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Geology and Geochemistry of High-Grade, Volcanic Rock-Hosted, Mercury Mineralisation in the Nuevo Entredicho Deposit, Almade´N District, Spain

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Geology and Geochemistry of High-Grade, Volcanic Rock-Hosted, Mercury Mineralisation in the Nuevo Entredicho Deposit, Almade´N District, Spain Mineralium Deposita (2002) 37: 421–432 DOI 10.1007/s00126-001-0222-y ARTICLE Michel Je´ brak Æ Pablo L. Higueras E´ ric Marcoux Æ Saturnino Lorenzo Geology and geochemistry of high-grade, volcanic rock-hosted, mercury mineralisation in the Nuevo Entredicho deposit, Almade´n district, Spain Received: 5 September 2001 / Accepted: 12 September 2001 / Published online: 2 February 2002 Ó Springer-Verlag 2002 Abstract The Nuevo Entredicho deposit contains the isotopic compositions differ from those measured in the richest concentration of mercury in the Almade´ n dis- syngenetic deposits of the Almade´ n district; they suggest trict, locally grading as much as 45% Hg. This ore de- a higher temperature of ore formation of about 300 °C, posit is hosted within an alkaline, conically shaped and a genesis related to a distinct hydrothermal flow diatreme, about 150 m in diameter, which was subse- path at the Nuevo Entredicho deposit. Deposition of quently filled with phreatomagmatic breccias. The dia- anomalously high-grade mercury ore at Nuevo Entred- treme cuts an Ordovician to Silurian clastic sedimentary icho is related to a combination of (1) an abundance of rock sequence that is intercalated with basaltic sills. black shale that provided sulphur and increasingly re- Structural analysis reveals a complex tectonic history ducing conditions with high sulphide/sulphate ratios, with three main phases of Hercynian deformation. (2) explosive Silurian–Devonian mafic magmatism that Mineralisation occurs as cinnabar replacements in vol- provided an initial source of mercury, (3) tectonic ac- canic tuffs and breccias and as recrystallised veins in tivity that lead to structurally favourable sites for ore tensions cracks associated with pyrophyllite and hy- deposition, and (4) replacement of secondary, carbon- drothermal pyrite, which is strongly enriched in Cu, Pb ate-rich volcanic rocks. and Hg. Lead isotopes in pyrite are characterised by high 207Pb/204Pb ratios (15.70–15.75), suggesting a con- Keywords Almade´ n Æ Cinnabar Æ Epigenetic tribution of ancient upper continental crust remobilised mineralisation Æ Stable isotope Æ Volcanic-hosted by Silurian–Devonian volcanism, with no mantle in- volvement. Sulphur isotopes of epigenetic cinnabar and pyrite range from +10.3 to +10.8& and from +10.6 to Introduction +11.9& respectively, suggesting a uniform sulphur source or a constant mixing ratio in the ore fluids. These The Almade´ n district is the largest mercury producer in the world and accounts for more than one-third of the total world output. The cinnabar deposits show a strong Editorial handling: G. Beaudoin spatial association with the Criadero quartzite unit of Ordovician–Silurian age (Prado 1855; Saupe´ 1990; Villas & M. Je´ brak ( ) et al. 1999), yet the equivocal nature of the genetic re- Universite´ du Que´ bec a` Montre´ al, De´ partement des Sciences de la Terre, lationship between mineralisation and epicontinental C.P.8888, Succ. Centre-Ville, clastic sedimentation has long presented a challenge to Montre´ al (Qc), H3C 3P8 Canada developing a genetic model for the mineralisation. Al- E-mail: [email protected] though a connection with magmatic rocks was proposed P.L. Higueras long ago (Van der Veen 1924), it wasn’t until the 1980s Universidad de Castilla La Mancha, that high grade mercury concentrations were found in Dept Ingenieria Geologica y Minera, 13400 Almade´ n, Ciudad Real, Spain association with volcanoclastic alkaline rocks (Hernan- dez 1984). Mercury production from an igneous rock- ´ E. Marcoux hosted deposit commenced in 1986 at Las Cuevas Universite´ d’Orle´ ans, De´ partement des Ge´ osciences, ISTO, UFR Sciences, 45072 Orle´ ans Cedex 2, France (Fig. 1). The ore has since been interpreted as the product of tectonic remobilisation from early strata- S. Lorenzo Minas de Almade´ n y Arrayanes bound mineralisation in the Criadero quartzite (Je´ brak S.A. Cerco de San Teodoro s/n, and Hernandez 1995, 1997; Ortega 1997), possibly be- 13400 Almade´ n (Ciudad Real), Spain cause of movement along a shear zone (Palero 1997). 