Vein and greisen Sn-W deposits are magmatic-hydrothermal systems and made up of simple to complex fissure filling quartz veins or replacement orebodies, both proximal and distal types in a wide variety of structural styles. These includes discrete single veins, sheeted veins or systems of veins, stockworks of interconnecting veins and fractures, breccias, and replacement zones in altered wall rocks adjacent to veins.
Vein Sn-W deposits are generally relative small, in the range of tens of thousands to hundreds of thousands of tonnes. Deposits that consists of several veins or stockworks may contain millions to tens of millions of tonnes. In vein deposits typical grades are in the range from 0.5-2.5 % Sn, and/or 0.3-1.8 % WO3. Sn greisen deposits are generally larger than Sn-W vein deposits and range from about 0.8 to dozens of millions of tonnes containing 0.2-0.5 % Sn. Stockwork deposits that can be mined using bulk mining methods have grades as low as 0.16 % Sn and/or 0.1 % WO3.
The host intrusions to vein and greisen Sn-W deposits are equigranular, felsic, and late-stage granites on the upper zones of large zoned plutons. The rocks are generally highly fractionated granites and leucocratic granites that are commonly referred to as tin granites or metallogenically specialized granites. Specialized granites are characterized by lithophile features and high contents of SiO2, alkalis, high field strength elements (HFSE, e.g. REEs, Zr, U, Nb, Ta), large-ion lithophile elements (LILE, e.g. Li, Rb, Cs), and Be, Ga, Mo, Sn, W, and volatiles such as F and B compared to normal granites. The rocks have S-type or A-type granite characteristics, but tend to be more highly fractionated and are characteristically peraluminous to metaluminuos. Following the ilmenite-magnetite classification, the rocks usually belong to the reduced ilmenite-bearing series. Granitoidic rocks associated with Sn-W mineralization comprise quartz, K-feldspar, sodic plagioclase, and minor modal amounts of biotite and/or muscovite. The are leucocratic and therefore poor in Fe-Ti oxide minerals. Accessory mineral phases may include topaz, fluorite, tourmaline, apatite, monazite, and Li-micas.
The granites were emplaced in multiple stages of intrusion that represent progressive degrees of fractionation. In most cases, the deposits are related to the most highly fractionated intrusion of vertically differentiated plutons. The deposits are spatially associated with the apical portions of mainly small to medium-sized cupolas and ridges of subsurface batholiths which have been emplaced at relatively shallow levels (1 to 4 km and up to 1 kbar) in the Earth's crust.
The tectonic settings of the emplacement of the granitic rocks are characteristically continental. They comprise continental collision belts, inner continental arcs, and continental rift zones. On a more local scale, the deposits are developed close to the contact zones of the intrusions. The deposits are hosted to varying degrees in the granitic rocks themselves, or in associated sedimentary, volcanic, metamorphic, or older intrusive rocks. Regions of Sn-W deposits commonly have a geologic history of multiple orogenic and tectonic phases. In these areas, genesis of highly-evolved, fractionated, and specialized granites often involves several stages of magmatism. Thus, regional structures may localize emplacement of favourable granitic rocks.
Monte Neme W-Sn mine, Spain. The W-Sn mineralization forms stockworks in two-mica monzonites, that are emplaced as part of a linear granitic body parallel to the principal structure. The endocontact veins are grouped into 3 stockworks that are up to 1,500 m long with near-vertical dip and with thickness from a less then 1 cm up to 30 cm. The stockworks zones are up to 95 metres wide and 150 metres in depth, totaling 42.7 Mt @ 0.16 % WO3 + Sn. Note the zone of widespread kaolinization, a type of argillic alteration.
A mine adit of an abandoned W-Sn mine, Spain. The adit continues along the strike direction of the sheeted veins in metamorphic rocks and the gallery is drifted where the thickest and most prominent veins prevails. The veins are from less to 1 cm up to 35 cm thick.
