Objetivo ESTABLISHMENT OF NEW EXPLORATION METHODOLOGIES CAPABLE OF LOCATING HIGH-GRADE ORE IN DEPOSITS OF MASSIVE SULPHIDES. THIS IS AN IMPORTANT REQUIREMENT OF EXPLORATION COMPANIES. The multidisciplinary studies conducted on the Chessy orebody and other volcanosedimentary mineral deposits in the Late Devonian Brevenne metamorphic belt have determined the main characteristics of the mineralization and of its setting, thus providing additional guides for regional exploration of base metal rich concentrations.Specific characteristics of the Brevenne volcanic hosted sulphide deposits can be summarized as follows:location of the orebodies within a thick felsic pile which was emitted during the first stage of ensialic rifting;sulphide deposition during short breaks in the submarine volcanic activity, which was proximal and effusive;absence of regional stratigraphic marker laterally to the massive sulphide deposits;homogenous lead isotope signatures of the main mineralizing event indicating similar deep seated sources, and probable contemporaneous hydrothermal systems, for the Chessy and Sain-Bel deposits (distinct, more radiogenic lead isotope signatures characterize the last, base metal poor mineralizing event);typical zonation of the hydrothermal systems which display, after metamorphic crystallizations, an outer chlorite biotite zone and an inner muscovite zone;concentration of the most intense hydrothermal transformations (with sulphide impregnation and stockworks) along highly deformed kilometre long corridors which correspond to former north south paleofractures (2 or 3 successive stages of hydrothermal activity along the same paleofracturation zone have been demonstrated at Chessy);concentration of base metals, barite and precious metals in stratiform massive sulphide bodies or layers which were deposited in local paloedepressions around the feeder zones (mineral and chemical ore zonation, well demonstrated at Chessy, displays a classical arrangement from proximal pyrite ore passing through polymetallic the distal barite sphalerite ore).The general approach which progressively prevails for the prospection of still undetecte d rich concentrations in old mining districts can be summarized as follows. Acquire a complete knowledge of the geological and structural evolution, reconstruct the regional models of ore deposition in their paleogeographic setting and then infer the specific markers of mineral concentrations which will be searched for using the most accurate scientific methods and exploration tools.The geodynamic environment of the Brevenne massive sulphide deposits has been more precisely recognized. The position of the basin close to a continent is acknowledged by the relative abundance, and the sialic component, of felsic volcanic rocks. The siltstone geochemistry shows that the bulk of detrital sedimentation came from a cratonized area, and that volcanic reworking played a minor role. Unexpected supplementary evidence came from recycled Archean zircons found in rhyodacite.Some efforts were made to identify, in the Brevenne belt, the remnants of an island arc related activity or the imprint of subduction on a back arc basin. From the above observations, constraints can be placed on island arc or back arc settings. The former should be of mature type, settled on a preexisting continental crust, the latter should be the product of rifting on a continent at the back of a distant arc, in response to subduction. The predominantly continental supply for pelites, the minor importance of epiclastic rocks and the lack of turbiditic deposits are not in favour of the arc hypothesis. Back arc identification should not be done on the sole basis of niobium and tantalum depletion of basalts, but from the occurrence of unusually depleted trace element contents.The diagnostic negative anomalies in niobium and tantalum are well documented in subduction related magmatism, but poorly explained. The presence in fusion restite of a niobium and tantalum bearing mineral such as ilmenorutile has been proposed and is controlled by redox parameters which are not necessarily restricted to subduction environments. It is proposed here that the geodynamic setting of the Brevenne belt is bound to a tensional episode of the Eo-Hercynian orogeny. The broken plate was a continental area previously cratonized and thickened during the early Devonian collision. This setting is similar to that of similar Late Devonian belts observed in the Vosges and Morvan where variable tholeiitic and calc alkaline affinities are related to variations in the crustal assimilation rate and in crustal thickness. None of these basins reached an oceanic stage and none required the direct intervention of subduction, but they probably opened in relation to movements along plate boundaries.The present layout of the belt came from later orogenic phenomena, but its elongated shape and parallel distribution of lithofacies cannot be the result of the sole tectonic closure. It is proposed that later structural features broadly followed primitive structures, which acted as guides from rifting to hydrothermal fluid flow, through fissural or ridge volcanism. The position