Paragenetic relations of some Cu-Fe-Sn-Sulfides in the Mangualde pegmatitie,North Portugal

The textures of chalcopyrite from the Mangualde pegmatite suggest: a) myrmekitic intergrowths of chalcopyrite and complex stannite-kesterite-hexastannite inclusions formed by reactions involving: 1, eutectoid unmixing of Sn-Zn-bearing chalcopyrite solid solution into chalcopyrite and Zn-bearing stannite, 2, formation of hexastannite by reaction of the stannite with chalcopyrite, whereby kesterite formed as an intermediate product; b) chalcopyrite-hexastannite myrmekites crystallized as cotectic precipitates from liquid; hexastannite was partly replaced and rimmed by mawsonite during subsequent cotectic crystallization of chalcopyrite-mawsonite myrmekites; c) stannite inclusions in cubanite-bearing chalcopyrite formed by exsolution; d) in bornite-rich samples bornite and hexastannite crystallized before chalcopyrite. Phase relations in the pseudo-ternary cubanite-stannite-bornite Zn-bearing system are depicted; the association bornite-stannite is incompatible at lower temperatures, as below about 500 °C hexastannite and mawsonite intervene as intermediate compounds.

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Zusammenfassung Im Kupferkies des Mangualde-Pegmatit treten a) myrmekitische Verwachsungen von Kupferkies mit komplexen Zinnkies-Kösterit-Hexastannit-Einschlüssen auf, die durch folgende Reaktionen entstanden sind: 1. eutektoide Entmischung von Sn-Zn-haltigen Kupferkiesmischkristallen in Kupferkies und Zn-haltigem Zinnkies, 2. durch darauf folgende Bildung von Hexastannit aus der Reaktion zwischen Zinnkies und Kupferkies, wobei Kösterit als Zwischenprodukt gebildet wurde. b) Kupferkies-Hexastannit Myrmekite kristallisierten als cotektische Präzipitate; während der subsequenten cotektischen Kristallisation von Kupferkies-Mawsonit-Myrmekiten wurde der Hexastannit durch Mawsonit verdrängt und von diesem umsäumt. c) Die Stannit-Einschlüsse in Cubanit-haltigen Kupferkiesen sind als Entmischungen zu deuten. d) In Bornit-reichen Paragenesen kristallisierten Bornit und Hexastannit vor dem Kupferkies. Die Phasenbeziehungen im pseudoternären, Zn-haltigen Cubanit-Zinnkies-Bornit-System werden diskutiert; in diesem System ist die Assoziation von Bornit mit Zinnkies bei tieferen Temperaturen nicht möglich, da unterhalb von etwa 500 °C Hexastannit und Mawsonit als intermediäre Phasen intervenieren.

     

Ore deposition in a proterozoic incipient rift zone environment: a tentative model for the Filipstad-Grythyttan-Hjulsjö region,Bergslagen, Sweden

The 1.9–1.8 Ga Bergslagen Supracrustal Series comprises: an Early Volcanic Stage represented by the Lower Leptite Group, an Initial Rift Stage by the Middle Leptite Group, a Rift Stage by the Upper Leptite-hälleflinta and Slate Group, metabasites and the Granite-Granophyre Suite, and a Post-rift Stage by conglomerate beds, remobilized granite-granophyres and the Hyttsjö Gabbro-Tonalite Suite. The formation and subsequent alteration of iron, manganese and sulfide skarn ores in the Supracrustal Series involve: (1) late Initial Rift Stage exhalative-sedimentary processes possibly related to ascending granitic magma, (2) early Rift Stage exhalative-sedimentary and seafloor hydrothermal processes related to basic volcanism and intrusion and subvolcanic granite intrusion, (3) late Rift Stage hydrothermal metasomatic alteration and mineralization around subvolcanic granites, (4) Post-rift Stage deformation and metamorphism, (5) Post-rift Stage post-deformation recrystallization and skarn formation related to Hyttsjö diorites, and (6) post-Supracrustal Series metamorphic modifications.     

Epigenetic lead-zinc mineralization in miocene pebbly mudstones,Sierra de Cartagena,Spain

The Pb-Zn-ores of Minas de Cartes, Sierra de Cartagena, Spain, occur in a zon along the lower boundary of a Miocene sequence and along post-Miocene faults. Pebbly mudstone beds in the Miocene are slump sediments, which by their lithological properties form preferred horizons for the spreading of the mineralization. Faulting and flexuring have caused cavernous fault breccias in the relatively competent pebbly mudstones, while dissolution of carbonate pebbles and carbonates in the matrix of these rocks has resulted in solution cavities. The brecciated cavernous pebbly mudstones form channelways for ore solutions and sites of deposition of ore streaks with marcasite, pyrite, galena, sphalerite and quartz. The mineralization is of typical cavity-filling nature and so-called “ore pebbles” are fillings of pebble-shaped voids resulting from selective dissolution of carbonate pebbles. The ores were formed by near-surface subvolcanic-hydrothermal processes in relation with Late Tertiary trachyandesitic volcanism in the region.     

