Gold deposit genesis in the Jiaodong gold district, eastern China: mineralogical and geochemical insights into Mesozoic gold in an Archean craton
thesisposted on 2016-11-29, 02:27 authored by Mills, Stephanie Elizabeth
The world-class Jiaodong gold district of northeast China contains many large Mesozoic gold deposits hosted by the Archeanâ€“Paleoproterozoic North China Craton. Peak metamorphism and devolatilisation of the cratonic basement occurred at 1.8 Ga, resulting in a gap of more than 1.5 b.y. between metamorphism and mineralisation, thereby excluding the possibility that metamorphic fluids played a role in the ore-forming system. Mineralisation in the Jiaodong district instead appears to represent a unique style of Au mineralisation that is related to widespread Mesozoic magmatism associated with reactivation of the eastern North China Craton. This thesis explores the mineralogical and geochemical signatures of the Jiaodong magmatic-hydrothermal system, and 1) provides the first detailed mineralogical and paragenetic characterisation of several of the largest gold deposits in the region, 2) evaluates the Jiaodong district in terms of modern genetic models for gold mineralisation, such as orogenic and intrusion-related systems, and 3) identifies the regional tectonic processes that were essential to forming the Jiaodong deposits. These findings are used to consider other areas worldwide that may be prospective for Jiaodong-style mineralisation. The mineralogy of gold deposits within the Jiaodong area records the evolution of the ore-forming hydrothermal system; initial barren pyrite precipitation changed to gold + base-metals precipitation geochemically focused in and around pre-existing pyrite, culminating in the formation of iron-oxide and sulfate minerals. This progression reflects the control of a â€˜self-oxidisingâ€™ magma source for hydrothermal fluids. The prevalence of fracture-hosted gold in pyrite is ascribed to localised adsorption and reduction of gold on the surface of pyrite, which had the effect of concentrating gold in zones of pre-existing disseminated pyrite. The trace element bulk geochemistry of ore-zone pyrite illustrates the broadly Ag-, base-metal-, Bi-Te-rich and As-poor nature of Jiaodong ores. Gold occurs as As-facilitated invisible gold, which formed during sulfide growth, and as As-independent gold grains, which formed after pyrite formation. Development of post-sulfide gold grains was the main gold enrichment event. He-Ar isotope analyses of pyrite indicate that ore-forming magmatic-hydrothermal fluids included mantle-derived components and a radiogenic Ar contribution generated by partial melting and incorporation of Archean basement into the magmatic system. It is proposed that the evolving mineralogy and geochemistry of the Jiaodong deposits sets them apart from modern genetic models for gold mineralisation. Crustal melting of Archean basement is interpreted to have been the key to accumulating world-class gold endowment in this area. Given that the extensive crustal melting was driven by loss of the Archean lithospheric keel in this part of the North China Craton, such delamination is considered here to have been another important factor in ore genesis.