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The Malé Vrbno magnetite occurrence of the Velké-Vrbno Unit, Czech Republic: petrology, mineralogy, geochemistry and genesis

2004; Elsevier BV; Volume: 66; Issue: 2 Linguagem: Inglês

10.1016/j.chemer.2004.08.003

1611-5864

Arno Mücke, Bohus Fojt, Jaroslav Skácel,

Geochemistry and Elemental Analysis

The mineralization investigated is that of a stretched magnetite-rich body consisting of three subparallel layers. These layers are each 200 m long, have in total a thickness of close to 2 m and are asymmetrically enveloped by amphibolite which is the host rock. Both, host rock and ore body belong to the Velké-Vrbno Unit which is a constituent of the Prevariscian Orogen and lie at the NW margin of the Bohemian Massif about 500 m SW of the village Malé Vrbno close to the town of Staré-Mešto (Czech Republic). The host rocks are composed of amphiboles (up to 90 vol%) with Mg# of 0.54–0.62 (pargasite, tschermakite and magnesiohornblende), Ab64An35K0.1 feldspar (up to 50 vol%), mica (eastonite) and Mg-dominated chlorite. The only oxide mineral is manganese-rich ilmenite. The ore body is composed of homogeneous, nearly pure magnetite (up to 50 vol%), amphibole with Mg# of 0.14–0.26 (ferrotschermakite, ferrogedrite and ferroanthophyllite), garnet (almandine-dominated with varying proportions of grossularite, andradite and pyrope), mica (annite), local Ab65−72An28−34K0.5−0.8 feldspar, apatite, secondary Fetot-dominated chlorite and quartz. Deduced from REE distribution patterns, the distribution of the HFS elements and the SPIDER diagram, host rocks and ore body originated from continental tholeiitic magmas. The magmatic origin is also supported by the occurrence of chromite and the abundance of apatite in the ore body. The rocks derived from two subvolcanic emplaced partial melts that originated from a deep-seated intrusion. The first melt, identical to the protolith of the host rocks, emplaced in an early stage of differentiation in which the Mg/Fe ratio was relatively high. In an advanced stage of differentiation a second melt was released from the deep-seated intrusion leading to the emplacement of the ore-body protolith. In this stage, the Mg/Fe ratio was remarkably lower and the oxygen fugacity higher causing not only the formation of Fe-rich minerals, but also the crystallization of magnetite. After their emplacement and consolidation, both rock types suffered the same geological development during which they were overprinted by various tectono-metamorphic events (which were not separately investigated).