on the underlying thickness of the volcanic sequence with preferential occurrence in association with thick pyroclastic units

(QUADE, 1970, 1976). These bodies therefore mark the flanks and not the top of the volcanic edifices as can be concluded from facies reconstruction (Fig. 2). Thus, the central facies were avoided as they comprise the least porous sections.

2) Mobilization of elements accompanied alteration of the pyroclasts (cf. WEDEPOHL, 1988) which can be correlated with calcite generations 1 and 2. An enrichment of Ca is paralleled by the depletion of Fe and Si (Fig. 5). A replacement of the particles by carbonates is especially high in the upper part of the unit. The loss of Fe in certain horizons was so substantial that later oxidization did not give rise to staining during stage 3 of cementation. Such horizons appear to result from bleaching and are known from other localities as well (L,PPn~T, 1951). This staining by oxidization gave rise to the violet colour of the pyroclastic sequence below the ore horizon. In the parts rather depleted of Fe at G~nsberg this violet staining makes tubes and streaks conspicuous which mark the paths of the compaction stream.

3) The iron bearing solutions which emerged upon the sea-floor were directly oxidized giving rise to massive ores with silica as the main gangue phase. Locally, as in the exposed section of the G~nsberg (the m o r e profitable massive o r e s have been mined away), Fe-impregnated fossiliferous limestones or Fe-enriched overlying ash layers demonstrate the dependence on the permeability.

4) Mobilization and fixation of Fe during the diagenetic stages 1 to 3 is confirmed by the development of the carbonate where the amount of substitution of Fe and Mn for Ca in the structure allows the estimation of the redox potentials throughout the pyroclastic sequence. Whereas stages 1 and 2 exhibit strong reducing environments in the course of stage 3 oxidization penetrated from above and resulted in violet staining of the upper part of the sequence. The absence of Fe and Mn substitution during carbonate cementation of stage 3 indicates that these elements were not mobile inside this stained part of the pyroclastic sequence anymore. The development of cements 2 and 3 demonstrates that the course of diagenesis was under near surface conditions.

Sulfides related to Lahn-Dill type iron ore formation are described from the northern Rhenish Mountains by MOLLER et al. (1986) where relationships suggest a diagenetic origin comparable to the results presented here. These deposits belong to a succession of pyroclastic rocks which can be assigned to a distal facies (cf. Fig. 2). The ores occur together with bituminous shales indicating a reducing environment within a local anoxic basin.

Recent oceanic enviromnents demonstrate the ira-

414 H. FLICK et al.

portance of diagenetic alteration leading to spilitiza- t ion of the volcanic rocks (WEDEPOHL, 1988). Thereby the process of seeping gives an effective mechanism for the mobilization of certain elements and driving them out at restricted localities such as smokers and chimneys (BEAUCHAMP et al., 1989; HOVLAND et al., 1987; KULM et al., 1986). The diagenetic development discernible at G~insberg corresponds with such seeping processes. Element con- centrations and element ratios depend on parameters like temperature and water/rock ratio (WEDEPOHL, 1988; ERZINGER, 1989). Thereby, the formation of the Lahn-Dill type iron ores fit best to a saline dominated environment which coincides with a high porosity indicated at G~insberg. The temperatures were distinctly lower than to produce sulfide smokers, according to oxygen isotope data less than 150 ~ (WEDEPOHL, 1988). On the other hand, temperatures were more elevated than 30 to 40~ described for the ,,cauliflower garden,, silica chimneys with very low metal content (HERZIG et al., 1988).

Alteration processes within the recent oceanic crust are considered as being driven by the circulation of ocean water guided through fissures and cracks in the rocks. This model has been applied by WEDEPOHL et al. (1983) and MIJLLER & STRAUSS (1984) for the Lahn-Dill area. However, it conflicts with the local

geological environment for the following reasons. Thick successions of pelitic rocks below the votcanics prevent circulation of water within the uppermost strata. The size of the volcanic edificies alone was too small for separate convection cells, and interior circula- t ion was hindered furthermore by fine-grained pyroclastic intercalations. It has been shown in this study that alteration started shortly after deposition of each volcanic unit and was most abundant within the pyroclastic rocks. Thus, &watering during compac- t ion seems the best fit for providing the driving forces of diagenetic alterations in the vokanics of the Lahn- Dill area.

A c k n o w l e d g m e n t s

Investigations gained support by a grant from the DFG (German Science Foundations). Cathodoluminescence was kindly provided by J. Mehl (Forschungsstelle inter- diszipliniire Paleontologic, Erlangen), and interpretation of the results was improved through discussions with W.-C. Dullo (Erlangen). Microprobe analyses were kindly provided by S. Meisl and M. Susic (Wiesbaden). The text was improved by L. Warr (Heidelberg).

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iron ore of the lahn-dill type formed by diagenetic seeping of pyroclastic sequences — a case...

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