Geology and Metallogeny of the Shizhuyuan Skarn-Greisen Deposit,Hunan Province,China |
| |
| Abstract: | The Shizhuyuan deposit is the largest among the economically important polymetallic tungsten deposits in China. The deposit occurs within the thermal aureole of Yanshanian felsic intrusions that were emplaced into Devonian carbonates and marls. The mineralization can be divided into three phases that are genetically associated with three episodes of granitic emplacement-pseudoporphyritic biotite granite, equigranular biotite granite, and granite porphyry. During the emplacement of pseudoporphyritic biotite granite, thermal metamorphism and subsequent skarnization developed around the stock. The pure limestone was transformed to marble, whereas marls and argillite interlayers were changed to a series of metamorphic rocks such as grossular-diopside hornfels, wollastonite hornfels, diopside hornfels, wollastonite-vesuvianite hornfels, muscovite-K-feldspar-anorthite hornfels, and prehnitevermiculite hornfels. Because of the subsequent strong skarn development, most hornfelses later were transformed into skarns. The skarns distributed around the granite stock are mainly calcic. They are massive in structure, and are composed mainly of garnet, pyroxene, vesuvianite, and wollastonite, with interstitial fluorite, scheelite, and bismuthinite. Although there is no cassiterite in the early skarns, their tin contents average 0.1%. The distribution and compositional and mineralogical relationships of skarn minerals suggest that they formed as a result of progressive reactions of a hydrothermal solution with a limestone of generally constant composition, and that the dominant process was progressive removal of Ca and addition of other constituents to the rocks. Following the primary skarn formation, some of the assemblages were retrograded to new assemblages such as fluorite-magnetite-salite rock, magnetite-fluorite-amphibole rock, and magnetite-fluorite-chlorite rock. The retrograde alteration of the skarns is characterized by a progressive addition of fluorine, alkali components, silica, tin, tungsten, and bismuth. A zonation from garnet-pyroxene skarn or garnet skarn, through fluorite-magnetite-salite rock, to magnetite-fluorite-chlorite rock frequently can be recognized in the deposit. All retrograde-altered rocks contain scheelite, cassiterite, molybdenite, and bismuthinite. During the emplacement of equigranular biotite granite, skarn veins several tens of centimeters wide were developed; they contain large crystals of garnet and vesuvianite, and interstitial scheelite, wolframite, cassiterite, and molybdenite. This second stage of mineralization occurs predominantly as coarse and fine stockwork greisens, which were superimposed on the massive skarns and surrounding marble. Such W-Sn-Mo-Bi-bearing greisens can be divided into topaz greisen, protolithionite greisen, muscovite greisen, and margarite greisen. Besides calcic skarn veins and greisens, manganese skarn veinlets also were developed; they consist of rhodonite, spessartine-almandine solid solution, spessartine, and helvite. The distribution of greisens is responsible for a metal zonation—i.e., W-Sn-Mo-Bi and Sn-Be-Cu-F zones from the contact boundary between the granite stock and skarns outward in the deposit. A third stage of mineralization is represented by lead-zinc veins, which also are accompanied by manganese skarns consisting of spessartine, rhodonite, manganese-rich pyroxene, helvite, tephroite, fluorite, tourmaline, and manganese-rich phlogopite. |
| |
| Keywords: | |
|
|
