Mineralium Deposita (1998) 33: 137±149
Ó Springer-Verlag 1998
ARTICLE
P. Nehlig á D. Cassard á E. Marcoux
Geometry and genesis of feeder zones of massive sulphide deposits: constraints from the Rio Tinto ore deposit (Spain)
Received: 10 Jaunary 1996 / Accepted: 10 April 1997
Abstract Vein-related data have been collected around the giant Rio Tinto orebody in southern Spain within the root zones of the massive sulphide deposits. Here, we report the main results of this study, concerning the geometry of the stockwork and the conditions of formation. Although ®eld and thin-section studies have shown that a wide range of vein con®gurations exist, from micro cracks (¯uid-inclusion planes) to large paleo¯ow channels, two groups seem to dominate. The ®rst corresponds to small, constricted micro cracks and capillary-¯ow channels, now mainly ®lled with quartz, whereas the veins of the second group have large widths, are continuous over several meters and are ®lled with quartz and sulphides. Most are tension veins and only very few ( quartz) tends to post-date the quartz-dominated veins (quartz > pyrite). The vein-thickness and -spacing distribution is modal rather than logarithmic, and their densities are not fractal, but are characterized by a Poisson distribution. From the immediate sub-surface zone to more than 100 m below the base of the massive sulphide deposits, most hydrothermal quartz-sulphide stockwork veins are sub-parallel to the base of the massive sulphide deposit. The assumption that the base of this deposit corresponds to a paleo-horizontal plane, implies that most veins were sub-horizontal. This is particularly evident for small veins, but the larger ones can be strongly oblique to the base of the deposit. The hydrothermal ¯uids that generated the massive sulphide deposits and underlying stockworks, were very saline and probably underwent sub- or super-critical phase separation in the
Editorial handling: DR P. Nehlig (&) BRGM-SGN, BP 6009, 45060 OrleÂans cedex, France e-mail:
[email protected] D. Cassard á E. Marcoux BRGM-DR, BP 6009, 45060 OrleÂans cedex, France
root zones of the system. This phase separation was the probable mechanism producing the periodic over-pressures of at least 20 MPa that were necessary to generate the sub-horizontal veins of the stockworks. Resumen (translated by E. Pascual) Se han recogido datos relacionados con las venas hidrotermales en las proximidades de la mineralizacioÂn gigante de Riotinto, en la zona de raõÂ z de los sulfuros masivos. Describimos aquõÂ los principales resultados de este estudio, referentes a la geometrõÂ a del stockwork y sus condiciones de formacioÂn. Aunque los estudios de campo y en laÂmina delgada han demostrado que hay un amplio rango de con®guraciones de venas, desde microfracturas (planos de inclusiones ¯uidas) hasta grandes canales de paleo¯ujo, dos grupos parecen dominantes. El primero corresponde a pequenÄas microfracturas y canales de ¯ujo capilar, rellenos de cuarzo, en tanto que las venas del segundo grupo son mucho maÂs anchas, continuas a lo largo de varios metros y rellenas de cuarzo y sulfuros. La mayor parte de venas son de tensioÂn, y soÂlo algunas (cuarzo) tiende a ser tardõÂ a respecto de las venas en que domina el cuarzo (cuarzo>pirita). La anchura y distribucioÂn espacial de las venas es modal maÂs que logarõÂ tmica y sus densidades no son fractales, sino que se caracterizan por una distribucioÂn de Poisson. Desde la zona inmediatamente subsuper®cial hasta maÂs de 100 m bajo la base de los sulfuros masivos, la mayor parte de las venas hidrotermales del stockwork con cuarzo y sulfuros son subparalelas a la base del depoÂsito de sulfuros masivos. La suposicioÂn de que la base de eÂste corresponde a un plano paleohorizontal implica que la mayor parte de las venas sean subhorizontales. Esto es particularmente evidente en el caso de las pequenÄas, pero las mayores pueden ser fuertemente oblicuas a la base del depoÂsito. Los ¯uidos hidrotermales que generaron sulfuros masivos y los stockworks infrayacentes fueron muy salinos y sufrieron probablemente separacioÂn de fases sub- o supercrõÂ tica en la zona de raõÂ z del sistema. Esta separacioÂn de fases fue
138
probablemente el mecanismo que produjo las sobrepresiones perioÂdicas de al menos 20 Mpa necesarias para generar las venas subhorizontales de los stockworks.
