Sulfide associations in garnet-melilite-wollastonite skarns of the Tazheran alkaline massif, Baikal region
Sharygin V.V., Starikova A.E.
V.S.Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia
Garnet-melilite-wollastonite skarns situated near western nepheline-syenite body within the Tazheran alkaline massif were studied by optical methods, electron microprobe (EMPA) and scanning electron microscopy (SEM photos, EDS, element mapping). We used skarn samples of L.I.Panina (IGM SB RAS, Novosibirsk) which were collected by her together with A.A.Konev in 1970. This type of high-temperature skarns was well documented by Konev and Samoilov (1974). These gray to greenish fine-grained rocks are variable in the content of five principal minerals: melilite, wollastonite, garnet (grossular-Ti-andradite), diopsidic clinopyroxene and calcite. Cuspidine, nepheline, kalsilite, graphite, Mg-rich kirschsteinite, perovskite, apatite, K-Na-feldspar and sulfides occur in some species (Konev, Samoilov, 1974). Here we present new data on chemistry of sulfides from the Tazheran clinopyroxene-rich skarns.
Sulfides (1-2 vol.%) in the studied skarns usually form rounded or anhedral assemblages (up to 100-150 µm) between the principal minerals, rarely occurs as inclusions in silicates (clinopyroxene, Ti-andradite). Such assemblages are mainly represented by troilite and/or pyrrhotite, whereas other sulfides (penlandite, chalcopyrite, K-sulfide) are minor and localized in outer parts (Fig. 1). The relations of troilite and pyrrhotite have shown that troilite is an earlier phase than pyrrhotite. Individual grains of pentlandite, chalcopyrite and K-sulfide occur rarely. In addition, some assemblages occasionally bear sphalerite, alabandite, NiAs- and NiSb-phases (Fig. 1) as well as pyrite.
Figure 1. Sulfides in the Tazheran garnet-melilite-wollastonite skarns, BSE images. Symbols: Tro - troilite; Po - pyrrhotite; Pn - pentlandite; Cpn - cobalt pentlandite; Djr - potassic sulfide (djerfisherite); Ccp - chalcopyrite; Sp - Fe-rich sphalerite; Nc - nickeline NiAs; Alb - Fe-rich alabandite; Wo - wollastonite; Adr - andradite-melanite; Mel - melilite; Cal - calcite; Ne - nepheline. 3088-1, -2, -3, -9, -10 - different skarn samples near western body of nepheline syenites.
Troilite is close to ideal FeS and virtually free in Ni (<0.05 wt.%), whereas pyrrhotite has ratio Me/S=0.91-0.93, contains Ni (0.3-0.5 wt.%) and Co (up to 0.2 wt.%) and seems to be hexapyrrhotite Fe9S10 (Table 1). Monoclinic pyrrhotite was previously described in metasomatic rocks of the Tazheran massif (Konev, Samoilov, 1974). Pentlandite is rich in Co (6.1-7.2 wt.%), and some grains contain very high Co (up to 12.0 wt.%) approaching cobalt pentlandite. Pyrite FeS2 is also rich in Co (up to 1.4 wt.%). Chalcopyrite is close to stoichiometric CuFeS2 with a minor predominance of Fe over Cu.
Among all sulfides only K-sulfide drastically varies in composition. Cl-bearing and Cl-free species were found in sulfide associations, even in one rock sample (T3088-10, Table 1). At present day three K-sulfides with the structure based upon Fe8S14 clusters are known: djerfisherite K6Na0-1(Fe,Ni,Cu)24S26Cl, bartonite K6Fe24S26(S,Cl) and chlorbartonite K6Fe24S26(Cl,S) (Czamanske et al., 1981; Evans, Clark, 1981; Yakovenchuk et al., 2003). Djerfisherite is a cubic phase, whereas bartonite and chlorbartonite are the tetragonal analogs of djerfisherite. Cubic Cl-free analog of djerfisherite supported by XRD is not described anywhere. Nevertheless, the existing compositional data argue for natural appearance of both Cl-bearing and Cl-free varieties of bartonite and djerfisherite, even within a single specimen; and isomorphic substitution between Cl and S is a possibility (Czamanske et al., 1981; Barkov et al., 1997; Stoppa et al., 1997; Azarova et al., 2006). Thus, the microprobe compositional data are insufficient to correctly classify K-Cl-sulfide as djerfisherite or bartonite-chlorbartonite without XRD supplement. However, Cl-bearing K-sulfide in the Tazheran garnet-melilite-wollastonite skarns was undoubtedly identified as djerfisherite and supported by XRD (Konev, Samoilov, 1974). Hence, we suggest that Cl-free K-sulfide coexisted with djerfisherite seems to be a cubic phase (Cl-free djerfisherite). There are no essential differences in composition between djerfisherite and Cl-free djerfisherite. Both species strongly vary in Ni (2.2-16.0 wt.%), Co (0.1-1.3 wt.%) and Cu (1.1-13.8 wt.%) (Table 1).
Table 1. Representative compositions (EMPA, wt.%) of sulfides from the Tazheran garnet-melilite-wollastonite skarns.
Symbols: Tro - troilite; Po - pyrrhotite; Pn - pentlandite; Cpn - cobalt pentlandite; Djr - potassic sulfide (djerfisherite); Ccp - chalcopyrite.
Alabandite and sphalerite as well as NiAs- and NiSb-phases were identified under scanning microscope and not analyzed by microprobe. EDS and element mapping for Zn and Mn sulfides have shown that they are ferroan sphalerite and ferroan alabandite, respectively. NiAs-phase (up to 5 µm, Fig. 1) is free in Fe and S, sometimes contains Co. It shows the Me/As ratio equal to 1 and may be labeled as nickeline. In some sulfide assemblages NiSb-phase (size - 1-3 µm) occurs together with nickeline. EDS and element maps indicate that this phase rich in Sb and Ni, free in Fe, Co and S, poor in As. It seems to be breithauptite NiSb or nisbite NiSb2.
The study of the Tazheran skarns has shown that sulfide associations are very diverse and represented by troilite, pyrrhotite, Co-rich pentlandite, chalcopyrite and djerfisherite with minor appearance of Zn-Mn-rich sulfides and Ni-rich arsenides and antimonides. The further detailed investigations of skarns and other rocks will promote the findings of minerals (including sulfides) which will be new for the Tazheran alkaline massif.
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