Remobilization of Ni in framboidal pyrite within sulfide nodules in Early Cambrian nickeliferous black shales

Abstract

Black shales commonly contain deposits extremely enriched in nickel (Ni), but the processes of Ni enrichment remain debated. Here we use laser ablation–multiple collector–inductively coupled plasma–mass spectrometry and transmission electron microscopy to present new micrometer- to nanometer-scale insights into the remobilization of Ni in framboidal pyrite from the Dazhuliushui Ni-Mo deposit, South China. Two types of framboidal pyrite have been identified in sulfide nodules: early Py-a with a core-rim texture in a MoSC phase and later Py-b in millerite aggregates. The Py-b microcrystals are larger and more tightly packed than Py-a microcrystals, indicating a decrease in FeS₂ supersaturation from sedimentation or early diagenesis to later diagenesis. This is consistent with the lighter S and Fe isotopic compositions of Py-a than Py-b (mean −6.99 ‰ vs. −2.13 ‰ and −0.31 ‰ vs. 0.09 ‰, respectively), produced via the preferential utilization of light S and Fe isotopes during pyrite formation within a semi-restricted basin and/or from pore water. Amorphous organic matter rich in gersdorffite nanoparticles via adsorption occurs within the interstices of Py-a cores. Gersdorffite and millerite nanoparticles within Py-a rims rapidly precipitated in response to continuous decomposition of organic matter. The Py-b is not associated with organic matter but with minor gersdorffite nano-veinlets that might form via the decay of residual organic matter during late diagenesis. These results reveal the complex remobilization processes of Ni in framboidal pyrite and highlight that nanoscale observations are necessary for fully understanding the detailed processes of metal enrichment in black shales.-