Tectonostratigraphic analysis of the Proterozoic Kangdian iron oxide - copper province, South-West China

Matthew Greentree

    Research output: ThesisDoctoral Thesis

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    Abstract

    The Cenozoic Ailaoshan – Red River shear zone marks the present day western margin of the South China Block. Along this margin are well preserved late Paleoproterozoic to early Neoproterozoic sedimentary and volcanic successions. This work examines the ages and tectonic environments for the formation of the successions, as well as significance of the regional tectono-magmatic events on the formation of widespread iron oxide-copper deposits. The oldest succession is the Paleoproterozoic Dahongshan Group. A new SHRIMP UPb age of 1675 [plus/minus] 8 Ma for a tuffaceous schist unit confirms its Paleoproterozoic age. Detrital zircon ages of the Dahongshan Group range between Archean to Paleoproterozoic (ca. 2780 – 1860 Ma). They include a population of ca. 2400 – 2100 Ma grains, which have no known source region on the exposed Yangtze Block. Previous geochemical studies of metavolcanic rocks from the Dahongshan Group have suggested that these rocks were erupted in an oceanic setting. However, this study shows that the metavolcanics are extremely altered and cannot be used for reliable tectonic discrimination. Based on the characteristics of sedimentary rocks in the Dahongshan Group, it is suggested that these rocks were deposited in a continental setting. Overlying the Dahongshan Group is a thick sedimentary sequence which has been variably termed the Kunyang, Dongchuan, Huili or Xide Groups. In the past, these rocks have been considered as a Mesoproterozoic rift succession. However, no precise age constraints were available for the succession. In this study, this sequence is found to contain at least two separate tectonostratigraphic units. The oldest (ca.1140 Ma) is comprised of alkaline basalt with a geochemical and isotopic character similar to that of modern intracontinental rift basalts. The presence of Cathaysia-derived sediments in this unit indicates sedimentary transportation from the southerly Cathaysia Block to the northerly Yangtze Block (in present coordinates) in South China at that time, which suggests an 'impactogen' scenario. The thick sedimentary sequence of what has traditionally been defined as the Kunyang Group has been found to have significantly younger depositional age of ca.1000 – 960 Ma. The composition of sedimentary rocks and the provenance of detrital zircons from the Kunyang Group are consistent with a foreland basin setting. The depositional age of this sequence coincides with the timing of Sibao Orogeny as determined elsewhere in the South China Block. Numerous iron oxide - copper (gold) deposits occur within the rocks of the Dahongshan and Kunyang Groups. Previous studies have classified these deposits into two deposit styles: the Dahongshan-type Paleoproterozoic VMS mineralisation hosted within the Dahongshan Group, and the Dongchuan-type diagenetic carbonate and shale-hosted deposits hosted within the Kunyang Group. However, both deposit types share similarities with the iron oxide – copper (gold) deposit class, such as stratabound disseminated and massive copper ores, abundance of iron oxide occurring mostly as low Ti - magnetite and haematite, and variable enrichments in Au, Ag, Co, F, Mo, P and REE. 40Ar/39Ar data from both deposit types indicate mineralisation ages of ca. 850 – 830 Ma and 780 – 740 Ma.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2007

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