| Nanling Range is a world famous REE, Nb, Ta, W, Sn, Mo, Bi, Pb, Zn metallogenic province. Skarn type tin-polymetallic mineralization occupies an important position in Nanling Range. Though a lot of significant progresses had been achieved in former research, some problems and deficiency are still existed. This paper focuses on two lately discovered large tin deposit, Hehuaping and Xitian, which represent for magnesian and calcic skarn type tin-polymetallic mineralization, respectively. Through the methods and means as field investigation, electron microprobe analysis, major and trace elements component analysis, fluid inclusion analysis, stable isotope analysis, radiogenic isotope geochronology, etc., we mainly studied the aspects of the host rocks characterics, skarn characterics, source of ore-forming material, source of ore-forming fluid, genesis of the deposit, mineralization process, etc, established the mineralization model, preliminary discussed the rules of skarn type tin-polymetallic mineralization. The main aehievements obtained were as follows:1. Magnesian skarn-type tin deposits are relatively rare in nature. The Hehuaping tin deposit in southern China is identified as such a type of cassiterite-sulfide mineralization and has a total reserve of approximately130,000tons of tin,50,000tons of lead and10,000tons of zinc. It is related to the Late Jurassic (157Ma) Hehuaping buried coarse-medium-grained biotite granite, which intruded the dolomite of the Middle Devonian Qiziqiao Formation and the sandstone of the Tiaomajian Formation. Four paragenetic stages of skarn formation and ore deposition have been recognized:Ⅰ. prograde skarn, Ⅱ. retrograde skarn, Ⅲ. cassiterite-sulfides, and Ⅳ. carbonates and clays. Alteration zoning through fresh granite to unaltered country rocks can also be clearly identified. A Mg-mineral assemblage of forsterite, spinel, diopside, tremolite, serpentine, talc, phlogopite, and other minerals typifies the skarn. The geochemistry of various skarn minerals indicates a tendency of gradually decreasing of Mg end member and correspondingly increasing of Fe and Mn (especially Mn) end members with the process of skarnization. Tin mineralization began during the late retrograde-skarn stage, forming Sn-bearing magnetite skarn. However, the deposition of cassiterite occurred predominantly as cassiterite-sulfide veins forming along fractures and interlayer fracture zones, in both proximal and distal skarn during stage III. The S and Pb isotopic analyses suggest that the ore-forming elements have a magmatic source and the magma was derived from the upper crust. The H-O isotopic and fluid-inclusion analyses indicate that high-temperature ore-forming fluids in early anhydrous skarn formation (stage I) are of magmatic origin. In comparison, the retrograde fluids are characterized by relatively low salinity (2to10wt.%NaCl equiv) and low temperature (220to300℃), which suggests a mixed origin of meteoric waters with magmatic fluids originating from the Hehuaping granite. The major ore-forming stage Ⅲ fluids are characterized by lower temperature (170to240℃) and salinity (1to6wt.%NaCl equiv), while meteoric waters are dominant in stage IV, resulting in a further lowering of temperature (130to200℃) and salinity (0.4to1wt.%NaCl equiv).2. Xitian is a typical calcic skarn type tin-polymetallic deposit with typical mineral assemblages of calcic skarn such as garnet, diopside, epidote, actinolite, chlorite, etc. Five paragenetic stages of skarn formation and ore deposition have been recognized:Ⅰ. prograde skarn, Ⅱ. retrograde skarn, Ⅲ. greisen and oxides, IV sulfides, and Ⅳ. late alteration. Main skarn zones are garnet-diopside skarn and diopside skarn. There are three types of tin mineralization with related with skarnization, which are cassiterite-chlorite type, cassiterite-magnetite type and cassiterite-sulphides type, respectively.Zircon LA-MC-ICP-MS U-Pb analysis is carried out on the Xitian granite in east Hunan province. Two samples from the first period of medium-(fine) grained porphyritic biotite monzonitic granite show ages of (220.9±0.6) Ma and (220.7±0.7) Ma, respectively. The age of the second period of medium-fine grained two-mica monzonitic granite is (154.4±0.7) Ma. Both two periods of granite are high potassium, rich in alkali and weakly peraluminous. Their compositions are high ΣREE, rich in U and Th, depleted in HFSE such as Ti, P and LILE such as Ba, Sr, with high104Ga/Al. These characteristcs indicating they are A-type granites. The Indosinian granite is rich in LREE, with slightly Eu negative anomaly. The Yanshanian granite shows no obvious fractionation between LREE and HREE, with significant Eu negative anomaly. Hf isotopic analysis indicates that the granite has low εHf(t)(-4.91～-11.04), depleted mantletwo stage Hf model ages concentrate in1.6-1.8Ga, accordance with the old metamorphic basement. Therefore, the Xitian granite is the product from partial melting of proterozoic crust materials of cathaysian block under the extensional setting. The confirmation of Xitian A-type granite has important significance to understanding the tectonic background of east part of Hunan province during Indosinian and Yanshanian movements. 3. Garnets in tungsten-tin skarns belong to andradite-grossularite series which have almandite+spessartine+pyrope component less than15%. Garnet from the Hehuaping deposit has a core rich in grossularite while rim rich in andradite. Garnet from the Xitian deposit has opposite variation tendency with which from the Hehuaping deposit. The REE distribution pattern of garnet from the Hehuaping deposit shows HREE enrichment characteristics while garnet from the Xitian deposit has high ΣSREE but shows no obvious differentiation between LREE and HREE.4. Though there are a lot of similarities of both skarn type tin-polymetallic deposits such as tectonic setting, ore-forming granite, ore-forming fluid, source of ore-forming material, etc, significant differences still hold the key position. The difference of host rocks lead to forming of two kinds of skarn. These two kinds of skarn ostensibly have diversities in mineral assemblages, alteration stages and alteration zoning. They indicates the activation, migration and mineralization of tin in two kinds of physiochemical environments. The skarn type tin mineralization in Hehuaping deposit is hosted by magnesian skarn, with both magnetite-rich and sulphide-rich type mineralization which often forming rich ore body by superposition on each other. The Xitian deposit is hosted by calcic skarn, with only sulphide-rich type mineralization which sometimes forming rich ore body by superpositioned by greisen veins. The Hehuaping deposit tends to form individual tin phase, cassiterite, while silicate skarn minerals hardly contain any tin. On contrast, silicate minerals in calcic skarn of the Xitian deposit show common occurrence of tin in the form of isomorphic replacement but with no large amounts of magnetite. In the Hehuaping deposit, Sn stay in fluid phase in prograde skarn stage and began crystallize until retrograde stage in the form of cassiterite with late sulfide veins. In the Xitian deposit, skarn minral "soaks" up considerably Sn in fluid in skarnization stage but later retrograde skarnization and greisenization released these tin, also formed cassiterite-sulfide veins. |
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