Textural evolution of W-Cu-Sn-bearing hydrothermal veins at Minas da Panasqueira, Portugal

Abstract

Combined structural and textural studies of the quartz veins of the Panasqueira W~Cu-Sn deposit of central Portugal have revealed the presence of coeval forms of crack-seal and cavity-fill mineral textures. The textures result from the dilational and infilling histories of brittle fractures produced within the host rock by the intrusion of pressurised fluids. Crack-seal textures were produced wherever or whenever crystal growth rates matched or were greater than fracture dilation rates, whereas cavityfill textures developed wherever or whenever dilation rates were quicker than those of crystal growth. Rates of dilation within the blade-like fissures were naturally lowest at the edges, where types of crackseal textures were produced, and highest in the cores, where cavity-fill texture formed. During propagation the linkage of adjacent echelon blades, by the rupture of the host-rock straps (bridges) between them, enhanced dilation rates and led to the widespread development of cavity-fill textures. Within the composite fissures so produced, environments in which types of crack-seal textures could form were restricted to the fissure margins and to areas where the tensile strain was sympathetically shared between closely spaced fissures. Textures and minerals characteristic of each fracture environment were able to form simultaneously within the fracture system.

Geochemistry of cassiterite and wolframite from tin and tungsten quartz veins in Portugal

“…The stages given by the latter authors are 1) crack-seal quartz seam, 2) muscovite selvage formation, 3) main oxide-silicate stage, 4) main sulphide stage, 5) pyrrhotite alteration stage, and 6) late carbonate stage. Stages 1, 2 and 3 are penecontemporaneous, but stage 1 precedes stage 2, which precedes stage 3 in any single locality (Foxford et al, 1991). Cassiterite and wolframite occur in stage 1 (Kelly and Rye, 1979), but cassiterite is earlier than wolframite.…”

The imprint of hydrothermal fluids on trace-element contents in white mica and tourmaline from the Panasqueira W–Sn–Cu deposit, Portugal

“…The paragenetic sequence 1,000,000 (LNEG 2010) and Carta Geológica de Portugal à escala 1:500,000 (Oliveira et al 1992); coordinate system: ETRS89/Portugal TM06 (EPSG: 3763)); (b) geological map of the Panasqueira mine area (adapted from Carta Geológica de Portugal à escala 1:500,000 (Oliveira et al 1992)); and (c) schematic cross section of the Panasqueira vein system, greisen and granite (modified after Thadeu (1979)] of the ore veins has been divided into six stages of mineralization: quartz-tourmaline alteration, muscovite selvage formation, main oxide-silicate stage, main sulfide stage, pyrrhotite alteration stage, and late carbonate stage. Tourmaline, white mica, and quartz coexist as gangue minerals from the early stages to the main sulfide stage (Kelly and Rye 1979;Polya 1989;Foxford et al 1991Foxford et al , 2000Polya et al 2000). Hydrothermal alteration of the wall rocks produced concentric zones with progressively greater distance from the veins, consisting of a proximal tourmaline-quartz-muscovite zone, 2 to 30-cm thick, containing more than 50 vol% of tourmaline, and a distal muscovite-quartz zone including minor tourmaline or topaz (Bussink 1984).…”

Incipient Wolframite Deposition at Panasqueira (Portugal): W-Rich Rutile and Tourmaline Compositions as Proxies for the Early Fluid Composition

“…As first noted by Bussink (1984), and later by Foxford et al (1991), thin quartz veinlets (1-5 mm 253 in average) are common in the tourmaline-bearing wall rocks adjacent to the main veins, and in 254 many cases define the vein margins. They display clear evidence for a crack-seal mechanism of 255 opening, and are labelled Qcs hereafter ( Fig.…”

“…3-B), but display well-281 characterized growth zoning in SEM images ( of the Qcs veinlets, displaying both Tur1a and Tur1b zones, the latter usually developed to a far 319 higher extent than in the wall rocks ( Fig.4-E, F). Thus, as pointed out by Foxford et al (1991), the 320 development of both the main Tur1a tourmaline prisms and the Tur1b rims was coeval with the first 321 increments of vein opening evident by the Qcs veinlets. This is pictured schematically in Fig.…”