Mineralogy and Metasomatic Evolution of Distal Strata-Bound Scheelite Skarns in the Riba de Alva Mine, Northeastern Portugal

Abstract

The metasediment-hosted distal scheelite skarns from the Riba de Alva mine, Barca de Alva-Escalhão re gion, northeastern Portugal, consist of three main levels of strata-bound, boudin-shaped, 0.5-to 4-m-thick bod ies, having a total resource of 380,000 tons (t) with an average grade of 0.51 percent W03. The boudinage of the Cambrian metasedimentary rocks of the Siate Greywacke Complex occurred early during the Hercynian orogeny, prior to the Granitic Complex intrusion and the skarn-forming process. The Granitic Cornplex corre sponds to reduced, highly evolved, and metallogenically specialized S-type leucogranites, comparable to those commonly associated with Mo-poor W-F skarns.Four different skarn types were recognized: pyroxene skarn with Fe diopside ± anorthite; amphibole-epi dote skarns with actinolite + clinozoisite ± bytownite ± phlogopite; ore skarn with clinozoisite + scheelite + flu orite + calcite ± labradorite ± apatite ± Fe axinite ± armenite ± scapolite; and late-stage veins with quartz (or fluorite or calcite) ± clinozoisite ± andesine (or albite) ± adularia.Clinopyroxene ranges from Di80Hd29 to Di60Hd49. Amphibole compositions range from actinolite to Mg hornblende and are F rich (0.5-1.9 wt %). Clinozoisite, often zoned with Fe-poor cores, shows little composi tional variation in the skarn types, having a low pistacitic component (5-13% Ps). Plagioclase compositions grade from anorthite in pyroxene skarn through andesine and albite in late-stage veins. Phlogopite, as amphi bole, is F rich (2.45-4.70 wt %). Au extremely pure Mo-free scheelite, the only ore mineral present, is re stricted to the ore skarn type.The Riba de Alva skarns were formed, after intrusion of the Hercynian Granitic Complex, by selective infil trative metasomatic alteration of argillaceous dolomitic limestone. An early stage of prograde alteration led to the formation of the pyroxene skarn, through reaction with an Si-rich and Fe-bearing fluid. Scheelite mineral ization occurred during retrograde alteration, which was rnainly fracture controlled and is characterized by a pervasive infiltration metasomatism with minor diffusion in the skarn walls. This alteration started with hydra tion reactions to form amphibole-epidote skarns, probably under lower X~09 conditions. Ore skarn formation, with scheelite deposition, took place later during this retrograde stage, mainly along veins within the strata bound boudins, as the result of a decrease in temperature and/or significant changes in the activity of some fluid cornponents such as M/, F, Na, Mn, P, and B, and particularly an increase in aca. Monomineralic late-stage veins represent the waning stage of the skarn-forrning process.

Polyphase scheelite and stanniferous silicates in a W-(Sn) skarn close to Felbertal tungsten mine, Eastern Alps

“…The lack of the garnet-clinopyroxene high-temperature assemblages can be explained with the distal position of skarn formation which is further corroborated by the lack of a contact aureole and nearby granite intrusion. Distal Sn-W skarns with similar mineralogy have been reported from Sangdong, South Korea, and Gejiu district, China (Kwak 1987) and the distal strata-bound scheelite skarns in the Riba de Alva Mine, Northeastern Portugal (Gaspar and Inverno 2000).…”

Relative Abundance and Compositional Variation of Silicates, Oxides and Phosphates in the W-Sn-Rich Lodes of the Panasqueira Mine (Portugal): Implications for the Ore-Forming Process

“…As a result, at a local scale and in the presence of suitable structural arrays, many primary features of early-formed mineralization could be significantly affected by late imprints related to the circulation of fluids with distinct compositions. These imprints could even lead to the generation of quite different mineralization types, spatially coexisting with previous lodes but often presenting a distinct structural control [107][108][109][110][111][112].…”

Fluid inclusions, stable isotopes (H–O–S), and age constraints on the genesis of the Basitielieke polymetallic tungsten deposit in the Altay, Xinjiang, NW China

“…(Lecumberri‐Sanchez, Vieira, Heinrich, Pinto, & Walle, 2017). Scheelite usually forms in the contact zone between igneous rocks and calcareous wallrock, and the wallrock provides calcium, especially skarn‐type scheelite deposits (Gaspar & Inverno, 2000; Mao & Li, 1996; Wu, Wang, & Xiong, 2014), it can form Ca skarn at prograde skarn stage. Scheelite formed at retrograde skarn stage, the most important source of Ca at reduced W skarn deposit should be pyroxene replacement by amphibole (Soloviev & Kryazhev, 2018).…”