Mineralogy and geochronology of the Proterozoic BIF-type Fe-Co-Cu ore district in Hainan Province, South China: implications for mineralization

Provinces and Case Studies

Dr. Deru Xu

Guangzhou Institute of Geochemistry

The Shilu Fe-Co-Cu ore district is situated in the western Hainan Province of South China, and mainly hosted within the Neoproterozoic low-grade metamorphosed rocks of Shilu Group. The
Banded Iron Formations (BIFs) and the interbedded barren rocks underwent multi-stages metamorphism hydrothermal alteration, which significantly reworked and/or upgraded the
original BIFs. Three types of metamorphism have been recognized: (1) the ca. 830-700 Ma greenschist facies metamorphism (T ≤ 300-400 °C and P ≤ 3-5 kbar) likely corresponding to the
Rodinia break-up, (2) the ca. 560-440 Ma amphibolite facies metamorphism (T = 450-650 °C and P = 5.4-6.4 kbar) probably affected by the Caledonian orogeny in South China, and (3) the ca. 270-190 Ma, local contact thermal-metamorphism (T =400-600 °C and P =1-4.5 kbar) likely resulting from the plutons emplaced during the closure of Paleo-Tethyan ocean. The amphibolite facies event was associated with simultaneous structural deformation (D1) and significantly refined the ore metals Fe, Co and Cu along favorable structural sites. The later magmatic activity as well as the retrograde metamorphism resulted in the oxidation of magnetite to hematite and late-stage sulfides formation. Three hydrothermal alteration zones were identified, namely the actinolite-Kfeldspar-quartz-rich alteration zone, the epidote-(chlorite)-rich alteration zone, and the calciteactinolite-(chlorite)-rich alteration zone. On the basis of paragenesis, the alteration minerals can
be classified into two sub-assemblages, the early assemblage formed at ca. 250-210 Ma related to oxidic fluid, and the later assemblage related to reduced fluid likely caused by the late
magmatic thermal event (ca. 130-90 Ma). Hydrothermal alteration most likely occurred at P/T conditions of T = 240-410 °C and P = 1.5-2.5 kbar. Based on the comparison in genesis and host
rocks with the worldwide, Neoproterozoic BIFs, the Shilu BIFs is ascribed to a Superior-type BIF but has been reworked and/or upgraded by structural deformation, metamorphism and
hydrothermal fluid.

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