44 New Data on Minerals. М., 2005. Vol. 40 UDC 549.351.12;548.735.4 EASILY OXIDIZABLE CHALCOPYRITE FROM BLACK SMOKERS OF THE RAINBOW HIDROTHERMAL FIELD (MIDATLANTIC RIDGE, 36°14'N) Farajallah Fardoost Lomonosov Moscow State University Nadezhda N. Mozgova Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Moscow, [email protected] Yury S. Borodaev Lomonosov Moscow State University Natalia I. Organova Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Moscow Lidia A. Levitskaya Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Moscow Anomalous chalcopyrite from newly discovered sulphide tubes of deep sea hydrothermal vents known as black smokers in the Rainbow field has been studied by a series of methods (Xray spectral microprobe analysis, miner- agraphy, scanning electron microscopy, and Xray microdiffraction method). In contrast to common chalcopyrite, the mineral quickly tarnishes in polished sections (highcopper sulphides of chalcocitedigenite series were detect- ed in the oxidized film). The newly polished surface is isotropic in reflected light; the reflectance spectra belongs to chalcopyrite type, but R coefficients are much lower than standard ones (by 1015%). The interval of values of microindentation VHN significantly exceeds those in common chalcopyrite (114235 to 181203 kgs/mm2). These characteristics indicate that the mineral is similar to two easily oxidizable cubic sulphides of the chalcopyrite group, talnakhite and putoranite. In the chemical composition, more copper than iron was noted in the limits expressed by the empirical formula Cu1x(Fe,Co,Ni)1+xS2, where x changes from 0 to 0.09 at a constant ratio Me/S=1. The min- eral is identified as standard chalcopyrite by Xray powder diffraction. There are two different reflection parts. The first ones are sharp and correspond to cubic cell with a=5.25 Å‘ . The second ones are wide that means disorder in chalcopyrite structure. Thus, Xray diffraction data also confirms the similarity of easily oxidizable chalcopyrite to talnakhite and putoranite. 3 tables, 4 figures, 24 references. In recent decades of the last century in chalcopyrite, that oxidized easily in polished hightemperature deposits of the Norilsk type sections in the open air, was encountered; simi- and in dunite pegmatites of Bushveld, four sul- lar to the abovementioned cubic minerals by phides have been found. They are close to chal- properties. The results of its detailed study are copyrite, but are distinguished from it by a defi- given below. ciency of sulphur: their ratio Me/S>1, whereas in chalcopyrite it is equal to 1. Two of these min- erals, talnakhite and putoranite, belong to the General characteristic of studied mineral cubic system. They can be oxidized easily in the open air in polished sections. Both minerals The samples of ores were picked up from a were originally described as cubic chalcopyrite depth of nearly 2300 m by deepwater inhabited (Bud'ko, Kulagov, 1963; Genkin et al., 1966; apparatus (DIA) «Mir1» during run 47 of the Filimonova et al., 1974). scientificresearch ship «Academician Mstislav In sulphide silts of the Red Sea in the Atlantis Keldysh» in 2002. They represent the small II deep, isotropic chalcopyrite was discovered tubes, branches of larger tubes. Their age, when and described; the proposed name was isochal- studied, was not more than two years, since dur- copyrite (Missack et al., 1989). This name ing the preceding run in 2000, the intense activ- appears in the Mineralogy Database on the ity of black smokers was not yet fixed in this sec- Internet as a mineral species that was not regis- tion of the ore field. tered in the IMA Commission on New Minerals Studied tubes belong to the copper type rep- and Mineral Names (CNMMN). resented mainly by minerals of the CuFeS and During study of oceanic sulphide ores from CuS systems in contrast to the zinc type, in black smokers of the Rainbow hydrothermal which sphalerite and iron sulphides predomi- field (MidAtlantic Ridge, 36°14'N), an isotropic nate. The tubes were small in sizes: their length Easily oxidizable chalcopyrite from black smokers of the Rainbow hydrothermal field (MidAtlantic Ridge, 36°14'N) 45 was 5–12 cm, diameter was from 2 up to 8 cm, the thickness of the walls of the largest speci- mens was 30 mm. In the centre of the tubes there were one or more channels (Fig. 1). Minerals and their structuraltextural correla- tions in tubes were studied in polished sections made without heating, in reflected light under ore microscope and scanning electron micro- scope JEM100C (IGEM). The results have shown that the tube walls had a distinct zoned structure that was described in detail in the spe- cial article (Borodaev et al., 2004). All copper tubes have a similar scheme of zoning; only the width of the zones varies. From the channel to the Fig. 1. Crosssection of zoned cooper tube with easily oxidizable tube surface are the following zones: I – a zone chalcopyrite (specimen FMM No. 4412M16). 