Introduction Studies of fossil and active hydrothermal systems have shown that volcanic massive sulphide deposits are underlain by stockworks, which mark the paleo-subsurface path of the ascending ore ¯uids that deposited massive sulphides (Davis et al. 1992; Nehlig et al. 1994; Tivey et al. 1995). However, few studies have attempted a geometric and geologic characterization of the physico-chemical parameters of this zone. In this study we examine the fracturing processes in several selected areas of the gigantic Rio Tinto ore deposits in southern Spain, corresponding to dierent levels and types of root zones. We then discuss the mechanisms involved in vein genesis, and propose a geometric and genetic model of fracture evolution in the root zones of massive sulphide deposits. The work was carried out by studying the material ®lling veins, and their continuity, spacing and orientation, and by large-scale ®eld sampling in the Rio Tinto and San Miguel areas. The Corta Atalaya open-pit was chosen for a more detailed study, the results of which are compared with data obtained from the Dehesa, Lago in Rio Tinto and San Miguel open-pits.
Geology
South Iberian Pyrite Belt (SIPB) belongs to the South Portuguese Zone in the southern part of the Iberian meseta (Fig. 1). It is one of the largest (if not the largest) of the world's massive sulphide provinces, with more than 80 known deposits containing >1300 Mt of sulphide ore (mined and reserves). Geologically, this belt of 250 ´ 25±70 km comprises rocks ranging from the Late Devonian to the Middle Carboniferous with, in particular, a thick volcano-sedimentary formation of acid and basic volcanic rocks, intercalated between Late Devonian phyllitic quartzites and the detrital Culm deposits of Dinantian age (Schermerhorn, 1971; Strauss and Gray 1984; Strauss and Madel 1974; Routhier et al. 1980; Munha 1983; Oliveira 1990). Paleontological data constrain the age of the volcano-sedimentary formation between the Early Tournaisian (360 Ma) and Middle Visean (342 Ma) (Oliveira 1990). The rocks were aected by Hercynian deformation during the Middle Westphalian (Schermerhorn 1971; Silva et al. 1990). Stockwork zones are generally well exposed in the footwall zones of the massive sulphide bodies, particularly ar Rio Tinto (San Dionisio, Lago and Dehesa in Cerro Colorado), San Miguel, Aznalcollar, Tharsis, San Telmo and ConcepcioÂn. They are composed of anastomosing, 2±3 cm-wide veinlets of quartz with disseminated sulphides (San Dionisio body and Dehesa open-pit in Rio Tinto), and pyritic-chloritic-siliceous matrix-supported breccias showing transitional boundaries with the host volcanic rocks (San Miguel north of Rio Tinto, Lago in Rio Tinto). These veins were channel-ways for hydrothermal solutions ascending through the alteration zones to the deposits. Two speci®c mineral parageneses are generally present and show an early assemblage with pyrite and cobalt-bearing minerals, the latter are fairly rare but highly characteristic and seem to indicate deep zones, and a late assemblage with Bi (Cu, Pb, Te) sulphides and sulfosalts that are much more common (Marcoux et al. 1995). These assemblages do not occur in distinct veins as at Hellyer in Australia (Gemmel and Large 1992), but generally are telescoped within the same veins, thus showing that the feeder channels had a certain longevity. These results con®rm the existence of late-stage hightemperature in¯ow of Cu (Bi, Te) solutions as proposed by Large's model (1992), which remobilized inter alia pre-existing Pb, Sb and Ag, and redistributed these elements in more outlying zones.
Regional geology
Geology of the Rio Tinto area
The Iberian Peninsula (Fig. 1) is largely underlain by a Hercynian belt that is approximately 750 km long in a NW-SE direction. The
The Rio Tinto orebody lies in an east-west anticline (Figs. 1, 2). It consists of three distinct mineralized zones that, though now dis-
Fig. 1 Location of the massive sulphide deposits (*) studied
139 The measuring of planes along a one-dimensional line greatly reduces the likelihood of intersecting any planes inclined at a low angle to the line (by an amount proportional to the cosine of the dip for a randomly distributed vein set, Terzaghi 1965). The data were stored in a GDM database, a computer-software package developed by the BRGM for the management and processing of geologic and mining data. GDM is particularly useful for the spatial representation of such data, i.e., the calculation of coordinates, maps and sections, and their structural processing into stereograms, iso-density contour maps, rotated measurements, etc. The data were manipulated to yield: 1. The true distance to the massive sulphide of each observation point, i.e., measured perpendicular to the base of the body; 2. The original orientation of the veins prior to the Hercynian tectonism of Westaphalian age (Schermerhorn 1971), seen as WNW±ESE to E±W folds that are overturned to the south.
