长江中、下游燕山期热液铜—金矿床成矿流体

Comparative Research on Ore-forming Fluids for theMain Types of Hydrothermal Copper-gold Deposits in the Middle and Lower Reaches of the Yangtze River

分别以安徽东狮子山隐爆角砾岩型、西狮子山矽卡岩型、沙溪斑岩型和铜牛井石英大脉型矿石作为实例 ,对长江中、下游燕山期热液铜 -金矿床的主要类型作了成矿流体对比研究 .这四类矿床的成矿作用早期均以高温 (>4 5 0℃ )高盐度 (>4 5wt %NaClequiv .)流体为特征 ,晚期流体温度和盐度均降低 .这些矿床的流体来源均以岩浆热液为主 ,但在斑岩铜矿床成矿作用的晚阶段有一定数量的雨水加入 .流体的沸腾对于各类矿床的形成均起过重要作用 ,尤其在狮子山矿床中存在着隐爆角砾岩阶段、矽卡岩阶段、石英 -硫化物阶段和石英 -碳酸盐 -硫化物阶段的多次沸腾 .长江中、下游燕山期热液矿床的大量产出 ,其原因之一是该区地壳富含流体 ,而流体的富集可能在很大程度上与扬子大陆板块在中生代向华北板块下方俯冲的过程中 ,俯冲带流体向前陆方向的回流有关 .下冲板块前峰拆沉所造成的板块后方撕裂 。

The Lower Yangtze district is famous in China for its great reserves of copper and gold ores. These ores were formed during two major mineralization periods: Carboniferous and Yanshanian (Mesozoic). The Carboniferous period produced a number of submarine exhalative massive sulphide deposits, whereas the Yanshanian event was characterized by emplacement of hydrothermal deposits genetically linked to intermediate or acidic intrusives. Four main types of the Yanshanian hydrothermal copper and gold deposits encompass the crypto?explosive breccia?, skarn?, porphyry? and quartz lode?types. Comparative research of ore?forming fluids is carried out on these four types based upon case studies on the ores from East Shizishan, West Shizishan, Shaxi and Tongniujing, respectively. Fluids of early mineralization of these deposits are characterized by high temperatures (>450 ℃) and salinities (>45 wt% NaCl equiv.), which decrease in the late stages. Melt?fluid inclusions, with homogenization temperatures exceeding 600 ℃ and salinities up to 56.2 wt% NaCl equiv., are recognized in the crypto?explosive breccia?type ore from Shizishan, and are interpreted to represent a water?rich magma. Ore?forming fluids of all these deposits are dominated by magmatic water, meteoric water being involved in a considerable amount only at the late stage of porphyry copper mineralization in the Shaxi deposit. Fluid boiling is significant to ore?formation of these deposits, and has happened multi?episodically in the Shizishan deposit corresponding to the crypto?explosion, skarn, quartz?sulphide and quartz?carbonate stages. The occurrence of great numbers of Yanshanian hydrothermal deposits in the Lower Yangtze district can be interpreted in terms of fluid enrichment in its crust, and the fluid enrichment in turn may have been induced largely by foreland?ward back?flow inside the down?going plate during subduction of the South China continental plate beneath the North China plate. The back?flow of deep fluids is likely to have been enhanced by fractures resultant from tearing effect of delamination on the plate segment behind the subducting front. Back?flown fluids gather to form fluid cells in favourable localities in the foreland crust, and are subsequently absorbed by the Yanshanian magmas and incorporated in the post?magmatic ore?forming fluids. The commonly occurring intracrustal low?velocity zones in the present Lower Yangtze district may be indicative of such fluid cells, which have contributed significantly to ore?forming processes in this district.