422 Fig. 1 Regional geological map of the Almade´ n mercury district including location of the Las Cuevas, El Entredicho, Alma- de´ n and Nuevo Entredicho de- posits In 1985, a drilling campaign near the El Entredicho quartzite (Upper Ordovician–Lower Silurian) and the Base deposit intersected the high-grade, dominantly volcanic quartzite (Lower Devonian). The Hirnantian–Llandoverian Cria- dero quartzite unit is the main host to mercury mineralisation. rock-hosted mineralisation of Nuevo Entredicho. Sedimentary rocks of Devonian age are composed of sandstone, Grades as high as 45.81% Hg over 1.84 m (true thick- shale and lenses of fossil-rich carbonate rocks, all of which were ness) represent some of the richest mercury concentra- deposited under conditions similar to those of the Ordovician tions ever drilled in the world. Calculated reserves of this rocks. Upper Carboniferous rocks near the synclinorium axis dis- small deposit total about 15,500 t, with an average grade cordantly overly Devonian rocks. The Almade´ n Synclinorium contains diverse types of magmatic of 17.5% Hg. This paper describes the geological envi- rocks (Higueras et al. 2000), including basaltic sills (both porphy- ronment of the Nuevo Entredicho deposit, together with ritic and doleritic), and diatremes (brecciated rocks, locally called new geochemical and isotopic data, which permit a Frailesca). These rocks are present throughout the stratigraphic better understanding of the cause and significance of succession, but are especially abundant in Silurian to Devonian sequences. The Criadero quartzite is intruded by both types of grade variations in the Almade´ n district. magmatic rocks. Diatremes cut the stratigraphic units at a slight angle in most of the mercury mines, and have a conical shape with a maximum diameter of several hundred metres. Saupe´ (1990) proposed that the diatreme magmatism was contemporaneous with Geology sedimentation and, therefore, may represent shallow submarine phreatomagmatic eruptions. The original magmas may have been The Almade´ n district is located 300 km south of Madrid on the derived from a garnet–lherzolite mantle source enriched in in- Iberian Peninsula and forms part of the Centro-Iberian Zone of the compatible elements and related to an intra-plate mantle plume Hercynian chain (Julivert et al. 1972; Dallmeyer and Martinez during lower Palaeozoic times (Higueras and Munha´ 1993; Higu- Garcia 1990; Fig. 1). Three cycles of sedimentation have been eras et al. 2000). distinguished: Late Precambrian, Palaeozoic and Late Cenozoic. Three episodes of Hercynian deformation have been recognised Mercury ores in the Almade´ n district are concentrated exclusively in the Almade´ n Synclinorium (Saupe´ 1973, 1990; Hernandez 1984; in Palaeozoic rocks. These rocks formed during lower Palaeozoic Garcia Sansegundo et al. 1987). The initial major phase of folding epicontinental marine sedimentation on a stable platform and were during homogeneous NNE–SSW shortening resulted in a narrow subjected to a main episode of deformation and low-grade meta- elongate synclinorium orientated 100–110°N, with an estimated age morphism during the Carboniferous. of 335±15 Ma (Na¨ gler et al. 1992). Related folds have subvertical A large synclinorium occurs in Palaeozoic rocks overlying the axial planes, subhorizontal axes and inconspicuous axial plane Precambrian basement. The Palaeozoic sequence is a 2,700- to schistosities. Reverse faults are commonly associated with folding 4,000-m-thick detrital suite of quartz–arenites, rhythmically inter- and are better developed in the southern limb. A second phase of layered sandstones and shales, and black shales, with minor hori- heterogeneous E–W shortening caused rotation of the previous zons of carbonate, volcanic and volcaniclastic rocks (Saupe´ 1973; folds along discrete shear zones. Deformation was concentrated Garcia Sansegundo et al. 1987). Four major units of quartzite have along two conjugate systems of predominantly brittle shear been recognised; the Armorican quartzite (Lower Ordovician), the bands oriented NW–SE (sinistral) and ENE–WSW (dextral). Canteras quartzite (Middle to Upper Ordovician), the Criadero Finally, a late N–S-shortening event created conjugate sets of 423 NNW–SSE- and NNE–SSW-oriented faults and caused reactiva- The oldest unit in the diatreme is composed of a black tion of earlier structures. shale interbedded with thin horizons of fragmented The entire sedimentary pile has been weakly metamorphosed to the pumpellyite facies (Saupe´ 1973; Higueras 1995). Illite associated volcanic and sedimentary rocks. Sedimentary clasts are with argillic alteration from the Las Cuevas and El Entredicho dominant, and as large as 20 cm in diameter. Accre- deposits provided an 40Ar/39Ar age of 361±2 Ma (Hall et al. 1997), tionary lapilli and bomb-impact sag structures in ash are which corresponds to the end of Frasnian time. New 87Sr/86Sr data characteristic of aerial transport during eruption and give a model age of 365±17 Ma (Higueras et al. 2001), consistent deposition in a water-rich environment (Cas 1989). with the argon age. Two interpretations have been proposed for this intermediate age between syn-sedimentary volcanic activity This episode may, therefore, correspond to a shallow and Hercynian deformation: (1) the age corresponds to isotopic submarine phreatomagmatic breccia event. resetting either by metamorphism during tectonic events that af- The overlying
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