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Fault-controlled magnetite-rich breccias in Jurassic limestones: Evidence for concealed epigenetic ore systems in the Lusitanian Basin (Portugal)?

2019; Elsevier BV; Volume: 114; Linguagem: Inglês

10.1016/j.oregeorev.2019.103125

1872-7360

Filipe Ribeiro, António Mateus, Ana C. Azerêdo,

earthquake and tectonic studies

The present work reports the first study on fault-controlled breccias cemented by low-Ti iron-oxides cropping out in Valverde (Maciço Calcário Estremenho, Lusitanian Basin, Portugal). The mineralized bodies: (i) occur in jogs of a WNW-ESE fault zone that comprises four segments, forming an irregular en échelon array of arched geometry; and (ii) are hosted in Jurassic limestones affected by a prominent metasomatic alteration halo macroscopically characterized by darker coloring, higher hardness and disseminations of pyrite, pyrrhotite, hematite and magnetite. Breccias are mainly composed of heterometric and variably modified angular clasts of limestone rocks along with minor dolerite clasts recording intense hydrothermal carbonate alteration. The breccia cements include magnetite and calcite, besides variable amounts of hematite, mixed-layered chlorite-smectite-mica, Ba-bearing phlogopite, fluorapatite, pyrite and chalcopyrite. These features are comparable with some types of Ti-poor iron oxide ores included in the “copper-deficient end-members” of the IOCG clan generated during the Phanerozoic. However, as far as can be observed, the hosting limestones at Valverde do not show calcic or magnesium skarn assemblages and massive Fe-oxide stratabound replacements are also absent. The evolution of the Valverde system can be synthetically described as a succession of two main ore-forming stages during which all of the aforementioned authigenic mineral phases grew in the following sequence: phyllosilicates, iron-oxides and redox reactions, and sulfides. Compared to the hosting limestones, the mineralized fault breccias display strong enrichments in Fe and Co (up to ≈200 times), Ba, Cu, P, Cr, Mn, Si and V (up to ≈33 times) and Rb, K, Zn, Mo, Al, Na, Pb, Mg, Sc, Zr and S (up to ≈8 times). Quantitatively, the iron grades are quite significant (rising up to ≈85 wt% Fe2O3) and the abundances in other base metals are anomalous (≈61 ≤ (Cu + Co + Zn) ≤ 280 ppm). The recognition of this exotic ore-system opens new mineral exploration perspectives for the Lusitanian Basin and other basins with similar characteristics.