Mineral features of Cu-Ag-Ba-Mn mineralisations of La Serena, Chile

Abstract

Three kinds of mineralisation appear in Lower Cretaceous volcanic units in the la Serena area, which have undergone very low metamorphism (prenhite+pumpellyite, with albite, epidote and chlorite): a) Stratiform Mn mineralisation enclosed in sandstones interlayered with andesites. The ore mineralogy consists mainly of silicates composed of braunite, piemontite and minor pyrolusite. b) Stratabound Cu-(Ag) mineralisations (“Mantos tipo Chile”), associated with lappilli tuffs with bitumen. The major phases are bornite and chalcopyrite, with minor fahlore, sphalerite, galena, stromeyerite, cobaltite and hematite. Besides stroomeyerite (AgCuS), there is Ag in minor quantities of Cu-bearing minerals, especially bornite (up to 0,33% at. Ag). c) Ba-Ag mineralisation in veins crosscutting volcanic rocks. The ore mineralogy consists of silver-bearing minerals (native Ag±Hg (87,4 - 93,4% at. Ag and 5,6 – 10,0 % at. Hg), Ag chloride (bromide), acanthite, and Ag-Cu sulphide (Ag0,77-2,69Cu0,90-1,44S1)), and copper-bearing minerals (bornite, Cu sulphide, and fahlore). The systematic spatial relationship among the different kinds of mineralisations, the basic magmatism, the very low-grade metamorphism, and the intrusion of granitic bodies suggest a genetic link among the different phenomena