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

Tornos, Fernando; Galindo, Carmen; Darbyshire, Fiona; Casquet, César; Noble, Stephen R.. 2021 Isotope geochemistry, age, and origin of the magnetite-vonsenite mineralization of the Monchi Mine, SW Iberia. Journal of Iberian Geology, 47. 65-84.

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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.

Item Type: Publication - Article
Digital Object Identifier (DOI):
ISSN: 1698-6180
Date made live: 26 Feb 2021 15:36 +0 (UTC)

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