Geological Characteristics And Genesis Of The Overprinting Mineralized Copper Deposit In The Jinjiwo Area,Jiangxi Province

The Jiurui mineralization district,which located in the northern margin of Yangtze Plate and to the south of the Dabie orogenic belt,is an important part of the Middle-Lower Yangtze Mineralization Belt.Although previous researchers have done some studies for the stratiform orebody within the bottom of the Upper Carboniferous Huanglong Formation consisting of dolomite and coarsely crystalline limestone,there have been some controversies on its genesis.The Jinjiwo copper deposit is located in the south-east end of the Jiurui ore district,where the Stratabound skarn Cu orebody and the stratiform copper-bearing pyrite orebody are developed.Here we report on an integrated study on geological,mineralogical and geochemical characteristics,with a view to understanding trace element compositions of pyrite,occurrence state and enrichment mechanism of elements,sulfur isotope compositions,ore-forming material sources and deposit genesis,and build up a metallogenic model.The major achievements are as following:(1)The Jinjiwo granodiorite porphyry is characteristic of the quasi-aluminous high potassium Ca-alkaline syntexis(? type)granite derived from the crust-mantle mixing source.Zircon LA-ICP-MS U-Pb dating yields a weighted mean U-Pb age of 144±1Ma,indicating that it was formed in the Early Yanshanian period.The Hf isotope shows that the?_Hf(t)values of zircon range from-4.09 to-8.61,with T^C_DM ages between 1.46Ga and 1.68Ga,suggesting that the genesis was derived from partial melting of crustal rock(>1.6Ga).(2)The metallic minerals in the stratiform orebody are dominated by pyrite,followed by chalcopyrite,pyrrhotite,sphalerite,etc.,accounting for 65%-85%of the total content.Textural observations suggested four types of pyrite corresponding to three metallogenic episodes:syngenetic sedimentation(Py?),metamorphism or deformation(Py?),and skarn-hydrothermal mineralization including the late skarn stage(Py?)and hydrothermal stage(Py?).Pyrite(Py?)can be further divided into two forms,namely colloform pyrite(Py?-1)and laminated pyrite(Py?-2).(3)Py?-1 and Py?-2 share similarities in behavior of pairs of elements and have low Co/Ni ratios,and the former contains the highest concentrations of Bi,Cu,Pb,Zn,Ag,Au,Mn,suggesting that rapid precipitation from highly saturated fluids.Py?shows high Co,Ni and As concentrations and Co/Ni ratios of 0.03 to 6.19.Py?and Py?have greatly varying Co/Ni ratios of 1.07 to 29 and typically low Au concentrations,suggesting a hydrothermal source.Py?is rich in Co and Se,but is depleted in As,Cu,Pb,Zn,Ag,and Au in comparison with Py?.(4)Most of the Cu,Pb and Zn in colloform pyrite are most probably restricted within the pyrite structure,while these elements in the other types of pyrite can be attributed to the presence of nanometer-sized inclusions of Cu-,Pb-,and Zn-bearing minerals.The Co vs.Ni plot also displays significant positive correlation for Py?,Py?,and Py?,but no significant pattern for Py?.The relatively uniform counts per second between Co and Ni,and consistent with the Fe counts,prove the existence of Co and Ni in the pyrite lattice.The majority of Au can occur as structurally bound Au⁺.(5)The results of XRD show pyrite peaks with the most intensive reflection peaks occurring mainly from the(311)and(200)plane.The sharp diffraction peaks indicate that the full width at half maximum(FWHM)is minor,and that pyrite crystal faces are well developed.A cubic cell and space groups(205)Pa³ with lattice parameters,a=5.4142(?)?5.4285(?),Vol=157.56?160.68(?)3,with large variation range,and its average value is respectively a=5.4221(?)and Vol=159.59(?)3,which is generally higher than the theoretical result.This may be due to the fact that trace elements(e.g.Co,Ni,As,Cu⁺,Au⁺)that enter the pyrite lattice via stoichiometric substitution.The Raman spectrum reveals subtle differences in band position and widths(FWHM)and displays the increasing formation temperature of from Py?,through Py?and Py?,to Py?.(6)There are two kinds of sulfur:one is that?³⁴S values are near zero,suggesting that the pyrite sulfur in the ores was derived from a mixed source in deep crust,and the other is that?³⁴S values are less than-39.1‰,indicating the sedimentary pyrite sulfur was originated from bacterial sulfate reduction.(7)The lead isotope compositions for sulfides show their contents of radiogenic Pb are relatively low,with?being 9.21?9.47(avg.9.39)and K being 3.49?3.85(avg.3.74),and their data points fall into the areas of the upper mantle and orogenic evolution lines,indicating that Pb-isotope has characteristics of crust-mantle mixed source.(8)The formation of the Jinjiwo copper deposit might have recorded the submarine exhalation sedimentation and the Mesozoic tectonic transition-deformation and hydrothermal overprint.Colloform pyrite was formed in the process of the late Paleozoic submarine hydrothermal sedimentation,and enriched in ore-forming elements,and acted as the source layer of the copper polymetallic deposit.The Yanshanian magmatic hydrothermal fluids not only caused the diversified and complicated ore texture but also induced skarnization in the Upper Carboniferous carbonate formations.As the hydrothermal system evolved,pyrite coexisting with chalcopyrite,bornite,sphalerite and galena were crystallized from the sulfide stage.