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Sn mineralization in W skarns from 'Douro Scheelite Belt': the example of Tabuaco (Portugal)
Abstract
In this contribution, we present the mineralogy and geochemistry
of Tabuaço skarn deposit, one of the most important tungsten
deposits of the “Douro Scheelite Belt”, Portugal. Tabuaço is a W-
(Sn) proximal skarn deposit located at the contact between
“Bateiras Formation” from the Schist Greywacke Complex and
Tabuaço Granite. The Tabuaço deposit has scheelite as the primary
target mineral. However, minor Sn minerals also occur. Three
types of skarn are distinguished: M-type, the richest in W
(>10000 ppm; scheelite average 1.7 vol%); F-type, the richest
in Sn (7770 ppm; malayaite and malayaite-titanite average 0.68
vol%, cassiterite 0.37 vol% and stokesite); and L-type, the
poorest in scheelite (130 – 7710 W ppm; Sn < 1840 ppm).
This study confirms that despite the occurrence of cassiterite other
Sn-bearing minerals, such as malayaite, malayaite-titanite and
stokesite, also occur. The whole rock geochemistry demonstrates
that enriched Sn and F are a typical characteristic of the W (Sn)
skarn of Tabuaço. Tungsten is more abundant in skarns richer in F
due to the presence of scheelite ± fluorite. These W (Sn) skarns
can be also distinguished by their rare earth elements (REE). The
higher values of ΣREE are registered in L-type skarn lithologies,
which include barren skarns, suggesting that the hydrothermal
metasomatism contemporaneous with the W mineralization stage
results in a decrease of ΣREE up to 36.40 ppm, a value found in
M-type skarns.
Keywords: tin, tungsten, skarn, “Douro Scheelite Belt”
... W occurs mainly in hydrothermal W quartz veins with wolframite and/or scheelite (e.g., Borralha (Noronha, 1983), Carris (Moura et al., 2014)); and W(Sn) deposits occur in hydrothermal quartz veins with wolframite and cassiterite [e.g., Panasqueira (Kelly & Rye, 1979), Vale das Gatas (Matos, 1991) and Argozelo (Neiva, 2008)]. W deposits are also found in scheelite-bearing skarns with some tin silicates produced by contact metasomatism on magnesian limestones [e.g., Riba de Alva (Gaspar & Inverno, 2000); Tabuaço (Ramos et al., 2017)]. ...
... The VNFC region has been a target of several studies since it is located inside the so-called "Douro Scheelite Belt" (Goinhas, 1985;Martins, 2012). The Douro Scheelite Belt (DSB) correspond to a WNW-ESE to W-E band of metamorphic rocks in a syncline, preserved between the two anticlines, the Vila Real-Carrazeda de Ansiães-Carviçais and the Lamego-Penedono-Escalhão ( Fig. 1) (e.g., Freire Ávila et al., 2005;Sanderson et al., 2008;Ramos et al., 2017, Teixeira et al., 2021and references therein). Both anticlines are marked by the occurrence of Variscan granitoids with several textures and mineralogy, emplaced during the D 3 phase. ...
... The rocks with calc-silicate minerals that occur in the FNA (Fig. 23, Table 1) were first affected by regional metamorphism and after by contact metamorphism and metasomatism giving origin to skarns with scheelite. The presence of Sn in the metasomatic fluids can also explain the presence of Sn silicates like malayaite in the skarns (Ramos et al., 2017). In the case of calc-silicate rock only affected by regional metamorphism, we cannot exclude the possibility that some W was already pre-concentrated in the sedimentary series before metamorphism because some deformed scheelite is sometimes present (Arribas, 1979;Noronha, 1976;Pellitero, 1980). ...
Geochemistry of granites from Vila Nova de Foz Côa coupled with fieldwork, metallography, and magnetic susceptibility (Km) studies provide a better knowledge about geology, magmatic differentiation degree, and relationships with tungsten and tin mineralizations. The region is characterized by three small isolated circumscribed Variscan plutons (Custóias, Numão and Freixo de Numão) intruding Lower Cambrian metasediments from Schist Greywacke Complex (SGC). Custóias (CG) is a homogeneous pluton, Numão pluton is composed of four granites (Cachão Arnozelo, CAG; Numão, NG; Arnozelo, AG and Quinta Silvã, QSG), and Freixo de Numão pluton is composed of Freixo de Numão (FNG) and Frei Tomé (FTG) granites. The geochemistry showed two magmatic differentiation trends: (a) FNG and CAG; and (b) NG, CG, FTG, AG and aplite. Km decreases with the increasing magmatic differentiation. The high values of Sn, Rb, Nb and W and low values of Sr, Zn, Zr, Ba and Th allowed distinguishing Sn-bearing granites. A single W and Sn mineralizing episode contemporary with the emplacement and magmatic fractionation of peraluminous magmas was established. The Sn mineralization is related to late magmatic phases and W is related to hydrothermal fluids. The similarities between the Freixo de Numão W(Sn) district and other mineralizations inside the Douro Scheelite Belt, support identical genetic links to mineralizations, with an origin related to late- to post-D3 peraluminous granites and the deposition of W-bearing minerals controlled by the interaction hydrothermal fluids-SGC formations with distinct compositions.
