Tin Mineralization Associated With Highly-fractionated Granite

The Nanling Range,South China,hosts huge volumes of Mesozoic granitoids and associated ore deposits,and is regarded as one of the most important Sn?W provinces in the world.The Furong Sn deposits is located at the western Nanling Range,and hosts a measured Sn reserve of about 530,000 t（metric tonnes）at an average ore grade of 0.8%Sn,and an inferred tin reserve of～700,000 t,ranking as the third largest Sn deposit in China.The giant Furong deposit occurs within and at the southern margin of the large Qitianling granitic batholith,representative of tin mineralization related to large granitic intrusions,which is distinct from those generally associated with small highly-evolved granite stocks and plutons.Thus,the Furong deposit has attracted considerable interest since it was discovered in late 1990s.The multiphase Qitianling granitic batholith consists of main-phase medium-to coarse-grained K-feldspar megacrystic hornblende biotite monzogranite and biotite monzogranite（main-phase granite）,and minor fine-grained late-phase alkali-feldspar granite（late-phase granite）.Previous researches mainly focus on the main-phase granite,while the petrogenesis of the late-phase granite and its correlations to the main-phase granite and tin mineralization remain unclear.Various types of tin mineralization occur in the Furong deposit,including skarn,altered granite,greisen,and quartz vein types,part of which has not been dated directly or precisely.The altered granite-type mineralization at the No.10 orebody at Furong defines the bulk mineable disseminated and fracture-controlled cassiterite–quartz–chlorite ore within granite affected by pervasive chlorite alteration.However,the co-precipitation of chlorite and cassiterite is still a matter of debate.Besides,the nature,source,and evolution of ore-forming fluids,and the ore-forming processes in the Furong deposit are poorly constrained.These unresolved questions limit a better understanding of the genesis of the giant Furong tin deposit.Based on detailed geological investigation and petrographic observations,a series of scientific researches are conducted regarding to the above issues,including（1）the mineralogy,zircon U–Pb geochronology and Hf–O isotopes,whole-rock elements and Nd isotope of the Qitianling late-phase granite,（2）cassiterite U–Pb,mica ⁴⁰Ar–³⁹Ar geochronology and cassiterite trace elements of the four ore types at Furong,and（3）textures,paragenesis,and in situ elemental and B–Sr–Nd isotopes of numerous minerals from the granites,altered rocks and tin ores at Furong.The obtained achievements and conclusions are summarized as follows:The Qitianling late-phase granite has SHRIMP zircon U–Pb ages of 155～154 Ma,coeval with the main-phase granite（163～153 Ma）,and is characterized by more evolved chemical compositions compared to the main-phase granite.The Qitianling granites have undergone a high degree differentiation in a crystal mush system,with the late-phase granite representing the highly-fractionated interstitial melt that was extracted from the crystal mush system,and the main-phase granites representing the residual crystal mush that consists of the‘cumulate crystals’and trapped interstitial melt.The four types of tin mineralization at Furong formed at 159～157 Ma,coinciding with the Qitianling granitic magmatism.Distinctive cathodoluminescence textures and trace-element patterns of various cassiterite types indicate that the four ore types resulted from pulsed ore-forming fluids and changes in temperature in a single magmatic-hydrothermal system.Several tourmaline and apatite generations are distinguished at Furong.Tourmaline（Tur）variants comprise pre-ore Tur-1 as disseminations and nodules in the late-phase granite,pre-to syn-ore Tur-2 as replacements in nodules and as veins crosscutting the late-phase granite and nodules,syn-ore Tur-3 in tin greisens,pre-to syn-ore Tur-4 as veins in the altered main-phase granite,and syn-ore Tur-5 from tin skarns in a distinct Ca-rich environment.Textural and compositional features suggest that all tourmaline variants are hydrothermal in origin with alkali and schorl to foitite composition and minor extensions to calcic and X-site vacant tourmaline groups.The narrow range of tourmalineδ¹¹B values（-14.8 to-10.4‰）suggests a single magmatic boron source in the ore-forming fluids.Apatite（Ap）generations include accessory Ap-G in the main-phase granite,and Ap-Ⅰto Ap-Ⅲfrom three stages related to skarn-type mineralization（garnet-diopside stage-Ⅰ,pargasite-phlogopite-cassiterite stage-Ⅱ,and sulfide-rich stage-Ⅲ,respectively）.All the apatite generations belong to fluorapatite with Ap-G crystallizing