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Inferences of Sr, Nd and O isotopic tracers on the origin and evolution of a gabbronorite-granodiorite sequence from southern Hercynian chain of Sardinia - A case study from the Arburèse igneous complex and its comparison with the erlier sequences of Sarrabus area.
No full textInto the depth of the Arburèse vein system: arsenide-sulfide evolution in the Ni-Co ores
No full textThe Arburese region (SW Sardinia) is well-known for a large swarm of mineralized veins hosted in low-grade metamorphic rocks, extending for about 20 km around the Late Variscan (304±1 Ma) Arbus pluton, made up of a core of cordierite-bearing biotite leucogranites and an external shell of granodiorites with subordinate mafic rocks (Cuccuru et al., 2015). The largest veins occur in the northern and western sectors (past Montevecchio district), with a primary mineral association of Pb-Zn (Cu, Ag) sulfides in quartz and siderite. Recent works on the southern branch of the vein system (Naitza et al., 2015) evidenced a more complex metallogeny, with Ni-Co-As-Sb-Bi-Pb-Zn-Cu-Ag ores. New microscopy, SEM-EDS and EPMA data allow to highlight the depositional history of these ores, defining two stages of mineralization. In the Arsenide stage, early precipitation of quartz with abundant Ni monoarsenide (nickeline) was followed by growth of Ni antimonide (breithauptite) on nickeline, by Ni, Ni-Co, Co, Fe diarsenides, triarsenides, sulfoarsenides (rammelsbergite, skutterudite, safflorite/löllingite, cobaltite, gersdorffite/ullmannite) and Bi minerals (bismuthinite and native Bi), replacing nickeline and forming rims and inclusions on previously crystallized phases. The Sulfide stage was preceded by cataclasis of the ores; initial abundant precipitation of quartz, siderite and sphalerite, cementing the fragments of the arsenide ores, was followed by Pb-Cu-Ag sulfides (galena, chalcopyrite, tetrahedrite/freibergite, bournonite, proustite/pyrargirite, stephanite), and, at last, by ankerite, calcite and pyrite. These sequences of mineralization suggest a relationship with distinct hydrothermal fluids: 1) an earlier fluid, carrying Ni, Co, As and Sb, deposited as arsenides and sulfoarsenides into structurally-controlled traps in response to variations in physicochemical conditions; 2) a late, carbonate and Pb-Zn-Cu-Ag -rich fluid, permeating fractures re-opened after a tectonic phase that involved the previous mineralization, and originating a “Montevecchio-type” mineral association. Depositional histories similar to the Arsenide stage have been reported in other hydrothermal Ni-Co deposits worldwide, as in the Bou Azzer district, Morocco (Ahmed et al., 2009). The geochemistry of these ores requires the presence in the geological sequence of a Ni-Co-rich source (e.g. mafic rocks) from which these elements could be leached by acidic and moderately oxidizing hot fluids. Analogous conditions may have been attained in the Arbus pluton, which includes an olivine-bearing end member (Secchi et al., 1991), possible Ni-Co source for fluids of magmatic origin. During the emplacement of the igneous complex, widespread uralitization of pyroxenes in granodiorites point to high fluid/rock interactions; moreover, quartz- Fe-cordierite graphic intergrowths in leucogranites support a late dehydration of magmas.
References
Ahmed A.H., Shoji A., Ikenne M. (2009). Mineralogy and Paragenesis of the Co-Ni Arsenide Ores of Bou Azzer, Anti-Atlas, Morocco. Econ. Geol. 104, 249-266
Cuccuru S., Naitza S., Secchi F., Puccini A., Casini L., Pavanetto P., Linnemann U., Hofmann M., Oggiano G. (2015): Structural and metallogenic map of late Variscan Arbus Pluton (SW Sardinia, Italy), Journal of Maps, DOI: 10.1080/17445647.2015.1091750.
Naitza S., Cuccuru S., Oggiano G., Secchi F. (2015). New observations on the Ni-Co ores of the southern Arburese Variscan district (SW Sardinia, Italy). Geoph. Res. Abs. 17, EGU2015-12659.
