On the Geology of the Rio-Tinto Mines, with some General Remarks on the Pyritic Region of the Sierra Morena

Abstract

The pyrites deposits of the Andevallo, as the mineral zone of the south-western branch of the Sierra Morena is called, are certainly the most extensive ever yet discovered. In addition to the world-famous mines of Rio Tinto and Tharsis in Spain, and of San Domingos in Portugal, they include many others of very considerable extent and importance, yielding in the aggregate, at the present time, upwards of two and a half millions of tons of ore per annum. The deposits in question are included within a band of country about 140 miles long and 30 miles wide, consisting for the most part of Palæozoic schists, now known to be principally, if not entirely, of Upper Devonian age, but which are often very highly metamorphosed locally into jasper, talc-schists, chiastolite-schists, &c., and are associated with quartz- and felspar-porphyries, diabase, quartz-syenite, and granite. The mineral strata to the northward abut against or rest upon the highly crystalline schists and gneissose rocks of the Sierra Alta, which have been assigned to the Huronian and even to the Laurentian period, while to the southward they are overlain by Tertiary limestones and sandstones. The chief mineral riches of this region consist of masses of cupriferous pyrites, such as those so largely worked at the above-mentioned mines; but there are, in addition, numerous veins of manganeseore, as well as of lead, copper, and zinc ores, some of which have been worked occasionally to considerble advantage, while a very large number of them could be so worked