The Deposits of Argentiferous Galena within the Bellerophon Formation (Upper Permian) of the Southern Alps* Carlo Brusca, Gabor Dessau, M. Leroy Jensen, and Giuliano Perna 1. Introduction During the past several years we have been engaged in the study of the argentiferous galena deposits of the Bellerophon formation. Our main results are included in this paper, while a more detailed paper is in pre-paration. There are two main hypotheses on the origin of these deposits: a magmatic-hydrothermal genesis, with metasomatic replacement of a sedimentary matrix (P o š e p n y , 1880; Andreatta, 1949; di C o 1 -bertaldo-Nardin, 1964; Morteani, 1965); and a sedimentary genesis, probably exhalative-sedimentary (Canaval, 1912; Trener, 1914; Tornquist, 1931; Munch, 1958; Maucher, 1959). For the diseussion of the literature we refer to a paper by Dessau and Perna (1968). 2. The geological background In the Bolzano area the "Bozen porphyric platform", a complex series of acid extrusive rocks of Permian age**, rests on the crystalline basement and reaches a thickness of over 2000 metres. At increasing distances from the centre the thickness decreases, as it is evident from the isopach map (Baccos, Brondi, Perna, 1971; from this paper Fig. 1 is re^-produced). The sedimentary series, which lies upon the "Bozen porphyric platform", begins with the "Grodener sandstone" (Middle Permian), which has a greater area and its maximum thickness in the Judicarian Alps to the SW, and in Carnia to the east. Another area where it attains a great thickness is at the centre of the Porphyric platform, where it is depressed because of volcano-tectonic collapse (Baccos, Brondi, Perna, 1971; Fig. 1). * Research carried out with the financial assistance of the Italian Council of Scientific Research and of the Autonomous Region Trentino-Alto Adige, and of the Laboratory of Isotopic Geology, University of Utah, Salt Lake City, Utah. ** In the following, for the sake of brevity, often called "Porphyric platform", "porphyric rocks" or "Porphyries". The Continental and deltaic surroundings, in which the Grodener sandstone was deposited, change to a partly lagoonal and partly shallow marine environment with the Bellerophon formation (Isopach and ore deposit map, Fig. 2). It reaches its greatest thickness in Carnia, where it is possible to distinguish (Selli, 1963) three complexes: the lower one composed of gypsum, argillite, marl, marly breccia, cellular dolomite; the middle one mainly of cellular marly breccia with dolomitic cement and of cellular dolomite with intercalated marly beds; and the upper one >of dark grey, slightly bituminous, well stratified limestone. The thickness decreases westward, where it changes to the so-called "Badiota facies" (Accordi, 1958, 1959), composed in its lower part of marl and gypsiferous sandstone, whereas its upper part is of neritic facies, with beds of bituminous limestone, marl, and dark grey limestone. Towards its western end, where its thickness is decreasing gradually to nil, the Bellerophon formation is of "Fiemmazza facies": above the cellular dolomite and the gypsiferous sandstone of the lower complex, t here are thick layers of calcareous dolomite with beds of oolitic texture, deposited in a lagoonal-evaporitic environment, as confirmed by small lenses of gypsum and tiny idiomorphous quartz crystals. This description, drawn mainly from the geological literature, is very much generalized. There are obviously varieties and exceptions, as can be expected in the čase of Continental, lagoonal and shallow sea sediments, transgressive over a continent of strong relief. In Chapter 4 we shall describe in detail a local series of reduced thickness, belonging to the "Fiemmazza facies". 3. The ore deposits Within the boundaries of Italy, rare and insignificant mineral occurrences are known also1 in places where the Bellerophon formation has considerable thickness. But the two areas of main interest, as the seat of important mining in the past and of recent exploratory activity, are those north-east of Trento (Monte Calisio and Lavis) and of the upper Val di Non, near Proves and Marcena, where the Bellerophon formation, in "Fiemmazza facies", has a thickness of not more than 50 metres (Fig. 2). In Slovenia, mineralization is evident within the Bellerophon formation, and from the limited bibliography we had at our disposal, we estimate that the paleogeographic