Zirconolite and Zr-Th-U minerals in chromitites of the Finero complex, Western Alps, Italy: evidence for carbonatite-type metasomatism in a subcontinental mantle plume

Zirconolite (CaZrTi2O7) associated with baddeleyite (ZrO2), thorianite (ThO2), uraninite (UO2), thorite or huttonite (ThSiO4) and zircon (ZrSiO4) has been discovered in chromitites of the Finero mantle-derived massif, in the Western Alps of Italy. The exotic minerals do not occur in late veins or fractures; they are part of the accessory assemblage (phlogopite, amphibole, apatite, ilmenite, geikielite, rutile, molybdenite, and Mg-Ca carbonates) formed as a result of metasomatism of the Finero mantle. Textural relations of zirconolite and the Zr-Th-U minerals indicate crystallization with the chromite - olivine - orthopyroxene assemblage between 800degrees and similar to600degreesC, at a given pressure of 1.0 GPa. Zirconolite is an indicator of silica-undersaturated conditions. Its composition, characterized by the prevalence of Th and U over Nb and Ta, is similar to that of zirconolite from some carbonatite complexes. The close association with Zr-Th-U minerals, and with the carbonatite-compatible elements Y, Hf, Pb, and LREE, is symptomatic of the carbonatitic character of metasomatism at Finero. The formation of a carbonatite-type liquid and related hydrous fluid is commonly related to the emplacement of mantle plumes in continental rift systems worldwide. This supports the inference that the metasomatism of the Finero mantle was a result of mantle diapirism at the base of the continental crust induced by extensional tectonics in pre-Hercynian times

[en] PHOTOPLASTICITY BEHAVIOR OF POLYESTER RESINS UNDER AND AFTER LOADING

The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ~5 km long and ~400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10-75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13-1,750 and Pd/Pt ratios of 1-73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4-6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic-chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24-1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 103-3.2 × 103) are lower than mantle (6.2 × 103) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ~1.3 × 103; komatiite: Cu/Pd ratio ~8 × 103) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1-8.8; gabbro unit: Nb/Ta = 11.5-13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5-16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif. © Springer-Verlag 2009.link_to_subscribed_fulltex

Another look at nagyagite from the type locality, Sacarimb, Romania: Replacement, chemical variation and petrogenetic implications

Extensive compositional heterogeneity is displayed by Pb-Sb-Au tellurides from the type locality at S $\check{\rm{a}}$ c $\check{\rm{a}}$ rîmb. These phases are collectively considered as varieties of nagyágite in the absence of crystal chemical data confirming the presence of distinct, but topologically closely related compounds. Chemical heterogeneity is seen relative to ‘normal’ nagyágite, with close to the ideal composition Pb3[Pb1.8(Sb1.1,As0.1)1.2]Σ3S6 (AuTe2), which is the primary and common type in the deposit. A modified formula, (Pb3S3)[(Pb2−x )(Sb,As,Te b )1+x (S3−y Te y )]Σ6(Au1−z−w Te2+z S w ), accounts for the chemical variation observed. Values of x (0.2 to 1.15) express substitution of Pb by Sb+As for Me2 in sulfosalt modules in the case of Au-depleted and low-Au nagyágite, and by Sb+As+Te b in high-As and low-Pb varieties (b = x+1−(Sb+As) = 0.24 to 0.29). Excess Te compensates for Au deficiency in the telluride layer, with substitution by S also observed; empirical values of z and w are 0 to 0.45 and 0 to 0.32, respectively. Minor substitution of Te for S (y < 0.17) is noted in all varieties except low-Au. These varieties are formed during replacement of the ‘normal’ type as seen in overprinting relationships in those veins reactivated during rotation of the duplex fault-system responsible for vein formation. Replacement is by coupled dissolution-reprecipitation reactions, as indicated by pseudomorphism of one nagyágite type by another in all cases. Variable rates of both molar-excess and -deficit reaction are invoked to explain the observed chemical and textural modifications. Low-Pb nagyágite is also present in zoned platelets where it grows over resorbed cores of ideal composition. Such platelets are instead interpreted as products of self-patterning in a residual precipitate. A marked depletion in the Au content of some nagyágite lamellae is considered to be a diffusion driven Te for Au substitution in the presence of Te-bearing fluid. Replacement of ‘normal’ nagyágite by other varieties can be linked to high fluid acidity, whereas replacement by galena-altaite symplectites relates to changes in the fTe2/fS2 within a narrow domain defined by coexistence of these two minerals. Nagyágite is a mineral with modular crystal chemistry and is able to adjust to variable rates of fluid infiltration by subtle chemical substitutions. The behavior of nagyágite will map and assist coupling between dissolution and precipitation during such reactions.C. L. Ciobanu, N. J. Cook, A. Pring, G. Damian and N. Căprar