Framboidal and spherulitic pyrite in sediment-hosted ore deposits of Iran

Abstract

Framboidal pyrite is common in marine sediments and organic matter-bearing sedimentary rocks. It has also been reported in many ‘sediment-hosted ore deposits’, such as shale-hosted massive sulfide (SHMS) or clastic-dominated Zn-Pb deposits, volcanogenic massive sulfides (VMS), Irish-type Zn-Pb, sediment-hosted stratabound copper (SSC), and sandstone-hosted Pb-Zn and U, as well as in coal deposits, whereas it is absent or rare in some others (e.g., Mississippi Valley Type, MVT). Spherulitic pyrites are more common in Cambrian pyrite-rich SHMS Zn-Pb deposits, hosted in organic matter-rich black shales and siltstones. Framboid textures can be observed in other minerals as well, such as magnetite, hematite, goethite, limonite, magnesium ferrite, chalcocite, cobaltite, digenite, and arsenopyrite. However, it is possible that these non-pyrite framboids are either formed due to the oxidation of pyrite or the replacement of pyrite by other minerals. The recognition of different morphology types of pyrite framboids and spherulites and their relationship with other sulfides are useful in determining the time of formation of these ore deposits, especially in sediment-hosted Zn-Pb mineralizations. Although framboidal pyrite usually has a sedimentary origin, in some sediment-hosted ore deposits (e.g., Koushk, Chahmir, Zarigan, Hossein-Abad, Eastern Haft-Savaran, Tiran, and Irankuh), where it occurs in association with fine-grained sphalerite and galena, and also in the hydrothermal alterations, its origin is influenced by hydrothermal fluid inputs. Whatever its origin, framboidal pyrite allows us to approach the redox conditions of the sedimentary environment, based on a detailed morphometric analysis in each (ore) facies. Suggesting an analogy with modern euxinic sedimentary basins, the large number of framboidal pyrite indicates euxinic to anoxic conditions in the Chahmir,Koushk, Zarigan, Hossein-Abad, Eastern Haft-Savaran, and Ab-Bagh ore deposits, which is consistent with the geochemical proxies of the host rocks