Mineralogical Features of Ore Diagenites in the Urals Massive Sulfide Deposits, Russia

Abstract

In weakly metamorphosed massive sulfide deposits of the Urals (Dergamysh, Yubileynoe, Yaman-Kasy, Molodezhnoe, Valentorskoe, Aleksandrinskoe, Saf’yanovskoe), banded sulfides (ore diagenites) are recognized as the products of seafloor supergene alteration (halmyrolysis) of fine-clastic sulfide sediments and further diagenesis leading to the formation of authigenic mineralization. The ore diagenites are subdivided into pyrrhotite-, chalcopyrite-, bornite-, sphalerite-, barite- and hematite-rich types. The relative contents of sphalerite-, bornite- and barite-rich facies increases in the progression from ultramafic (=Atlantic) to bimodal mafic (=Uralian) and bimodal felsic (=Baymak and Rudny Altay) types of massive sulfide deposits. The ore diagenites have lost primary features within the ore clasts and dominantly exhibit replacement and neo-formed nodular microtextures. The evolution of the mineralogy is dependent on the original primary composition, sizes and proportions of the hydrothermal ore clasts mixed with lithic serpentinite and hyaloclastic volcanic fragments together with carbonaceous and calcareous fragments. Each type of ore diagenite is characterized by specific rare mineral assemblages: Cu–Co–Ni sulfides are common in pyrrhotite-rich diagenites; tellurides and selenides in chalcopyrite-rich diagenites; minerals of the germanite group and Cu–Ag and Cu–Sn sulfides in bornite-rich diagenites; abundant galena and sulfosalts in barite- and sphalerite-rich diagenites and diverse tellurides characterize hematite-rich diagenites. Native gold in variable amounts is typical of all types of diagenites.

Authigenesis at the Urals Massive Sulfide Deposits: Insight from Pyrite Nodules Hosted in Ore Diagenites

“…Host rocks are altered with formation of talc, quartz, carbonates, and chlorite. The main ore body was interpreted as a strongly eroded sulfide mound with fragments of numerous chalcopyrite-pyrite and calcite-pyrite-rich chimneys, diffusers-like structures, colloform sulfides, and fossilized tube worms [24,25]. The upper part of the main ore body is composed of coarse-clastic pyrite breccias with fragments of chalcopyrite-pyrite smoker chimneys, which are crowned by fine (1-10, rare, up to 20 cm thick) interlayers of sulfide gravelites and sandstones intercalated with siltstones.…”

“…Pyrite nodules of the Dergamysh deposit were recognized in the satellite-1 ore body at the northwest part of the deposit, which is represented by a 20-cm thick pyrite-chalcopyrite-pyrrhotite layer (pyrrhotite-rich diagenite [13]) mixed with serpentinite fragments up to 5 cm across [27,40]. Macroscopically, this layer exhibits a clastic structure with angular fragments of pyrite and pyrrhotite 1-4 mm in size and interstitial chalcopyrite aggregates in a rock-forming and small-grained sulfide matrix (Figure 5a), however, pyrrhotite, chalcopyrite, and cubanite have pseudomorphic nature and probably replaced former pyrite clasts (Figure 5b-d) [13]. The sulfide aggregates are also replaced by quartz and magnetite.…”

“…The formation of abundant authigenic sphalerite occurred in higher alkaline conditions because of the presence of carbonate material (cf. [25]). In comparison with previously studied pyrite nodule with clear zoning [44], the pyrite nodule from this study exhibits much lower trace element contents.…”

“…Association IV with strong correlation between Au, Se, and Mn is rather unusual, but it reflects the high contents of these trace elements in the core of the nodule (Figures 8 and 9). Enrichment in Mn is inherited from framboidal pyrite, which typically contains Mn [66,67] similarly to colloform pyrite suggesting its growing of pyrite under relatively oxygenated subalkaline conditions of seawater [13]. The Au and Se are most likely present as nanoparticles [10].…”

Sulfide Breccias from the Semenov-3 Hydrothermal Field, Mid-Atlantic Ridge: Authigenic Mineral Formation and Trace Element Pattern

“…Mercury was detected in sphalerite (up to 1.0 wt %), enargite (up to 0.15 wt %), tennantite-tetrahedrite (up to 0.54 wt %) and covellite (up to 0.02 wt %) of hydrothermal precipitates from the Hook Ridge of the Bransfield Strait [73]. In the Uralian VHMS deposits, Hg is also mostly concentrated in sphalerite and, to a lesser extent, tennantite-tetrahedrite and native gold [74]. According to LA-ICP-MS analyses, sphalerite from the nearby Semenov-2 hydrothermal field contains 9 to 47 ppm Hg (in-house unpublished data).…”