of all the known sulphide deposits close to thrust faults, themselves broadly parallel to the belt trend, shows that structural controls of volcanic, hydrothermal and tectonic activities stayed the same for a long time. These early structures are those of rift opening, but may be older and related with a previous weakening of the continental crust during the Cambro-Ordovician distension.The Brevenne volcanosedimentary series, probably of late Devonian age, comprises bimodal, basaltic and rhyolitic volcanic rocks dominated by lavas and intrusions with subordinate siltstone. In the southeast, this series is thrust over the basement gneiss and migmatite of the Lyonnais basement. In contrast, to the north and northwest, its relations with the underlying, retromorphosed Affoux paragneiss and orthogneiss are still controversial being either overthrust of unconformable. The Saint-Laurent-de-Chamousset granite, dated at 318+10 Ma, intrudes the Brevenne on its northwestern edge separating it from the Violay group, which could be its northern equivalent.The piling up of lithotectonic units, composed mainly of quartz keratophyre, siltstone and basic volcanic rocks, separated by shear planes along which slices of basement occur, is the main feature of the Hercynian architecture of the Brevenne series. The limits of the various isopic zones coincide with those of the lithotectonic units. This arrangement underlines the major role played by these crustal dislocations. Having guided the extensional preorogenic evolution of the area, they could have been reactivated during the orogeny by thrusting. This thrust tectonic episode can be compared to the Eo-Carboniferous events of the Variscan orogeny.The direction of displacement is clearly shown by the roughly north east south west mineral lineation, and the shear criteria indicate a northeasterly displacement. This northeasterly thrusting fits poorly into recently proposed schemes of geodynamic evolution of the belt. The age of the deformation and the presence of slices of basement and serpentinite along the shear planes lead to interpretation of the se discontinuities as thrust, brought about by a compressive tectonic regime.The polymetallic of Chessy can be classified as a massive exhalative sulphide deposit.Its specific characteristics are as follows:During a quiescent period of the submarine volcanic activity, the main mineralized horizon was deposited in an eruptive rhyolitic to rhyodacitic cycle where volcanoclastic rocks are very rare.The barite ores are abundant in various stratigraphic positions. The elongation axis of the massive sulphide deposit probably corresponds to a corridor of early paleofractures. Basal mineralized breccias were probably centredon the highly permeable paleofracture zones.The feeding stockworks are numerous and generated several sulphide or barite lenses at various levels of the acid volcanic pile.The texture of the mineralized stockworks, the intensity of the hydrothermal alteration and the mineralogical composition make it possible to roughly define a zonality with internal stockworks containing silica, sericite, sulphides, barite and andalusite, and external stockworks containing chlorite, sulphides, silica, sericite and rare andalusite.The northern and northwestern extremities of the deposit are well delineated. Its closure is indicated by the thinness of the mineralized layer and the passage to barite facies with disseminated sulphides. The deposit is open to the south where the distal barite ores have not been identified. To the east, beyond the north north east south south west fault, the downthrown block beneath the Mesozoic sediments has not yet been investigated.Hydrothermal alterations related to the Chessy and Sain-Bel Paleozoic massive sulphide deposits have been studied through mineralogy, mineral chemistry and geochemistry, in order to identify potential markers for hidden massive sulphide deposits and to determine the effects of various later geological processes on early hydrothermal alteration. The purpose of this study was to provide early diagnosis of hydrothermally altered zones in a volcanosedimentary environment (ie to estimate their mineral potential and to focus exploration drilling on their most promising parts).The mineralogical and geochemical study of hydrothermal systems from the Saint-Antoine open pit and the Chessy mine suggests that numerous minerals can be used as guides for the exploration of massive sulphide deposits in other similar environments. The evolution of the chemical composition of these minerals has provided additional diagnostic markers for exploration, all of the which are related to the distance from the ore deposit and define hectometric haloes. These markers are the FM ratios of muscovite, biotite and phlogopite, and chlorite, the barium content of muscovite and the iron content of andalusite.These guides have been tested on 2 other hydrothermally altered zones and have allowed the determination of which of these zones has the greater degree of hydrothermal alteration.The results of this comparison indicate that:the hydrothermally altered rhyolites from Bernay have characteristics similar