Abstract
The Monchi Mine (Ossa Morena Zone, SW Iberia) is a rather unique ore deposit characterized by unusually high Fe grades and an ore assemblage that includes dominant magnetite but with abundant B (vonsenite), U (uraninite), Co (cobaltite), As (löllingite, safflorite) and rare earth elements (allanite). The mineralization occurs at the western edge of a Variscan concentrically zoned gabbro to granodiorite pluton, the Burguillos del Cerro Plutonic Complex. Moreover the western side of the complex is within a large N–S trending dextral strike-slip shear zone in which Ediacaran to early Cambrian metapelitic and calc-silicate hornfels and marble constitute a vertical screen between an outer syn-tectonic sheet of foliated biotite monzogranite and an inner post-tectonic amphibole-biotite diorite unit. The magnetite-vonsenite mineralization is adjacent to the screen and forms large lens-shaped bodies with sharp contacts with the intrusive rocks and is directly related with a granoblastic U-REE-bearing Fe-pyroxene-rich hornfels which is locally brecciated and cemented by pegmatite dominated by albite, K feldspar, quartz, clinoamphibole/biotite and axinite. Within the enclave there is a large post-tectonic exoskarn, including calcic and magnesian types which predates the diorite that mainly replaced the calc-silicate hornfels and the marble. The calcic exoskarn is dominated by grandite and hedenbergite and was retrogressed to actinolite, epidote, calcite and magnetite with variable amounts of pyrite and chalcopyrite. U–Pb TIMS dating of allanite from the U-REE-rich hornfels yielded 337.13 ± 0.99 Ma, i.e., within the range of ages of the Burguillos Plutonic Complex (335–340 Ma). Sr–Nd isotope geochemistry shows that the mineralization (including skarn and massive ore) has isotope signatures (εNd338 between -0.8 and -4.1; 87Sr/86Sr338 = 0.7071–0.7112) mostly intermediate between those of the igneous (− 6.8 to − 2.3; 0.7047–0.7097, respectively) and the sedimentary (− 11.7 to − 8.3; 0.7090–0.7164, respectively) rocks. The massive high grade mineralization could be the result of a syn-magmatic interaction of an unknown protolith with deep sourced fluids that were focused along early thrusts and shear zones probably rooted at a magma chamber in the middle crust. Alternatively it could also be the product of crystallization of a boron-bearing iron melt. This melt would be somewhat similar to the magnetite-(apatite) deposits elsewhere but in which the fluxing agent is boron. Fluids exsolved from these rocks produced a high-temperature magmatic-hydrothermal system that formed the post-tectonic exoskarn. The ultimate origin of these immiscible Fe-B melts could hypothetically be the assimilation at depth of former shallow marine metasediments.
Resumen
La Mina Monchi (Zona de Ossa Morena, SO Iberia) es un depósito inusual por sus elevadas leyes en uranio y una asociación mineral que incluye abundante magnetita junto con abundante boro (vonsenita), U (uraninita), Co (cobaltita), As (löllingita, safflorita) y tierras raras (allanita). La mineralización se encuentra en el límite oeste de un plutón Varisco zonado con gabro a granodiorita, el Complejo Plutónico de Burguillos del Cerro. Este límite occidental del complejo está controlado por una gran cizalla de desgarre dextral de dirección N-S y en donde se localiza un enclave vertical de corneanas metapelíticas y calcosilicatadas y mármoles que se localiza en el contacto entre una zona externa de un monzogranito biotítico foliado y una zona interna de diorita con anfíbol-biotita postectónica. La mineralización de magnetita-vonsenita es adyacente al enclave sedimentario y forma grandes cuerpos lentejonares con contactos netos con las rocas intrusivas y está directamente relacionada con una corneana piroxénica granoblástica rica en U-REE, que está localmente brechificada y cementada por una pegmatita dominada por albita, feldespato potásico, cuarzo, clinoanfíbol/biotita y axinita. En el enclave hay un importante exoskarn postectónico que incluye tipos cálcicos y magnésicos que precede a la diorita y que se desarrolla sobre las corneanas calcosilicatadas y el mármol. El exoskarn cálcico está dominado por grandita y hedenbergita y ha sido retrogradado posteriormente a actinolita, epidota y magnetita con cantidades variables de pirita y actinolita.
