The Metallogenic Processes Of Huangshan Cu-Ni Sulfide Deposit In Xinjiang, Western China

The Huangshan Cu-Ni sulfide deposit is hosted in a mafic-ultramafic layered intrusion(269±2Ma),which is one of the Ni-Cu-(PGE) sufide deposits in Huangshan-Jing'erquan Permian magmatic metallogenic belt,belong to southern part of the Central Asian orogenic belt.The Huangshan intrusion was intruded into sandstones,carbonaceous siltstones and interlayered limestones of the Gandun formation.It forms a lozenge-shaped multiphase intrusive body with a area of 3.8×0.8m².It is composed of peridotite,pyroxenite,gabbro and diorite.The peridotite hosts most of the Ni-Cu-(PGE) sulfide ores,and the pyroxenite hosts minor disseminated sulfide ores.In this study,peridotite,pyroxenite and gabbro samples were collected from two new drilling cores for the petrological,major elements,trace elements and platinum group elements(PGE) analysis to reveal the metallogenic processes.The following results have been obtained.(1) Huangshan intrusion is composed of three well-defined petrological units:The top unit is composed entirely of relatively homogeneous peridotite(20-40m thick) with commonly poikilitic texturex.The middle unit is composed of various types of rocks,which are gabbro,pyroxenite and peridotite from top to base.The basal unit is composed of a free-grained gabbronorite and gabbro.It is about 300m thick and occurs along the lower margins of the intrusion.There is a sharp boundary between the basal peridotite and the peridotite in middle unit,marked by a hydrothermal alteration zone, as well as the gabbro in middle unit and the top peridotite.The peridotite in midle unit hosts most of the Ni-Cu-(PGE) sulfide ores,but the top peridotite hosts few ore.(2) Three units are formed in different magam intrusive stages with different characteristics of major and trace elements.The top peridotite has higher MgO,Al₂O₃ and CaO contents than the peridotite in middle unit.The peridotite and pyroxenite in middle unit have similar trace element patterns and REE patterns with the basal gabbro.However the basal gabbro is more enriched than the middle units,and shows higher SiO₂ and FeO contents,and lower CaO content than the gabbro in middle unit.(3) The different types of rocks in the Huangshan Cu-Ni sulfide deposit display systematical variations of major element oxide,similar distribution patterns of trace element and REE,and correlationship between Ta/Yb and Nb/Y artios as well as between Zr/Nb and Hf/Nb ratios,suggested that they could be derived from the same source of the parent magmas,which were high-MgO(15wt.%) basaltic magmas.(4) Primitive mantle-normalized trace element distribution patterns diplay significantly Nb- and Ta- depleted,and Sr-riched.Above evidences combined with negativeδEu values sugested the basaltic magma related to the Huangshan Cu-Ni sulfide ores undergone several important processes including fractional crystallization of olivine pyroxene and plagioclase minerals in the deep magma chamber,crustal contamination and the addition of Si-rich volatiles.(5) Some sulfide droplets are found in olivine of peridotite,and the ratios of Cu/Pd and Ti/Pd are higher than primitive mantle value.The metallogenic magma is contaminated by crustal components.These evidences imply the contamination of Si-rich crustal components could be responsible for sulphur supersaturation as well as sulfide melt segregation in deep magma chamber.