Tracing metal sources in W-Sn deposits: Sr-Nd isotope geochemistry in wolframite, scheelite, and cassiterite from the Iberian Variscan Massif, Spain and Portugal

Abstract

Abstract This study presents new TIMS Sr and Nd isotopic analysis of cassiterite, wolframite and scheelite from nine W-(Sn) and six Sn-(Ta-Li) vein-type deposits in the Iberian Variscan Massif, with the aim of constraining the source of metals in these deposits. Scheelite displays high Sr and Nd contents, in the order of tens or hundreds of ppm. In contrast, cassiterite, and wolframite contain much lower concentrations of Rb, Sr, Sm, and Nd (generally < 5 ppm and close to 1 ppm). Despite these low abundances, Sm and Nd are incorporated into their crystal structures through distinct substitution mechanisms, whereas for Rb and Sr it remains uncertain whether they are structurally bound or occur as nanoscale inclusions. On the εNd i vs. 87 Sr/ 86 Sr i diagram at 300 Ma, wolframite, scheelite and cassiterite plot within the isotopic fields defined by nearby Variscan granitoids and pre-Variscan metapelitic and metaigneous rocks of western Iberia. In NW Iberia, εNd 300 Ma values of the mineralization overlap those of S-type and I-type granitic rocks and are less radiogenic than the pre-Variscan metapelitic series, whereas cassiterite and wolframite from southern Iberia match the values of spatially associated S 2 granites. The 87 Sr/ 86 Sr 300 Ma ratios of the ore minerals are more variable and overlap those in Variscan granitoids or align with the more radiogenic isotopic signatures in the host metaigneous and metapelitic rocks. Sulfides, including arsenopyrite, pyrite, chalcopyrite, molybdenite, sphalerite, and stannite, associated with the Sn-W mineralization display δ 34 S values ranging from − 6.1 to + 8‰, overlapping the isotopic ranges of both granites and metapelites, although the extreme values correspond only to metapelitic sources. These results support a polyphasic origin for the deposits, with an early oxide stage formed from magmatic hydrothermal fluids derived from evolved Variscan granitoids, which acted as the main metal source. In contrast, sulfur was largely derived from leaching of metasedimentary and granitic rocks by external fluids and introduced into the system during a later post-magmatic, hydrothermal stages.