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Isotope geochemistry, age, and origin of the magnetite-vonsenite mineralization of the Monchi Mine, SW Iberia

Geoquímica isotópica, edad y origen de la mineralización de magnetita - vonsenita de la mina Monchi, SO de Iberia

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.

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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.

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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

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Keywords

  • Ossa Morena Zone
  • Magnetite
  • Skarn
  • Iron-rich melt
  • Geochronology
  • Isotope geochemistry

Palabras clave

  • Zona de Ossa Morena
  • magnetita
  • skarn
  • magmas ricos en hierro
  • geocronología
  • geoquímica isotópica