Formation mechanism of skarn iron deposit: Evidence from Fe-rich enclave in porphyritic monzonite

Abstract

Skarn deposits constitute a significant reservoir of high-grade iron (Fe) ores in China, yet the metallogenic processes remain debated. This study focuses on the iron-rich enclaves embedded within porphyritic monzonite, aiming to unravel the metallogenic mechanisms underlying skarn Fe deposits. The host porphyritic monzonite intrudes into porphyritic diorite. Within the iron-rich enclaves three types of magnetite (Mt) are discerned. Mt-I, ranging from 50 to 100 μm in size, predominantly occupies the core of the enclave, enshrouding diopside at its center. Mt-II, with a size range of 20–40 μm, is primarily distributed in the middle of the enclave. The smallest variant, Mt-III, measuring approximately 1 μm, typically manifests at the peripheries of Mt-I and Mt-II. Comparative analysis reveals that Mt-II exhibits higher total rare earth elements (REE), Sr, Ca, alongside lower TiO2 content in contrast to Mt-I. Notably, Mt-II displays a characteristic W-type tetrad effect of REE, indicative of crystallization in a fluid-saturated environment. In contrast, Mt-III, characterized by microcrystalline magnetite, is inferred to have formed under conditions of rapid cooling.Our interpretation posits a two-stage genesis for these enclaves. The initial stage involves the formation of iron-rich magma within a magma chamber situated at depths of 8–10 km, during which Mt-I crystallizes. Subsequently, the second stage unfolds as mantle-derived fluids infiltrate the magma chamber, leading to the formation of Mt-II. The fluid overpressure within the magma chamber triggers a swift ascent of iron-bearing melt-fluid, localized at depths ranging from 1.5 to 2.6 km, resulting in the crystallization of Mt-III. Our results provide valuable insights into the metallogenic processes governing skarn Fe deposits, and the complex geological evolution of these deposits.