Magmatic–hydrothermal molybdenum isotope fractionation and its relevance to the igneous crustal signature

We analysed the Mo isotope composition of a comprehensive series of molybdenite samples from the porphyry- type Questa deposit (NM, USA), as well as one rhyolite and one granite sample, directly associated with the Mo mineralization. The δ98Mo of the molybdenites ranges between −0.48‰ and +0.40‰, with a median at −0.05‰. The median Mo isotope composition increases from early magmatic (−0.29‰) to hydrothermal (−0.05‰) breccia mineralization (median bulk breccia = −0.17‰) to late stockwork veining (+0.22‰). Moreover, variations of up to 0.34‰ are found between different molybdenite crystals within an individual hand specimen. The rhyolite sample with 0.12 μg g−1 Mo has δ98Mo = −0.57‰ and is lighter than all molybde- nites from the Questa deposit, interpreted to represent the igneous leftover after aqueous ore fluid exsolution. We recognize three Mo isotope fractionation processes that occur between about 700 and 350 °C, affecting the Mo iso- tope composition of magmatic–hydrothermal molybdenites. Δ1Mo: Minerals preferentially incorporate light Mo isotopes during progressive fractional crystallization in subvolcanic magma reservoirs, leaving behind a melt enriched in heavy Mo isotopes. Δ2Mo: Magmatic–hydrothermal fluids preferentially incorporate heavy Mo iso- topes upon fluid exsolution. Δ3Mo: Light Mo isotopes get preferentially incorporated in molybdenite during crys- tallization from an aqueous fluid, leaving behind a hydrothermal fluid that gets heavier with progressive molybdenite crystallization. The sum of all three fractionation processes produces molybdenites that record heavier δ98Mo compositions than their source magmas. This implies that the mean δ98Mo of molybdenites published so far (~0.4‰) likely represents a maximum value for the Mo isotope composition of Phanerozoic igneous upper crust.

Episodic and long-lasting Paleozoic felsic magmatism in the pre-Alpine basement of the Suretta nappe (eastern Swiss Alps)

The Suretta nappe of eastern Switzerland contains a series of meta-igneous rocks, with the Rofna Porphyry Complex (RPC) being the most prominent member. We present LA-ICP-MS U–Pb zircon data from 12 samples representing a broad spectrum of meta-igneous rocks within the Suretta nappe, in order to unravel the pre-Alpine magmatic history of this basement unit. Fine-grained porphyries and coarse-grained augengneisses from the RPC give crystallization ages between 284 and 271 Ma, which either represent distinct magma pulses or long-lasting magmatic activity in a complex magma chamber. There is also evidence for an earlier Variscan magmatic event at ~320–310 Ma. Mylonites at the base of the Suretta nappe are probably derived from either the RPC augengneisses or another unknown Carboniferous–Permian magmatic protolith with a crystallization age between 320 and 290 Ma. Two polymetamorphic orthogneisses from the southern Suretta nappe yield crystallization ages of ~490 Ma. Inherited zircon cores are mainly of late Neoproterozoic age, with minor Neo- to Paleoproterozoic sources. We interpret the Suretta nappe as mainly representing a Gondwana-derived crustal unit, which was subsequently intruded by minor Cambrian–Ordovician and major Carboniferous–Permian magmatic rocks. Finally, the Suretta nappe was thrust into its present position during the Alpine orogeny, which hardly affected the U–Pb system in zircon.

The Yanque prospect (Peru): from polymetallic Zn-Pb mineralization to a nonsulfide deposit

The Yanque nonsulfide Pb-Zn deposit (inferred resources 12.5 Mt @ 3.7% Pb and @ 3.5% Zn) is located in the Andahuaylas-Yauri ore province (Cuzco, southern Peru). The deposit occurs within a base metal mineralized district, centered on the medium-sized Dolores porphyry copper. A thorough geological, mineralogical and geochemical study has carried out in order to define: the relationships between the Dolores Cu-porphyry ore and the Yanque Zn-Pb polymetallic mineralization, and the characteristics of the economic nonsulfide concentrations. Both sedimentary and igneous rocks constitute the backbone of the Yanque-Dolores area. The sedimentary lithologies belong to the Soraya, Mara and Ferrobamba Fms. (upper Jurassic-middle Cretaceous). The Yanque orebody is hosted by the Mara Fm., which prevailingly consists of a siliciclastic sedimentary breccia. The original sulfide mineralization consisted of galena, pyrite and sphalerite. The host rock has been affected by a strong hydrothermal alteration, characterized by prevailing sericite/illite, as in the typical porphyry-related phyllic-argillic alteration stage, and by minor kaolinite, dolomite and quartz. Minor element geochemistry, characterized by Sb, As, Mn, Ag and locally also by Cu, points to magmatic-hydrothermal related mineralizing fluids. The Pb isotopic compositions from Dolores and Yanque sulfides are similar, and are typical of the Tertiary magmatically-derived ores in this part of Peru. The hydrothermally altered rocks at Yanque have the same Pb isotopic compositions as the sulfides, thus confirming the hypothesis that the Yanque primary Zn-Pb mineralization may have been produced by hydrothermal circulation related to the emplacement of the Dolores Cu-porphyry, as it is the case of other porphyry Cu systems associated with polymetallic mineralization elsewhere. However, no simple genetic model for the mineralization involving just one fluid circulation episode is able to explain the data. The Yanque economic nonsulfide ore association consists of sauconite, hemimorphite, smithsonite and cerussite, which result from the weathering and alteration of the original sulfide mineralization. Zinc is allocated mainly in sauconite (Zn-smectite), rather than in carbonates: a factor strictly related to the prevailing siliciclastic character of the host rock. Distinctive features of the Yanque orebody are the comparable ore grades for both Pb and Zn (3.5% Zn and 3.7% Pb), and the inverse supergene chemical zoning. In fact, contrary to other supergene ores of this type, zinc prevails in the top zone of the Yanque deposit, whereas lead content increases with depth. Considering the different mobility of the two metals in solution, it may be assumed that most of the primary zinc that was the source for the Yanque nonsulfides was originally located far from the position occupied by the galena mineralization, whose remnants have been observed on site. Zinc sulfides may have been originally contained in the eroded rock volumes that surrounded the actual deposit: the zinc-rich solutions have possibly migrated through the siliciclastic Mara Fm. and precipitated the nonsulfide minerals by porosity filling and replacement processes. In this sense, the Yanque secondary Zn-Pb deposit could be considered as a special type of “Exotic” mineralization.