Iberian Sn-W deposits

In southwest Europe the Sn-W metallogenic provinces are bound to the European Variscan Belt, namely the Hesperian Massif in Iberia and the Massif Central in France. The metallogenic provinces are composed of various W, W-Sn, W-Mo, Sn-W and Sn deposits and occurrences. These deposits are generally formed by magmatic-hydrothermal processes and are commonly related to magmatic rocks of granitic composition.

 

The various deposits and mines are located in the Galicia-Tras-os-Montes Zone, in the West Asturian-Leonese Zone and in the Central Iberian Zone in Spain and Portugal. Although tungsten occurrences and deposits in France exist in all crystalline massifs, the Massif Central and the Pyrenees shows the highest concentration of deposits and mines.

Sn-W metallogenic provinces of the European Variscan Belt with Sn and W dominant bands.

The Variscan orogeny is characterized by extensive magmatism, that resulted in numerous granitic intrusions emplaced into a thick marine metasedimentary sequence. The Sn-W metallogenic province contains various veins of quartz with cassiterite (SnO2) and/or wolframite ((Fe, Mn)WO4) associated mostly with highly evolved stanniferous S-type granites, derived by partial melting of metasedimentary materials. Others have mixed characteristics of I- and S-type granites and a few are of I-type of mantle origin.

 

The granites can be divided into two main groups: two-mica (muscovite > biotite) granites and biotite granites.  Usually the two-mica granites are of leucocratic origin with primary muscovite and biotite. These granites were crystallized from wet peraluminous magmas originated at mesocrustal level. The metaluminous to peraluminous biotite granites originate from relatively dry magmas on deeper crustal levels. When the granites exhibits muscovite, it has a secondary origin.

 

Following this classification, Sn mineralization in pegmatites and aplites are appendant to the granite-aplite-pegmatite sequence related mainly to two-mica evolved granites. Sn and W hydrothermal lode deposits are related to both granite groups.

 

In intrusion-related deposits, ore element ratios are part of a function of relative oxidation state and the degree of fractionation in the associated granite suite. Therefore, the oxidiation state in a granitic magma is more important than the origin of the granitic source rocks when considering the type of corresponding mineral deposits. Sulphide-rich deposits, such as Cu, Pb, Zn, and Mo, tend to develop in association with magnetite-series granitic rocks, while oxide-rich deposits, such as Sn and W, are related to ilmenite-series granitic rocks. In the Iberian Sn-W metallogenic province, Sn and W deposits are linked to two-mica and biotite-rich granites of reduced ilmenite series, whereas W-Mo deposits are linked to biotite-rich granites of magnetite series.

 

The Variscan orogeny is marked by three deformation phases (D1, D2, and D3) and later extensional fragile phase D4. The granites emplacement is mostly syn-D3 (320–313 Ma), late-D3 (311–306 Ma), late-to-post-D3 (300 Ma), or even post-tectonic (299–290 Ma), as supported by their geological, petrographic and geochemical characteristics. Geochronological data on Sn-W ore deposits in southwest Europe have indicated good agreement between the timing of granite emplacement and mineralization.

 

The two-mica granites can be considered as syn-D3 and are commonly emplaced along the core of D3 regional folds, whereas the emplacement of the biotite granites is mostly controlled by D3 shear zones and late Variscan tectonic structures, so their emplacement can be syn-D3, late-D3, late-to-post-D3, or even post-tectonic. Therefore, Sn and W mineralizations occur almost exclusively sub-parallel to the regional Hercynian structures and associated with shear zones, following the same trend as the granites.

 

The distribution of the Sn-W deposits is related to granite outcrops. The deposits can be found either within the peripheral part of the intrusive itself (greisen, stockwork and vein deposits), or in its vicinity in metasedimentary sequences (stockwork, vein and skarn deposits). In general, breccia pipes, sulfide replacement or skarn types are less often developed. Sn-W deposits or showings are normally not found within large granitic outcrops. When present, they mark the occurrence of granitic intrusions into older granites or features of the granite-aplite-pegmatite sequence. 

Exploited discrete quartz vein (lode-type; up to 1.5 m thick) in granite with wolframite, sulphides, cassiterite, and flourite.

Discrete wolframite-bearing quartz vein in metasedimentary rocks.

Sheeted quartz veins (dm thick) in metasedimentary rocks with wolframite and subordinate cassiterite.

Sheeted quartz veins (up to 3 dm thick) in metasedimentary rocks with scheelite.

Gently dipping exploited Sn-W pegmatite (room and pillar mining; height of stopes up to 5 m) with cassiterite and wolframite.

In the Iberian Sn-W metallogenic province, the Sn-W and W-Sn bearing veins have a similar mineralogical association . The tin mineral of economic interest, cassiterite, occurs mainly in granitic pegmatites, quartz veins and greisenized granites. It is also found in small quantities in some aplites and locally in granite. Wolframite and scheelite, the two main tungsten-bearing minerals, appear mainly in quartz veins, but scheelite is also found in skarns originated by granite metasomatism on magnesian marbles. Generally, wolframite is later than cassiterite. Scheelite also appears in the veins, but in much smaller quantity than wolframite; as a rule, it also postdates wolframite. In general, these deposits have multiple stages of mineralization, but cassiterite and wolframite commonly occur in the oxide silicate stage, which occurs early in the paragenesis and is often postdated by a sulphide-rich stage.

 

Ta-oxides were only found in the veins containing more cassiterite than wolframite, whereas rutile and ilmenite are only significant in the veins containing more wolframite than cassiterite. In general, cassiterite and Ta-oxides occur close to, and associated with, muscovite selvages along the vein–wallrock contacts, particularly along vein–schist contacts. Cassiterite is also associated with muscovite within the vein, whereas wolframite is distributed throughout the veins.

 

All the veins contain carbonates (siderite, dolomite and calcite), which belong to the last hypogene stage. At the walls of the quartz veins, there are notable enrichments in quartz, muscovite, sulphides and locally also in tourmaline. Scheelite also occurs here, in greater abundance than in the quartz veins themselves. It replaces calcite within the schist.

Detail of quartz vein with massive wolframite (black) in typical thick tabular and conchoidal-lamellar crystals with minor scheelite (whitish-yellowish spots in wolframite).

Detail of discrete quartz vein (white) in granite with wolframite (black), sulphides, and fluorite (greenish).

Detail of sheeted quartz vein in metasedimentary rocks with wolframite (black) and cassiterite-bearing muscovite selvages.