to those of the distal zone of Chessy (high FM ratio of biotite, low barium content of muscovite, iron bearing andalusite);the quartz sericite schists from Bully have characteristics similar to those of the proximal zones of Chessy and Sain-Bel, particularly marked in the Gruges area where barium rich muscovites (up to 4% barium) have been analysed.According to the results of the mineralogical and geochemical studies, it appears that the Bernay zone, despite the large volume of hy drothermally altered rocks, is an aborted system, None of the analysed minerals are representative of a proximal zone and recent drilling intersected only loose sulphide bearing stockworks. On the other hand, the Bully Gruges zone has remarkable analogies with the proximal zone of Chessy, thereby suggesting a high mineral potential which should be confirmed by future drilling.These 2 examples show the value of studying the chemical composition of hydrothermal minerals for the exploration of hidden massive sulphide deposits. Electron microprobe analysis of alteration minerals could also be applied, together with more classical geochemical and geophysical methods, in other volcanosedimentary belts, provided that typical hydrothermal zonations around known deposits are well characterized.The longitudinal section of the Chessy orebody integrates all the criteria concerning ore zonation and primary structures. True thicknesses of ore were used and the top of the deposit was considered to be horizontal. As fold axes (D2 tectonic stage) are parallel to this section and because the southern plunge of the orebody is no more than 8 degrees, its measured length (780 m) can be considered as similar to that of the initial deposit before Hercynian deformations. Maximum ore thicknesses (up to 40 m) are found in the vicinity of the 2 feeder pipes. Distal barite rich ore facies mark the northern extremity of the deposit. Southwards, it is still open despite a probable culmination of the footwall rhyolites which interrupted ore deposition. However, hydrothermal processes were still active in this area as indicated by numerous sulphide stringers in the footwall acid lavas, and the existence of southernmost massive sulphide lenses cannot be excluded. It should also be noted that drilling data in the southern part of the deposit are sparse and that numerous faults make reconstruction of the initial geometry of the orebody particularly difficult in this area.The extent, main facies and (true) thickness variations of the 2 ore sequences of the Chessy massive sulphide deposit can be represented. Because the ore intersections are projected on the horizontal plan of the development works, the resulting maps do not take into account the deposit plunge and folding (D2), and the east west extent of the orebody is therefore underestimated. The lower ore sequence is clearly centred on the 2 breccia pipes which represent former hydrothermal vents. Barite rich ore mainly accumulated, up to 25 m thick, west of these feeder zones in probable paleotroughs of the volcanic seafloor. These small basins have a general north 10 degrees east strike which is roughly parallel to the eastern fault. East of this fault, which is a probable reactivated paleofracture, possible extensions of the orebody are conjectural because the amount of downward vertical or horizontal displacement is unknown. The fault which limits the southern breccia pipe and shifts the main north south fault is inferred from limited drilling data and from surface geology.The upper ore sequence broadly overlaps the lower one, but was not deposited over the former breccia pipes which probably formed local highs. This is demonstrated by slump directions around the northern pipe. Feeder stockworks of the upper ore sequence are more irregularly distributed than feeder vents of the lower sequence. Pyrite rich ore accumulated in 4 or 5 small basins which display locally a north east south west direction. Distal barite rich ore extends northwards (northern limit of the deposit) and westwards (mine jaune barite sphalerite chalcopyrite layer). The thickness of the orebody considerably increases east of the north south fault which separates the mine jaune layer from the downwarped main deposit. This suggests that the fault could be the reactivated western limit of the paleotroughs where proximal ore was deposited. The most complete and typical zonation is found in these troughs from a pyrite (chalcopyrite) rich base to a sphalerite barite (silica) rich top.The Chessy massive sulphide body results from a classic submarine hydrothermal event, namely, the uprise of mineralized solutions which deposited an abundance of sulphides and sulphates in seafloor hollows. The 2 successive mineralized sequences were formed during the same process, the internal zonation of eachsequence resulting more from variations in the conditions of deposition than from a chronological polarity.Lead isotope geochemistry proves the deep origin of the fluids and of part of their metal contents, with dilution occurring during the passage through the overlying crust. The isotopic signatures support the hypothesis of mantle fusion in an intracratonic