Significance Of Apatite For Metallogenic Mechanism And Prospecting Evaluation Of Gejiu Sn-Cu Polymetallic Deposit In Yunnan Province

The Gejiu tin-copper polymetallic deposit in Yunnan Province is a world-renowned huge tin-polymetallic deposit.The mineralization is related to the Cretaceous highly differentiated granite,but only the Masong and Laoca intrusions have formed economic deposits in the mine area.Therefore,comparing the mineralization potential of different intrusions and precisely analyzing the mineralization process have been the focus and difficulty of scientific research and exploration.In this thesis,based on the summary of previous studies,apatite was selected as an indicator mineral.The texture and chemical compositions of apatite in different strata,intrusions,and mineralized zones in the Gejiu area were systematically investigated by using electron microprobe（EPMA）and laser ablation inductively coupled plasma-mass spectrometry（LA-ICP-MS）.Then,distinguishes magmatic apatite from altered apatite and deeply analyzes the significance of the two types of apatite in the diagenesis and diagenesis of the Gejiu tin-copper polymetallic deposits,and finally concludes the role of apatite in guiding tin exploration.Magmatic apatite can indicate magma origin,magma oxidation state,magma volatile and magma evolution.The apatites in different intrusions in the Gejiu area are magmatic fluorapatite,reflecting the high F and low Cl contents of the primary melt.And the diagenetic temperature of the intrusions in Gejiu was calculated by the whole-rock zirconium saturation temperature and the apatite saturation temperature.The results showed that the intrusions in the Gejiu area have high crystallization temperatures and have the prerequisite of tin enrichment.The apatite trace elements characteristics reflect that the ore-bearing Laoka intrusion and Masong（Beipaotai）intrusion are mainly derived from the partial melting of crustal materials with limited contribution from mantle materials,and their parent magma is highly differentiated and reduced,which is conducive to the initial enrichment of tin.The Shenxianshui,Baishachong,and Longchahe intrusions,which showed weaker tin mineralization,were formed in a reduced-moderate oxidized environment with more mantle materials involved than the ore-bearing intrusions.The monzonite and gabbro in Jiasha are barren intrusions that formed in an oxidized environment and have significant mantle input.Meanwhile,apatite in ore-bearing intrusions has high Li,Fe O,and Mn O contents with very low（Eu/Eu*）N ratios,reflecting its more reduced and higher degree of differentiation,which can be used to distinguish ore-bearing and barren intrusions.The texture and compositional characteristics of altered apatites can be used to constrain magmatic-hydrothermal processes.The texture and composition of altered apatites in the granite from the Gejiu tin-copper polymetallic deposit are significantly changed relative to that of unaltered apatites.The changes of oxidation state sensitive elements（e.g.,Mn,Fe,Ce,Eu,Ga,etc.）contents imply that the mineralization process occurred in a more oxidized environment,which favors reduced tin solubility in fluids and subsequently promotes the precipitation of cassiterite.The altered apatites in basalt have significantly lower Cl and light rare earth elements（especially La and Ce）contents than unaltered apatites,suggesting that Cl in basalt was partitioned into fluids during magmatic-hydrothermal processes,providing Cl for the migration of metal elements.The hydrothermal quartz trace elements can reflect the temperature and acidity changes of mineralizing fluids.The hydrothermal quartz in the granite alteration zone is characterized by low Ti and Al contents and high alkali metal contents,whereas the hydrothermal quartz in the skarn has high Ti,Al,and Li contents,reflecting the gradual decrease of p H during hydrothermal fluid evolution,which is favorable to increase the solubility of tin in the hydrothermal fluid.Moreover,the cathodoluminescence characteristics and oscillation-bands of hydrothermal quartz indicate significant changes in external physicochemical conditions throughout the mineralization process.Thus,the precipitation of cassiterite may be related to the changes in environmental physicochemical conditions.Apatites in the ore-bearing strata show high Cl and SO₃ content and low F content,whereas apatite in the non-mineralized strata show low Cl and SO₃ content,suggesting that the strata may have provided part of Cl and S for metal transportation and precipitation during mineralization through water-rock interaction or mineral-fluid interaction,which facilitated the increase of tin solubility and transport to sites far from the intrusions for precipitation.The variation of the apatite composition in the pits and drills indicates a relationship between the apatite composition and its relative distance from the ore body,which is indicative of the localization of the ore body.The apatite in the ore body or mineralized zone has high Cl and REE₂O₃ contents and low F/Cl and Mn/Fe ratios.The Cl and REE₂O₃ contents gradually decrease and the F/Cl and Mn/Fe ratios gradually increase from the ore body to the surrounding rocks and the interior of the intrusions.Moreover,apatite in tin deposits has high F,Y,and Mn contents and F/Cl ratio,low Sr content,and low（Eu/Eu*）_N ratios,which is different from other deposit types.Therefore,the Sr,Y,Mn,Fe,and F contents and F/Cl,（Eu/Eu*）_N ratios of apatite can be used to distinguish tin deposits from other types of deposits.Apatite is an efficient tool in tin deposit exploration.But using apatite to guide tin exploration needs to be established on a certain degree of fundamental geological work.This thesis summarizes the application methods of apatite in tin deposit exploration through the above studies.The main steps are as follows:1)using magmatic apatite to identify ore-bearing intrusions;2)using apatite in strata to identify favorable strata;3)combining with other favorable geological conditions to find favorable locations for mineralization;4)identifying sites of strong hydrothermal activity by horizontal or vertical sampling,and further indicating where ore bodies are located.