Genesis Of Cretaceous Porphryry Skarn Type Deposit And Their Magmatic Potential From East Of Zhangbaling Area, Anhui Province

Porphyry copper polymetallic deposit is a significant copper genetic type worldwide.Middle-Lower Yangtze River Valley Metallogenic Belt?MLYB?is one of the most important copper polymetallic mineralization belt of China with the porphyry and skarn deposit as the main ore deposit type.This research focuses on the copper polymetallic deposits and their related intrusions in east of Zhangbaling area,north of MLYB.Based on the detailed field observation,analysis like petrography,petrogeochemistry,zircon U-Pb dating,mineral geochemistry and whole rock Sr-Nd isotopes were carried out for the petrogenesis of the intrusions and their geodynamic background.Then molybdenite Re-Os dating,S-Pb-H-O isotope and in-situ S isotope analysis were carried out for tracing the material and fluid source as well as the ore-forming geochemical conditions.Combined with the previous study in the MLYB,this study modified the geodynamic model of this region and then evaluate the magmatic potential of these ore-related intrusions.The main results and conclusions are as follows:1.The study area is in the east of Zhangbaling uplift and is part of north MLYB metallogenic belt.The basement of this region is the ancient Yangtze basement.Mesozoic magmatic rocks and deposits in this region were controlled by the NE-SW trend fault zones.The main deposit in the study area is Chuzhou skarn copper ore deposit,Dongsunjia skarn copper ore deposit and Huangdaoshan copper mineral deposit.The ore-related intrusions of these three deposits are Chuxian,Machang and Huangdaoshan pluton and mainly composed by quartz-diorite porphyry,diorite porphyry,quartz-monzonite porphyry,diorite etc.Porphyry mineralization is formed inner the porphyry or as veins in the fissuring.The orebody of the skarn ore deposits are exhibits as veins or lenticular occurred in the contact zone of the intrusion and the carbonate formations.The ore textures are mainly massive,disseminated,banded and veinlets structures.The ores show idiomorphic or hypidiomorphic granular structures,disseminated textures,poikilitic textures,metasomatic textures.The main metallic minerals are chalcopyrite,pyrite with a small number of bornite,tetrahedrite and chalcocite.Native gold can be found by micro observation,although it is rare.The alterations of porphyry deposit are potassic,argillization,advanced argillization,sericitization and chloritization.The alterations of skarn deposit are skarnization,pyritization,chloritization and sericitization.Quartz and multi-metal mineralization stage is the main mineralization process of these deposits.2.LA-ICP-MS zircon U-Pb dating results show that the Chuxian intrusion was emplaced at around 129±2 Ma;Machang quartz-diorite and quartz-monzonite was emplaced at 132±2 Ma and 133±2 Ma,respectively;the Huangdaoshan intrusion was emplaced at 130±1 Ma.All these intrusions were formed at early Cretaceous.The intrusions of study area display high silica,Na and Al contents,low Mg and Ti.Contents.These intrusions both show great fractionation of HREE and LREE,depleted in HREE and enriched in LREE with high Rb,Ba and Sr and low Y and Yb contents.Both intrusions all have high Sr/Y and La/Yb ratios and show adakitic characters.The moderate Sm/Yb and high La/Sm ratios and obvious positive Eu anomalies indicate the fractionation of amphibole,which may result to the relatively high Sr/Y and La/Yb ratios of these intrusion.The massive acicular apatites,quartz and potassium feldspar ocellis,biotite inclusions in the amphiboles suggest that hot mafic magma has injected into the felsic magma.Before that,the felsic magma should have undergone fractionation to some degree.The intrusions show various(⁸⁷Sr/⁸⁶Sr)_i,?_Nd?t?and?_Hf?t?indicated a magma mixing process.The quantitively Sr-Nd calculations suggest65%-90%magma from lower crust mixed with 35%-10%mantle derived magma can account for the isotope composition of these intrusion.3.The molybdenite Re-Os model ages of Zhangbaling area are between128.7¹29.8 Ma,which are coeval with the emplacement age of these intrusion within the analysis uncertainties.It means that the mineralization of this region is occurred in the second mineralization episode?135¹23 Ma?of Middle-Lower Yangtze Valley Metallogenic belt.The?¹⁸O_SMOW?‰?values of fluids from Chuzhou ore deposit,Dongsunjia ore deposit and Huangdaoshan mineral deposit are 6.7¹7.1,6.8⁷.0 and5.1⁷.0,respectively.Their?¹⁸D_SMOW?‰?are plotted into the magmatic water field,with values of-65.3^-78.1,-48.3^-63.7 and-47.0^-72.8,indicating an magmatic water origin.Sulfur mineral S and in-situ?³⁴S values of these ore deposits are varied around 0,which indicates a deep magmatic sulphur source.The lead isotope analyses results show thatleadisotoperatiosoftheoresare²⁰⁶Pb/²⁰⁴Pb=16.086¹7.155,²⁰⁷Pb/²⁰⁴Pb=15.333¹5.455,²⁰⁸Pb/²⁰⁴Pb=36.793³7.258.It means that the lead of these deposit are mainly from the deep mantle with minor crustal lead contamination.4.The oxygen fugacity calculated by the Mg contents of the amphibole and the qualitatively estimated by the Fe²⁺,Fe³⁺and Mg contents of biotite indicate that the oxygen fugacity of the ore-related intrusion are higher than the Ni-NiO buffer,suggesting a high oxygen state of the intrusion.Based on the Al in amphibole geobarometer,the emplaced depth of Chuxian,Machang and Huangdaoshan intrusion should be 1.2⁶.3 km,1.0⁵.7 km and 4.2⁹.8 km,respectively.5.Based on the previous study about the magmatism and mineralization of MLYB,combined with the movement of Tanlu fault zone,this study modifies a slab subduction-roll back-extension-underplating model for the east of Zhangbaling area.Then proposed an ore-forming model of this area.Comparing the ore-related intrusion in the study area with Jiurui region of MLYB from the aspect of emplaced depth,oxygen fugacity and structure development,we suggest that the future exploration work should pay more attention to the periphery of these deposit.