Study On Genesis Of Luodian Nephrite Deposit,Guizhou Province

Luodian nephrite deposit,occurs in the Mg-poor limestone and siliceous strata of the second section of the Permian Sidazhai Formation but has as high quality as that of the Hetian nephrite in Xinjiang.However,the previous studies were mainly focused on gemology,petrology,mineralogy and petrochemistry with them,few was involved geology affecting the mineralization of the nephrite deposit.The origin of the ore-forming element Mg remains controversial,the genetic type of the deposit has not been determined,and the mineralization mechanism remains to be elaborated.In this paper,the stratigraphic chemical composition,magmatism,metamorphism,geological and geochemical characteristics of the deposit are comprehensively studied,so as to reveal the genetic types and metallogenic mechanism of the deposit,and provide theoretical support for discovery of higher-quality Luodian nephrite deposits.The research has achieved the following results:1.Confirming that the metallogenic type of Luodian nephrite is the contact-hydrothermal-metasomatic superimposed nephrite deposit,rather than the contact metasomatic type proposed previously.This contact-hydrothermal metasomatic superimposed deposit has not been reported both in domestic and international,thus is a new genetic type for nephrite deposit.The metallogenic processes is prolonged and complex for ca.200Ma with multi-events,beginning with deposition of limestone and siliceous rocks of the Sidazhai Formation followed by intrusion of the basic sill along with thermal contact metamorphism,the autometamorphism of the sill and the skarnization of the wall-rocks at?260Ma,and ending with intrusion of the granitic magmatic intrusion accompanying propylitic,hydrothermal alteration and metasomatic mineralization at?86Ma.2.Systematically determining the characteristics of the wall-rocks of the nephrite orebody,and identifying the composition,peculiarity,and timing of the three magmatic events and the two metamorphic events over the whole mineralization.The ore-body wall-rocks of the Sidazhai Formation deposited in the Middle and Late Permian,and the main lithology consists of the high purity limestone but poor in Mg,Fe,Al and other components and siliceous rock with various amount of lime composition but also poor Mg.The first stage magmatism occurred in the Late Permian aged in 260Ma?256Ma,the same ages with similar rocks in the Emeishan Large Igneous Province.It consists of remote transportation-magma-forming diabase sills,intermediate plugs and acid dikes.The magma is evolutioned due to episodic transport and emplacement.The intermediate plugs are the felsic magmas formed by differentiation crystallization of basaltic magma that penetratively upward moved into the newly arrived basaltic magma,and the acid veins are invading of the residual magma from the latest differentiation crystallization.The second and third intermediate-acid magmatic events occurred in 160Ma?170Ma and 86Ma?90Ma,respectively,with Na-rich for the former and K-rich for the latter.During the first metamorphic event around 260Ma,the episodic intrusive basic magma continued the contact thermal metamorphism to the wall rocks,along with that skarnization alteration occurring in the interval,and the autometamorphism of diabase sill in the last.The second metamorphic event of strong propylitization was related from the third stage intrusion of K-rich intermediate-acid veins at 86Ma?90Ma,and ended with completion of the hydrothermal metasomatic mineralization.3.Confirming that diabase sills are the supplier of key metallogenic element Mg for the Luodian nephrite,and decomposition of the clinopyroxene consiute is the sole source of that Mg.During the whole mineralization process,diabase sills has supplied Mg to the wall rocks for three times.In the interval of multi-episode intrusion of the basaltic magma of the first stage of metamorphism,hydrothermal fluid generated by certain degree of magma duing cooling and led to skarnization through unidirectional metasomatism,making the Mg migrate from the magma to the wall-rocks for the first time.However,the second migration of Mg was controlled by the autometamorphism post-formation of the sills And the third transport was related to the second stage propylitic hydrothermal metasomatism.The first supply of Mg occurred from the unconsolidated basaltic magma.The second and third transports were those Mg²⁺released from discomposition of the clinopyroxene to chlorite and epidote successively and dissolved into the hydrothermal liquid.4.Confirming that the mineralization of Luodian nephrite occurred in the Early Himalayan period rather than the Late Hercynian one.The key mineralization was caused by the intrusion of?86Ma K-rich granitic veins,leading to propylitication hydrothermal metamorphism of both the already altered?260Ma diabase and granitic dikes,and the 164Ma?172Ma intermediate plugs and?86Ma acid veins as well.The hydrothermal metasomatic mineralization of the Luodian nephrite occurred in two stages.In the first stage,the clinopyroxene continued to decompose,releasing Mg into a alkali metal K-and Na-rich hydrothermal liquid mixed with magmatic,metamorphic and meteoric waters that flowed to the wall-rocks,dissolving the original Mg minerals like diopside and released further Mg to the liquid to generate Mg-rich ore solution.In the second stage,the nephrite is formed by the ore-solution under suitable physical and chemical conditions.Therefore,formation of the Luodian nephrite is different from that of the Xinjiang Hetian nephrite(has a so-called"rock-forming stage"refering to the metasomatic reaction transition of diopside to tremolite)in that it has a high-Mg ore solution formation statge via dissolution of diopside,and the last stage is the nephrite formation stage in which the high-Mg ore solution is transformed to nephrite assemblages,i.e.the Ludian Nephrite.5.This study for the first time proposed such a Mg-purification mechanism for hydrothermal liquid mixed with Mg,Fe,Al and other compositions released from decomposing of clinopyroxene in the alteration,the formation of high Fe²⁺/(Fe²⁺+Mg)Fe-chlorite to take in large amount of Fe and increase the Mg-concentration and the Mg-purity of the liquid,providing a compositional premise for formation of the high-quality white nephrite and green-white nephrite.In a word,this genetic study of the Luodian nephrite come into proposes of the genetic type of contact-hydrothermal metasomatic superimposed nephrite deposit,the unidirection metasomatic skarnization between basic magma and siliceous limestone in inter-episodic magma transports in a major contact thermal metamorphism,and division of ore-solution formation stage and nephrite formation stage for the hydrothermal metasomatic mineralization,the viewpoint of Mg-purification mechanism for formation of the high-quality via formation of Fe-chlorite and epidotite,and the recognition of formation of the Luodian nephrite in the Early Himalayan period for the first time,etc will refresh the understanding of the mineralization mechanism of nephrite.