Abstract:

The Bailashui tin deposit is located in Qitianling Yanshanian biotite granite, Southern Hunan province. Chloritization
is a universal hydrothermal alteration in this ore-bearing granite. Based on the research of the samples collected from the
drillhole, we found a notable variation in the chloritized degree of the granite. This study was carried out by selecting three groups
of granite samples which displayed notable different degree of chloritization. The chloritization nhich closely related to oreforming
in the Sn-bearing granite had been discussed. The results indicated that, in the granite samples of different degrees of
chloritization the petrography and chemical composition of chlorite changed with a series of obviously variable characteristics.
According to the analysis of micropetrography and relevant geochemical data, some conclusions could be drew as follows. Themetasomatic sequence of rock-forming minerals in the granite replaced by chlorite is: amphibole and biotite→plagioclase and
potassium feldspar→quartz. The variable shape of chlorite was, on the one hand, related to the composition of chlorite; on the
other hand, related to the crystal texture of rock-forming minerals which were replaced by chlorite. As the chloritization of granite
was stronger, the LOI and Zn content of whole-rock samples would be higher, and the Cl contents losing to hydrothermal liquid
from original rock would be more. With the chloritized degree of granite becoming stronger, main compositions of chlorites would
present regular variations. The Si content of chlorite decreased in the process of chloritization. Al elements were released after
the breakdown and dissolution of feldspar. The mobilization and redistribution of Al on the thin-section scale increased the
aluminum content of chlorite. Fe element were brought in to the chrorite by hydrotherm. The amounts of chlorite formed depended
on how much Fe had been introduced into the hydrothermal system. The Mg for the formation of chlorite was primarily related to
the redistribution of Mg which was released from mafic minerals. Mn element was mainly brought in to the chlorite by the outer
hydrotherm.

Key words: Southern Hunan Province, Bailashui, Qitianling altered granite, Tin deposit, chloritization