In-situ LA-ICP-MS and EMP trace element analyses of hematite : insight into the geochemical signature of the Neoproterozoic Urucum iron formation, Brazil.

Abstract

Hematite hosted in iron formations (IFs) is widely held as a transformation of (bio)chemical precipitates, which may register depositional conditions and sources responsible for the origin of these enigmatic rocks. Consequently, this is an attractive target to mineral-specific geochemical investigations aiming at distinguishing primary signatures from ore-related transformations and terrigenous contamination. This study reports in situ Electron Microprobe (EMP) and Laser AblationInductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses of hematite grains from the Neoproterozoic Urucum IF, Jacadigo-Boqui Gr. The hematite grains were divided into three main morpho-textural types: (i) anhedral, micro-crystalline hematite (Hm1); (ii) coarser, subhedral to euhedral, microspecular (Hm2) and (iii) microplaty (Hm3) hematite. Factor analysis (FA) was used to trace underlying relationships among the trace elements (TEs). The identified factors were correlated to: (i) substitutions in the hematite structure (F3a, MgO-MnO-Al2O3-TiO2; and F1b, Al-Ti-V); (ii) mineral inclusions (F1a, SiO2; F2a, P2O5-CaO-MgO; and F2b, Mg-Mn-Sr); (iii) hydrogenous and diagenetic incorporation (F3b, Th-Zr-Cu and Ba-REE; F4b, Si-U-Hf). The TE compositions are broadly consistent in all hematite types; this trend suggests limited post-depositional overprinting. Nearly isochemical signatures show that transformation of ferrihydrite to hematite and the subsequent recrystallization of this mineral were dominated by solid-state processes under high Eh. Elements exclusively incorporated in the hematite structure (Al, Ti, V) did not show significant fractionation with increasing recrystallization. Replacement and solution precipitation also occurred, albeit to a lesser extent. The recrystallization of Hm1 to Hm2 and Hm3 was likely controlled by texture and assisted by pressure and fluid circulation. Warm, alkaline, basin brines were also responsible for the dissolution of gangue minerals, particularly chert. Consequently, elements associated with mineral inclusions (Si, P, Ca, Mg, Mn, and Sr) decrease in abundance from Hm1 to Hm2 and Hm3. Ptygmatic quartz veinlets and chert dissolution pods attest to silica leaching and volume reduction leading to hypogene enrichment. Supergene enrichment resulted in ubiquitous secondary porosity, but generally did not affect the composition of the hematite grains. Non-CHARAC Zr/ Hf ratios suggest active HFSE fractionation processes. Super-chondritic ratios record basin seawater enriched in Zr as a consequence of the fractionation of Hf onto amorphous silica, registered by sub-chondritic Zr/Hf ratios and a correlation between Si and Hf in factor F4b. Distinctively seawater-like REE pasterns suggest a common precursor hydrogenous phase. Consistent true negative Ce anomalies and generally low Th/U ratios, together with the relatively common occurrence of hematite peloids, suggest that the precursor ferrihydrite particles were deposited and accumulated near the water-sediment interface, in a well-oxygenated shallow water setting. This implies the existence of a discrete chemocline separating the deep ferruginous waters from oxygenated shallow waters.