Geological characteristics and metallogenesis of the shilu Fe-ore deposit in Hainan Province, South China

摘要

The Shilu Fe-ore deposit, a large-scale hematite-rich ore deposit in Hainan Province, South China, contains proven reserves of over 460 Mt Fe-ore at 51% FeO, 4.07 Mt Co-ore at 0.29% and 6.65 Mt Cu-ore at 1.18%. The deposit is largely hosted within the Meso- to Neoproterozoic (ca. 0.85-1.30 Ga) Shilu Group, a suite of metamorphosed neritic siliciclastic rocks and sedimentary carbonates, in which original sedimentary tex tures and structures are elegantly preserved. The Shilu Group, which hosts banded iron formations (BIFs), locally called amphibolitic itabirites, comprises six lithostratigraphic sequences. Fe-rich ores mainly reside in pyroxene-amphibole rocks of the middle sixth sequence, whereas the Co-Cu ores are located about 30-60 m below the Fe-ore horizons and are primarily hosted by banded or impure dolostones of the lower sixth sequence. These ores are present mainly as lens-like or sheet-like bodies trending along troughs and/ or transitional zones from limbs to troughs of the NW-SE trending synclinorium, and generally show banded or massive structures with lepidoblastic, cryptocrystalline or microcrystalline textures. Occasional oolitic and blastopsammitic textures are present, indicating an original sedimentary source for the Shilu Fe-polymetallic ores. At least two stages of deformation and associated metamorphism can be recognized. D_1 deformation corresponds to a NW-SE trending synclinorium with associated regional greenschist facies metamorphism. D_2 deformation records an early NW-SE orientated contraction (D_(2a)), followed by doming with apparent skarnization (D_2b) and finally transitional to a nearly E-W orientated extension (D_(2c)), resulting in various amounts of folding, faulting and mineral recrystallization.Radiogenic isotopic analyses conducted on ores and host rocks record three metamorphic ages, including two Sm-Nd isochronages of 830±16 Ma and 212.9±6.6 Ma, and a ~(40)Ar-~(39)Ar plateau age of 132.08± 1.57 Ma cor responding to D_1, D_(2b), and D_(2c), respectively. Combined with previous U-Pb SHRIMP ages from ca. 960 Ma to ca. 1300 Ma and isotopic geochemical data (Xu et al, 2009), as well as other regional metamorphic events, an updated four-stage metallogenic model is proposed here: (1) deposition of the BIF-type ore source horizons be tween ca. 830 and 960 Ma, (2) formation of a metamorphic sedimentary-type ore deposit from ca. 830-360 Ma, (3) refinement of the deposit due to D_2 deformation between ca. 250 and 210 Ma and (4) superposed mineral izing stage formed by magma-related hydrothermal fluids between ca. 130 and 90 Ma. The Shilu Fe-ore deposit can thus be better regarded as a structurally and hydrothermally reworked and re-enriched BIF-type ore deposit