from the magma and Ap-Ⅰto Ap-Ⅲbeing hydrothermal in origin.The similar rare earth element patterns andε_Nd（t）values（-8.2 to-5.9 for Ap-G and-8.0 to-7.3 for Ap-Ⅰ）between magmatic and hydrothermal apatite indicate that the skarn-forming fluids are dominantly derived from granites.The ⁸⁷Sr/⁸⁶Sr ratios of Ap-Ⅰto Ap-Ⅲ（0.70733–0.70795）are similar to the carbonate wall rocks,but distinctly different from the more radiogenic granites,indicating strong Sr exchange with carbonate rocks.Integrating previous H-O isotopic data,the tourmaline and apatite elemental and B-Sr-Nd results suggest that the greisen-type ore formed by interaction of B-,Na-,Li-,Zn-,and Sn-rich magmatic fluids with the late-phase granite in a closed and reduced feldspar-destructive environment,whereas the tin skarns resulted from mixing of magmatic fluids with meteoric water and interaction with the carbonate wall rocks in an open system where oxygen fugacity changed from reduced to oxidized conditions.During fluid-rock interactions and fluid mixing,considerable Ca,Mg,V,Ni,and Sr from the host rocks were introduced into the ore system.Tourmaline B isotopes by themselves are ineffective in discriminating the entrainment of the B-poor,wall rock-derived fluids in granite-related tin deposits.But coupled hydrothermal minerals such as tourmaline and apatite have great potential to fingerprint the nature,source,and evolution of fluids in granite-related ore systems.Four types of chlorite are identified at Furong:chlorite-1（Chl-1）partially or entirely replacing biotite with biotite relicts or pseudomorphism mainly in the weakly chloritized granites,chlorite-2（Chl-2）without biotite relicts or pseudomorphism expressing an irregular interlocking texture with quartz,cassiterite,and magnetite in the moderately and/to intensely chloritized granites,comb-shaped chlorite-3（Chl-3）coexisting with comb-like quartz and potassium feldspar in the quartz?Chl-3 vein crosscutting the intensely chloritized granites,and chlorite-4（Chl-4）replacing garnet,diopside,and vesuvianite in the skarns.Major elements of chlorite suggest that Chl-1 is mostly brunsvigite and chamosite,Chl-2 and Chl-3 are mainly thuringite with high Fe contents,and Chl-4 belongs to brunsvigite and ripidolite.The calculated temperatures decrease from 321–422℃of Chl-2 and 397–420℃of Chl-3 to 260–365℃of Chl-1 and 285–366℃of Chl-4,with decreasing intensity of chloritization from the proximal to distal ends of the orebody.Chl-1 formed by the layer-by-layer replacement when two biotite layers converted to one Chl-1 layer under low fluid-rock ratios,Chl-2 resulted from the dissolution-transport-precipitation processes during the interactions between the Sn-,Fe-,and Zn-rich fluids and granites under high fluid-rock ratios,Chl-3 precipitated directly from the Sn-,Fe-,and Zn-rich fluids,and Chl-4 formed by the dissolution-transport-precipitation mechanism under low fluid-rock ratios.Compared to other chlorite variants,Chl-2 and Chl-3 coexisting with cassiterite are featured by high temperatures and elevated Fe/（Fe+Mg）ratios,forming an important indicator for chloritization-dominated,altered granite-type tin mineralization.In combination with previous researches,the chlorite occurrence,texture,and elemental compositions indicate that the co-precipitation of abundant Fe-rich chlorite（Chl-2 and Chl-3）and cassiterite along the NNE-trending fault zones was related to the mixing of the rising ore-bearing magmatic fluid with meteoric water and their interactions with the main-phase granite.In addition to cassiterite,hornblende and biotite in fresh granite,hydrothermal muscovite in altered granite,and most silicates（garnet,diopside,vesuvianite,pargasite and epidote）and magnetite from tin skarn are the main Sn-bearing phases at Furong（Sn=44～44241 ppm）,whereas K-feldspar,plagioclase in fresh granite,chlorite in altered granite,and phlogopite and scapolite from tin skarn are poor in Sn（mostly<10 ppm）.Tin generally enters the stannous silicates and magnetite through substitutions for Al^vi（Al at the octahedral site）and Fe³⁺,with a preference for Al^vi in Al^vi-rich silicates.Comparisons of Sn contents between magmatic and hydrothermal minerals in granite,prograde and retrograde minerals related to tin skarn indicate that remobilization of Sn from granite and prograde skarn is not a pre-requisite to form tin mineralization.Based on the above results,an ore-forming model is proposed for the Furong Sn deposit,which integrates the four ore types into one single magmatic-hydrothermal system genetically related to the highly-fractionated late-phase granite of the Qitianling batholith,rather than the main-phase granite as was proposed previously.