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

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 (lollingite, 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 (epsilon Nd-338 between -0.8 and -4.1; Sr-87/Sr-86(338) = 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.

Automated summary

The Monchi Mine in SW Iberia is characterized by high iron grades and a unique ore assemblage, related to a large plutonic complex. The ore formation is likely the result of deep sourced fluids interacting with protoliths, leading to the formation of high-grade mineralization through a high-temperature magmatic-hydrothermal system.