安徽沿江地区中生代原地和异地矽卡岩岩浆－热液成矿作用

在安徽沿江地区，既分布有大量接触交代成因和叠加复合成因的矽卡岩矿床，也分布有岩浆成因的矽卡岩矿床。本文给出了不同类型岩浆矽卡岩及其矿床的定义，论述了安徽沿江地区中生代岩浆矽卡岩及其矿床的特征，并在此基础上分析了区域中生代矽卡岩岩浆－热液成矿作用。根据矽卡岩岩浆就位位置的不同可将岩浆矽卡岩分成原地矽卡岩和异地矽卡岩两类，相应地将岩浆矽卡岩矿床分成原地矽卡岩矿床和异地矽卡岩矿床两大类。两类矽卡岩及其矿床具有明显不同的特征。在地质产状上，原地矽卡岩岩体常与壳幔同熔岩浆侵入体紧密伴生而分布在壳幔同熔岩浆侵入体与碳酸盐围岩的接触带上，在矽卡岩岩体边缘一般没有冷凝边和烘烤边，但常能见到因同化混染作用不彻底而留下的围岩残留体（多已变质成角岩或大理岩）。与此明显不同的是，异地矽卡岩岩体常分布在断裂带或地层虚脱带中，附近一般没有壳幔同熔岩浆侵入体与其紧密伴生，在矽卡岩岩体边缘一般有冷凝边和烘烤边，有时能见到气孔构造以及石榴石或透辉石堆积岩，但见不到围岩残留体。同时，原地矽卡岩和异地矽卡岩均具有明显的水平分带，但两者明显不同。原地矽卡岩的水平分带常表现为从侵入岩体经矽卡岩往碳酸盐围岩方向依次出现侵入岩→同化混染侵入岩→富铁矽卡岩→富钙矽卡岩→同化混染碳酸盐岩→碳酸盐岩，反映同化混染作用逐渐减弱，而异地矽卡岩的水平分带常表现为从矽卡岩体中央往两边依次出现中粗粒矽卡岩－中细粒矽卡岩，反映随降温速度逐渐增加结晶速度逐渐降低。在矿物组成上，原地矽卡岩中的石榴石包含钙铁榴石、钙铁铝榴石和钙铝榴石，辉石主要为透辉石和钙铁辉石，而异地矽卡岩中的石榴石几乎全是钙铁榴石，辉石全是钙铁辉石。在地球化学方面，相对于原地矽卡岩，异地矽卡岩明显富集W、F、Rb、Be、Fe，而亏损Al、Sr、Ba、Cu、Pb、Zn、Cr、Co、Ni等元素。在岩相学上，原地矽卡岩和异地矽卡岩大多具有自形等粒结构，其中大多能见到熔融包裹体，但原地矽卡岩中的石榴石和辉石更为自形且常发育良好环带，熔融包裹体的均一温度明显高于异地矽卡岩中熔融包裹体的均一温度。在矿床类型上，原地矽卡岩一般为铜矿床，矿石矿物主要是铜铁硫化物，而异地矽卡岩一般为铁矿床，矿石矿物主要是铁氧化物。通过综合分析，认为原地矽卡岩和异地矽卡岩是由矽卡岩岩浆在原地或异地发生冷却结晶作用形成的，而原地矽卡岩矿床和异地矽卡岩矿床是由原地矽卡岩岩浆和异地矽卡岩岩浆发生熔离作用和结晶分异作用，熔离出的矿浆和分异出的热液发生冷却结晶和交代蚀变作用形成的。

Mesozoic In-situ and External Skarn Magmatic-Hydrothermal Mineralization in the Anhui Segment of the Lower Yangtze Metallogenic Belt

In the Anhui segment of the Lower Yangtze metallogenic belt are distributed not only a great number of contact metasomatic and superimposed and composite skarn deposits, but also some magmatic skarn deposits. Definitions are given for different magmatic skarns and associated deposits, and discussions are held on the characteristics of Mesozoic magmatic skarns and associated deposits in the area, with a focus on an analysis of Mesozoic skarn magmatic-hydrothermal mineralization. Based on emplacement location of skarn magma, magmatic skarn can be divided into in-situ skarn and external skarn, and magmatic skarn deposit correspondingly into in-situ skarn deposit and external skarn deposit. Characteristics are evidently different between the two skarns and associated deposits. Geologically, an in-situ skarn body is closely associated with a crust-mantle syntactic magmatic intrusion and occurs in the contact belt between the intrusion and carbonate wall rock. This in-situ skarn body generally has no chilled or optalic border but contains residuals of wall rock at its margin. These xenoliths were formed by incomplete assimilation of crust-mantle syntactic magma with the wall rock and mostly metamorphosed as hornfels or marble. In contrast, an external skarn body is not associated with a crust-mantle syntactic magmatic intrusion and generally located in fault belt or detachment belt within strata. This external skarn body often has a chilled or optalic border, vesicular structure and garnet or pyroxene cumulate, but contains no residuals of wall rock at its margin. In addition, clear horizontal zoning occurs in both of the in-situ and external skarn bodies, but their style of zoning is evidently different. Granitoid, hybrid granitoid, rich-Fe skarn, rich-Ca skarn, hybrid carbonate, and carbonate occur in order from crust-mantle syntactic magmatic intrusion through in-situ skarn body to carbonate wall rock, indicating gradual weakening of assimilation and contamination. In contrast, lithologies from the centre to the margin of external skarn body vary from medium-coarse-grained skarn to medium-fine-grained skarn, showing gradual decreasing of crystallization speed with gradual increasing of temperature lowing speed. Mineralogically, in the in-situ skarn, garnet includes andradite, gralmandite and grossular, and pyroxene does diopside and hedenbergite. In contrast, in the external skarn, garnet is almost andradite, and pyroxene is all hedenbergite. Geochemically, compared with the in-situ skarn, the external skarn is evidently rich in W, F, Rb, Be and Fe, but poor in Al, Sr, Ba, Cu, Pb, Zn, Cr, Co and Ni. Petrographically, both of the in-situ and external skarns have automorphic granular texture and mostly contain melt inclusions. However, compared with those in the in-situ skarn, garnet and pyroxene in the external skarn are more automorphic and have better zonal structure, and melt inclusions indicate evidently lower homogenization temperature. In ore deposit, the in-situ skarn is generally associated with copper deposit with copper and iron sulfite ore, while the external skarn with iron deposit with iron oxide ore. A comprehensive analysis is consistent with formation of the in-situ and external skarns by cooling and crystallization of the in situ and external skarn magmas, and supports formation of deposits associated with the in-situ and external skarns by cooling and crystallization of the ore pulp formed through liquation of the skarn magmas and by alteration and replacement of the hydrothermal solution formed through crystalline differentiation of the skarn magmas.