The jacobsite-magnetite series at the Tumurtijn-ovoo Fe-Mn-Zn skarn deposit, Mongolia
Gottesmann, Wolfram; Gottesmann, Bärbel; Seifert, Wolfgang; Unger, Hans
published: Jul 1, 2015
ArtNo. ESP154019203004, Price: 29.00 €
Members of the jacobsite-magnetite series (Mn-Fe spinels) are main constituents of the andradite-skarn body which contains the rich submarine hydrothermal sphalerite mineralization at Tumurtijn-ovoo in Eastern Mongolia. They have been investigated by microscopy, X-ray diffractometry and electron microprobe analysis. Their composition varies along the whole join between ideal jacobsite (MnFe2O4) and pure magnetite (FeFe2O4). A correlation is found between Mn2+-Fe2+ proportions and lattice constants. Whereas magnetites occur in the whole skarn body, Mn-rich species (jacobsite and manganoan magnetite) are limited to the Zn-rich lithostratographic Upper Skarn Unit. The Mn-Fe spinels of the Upper Skarn Unit are distinguished not only by their Mn-content, but also by their minor elements. Aluminium (0.01–1.16 wt% Al2O3), magnesium (0.01–0.81 wt% MgO) and zinc (0.01–0.48 ZnO) decrease from jacobsites through manganoan magnetites to magnetites whereas silicon and titanium do not. Magnetites of the Lower Skarn Unit show increased contents of silicon (0.29–4.22 wt% SiO2) compared with those of the Upper Skarn Unit (0.01–0.26 wt% SiO2). This indicates chemical equilibrium with the abundant coexisting quartz. Within grains of jacobsite and manganoan magnetite, Mn is homogeneously distributed or decreases significantly from core to rim. This decrease of Mn accompanied by decrease of Al, Mg and Zn is ascribed to slow cooling near a centre of persistent fluid discharge, which permitted longer-lasting crystallization down to lower temperature. Homogeneity probably developed at a distance from such a centre due to rapid cooling. Rhythmic, optical zoning has been observed in grains of a Si-rich magnetite. Zones rich or poor in silicon-aluminium-magnesium alternate with each other. A secondary hydrothermal origin is suggested from the microstructure of the rock and compositional differences to a modern analogue. Based on stoichiometry, the jacobsites from Tumurtijn-ovoo contain Mn and some of the Fe in a divalent state. In this respect they are similar to the jacobsites of other submarine hydrothermal deposits. Such jacobsites formed in slightly reducing conditions in a near-vent position. Jacobsites containing trivalent manganese probably formed in oxidizing conditions at greater distance from the vent sites. The Tumurtijn-ovoo jacobsites are Zn-poor (0.18–0.48 wt% ZnO), although intimately associated with sphalerite. This is in contrast to jacobsites from several other deposits, which show higher levels of zinc (0.74–4.52 wt% ZnO) and are not associated with sphalerite.