Laba Porphyry-skarn Copper-molybdenum Deposit In Northwestern Yunnan: Alteration-mineralization Characteristics And Metallogenic Model

The Laba Mo-Cu deposit located at the northwestern margin of the Yangtze Terrane,adjacent to the junction between the Yidun and Yangtze Terrane in northwestern Yunnan province,SW China.It is a super-large magmatic hydrothermal-type deposit associated with granitoids during Late Cretaceous.The ore-bearing pluton intruded into the coastal-shallow marine carbonate sedimentary of Beiya Formation in the midlle Trissic,and the shallow-bathyal volcaniclastic association of Heinishao Formation in the upper Permian.The deposit consists of porphyry-type and skarn-type orebodies,and there is lack of constraints on their genetic relationships.On the basis of previous studies and detailed field works,we reveal the petrogensis and tectonic setting of the Laba ore-bearing pluton,and identify the temporal and spatial links among the mineralization,the alteration and the related veins of different ore types,and the ore-forming process,and establish the ore genetic model for the Laba deposit.We come to the major conclusions as follows.?1?The stratigraphic lithology,structures,spatial distributions of ore-bearing pluton and orebodies of the Laba deposit have been identified.The ore-bearing pluton was granitic porphyry belonging to high-K calc-alkalic and/or shoshonitic series,and the orebody is major in Mo,distributed within the granitic porphyry and the garnet-diopside skarns,while Cu-rich oredodies were distributed within the garnet-diopside skarns and chlorite-epidote-magnetite skarns.The pluton and orebodies were both controlled by the fault F₁ and F₂ with NE trend and lateral trending to north.?2?Mo-mineralization mainly concentrated in the porphyry,and always the apophysis mineralized absolutely,while Cu-mineralization centered within the different kinds of skarns in vein and layer-style.The potassic alteration,chlorite-phyllic alteration,propylitic alteration spatially distribute from the plutons to the wall rock in the Laba deposit.The location of argillic alteration is controlled by the structural fissures.The skarnization are distributed in the contact between the the plutons and wall rocks,and the?tremolite-?diopside-garnet skarn,the tremolite-chlorite-epidote-magnetite skarn are spatially distribute from the plutons to the wall rocks in the Laba deposit.Mo-mineralization mainly distributed in the potassic alteration,while Cu-mineralization distributed within the chlorite-phyllic alteration.Comparing with the collision-type porphyry deposit,the Laba deposit had wide chlorite-phyllic alteration,and narrow potassic alteration.The early potassic and propylitic alteration of porphyry-type oerbodies are respond to the wet-skarn formation,and oxide and quartze-sulfide stage,the chlorite-phyllic alteration is respond to the quartze-sulfide stage and the argillic alteration is respond to the carbonate stage.Major Mo mineralization is always associated with the potassic alteration,while Mo and Cu mineralization is mostly associated with the chlorite-phyllic alteration.The Laba deposit can be divided into pre-mineralization phase,syn-mineralization phase.The syn-mineralization phase can be divided into the K fledspar-biotite-molybdenite stage,the chlorite-epidote-magnetite stage,the chlorite-sericite-quartze-sulfide stage,the carbonate-clay minral stage,respectively.It is same with the collision-type porphyry deposit in this sense.?3?From early to late phase,the temperature of the ore-forming fluids decreased gradually,while the salinity varation has no definite tendency.The temperature,salinity and density of the ore-forming fluids is 291.7⁴59.5?,10.36²1.61%NaCleqv,1.08¹.18g/cm³ in the potassic stage;263.9⁴03.7?,8.12²2.14%NaCleqv,1.06¹.19g/cm³ in the propylitic stage;217.9³70.8?,5.11²4.08%NaCleqv,1.04¹.20g/cm³ in the chlorite-phyllic stage;167.2²92.5?,3.23¹3.72%NaCleqv,1.02¹.11g/cm³ in the argillic stage,respectively.And the features of different stages of skarn-type mineralization also have the similar evolution tendency.The fluid salinity of the K fledspar-biotite-molybdenite stage and chlorite-sericite-quartze-sulfide stage changed dramatically and different fluid inclusions type distributed in the stages,suggesting the molybdenite and chalcopyrite deposited associating with the fluids boiling and different fluids mixing.Comparing to the collision-type porphyry deposit,the magmaic hydrothermal contribution dominated the main ore-formation in the Laba deposit and the participation degree of non-magmatic hydrothermal fluid was higher.And the regional upliftting and erosion can trigger and define the temporal windows for mineralising events.?4?The Laba granitic potphyry shows high SiO₂,Al₂O₃,CaO,?Na₂O+K₂O?,Sr content,high degree of REE fractionation,high(⁸⁷Sr/⁸⁶Sr)_i values,and negative?_Nd?t?and?_Hf?t?values.These fearture suggest that the Laba pluton is I-type one with adakitic affinities,and formed by partial melting of ancient thickened lower continent modified by the SCLM with garnets as residual phases in the magmatic origins.The zircon U-Pb age of the Laba porphyry is 89⁸7Ma.Combined with the contemporary plutons in different part of the Tibetan Plateau regions,the Laba porphyry is remotely respond to the Neo-Tethyan subduction processes.The Laba deposit was formed under an intra-plate setting.It is neither similar with the subduction-type porphyry deposit nor with the collision-type porphyry deposit.The temperature,pressure,depth and oxygen fugacity of the Laba pluton are 705⁹03?,59⁴49MPa,2.2¹7.0km,and-11.8^-10.5,respectively.The positive Ce-anomaly further proved the high oxygen fugacity.The high stable oxygen fugacity,wide ascended range,and the fluids exsolved the magma are rich in F and Cl,all these are conducive for the metals enrichment and precipitation including the Mo and Cu.?5?The ore genetic model of the Laba porphyry-skarn type Mo-Cu deposit is established as follows.The magma primarily is origined from the thickened lower crust modified by the SCLM,ascend to the shallow,and/then exsolved the magmatic fluid mixing with different degree meteotic water?early stage is major in magmatic fluids,and the late stage is major in meteotic water?and with the local contribution of wall rocks,then formed the super-large Laba porphyry-and skarn-type Mo-Cu deposit.The mineralization metals are mostly origin from the magmas,with a little contribution of the direct wall rocks.The primary hydrothermal fluids are mostly origined of the magma in the early stage,and mostly of non-magmatic hydrothermal fluids in the late stage.The Laba deposit is belong to subduction-collision type according to tectonic setting.The bottom of Lianban and Tongchangpin still show great prospecting potential for Mo orebodies.