Original paper

Sphalerite composition and ore genesis at the Tumurtijn-ovoo Fe-Mn-Zn skarn deposit, Mongolia

Gottesmann, Wolfram; Gottesmann, Bärbel Seifert

Abstract

Sphalerite is a major constituent of the Tumurtijn-ovoo skarn deposit. The deposit is hosted in a block of metamorphosed Devonian sediments enclosed within a polyphase granite intrusion. In the skarn, three stages of mineral formation are distinguished. Stage 1 is the main stage. It has a granoblastic texture and is therefore assumed to be the result of recrystallization of precursor lithologies. The Stage 1 assemblages consist chiefly of andradite, MnFe-spinels, sphalerite, quartz and calcite in various proportions. These minerals did not replace each other. Later stages characterized by replacement are formed by assemblages of rhodochrosite - sphalerite - pyrite and bustamite - sphalerite. The sphalerites show a wide range of chemical composition (Mn 0.01-9.32 wt%, Fe 0.10-6.54 wt%, Cu 0.00-4.79 wt%, Cd 0.00-0.32 wt%).In Stage 1 sphalerites (from the jacobsite-type sphalerite ore through Mn-magnetite-type sphalerite ore to magnetite-type sphalerite ore) the Mn content decreases from 9 to 0.2 wt% and the Fe content from 6 to 1 wt%, whereas the Fe/(Fe+Mn) ratio increases from 0.3 to 0.9 and the Zn/Cd ratio from 500 to 1000.These trends reflect equilibrium between the sphalerites and the MnFe-spinels. The equilibrium is suggested to be of pre-contact-metamorphic origin, because contact metamorphism has not erased the differences in sphalerite composition. The similarity between the composition of sphalerites included in andradite and in the MnFe-spinels and those not included in andradite and in the MnFe-spinels provides evidence that the crystallization of both kinds of Stage 1 sphalerite and the associated matrix minerals was coeval. The Fe and Mn contents of the cores and rims of Stage 1 sphalerite grains display variations that depend on the adjacent mineral. Thus, for example, the Fe content increases in the marginal part of sphalerite 1 where in contact with magnetite or jacobsite, but decreases where in contact with andradite. Mn shows analogous patterns of distribution. The relations between the paragenesis, texture and chemistry of Stage 1 sphalerites indicate cogenetic crystallization and argue against the introduction of ZnS into pre-existing Fe-Mn-rich calcsilicate-oxide rocks.The composition of sphalerites of the later replacement stages differs distinctly from that of Stage 1 sphalerites. These sphalerites are characterized by Zn/Cd ratios lower than 400 and low contents of Mn and Fe. They are considered to have formed as a result of remobilization of the components from Stage 1 sphalerites showing Zn/Cd ratios higher than 500 by migrating solutions. The high Zn/Cd ratios of Stage 1 sphalerites are believed to have been determined by basaltic source rocks.Drawing attention to the sedimentary features, notably the stratigraphic lamination, slumping textures, and intraclasts, and the general abundance of Fe, Mn, and Zn in the minerals of the deposit as a whole, and in Stage 1 sphalerites in particular, it is suggested that Tumurtijn-ovoo mineralization formed by a submarine hydrothermal process. This interpretation is consistent with the stratification of the ores, the conformable interdigitation of skarn and marble beds, and the tuffitic character of the footwall and the hangingwall rocks enclosing the skarn-marble lithostratigraphic unit. Contemporaneous vent-proximal precipitation of the elements Si, Fe, Mn, Zn, and S from high-temperature solutions emanating from a basaltic source into a lime-saturated environment with moderate oxygen activity could have provided appropriate conditions for the formation of the primary ores by reaction with the calcareous ooze. The minute grain size of Stage 1 ores and the zoning of Fe and Mn in the margins of Stage 1 sphalerite grains relate the present Stage 1 textures to a pre-contact-metamorphic crystallization process, a diagenetic to anchimetamorphic one.The position of the Tumurtijn-ovoo deposit within the scope of other submarine hydrothermal ore deposits is outlined by discussing (1) the close association of the elements Fe-Mn-Zn, (2) the high concentration of Mn in sphalerite, (3) the incorporation of Mn and Fe together in the structure of the chief minerals, and (4) the paragenesis of sulfide and oxide minerals, which formed in relation to the Eh-pH constraints that can be placed on the system.The closest analogues of the Tumurtijn-ovoo deposit are some Swedish sphalerite-bearing manganiferous skarn iron ores, although there are still significant differences.

Keywords

sphalerite oressphalerite compositioncompositional zoningnon-separation of fe-mncadmium variationsphalerite formationsphalerite remobilizationfe-mn-zn-type depositstumurtijn-ovoomongolia