Vein-stockwork and greisen Sn-W deposits are structurally controlled and may be endo- or exocontact. Endocontact deposits comprise Sn greisen bodies and Sn-W veins. These are developed on the roof or along margins of granitic rocks in or near cupolas and ridges of granitoids or beneath internal contacts within granitic rocks. Many deposits are lenticular bodies of disseminated, relatively low-grade ore. Typically the form sheet-like lenses sub-parallel to the upper contact of granitoids.
More common and in most areas more important than greisen deposits are the exocontact deposits of transgressive veins. These occur in associated overlying wall-rocks of sedimentary, volcanic, metamorphic, or older intrusive rocks within the contact metamorphic aureole of a pluton. The setting of the veins can be sub-horizontal or sub-vertical. Both disseminated and vein ore may be present at a single intrusion. The thickness of the veins may be quite variable ranging from less than 1 cm to several metres, but most are on the order of 10 to 20 cm. Veins may bend, branch, or pinch out over tens to hundreds of metres both laterally and vertically. Some mines and districts contain more than 650 veins systems in sheeted vein systems. For instance, some vein systems in sheeted veins and stockwork deposits are hundreds of metres wide are more than a thousand metres long.
Detail of an unexploited wolframite-cassiterite bearing vein with arsenopyrite, chalcopyrite, pyrrhotite, and tantalite in an underground mine, Spain. The deposit is of the vein-sheeted vein type and has a total of 200 vein-veinlets of which only 10 are wider than 0.1 metres.
View into a stope with the remaining portion of a vein in foreground (direction of view is upwards). The vein-sheeted vein system is about 1,000 metres long, 250 metres wide and 300 metres in depth and the geological resource (exploited and reserve) of the deposit is 1.35 Mt @ 0.25 % WO3 and 0.05 % Sn.
Closed adit of an abandoned Sn-W mine, Portugal. The veins have an average thickness of 30 cm and they have been exploited to a depth from 100 to 150 m.
Wolframite-bearing vein in metasedimentary rocks with minor scheelite and bismuthinite, France. The vein is 0.2-0.5 metres thick and shows wolframite in narrow bands and in minor spots. Weathering of the sulphides, e.g. pyrrhotite, chalcopyrite, and pyrite, has produced yellowish-brownish staining adjacent to the vein. The inferred mineral resource is estimated to be 5,000 t WO3 (0.83 Mt @ 0.6 % WO3 ).
The composition of ores in vein and greisen Sn-W deposits is highly variable. Most deposits contain especially either tin or tungsten, with minor amounts of the other. Vein mineralogy varies from simple consisting almost entirely of quartz and Sn (mainly cassiterite) or W (wolframite and/or scheelite) to complex. In addition to Sn and W, other elements may be enriched, e.g. Li, F, Rb, B, and Be. Moreover, they may contain other sulfide and sulfosalt minerals of Cu, Pb, Zn, Bi, Ag, As, and Sb, present in may be economically significant amounts. The most common minerals in W vein deposits wolframite, molybdenite, pyrite, pyrrhotite, arsenopyrite, chalcopyrite, scheelite, cassiterite, mica, and fluorite, the most common minerals in Sn vein deposits are cassiterite, wolframite, arsenopyrite, molybdenite, hematite, scheelite, chalcopyrite, galena, sphalerite, and bismuthinite.
In contrast, in most greisen deposits ore minerals are polyphase and multiple mineralizing centers control ore distribution. The idealized disseminated greisen deposit in an individual cupola is zoned with respect to the distribution of Sn, Mo, As, Bi, and W in hydrothermally altered rocks near mineralized cupola and Ag, Pb, and Zn in more distal parts of mineralized areas. In addition to cassiterite and wolframite, pyrite, arsenopyrite, molybdenite, pyrrhotite, chalcopyrite, tetrahedrite-tennanite, spalerite, galena, enargite, hematite, siderite, and calcite are common minerals.
Grain size in vein and greisen deposits ranges from fine to coarse, whereas the grain size in Sn vein deposits is more fine, wolframite crystals in some quartz veins frequently are as much as 10 to 20 cm long. Ore in gangue minerals in Sn greisen are fine-grained. Quartz is the most common gangue mineral in both vein and greisen deposits and may account for 90 % or more of vein fillings.