Fig. 2 On top: Geological map of the Rio Tinto anticline (modi®ed from Garcia Palomero, 1980) (see location on Fig. 1). Below: Schematic cross section of the Rio Tinto anticline in the Cerro Colorado area (from Garcia Palomero et al. 1986) connected, originally formed a single body of about 5 km ´ 750 m ´ 40 m, or about 500 Mt of sulphides. The three bodies are San Dionisio (about 45 Mt of sulphides) in the south limb, San Antonio (about 9.5 Mt) in the east-plunging axis, and the Cerro Colorado unit. The last includes the ``Filon Sur'' and the ``Filon Norte'' (comprising the Lago and Solomon bodies) that, respectively, lie in the south and north limbs of the anticline, are interconnected by the central stockwork in the fold hinge (Cerro Colorado sensu stricto) and are opened by the ``Corta Dehesa'' (Corta is a local term for open-pit) (Fig. 2). Solomon et al. (1980) showed more or less continuous stockwork-containing zones along the north and south margins of the pyritite sheet. Intense chemical weathering (undated) of the sulphides led to the formation of a gossan that can be up to 70 m thick, and which is still mined for its gold content. The massive and stockwork sulphides were extensively mined from open-pits, such as the Corta Atalaya in the San Dionisio body, the Cortas Lago, Dehesa and Solomon in the Filon Norte of the Cerro Colorado, and the Corta Sur on the Filon Sur. Underground mining took place from the Alfredo mine in the San Dionisio orebody. The dierent orebodies are autochthonous and underlain by three main stockwork zones: San Dionisio, much dissected by the Atalaya pit, Lago, and the Dehesa-Cerro Colorado pit that is directly overlain by the gossan, in which the sulphides of the massive body have been completely oxidized. Our observations on stockworks were made in all three sites, with the added advantage that the San Dionisio massive sulphide could be studied on several levels in the Corta Atalaya. The underground workings in the San Dionisio body are no longer accessible since pyrite mining was halted.
The parameters for such rotation depend upon the choice of the reference paleo-horizontal. In the absence of other, more reliable, markers (no bedding is seen in this area), this was taken to be the base of the San Dionisio body, even though this plane may not have been strictly horizontal. Its orientation is approximately constant throughout the study area (Table 1) and can be estimated to have an average strike and dip of N100°±87°S, which is coherent with the geometry of the south limb of the Rio Tinto anticline. This structure is unfolded by rotation along a sub-horizontal axis trending N100°, parallel to the main fold axes and the bottom of the orebody (Figs. 3, 4). Within the stockwork, no evidence was found for second-order folding of the main deformation, and the same rotation was thus applied to all measurements.
Table 1 Principal characteristics of the three studied pro®les (level 6, level 15 and level 19±20), with indication of the absolute altitude, the length of the pro®le, the number of measurements, and the attitude of the base of the massive sulphide deposit at the start of the pro®le Pro®le
Altitude Length (m) (m)
Level 6 406 Level 15 285.8 Level 19±20 209
179 297.6 119.7
Number of Attitude of the measurements base of the massive sulphide deposite 242 348 69
N95°E±85°S N96°E-vertical N105°E-vertical
Methods of study and paleo-tectonic framework The stockwork was studied in the Corta Atalaya, along three sections that were chosen on the basis of accessibility of the various benches (Table 1; Fig. 15). For each section, a line was drawn on the wall at about 1.2 m from the bench ¯oor, and the length and orientation of the various sections were recorded. All veins intersecting this line were measured (strike, dip, thickness along the intersecting plane, type of in®lling, distance from preceding vein).