1.5x composed of recently discovered Y phase, that is close to isocubanite (Mozgova et al., 2002); II – a chalcopyrite zone represented by an easily oxi- dizable variety of this mineral, which is described in this work; III – bornite zone, and IV – a zone formed by an assemblage of copper sulphides (chalcocite, digenite, etc.). The chalcopyrite zone that is of interest to us in comparison with other zones has a signifi- cantly larger width reaching 58 mm in some tubes. The structure of this zone (as well as the first one) is radiatefibrous with the size of sepa- rate elongated grains up to 400 mm. In newly polished sections in reflected light, a mineral composing this zone has a yellow colour and looks like common chalcopyrite, and is not eas- ily distinguished by reflection and colour from the Y phase in contact with it. Because of quick b oxidation in the open air, chalcopyrite looks pinkishbrown, and a border with the Y phase becomes clearly visible (Fig. 2a). At the contact between them, the lattice structure formed by lamellas of tarnished chalcopyrite in the matrix of Y phase is observed (Fig. 2b). During the study of easily oxidizable chal- copyrite in newly polished sections it was deter- mined that in reflected light the mineral behaves like an isotropic one: anisotropy was not observed even in immersion. Reflection was measured with automatic spectrophotometer MSFU3121; Si was the standard; objective was 20x0.40. The results have shown (Table 1, Fig. 3a) that the reflectance spectrum of chalcopyrite from the Rainbow is very close to the same spec- trum of chalcopyrite and cubic minerals of this group. At the same time, the values of reflection of studied mineral are much lower (on average 1015%) than the corresponding values of com- mon chalcopyrite and slightly less than the Fig. 2. Contact of a zone of easily oxidizable chalcopyrite with a reflection coefficients of talnakhite (on average zone of Y phase. Polished section in reflected light: a – zone of Y 23%) and putoranite (on average 12%). phase (white), zone of easily oxidizable chalcopyrite (grey); b – Measurement of reflection values of chalcopy- lattice structure of an aggregate directly on the contact of both rite covered by an oxidation film has shown the zones: lamellae are easily oxidizable chalcopyrite (grey), matrix is Y phase (white) 46 New Data on Minerals. М., 2005. Vol. 40 a Fig. 3. eflectance spec tra for easily strong decrease of R coefficients in comparison oxidizable chalcopy- with a newly polished surface (on an average of rite in comparison 2025% in the longwave part of the spectrum) with published data and opposite inclination of reflectance spectrum, (Chvileva et al., 1988): a) newly polished which is similar to spectra of digenite and chal- easily oxidizable cocite by configuration (Table 1, Fig. 3b). chalcopyrite from Microindentation VHN of the studied Rainbow (ChpR), chalcopyrite, which was obtained with PMT3 putoranite (Put), instrument at load 30 g, is was within the limits talnakhite (Taln), 2 and common chal- 114235 kgs/mm ; according to reference data copyrite (Chp); (Anthony et al., 1990), it is lower than corre- b) reflectance spec- sponding values of talnakhite (261277 tra for chalkopy- 2 2 rite covered by oxi- kgs/mm ) and putoranite (263 kgs/mm ) and dation film from significantly exceeds the fluctuation of values Rainbow (ChpR), of chalcopyrite (181203 kgs/mm2). digenite (Dig), and Thus, easy oxidizability in the open air as chalcosine (Cc) b well as optical characteristics indicate a consid- erable distinction of described chalcopyrite from Rainbow from common chalcopyrite and its similarity to cubic minerals of the chalcopy- rite group. Chemical composition Chemical analyses were performed with Xray spectral microprobe instrument CAME- BAXSX50 and energydispersive spectrometer Link ISIS on electron microscope JEM100C. Conditions of mesurements on CAME- BAXSX50 were: 20 kV, 30 nA; standards (ele- ment, line) were: CuS (CuKa), FeS (FeKa, SKa). Results obtained in comparison with data on Table 1. Reflectances of newly polished chalcopy- isochalcopyrite and theoretical chemical compo- rite from Rainbow (1), putoranite (2)*, tal- sitions of the chalcopyrite group minerals are nakhite (3)*, and common chalcopyrite (4)* given in Table 2. Chemical composition of the l, nm 1 2 3 4 studied mineral varied within small limits (wt %): Cu 31.2334.75; Fe 27.8732.26; S 34.9836.03. 400 12.3 14.6 13.0 – Admixtures of Co (to 0.34, in one analysis up to 420 14.1 16.1 15.2 21.3 3.56 wt %) and Ni (0.251.49 wt %) were noted in 440 16.6 18.7 19.2 27.9 insignificant amounts. In single instances Au (to 460 20.2 22.3 23.9 34.2 0.4 wt %) and Ag (0.1 wt %) were detected.