La allanita de la corneana piroxénica ha sido datada por U-Pb (TIMS) en 337.13 ± 0.99 Ma, i.e., coetánea con las edades del Complejo Plutónico de Burguillos (335–340 Ma). La geoquímica isotópica Sr-Nd muestra que la mineralización (incluido el skarn y la mineralización masiva) tiene firmas isotópicas εNd338 entre −0.8 y −4.1; 87Sr/86Sr338 = 0.7071–0.7112) que se localizan dominantemente entre las de las rocas ígneas (−6.8 a −2.3; 0.7047–0.7097, respectivamente) y sedimentarias (−11.7 a −8.3; 0.7090–0.7164, respectivamente).
La mineralización masiva de alta ley podría ser el resultado de la interacción sin-magmática de un protolito desconocido con fluidos de origen profundo que se canalizaron a lo largo de zonas de cizalla y cabalgamientos tempranos quizás enraizados en una cámara magmática en la corteza media. Alternativamente, podría ser el producto de la cristalización de un magma rico en hierro y boro. Este fundido podría ser equivalente al que forma los depósitos de magnetita-(apatito) en otros sitios, pero aquí el boro facilitaría ha fusión y flujo de los magmas. Los fluidos exsueltos durante la cristalización de estos magmas sería los responsables de la formación de un sistema magmático-hidrotermal que daría lugar al skarn post-tectónico. El origen último de estos magmas inmiscibles ricos en Fe-B podría estar ligado a la asimilación en profundidad de metasedimentos marinos someros.
Similar content being viewed by others
References
Arribas, A. (1962). Mineralogía y metalogenia de los yacimientos españoles de uranio: Burguillos del Cerro (Badajoz). Estudios Geológicos, 28, 173–192.
Arriola, A., Eguiluz, L., Fernandez Carrasco, J., & Garrote, A. (1981). Individualización de diferentes dominios y unidades en el anticlinorio de Olivenza-Monesterio. Cuadernos Laboratorio Xeoloxico Laxe, 8, 147–168.
Bachiller, N., Galindo, C., Darbyshire, D. P. F., & Casquet, C. (1997). Geocronología Rb-Sr de los leucogranitos del complejo plutónico de Burguillos del Cerro (Badajoz). Geogaceta, 21, 29–30.
Bandrés, A., Eguíluz, L., Pin, C., Paquette, J. L., Ordóñez, B., Le Fèvre, B., et al. (2004). The northern Ossa-Morena Cadomian batholith (Iberian Massif): magmatic arc origin and early evolution. International Journal of Earth Sciences, 93, 860–885.
Bath, A. B., Cooke, D. R., Friedman, R. M., Faure, K., Kamenetsky, V. S., Tosdal, R. M., & Berry, R. F. (2014). Mineralization, U–Pb geochronology, and stable isotope geochemistry of the lower main zone of the lorraine deposit, North-Central British Columbia: A replacement-style alkalic Cu–Au porphyry. Economic Geology, 109, 979–1004.
Borg, S. G., Depaolo, D. J., & Smith, B. M. (1990). Isotopic structure and tectonics of the central transantarctic mountains. Journal of Geophysical Research Solid Earth, 95, 6647–6667.
Brun, J. P., & Pons, J. (1981). Strain patterns of pluton emplacement in a crust undergoing non-coaxial defromation Sierra Morena, SE Spain. Journal Structural Geology, 3, 219–229.
Cambeses, A. (2015). Ossa-Morena Zone Variscan ‘calc-alkaline’ hybrid rocks: interaction of mantle- and crustal-derived magmas as a result of intra-orogenic extension-related intraplating. Ph.D. thesis, University of Granada.
Carriedo, J. (2016). Geología y geocronología de los yacimientos de oxidos de hierro (Cu-Au) en la Zona de Ossa Morena (Suroeste Ibérico). PhD thesis, Universidad de Oviedo.
Carriedo, J., & Tornos, F. (2010). The iron oxide copper gold belt of the Ossa Morena Zone, SW Iberia: Implications for IOCG genetic models. In T. M. Porter (Ed.), Hydrothermal iron oxide copper-gold and related deposits: A global perspective (Vol. 3, pp. 441–460). Adelaide: PGC Publishing.
