Abstract

El Romeral iron mine is in the coastal iron province of northern Chile, a north-trending belt of magnetite deposits about 30 km wide and 600 km long.Iron at El Romeral is mined from two orebodies in which magnetite is microscopically intergrown with actinolite. The Main orebody is a lenticular, steeply dipping magnetite-rich mass in a reentrant between two lobes of the Romeral diorite pluton. The North orebody consists of conformable pods of magnetite in actinolitized biotite schists with steeply dipping foliation. Both orebodies trend northerly, adjacent and subparallel to the eastern contact of the western lobe of the diorite pluton and within a north-trending network of anastomosing strike-slip faults.Ore-zone boundaries are gradational, and altered rocks within and around the ore-bodies contain magnetite, actinolite, plagioclase (An (sub 22-32) ), diopside, clinozoisite, sphene, chlorapatite, marialitic scapolite, tourmaline, chlorite, pyrite, calcite, micas, and clays.The schists, phyllites, and quartzites of El Romeral probably are late Paleozoic, whereas the andesite porphyry, diorite, and magnetite deposits are early Cretaceous in age.Emplacement and crystallization of the Romeral diorite pluton preceded ore deposition. During late stages of diorite crystallization, quartzite and andesite porphyry near diorite contacts were dioritized by metasomatic additions of plagioclase (An (sub 20-30) ), hornblende, and diopside. Iron in hematite-banded quartzite was mobilized and redeposited as hematite 80 to 145 m from the diorite contact. Cordierite porphyroblasts grew in phyllite and schist, and diopside and oligoclase partially replaced andesite porphyry between the east and west lobes of the diorite pluton.Magnetite ore deposition was hydrothermal and was accompanied by pervasive actinolitization, probably at temperatures mainly in the range 550 degrees to 475 degrees C. Left-lateral movement on the Romeral fault was concurrent with ore deposition and with emplacement of intramineral diorite aplite dikes into en echelon, northwest-trending, steeply dipping gash fractures, which opened in response to the left-lateral faulting.Ore deposition was followed by chloritization, by emplacement of a few minor granitic dikes, and by alteration of previously actinolitized diorite and phyllite to aplitic rocks consisting of secondary plagioclase (An (sub 8-25) ), quartz, microcline, and minor tourmaline. Aplitization was followed by emplacement of many biotitic diorite aplite dikes, by formation of late actinolite-magnetite-apatite veins, by locally intense argillization and martitization, by right-lateral movement on the late North-northeast fault, and by emplacement of the postore batholith east of El Romeral.

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