Ore minerals in the telluride-bearing gold-silver ores of Salida,Indonesia, with special reference to the distribution of selenium

Te-Se-bearing gold-silver ores from Salida in Sumatra, Indonesia, show carbonate and sulfidic diffusion bands in quartz incrustations. The sulfidic diffusion bands show a fine diffusion zoning with: 1. an earliest or inner zone with concentrations of Zn, Cu, Fe, S (sphalerite, chalcopyrite, pyrite); 2. an intermediate zone with concentrations of Pb, Au, Ag, S, Te, Se (galena and Au-Ag-tellurides in Te-bearing parageneses; galena, electrum and acanthite in Te-free parageneses); and 3. a zone of As-bearing minerals (tennantite, enargite or luzonite, arsenpolybasite) superimposed on the first two zones. In the telluride-bearing ores paragenetic relations suggest that galena, altaite, hessite, and -phase and x-phase solid solutions originally crystallized above about 120 °C; on cooling this assemblage equilibrated into one stable below 50 °C and consisting of galena, altaite, hessite, hessite-sylvanite intergrowths, and hessite-petzite aggregates. Se is concentrated in sulfides of the intermediate diffusion zones; microprobe analyses indicate up to 4.0 weight percent Se in some acanthites and up to 0.4⁵ weight percent Se in some galenas; arsenpolybasite also contains up to 4.0 weight percent Se but only when replacing seleniferous acanthite. The concentration of Se with Pb, Au, Ag, S, Te in intermediate diffusion zones is ascribed to solution differentiation during solute diffusion. Microprobe analyses are given of hessite, sylvanite, petzite, minerals of the pearceite-polybasite group, mckinstryite, and the Fe-, Mn- and Cd-contents in sphalerites.

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Zusammenfassung Te-Se-führende Gold-Silber Erze von Salida in Sumatra, Indonesien, zeigen karbonatische und sulfidische Diffusionsbänder in Quarzkrusten. Die sulfidischen Diffusionsbänder zeigen eine feine Zonierung mit: 1. einer ältesten, inneren Zone mit Konzentrationen von Zn, Cu, Fe, S (Zinkblende, Kupferkies, Pyrit); 2. einer intermediären Zone mit Konzentrationen von Pb, Au, Ag, S, Te, Se (Bleiglanz und Au-Ag-Telluride in Te-haltigen Paragenesen; Bleiglanz und Akanthit in Te-freien Paragenesen); und 3. einer die erstgenannten Zonen überlagernden Zone von As-haltigen Mineralen (Tennantit, Enargit oder Luzonit, Arsenpolybasit). Die paragenetischen Beziehungen der Telluride deuten auf eine ursprüngliche Kristallisation von Bleiglanz, Altait, Hessit und - und X-Phase Mischkristalle bei Temperaturen oberhalb 120 °C; bei der Abkühlung wurde diese Mineralassoziation ersetzt durch eine unterhalb 50 °C stabile Paragenese von Bleiglanz, Altait, Hessit, Hessit-Sylvanit-Verwachsungen und Hessit-Petzit Aggregaten. Selen hat sich in den Sulfiden der intermediären Diffusionszonen angereichert. Mikrosondeanalysen zeigen Se-Gehalte bis 4.0 Gew. % in Akanthit und bis 0.4⁵ Gew. % in Bleiglanz. Arsenpolybasit kann ebenfalls bis 4.0 Gew. % Se enthalten, aber nur wenn er als Verdränger von Se-reichen Akanthiten auftritt. Die Ursache der Anreicherung von Se zusammen mit Pb, Au, Ag, S, Te in den intermediären Diffusionszonen ist in einer Art von Lösungsdifferentiation während des Diffusionsprozesses zu suchen. Mikrosondeanalysen von Au-Ag-Telluriden, Mineralen der Pearceit-Polybasit Gruppe, Mckinstryit, und den Fe-, Mn- und Cd-Gehalten der Zinkblenden werden gegeben.

     

Isotopic dating of the emplacement of the ultramafic masses in the Serrania de Ronda,Southern Spain

A Rb-Sr analysis of suites of samples from a small intrusion of cordierite-bearing alkali granite into the peridotite of the Sierra Bermeja (Serrania de Ronda) yields an age of 22± 4 Ma ( = 1.42×10^–11 a^–1): Late Oligocene/Early Miocene. It is believed that the intrusion was derived from contact-anatectic melts produced along the hot ultramafic mass during and/or directly following its tangential, tectonic dislocation from a mantle diapir. Its age can thus be taken as dating the termination of the hot emplacement of the ultramafic masses. K-Ar dates of biotites and Rb-Sr dates of biotite/whole-rock pairs in contact-metamorphic wall rocks along the ultramafics mostly lie between 19.5 and 18.5 Ma. This probably indicates that about 19 Ma ago the contact-zones of the ultramafic masses had cooled down to the blocking temperature of biotite to Rb-Sr and K-Ar.