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 Gold Belt, Asturias, Spain
Fuertes-Fuente, M; Martin-Izard, A; Nieto, JG; Maldonado, C; Varela, A
The Ortosa deposit (NW Spain) in the northern part of the Rio Narcea Gold Belt (RNGB) is located in the Cantabrian Zone of the Iberian Massif. This zone corresponds to the westernmost exposure of the European Hercynides. The deposit is hosted by marine shales, siltstones, calcareous siltstones and interbedded sandy limestones of the upper part of the Silurian Furada Formation. These rocks are intruded by a main stock and numerous sills and dikes consisting of a reduced, ilmenite-bearing quartz-monzodiorite (Ortosa intrusion). Skarn metasomatism and associated gold mineralization overprinted these 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. Subsequently, quartz, pyroxene (Hd(30-45)), and sulfides (mainly arsenopyrite and pyrrhotite) formed, followed by a second generation of amphibole (ferroactinolite 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 minerals with minor fluorapatite, K-feldspar, albite, epidote-clinozoisite, vesuvianite and calcite. A final stage of retrograde alteration is characterized by calcite, quartz, and chlorite. Pyrrhotite and arsenopyrite are the more abundant metallic minerals, and lollingite, chalcopyrite, pyrite and sphalerite are present in smaller amounts. The gold occurs as native gold and maldonite, and is accompanied by hedleyite, 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 and stability fields of major calc-silicate and sulfide minerals indicate that the hedenbergite-rich pyroxene and the earliest sulfides (lollingite-pyrrhotite-arsenopyrite) crystallized at temperatures between 470 and 535 degreesC at low logfS(2) between -10 and -6.5 and low log fO(2) of -22. The Ortosa skarns can be included in the reduced gold skarn subtype defined by Meinert (Mineralogical Association of Canada, Quebec city, Que., Canada, 1998, 26,359-414). (C) 2000 Elsevier Science B.V. All rights reserved.