Sulfide Breccias from the Semenov-3 Hydrothermal Field, Mid-Atlantic Ridge: Authigenic Mineral Formation and Trace Element Pattern
Irina Yu. MelekestsevaВ. В. МасленниковN. P. SafinaPaolo NimisS. O. MaslennikovaV. E. Bel’tenevI. I. RozhdestvenskayaL DanyushevskyRoss R. LargeDmitry A. ArtemyevVasily A. KotlyarovLuca Toffolo
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Abstract:
The aim of this paper is the investigation of the role of diagenesis in the transformation of clastic sulfide sediments such as sulfide breccias from the Semenov-3 hydrothermal field (Mid-Atlantic Ridge). The breccias are composed of marcasite–pyrite clasts enclosed in a barite–sulfide–quartz matrix. Primary hydrothermal sulfides occur as colloform, fine-crystalline, porous and radial marcasite–pyrite clasts with inclusions or individual clasts of chalcopyrite, sphalerite, pyrrhotite, bornite, barite and rock-forming minerals. Diagenetic processes are responsible for the formation of more diverse authigenic mineralization including framboidal, ovoidal and nodular pyrite, coarse-crystalline pyrite and marcasite, anhedral and reniform chalcopyrite, inclusions of HgS phase and pyrrhotite–sphalerite–chalcopyrite aggregates in coarse-crystalline pyrite, zoned bornite–chalcopyrite grains, specular and globular hematite, tabular barite and quartz. The early diagenetic ovoid pyrite is enriched in most trace elements in contrast to late diagenetic varieties. Authigenic lower-temperature chalcopyrite is depleted in trace elements relative to high-temperature hydrothermal ones. Trace elements have different modes of occurrence: Se is hosted in pyrite and chalcopyrite; Tl is related to sphalerite and galena nanoinclusions; Au is associated with galena; As in pyrite is lattice-bound, whereas in chalcopyrite it is related to tetrahedrite–tennantite nanoinclusions; Cd in pyrite is hosted in sphalerite inclusions; Cd in chalcopyrite forms its own mineral; Co and Ni are hosted in chalcopyrite.Keywords:
Marcasite
Authigenic
Bornite
Chalcocite
Anhydrite
The sulfide mineralization of die Camaqua Mines (Cu) and of the Santa Maria Deposit (Pb-Zn). State of Rio Grande do Sul, southern Brazil, is associated with red bed conglomerates and sandstones, deposited hi an alluvial fan environment at the end of the Brasiliano Event. The non opaque cement and ore mineral analysis allow to identify four main phases hi the mineralization process occurred after the sedimentation: the eodiagenetic and mesogenetic phases are represented by the formation of hematite I-rutile and pyrite I-marcasite, respectively; the late diagenetic to epigenetic phase has occurred at deep burial levels and it was superimposed to the least non opaque cements. It comprises the galena-sphalerite-chalcopyrite paragenesis in the Santa Maria Deposit and the pyrite Il-chalcocite I-bornite I-chalcopyrite paragenetic sucession in the Camaqua Mines; the paleo-oxidation/cementation phase took place after uplift and unroofing. It is well observed in the Camaqua Mines where the previous sulfide phases were altered and replaced by die hematite IT-bomite Il-chalcocite TI-covellite. In the Santa Maria Deposit occur the hematite n-stephanite-chalcocite II-covellite-native silver paragenesis; and the tectonic remobilization phase of the previous ore cements has occurred during bed tilting and leaded to the formation of pvrite-bomite-chalcopyrite-quartz and hematite-bornite-chalcocite-barite-calcite veins in the Camaqua Mines, and only of veinlets with galena-sphalerite in die Santa Maria Deposit. The deposition of die Guaritas Formation occurred when die mineralization process had finished, i.e. after the tilting of die Bom Jardim Group beds.
Chalcocite
Bornite
Covellite
Marcasite
Paragenesis
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A relatively simplified calculation method, presented on two and three dimensional diagrams of log a Cu – log a FeS2 - log f S2 for the Cu-Pe-S system (T = 298 K, p = 1 bar), includes the regions and fields of stability for chalcocite, digenite, bornite, chalcopyrite, covellite, and pyrite (marcasite). The utility of the aforementioned diagrams for the interpretation of sulphide mineralization in black shales is demonstrated with Fore-Sudetic copper resources as an example.
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Marcasite
Arsenopyrite
Sulfide Minerals
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Bornite
Covellite
Chalcocite
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Cassiterite
Hypogene
Pentlandite
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The Number One orebody is the largest and most sulfide-rich ore zone (at least 200,000 tons containing 8.4 wt % Cu) at the Ruby Creek copper deposit, in the southwestern Brooks Range of Alaska. Pyrite and copper-bearing sulfide minerals are concentrated within the matrix of a dolostone breccia body, which is enclosed by phyllite and calcic marble of Middle to Late Devonian age. The Number One orebody has three mineralogical zones that grade into each other: (1) an outer zone, widest toward the hanging wall, containing mostly pyrite with minor amounts of chalcopyrite and traces of carrollitc and sphalerite; (2) an intermediate zone containing major chalcopyrite and pyrite, minor tennantite-tetrahedrite, bornitc, carrollitc, and sphalerite, and traces of galena; and (3) a core zone containing major bornitc, chalcopyrite, pyrite, and chalcocite, minor carrollitc, digenite, and sphalerite, and traces of galena, covellite, and the germanium-bearing sulfides renierite and germanitc. Small clots of anthracitelike organic material (anthraxolite) are found throughout the ore. Much of the pyrite is fine grained and was deposited before the other sulfides, being increasingly replaced by Cu-bearing sulfides from the outer zone to the core. Some of this pyrite recrystallized into coarser grains having cobaltiferous rims, and these grains were generally not replaced. The sulfide minerals are commonly pseudomorphous after lath-shaped crystals within the dolomite clasts; no unreplaced examples were found, though the crystals closely resemble those of marcasite. The abundant fine-grained, porous pyrite also may represent replacement of marcasite. The origin and timing of brecciation and ore deposition remain unknown, though dolomitization and ore deposition appear to have occurred in preexisting carbonate breccia. Close mineralogical and geologic similarities are noted with the Kipushi Cu-Zn-Pb deposit in Zaire and with several other carbonate-hosted copper-rich organic-bearing deposits. The common concentration of the rare metals Co, Ge, and Ga in these structurally and mineralogically complex ores should increase their economic attractiveness.
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