Casquet, C., Galindo, C., Darbyshire, D.P.F., Noble, S.R., & Tornos, F. (1998). Fe–U-REE mineralization at Mina Monchi, Burguillos del Cerro, Spain: age and isotope (U–Pb, Rb–Sr and Sm–Nd) constraints on the evolution of the ores, GAC-MAC-APGGQ: Quebec, p. A28
Casquet, C., Galindo, C., Tornos, F., & Velasco, F. (2001). The Aguablanca Cu–Ni ore deposit (Extremadura, Spain), a case of synorogenic orthomagmatic mineralization: Isotope composition of magmas (Sr, Nd) and ore (S). Ore Geology Reviews, 18, 237–250.
Clark, A. H., & Kontak, D. J. (2004). Fe–Ti–P oxide melts generated through magma mixing in the Antauta Subvolcanic Center, Peru: Implications for the origin of nelsonite and iron oxide-dominated hydrothermal deposits. Economic Geology, 99, 377–395.
Corfu, F., & Noble, S. R. (1992). Genesis of the southern Abitibi greenstone belt, Superior Province, Canada: evidence from zircon Hf isotope analyses using a single filament technique. Geochimica et Cosmochimica Acta, 56, 2081–2097.
Cuervo, S., Tornos, F., Spiro, B., & Casquet, C. (1996). El origen de los fluidos hidrotermales en el skarn férrico de Colmenar-Santa Bárbara (Zona de Ossa Morena). Geogaceta, 20, 1499–1500.
Cueto, A., Ruiz, C., & Arevalo, P. (1971). Presencia de vonsenita en la mina Monchi, Badajoz (España). Boletin Geologico Minero, 92–2, 186–190.
Dallmeyer, R. D., Garcia Casquero, J. L., & Quesada, C. (1995). 40Ar/39Ar mineral age constraints on the emplacement of the Burguillos del Cerro Igneous Complex (Ossa Morena Zone, SW Spain). Boletín Geológico Minero, 106, 203–214.
Dixon, S., & Rutherford, M. J. (1979). Plagiogranites as late-stage immiscible liquids in ophiolite and mid-ocean ridge suites: An experimental study. Earth and Planetary Science Letters, 45, 45–60.
Einaudi, M.T., Meinert, L.D., & Newberry, R.J. (1981). Skarn deposits, in Geologists, S.E., ed., Economic Geology 75th Anniversary Volume, p. 317–339.
Exposito, I., Simancas, J. F., Gonzalez Lodeiro, F., Bea, F., Montero, P., & Salman, K. (2003). Metamorphic and deformational imprint of Cambriana Lower Ordovician rifting in the Ossa-Morena Zone (Iberian Massif, Spain). Journal of Structural Geology, 25, 2077–2087.
Galindo, C., & Casquet, C. (2004). Magmatismo varisco y postvarisco en la Zona de Ossa Morena. In J. A. Vera (Ed.), Geología de España (pp. 194–199). Madrid: Instituto Geológico y Minero de España.
Galindo, C., Tornos, F., Darbyshire, D. P. F., & Casquet, C. (1994). The age and origin of the barite-fluorite (Pb–Zn) veins of the Sierra del Guadarrama (Spanish Central System): a radiogenic (Nd, Sr) and stable isotope study. Chemical Geology, 112, 351–364.
Garcia Casquero, J. L. (1995). Intrusión múltiple y cuerpos ígneos politípicos: El Complejo Igneo de Burguillos del Cerro, un macizo diorítico zonado en el basamento varisco de la Península Ibérica. Boletin Geologico Minero, 106, 379–398.
Gascon, A. (1904). Los criaderos de hierro de Burguillos del Cerro (Badajoz). Madrid: Ricardo Rojas.
Halverson, G. P., Wade, B. P., Hurtgen, M. T., & Barovich, K. M. (2010). Neoproterozoic chemostratigraphy. Precambrian Research, 182, 337–350.
Henriquez, F., & Martin, R. F. (1978). Crystal growth textures in magnetite flows and feeder dykes El Laco, Chile. Canadian Mineralogist, 16, 581–589.
