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Ore-hosting metasomatic rocks of hydrothermal fields in the ore cluster «Semenov» region (Mid-Atlantic ridge, 13°31')

Vlasov E.A.*, Pertsev A.N.**, Kotova M.S.*, Beltenev V.E.***

* Moscow State University, Moscow Russia;**IGEM RAS, Moscow, Russia;*** Polar Marine Geosurvey Expedition, St. Petersburg, Russia.



The Semenov ore cluster discovered in 2007 as a result of the 30th Cruise of R/V Professor Logatchev comprises five ore fields: Western, Northwestern, Central, Northeastern, and Eastern. Currently, the Semenov ore cluster is the largest in the Mid-Atlantic Ridge [Beltenev et al., 2009].

The Semenov ore cluster located in non-transform fault of rift valley is related to extended near-latitude volcanic rise with serpentinized peridotite as basement [Beltenev et al., 2007]. Hydrothermal mineralizing processes are spatially related to both serpentinized peridotite and basalt. On the one hand, this is reflected in the localization of ore fields; for example, the Western field is spatially related to peridotite, whereas the other fields, to basalt [Beltenev et al., 2007]. On the other hand, both serpentinezed peridotite and basalt were hydrothermally altered.

Hydrothermal alteration of serpentinized peridotite is exhibited as substantially talc rock. Content of talc reaches 70% and occasionally 100%. In addition, there are carbonate, chlorite, and relic serpentine in the rock. Talc rock frequently inherits texture of replaced serpentinized peridotite. In general, talc flakes are very fine (not more than first microns in size). Locally, talc occurs as microflaky and microspherulite aggregates. In comparison with primary serpentinite, talc metasomatic rock is enriched in sulfides, which are dominated by pyrrhotite and pyrite. Ni sulfide whose composition is (Ni,Fe)4S5 occurs as inclusions in pyrite crystals. Pyrrhotite is partly or completely replaced by fine aggregates of Fe oxides. Ingrowths of atacamite and Cu sulfide determined as  yarrowite were identified in these aggregates. Total content of sulfides in serpentinite metasomatic rocks reaches 15 vol.%.

Basalt occurred in the Semenov ore cluster are irregularly altered. In addition to newly formed chlorite and talc in the least altered basalt, abundant segregations of native copper were identified; these segregations cause reddish hue on the fresh chip. Locally developed native copper occurs as plates and flattened dendrites up to 3 mm in size in fractures and occasionally as thin veinlets up to 200 microns long and 3-5 microns thick.

Basaltic metasomatic rocks of the Semenov ore cluster are predominantly talc-chlorite or quartz-chlorite. Chlorite and quartz occur as both close intergrowths and monomineralic segregations; quartz also occurs as micropockets and veinlets. In some cases, chlorite occurs as regular-shaped spherulites up to 1.5 mm in size. Chlorite of basalt metasomatic rocks is intermediate member of the chamosite-clinochlore series. Chlorite segregations are heterogeneous in composition. Elevated K content in chlorite suggest partial replacement by mica or hydromica.  

Actinolite-quartz-chlorite alteration is formed at the contact with relicts of primary rock (hornblende gabbro), Actinolite occurs as inclusions in quartz and replaces grains of magmatic hornblende. Gabbroic plagioclase is replaced by epidote, ilmenite, by titanite and rutile.

In general, mineral assemblages of basaltic altered rocks allow attributing them to propylitic type. The formation temperature of basaltic quartz-chlorite propylite estimated from chlorite geothermometer [Cathelineau, 1988] is 260-280°С. Actinolite-quartz-chlorite propylite with gabbroic relicts was formed at higher temperature 320-350 °С.

Magnetite, pyrite, and chalcopyrite are predominant ore minerals in basaltic alteration. Oriented intergrowths of chalcopyrite and cubanite were identified in one sample; such intergrowths are resulted from exsolution. Cubanite plates are selectively replaced by pyrite.

Sulfides of basaltic alteration are partially replaced by aggregates of covellite and hematite. Atacamite occurred as pockets and fine intergrowths with chlorite is confined to altered sulfides. The formation of late sulfides is caused by increasing oxygen fugacity as a result of mixing oceanic water ore-bearing fluid that is supported by intimately associated copper sulfides and atacamite. The similar processes were established in massive sulfide ore of the Semenov ore cluster.


This study was based on material collected during the 30th and 32nd Cruises of R/V Professor Logatchev of Polar Marine Geosurvey Expedition. Laboratory investigation was supported by RFBR grant (project no. 08-05-00825-а).



Beltenev V., Ivanov V., Rozhdestvenskaya I. et al. A new hydrothermal field at l3º30' N on the Mid-Atlantic Ridge // InterRidge News. 2007. Vol. 16. P. 9-10.

Beltenev V., Ivanov V., Rozhdestvenskaya I., Cherkashov G., Stepanova T., Shilov V., Davydov M., Laiba A., Kaylio V., Narkevsky E., Pertsev A., Dobretzova I., Gustaytis A., Popova Ye., Amplieva Ye., Evrard C. New data about hydrothermal fields on the Mid-Atlantic Ridge between 11° - 14° N: 32nd Cruise of R/V Professor Logatchev // InterRidge News. 2009. Vol. 18. P. 14-18.

Cathelineau M. Cation site occupancy in chlorites and illites as a function of temperature // Clay. Miner. 1988. V. 23. P. 471-485.