| Tourmaline and tourmaline breccia commonly occur in various types of hydrothermal ore deposits.The correlationship between tourmaline,B-isotope and ore-forming process has been investigated in many Sn or Sn-W deposits,while limited studies focused on Au deposits.Pervasive tourmaline exists among Longtoushan Au deposit in Guangxi,Hadamiao Au deposit and Bilihe Au deposit in Inner Mongolia.In the former two deposits,massive tourmaline breccias were developed and are intimately related to gold mineralization.Investigating the petrographic and compositional features of tourmaline in these gold deposits,and revealing the genesis of tourmaline breccia and its relationship to gold mineralization are significant for evaluating the potential of tourmaline breccia as geochemical indicator for gold exploration.In this paper,comprehensive fieldworks and detailed petrography,in situ composition analyses of major,trace elements and B-isotope of tourmaline from the three gold deposits mentioned above,as well as from other Au-barren areas?including Pingtianshan from Guangxi,Narenwula and Beidashan from Inner Mongolia?have been conducted,and major achievements are listed as following:?1?Tourmaline samples mainly occur as breccia cements,orbicular aggregates,and hydrothermal veins in the Longtoushan deposit and Hadamiao deposit,and occur as disseminated complexes in granodiorite porphyry and as stockworks in sandstone when proximal to or distal from the orebody in Bilihe deposit,respectively.Orbicular tourmaline aggregates in the rhyolite porphyry from Longtoushan were formed in pre-ore stage,which belonged to magmatic period.Tourmaline cements and aggregates in the Au-mineralized breccia and granite porphyry were formed in ore stage.Tourmaline in Bilihe and Hadamiao shares a post-magmatic hydrothermal origin.?2?All the tourmaline samples belong to the schorl-dravite series of alkali group.Tourmaline from Longtoushan is featured by high Al?avg.6.52 apfu?and low Na?avg.<0.63 apfu?contents.The pre-ore tourmaline is enriched in Fe and depleted in Mg?Fe#:0.99–0.84?,and the ore stage tourmaline is enriched in Mg?Fe#:0.78–0.14?.These tourmalines mainly show MgFe-11 and X?Al(NaR2+)-11 exchange vectors.Tourmaline from Hadamiao is featured by low Al?avg.<5.50 apfu?,medium-high Na?avg.>0.71 apfu?contents and low-medium Fe#values?<0.80?,and follows the Fe3+Al-11 exchange vector.The?11B values of the tourmaline from Longtoushan and Hadamiao range from–14.5 to–2.3‰,indicating dominant magmatic-hydrothermal fluids of crustal origin.In the Bilihe deposit,tourmaline is enriched in Fe and depleted in Mg?Fe#:0.97–0.61?in the orebody,and enriched in Mg and depleted in Fe?Fe#<0.73?in the sandstone.In contrast to Au-barren areas,tourmaline from gold deposits generally has high As contents?>10 ppm?,but shows no significant differences in major elements.?3?Tourmaline breccias from Longtoushan and Hadamiao are probably formed by open-space infilling of boron-rich fluids violently released from felsic magmas.Vapor fluids dominate in Longtoushan,while hydrothermal fluids are more likely in Hadamiao.Fluid boiling and brecciation would be critical to gold mineralization.Tourmaline breccia with high As?>10 ppm?,high Fe3+?>0.30 apfu?and Na?>0.70apfu?contents,medium-low Fe#values?<0.80?,and magmatic boron source is an prospective indicator for porphyry gold mineralization in depths or distal locations.Tourmaline breccia with high As?>10 ppm?and Cu?>1 ppm?contents,low Fe#values?<0.80?,and magmatic boron source would be a prospecting guide to proximal epithermal gold mineralization or porphyry Cu mineralization in depths. |
PDF Full Text Request