Secchi F. A., Brotzu P., Callegari E. (1991). The Arburèse igneous body (SW Sardinia, Italy) - An example of dominant igneous fractionation leading to peraluminous cordierite-bearing leucogranites as residual melts. Chem. Geol., 92, 213-249
Redox state of magmas and granite-related Mo mineralization: evidences from Late Variscan F- bearing granites from Southern Sardinia (Italy)
No full textRedox state of magmas has been invocated to explain the metallogenic behaviour of intrusive magmatism (Ishihara,
1981). Thus, Sn deposits are linked to low-fO2, ilmenite series, while Mo deposits are better bracketed into high-fO2,
magnetite series. In Variscan Europe Mo deposits are represented in a few districts, including Eastern Erzgebirge,
France and Sardinia. In Sardinia, small Mo deposits are related to a post-collisional F-bearing granite suite dated close
to 290 Ma by Re/Os on molybdenite (Boni et al., 2003) and 40Ar/39Ar on biotite (Dini et al., 2005). This late Variscan
suite is dominated by metaluminous biotite leucogranites grading to hololeucratic microgranitic to granophyric
varieties; amphibole biotite monzogranites are locally observed (Monte Sette Fratelli). They emplaced at shallow crustal
levels and locally (Ogliastra) grade to felsic volcanics. Thermal effects of the intrusions are limited to narrow contact
aureolas (andalusite zone) around the plutons. In SW Sardinia (Sulcis and Monte Linas intrusions), both Mo and Sn
deposits occur. Magmatic bodies show a distinctive magmatic zonation, with medium-grained granites dimembered
upward by thick, flat-lying fine-grained to porphyritic varieties, including fayalite-bearing pegmatite layers. Finegrained
to porphyritic rocks suffered various degrees of greisening, and host Mo ores, related to numerous small endoand
exo- quartz-muscovite greisens (e.g. Perda Lada), grading to quartz vein and stockwork systems (Perd'e Pibera, Su
Seinargiu, Flumini Binu). Ores are commonly dominated by molybdenite, with subordinate pyrite, chalcopyrite and
wolframite. The studied deposits are related to ilmenite rock -series, as evidenced by petrography (opaque contents of
granites <1%; ilmenite>>magnetite), and confirmed by geochemical ratios (Rb/Sr, FeO/Fe2O3, K/Rb). This behavior
seems to contrast with the Mo/magnetite series association, and is more coherent with the presence of Sn vein deposits
in the same areas (Naitza et al. 2015). A possible explanation involve changes of physicochemical parameters (in
particular, fO2 and HF activity) from magmatic processes during magma emplacement to greisening; variation of redox
conditions are confirmed by discontinuous reverse zoning of plagioclase laths in fine-grained facies, indicating a
progressive PH2O increase with magmatic evolution. Mineralization prevalently occurred in close system conditions,
although in some areas (Oridda, Su Seinargiu, Flumini Binu) Mo ores are centered on bodies of porphyritic rocks,
showing features close to porphyry-style mineralization (Fiori et al., 1986).
Boni M., Stein H.J., Zimmerman A. & Villa I.M. 2003. Re-Os age for molybdenite from SW Sardinia (Italy): a
comparison with 40Ar/39Ar dating of Variscan granitoids. In: Eliopulos K. et al., Eds., Mineral exploration and
sustainable development. Millpress, Rotterdam, 247-250.
Dini A., Di Vincenzo G., Ruggieri G., Rayner J. & Lattanzi P. 2005. Monte Ollasteddu, a new gold discovery in the
Variscan basement of Sardinia (Italy): first isotopic (40Ar-39Ar, Pb) and fluid inclusion data. Miner. Deposita, 40,
337-346.
Fiori M., Garbarino C., Padalino G. & Masi U. 1986. Chemical features of wallrocks from Mo-showings of Sardinia
(Italy). Rend. Soc. It. Mineral. Petrol., 41, 25-39.
Ishihara S. 1981. Granitoid series and mineralization. Economic Geology, 75th anniversary volume, 458-484.
Naitza S., Secchi F., Oggiano G. & Cuccuru S. 2015.
The search for the parental magma of the high-K calc-alkaline igeous rock series in the southernmost Sardinia Batholith
No full textThe Arburese igneous complex (SW Sardinia, Italy)- an example of dominant igeous fractionation leading to peraluminous cordierite-bearing leucogranites as residual melting.
No full textThe search for the parental magma of the high-K calc-alkaline igeous rock series in the southernmost Sardinia Batholith
No full textThe Arburese igneous complex (SW Sardinia, Italy)- an example of dominant igeous fractionation leading to peraluminous cordierite-bearing leucogranites as residual melting.
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