Preliminary mineralogical and petrological study of the Ortosa Au-Bi-Te ore deposit: a reduced gold skarn in the northern part of the Rio Narcea GoldBelt, Asturias, Spain

The Ortosa deposit (NW Spain) in the northern part of the Rio Narcea Gold B elt (RNGB) is located in the Cantabrian Zone of the Iberian Massif. This zo ne corresponds to the westernmost exposure of the European Hercynides. The deposit is hosted by marine shales, siltstones, calcareous siltstones and i nterbedded sandy limestones of the upper part of the Silurian Furada Format ion. These rocks are intruded by a main stock and numerous sills and dikes consisting of a reduced, ilmenite-bearing quartz-monzodiorite (Ortosa intru sion). Skarn metasomatism and associated gold mineralization overprinted th ese sedimentary and igneous rocks, forming endo- and exoskarns. The earliest stage of alteration involved potassium metasomatism from which metasomatic biotite developed in the hornfels around the intrusion. In the endoskarn, the first metasomatic mineral to form is actinolite. Subsequent ly, quartz, pyroxene (Hd(30-45)), and sulfides (mainly arsenopyrite and pyr rhotite) formed, followed by a second generation of amphibole (ferroactinol ite and ferrohornblende). The exoskarn is a pyroxene-garnet skarn, which is often banded. The prograde minerals are pyroxene (Hd(10-30)) and grossular garnet. The retrograde mineralogy consists of hedenbergite-rich pyroxene ( Hd(50-87)), amphibole (ferroactinolite-ferrohornblende), and the metallic m inerals with minor fluorapatite, K-feldspar, albite, epidote-clinozoisite, vesuvianite and calcite. A final stage of retrograde alteration is characte rized by calcite, quartz, and chlorite. Pyrrhotite and arsenopyrite are the more abundant metallic minerals, and lo llingite, chalcopyrite, pyrite and sphalerite are present in smaller amount s. The gold occurs as native gold and maldonite, and is accompanied by hedl eyite, native bismuth, and bismuthinite. These Au-Bi-Te mineral assemblages occupy cavities and fractures in the arsenopyrite or in the pyrrhotite. Estimated physiochemical conditions of formation based on the composition a nd stability fields of major calc-silicate and sulfide minerals indicate th at the hedenbergite-rich pyroxene and the earliest sulfides (lollingite-pyr rhotite-arsenopyrite) crystallized at temperatures between 470 and 535 degr eesC at low logfS(2) between -10 and -6.5 and low log fO(2) of -22. The Ort osa skarns can be included in the reduced gold skarn subtype defined by Mei nert (Mineralogical Association of Canada, Quebec city, Que., Canada, 1998, 26,359-414). (C) 2000 Elsevier Science B.V. All rights reserved.