situation may be similar to that prevailing in the Trento area. The Italian deposits have been exploited for argentiferous galena, which is accompanied by chalcopyrite, pyrite and marcasite, probably As-rich fahlerz, ali very scarce. The sphalerite is always anhedral and sometimes shows concretionary textures. It is characterized by a very low iron content, as revealed by the light-coloured internal reflexes and by the spectrographic analyses. A sulphosalt is found in trace amounts, most probably a lead sulphoantimonite, as confirmed by the optical characteristics and by the trace elements detected when analyzing the galena. Only at Transacqua is a sulphosalt important, which has been Fig. 1. Isopachs of the "Grodener sandstone" and of its mineral deposits tentatively classified by us (D e s s a u and P e r n a , 1968) as belonging to the Boulangerite-Falkmanite group. Our spectrographic analyses for trace elements have shown that on the average the galenas contain 800 ppm of Ag, traces of Ni, a charac-teristic and always present percentage of antimony of about 3000 ppm, and As varying between traces and a few hundreds of ppm. Sn is present only in small traces, and Bi and Co are absent. Less meaningful are our few analyses of sphalerites; noteworthy is the low content of Cd, often below 1000 ppm. We have not obtained the extremely high measurements in silver, up to 10 and even'19 kg in a metric ton of lead, mentioned in ali previous literature, and which certainly refer to local supergene enrichment. Neither are we able to specify which is the "silver bearer" in the galena we examined and which, as already stated, is not very high in silver. The isotopic composition of the sulphur of galena, sphalerite and pyrite is normally characterized by low positive values of <5S84, averaging about + 2,5 %o, but with occasional aberrant values. Some peripheral deposits, however, exhibit negative and strongly variable values, between —0,10 and —13 %o. Barite and gypsum show strongly positive values, between + 9 and +14%o. For the sake of comparison, isotopic analyses have been done on the sulphides of the hydrothermal ore veins within the Porphyric platform, and of the "Tregiovo slates". The latter are faintly mineralized with galena and sphalerite and represent a lacustrine episode interlayered with the Upper volcanic complex of the platform. The values of <5S34 for the sulphides of the ore veins of Quadrate and Nogare are very near to +5,5%o, whereas the sulphides of the Tregiovo slates are strongly variable. The ores of the Bellerophon formation generally are devoid of any gangue, with the exception of barite, which is sometimes present. Only in the Monte Calisio area and at Transacqua is barite abundant and generally crystallized in great blades. It has been exploited in the past. At Transacqua siderite is abundant. The sulphide^bearing ročk is a calcareous dolomite. In this section the more or less idiomorphic grains of dolomite vith the space between them occupied by calcite can be noticed. Ferriferous carbonates and sparse large oolites are present. As far as the small-scale textures are concerned, the galena occurs in clouds of small grains disseminated in the calcareous-dolomitic ročk, Fig. 2. Map of isopachs of the Bellerophon formation and of its ore deposits Explanation of Figure 2 Val di Non: 1 Rio del Bavaro, 2 Marcena, 3 Proves and Lauregno, 4 Forcella di Brez Lavis: 5 Maso Furli Monte Corona: 6 Faedo, 7 Masen Monte Calisio: S Agli Orti and Maso Saracino, 9 Doss delle Grave, Santa Colomba and Monte Piano Other areas: 10 Roncogno, 11 Povo and Villazzano, 12 Transacqua and Valle Uneda, 13 Passo di Oeclini (?), 14 Ortisei (?), 15 Telpeit e Pieeolino, 16 Montauro, 17 Calalzo (?), 18 Passo Mauria, 19 Val Bartolo, 20 Passo del Zovo (?), 21 S. Lucia and in tiny cross-cutting veins. Typical replacement textures are evident. Relatively big expanses of galena have completely replaced the host ročk, with the exception of small idiomorphic quartz grains, which are now completely surrounded by ore. Elsewhere some of the concentric layers of the oolites have been selectively replaced by sphalerite and galena (Fig. 3). Sometimes the sphalerite appears to be definitely subsequent to the galena. Supergene minerals are present in traces. Microscopic studies on the ores of the Bellerophon formation have been published by Munch (1958), Maucher (1959) and d i Col-bertaldo> and Nardin (1964); according to the latter the sphalerite is of marmatitic type. In the rare outcrops, almost always without economic interest, where the Bellerophon formation rests on the phvllites of the basement, copper ores seem comparatively more abundant. 