rift environment. This must have been very extensive for the isotopic homogenization to have occurred over a distance of more than 10 kilometres. Such uniformity also implies a contemporaneity of the mineralizing phases which formed these orebodies.Although Chessy and Sain-Bel deposits show similarities in environment of deposition, morphology and mineralogy with Kuroko type massive sulphide bodies, comparison of their lead isotope compositions brings out contrasting characteristics, probably due to the differing geodynamic settings of the mineralizing hydrothermal activity. The isotopic compositions of the Kurokos, although constant within a single body, vary from one deposit to another because of the very marked lithological contrasts of the host rocks in an arc context where the crust mantle ratios are completely different from those found in the Brevenne extensional context.After the emplacement of the Chessy and Sain-Bel sulphide bodies, the submarine hydrothermal activity associated with the Brevenne acid volcanism continued, locally affecting large volumes which became impregnated with pyrite (sphalerite barite). The isotopic compositions of the sulphide impregnations, poor in lead, are characterized by wide variations and by more highly radiogenic signatures than those of the massive sulphi de bodies, showing a different probably shallower, origin for the mineralizing fluids. The discovery to 2 lead isotope signature groups, indicative of the different Brevenne volcanosedimentary deposits, provides an additional tool for target selection during mineral exploration.Multielement analyses have been performed in order to define the geochemical characteristics of the various sample settings in the vicinity of the Chessy deposit. The aim was to identify possible multielement signatures in rocks or soils which would reflect the presence of the Chessy orebody at depth.Each sampling site contributes to the definition of geochemical guides which can be used in the vicinity of a massive sulphide deposit.In the hydrothermally acid volcanic rocks, the sulphides are accompanied by silver, antimony, gold, mercury, bismuth and molybdenum with the hydrothermalism being marked by potassium oxide, titanium oxide and phosphoric oxide. No variation has been observed in the distribution of the elements, and their values at various depths, except for the mercury content which is higher in the vicinity of the sulphide deposit.In the unaltered acid volcanic rocks, a zinc and mercury halo can be observed around the hydrothermally altered and mineralized acid volcanic rocks with a few isolated barium and copper containing deposits.The soils clearly reflect the various lithological facies and make it possible to delineate the boundary of the hydrothermally altered acid volcanic rocks. They also enable the detection of a potassium oxide, mercury, arsenic, copper, zinc anomaly in the Mesozoic sediments, which may be related to a leakage process.The iron oxide rich facies can be classified into 3 groups. The first corresponds to the oxidation of barite and sulphide stockworks in hydrothermally altered acid volcanic rocks. The second is located in the southern part of the prospect and represents the outcrop of a submassive barite and sulphide mineralization which is situated at the contact between mafic and acid volcanic rocks. The third group belongs to the mine rouge facies that is located along the main fault and appears to correspond to sulphides. This last group comprises only 4 samples. A more extensive sampling is indispensable to confirm its particular arsenic, tungsten, gold signature and whether it is favourable for gold.In the new exploration areas of the Brevenne belt, the identification of the hydrothermalized acid volcanic rocks is a priority. In the Arbresle area, south of Chessy, the outcrops are isolated and direct observation is not sufficient to define the extent of particular facies. Since the soils are residual. their geochemistry may be very useful in the preliminary exploration phases.A soil geochemistry survey, on a 500 x 250 m grid, was conducted on an area of 50 square kilometres, the centre of which is located at 12 km to the southwest of Chessy. The samples were collected in horizon B, at 20-30 cm depth. The fraction of size less than 0.125 mm of the 423 samples was analyzed for 34 elements by the inductively coupled plasma (ICP) method.In addition to its contribution to the geological mapping of the Arbresle sector, the soil geochemistry survey highlighted the possibility of finding hydrothermally altered acid lavas in the south south west extention of the Chessy deposit. This possibility was confirmed after reexamination of the outcrops and multielement analysis of the various acid lava facies.The helicopter borne DIGHEM survey conducted in the Chessy-les-Mines prospect in 1978 detected a magnetic anomaly on the western border of the deposit.In order to explain the origin of this magnetic anomaly the following work was undertaken:a ground check of the magnetic anomaly;measurements of the magnetic susceptibility on samples from drifts and drill holes;data analysis and modelling.The analysis of the magnetic