Framboidal chalcopyrite and bornite constrain redox conditions during formation of their host rocks in the copper stratabound mineralization of Picachos, north-central Chile

Abstract

The Picachos Project is a copper deposit that occurs in manto-type orebodies in north-central Chile and it exhibits common features of Chilean stratabound or manto-type copper deposits: (1) an increase of copper contents from the margins to the centre of the orebodies; (2) sodic and potassic hydrothermal alteration of host rocks with subsequent chloritization and sericite alteration; and (3) its occurrence in sequences of limestones that are intercalated with volcanic rocks. The most salient characteristic of the Picachos Project is the presence of pervasive framboidal pyrite in both the non-mineralized limestones and the host rocks of the copper mineralization. These framboidal pyrites are well preserved, probably due to their close relationship with organic matter, and have no evident textures of overgrowth, recrystallization or dissolution. Moreover, framboidal chalcopyrite and bornite are formed in the external and internal areas, respectively, of the orebodies, sharing common morphological characteristics with framboidal pyrite, and are formed by the replacement of the original pyrite framboids, without changing their shape and size distribution. Representative size distributions of framboidal pyrite, chalcopyrite and bornite are determined using high-resolution X-ray tomography (micro-CT). This study highlights the effectiveness of these analytical techniques to obtain the representative size distribution of framboids in mineralized rocks. The preservation of the original framboidal texture of pyrite makes it possible to infer anoxic to near-dysoxic conditions. The size distributions of framboidal chalcopyrite and bornite are hypothesized to be additional potential paleo-redox indicators, referring to the same formation dynamic as typical framboidal pyrites.