Fig. 3 Sub-north-south section through the San Dionisio orebody (from Garcia Palomero 1980) with indication of the rotation parameters that were applied to the base of the massive sulphide, and the location of the studied sections
140
Fig. 4 Stereographic plot of the magmatic ¯ow planes in the hypovolcanic rhyolite, before and after rotation of the base of the San Dionisio body to a horizontal plane. Schmidt projection, lower hemisphere
Vein geometry The hydrothermal vein systems Although ®eld and thin-section studies have shown that a wide range of vein types exists, from micro-cracks (¯uid-inclusion planes) to large paleo-¯ow channels, two groups dominate. The ®rst consists of small narrow micro-cracks and capillary ¯ow channels (now mainly ®lled with quartz), whereas the veins of the second group have large widths, are continuous over meters, and are ®lled with quartz and sulphides. In this study we shall discuss only the macroscopic veins, i.e., those visible to the naked eye, because they have the largest eect on permeability. Most are extension veins and only few (9 cm), steeply intersect the base of the massive bodies. Width and spacing of veins
Fig. 7 Stockwork of the San Dionisio body, Corta Atalaya. Stereogram after rotation, of the veins by type of their in®lling; 294 data points. Schmidt projection, lower hemisphere
The transition zone between stockwork and massive sulphide deposit: examples from the cortas Dehesa and Lago Near the base of the massive bodies, no stockwork sensu stricto (i.e., a three-dimensional system of intersecting planar to curviplanar veins) is preserved. The bleached and altered rhyolitic host rock is intensely fragmented, with the development of a characteristic crust texture (Fig. 8, point 1), the partings of which are preferentially sub-parallel to the base of the massive sulphide. The main blocks in the mass can be several decimeters in size and the structures between them, which can be up to
Vein-width distribution presents a mode between 1 and 2 cm (Fig. 9) within the Corta Atalaya stockworks at level 15 and though this is not the case at level 6, it must be stressed that even there the distribution of widths is not exponential. While the quartz-dominated veins are characterized by small widths (i.e., only 5% of the veins are wider than 3 cm), and a mode between 1 and 2 cm, 75% of the pyrite-dominated veins are larger than 3 cm, and have a mode between 3 and 4 cm. This is a rather surprising result because we expected an exponential rather than modal distribution of vein thickness. Vein spacing, measured along a virtual line, has a mode between 20 and 30 cm in Corta Atalaya in levels 6 and 15 (Fig. 10). Ideally, the total surface of veins divided by the total volume (m2/m3) would provide the closest approximation of vein density. However, such measurements being impractical, we chose to calculate vein density by
142
Fig. 8 Schematic block diagrams by zone, in the ®rst meters of crust, below the massive sulphides of the cortas Dehesa and Lago. 1 Fragmented rhyolite with a crusty texture; the open fractures are ®lled by pyrite; 2 ¯at, lens-shaped sulphide body with ribbon texture of alternating ®ne and coarse pyrite; lateral passage possible to, 3 ¯attened lens-shaped sulphide body with breccia texture with dierent types of elements such as (a) host-rock (in white) mostly found in the hanging wall of the body, and (b) pyrite aggregates from a fragmented earlier in®lling. 4 Roof of the body in the process of brecciation. 5 Sulphide body with ribbon texture identical to 2; ``late'' reopening ®lled with white quartz is found in the coarse pyrite ribbons. 6 Sulphide-breccia body, several meters thick, enclosing host-rock elements. 7 Impregnation zone with massive pyrite and relics of host rock. 8 Feeder veins of the massive sulphide, later remobilized in reverse faulting Fig. 9 Histogram showing the modal distribution of the widths (cm) of hydrothermal veins within the three studied levels of the Corta Atalaya
measuring it along pro®les (1/m mÿ1 ). The results were compared with measurements of the total length of veins on a plane that were divided by the surface of this plane (m/m2 mÿ1 ). Figure 11 shows that the vein
density is around 5 veins/m over large distances. When taking the cumulative vein width into account, it appears that, at least for level 15, the vein density is homogeneous and decreases slightly with depth. In addition, there is a strong linear correlation between the sum of vein widths over ®xed intervals and the number of veins
143
Fig. 10 Histogram showing the modal spacing (cm) of hydrothermal veins for the three studied levels of the Corta Atalaya
Fig. 13 Results of the fractal analysis along the two transects (level 15 and 6 of the Corta Atalaya) plotted in a log1/r -log Nr diagram. The spatial scale r used for normalization varies between 10 and 400 cm (see Velde et al. (1991), and Manning (1994) for details of calculation)
with log Nr is linear. Figure 13 shows that this is not the case in this study, and the curvature instead implies that the veins de®ne a Poisson distribution. Variation with depth
Fig. 11 Density of veins (expressed as number of veins) for pro®le 15, from 0 m (base of massive sulphide deposit) to 138 m below the massive sulphide deposit. The ®gure shows an average density of 5 veins per 2 m. Negative values stand for (parts of) segments without measurements (talus rubble, drifts, etc.)
