Metallogeny Of The Hongniu-Hongshan Porphyry-Skarn Copper Deposit And The Porphyry-Skarn Metallogenic System Of The Yidun Island Arc,Yunnan,SW China

The Hongniu-Hongshan porphyry-skarn copper deposit, formed in post-subduction settings, is located in the Zhongdian area, northwester Yunnan province, China. Here we describe the igneous geology, phenocryst, skarn and retrograde mineralogy, alteration and mineralization history of the deposit and establish two different porphyry-skarn mineralization model in Yidun arc, which is based on field observation and a large number of drill core information, and combined with modern analytical technology such as Cathodoluminescence(CL), Back Scatter Electron(BSE), Electron Probe Microanalysis(EPMA), La-ICP-MS, Laser Raman Microprobe and Proton induced X-ray emission (PIXE) technique.The following are major achievements obtained in this dissertation.The mineralization and alteration zoning were controlled by the quartz monzonite porphyry, granite porphyry and lithostratigraphy. Single-zircon laser ablation inductively coupled plasma mass spectrometry U-Pb dating and Re-Os isotopic date of molybdenite from the ore suggests that the ore formed at 78.9 Ma, which is accordance with the age of quartz monzonite porphyry and quartz monzonite porphyry. The mineralization of the Hongniu-Hongshan deposit is closely related to the Late yanshanian magmatic activities, and the discovery of this deposit provided a successful model for searching Late Cretaceous deposits in the Zhongdian arc.It is possible to recognize two kinds of alteration zones in the Hongniu-Hongshan deposit: (1) Quartz monzonite porphyry is exposed and intruded into marble, and from the porphyry to marble are, in order, the endoskarns (Gar-Pyx-Ves), garnet skarns (Gar>Pyx), pyroxene skaras (Pyx>Gar) and Wollastonite skarns (Wo>Pyx-Gar). (2) Granite porphyry emplacement in hornfels under the surface, and the overlying country rocks are characterized by hornfels interbedded with marble. In this case, the alteration zones were mainly determined by lithological characters of wall rocks, and from the shallower to the deeper on the alteration zones are, in order, biotite hornfelsâ†'banded biotite hornfelsâ†'diopside hornfelsâ†'dioside-garnet skarnâ†'marber. However, the garnets also show the regularity on a certain scale (< 1km), garnet in the proximal zones are brown, coarse grain and in the intermediate zones are reddish to red, fine grain.The formation of the ore deposit has undergone five stages, and here we summarizes the sources and physical-chemical conditions of the fluid of each stage according to the researches of mineralogy, fluid inclusion and H-O isotope. We also set up the porphyry-skarn type mineralization and alteration system in this dissertation:(1)early skarn stage, country rocks altered into hornfels and marble owing to emplacement of the quartz monzonite porphyry, and formed endoskarn minerals represented by Al-rich garnets (Adr22-57Grs78-43) and salite (Hd7-13). In this stage, single-phase magmatic fluid with low salinity(566～650℃,10.8 wt% NaCl eq.) were derived from the exsolution of intruding magmatic melts (2.5-3.5km) and phase separated at 18-34 MPa,0.7～1.3 km, formed high temperature and high salinity liquid phase and low density of the gas phase. (2) Single-phase magmatic fluid (580～600℃,18.7wt% NaCl eq.,155 MPa) were directly derived from the exsolution of magma chamber in the late skarn stage, which isothermal ascented to shallower depths concurrent with intrusion of granite porphyry. Phase separation of the supercritical fluid occurred at ～55MP, metasomasised with carbonate surrounding rock at 2.2 km (the depth of boiling inclusion formed) and formed exoskarn minerals such as Fe-rich garnets (Adr75-98Grs25-2), diopside-hedenbergite (Hd 10-99) and Wollastonite, and leaded to potassium alteration of the potassium alteration and hornfels. (3) In the retrograde stage, as the underlying vapor-saturated magma chamber continued to cool and crystallize, fluid continually exsolved. Single-phase magma dissolving fluid with low salinity (350-400℃,9wt% NaCl eq.) formed from the magma chamber rised with cooling and condensing, which caused propylitization for the granite porphyry and metasomasised skarn minerals, formed hydrous minerals such as epidote, amphibole, and chlorite. (4) In the quartz-sulfide vein stage, the fluid shows multiple sources:H-O isotope results suggests the existence of residual metasomatic fluids; a small number of high temperature and high salinity liquid phase (322～389℃,39.9～45.3wt% NaCl eq.,110MP)and low density of the gas phase (489℃,<5wt% NaCl eq.) came from the magma chamber, which leaded to sericitization alteration on the granite porphyry; Boiling inclusion group and bimodal distribution of Al in quartz indicates the mixing of atmospheric water. (5) The fluid of carbonate phase were residual metasomatic fluids mixed with atmospheric water.Three garnet generations have been identified in this deposit:(1) Al-rich garnet (Grt â… ) is most likely associated with a low- to medium-salinity liquid resulting from the contraction of an ascending vapor phase, and formed in diffusive metasomatism by pore fluids equilibrated with the host rocks under low W/R ratios. (2) Fe-rich garnet (Grt â…¡) is associated with hypersaline brine, growing rapidly during advective metasomatism and under relatively high W/R ratios.(3) The Al-rich garnet veins (Grt III) most likely formed from the residual metasomatic fluids.The Raman and PIXE analysis of single fluid inclusions and S isotope results indicated that the S and Cu in the Hongniu-Hongshan deposit came from magma chamber, and entered watery volatile phase during the solution process. Cu and S as the complexes got into the gas phase in the phase separation of the supercritical magmatic fluid in skarn stage. However, Cu and Cl as the complexes existed in the single phase magmatic solution fluid in the retrograde stage. Owing to sulfate reduction, Cu-Cl complexes changed into sulfide shch as pyrrhotite, pyritic and chalcopyrite, and the increase of fluid pH value lesded to precipitation of metal elements. In the quartz-sulfide vein stage, Cu-S complexes dissociated to form sulfide because of oxygen fugacity increased of the ore-forming fluid, and the precipitation of metal elements caused by pressure fluctuations, which due to the repeated boiling of the fluid.Granites of the late Yanshanian period, including Hongniu-Hongshan, Xiuwacu and Relin, show the characteristic of I type granite, with high concentration of Al, K, Rb, Ba, and Pb, low amount of Ta, Ti, and Yb, a strong fractionation between light and heavy rare earth elements, which is similar to those of the Triassic subduction-related porphyry in the Zhongdian arc, and show the high ratios of Sr/Yand La/Yb, the ratios of LREE/MREE are higher than MREE/HREE, which indicated the primary magma of the late Yanshanian were riched in water, and fractional crystallization of amphibole took place during the magmatic evolution.On the basis of prevenient research, this paper summarized the space-time distribution of the deposit in the Yidun arc, and compared the Granite lithology and element geochemical characteristics formed in Indo-Chinese epoch and Late Yanshanian epoch at Changtai arc and Zhongdian arc. In the Indo-Chinese epoch, the steep subduction of oceanic plates in the Changtai arc caused the forming of the arc rift and back-arc basin, and developed related VMS deposit and epithermal Au-Ag-Hg deposits; arc andesite and related porphyry-skarn Cu-Au deposit in the Zhongdian arc caused by the gentle subduction. In the Late Yanshanian epoch, A-type granite and related skarn Sn-Ag deposit formed owing to continental crust remelting in the Changtai arc; I-type granite and related porphyry-skarn Cu-Mo deposit formed owing to the remelting of the residual oceanic slab combined with contributions from subduction-modified arc lithosphere and continental crust in the Zhongdian arc.