Tectonic-depositional Features Of The Shihuiding Formation, And Its Constraint On Metallogensis Of The Shilu Fe-polymetallic Ore Deposit In Hainan Province, South China

Hainan Island is located on the joint of the Eurasian plate, the Australia- Indian plate and the Pacific plate, which are dominated by the Tethys tectonic domain and the Pacific tectonic domain. There are three Precambrian strata, i.e. Mesoproterozoic Baoban Group, late Mesoproterozoic- Neoproterozoic Shilu Group and Neoproterozoic Shihuiding Formation, were found on the Hainan Island. Of these three strata, the Baoban Group and the Shilu Group own higher degree of researches, and the Shihuiding Formation has lower research degree. Compared with the dominated ore – hosting strata named Shilu Group, the researches carried out on the subordinate ore – hosting strata- Shihuiding Formation were focused on studies of petrography and chronology. However, the deposit time of Shihuiding Formation has been controversial. Eeven worse, the research on deposit environment, provenance and source of metallogenic material is still on the blank stage. The studies on the Shihuiidng Formation not only provide clues for origin of metallogenic material of the Fe deposit, provenance and tectonic setting of the Shihuiidng Formation, but also constraints on the Precambrian evolution of the Hainan Island, rebuild the tectonic environment and ancient geography etc.In this paper, on the basis of field survey, major and trace elements analyses, Sm- Nd istope composition ananlyses, zircon U- Pb dating, zircon Lu- Hf isotopic composition tests, bulk Fe isotope composition test and X- ray rock fabric analysis were implemented on Shihuiding Formation.In this paper, the main results and achievements are as follows:(1) The results of X- ray rock fabric analyses show that deformation of minerals from the Upper Carboniferous Qingtianxia Formation, the Lower Carboniferous Nanhao Formation and Silurian Kongliecun Formation is not obvious with weak mineral preferred orientation. The quartz grains of the Shihuiding Formation have obvious preferred orientation. Optic axes of quartz grains distribute as point pole density, and sandstones of Shihuiding Formation are S type structure rock. The deformation temperature of Shihuiding Formation is 300-400 °C. Hematite crystal face(1120) presents perfect point pole density in Sixth Unit of Shilu Group, and its deformation temperature is about 400 °C. Accordingly, deformation of the Shihuiidng Formation and the Shilu Group is mainly produced in Pan-African or Caledonian tectonic event, or even earlier.(2) Mineral assemblage of sandstones from Shihuiding Formation is quartz(70-99%), hematite(0-30%) and sericite(1-2%). The main accessory mineral is zircon, pyrite and magnetite. Sandstones show banded structure.(3) The age of Shihuiding Formation arrange from 2946 to 911 Ma with dominated age peak of 1485-1330 Ma, on behalf of the product of Greenville orogeny. The minimum age peak of ~ 970 Ma, can be regarded as the lower limited time of deposition.(4) Zircon εHf(t) values of Shihuiding Formation sandstones arranggef rom-28.7 to 11.4 with mean value of 1.9. Zircon TDM C age distributes from 3420 to 1221 Ma, and 80% Precambrian crustal of Hainan Island formed from 2.4 to 1.4 Ga.(5) Averaged CIA value of is 68. In combination with rare earth element characteristics, provenance discrimination diagrams and provenance discriminant function inferenced that the provenance of Shihuiding Formation was dominated by recycled sediment. Though age characteristics and Nd isotopic characteristics, we speculated that the Baoban Group is main source of Shihuiding Formation, and 67% sediment and 33% granite from Baoban Group is preferred.(6) Tectonic discrimination diagrams show Shihuiding Formation deposit on the passive continental margin. Combined with age and tectonic analysis, Shihuiding Formation is considered to deposit in foreland basin of Greenville orogenic.(7) The formed tectonic setting is different fron South Chiana Block whose tectonic setting is active continental margin at early Neoproterozoic. In comparison with the South China and other pre-Grenvillian land, the detrital zircon age populations and TDM C age populations reveal that Hainan Island had crystalline basement similar to Laurentia rather than the Yangtze nor the Cathaysia Blocks. Hainan Island probably may be a part of Lauren continent before Neoproterozoic.(8) Shihuiding Formation iron deposit has typical banded iron formation, and it contain a mount of clastic particles. According to these characteristics, the Shihuiding Formation iron deposit is considered to be GIF type iron deposit. PAAS normalized REE patterns imply that metallogenic fluid of Shihuiding Formation is low temperature reducing hydrothermal fluid which is similar to metallogenic fluid of Shilu Group. The Shilu Group iron deposit displays enrichment of heavy Fe istope, and the δ57Fe of Shilu Group iron deposit arrange from 1.643 to 1.724; The Shihuiding Formation iron deposit is enrichment of light Fe istope, and the δ57Fe of Shihuiding Formation iron deposit arrange from-1.142 to-0.876. The Shilu Fe polymetallic deposit, including Shihuiding Formation GIF typle iron deposit and Shilu Group BIF typle iron deposit, yielded obvious Fe isotope fractionation, implying it form at closed system. Combining with the sedimentary environments and tectonic deformation, the Shilu iron polymetallic deposit is Superior type BIF, and it was restructured and transformed by later tectonic events.