Fig. 12 Binary diagram showing a strong positive correlation between the sum of vein widths (cm) over 2 m intervals (y axis) and the number of veins over the same interval (x axis). The ®gure shows that there is no correlation between the density of veins and their width
over the same intervals (Fig. 12). This implies that there is no correlation between vein density and width. In order to quantify the heterogeneity of vein distribution, Cantor's method for fractal analysis of veins was applied to the quartz-sulphide vein networks. Detailed descriptions of this analytical method are given by Velde et al. (1991) and Manning (1994), in which a vein set is characterized by fractal clustering if the change in log 1/r
In Fig. 14, drawn for the section of level 15 in Corta Atalaya, the distance to the massive sulphide, measured at right angles to the base, is plotted against the spacing between veins (Fig. 14a), and their apparent thickness (Fig 14b). In both cases, the values were measured along the section. The values for the spacing variable are relatively stable down to 87 m, but beyond this depth they increase rather abruptly. The evolution of the thickness variable is inverse: structures that are thicker than 5 cm occur almost exclusively in the interval 0±87 m while the thickest veins (>20 cm) only occur in the 0±50 m interval. Pro®le 6, although shorter, gives a similar result, whereby the transition depth now lies at about 63 m below the massive body. Except that the thickest and steepest structures preferentially occur close to the base of the massive sulphide, no other change seems to occur with depth. Sequential stereographic plotting of the data by slices of 50 m, combined with the drawing of sub-N-S sections where the veins are shown by class of thickness, does not show a clear change in the spatial organization of veins; on the contrary, the sub-horizontal veins, in particular those with a thickness r 3 T 0 In the present case, most veins being sub-horizontal, it can be considered that the minimum principal stress r3 probably was close to vertical:
2 r3 r v The two other main stresses (r1 and r2) then had to be sub-horizontal, which seems to be compatible with the tectonic regime of plate convergence (Silva et al. 1990). In view of the fact that the massive sulphides were formed at the bottom of a basin, the lithostatic pressure rv at depth z in the rhyolite body, which was vertically oriented and created by the weight of the overlying formations, can be written as: rv qr g z qw g h
3 qr and qw being the average speci®c gravity of rhyolite and sea water, g being the gravitational acceleration, z being the depth, and h being the height of the water column, i.e. the depth of the basin. We can now estimate the value of rv at 10 m and 150 m below the massive sulphides. If h 200 m, z will respectively be 210 m and 350 m. qw can be estimated to be 1, and in the absence of speci®c data for the Rio Tinto rhyolite, qr takes the standard value of 2.6 (Olhoeft and Johnson 1989). This gives the values rv210 2:46 MPa and rv350 6:1 MPa. If no new fractures were created
147
and only pre-existing ones were reactivated, these values represent the thresholds of Pf at the depths considered. If, however, such fractures were newly formed, these values have to be added to the tensile strength T0 of the rhyolite (1). The rock being strongly altered and weathered, it seems to be doubtful that a realistic value for T0 could be obtained, and we thus attributed a value of 12.5 MPa to T0, which is the average value obtained from 18 tests on the rhyolite around the massive sulphide stockwork at Jabal Sayid, Saudi Arabia (F. Wojtkowiak, personal communication). This gives the following two estimates for Pf : Pf 210 > 15 MPa Pf 350 > 18:6 MPA This pressure estimate is also close to that provided by the ¯uid inclusions: the highest homogenization temperature that is in liquid phase (CA67), requires a minimum pressure of 13 MPa. Thus, in order to generate the horizontal stockwork fractures requires an over-pressure of approximately 20 MPa for the out¯owing ¯uid in respect to the in¯owing ¯uid, in order to generate the horizontal stockwork veins. Several theoretical mechanisms allow to achieve such an over-pressure: 1. Assuming that the hydrothermal system responsible for depositing the Rio Tinto massive sulphides was broadly analogous to an open thermosyphon with an in¯ow zone, a heat source and a discharge zone enable the calculation of a minimum hydrothermal¯uid-circulation depth, in order to obtain the overpressure required for generating the horizontal veins. Taking an in¯owing seawater density of close to 1 (present-day seawater is about 1020 kg/m3 at 2 °C) and a hydrothermal-¯uid density of 0.7 (350 °C, 20 MPa), an over-pressure of 20 MPa requires a siphon at least 4400 m high, not taking into account the pressure drop due to friction. This simple calculation, which provides an unrealistic size of the thermosyphon in view of the present knowledge of geothermal systems, shows that the hydrothermal system cannot be treated as a simple open thermosyphon. Other models are necessary to explain the 20 MPa over-pressure required to generate the horizontal veins. 