Geology, and genesis of auriferous hydromagmatic breccias and related deposits in northwestern Spain

Jahoda, Rudolf (1988) Geology, and genesis of auriferous hydromagmatic breccias and related deposits in northwestern Spain. University of Southampton, Doctoral Thesis.

Abstract

Late Hercynian gold-bearing hydromagmatic breccias occur in dolomi-tized Vegadeo Limestone and Cabos Series metasediments, both of early Palaeozoic age. The breccias are confined to jogs (side-steps) and areas of imbrication along major strike-slip faults.

Hydrothermal alteration leading to hydromagmatic brecciation in dolomi-tized Vegadeo Limestone (Abaniella-Iboyo) commenced with biotitization and albitization and was followed by the replacement of dolomite by silica (jasperoid). The silicification generated an impermeable cap, and the ensuing increase in PH3O and Pcp3 culminated in hydromagmatic brecciation. The early stages of brecciation were characterized by the production of low-energy solution breccias in which fragments were corroded but show very little displacement. The main stage of hydromagmatic brecciation was caused by high-velocity fluid-streaming of a CC>2-rich aqueous fluid and the resultant breccias contain angular and subrounded fragments of various lithologies. The breccias are commonly cemented by the hydrothermal minerals dolomite, quartz, albite, biotite, and sericite, accompanied by pyrite and minor tetrahedrite. Late-stage fluids altered biotite to chlorite, deposited chalcedony, and caused locally intense serialization.

Hydromagmatic breccias in the Cabos Series metasediments (Monterroso) were also emplaced by high-velocity fluid-streaming and are composed of dolomite-albite (-quartz-apatite) plus quartzitic rock fragments. Pyrite and arsenopyrite occur disseminated throughout the breccia; the gold occurs mostly as free gold and as grains up to 3 mm. The presence of coarse grained gold in these deposits introduces a significant nugget variance into any sampling programme but this can be recognized and minimized by the assessment of sample errors at 95% confidence levels.

Both the Abaniella-Iboyo and Monterroso breccias are accompanied by gabbroic intrusions exhibiting chemical characteristics equivalent to those of continental basaltic magmas.

6l*O values of albite from both prospects range between 10.25 and 10.68 per mil, and isotope temperatures based on a primary magmatic water-source indicate crystallization temperatures between 350° and 400°C. 618O values of dolomite from dolomite-albite breccias average 15.7 per mil and isotope temperatures based on mixed magmatic-metamorphic fluids indicate temperatures between 270° and 350°C.

In conclusion, identification of gold exploration targets in northwestern Spain requires an evaluation of the interactive roles of magmatism, fluid evolution and host rock-type within fault jogs and zones of strike-slip imbrication. (DX77116)

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