4. The ore deposit of Lavis A description of an area recently explored, near Lavis 9 km south of Trento, provides a better understanding of the characteristic features of the ore deposits within the Bellerophon formation. At Lavis, the Societa Fig. 3. Maso Furli (Lavis) — Polished section, polarizer only, 100 X. Great area of galena (partly altered to cerussite) which replaces the carbonatic ročk. The galena is locally idiomorphic and rimmed by sphalerite. Carbonatic oolites are partly or wholly replaced by galena and sphalerite Fig. 4. Monte Rosa — Conglomerate of porphyric gravel cemented by iron-bearing dolomite, and transgressive on the Bellerophon formation Mineraria e Metallurgica di Pertusola has carried out, during the years 1967 and 1968, a detailed geological &urvey, followed by a drilling program (14 holes with continuous coring and 27 with recovery of the cuttings), and finally by an ore-dressing test on 200 tons of material. Unfortunately the low reserves have prevented further developments. Attention had been drawn to this area by an outcrop of Bellerophon formation, carrying galena and sphalerite, on the eastern flank of the small hill of Maso Furli. This area corresponds to a paleogeographic high. disclosed by the thinning of ali Permo-Triassic sediments from the trans-gression surface upwards. The leading hypothesis has indeed been that the metallic ions found in the Bellerophon formation originated from the degradation of the volcanics of the Porphyric platform; thus the area of Lawis, placed on the border of a buried hill of these volcanic rocks, should have been specially favourable to the deposition of ore-bodies of economic interest. In this area the Bellerophon formation crops out along a belt striking NNE—SSW and dips in a WNW-direction belov the Werfenian strata, which are discontinuously covered by fluvioglacial deposits. Immediately to the east, the quartz porphyries crop out. We consider of importance in regard to the genesis: 1. The absence of the Grodener sandstone. EE3 ^ EvfffrJ 2a Fig. 5. Geological sketch map of the Lavis area Explanation of Figures 5, 5a, and 5b 1 Werfenian 2 Bellerophon formation 2a Conglomerate of porphyry pebbles with fossiliferous dolomitic-ferruginous cement 3 "Grodener sandstone" 4 Quartz porphyries 5 Ore body S Cored bore holes 2. The local appearance of a conglomerate just below the Bellerophon formation, which in its proximity contains barite. The conglomerate is composed of coarse (1 -i- 50 cm) rounded porphyry gravel, cemented by an iron-bearing dolomite (Fig. 4) containing fragments of lamellibranchs*. 3. The transgression of the Werfenian, immediately to the north, di-rectly over the Porphyries (Figs. 5, 5a and 5b. 4. The presence, in the Maso^ Furli hill, of tabular blocks of porphyry buried in the Bellerophon formation (Fig. 6). * Further to the north-east, on the western slope of Monte Rosa, there is another outcrop of porphyric conglomerate, which however must be of Wer-fenian age, on account of the presence in the cement of Holopella gracilior. pressano /o o O-V 'O O O cl OOOOrf\ Po ooo' \oo.oo X Y Fig. 5a. Geological section NW—SE of the Lavfs area Please see the explanation on the opposite page Fig. 5b. Geological section N—S of the Lavis area Please see the explanation on the opposite page Ali these facts, together with the presence of NW—SE striking faults*, suggest a complex paleogeography formed by the interference of two structures, viz.: 1. A main structure, striking NNE—SSW, has given rise, during the deposition of Bellerophon formation and Werfenian, to a cliff on the edge of the volcanic rocks to' the east. Blocks from this cliff, falling into' the sea, were then embedded in the Bellerophon formation. 