2. Solomon (1984) has developed an alternative model to explain the formation of stockworks beneath massive sulphide deposits in boiling free hydrothermal systems. He has shown that reaction between ¯uids and volcanic rocks generally reduces the permeability. The ¯uids then circumvent the partially sealed rock by expanding the width of the ¯uid column and they overcome the increased resistance by increased ¯uid pressure due to an increased temperature, until fracturing occurs. Typical values of the isochoric coecient of thermal expansion of H2O (Johnson and Norton 1991), indicate that to obtain
an over-pressure of 20 MPa in a closed system, an approximate temperature increase of 20 °C is necessary. However, this temperature increase in the root zone of the system is likely to be counterbalanced by conductive cooling in other parts of the system. In addition, although this mechanism is likely to generate small veins, it is highly improbable to have generated the wide and >10-m-long veins of the stockwork. 3. The most probable model is suggested by the measured high salinities of the hydrothermal ¯uids, which are interpreted as the result of phase separation within the root zones of the hydrothermal system. Sub- or super-critical phase separation generates large volume increases of the hydrothermal ¯uids, and is likely, when occurring in the root zone of the hydrothermal system, to create the over-pressures of 20 MPa needed to develop the horizontal stockwork veins. Fluid-inclusion measurements within the plutonic root zones of massive sulphide deposits (not detailed here as they fall beyond the scope of this work), have provided evidence of very-high-salinity ¯uids, similar to those measured within oceanic plagiogranites in the root zones of black-smoker-type hydrothermal circulation, or associated with porphyry-copper deposits and interpreted as the result of super-critical phase separation of hydrothermal or magmatic ¯uids (Nehlig 1991). This mechanism has been investigated recently by Germanovich and Lowell (1995) in a study of the mechanisms for initiating phreatic eruptions following the emplacement of a shallow magmatic intrusion into water saturated permeable rock which contains subsidiary low-permeability crack networks and disconnected cracks. In their model, heat from the intrusion causes the local groundwater to boil. As the ascending superheated steam heats the overlying rock, the water in the subsidiary and disconnected cracks will boil. The pressure exerted by the vapor in the subsidiary and disconnected cracks can lead to rapid horizontal crack propagation, resulting in an increase in crack length by more than an order of magnitude. The presence of a heat source (magma chamber) is suggested close to Campofrio, north of the Rio Tinto anticline. Here, spectacular trondhjemite-tonalite magmatic breccias have been subjected to intense hydrothermal alteration (Halsall and Sawkins, 1989), as shown by porphyry-style iron-sulphide mineralization with pegmatitic amphibole-bearing pockets. Schutz et al. (1987) and ThieÂblemont et al. (1994) have shown that these intrusions are the co-magmatic equivalents of the felsic volcanic group of the volcano-siliceous complex at Rio Tinto. The heated hydrothermal ¯uids were subject to sub- and/or super-critical phase separation associated with slight phase segregations. It is unknown whether a magmatic ¯uid-component was added to the hydrothermal ¯uid, but this is suggested by the oxygen-isotope studies of Halsall and Sawkins (1989). A periodic functioning of such phase separation and a critical value of
148
the generated over-pressure, might explain the modal distribution of the vein widths (Fig. 9). The physical behavior of hydrothermal solutions entering seawater is a direct function of the density contrast between the two ¯uids. The rather low homogenization temperatures and high salinities measured in the ¯uid inclusion indicate that the hydrothermal ¯uids were much denser than previously thought and have thus a lower density contrast with seawater than previously thought (Solomon et al. 1980). They are thus likely to pond easier upon mixing with seawater (Sato 1972) and generate smaller hydrothermal plumes (Solomon et al. 1980).
Conclusion Our study of a massive sulphide deposit and its fossil feeder zone has led to the ``artists impression'' (Fig. 17) of the San Dionisio massive sulphide during its creation, showing the preferential orientation of the hydrothermal veins. The main result of our work lies in the observation and quanti®cation of an apparent order in the stockwork, which at ®rst sight consists of a chaotic interlacing of veins. Most veins were found to be (sub)-parallel to the base of the massive sulphides; the rare thick veins are generally secant to, and were seen to be feeders of, the thin veins.
