Abstract

The Xinlu tin polymetallic ore field in Guangxi Zhuang Autonomous Region, southern China is associated with the Mesozoic Guposhan granitic batholith, which intruded pre-Devonian metasedimentary rocks and late Paleozoic sedimentary rocks including large amounts of carbonates. Dikes of intermediate to granitic compositions similar in age to the granites are widespread. Five tin polymetallic deposits occur in the ore field; among them, the Dachong and Liuhe'ao deposits are located at the contact zone between the granitic intrusions and the Devonian sedimentary rocks and are therefore 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 dikes 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. A correlation between the vertical distance from orebodies to iutrusion top surface and the depth of emplacement of the intrusions can be established: proximal deposits are found in the northern part of the ore field, where the granites were emplaced at relatively high levels, in contrast to the distal deposits which are found in the southern part of the ore field, where the granites were emplaced at deeper levels. Both proximal and distal deposits are mainly composed of sulfides, with pyrrhotite and sphalerite being the major components coexisting with cassiterite. The localization of the ore deposits does not appear to be controlled by a specific sedimentary bed at the ore field scale, nor can it be attributed to the effect of metal zonation.Fluid inclusions in cassiterite from proximal and distal ores and in quartz from granites associated with the proximal deposits have been studied. The chemical system of the ore-forming fluids is H 2 O-CO 2 (-other gases)-NaCl-CaCl 2 . The common coexistence of relatively CO 2 -rich gas inclusions (X (sub CO 2 ) = 0.07-0.18) and relatively saline liquid inclusions (NaCl + CaCl 2 = 17.1-44.5 wt %) in cassiterite indicates that fluid phase separation is probably an important mechanism of ore deposition. Homogenization temperatures of fluid inclusions are similar between proximal and distal deposits: 250 degrees to 450 degrees C for the Baimianshan deposit, 325 degrees to 450 degrees C for the Shimen deposit, and 350 degrees to 475 degrees C for the Liuhe'ao deposit. This indicates a small temperature gradient in the conduits of the ore-forming fluids. Fluid pressures estimated from isochores range from 70 to 546 bars for the ore deposits and from 123 to 2,822 bars for the granite at Liuhe'ao. The thickness of the overburden at the time of mineralization is estimated to be 3,900 m according to stratigraphic data, the corresponding hydrostatic and lithostatic pressures being 382 bars and 1,032 bars, respectively. Thus the fluid pressure data derived from fluid inclusion study indicate that there was probably a high-pressure contrast between the source of the fluids within the intrusions (approximated by lithostatic pressure) and the contact zone and conduits (approximated by hydrostatic pressure).A model is proposed to explain the separation of proximal and distal deposits in relation to the depth of emplacement of the intrusions. It is shown that the migration capacity and phase separation fields of the ore-forming fluids are associated with the change from a lithostatic pressure system within the intrusions to a hydrostatic pressure system outside the intrusions and in turn are related to the depth of emplacement of the intrusions. Therefore, when the intrusion is emplaced at high levels, mineralization tends to take place near the contact zone, forming proximal deposits, whereas when the intrusion is emplaced at deeper levels, mineralization is more likely to occur some distance from the intrusion, forming distal deposits.

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