2. A secondary order of structures, striking NW—SE, manifests itself in the alternating raised and lowered blocks aligned transversaly to1 the cliff. In the trenches, the pebbles of Continental origin were collecting. The uplifts were covered only at a later time by the Werfenian deposits. The many bore-holes have provided a very detailed reconstruction of the paleogeography and the clarification of the relationships between the latter and the ore bodies. A buried positive structure has been located, striking NW—SE and therefore parallel to the seco\ ® V ® 68-2 68 12 \\\\ ® 66-11 Fig. 7. Isopach maps of "Grodener sandstone" (a) and Bellerophon formation (b) in the Lavis area Facies of Bellerophon formation as encountered by the bore-holes: A Carbonatic-sandy facies, barren B—C Mainly carbonatic facies, ore-bearing C Transitional facies, ore-bearing C Mainly marly-clayey facies, practically barren D Bellerophon formation missing On the other hand, the regional spread and the stratigraphic persistence of the ore-bearing horizon are considered by us to be a control of a sedimentary deposition. In addition, the lagoonal, moderately euxinic environment, with precipitation of the first terms of the evaporitic series (dolomite, gypsum, anhydrite), seems favourable for the deposition of galena. What has to be explained. are the sources both of the metallic ions and of the sulphur of the sulphides. As far as the first are concerned, one can assume either a leaching of the rocks of the Porphyric platform and of their mineral deposits, or a supply by thermo-mineral springs issuing at the bottom of the sea. In favour of the first hypothesis stands the fact, that there were por-phyric land-surfaces (Monte Rosa near Lavis and Monte Luco in the upper Val di Non) during the deposition of the Bellerophon formation, and near to the areas containing metal minerals. Moreover the trace--element content and the 5S34 composition of the sulphides of the Bellerophon formation show analogies with the corresponding values for the sulphides of the hydrothermal veins in the Porphyri.es. We, however, do not attribute too much probative value to this control, because of the possibility of changes during solution transport and re-deposition. In regard to the second hypothesis, there is an equal lack of direct evidence. But an exhalative-sedimentary origin, already suggested by several authors beginning with Trener (1914) and C a naval (1916), would be in agreement with the general characteristics of these deposits and especially with the isotopic composition of the sulphides. At any rate, such an important volcanic activity, as the Permian one in our area, may weTl have had a long hydrothermal "tail"; it may also be remembered, that in the neighbouring district of Predazzo there has been a resumption of volcanic activity during Triassic times. For the greater part of the deposits of the Bellerophon formation examined, the dS34 of the ores is about + 2,5 %o, with small fluctuations of this average. This is strong evidence in favour of a direct magmatic origin, at least for the sulphur. One has, however, stili to explain the values of <5S34 of the ores of the upper Val di Non and of S. Lucia, values which are almost entirely negative, and varying over a wide range. These two features are strongly suggestive of a microbiological origin, through reduction of sulphates by sulphate reducing bacteria. The environment where the last mentioned deposits were formed, some distance from the likely center of activity of the submarine sulphurous springs, was probably out of their reach, and more favourable to the thriving of the sulphate-reducing micro-organisms, on account of lower temperatures and diluted solutions. Amongst the various hypotheses, while acknowledging the difficulty of a proper choice, we are in favour, as far as the metallic ions are con-cemed, of an origin by meteoric leaching of the volcanic rocks of the Bozen Porphyric platform. Even disregarding the circumstance, that no feeder channels of the supposed submarine thermo-mineral springs have been found, we consider circumstancial evidence that metallic sulphides appear, even if exceptionally, in the Werfenian where, as for instance to the north of Lavis, it is transgressive over the Porphyries and in the same facies as the ore-bearing horizons of the Bellerophon formation. As far as the source of the sulphur of the ores is concerned, according to the present state of knowledge about sulphur isotopes, whereas in Val di Non and at S. Lucia it derived from biochemical processes, in the other areas it is of inorganic origin, and most probably supplied to the se-dimentary basins by sulphurous springs. It may be noted, that such springs are known in the district even to-day. * 6. Acknovvledgments We have made use of