Fig. 17 Interpretative reconstruction of the formation of a Rio Tintotype massive sulphide body. The axial part has been drawn from the indications by Garcia Palomero (1980). Outside the axis, the veins have a preferential horizontal orientation, but the thick massive sulphide veins in the axis tend to be sub-vertical. The passage from stockwork to massive sulphide is characterized by a progressive substitution of host-rock relics by sulphides. The paleo-location of the zones studied in the cortas Atalaya and Lago has been indicated
The formation of the stockwork can be best explained by horizontal propagation of subsidiary and disconnected cracks following periodic phase separations during heating and mineral precipitation during cooling. Acknowledgements This work was supported by the Bureau de Recherches GeÂologiques et MinieÁres (France) (Project: Fracturation, permeÂabilite et instabilite dans la crouÃte supeÂrieure). Special thanks are extended to the geological sta, Felix Garcia Palomero and Frederico Sobol, of Rio Tinto Minera, and to J-F. BecqGiraudon, D. Bonijoly, D. Burlet, G. Courrioux, L. Germanovich, M. JeÂbrak, J.-M. Leistel, J.-L. Lescuyer, R. Lowell and C. Ramboz who helped to clarify several aspects of the study. H.M. Kluyver translated and edited the paper for publication.
References Boulter CA (1993) Comparison of Rio Tinto, Spain, and Guyamas Basin, Gulf of California: An explanation of a supergiant massive sulphide deposit in an ancient sill-sediment complex. Geology 21: 801±804
149 Cornet FH (1992) Fracture processes induced by forced ¯uid percolation, In: Gasparini P., Scarpa R., Aki K (eds) Volcanic seismology. IAVCEI Proceedings in volcanology 3, SpringerVerlag, Berlin Heidelberg New York, pp 407±431 Davis EE, Mottl MJ, Fischer AT (1992) Proc ODP, Init Rep 139 Etheridge MA, Wall VJ, Cox SF Vernon RH (1984) High ¯uid pressures during regional metamorphism and deformation: implications for mass transport and deformation mechanisms. J Geophys Res 89: 4344±4358 Garcia Palomero F (1980) Caracteres geoloÂgicos y relationes morfoloÂgicas y geneÂticas de los yacimientos del ``Anticlinal de Riotinto''. Ph D Thesis, University Salamanca (Spain). Excma DiputacioÂn Provincial de Huelva Ed, 262 p Garcia Palomero F, Bedia Fernandez JL, Garcia Magarino M, Sides EJ (1986) Nueves Investigaciones y trabajos de evaluacion de reservas de gossan en minas de Rio Tinto. Bol Geol Mi Esp 99(1): 83±101 Gemmel JB, Large RR (1992) Stringer system and alteration zones underlying the Hellyer volcanic-hosted massive sulphide deposit, Tasmania, Australia, Econ Geol 87: 3, 620±649 Germanovich LN, Lowel RP (1995) The mechanism of phreatic eruption. J Geophys Res 100(B5): 8417±8434 Gratier JP (1984) La deÂformation des roches par dissolutioncristallisation. Aspects naturels et expeÂrimentaux de ce ¯uage avec transfert de matieÁre dans la crouÃte supeÂrieure. PhD Thesis, 315 pp, University Scienti®que et MeÂdicale, Grenoble, France Halsall C, Sawkins F (1989) Magmatic-hydrothermal origin for ¯uids involved in the generation of massive sulphide deposits at Rio Tinto, Spain. In: Miles L (ed) Water rock interaction. International Association of Geochemistry and Cosmochemistry and Alberta Research Council, Edmonton, pp 285±288 Jaeger JC, Cook NGW (1971) Fundamentals of rock mechanics, 515 pp. Chapman and Hall Ltd, London, UK Johnson JW, Norton D (1991) Critical phenomena in magma-hydrothermal systems: I. State, thermodynamic, transport and electrostatic properties of H2O in the critical region, Am J Sci 291: 541±648 Large RR (1992) Australian volcanic-hosted massive sulphide deposits: features, styles, and genetic