... The metallogenic province contains many quartz-cassiterite and quartz-wolframite ores associated with highly evolved Variscan Stype granites and to a lesser extent, I-type granites, which consist of granite/greisenhosted or metasedimentary-hosted mineralization in close association with granitic cupolas. Breccia pipe-type (e.g., Breiter et al., 1999;Gonçalves et al., 2018a;Harlaux et al., 2015), and skarn deposits are also recognized (e.g., Cheilletz et al., 2005;Gomes, 2016;Lefebvre et al., 2019;Ramos et al., 2017;Spruzeniece et al., 2018). ...
... Panasqueira: Kelly and Rye, 1979). Moreover, given the presence of some carbonate layers within the host metasedimentary units, some skarns may also appear (e.g., Tabuaço: Gomes, 2016;Ramos et al., 2017;Spruzeniece et al., 2018;Los Santos: Tornos et al., 2008). Derré, 1982). ...
This dissertation combines field and geochemical observations and analyses with numerical modeling to understand the formation of vein-hosted Sn-W ore in the Panasqueira deposit of Portugal, which is among the ten largest worldwide. The deposit is located above a granite body that is altered by magmatic-hydrothermal fluids in its upper part (greisen). These fluids are thought to be the source of metals, but that was still under debate. The goal of this study is to determine the composition and temperature of hydrothermal fluids at Panasqueira, and with that information to construct a numerical model of the hydrothermal system. The focus is on analysis of the minerals tourmaline and white mica, which formed during mineralization and are widespread throughout the deposit. Tourmaline occurs mainly in alteration zones around mineralized veins and is less abundant in the vein margins. White mica is more widespread. It is abundant in vein margins as well as alteration zones, and also occurs in the granite greisen. The laboratory work involved in-situ microanalysis of major- and trace elements in tourmaline and white mica, and boron-isotope analysis in both minerals by secondary ion mass spectrometry (SIMS).
The boron-isotope composition of tourmaline and white mica suggests a magmatic source. Comparison of hydrothermally-altered and unaltered rocks from drill cores shows that the ore metals (W, Sn, Cu, and Zn) and As, F, Li, Rb, and Cs were introduced during the alteration. Most of these elements are also enriched in tourmaline and mica, which confirms their potential value as exploration guides to Sn-W ores elsewhere.
The thermal evolution of the hydrothermal system was estimated by B-isotope exchange thermometry and the Ti-in-quartz method. Both methods yielded similar temperatures for the early hydrothermal phase: 430° to 460°C for B-isotopes and 503° ± 24°C for Ti-in-quartz. Mineral pairs from a late fault zone yield significantly lower median temperatures of 250°C. The combined results of thermometry with variations in chemical and B-isotope composition of tourmaline and mica suggest that a similar magmatic-hydrothermal fluid was active at all stages of mineralization. Mineralization in the late stage shows the same B-isotope composition as in the main stage despite a ca. 250°C cooling, which supports a multiple injection model of magmatic-hydrothermal fluids.
Two-dimensional numerical simulations of convection in a multiphase NaCl hydrothermal system were conducted: (a) in order to test a new approach (lower dimensional elements) for flow through fractures and faults and (b) in order to identify conditions for horizontal fluid flow as observed in the flat-lying veins at Panasqueira. The results show that fluid flow over an intrusion (heat and fluid source) develops a horizontal component if there is sufficient fracture connectivity. Late, steep fault zones have been identified in the deposit area, which locally contain low-temperature Zn-Pb mineralization. The model results confirm that the presence of subvertical faults with enhanced permeability play a crucial role in the ascent of magmatic fluids to the surface and the recharge of meteoric waters. Finally, our model results suggest that recharge of meteoric fluids and mixing processes may be important at later stages, while flow of magmatic fluids dominate the early stages of the hydrothermal fluid circulation.
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