Mineralogical Characteristics Of Pyrrhotite In Jiama Copper Polymetallic Deposit,Tibet

Jiama Super-large Copper Polymetallic Deposit is the product of typical porphyry metallogenic system in the eastern Gangdise metallogenic belt,which has great economic and social benefits and precious scientific research significance.Proven and controlled resource of copper in the deposit is nearly 9 million tons,associated with molybdenum,lead,zinc and gold resources,etc.,all of which have reached large scale.The ore-forming elements of the deposit are abundant and the orebody types are complex,which mainly include four types:skarn-type copper-polymetallic orebody,hornfels copper-molybdenum orebody,porphyry-type molybdenum(copper)orebody and independent gold orebody.Although the deposit has a high degree of research,the study on the typical mineralogy of pyrrhotite is weak.Pyrrhotite is an important metal sulfide in the deposit,mainly distributed in the periphery of the mining area,and some of it is closely associated with gold mineralization.It has important research significance and practical value in genetic mineralogy,prospecting mineralogy,etc.Therefore,in this paper,the pyrrhotite in Jiama deposit is studied by means of geological logging,microscopic identification,x-ray powder diffraction,electron probe microanalysis,LA-ICP-MS in situ analysis and stable isotope analysis.The structure and composition of different types of pyrrhotite are found out,the forming environment and material source of pyrrhotite with different occurrence are discussed,and the relationship with gold and the genesis of the deposit are also discussed.The main results and knowledge obtained in this paper are as follows:(1)The occurrence characteristics of pyrrhotite in Jiama deposit are identified.There are two main types of pyrrhotite occurrence in Jiama Deposit:1)occurs in hornfels far from porphyry body,primarily vein-like and veinlet-disseminated,with the mineral assemblage of pyrrhotite±pyrite±pyrite±chalcopyrite±quartz;2)occurs in the distal skarn of south pit section,mainly massive or dense disseminated,with the mineral assemblage of pure pyrrhotite±chalcopyrite±sphalerite±galena.(2)The structure types,major and trace components of pyrrhotite with different occurrences have been determined.The one occurred in skarn is purely hexagonal,with content of Fe element of 60.09%?60.71%and S element of 38.18%?38.69%,with an average of 60.38%and 38.35%,respectively,and it's mainly cobalt-rich and nickel-poor Pyrrhotite(Co/Ni more than 1)with chemical formula Fe₈S₉?Fe₁₀S₁₁.Another in hornfels is an intergrowth of monoclinic and hexagonal pyrrhotite,with content of Fe element of 59.05%?59.57%and S element of 39.28%?39.95%,with an average of 59.10%and 39.59%,respectively,and it's mainly nickel-rich and cobalt-poor Pyrrhotite(Co/Ni<<1)with chemical formula Fe₅S₆?Fe₇S₈.The result of deposit genesis discrimination diagram(Co/Ni ratio)shows that Jiama deposit belongs to skarn-type deposit related to magmatic hydrothermal.(3)The metallogenic environment and material source of pyrrhotite with different occurrence are revealed.The pyrrhotite in skarn was formed in the reduction environment of sulfur-rich and medium-high temperature(>254?),while the pyrrhotite in hornfels was formed in the reduction environment of sulfur-rich and medium-low temperature(<254?).The results of sulfur and lead isotopes show that the S and Pb isotope compositions in Jiama pyrrhotite are similar to those in magmatic rocks,indicating that the S and Pb sources of pyrrhotite are mainly magmatic.Nevertheless,the?34S of some pyrrhotites shows negative value,to some extent suggesting the addition of reductive S in carbonaceous argillaceous host rocks(hornfels).Moreover,the modification degree of pyrrhotite in hornfels(some samples show highly negative?34S value)by reductive S in carbonaceous argillaceous host rock(hornfels)is greater than that in skarn.(4)The enrichment mechanism of gold in distal skarn has been preliminarily discussed.The relatively high temperature pyrrhotite(Co/Ni>1)in skarn is closely related to gold mineralization,while the hexagonal pyrrhotite and monoclinic pyrrhotites in Hornfels(Co/Ni<<1)are not.Au mineralization in pyrrhotite is positively correlated with Co/Ni value(or temperature)in the mineral.Meanwhile,the distribution of Au in gold-bearing hexagonal pyrrhotite is closely relevant to that of Bi.Bismuth minerals,containing relatively high Au,Ag,As,Te and so on,are associated with natural gold,indicating that gold may be enriched in the form of Bismuth-gold melt,and Au mineralization is closely related to the trapping mechanism of Bismuth melt.The possible metallogenic mechanism is as follows:when the temperature of the fluid is above 271?,the bismuth in the fluid continuously captures the Au element in the fluid and forms the bismuth gold melt.When the fluid temperature is reduced to271??254?,the bismuth gold melt melts out,forming natural gold and natural bismuth/bismuth sulfide,which are encased in hexagonal pyrrhotite,forming bismuth gold mineralization in skarn.The fluid temperature in the hornfels is relatively low(<254?),the bismuth gold melt has already melted,and the content of Au in the fluid is low.Therefore,no gold mineralization is developed in the intergrowth of hexagonal pyrrhotite and monoclinal pyrrhotite formed in the hornfels.