Abstract

The igneous complex that hosts the Yanbei porphyry tin deposit is situated along the eastern margin of the southeast China Caledonian fold belt. It intrudes a Late Jurassic granitic batholith and was unconformably covered by Late Cretaceous red beds. The complex consists mainly of volcanic (ignimbrites and minor rhyolitic lavas, tuffs, and subvolcanic rhyolite porphyry) and intrusive rocks (topaz-bearing granite porphyry and granite). These rocks are generally peraluminous with F- and VI Al-rich biotite as the main mafic mineral, and F-rich and OH-poor topaz as an accessory mineral in the granite porphyry and granite, and as an essential mineral in the topazite. Topaz in the granites is usually interstitial and crystallized during the late magmatic stage. The volcanic rocks and granites are rich in Si, K, F (SiO 2 > 75 wt %, Na 2 O/K 2 O = 0.03-0.72, F = 0.06-6.48 wt %) and incompatible trace elements (Rb, Zr, Nb, W, Sn ) and poor in Mg, Ti, Ca, P and compatible trace elements (Sr, Eu ). The chondrite-normalized rare earth element (REE) patterns for the granites generally have a distinct V shape, with strong Eu depletion (Eu/Eu (super [whitesunwithrays]) = 0.04-0.20). The main rocks of the complex have low epsilon Nd (T) values (-9.3 to -8.9 for volcanic rocks, -4.0 to -4.8 for granites), and high delta 18 O values of quartz from dacite (9.5ppm) and granites (9.6-10.1ppm). The difference in epsilon Nd (T) between the volcanic rocks and granites indicates that they are not comagmatic. The source of the volcanic rocks probably contains a greater metasedimentary component than that of the granites.The Yanbei porphyry tin deposit is located along the contact zone between topaz-bearing granite porphyry and rhyolite porphyry, and consists principally of one orebody which makes up 99 percent of the total reserves. Of the metallic minerals (5% by volume), cassiterite, pyrite, and chalcopyrite are dominant, sphalerite, wolframite, and galena are less important, and siderite, molybdenite, and stannite are minor. Cassiterite accounts for 96 percent of the total Sn reserves and stannite is the subordinate Sn-bearing mineral that occurs as rims around chalcopyrite. Cassiterite occurs mostly in disseminated form and as veinlets. The wall-rock alteration is intense and can be roughly grouped into three zones spatially arranged around the granite porphyry, namely: topaz-quartz, chloritic, and sericitic zones. Most of the orebody occurs in the topaz-quartz and chloritic zones. Hydrothermal alteration caused a decrease in K and Na and an increase in Al, Fe, Ca, Sn, W, and Cu. The delta 18 O values of quartz and cassiterite from the mineralized zones vary from 6.0 to 11 per mil, and the calculated delta 18 O (sub H 2 O) values of the ore-forming fluid range from 4.2 to 7.8 per mil. The altered rocks have elevated delta 18 O (sub H 2 O) values (14.5-9.7ppm) relative to fresh rocks. The pyrite from the ore has a limited range of delta 34 S values (-0.3-+1.5ppm). These data suggest a magmatic source for the ore-forming hydrothermal fluid.

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