models, Econ Geol 87: 477±510 Manning CE (1994) Fractal clustering of metamorphic veins. Geology 22: 335±338 Marcoux E, MoeÈlo Y, Leistel JM (1996) Bismuth and cobalt minerals as indicators of stringer zones to massive sulphide deposits, South Iberian Pyrite belt. Mineralium Deposita 31: 1±26 Munha J (1983) Hercynian magmatism in the Iberian pyrite belt, in: Lemos de Souza J, Oliveira JT (eds) The Carboniferous of Portugal Mem Serv Geol Port 29: 39±81 Nehlig P (1991) Salinity of oceanic hydrothermal ¯uids; a ¯uid inclusion study, Earth Planet Sci Lett 102: 310±325 Nehlig P, Juteau T, Bendel V, Cotten J (1994) The root zones of oceanic hydrothermal systems: constraints from the Semail ophiolite (Oman). J Geophys Res 99 (B3): 4703±4713 Olhoeft GR, Johnson G (1989) Densities of rocks and minerals, In: Carmichael RS (ed) Practical handbook of physical properties
of rocks and minerals. CRC Press, Boca Raton, Fla., pp 139±176 Oliveira JT (1990) South Portuguese zone. In: Dallmeyer, Martinez Garcia E (eds) Pre-Mesozoic geology of Iberia. Springer-Verlag, Berlin Heidelberg New York, pp 333±347 Paterson MS (1958) Experimental deformation and faulting in Wombeyan marble. Bull Geol Soc Am 69: 465±475 Potter RW, Babcook RS, Brown DL (1978) Freezing point depression of aqueous sodium chloride solutions. Econ Geol 73: 284±285 Ramsay JG (1980) The crack-seal mechanism of rock deformation Nature 284: 135±139 Roedder E (1984) Fluid inclusions. Mineral Soc Am, 644 pp Routhier P, Aye F, Boyer C, Lecolle M, Moliere P, Picot P, Roger G (1980) La ceinture sud-ibeÂrique aÁ amas sulfureÂs dans sa partie espagnole meÂdiane. Tableau geÂologique et meÂtallogeÂnique. SyntheÁse sur le type amas sulfureÂs volcano-seÂdimentaires. 26eÁme Congr GeÂol Int Paris MeÂm BRGM. 94, 265p Sato T (1972) Behaviours of ore-forming solutions in seawater. Minlogy Geol 22: 31±42 Schermerhorn LJG (1971) An outline stratigraphy of the Iberian pyrite belt. Bol Geol Min Spain 82: 238±268 Schutz W, Ebneth J, Klaus Dieter M (1987) Trondhjemites, tonalites and diorites in the south Portuguese zone and their relations to the vulcanites and mineral deposits of the Iberian Pyrite Belt. Geol Rundsch 76/1: 201±212 Secor DT (1965) Role of ¯uid pressure in jointing, Am J Sci 263: 633±646 Silva JB, Oliveira JT, Ribeiro A (1990) South Portuguese Zone: structural outline, In: Dallmeyer RD, Martinez Garcia E (eds) Pre-Mesozoic Geology of Iberia. Springer-Verlag, Berlin Heidelberg New York, pp 348±362 Solomon M, Walshe JL, Garcia Palomero F (1980) Formation of massive sulphide deposits at Rio Tinto. Spain Inst Min Met 89: 16±24 Solomon M (1984) The formation of stockworks beneath massive sulphide deposits (abstract). IGC Abstr 27(6): 308 Strauss GK, Gray KG (1984) Base metal deposits in the Iberian pyrite belt, In: Friedrich et al. (eds) Geology and metallogeny of copper deposits. Springer Verlag, Belin Heidelberg New York, pp 304±324 Strauss GK, Madel J (1974) Geology of massive sulphide deposits in the Spanish-Portuguese pyrite belt. Geol. Rundsch 63: 191±211 Terzaghi RD (1965) Sources of errors in joint surveys. Geotechnique 15: 287±304 ThieÂblemont D, Marcoux E, Tegyey M, Leistel JM (1994) GeneÁse de la Province Pyriteuse Sud-IbeÂrique dans un plaeÂo-prisme d'accreÂtion: arguments peÂtrologiques. Bull Soc GeÂol 165: 407±423 Tivey MK, Humphris SE, Thompson G, Hannington MD, Rona PA (1995) Deducing patterns of ¯uid d¯ow and mixing within the active TAG hydrothermal mound using mineralogical and geochemical data. J Geophys Res 100: 12527 Velde BJ, Dubois J, Moore D, Touchard G (1991) Fractal patterns of fractures in granites. Earth Planet Sci Lett 104: 25±35