Honour, V. C., Holness, M. B., Partridge, J. L., & Charlier, B. (2019). Microstructural evolution of silicate immiscible liquids in ferrobasalts. Contributions to Mineralogy and Petrology, 174, 77.
Hou, T., Charlier, B., Holtz, F., Veksler, I., Zhang, Z., Thomas, R., & Namur, O. (2018). Immiscible hydrous Fe–Ca–P melt and the origin of iron oxide-apatite ore deposits. Nature Communications, 9, 1415.
IGME, (1994). Mapa Metalogenético de España escala 1/200000, hoja no. 67–68, Cheles-Villafranca de los Barros, IGME, Volume IGME.
ITGE, (1994). Mapa Metalogenético de España a escala 1/200.000 núm. 67–68 (Villafranca de los Barros-Cheles), Informe Interno ITGE, 190 pp., Volume Informe Interno ITGE, 190 pp.
Jaffey, A., Flynn, K., Glendenin, L., Bentley, W. T., & Essling, A. (1971). Precision measurement of half-lives and specific activities of U235 and U238. Physical Review C, 4, 1889.
Kamenetsky, V. S., Charlier, B., Zhitova, L., Sharygin, V., Davidson, P., & Feig, S. (2013). Magma chamber–scale liquid immiscibility in the Siberian Traps represented by melt pools in native iron. Geology, 41, 1091–1094.
Krogh, T. E. (1973). A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochimica et Cosmochimica Acta, 37, 485–494.
Kubaschewski, O. (2013). Iron—binary phase diagrams. Berlin: Springer Science & Business Media.
Lester, G. W., Clark, A. H., Kyser, T. K., & Naslund, H. R. (2013a). Experiments on liquid immiscibility in silicate melts with H2O, P, S, F and Cl: implications for natural magmas. Contributions to Mineralogy and Petrology, 166, 329–349.
Lester, G. W., Kyser, T. K., Clark, A. H., & Layton-Matthews, D. (2013b). Trace element partitioning between immiscible silicate melts with H2O. P, S, F, and Cl: Chemical Geology, 357, 178–185.
Ludwig, K.R., (1993). ISOPLOT: a plotting and regression program for radiogenic-isotope data. Version 2.82, USGS Open File Report 91-445, 45 pp., Volume USGS Open File Report 91-445, 45 pp.
Ludwig, K. R. (2003). Isoplot 3.00: A geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication, 4, 70.
Montero, P., Salman, K., Bea, F., Azor, A., Exposito, I., Lodeiro, F., et al. (2000). New data on the geochronology of the Ossa Morena Zone. Iberian Massif: Basement Tectonics, 15, 136–138.
Nägler, T., (1990). Sm–Nd, Rb–Sr and common lead isotope geochemistry on fine-grained sediments of the Iberian Massif. Doctoral thesis Zurich, Swiss Federal Institute Technology.
Naslund, H. R., Henriquez, F., Vivallo, W., & Dobbs, F. M. (2002). Magmatic iron ores and associated mineralisation; examples from the Chilean High Andes and Coastal Cordillera. In T. M. Porter (Ed.), Hydrothermal iron oxide copper–gold and related deposits: a global perspective (Vol. 2, pp. 207–226). Adelaide: PGC Publishing.
Ochsner, A., Schafer, H. J., & Gebauer, D., (1992). The geochemistry and age of granitoids of the Ossa Morena Zone (SW Spain): Implications for the late precambrian and early paleozoic geodynamic evolution, En: Libro de Resúmenes. VIII Reunión de Ossa Morena, Rábano, I., Gutierrez Marco, J.C. (eds.)., pp.11, Volume En: Libro de Resúmenes. VIII Reunión de Ossa Morena, Rábano,I., Gutierrez Marco, J.C. (eds.)., pp. 11.
Ordonez-Casado, B., Martin-Izard, A., & Garcia-Nieto, J. (2008). SHRIMP-zircon U-Pb dating of the Ni-Cu-PGE mineralized Aguablanca gabbro and Santa Olalla granodiorite: Confirmation of an Early Carboniferous metallogenic epoch in the Variscan Massif of the Iberian Peninsula. Ore Geology Reviews, 34, 343–353.
