Original paper

The Tumurtijn-ovoo Fe-Mn-Zn skarn deposit, Mongolia: Sedimentary features, distribution of main minerals and the whole-rock chemical composition

Gottesmann, Wolfram; Kampe, Aribert


The Tumurtijn-ovoo deposit in eastern Mongolia is a big concentration of magnetite, jacobsite and sphalerite. It is hosted in Devonian sedimentary rocks enclosed in a polyphase granite batholith of Triassic-Jurassic age. The ores are strictly linked to a large skarn body, which consists of a lower part rich in quartz and poor in manganese and zinc, and an upper part with the opposite relationship. The upper skarn borders a marble with an interdigitation zone. Hornfelses, originated from tuffites, lie above and below the skarn body. The main minerals show the following distribution regularities. Magnetite occurs in the whole skarn body. In the lower part it constitutes massive lenses of up to 20 m in thickness. Jacobsite is restricted to the upper part. Sphalerite is concentrated in three strata-like zones in the upper part and also forms locally a low-grade mineralization at the top of the lower part there being related to quartz-rich layers. Galenite is bound to a zone marginal to the major Zn mineralization. Garnets of the grossularite-andradite series and bustamite are the dominant calcsilicate minerals. Andradite is the most abundant garnet. Grossularite is typical for the hangingwall skarn, peripheral parts and local intercalations in the upper skarn. Bustamite preferentially occurs along the marble-skarn contact zone. Sphalerite coexisting with jacobsite contains more Mn than sphalerite coexisting with magnetite, indicating that the minerals crystallized in chemical equilibrium. The Zn mineralization does not extend into the sedimentary wall rocks and the surrounding granite. Even the lowest and the uppermost parts of the upper skarn are sphalerite-free. The strata-bound position of the zinc ores, the stratified structures of the skarn and the conformable alternation of marble and skarn beds in the interdigitation zone indicate sedimentary deposition of the substances for the formation of skarn and ore. Therefore, with regard to the following arguments, the origin of the deposit is classi- fied as hydrothermal sedimentary. The ores are extremely poor in Al. This feature is characteristic for several deposits accepted to be of submarine exhalative origin. On the other hand certain barren parts of the skarn body are Al-rich. In analogy to other deposits the association of Al-poor and Al-rich skarns can be caused by the participation of Al-rich detrital or tuffaceous material in the skarn-forming process. The metals Fe, Mn and Zn at Tumurtijn-ovoo display proportions similar to those of the modern ore precipitates in the Red Sea. In both deposits, Fe ores occur in the footwall of the base metal ores. These ores show Fe-Mn-Zn signatures like basalt suggesting that the initial ore solutions derived from basalt. A link with basalt is also supposed for the overlying Zn ores at Tumurtijn-ovoo on the basis of similar Zn/Cd ratios of basalt and the ores.


fe-mn-zn depositmineral distributionsmongoliaore compositionsore-free skarnssedimentary successionsskarnsphalerite-ore analoguestumurtijn-ovoo