| 其他摘要 | The Hetai gold field, located in western Guangdong Province, is the biggest found in the Yunkai Terrain, southern China. The objective of this study is to characterize the geological and geochemical features of the Hetai gold field, to understand the geochemical behvior of gold in the history of mineralization, and to establish a reasonable metallogenetic model. The country rocks are principally metamorphosed Sinian (Late Precambrian) series, which developed in a medium or medium-low pressure metamorphic facies series during the Caledonian period. The characteristic mineral paragenesis for the dominant schist is quartz + muscovite + biotite ± staurolite ± sillimanite ± almandine garnet, indicating a metamorphic temperature range from about 550° to 670 ℃ and pressure range from 230 to 600 MPa. The protolith is mainly pelitic or semi-pelitic rocks, intercalated with bedded cherts. Petrographic evidence suggests that the bedded cherts within the Sinian strata are the product of a fossil geothermal system, which is favored by geochemical analysis. The cherts have a geochemical affinity to typical hydrothermal cherts. The embedded sink mosaic model is established to describe the migration pattern of impurity trace elements in source rocks. According to this model, impurity trace elements tend to migrate out of bulk solid cells and trapped in various sinks. Trace elements also tend to migrate from high temperature fields to lower ones. High temperature accelerates the realization of these two tendencies. Diffusion is an important type of element reactivation, which is essentially a kind of oriented short-range mass transport within bulk solid cells and lower ranks of sinks. The ore is predominantly auriferous altered mylonites within hydrothermally altered ductile shear zones. At all scales, the characteristic pattern of shear deformation is anastomosing domains of higher deformation separating rhomboid domains of lower deformation. Hydrothermal alteration is both spatially and temporally associated with the development of the shear deformation system. The alteration products are principally quartz and sulfides. Gold precipitation is an integral part of the hydrothermal alteration system, and culminated during the formation of the second generation of sulfidation. According to the mass equilibrium estimation on the basis of the chemical composition of altered rocks, silica, precious metals and most thiophile elements were regularly introduced with the advance of alteration, and the mass addition reached its maximum in the ore bodies, with an average mass factor of 1:0.84. Fluid inclusion study demonstrates that the primary ore-forming hydrothermal solution is a H_2O-NaCl-CO_2-dominated system, characterized by a low-salinity (about 1.5 -6 equiv. wt.% NaCl) and bearing CO_2. Moderate-salinityaqueous fluid and CO_2-dominated fluid were caused by CO_2 effervescence and unmixing of the primary hydrothermal fluid. The Hetai gold deposits are the result of the integration of polystage geological and geochemical processes. Sedimentation, regional metamorphism, granitic magmatism, deformation and hydrothermal activities, have all contributed to the formation of the gold deposits. During the Precambrian to Cambrian period, a thick potential source strata was deposited. Coeval hydrothermal activity may have participated in the preliminary enrichment of gold in the source rock. The Caledonian regional metamorphism played a basic role to redistribute the gold in the source strata. There are two general tendencies for gold to migrate during metamorphism. One is from within host minerals toward the weaknesses of rocks, where gold is more accessible in subsequent events. The other tendency is from higher grade metamorphic domains toward low ones. The Hercyno-Indosinian period is the main stage of the entire gold mineralization history of the Hetai gold field. Gold emplacement occurred along altered shear deformation zones. The temperature drop, sudden release of pressure, and the accompanying unmixing of the primary low salinity hydrothermal fluid are key factors leading gold precipitation out o the hydrothermal solution. The Yunluogan and Wucun granites are not the main source of gold, but provided the heat source necessary for the circulation of the hydrothermal fluid, which is a mixture of meteoric water and metamorphic water. |
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