Page, R. W. (1983). Chronology of magmatism, skarn formation and U mineralization Mary Kathleen, Australia. Economic Geology, 78, 838–853.
Philpotts, A. R. (1967). Origin of certain iron-titanium oxide and apatite rocks. Economic Geology, 62, 303–315.
Pons, J., (1982). Un modele d'evolution de complexes plutoniques: Gabbros et granitoides de la Sierra Morena Occidentale (Espagne) [Doctoral thesis]: Tolouse, Université Paul Sabatier.
Quesada, C. (1990). Ossa Morena Zone: introduction. In E. Martinez & R. D. Dallmeyer (Eds.), Pre mesozoic geology of Iberia (pp. 249–251). Heidelberg: Springer Verlag.
Roedder, E. (1951). Low temperature liquid immiscilbility in the system K2O–FeO–Al2O3–SiO2. American Mineralogist, 36, 282–286.
Roedder, E. (1978). Silicate liquid immiscibility in magmas and in the system K2O–FeO–Al2O3–SiO2: an example of serendipity. Geochimica et Cosmochimica Acta, 42, 1597–1612.
Roedder, E. (1979). Silicate liquid immiscibility in magmas. In Y. H. S. Yoder (Ed.), The evolution of the igneous rocks. Fiftieth Anniversary Perspective (pp. 15–57). Princeton: Princeton University Press.
Romeo, I., Lunar, R., Capote, R., Dunning, G. R., Piña, R., & Ortega, L. (2004). Edades de cristalización U–Pb en circones del complejo ígneo de Santa Olalla de Cala: implicaciones en la edad del yacimiento de Ni-u-EGP de Aguablanca (Badajoz). Macla, 2, 29–30.
Rose, A. W., Herrich, D. C., & Deine, P. (1985). An oxygen and sulfur isotope study of skarn type magnetite deposits of the Cornwall type SE Pennsylvania. Economic Geology, 80, 448–463.
Ruiz, C. (1976). Génesis de los depósitos de hierro del suroeste de la provincia de Badajoz: Mina Monchi. Boletín Geológico y Minero, 87, 15–31.
Ryerson, F., & Hess, P. (1978). Implications of liquid-liquid distribution coefficients to mineral-liquid partitioning. Geochimica et Cosmochimica Acta, 42, 921–932.
Salman, K. (2004). The timing of the Cádomian and Variscan cycles in the Ossa-Morena Zone SW Iberia: granitic magmatism from subduction to extension. Journal Iberian Geology, 30, 119–132.
Sanabria, R., Casquet, C., Tornos, F., & Galindo, C. (2005). Mineralizaciones ferríferas del coto minero San Guillermo (Jérez de los Caballeros, Badajoz, España). Geogaceta, 38, 223–227.
Sánchez Carretero, R., Eguíluz, L., Pascual, E., & Carracedo, M. (1990). Ossa Morena Zone: Igneous rocks. In E. Martinez & R. D. Dallmeyer (Eds.), Pre Mesozoic Geology of Iberia (pp. 292–313). Heidelberg: Springer Verlag.
Simancas, J. F., Carbonell, R., Gonzalez-Lodeiro, F., Perez-Estaun, A., Juhlin, C., Ayarza, P., et al. (2003). Crustal structure of the transpressional Variscan orogen of SW Iberia: SW Iberia deep seismic reflection profile (IBERSEIS). Tectonics, 22, 1962–1974.
Smye, A. J., Roberts, N. M., Condon, D. J., Horstwood, M. S., & Parrish, R. R. (2014). Characterising the U-Th–Pb systematics of allanite by ID and LA-ICPMS: implications for geochronology. Geochimica et Cosmochimica Acta, 135, 1–28.
Spiering, E.D., Rodriguez Pevida, L., Castelo, J.M., Garcia Nieto, J., & Martinez Chaparro, C.,(2005). Aguablanca: a new nickel mine in a potential new Ni/Cu and IOCG belt of southern Spain and Portugal. in: Rhoden, H.N., Steininger, R.C., and Vikre, P.G., eds., Geological Society of Nevada Symposium 2005: Window to the World, Reno Nevada.
Stacey, J.S., Kramers, J.D. (1975). Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters, 26, (2):207–221.
