Petrogenesis, metallogenesis and lithogeochemical prospecting of metabasic rocks near Santiago de Compostela, N.W. Spain

Abstract

The crystalline core of the Hercynian Orogen in N.W. Iberia contains five allochthonous, polymetamorphic complexes. One of these, the Ordenes Complex, consists of a large area of metasediments, flanked by a discontinuous belt of mafic and ultramafic rocks. The Santiago Unit, a segment of the peripheral belt of the Ordenes Complex, is dominantly composed of highly deformed, variably retrogressed, medium pressure amphibolite facies metabasites and pelitic metasediments. The metabasites locally contain disseminated and massive Fe-Cu sulphides which have been mined at three localities. The metabasites have the immobile trace element characteristics of mid-ocean ridge basalts. Their diverse primary metamorphic mineral assemblages were produced by predominantly isochemical metamorphism of more or less undifferentiated igneous rocks, some of which had previously been affected by submarine hydrothermal processes. Metamorphosed chemical sediments occur as bands within the metabasites. The mineralisations were integral components of the original rock assemblage. This assemblage may be deduced from the field and petrochemical evidence and is uniquely consistent with the mafic rocks and ores being part of a metamorphosed ophiolite complex. The sulphide mineralisations contain economic Cu, some Zn but no Ni. They are interpreted as being of volcanogenic-exhalative origin. The economically important disseminated minoralisations occur in a discrete horizon of unusual composition which is dominantly composed of almandine-gedrite-quartz-sulphide. These rocks were probably formed in a submarine fault-zone as a system of stockwork deposits in intensely altered mafic volcanic rocks. The smaller, massive mineralisations are metamorphosed sulphide facies chemical sediments.The genetic models for the rocks and mineralisations may be used as a basis for, and in conjunction with, lithogeochemical prospecting techniques. The application of such techniques in the Santiago Unit identifies original (igneous) stratigraphy and lithogeochemical anomalies which were derived through processes directly related to are-formation. Both types of feature have considerable potential as prospecting tools. In the Santiago Unit, lithogeochemistry outlines areas having potential for hitherto undiscovered mineralisation. Similar methods are applicable to the metabasic rocks of all the Iberian polymetamcrphic complexes and also to metavolcanic terrains elsewhere.

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