62 New Data on Minerals. М., 2007. Volume 42 FEATURES OF TRANSFORMATION OF THE MINERAL PARAGENETIC ASSEMBLAGES FROM COPPER SULFIDE ORES OF THE KRASNOV HYDROTHERMAL FIELD (16°38' N MidAtlantic Ridge) Nadezhda N. Mozgova Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS Moscow, Russia, [email protected] Yury S. Borodaev Geology Department, Lomonosov Moscow State University, Moscow, Russia Irina F. Gablina Geological Institute, Russian Academy of Sciences, Moscow, Russia Tamara V. Stepanova AllRussia Research Institute for Geology and Mineral Resources of the World Oceans, St. Petersburg, Russia Georgiy A. Cherkashev AllRussia Research Institute for Geology and Mineral Resources of the World Oceans, St. Petersburg, Russia Tat’yana Yu. Uspenskaya Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia Copper sulfide ores of the 5–80 thousands age from the Krasnov relic hydrothermal field (16°38' , MidAtlantic Ridge) have been studied with optical microscope, electron microscope equipped with energy dispersion system (SEMEDS), electron microprobe, and Xray diffraction. According to the mineralogy, three types of paragenetic assemblages are recognized. Type I is homogeneous isocubanite with Curich sulfides (bornite and copper sulfides of the chalcocitedigenite series). Type II is exsolved isocubanite and its unique products of subsequent transformation. Type III is oxidesulfatesulfide, which is char- acterized by iron specialization and is divided into two subtypes: with predominant pyrite (IIIa) and with predom- inant FeCu sulfates and Fe oxides (IIIb). Type III contains relics of the highly modified type II paragenesis. The age relationship of type I has not been established. 6 tables , 8 figures, 23 references. General review of the Krasnov ley and is controlled by the intersection of the hydrothermal ore field nearmeridian deep margin fault and the sublat- itudinal fault. Approximately 60% of the bot- The new hydrothermal field at 16°38' N of tom surface, which was studied with teleprofil- the MidAtlantic Ridge (MAR) was discovered ing is covered by carbonate sludge up to in 2004 by the Polar Marine Geosurvey 160 cm thick. Two sulfide ore bodies of 370 x Expedition and AllRussia Research Institute 580 m and 110 x 130 m in size occur over the for Geology and Mineral Resources of the outcrops of primary basalts and occur as fine World Ocean, (St. Petersburg, Russia) during and medium blocks and relics of sulfide Cruise 24 of the R/V Professor Logachev. mounds covered by sediments and red crusts. During a subsequent voyage in 2006, this field Sulfide bodies are composed mainly of was extended and was named Krasnov in honor pyrite; copper and copper massive sulfide ores of Sergei Gelievich Krasnov (19521996), a are less abundant and zinc massive sulfide ores marine geologist, who investigated oceanic are practically absent. According to texture hydrothermal mineralization for the last 15 and morphology, massive, porous, and vein- years of his brilliant professional career. Brief letimpregnated ores, as well as ores of the information on geology, zoning and mineralo- chimney complex were defined in the ore field. gy of ores was previously reported by Bel'tenev In addition to massive sulfides, the orebearing et al. (2006). sediments with Fe > 10% and Cu+Zn > 0.25% The Krasnov ore field, located in the east- occurred as a band 30–150 m wide north and ern margin of the rift valley at a depth of northeast of Orebody 1, and are found within 3700–3760 m is confined to the junction zone the ore field. Pyrite, atacamite, aragonite, chal- of the axial rift rise and the slope of the rift val- cocite, native copper, hematite, barite, and Fe Features of Transformation of the Mineral Paragenetic Assemblages from Copper Sulfide Ores of the Krasnov Hydrothermal Field (16° 38' N MidAtlantic Ridge) 63 and Mn hydroxides were identified in these 200, Geological Institute, Russian Academy of sediments. Sciences, Moscow. In the latter case, the uti- The high oxidation of the ores, weak anom- lized software requires the normalization pro- alies of metals content in dissolved and sus- cedure to 100% of the total elements recorded pended phases in water samples, as well as the in spectrum. A number of analyses was per- absence of hydrothermal fauna, indicate the formed with a Camebax SX50 electron micro- relict nature of the ore field, of which the probe, Lomonosov Moscow State University. hydrothermal activity has actually terminated. Accelerating potential was 20 kV, current, This is confirmed by the dating of sulfide ores; 30 nA. The following standards were used (Ka the U/Pb age determined for 16 samples Xray lines): FeS(Fe, S), CuS (Cu), ZnS (Zn). ranges within 5–80 kyr (Kuznetsov et al., Most analyses were made with SEMEDS since 2007). Iron specialization, continuance of for- the low current allowed measurements of mation, and the presence of sedimentary cover numerous points in the fine grains of