The Larong Giant W-(Mo) Deposit In The Eastern Tibet: Magmatism And Mineralization

Porphyry W deposits as an important type of W deposits.At present,researches on the deposit geology,magmatism,ore-forming fluid characteristics,and mineral precipitation mechanisms of porphyry W deposits occurred in magmatic arc and intraplate settingts have got some progressions.In contrast,little is known about the above characteristics of porphyry W deposits formed in a continental collision regime.The Larong deposit,as the first giant collision-type porphyry W-?Mo?deposit discovered in the Leiwuqi-Zogang metallogenic belt?LZMB?of eastern Tibet,provides an important window for us to solve the above problems.In this paper,a detailed study of the Larong deposit was carried out and the regional metallogenic regularity and prospecting direction were preliminarily studied.The main results achieved are as follows:1.Systematic identification of the formation ages of various intrusive rocks and the metallogenic age of the Larong deposit.LA-ICP-MS zircon U-Pb dating of the dacite porphyry,granodiorite porphyry,biotite granite porphyry,monzogranite porphyry,biotite granite,and granite aplite yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 214.0±0.7 Ma,213.8±1.3 Ma,104.4±0.9 Ma,93.9±1.3 Ma,91.7±0.5 Ma,and 36.2±0.6 Ma,respectively.The Larong composite pluton is composed of the latter five,and the magmatic activity contains at least three periods and spans?180 Ma.Samples of molybdenite yielded a weighted mean age of 91.8±0.5 Ma and an isochron age of 90.6±2.1 Ma,indicating that the Larong deposit was formed in Late Cretaceous and the W-Mo mineralization of the Larong deposit was genetically associated with the monzogranite porphyry.2.A detailed exploration of the petrogenesis and geodynamic background of various intrusive rocks.The Late Triassic porphyries should be classified as peraluminous granodiorites.They are characterized by lower oxygen fugacity and degree of differentiation,and enriched Sr-Nd-Hf isotopic compositions(⁸⁷Sr/⁸⁶Sr?i?=0.714785-0.728418;?Nd?t?=-10.01 to-13.92;?_Hf?t?=-8.5 to-16.6),with relatively older two-stage Nd-Hf model ages of 1.79–2.30 Ga,indicating that they were mainly derived from partial melting of ancient crust crystalline basement after break-off of the Paleo-Tethyan oceanic slab.The Cretaceous and Eocene intrusive rocks belong to peraluminous to weak peraluminous granitoids.They have higher oxygen fugacity and degree of differentiation,and relatively depleted Sr-Nd-Hf isotopic compositions(⁸⁷Sr/⁸⁶Sr_?i?=0.71192-0.72932;?Nd?t?=-11.81 to-9.11;?Hf?t?=-10.4 to 1.4),with relatively younger two-stage Nd-Hf model ages of 1.03-1.86 Ga,suggesting that they were closely related to mixing of crust-mantle source magmas and subsequent extensive fractional crystallization.The biotite granite porphyry was produced related to the slab roll-back of the Bangong-Nujiang Tethyan ocean?BNTO?,the monzogranite porphyry and biotite granite were formed under a post-collisional tectonic setting related to slab break-off of the BNTO,and the granite aplite was likely associated with delamination of the thickened eclogitic lower crust together with lithospheric mantle.The Cretaceous granitoids of the Larong composite pluton have relatively higher oxygen fugacity and degree of differentiation,and thus are conducive to the formation of W-Mo deposit.3.Preliminary ascertaining characteristics of ore-forming fluids and sources of ore-forming materials of the collision-type Larong porphyry W?Mo?deposit.The ore fluids of the Larong deposit belong to the Na Cl-H2O-CO2 immiscible system.The ore fluids of the main mineralization stages are characterized by CO2 enrichment,high-medium temperature,and medium-low salinity.Sulfides from the Larong deposit have calculated?³⁴S_VCDT values ranging from 0.25‰to 6.37‰,suggesting a predominantly magmatic origin for sulfur.The trace elemental and Pb isotopic compositions of sulfides indicate that ore-forming metal elements were mainly derived from crust-derived magmatism.Based on the relationship between scheelite mineralization and wall-rock alteration,Sr isotopic characteristics of scheelite,and the characteristics of major elements in plagioclase,we propose that the alteration and decomposition of plagioclase in granodiorite porphyry,biotite granite porphyry,monzogranite porphyry and greenschist might have provided abundant calcium for the precipitation of scheelite.4.A brief analysis of the metallogenic mechanism and the establishment of the metallogenic model of the Larong deposit.During the collision orogenic stage of the Lhasa Qiangtang block,slab break-off of the BNTO at?94 Ma triggered mantle upwelling that induced the remelting of ancient crustal materials and mixed with crust-derived magma.The mixed magma subsequently underwent intensive crystalline differentiation,resulting in the formation of highly evolved,relatively oxidized,W-rich,and volatile-rich magma.With the cooling of magma and the emplacement of the monzogranite porphyry in the shallow crust,a large amount of hydrothermal fluids dissolved from the ore-causative porphyry,migrated upward,filtered through and intensively reacted with wall rocks,causing significant addition of Ca²⁺into the fluids.Intense fluid-rock interactions and CO2 loss decreased the temperature and oxygen fugacity and increased the S^2-activity and p H values of the fluid systems,which could account for the successive mineralization of W and Mo.5.Based on the study of metallogenic regularity and the extraction of the related prospecting factors,the prospecting models of typical W deposits in the LZMB have been established.Though the analysis of ore-forming geological characteristics and theoretical calculations,the great prospecting potential of the deep and southeast parts of the Larong mining area has been proposed.According to relevant prediction parameters,five W polymetallic metallogenic prospective areas have been divided in the LZMB.