unpublished data from the archives of the Bureau of Mineš of Trento, and we thankfully acknowledge the permission granted by the Societa Mineraria e Metallurgica di Pertusola to discTose information about their exploration work at Lavis. Explanation of Fig. 8 A Carbonatic-sandy facies, found near to the Permian Quartz porphyry highs B Mainly carbonatic facies, bordering facies "A" C Mainly marly-clayey facies, found in depressed or rapidly subsident areas A-l Werfenian A-2 Bellerophon formation: Sandy limestone with detrital elements (quartz and glauconite) and gypsum veins. Sparse nodules of barite A-3 Red dolomite with glauconite grains. Sparse gypsum nodules A-4 Cellular grey marly dolomite A-5 Light grey dolomite containing thin detrital beds with clastic smoky quartz; upwards the ročk gradually changes to red sandy marl A-6 "Grodener sandstone": Red sandy marl alternating with thin sandy layers, green at the bottom A-7 White sandstone with clastic smoky quartz. This complex grows micaceous and more grey downwards A-S Quartz porphyries B-l Werfenian B-2 Bellerophon formation: Grey dolomitic limestone with many thin black marly partings B-3 Grey dolomite with very abundant grains and veinlets of galena, sphalerite, pyrite and chaleopyrite B-4 Light grey cellular dolomite, with thin beds of microbreccia, containing small vegetable fragments. These beds bear galena in scattered spots B-5 Arkose with light-coloured fragments of Porphyries. Downwards the arkose gradually changes to a conglomerate of limestone pebbles B-6 Sandy altered porphyry B-7 Quartz porphyries C-l Werfenian C-2 Bellerophon formation: Alternations of intraformational glauconitic breccia and of marl and marly dolomite C-3 Alternations of grey and pink marly, clayey and dolomitic beds, with nodules, veins and thin strata of gypsum C-4 Grey, oolithic, the upper, lagoonal section of the Bellerophon formation, and for their greater part are found where the sedimentary series thin out above or in the vicinity of the volcanic rocks of the "Bozen porphyric platform". Microscopic examination reveals re-placement textures, which are, however, attributed to diagenetic phenomena. The stratigraphic persistence is considered evidence for a sedimentary deposition. The metallic ions could have been derived either from the meteoric leaching of the volcanic rocks and of the ore veins which they contain, or from submarine thermo-mineral springs (exhalative-sedimentary origin); the authors incline toward the first of the two hypotheses. The isotopic composition of the sulphur of the ores is suggestive of a magmatic source, it is thought, from submarine sulphurous springs; only in peripheral areas does the isotopic composition suggest a microbiological origin through reduction of sulphates. RI ASSUNTO La «Formazione a Bellerophon», del Permiano superiore, oompren-dente calcari, calcari marnosi, dolomie e gessi, affiora su vasta estensione nelle Alpi Orientali italiane. In molte localita essa risulta mineralizzata a galena argentifera, ed ha dato luogo in passato ad un'intensa attivita estrattiva, in particolare nella zona del Monte Calisio a NE di Trento. Le ricerche minerarie e gli studi degli ultimi anni hanno permesso di perfezionare le conoscenze sulla mineralizzazione, che e a galena argenti- fera prevalente, cui si accompagna spesso la barite; piu rare e sporadiche sono blenda, siderite, calcopirite, pirite e marcasite, tetraedrite e solfo-antimoniti di piombo. Vengono esposti i risultati degli studi stratigrafici, paleogeografici, mi-nerografici e delle analisi degli elementi traccia dei solfuri ed isotopiche dello zolfo, che hanno portato a formulare differenti ipotesi sulla genesi delle mineralizzazioni. Queste ultime sono confinate al livello superiore, lagunare, della For-mazione a Bellerophon, e sono in gran prevalenza concentrate dove la serie si assottiglia, al di sopra o in vicinanza delle vulcaniti della «Piatta-forma porfirica atesina». Per quanto studi minerografici rivelino strutture metasomatiche — attribuibili pero a fenomeni diagenetici —- la costanza stratigrafica viene considerata prova di deposizione sedimentaria. Gli ioni metallici potrebbero provenire o da dilavamento meteorico delle vulcaniti e dei filoni che queste contenevano, oppure da sorgenti termo-minerali sottomarine (origine esalativo-sedimentaria); ma gli autori propendono' per la prima ipotesi. La composizione