minerals. differentiate this field from the other Total analyses were about 500. Xray study was hydrothermal fields within the MidAtlantic performed with a Rigaku D/Max 2200 diffrac- Ridge. tometer, CuKa radiation in IGEM RAS and a According to Bel'tenev et al. (2004), the ore RKD 57.3 camera (Fe radiation, 6 h exposure) types and major minerals were recognized as a in Fersman Mineralogical Museum, Russian result of the initial mineralogical and geo- Academy of Sciences, Moscow. chemical studies of the new field. The aim of this study is to provide more detailed textural, structural, and chemical investigation of ores Mineralogy of the copper ores from from the chimney complex of the copper min- the Krasnov field eralization and to reveal the features of trans- formation of the mineral paragenetic assem- Sulfides common for the oceanic ores from blages in the hydrothermal field that is uncom- the MidAtlantic Ridge were identified in the mon for this region. studied samples. Isocubanite, chalcopyrite, and pyrite are the major minerals; bornite and copper sulfide are minor and sphalerite and Materials and experiments marcasite are rare. Prevalence of idaite, Cu3FeS4, previously described from the subma- Samples of the copper mineralization have rine ores of the black smokers at 21o N East been studied as the most interesting in terms of Pacific Rise (Oudin, 1983) and the presence of mineralogy and the easiest transformation due new phases of the CuFeS system with provi- to the high mobility of copper. The samples sional names X, Y, and M are noticeable. In occur as irregularlyshaped fragments contain- addition to sulfides, FeCu sulfate and Fe ing copper minerals and are frequently cov- oxides are important; quartz, opal, barite, and ered by the black soot crust. Small pieces of native sulfur are also abundant. these samples were fixed by epoxy to prepare The size of studied sulfide grains ranges polished sections (without heating). from 0.3–0.5 mm (occasionally up to 1 mm), After study under reflected light, but usually ranges from 1–2 to 50 mm. microsamples and polished sections were Diagnostics of the minerals have been per- investigated with a JEM100C scanning elec- formed by electron microprobe and SEMEDS; tron microscope and a CamScan MX 2500S in some cases, an Xray diffraction study was scanning electron microscope equipped with a applied. The representative data are given in Link10000 energy dispersion system (acceler- Tables 1–6, in ternary plot CuFe(Zn)S ating potential 25 kV, current 0.4 nA) (Fig. 1), and in photomicrographs of polished (SEMEDS), Institute of Geology of Ore sections and microsamples (Figs. 2–8). Deposits, Petrography, Mineralogy, and Isocubanite, the cubic analogue of rhombic Geochemistry, Russian Academy of Sciences cubanite, plays the most important role in the (IGEM RAS), Moscow and a CamScan MV2300 copper ores of Krasnov. Some explanations in scanning electron microscope equipped with regard to this mineral appear to be necessary. As an energy dispersion system – INCA Energy a new mineral species, it was found for the first 64 New Data on Minerals. М., 2007. Volume 42 Table 1. Chemical composition of isocubanite (polished sections and microsamples) Anal. Sample Cu, wt.% Fe S Total Formula Fig. 1** 12714 25.56 38.57 35.89 100.02 Cu1.1Fe1.9S3.0 5d 2 12696/1 23.42 40.16 35.98 99.56 Cu1.00Fe1.95S3.05 3* 12696/1 23.09 42.27 35.29 100.65 Cu0.98Fe2.05S2.97 4* 12696/1 22.77 42.45 35.75 100.98 Cu0.96Fe2.04S3.00 4d 5* 12696/2c 22.92 42.03 35.32 100.27 Cu0.98Fe2.04S2.98 6 12696/1 22.67 44.87 34.64 99.18 Cu0.98Fe2.06S2.96 4b 7 12696/1 21.97 42.07 36.17 100.21 Cu0.93Fe2.03S3.04 8 12696/1 21.88 42.01 34.58. 98.47 Cu0.95Fe2.07S2.98 4b 9 12714 21.34 42.57 36.26 100.17 Cu0.91Fe2.05S3.04 7b 10* 12714 21.29 43.46 36.39 101.14 Cu0.89Fe2.08S3.03 8c Notices: * is EDSSEM analysis, without *, is electron microprobe analysis. ** is microsample. Formula coefficients for phases ana- lyzed in polished sections are accurate within two decimal digits, in microsamples, to one Table 2. Chemical composition of chalcopyrite and phase X (polished section and microsamples) Anal. Sample Cu, wt.% Fe S Total Formula Fig. Chalcopyrite 1* 12696/1 36.73 29.51 33.77 100.01 Cu1.07Fe0.98S1.95 4а 2* 12696/1 35.21 30.69 34.09 99.99 Cu1.02Fe1..02S1.96 3* 12696/2c 34.50 31.17 34.33 100.00 Cu1.00Fe1..03S1..97 4c Phase X 4 12696/2c 33.12 35.58 34.72 103.42 Cu0.93Fe1.14S1..93 5 1356/1 32.86 29.58 37.40 99.84 Cu0.94Fe0..96S2.11 6а 6 12696/2c 32.54 35.42 33.64 102.12 Cu0.93(Fe1.15Zn0..02)1.17S1.90 3e 7 12696/2c 32.49 37.14 33.65 103.28 Cu0.92Fe1.19S1..89 3e 8 12696/2c 31.41 31.76 35.41 98.58 Cu0.91Fe1..05S2.04 9* 12696/2c 31.20 32.64 34.66 98.50 Cu0.91Fe1..08S2.01 10* 12696/2c 30.98 34.18 34.37 99.53 Cu0.90Fe1.12S1.98 11 12696/2c 30.72 35.50 34.57 100.79 Cu0.88Fe1.16S1.96 12 1356/1 32.41 33.10 33.96 99.99 Cu0.94(Fe1.09 Zn0.02)1.11S1.95 6а 13** 12696/1 29.21 32.14 38.65 100.00 Cu0.8Fe1.0S2.2 Notices: 0.62 (anal.