Textural Evolution of W-Cu-Sn-bearing Hydrothermal Veins at Minas da Panasqueira, Portugal

K. A. Foxford, R. Nicholson and D. A. Polya
Department of Geology, University of Manchester, Manchester M13 9PL

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 cavity-fill 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 crack-seal 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.

Keywords: crack-seal • cavity-fill • textures • blade-like fractures • bridges • dilation • hydrothermal

Mineralogical Magazine; September 1991 v. 55; no. 380; p. 435-445; DOI: 10.1180/minmag.1991.055.380.13
© 1991, The Mineralogical Society
Mineralogical Society (www.minersoc.org)