isotopica dello* zolfo dei solfuri e generalmente a favore di una provenienza magmatica dello zolfo, da sorgenti solfidriche sottomarine; solo in zone periferiche la composizione isotopica suggerisce che lo zolfo dei solfuri abbia origine dalla riduzione microbiologica di solfati. DISCUSSION Di Colbertaldo: Se bene ho capito, dal punto di vista genetico, voi siete piu propensi a ritenere una origine di questi giacimenti, situati nel Per-miano superiore, da lisciviazione della piattaforma porfirica e dei filoni eventualmente presenti nella piattaforma porfirica. Pero desidero far osservare che il calcare a Bellerophon non e mineralizzato soltanto in Trentino, dove ha forse la maggiore estensione, ma anche nella Val Bar-tolo presso' Tarvisio. Nella Val Bartolo la manifestazione e a piombo, zinco e rame; in vicinanza si trovano anche masse di calcite bianca, spatica, molto fetida, molto> odorante di acido solfidrico. In questa zona non esiste alcuna manifestazione eruttiva a cui legare una mineralizzazione. Ora, come nelle Alpi noi abbiamo molti livelli mineralizzati che vanno dal Trentino al con-fine col Austria, Jugoslavia, e nell'interno di questi due Stati, vien di pensare che per uno stesso livello mineralizzato noi dovremo invocare uno stesso tipo di genesi. Ecco perche io mi trovo imbarazzato neH'accettare una origine per lisciviazione di mineralizzazioni preesistenti nella piattaforma porfirica Trentina, quando invece nel Tarvisiano mancano questi elementi a cui riferire un analogo tipo di mineralizzazione. Dessau: I well understand what Professor di Colbertaldo wants to teli us. It is also shown on Fig. 2 of our paper that in Val Bartolo, in the vicinity of the boundaries with Austria and Yugoslavia, there is another small ore deposit within the Bellerophon formation; I believe I also mentioned it during my talk. This small deposit is very far away from the Bozen porphyric platform or, more generally, it is far away from outcrops of eruptive rocks. As it is difficult to> admit a different origin for this deposit with respect to the other ones of the main area, this would be evidence in favour of the sedimentary origin of ali the deposits. My answer to Professor di Colbertaldo is that we really have not been able to make up finally our minds between the two possible origins. We have stressed in our paper that we are stili doubtful about this subject, although we give a slight preference to the hypothesis of the origin of the metallic ions from the weathering of the porphyric platform. Maucher: Wenn ich Sie richtig verstanden habe, dann sind Sie der Ansicht, daB die Erze im Wesentlichen sedimentiert wurden, und daB der Erzbestand aus der Verwitterung der Quarzporphyre kommt. Fiir diese Hvpothese spricht jawohl auch die Zusammensetzung der Schwefelisotope, die auf der einen Seite in der Nahe von Quarzporphyren mehr den Cha-rakter von vulkanischem Schwefel, oder magmatischem Schwefel hat, und je weiter wir uns davon entfernen, um so> mehr veranderten S34 Gehalt zeigt. Wenn ich Herrn Professor di Colbertaldo richtig verstanden habe, so sagte er, die Erze konnten nicht von den Quarzporphyren stammen, da in diesem Gebiet bei Tarvis die Quarzporphyre fehlen. Nun, ich glaube, man kann folgendes sagen. Das Milieu des Bellerophonkalkes mit seiner salinaren Fazies ist ein Milieu, in dem Metallgehalte, wenn sie ins Meer gelangen, sedimentiert werden. Ob diese Metallgehalte aus der Vervvitte-rung von Quarzporphyren stammen oder aus der Abtragung der Erzgange im Quarzporphyr, oder ob die Verwitterungslosungen aus den karbonischen Erzgangen stammen, wie Sie ja gerade in den Karnischen Alpen gezeigt haben, ist dafiir vollkommen gleichgiiltig. Ich glaube, wir sollten uns zu-nachst einmal einfach dariiber unterhalten, daB das Sedimentationsmilieu, die Fazies des Bellerophons eine salinare Fazies ist, die gerade fiir solche Ausscheidungen von Metallen giinstig ist. Und zwar eine besondere karbo-natische Fazies, in der hauptsachlich das Blei fallt, sehr wenig das Kupfer und sehr wenig das Zink. Erst in dem Augenblick, so war wenigstens u.nsere Beobachtung, wenn die Kalke etwas mergeliger werden, wenn sie etwas toniger werden, wenn also das Sedimentationsmilieu sich andert, dann fallt auch mehr Kupfer. Es liegt also nicht nur am Angebot der Metallionen, sondern es liegt an der Fazies, in der sie ausgeschieden werden. Dafiir spricht auch Ihre Zinkblende, die kaum