Controle polygenetique sur la localisation des gisements proximaux et distaux dans le champ polymetallique-stannifere a Xinlu Guangxi Chine meridionale

Abstract

The Xinlu tin-poly metallic ore field in the Guangxi Autonomous Region of the Zhuang Nationality of southern China is associated with the Mesozoic Guposhan granitic batholith. The batholith consisting of at least three phases of intrusion intrudes Pre-Devonian meta-sedimentary rocks and Late Paleozoic sedimentary rocks including large amounts of carbonates. The ore field is situated at the southern margin of the batholith, where sedimentary rocks are surrounded by granites on three sides (west, north and east) and underlain by unexposed granites, exhibiting a concave intrusion surface. Dykes of intermediate to granitic compositions similar in age to the granites are widespread.Five tin-polymetallic deposits occur in the ore field; among them two (the Dachong and Liuhe'ao deposits) are located at the contact zone between the granitic intrusions and the Devonian sedimentary rocks and are considered as proximal deposits. The Baimianshan and Shimen deposits, classified as distal deposits, are located in the Devonian sedimentary rocks relatively far above the intrusions and are spatially related to dykes and/or fractures. The Muqiaomian deposit is situated in the sedimentary rocks not far from the contact zone, forming a transitional type between proximal and distal deposits.This study is an attempt to understand the mechanism of emplacement of proximal versus distal deposits, through factors such as ore-forming sources, migration and deposition processes.The proximal and distal deposits in the Xinlu ore field are mainly composed of sulfides, with pyrrhotite and sphalerite being the major components coexisting with cassiterite. A major difference between proximal and distal deposits is that the proximal deposits are associated with ore skarns superimposed on preore skarns, whereas the distal deposits are hosted by carbonates with relatively weak and restricted alterations. Both types have been considered to be related to late-stage intrusions of the Guposhan batholith. The localization of the ore deposits do not appear to be controlled by a specific sedimentary bed at the ore field scale. A correlation between the vertical distance from ore bodies to intrusion surface and the depth of emplacement of the intrusions can be established: proximal deposits are developed in the northern part of the ore field, where the granites were emplaced at higher levels; on the contrary distal deposits are developed in the southern part of the ore field where the granites were emplaced deeper. The transitional type of deposit (Muqiaomian), on the other hand, is developed in the western part of the ore field where the depth of emplacement of the intrusions is intermediate.The geochemical characteristics of the host rocks (granites, dykes and sedimentary rocks), including their major elements, trace elements, REE patterns, and Sr isotopes have been examined. It I I is indicated that the second and third phases of the Guposhan batholith belong to S-type granites, and fractional crystallization is most likely to be the process res...