High pressure and temperatures during the early stages of tungsten deposition at Panasqueira revealed by fluid inclusions in topaz

Abstract

The Variscan vein-type Panasqueira W-Sn(Cu) deposit, one of the main tungsten deposits in Western Europe, has a long and complicated geological history. The first vein infillings, which consist of the quartz-wolframite association as well as the first generation of topaz, underwent significant deformation. As a consequence, most fluid inclusions of the earliest hydrothermal event are deformed and destroyed. Two preserved fluid inclusion assemblages are, however, found in the topaz overgrowth band and are dense aqueous-carbonic inclusions as well as dense CO2 dominated fluid inclusions. The P-T conditions of fluid trapping are constrained by using the intersection between isochores, as well as graphite-water equilibrium data and yield the following trapping conditions: 500 ± 20°C and 250 ± 20 MPa. These P-T conditions are incompatible with fluid unmixing. Fluid chemistry results from water-graphite equilibrium, probably in metapelites, at two distinct temperatures: around 450-500°C for the predominant aqueous-carbonic fluid, and higher temperatures of maximal 550°C for the CO2-rich fluid enriched in N2. These P-T estimates are consistent with deep crustal levels around 8-10 km depth and a high geothermal gradient around c. 60°C/km-1. The ascending non-magmatic fluids, enriched in volatiles, are essential in the ore genesis.The high thermal gradients may be related either to new magma pulse after the formation of the Panasqueira granite intrusion or to anomalous heat flux produced by the hot fluids ascending from migmatitic levels present at greater depth. This hypothesis necessitates to consider the role of a crustal weakness, which is attested both by the successive intrusions of several granitic magmas at the same place, and the presence of inherited quartz filled structures so-called Seixo-Bravo found only in the Panasqueira area.

Fluid Processes of Wolframite-Quartz Vein Systems: Progresses and Challenges

“…Densely developed wolframite-bearing subhorizontal hydrothermal quartz veins and the greisenized cupola (greisen) of a Variscan S-type granite provide the main W resources in Panasqueira. These quartz veins are mainly hosted in the Cambrian schist-metagraywacke complex, and are mainly composed of quartz, tourmaline, muscovite, topaz, apatite, wolframite, cassiterite and minor arsenopyrite and galena [8,[104][105][106]. Rutile that coexists with wolframite yielded LA-ICP-MS U-Pb ages of 305 Ma and this is suggested as the mineralization age for wolframite [8].…”

“…For example, [6] reported a large number of CO 2 -rich fluid inclusions in quartz from the Yaogangxian and Huangsha tungsten deposits in the Nanling region (Figure 6A-C). Coincidentally, the Panasqueira tungsten deposit in Western Europe also has a significant amount of volatiles dominated by CO 2 in the ore-forming fluid [106]. However, it is also important to note that many other wolframite-quartz vein-type tungsten deposits do not contain CO 2 -rich fluids, as exemplified by the Maoping (Figure 6E), Xingluokeng (Figure 6F), Piaotang (Figure 6G) and Taoxikeng (Figure 6H) tungsten deposits, all of which are large to super-large deposits.…”

Tungsten (VI) speciation in hydrothermal solutions up to 400°C as revealed by in-situ Raman spectroscopy

“…(2004) the dominant presence of CO2 (Higgins, 1980;Ball et al, 1985;Wood and Samson, 2000;Rios et al, 2003;Naomov et 486 al., 2011 andNi et al, 2015). According to Cathelineau et al (2020), W-ores at Panasqueira W deposit (Portugal) 487 precipitated from low salinity, aqueous-carbonic fluids with CO2 -CH4 -N2 present in the volatile phase. Tungsten does 488 not form complexes with carbonate species over the whole pH and T ranges investigated in the present work.…”

Evaporitic brines and copper-sulphide ore genesis at Jbel Haïmer (Central Jebilet, Morocco): A reply

“…Late Variscan magmatic Sn-W-Mo deposits and orogenic Au-deposits: Contrary to the comment, our paper does not examine at all the Au-deposits in Morocco, but is focused on base metal and silver veins. It is evident that Sn-W (Mo), Au-As deposits in ductile to brittle structures are related to fluid movements associated to late Variscan exhumation and deformation, frequently driven by late magmatism in the whole Variscan belt including Portugal, Galicia, South-West England, French Massif Central, Bohemia (Sn-W: Harlaux et al, 2018, Burisch et al, 2019, Moscati and Neymark, 2020, Cathelineau et al, 2020Au-As deposits: Boiron et al, 1996, Essarraj et al, 2001.…”