Geological Characteristics And Ore Genesis Of Jiguanzui Cu-Au Deposit In Southeastern Hubei

The Jiguanzui copper-gold deposit, located in the Southeastern Hubei Province, the westermmonst portion of the Middle-Lower Yangtze River Fe-Cu-Au metal logenic belt, is one of the largeset Cu-Au deposit in E’dong district. Mineralization mainly occurs as stratabound and tabular bodies in the host Triassic marine limestones, and as massive sulfide accumulations and veins within the contact zone between the Lower Triassic Daye Group and the Yanshanian intrusion consisting of quartz diorite and dioritic porphyrite. The ores in the Triassic limestones share common features of the Andean Manto-type ores. In view of the main commodity in the ores, the deposit can be classified as Au-Fe ores, Cu-Au ores and copper ores. The main ore minerals include chalcopyrite, bornite, pyrite, marcasite, hematite, magnetite and native gold and electrum. Native gold and electrum are droplet-like and hosted in chalcopyrite and carbonate. Naumannite is closely associated with pyrite coexists in the carbonate bearing native gold, indicating the intimate relationship between Se and Au-Ag. The gangue minerals are mainly carbonate, quartz, phlogopite, chlorite and less skarn minerals.Three main paragenetic skarn stages of skarn type ore have been recognized based on the petrographic observation, which are the skarn, quartz-sulfides and carbonate stage stage. The main mineralization occurs in the quartz-sulfides stage. Zircon grains from two quartz diorite samples have U-Pb ages of 139.3± 0.8 Ma and 139.6±1 Ma, respectively, whereas gold-bearing pyrite grains from the Manto ores yield a well-constrained 187Re-187Os isochron age of 141±18 Ma (MSWD = 8.6). The age constraints suggest the the Manto ores and skarn ores formed coevlally and thus the two types of ores were products of the same magmatic-hydrotherml fluids.Microscope, scanning electron microscopy observations and electron microprobe analysis suggest the main occurrence of Au and Ag are visible Au and Ag minerals, with minor amounts of invisible Au and Ag in sulfides minerals. Visible gold is widespread as irregular electrum minerals, and occurs mainly as inclusions in chalcopyrite and porosities and microfractures filling in pyrite. And the occurrence of invisible gold is nanoparticle native gold in pyrite. Visible silver mainly occur as Ag mineral (naumannite, hessite and argentite) distributing in chalcopyrite and bornite whereas invisible Ag occurs mainly as nano-submicron particles in sulfides. In high temperature (>400℃),Cu and Au is extracted from the magma and possibly transported as Au-Telluride melts or complexes like [AuCl2]- or [AuTe2]3-.As hydrothermal fluid altered with the wall rocks, large numbers of Cl precipitate with skarn minerals. In addition, the temperature of the fluid decreases slowly. In the early quartz-sulfides stage, Bi-Te-S minerals precipitate as inclusions in pyrite. Then numerous Ag, Bi and Cu combine with S and Te respectively as Ag2Te, Cu3BiS3 with sulfides, quartz and carbonates. After hydrothermal fluid migrates into the favorable fault belts, the changes of pressure, temperature, pH and fO2 lead to instability of the metal complex. On the condition of log fTe2<-18 and-14<log fSe2<-10, Au and Ag precipitate as native gold bearing Ag, electrum and kustelite, with chalcopyrite, pyrite and selenides. During the process of metallogenesis Bi and Te may be quite significant for the enrichment of Au.Four stages of fluid evolution were recognized in light of phase proportions at room and subzero temperatures, textural relationships, and microthermometric results:(1) Early prograde and retrograde stage fluids are brine (homogenization = 420～>500℃,salinity = 40～>55 wt.% NaCl) and less liquid-rich inclusions (homogenization = 420～>500℃ 15～25 wt.% NaCl equivalent). Compared with stage 1 and stage2, there are relatively more liquid-rich inclusions but the leading is still daughter mineral-bearing inclusions at main ore stage3. Main ore stage fluid inclusions were trapped under three phase conditions as indentified by the conexistence of vapor, liquid rich and brine fluid incluisons; liquid rich inclusions homogenization temperatures is 300～400℃ and the salinity of the liquid-rich inclusions and brine inclusions are 16～24 wt.% and 40～54 wt.% NaCl equivalent, respectively. The conexistence of gas-rich inclusions, liquid-rich inclusions and daughter mineral-bearing with the similar homogenization temperatures (300～450℃),which indicates the fluid boiling, one of the possible factors resulting in the mineral precipitation. The stage4 fluids are represented by liquid-rich inclusions in the calcite, characterized by homogenization temperatures ranging from 140.5 to 287.5℃ and salinities between 0.2 to 1.4 wt.% NaCl equivalent. Oxygen and hydrogen isotopes for ore fluids range from +4.77 to +12.7‰ and -49.2 to -79.5‰, respectively, compatible with magmatic components. Sulfur isotope compositons of pyrite and chalcopyrite form the skarn ore and manto type ore have a narrow range of δ34S values from -0.7‰-+4.5‰, consisitent with a magmatic origin. Lead isotope compositions of sulfides (206Pb/204Pb:17.9716-18.2716, 207Pb/204Pb:15.5329～15.6035,208Pb/204Pb:38.1205～38.4357) are similar to the granitic intrusions in this area, confirming the magmatic origin.Geologic, geochemical, and isotopic data indicate that Manto type ore and skarn type ore have a close genetic relationship with the～140 Ma intusive magmatisim and belong to the same hydrothermal ore-forming system. Based on the geological new geochemical data, a genetic model for the Jiguanzui Cu-Au deposits and other equivalents in the Southern Hubei Province is presented, which has implications for guture exploration of Cu-Au ores in this district.