Detail of quartz vein with massive wolframite (black) in typical thick tabular crystals, France. The biggest crystals have a length of 5 cm, but they commonly appear in aggregates measuring up to 10 to 15 cm.
Detail of euhedral cassiterite crystals at the selvage in a Sn-bearing quartz vein, Portugal. Compared to wolframite crystals, cassiterite crystals are fine grained and commonly exhibit length up to a few mm and less than 1 cm.
In many vein Sn-W deposits the distribution of the main ore phases is systematic, but in others it is irregular or random to complex. For example, some veins display comb texture indicating continuous precipitation from the wall-rocks inward. Cassiterite commonly occurs at, or close to, the selvages, and is succeeded inward by wolframite. On the other hand some veins are banded and appear to have undergone repeated opening of the vein and subsequent precipitation of ore minerals. Crosscutting veins and fractures typical of many stockwork deposits also indicate that mineralization occurred in multiple stages.
Granite-bound sheeted quartz vein in comb texture with cassiterite on the selvages and wolframite in quartz, Spain.
Detail of granite-bound discrete W-Sn vein with banded texture containing wolframite, cassiterite, pyrite, arsenopyrite, and chalcopyrite indicating repeated opening of the vein and subsequent precipitation of ore minerals, Portugal.
Crosscutting veins and fractures as part of a typical W-Sn-bearing stockwork deposit in granite, Portugal. The veins are up to 15 cm thick and exhibit K dominated feldspathic alteration (microclinisation in the yellowish staining), silicification, and muscovitisation. Where the veins and fractures are tightest, the alteration is most prominent.
A 20 to 30 cm thick granite-bound W-Sn-bearing vein in comb texture is cut by a mineralized younger fracture, Portugal. Alteration dominated by microclinisation and muscovitisation (strongest in the hanging wall).
In apparently unaltered granitic rocks the concentrations of cassiterite and tungstates implies that high concentrations of the ore minerals were at least in some cases reached as a result of successive magmatic fractionation and independently of partitioning into a magmatic-hydrothermal fluid. In the most cases, however, it is apparent that Sn-W greisen and vein deposits and associated alteration formed from magmatic-hydrothermal fluids that were derived from volatile phase separation during magma crystallisation processes in the pluton and therefore both highly saline and low-salinity fluid inclusions are present in ores. The volatile phase cooled as they percolated upwards into the pluton roof zones and overlying wall-rocks. In some cases, a possible role for meteoric and/or highly exchanged fluids resulting from the interaction between magmatic water and C-bearing metamorphic wall-rocks for precipitation of ore minerals is guessed. Typical temperatures during Sn-W precipitation in vein and greisen Sn-W deposits were moderate to high in the range of 250-450 °C.
Model of vein and greisen Sn-W systems. Scale very approximate.
Alteration in different styles is most pronounced in and around granite cupola and in overlying zones. Granite flanks may be unaltered. Alteration directly associated with ore includes pervasive greisenization, a type of phyllic alteration characterized by F-, Li-, and/or B-rich minerals such as topaz, tourmaline, fluorite, lepidolite, and Li-F-bearing micas (including sericite), biotite, and quartz. Other alteration assemblages include feldspathic alteration (Na or K dominated), i.e. albitization, mostly developed in granite underlying greisen, and microclinization; tourmalinization, silicification in the contact aureoles of granitic plutons and cupolas, chloritization and propylitisation in lower temperature and more distal portions, and kaolinization, a type of argillic alteration.
A gently dipping and 40 cm thick Sn-bearing vein in an abandoned mine, Portugal. Note the prominent muscovite-bearing selvages. Cassiterite occurs at, or close to, the selvages. K dominated pervasive feldspathic alteration and silicification is widespread.
Detail of sheeted quartz vein with wolframite (black crystals in quartz) in mica schist with prominent vein-veinlet tourmaline alteration gradually passing outwards into silicification and sericitization in an abandoned mine district, France.