Tornos, F., Boixereu, E., Florido, P., Gumiel, P., Locutura, J., Lopera, E., et al. (2006a). Mapa Metalogenético de la provincia de Badajoz escala 1/200000 (p. 183). Madrid: IGME-Junta de Extremadura.
Tornos, F., & Casquet, C. (2005). A new scenario for related IOCG and Ni-(Cu) mineralisation: the relationship with giant mid-crustal mafic intrusion Variscan Iberian Massif. Terra Nova, 17, 286–290.
Tornos, F., Casquet, C., & Relvas, J. (2005). Transpressional tectonics, lower crust decoupling and intrusion of deep mafic sills: A model for the unusual metallogenesis of SW Iberia. Ore Geology Reviews, 27, 133–163.
Tornos, F., Casquet, C., Relvas, J., Barriga, F., & Saez, R. (2002). The relationship between ore deposits and oblique tectonics: the SW Iberian Variscan Belt. In D. Blundell, F. Neubauer, & A. von Quadt (Eds.), The timing and location of major ore deposits: an evolving orogeny (Vol. 206, pp. 179–198). London: Geological Society Special Publications.
Tornos, F., Galindo, C., Casquet, C., Pevida, L. R., Martinez, C., Martinez, E., et al. (2006b). The Aguablanca Ni–(Cu) sulfide deposit SW Spain: geologic and geochemical controls and the relationship with a midcrustal layered mafic complex. Mineralium Deposita, 41, 737–769.
Tornos, F., Hanchar, J.M., Munizaga, R., Velasco, F., & Galindo, C. (2020). The role of the subducting slab and melt crystallization in the formation of magnetite-(apatite) systems, Coastal Cordillera of Chile: Mineralium Deposita.
Tornos, F., Inverno, C., Casquet, C., Mateus, A., Ortiz, G., & Oliveira, V. (2004). The metallogenic evolution of the Ossa Morena Zone. Journal Iberian Geology, 30, 143–180.
Tornos, F., Velasco, F., & Hanchar, J. (2016). Iron-rich melts, magmatic magnetite and superheated magmatic-hydrothermal systems: The El Laco deposit, Chile. Geology, 44, 427–430.
Veksler, I. V. (2004). Liquid immiscibility and its role at the magmatic-hydrothermal transition: a summary of experimental studies. Chemical Geology, 210, 7–31.
Velasco, F., & Amigo, J. M. (1981). Mineralogy and origin of the skarn from Cala (Huelva, Spain). Economic Geology, 76, 719–727.
Velasco, F., Tornos, F., & Hanchar, J. M. (2016). Immiscible iron- and silica-rich melts and magnetite geochemistry at the El Laco volcano (northern Chile): Evidence for a magmatic origin for the magnetite deposits. Ore Geology Reviews, 79, 346–366.
Xie, Q., Zhang, Z., Hou, T., Jin, Z., & Santosh, M. (2017). Geochemistry and oxygen isotope composition of magnetite from the Zhangmatun deposit, North China Craton: Implications for the magmatic-hydrothermal evolution of Cornwall-type iron mineralization. Ore Geology Reviews, 88, 57–70.
Zaky, A. H., Brand, U., Buhl, D., Blamey, N., Bitner, M. A., Logan, A., et al. (2019). Strontium isotope geochemistry of modern and ancient archives: tracer of secular change in ocean chemistry. Canadian Journal of Earth Sciences, 56, 245–264.
Acknowledgements
This paper is a tribute to Dr Carmen Galindo, who passed away in 2019. Contribution by FT has been funded by grant RTI2018-009157-A-100 MCI/AEI/FEDER-UE. Radiogenic isotope work and CC and CG field work were financed in part by former Spanish CICYT grant AMB92-0918-CO2-01. Francisco Javier González del Amo and Manuel Lima were invaluable contributors to the field work. We also thank John Hanchar and Jorge Carriedo for their reviews of the original manuscript.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Tornos, F., Galindo, C., Darbyshire, F. et al. Isotope geochemistry, age, and origin of the magnetite-vonsenite mineralization of the Monchi Mine, SW Iberia. J Iber Geol 47, 65–84 (2021). https://doi.org/10.1007/s41513-020-00159-4
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s41513-020-00159-4