susceptibility data, and the modellings performed, in 3 east west oriented cross sections showed that the magnetic anomaly is probably caused by:part of the basic rocks with a strong magnetic susceptibility that forms an east dipping body of about 75 m thick (this magnetic body corresponds to the external portion of the basic unit);part of the rhyolites with a medium magnetic susceptibility located at the eastern contact of the magnetic basic rocks with a thickness of about 30 m (this magnetic marker may correspond to a shear zone located between the basic and acid unit).Given its magnetic susceptibility values, geometry and depth, the orebody does not appear to have generated the surface anomaly. The geophysical interpretation has demonstrated that the magnetic anomaly is not directly related to the mineralization. Nevertheless, the anomaly is located in the vicinity of the rhyolite basic rock contact. Finally, it must be noted that, because of the lack of data on the magnetic susceptibilities, particularly in the first 50 metres, and the eventual presence of remanent magnetization, the models that are proposed for the various bodies are not unique.The aim of the study was to complement conventional massive sulphide prospecting methods by combining volcanostructural analysis, isotopic geochemistry and detailed mineralogy of the alteration phase in order to define distinctive parameters between base and/or precious metal enriched bodies and low grade deposits.The multidisciplinary studies conducted on the Chessy orebody and other volcanosedimentary mineral deposits in the Late Devonian Brevenne metamorphic belt have determined the main characteristics of the mineralization and of its setting, thus providing additional guides for regional exploration of base metal rich concentrations.Diagnostic markers for base metal rich concentrations in Brevenne or similar volcanosedimentary series are paleoalteration zones within the lower acid volcanic rocks which have typical textures, mineralogy and geochemical composition. Where not outcropping, these zones can be detected by specific multielement geochemical signatures in residual soils. Through mineral chemistry of the hydrothermal paragenesis, it is possible to focus on the most intensely altered parts of the former hydrothermal system. Once the inner part of the alteration column is defined, lead isotope geochemistry of sulphides, which are usually present in this central zones, can be used by reference with the Chessy model.In each case, a detailed structural analysis is necessary to determine the primary layering of the volcanic sequence determinethe primary layering of the volcanic sequence and the effects of the successive deformation stages on the geometry of a possible stratiform concentration.This multidisciplinary methodolgy of exploration, which combines classical prospection tools with specific methods, such as mineral chemistry, structural analysis or isotope geochemistry, is not directly transposable to other volcanosedimentary belts.In conclusion, the general approach which prevails for prospection requires the acquisition of the comple te knowledge of the geological and structural evolution and reconstruction of the regional models of ore deposition in their paleogeographic setting, which will then infer the specific markers of mineral concentrations.THE RESEARCH WILL BE CARRIED OUT ON THE CHESSY OREBODY AND IN THE BREVENNE DISTRICT OF FRANCE, AND IN TWO OREBODIES OF THE ALJUSTREL AREA OF PORTUGAL. FACTORS DETERMINING THE ACCUMULATION OF LARGE AMOUNTS OF BASE METAL SULPHIDES ON THE SEA FLOOR WILL BE INVESTIGATED BY STUDIES ON THE MINERALOGY AND GEOCHEMISTRY (INCLUDING FLUID INCLUSIONS, MINERAL CHEMISTRY AND ISOTOPE CHEMISTRY) OF THE ORES AND THEIR HOST ROCKS, PARTICULARLY THOSE AFFECTED BY HYDROTHERMAL ALTERATION. ZONATIONS OF VARIOUS NATURES AND ON ALL SCALES WILL BE SOUGHT, AS WELL AS A CORRECT AND DETAILED RECONSTRUCTION OF THE SITES OF ORE ACCUMULATION. THE DISCOVERY OF EMPIRICAL GUIDES TO HIGH-GRADE ORE IS ALSO ANTICIPATED (ANOMALOUS ABUNDANCES OF MINOR AND TRACE ELEMENTS, CRISTALLOGRAPHIC AND CHEMICAL FEATURES OF MINERALS SUCH AS PHYLLOSILICATES). Ámbito científico natural scienceschemical sciencesinorganic chemistrytransition metalsnatural scienceschemical sciencesinorganic chemistryalkaline earth metalsnatural sciencesearth and related environmental sciencesgeochemistryisotope geochemistrynatural sciencesearth and related environmental sciencesgeologymineralogynatural scienceschemical sciencesinorganic chemistrymetalloids Programa(s) FP1-RAWMAT 3C - Research programme (EEC) on materials (raw materials and advanced materials), 1986-1989 Tema(s) Data not available Convocatoria de propuestas Data not available Régimen de financiación CSC - Cost-sharing contracts Coordinador Bureau de Recherches Géologiques et Minières (BRGM) Aportación de la UE Sin datos Dirección 39-43 Quai André Citroën 75739 Paris Francia Ver en el mapa Coste total Sin datos Participantes (1) Ordenar alfabéticamente Ordenar por aportación de la UE Ampliar todo Contraer todo UNIVERSIDADE DE LISBOA Aportación de la UE Sin datos Dirección Ver en el mapa Coste total Sin datos
