Deciphering ore-forming conditions and ore genesis in the Devonian Xierqu iron deposit (East Tianshan, NW China): Insights from in-situ magnetite and apatite geochemistry

Abstract

The Xierqu Fe deposit in the Kalatag district offers key insights into Devonian magmatic-hydrothermal iron mineralization within the East Tianshan Orogenic Belt. This study combines detailed microtextural observations, electron probe microanalysis (EPMA), and LA–ICP–MS analyses of magnetite and apatite to decipher the genetic processes and fluid evolution that led to the formation of high-grade Fe ores in the Xierqu deposit. Magnetite mineralization is genetically and spatially linked to quartz diorite porphyry intrusions that were emplaced into volcanic-sedimentary sequences of the Lower Devonian Dananhu Formation. Three magnetite generations are identified: (1) Mag-1 (disseminated) from the prograde skarn stage, characterized by high V (avg. 190 ppm), Cr (avg. 504 ppm), and low Ni/Cr ratios (avg. 0.08), indicative of a high-temperature magmatic-hydrothermal origin; (2) Mag-2 (banded) from the retrograde skarn stage, showing transitional Ni/Cr (avg. 1.57) and moderate V depletion (avg. 92 ppm), reflecting magmatic-hydrothermal interaction; and (3) Mag-3 (massive) from the retrograde skarn stage, marked by hydrothermal signatures (Ni/Cr > 5, V < 73 ppm) and oscillatory zoning linked to fluctuating oxygen fugacity (fO₂) and fluid-rock interaction. Systematic Sn increases (2.2–19.2 ppm) and V/Cr decreases from Mag-1 to Mag-3 record progressive oxidation during retrograde skarn alteration, driven by external oxygen influx and hematite coexistence. Apatite geochemistry further constrains fluid evolution. Retrograde skarn apatite (Apt-1) exhibits high F/Cl (avg. 24.4), negative Eu anomalies (Eu/Eu* = 0.07–0.33), and elevated Sr/Y (avg. 4.88), consistent with oxidizing, F-rich fluids derived from continental crustal melts. In contrast, quartz diorite porphyry apatite (Apt-2) displays lower F/Cl (avg. 1.38), weak Ce anomalies (Ce/Ce* = 1.35), and higher Cl (avg. 0.74 wt%), indicative of subduction-related magmatism with elevated salinity. Fluid-rock interaction, evidenced by increasing Mg + Mn (245–1937 ppm) and Ca (2442–10361 ppm) in magnetite, promoted Fe-Cl complex destabilization and magnetite deposition during carbonate metasomatism. The Xierqu deposit exhibits diagnostic skarn attributes, including: (1) spatial-genetic links to quartz diorite porphyry intrusions, (2) prograde garnet-pyroxene-magnetite assemblages overprinted by retrograde epidote-chlorite-quartz-sulfide mineralization, and (3) magnetite geochemical trends (Ca + Al + Mn vs. Ti + V) aligning with global skarn systems. These features distinguish it from submarine volcanogenic or iron oxide copper–gold (IOCG) models, affirming a skarn origin. The identification of Devonian iron mineralization extends the metallogenic framework of the East Tianshan beyond Carboniferous systems, underscoring the importance of Paleozoic arc-related magmatic-hydrothermal processes. Targeting concealed intrusions along carbonate interfaces within the Kalatag structural corridor may reveal additional skarn-hosted iron deposits, thereby improving exploration targeting in accretionary orogenic settings.