Cadmium enthalt, wahrend j a Zinkblenden in den Porphyrgangen noch hohe Cadmium-gehalte haben. Die entscheidende Frage ist doch einfach die nach dem Milieu. Der Nachweis, wo die Metallionen hergekommen sind, ob nun aus einer Verwitterung des Grodener Sandsteines, aus einer Venvitterung des Porphyrs, oder aus der des karnischen Gebietes, das, glaube ich, ist sehr schvver zu entscheiden. Das konnte man vielleicht mit Spurenelementen machen, aber auch da ist wieder die Schwierigkeit, daB auch die Spuren-elemente nicht nach dem Angebot fallen, sondern nach dem Milieu. Wir miissen uns immer dariiber klar sein, daB die Fallungsbedingungen das 12 — Geologija 15 177 Wesentliche sind und nicht das Losungsangebot allein. Es kann nattirlich nur das fallen, was in der Losung ist, aber es muB nicht fallen, was in der Losung ist. Insofern ist, glaube ich, die Frage, oh der Porphyr in der Nahe ist oder nicht in der Nahe ist, fiir die Erz-Ausscheidung als solche gar nicht so wesentlich. Dessau: Wenn ich in Kiirze darf, mochte ich sagen, daB wir einfach mit dem, was Professor M a u c h e r gesagt hat, vollig einverstanden sind. Ich errinere mich an alte Arbeiten iiber Oberschlesien, wo es bewiesen ist, wie sehr eine Salzzirkulation fiir den Transport und die Fallung besonders von Bleiglanz wichtig ist. tiber den Ursprung der Metallionen sind wir eigentlich zu keinem sicheren SchluB gekommen. Wir haben aber in Betracht gezogen, daB die Zuhorer im allgemeinen doch am Ende wissen wollen »Was denkt Ihr eigentlich daruber?«. Auch fiir uns sind die Fal-lungsbedingungen viel wichtiger als die Herkunft. Was die Herkunft betrifft, ist es auch bemerkenswert, daB dort, wo die Porphyre: fehlen und die Bellerophon-Formation auf Altkrystallin zu liegen kommt, die Erze kupferreicher und weniger bleihaltig sind, also von dem anstehenden Ge-stein beeinfluBt worden sind. Mittempergher: I quite agree with the explanation of Professor M a u -cher, that the Bellerophon beds may be very favourable host rocks; but we have also a lot of geochemical evidence about the backgroung of lead in the quartz porphyry and also in the Carboniferous beds. The Carboniferous beds in the eastern part are very rich in lead and zine, especially lead. There is a lot of smal1 mineralization, but also the back-ground is very high. At the other hand, in the Trento- and the Bo-zen areas, we have the quartz porphyry with very high backgroung of lead, and in the same area we have the lead mineralization not only in quartz porphyry, in the propylitic mineralizations of the Val Sarentino. Terlano, but also in the Tregiovo beds and in the upper part of the Groden sandstone. Here there are many places where we have syngenetic lead minerals. For this reason I believe that the Permian volcanics and the Carboniferous shales may be regarded partially as sources for the lead and zine contained in the Bellerophon mineralizations. Emberger: Une question sur les isotopes de soufre. Dans votre texte vous avez mentionne comme valeurs du rapport isotopique du soufre pour la galene des groupes de chiffres, des valeurs positives faibles et quelques valeurs negatives fortes. Je voudrais savoir si ces valeurs ont ete mesurees sur des echantillons presentant des conditions de gisement ou un aspect morphologique identique ou different? Dessau: Le collaborateur Professeur P e r n a confirme que les facies des echantillons sont exactement les memes; mais les echantillons avec les valeurs aberrantes viennent d'une region bien definie. Emberger: Je vous pose cette question parce que j'ai eu 1'occasion de 1'aire des etudes de ce genre sur des gisements marocains qui appartiennent en fait a la zone alpine et dans lesquelles j'ai constate que les variations importantes du rapport isotopique peuvent etre enregistrees la, ou il y avait des mineralisations voisines (a des distances de l'ordre du decimetre cu de la dizaine de metres), mais presentant des caracteristiques diffe-rentes (morphologie, roche encaissante, association minerale...). Les analyses ont parfois donne des resultats tres differents. Ainsi, de la galene dispersee en nuage dans le ciment d'un gres montre un ^S31 negatif tres faible tandis que de la galene en remplissage de fissure dans ce meme gres montre au contraire une valeur negative tres elevee. Cette difference de valeur traduit tres probablement des differences dans le mode de formation de ces deux types de galene.