Various chlorite alteration styles in the Furong Sn deposit, South China: Chlorite chemistry and implication for tin mineralization

Abstract

The altered granite- to skarn-type Furong tin deposit (530,000 t Sn @ 0.8% Sn) lies at the southern margin of the multiphase Qitianling granite batholith, South China. Chlorite alteration is a distinct feature at Furong, and four chlorite variants are identified: (i) chlorite partially or pseudomorphically replacing biotite in weakly chloritized granites (Chl-W), (ii) chlorite aggregates interlocking irregularly with quartz, cassiterite, and magnetite in intensely chloritized granites (tin ores) (Chl-I), (iii) comb-shaped chlorite in quartz veins crosscutting the intensely chloritized granite (Chl-V), and (iv) chlorite replacing garnet, diopside, and vesuvianite in skarns (Chl-S). Compositionally, all the Furong chlorite samples belong to trioctahedral Fe-chlorite (chamosite). Empirical chlorite geothermometry without Fe/(Fe + Mg) correction yielded formation temperatures of 321–422℃ and 397–420℃ for the granoblastic chlorite variants Chl-I and Chl-V, respectively, and lower temperatures of 273–343℃ and 285–366℃ for the pervasive chloritization variants of Chl-W and Chl-S, respectively. Chl-W is the result of layer-by-layer replacement with two biotite layers being converted to one Chl-W layer, and Chl-S formed by a dissolution-transport-precipitation mechanism. Both variants formed in a rock-dominated fluid environment, with their elemental compositions controlled dominantly by the precursor minerals and temperature. In contrast, Chl-I resulted from the dissolution-transport-precipitation process in a fluid-dominated system, and Chl-V precipitated directly from the fluid. During intense fluid-rock interaction, the fluid provided considerable Fe, Zn, and Sn to form granoblastic Chl-I and Chl-V together with cassiterite. Fluid flow was controlled by NNE-trending fault zones which were the preferential sites of mixing of ascending metal-rich magmatic fluid with meteoric water and fluid interaction with the granites.