A textural and trace-element study of pre-ore pyrite types at the Kamoa-Kakula sedimentary-rock-hosted copper deposit, Democratic Republic of Congo

Abstract

Pyrite is known or assumed to be an important precursor phase in a variety of sedimentary-rock- hosted ore deposits. The probable role of framboidal pyrite has been identified in the origin of the giant Kamoa-Kakula Cu deposits in Democratic Republic of Congo, which are hosted by multiple-km-thick mid-Neoproterozoic diamictite, but the nature and paragenesis of pre-ore pyrite(s) in the system is poorly known. Using samples from a least-altered, least-mineralised drill-hole, this study identified and characterised four types of pre-ore pyrite in the Kamoa- Kakula system: (a) framboidal (‘sedimentary’) pyrite; (b) aggregate pyrite, consisting of irregular patches of pyrite microcrysts; (c) microcrystalline euhedral pyrite, consisting of independent very small euhedra; and (d) cluster pyrite, characterised by inclusion-rich amorphous centres overlain by groups of large, interlocking, inclusion-free euhedra. Textural relationships indicate that framboidal pyrite formation was followed by euhedral pyrite, aggregate pyrite, and then cluster pyrite. There is no textural evidence that euhedral or aggregate pyrite formed through recrystallisation of framboids. Trace element geochemistry indicates that framboidal, aggregate, and (some) euhedral pyrite formed under related, early diagenetic conditions, whereas (most) euhedral pyrite and cluster pyrite formed later, under different (“hydrothermal”) geochemical conditions. The trace element geochemistry indicates that, in contrast to the textural evidence, the paragenetic order of the pre-ore pyrite types is framboidal, aggregate, euhedral, and cluster. An ore-phase pyrite (clast-rimming pyrite) is texturally and geochemically distinct from all four pre-ore pyrite types.