贵州西北部泥盆系镁菱铁矿床成因研究

Study on the Genesis of Devonian Magnesian Siderite Deposits in the Northwestern Guizhou Province

贵州西北部地区(主要包括水城地区和赫章地区)位于扬子地块东南缘,产出有多个大型、中型镁菱铁矿床和菱铁矿床。赫章和水城地区是贵州省最重要的两大铁矿富集区,矿床赋存于泥盆系和石炭系碳酸盐岩地层中。本文报道了贵州赫章水塘、雄飞和铁矿山矿段泥盆系镁菱铁矿床岩石学、矿物学及矿床地球化学结果:矿体呈层状、似层状和脉状产出,矿石类型以米黄色和灰色镁菱铁矿为主,其构造类型有块状、条带状及米粒状构造。与镁菱铁矿共生产出有白云石、长石、黄铁矿及石英等矿物。与长石紧密共生的白云石具有典型的鞍状结构,表明其遭受了明显的热液叠加改造作用。水塘和雄飞镁菱铁矿床的钍(0.4×10-6~2.3×10-6,平均1.1×10-6;0.2×10-6~3.2×10-6,平均1.3×10-6)、锆(2.7×10-6~20.6×10-6,平均8.1×10-6;1.3×10-6~17.8×10-6,平均7.5×10-6)和钪(0.5×10-6~3.9×10-6,平均1.3×10-6;0.1×10-6~3.6×10-6,平均1.4×10-6)及Al2O3(0.24%~1.78%,平均0.85%;0.06%~2.77%,平均1.18%)含量较铁矿山铁矿(Th含量为0.2×10-6~0.8×10-6,平均0.4×10-6,Zr含量为0.6×10-6~9×10-6,平均4×10-6,Sc含量为0.1×10-6~1.7×10-6,平均0.5×10-6,Al2O3含量为0.01%~1%,平均0.38%)偏高,表明水塘和雄飞铁矿成矿过程中有较强的陆源风化物质参与。铁矿山铁矿具有较高Eu/Eu*值(6.73~8.94,平均7.68)和低Y/Ho值(26.67~33.33,平均30.01),暗示热液作用对铁矿山铁矿成矿贡献更大。结合矿床硫同位素及有机碳分析结果,认为区内镁菱铁矿床属于沉积——改造成因。

Northwestern of Guizhou Province( it mainly includes Shuicheng area and Hezhang area),where plenty of middle——large scale magnesian siderite deposits and siderite deposits distributed,is located at the southwest margin of the Yangtze block. Hezhang area and Shuicheng area are the two largest iron ore clusters in Guizhou. The iron formation occurs in the carbonate of the Devonian and Carboniferous. The petrological,mineralogical,and geochemical characteristics of the Devonian magnesian siderite deposits in Shuitang,Xiongfei and Tiekuangshan at Hezhang county,Northwest Guizhou were reported in this paper. The ore bodies are mainly layered,stratoid or lode-shaped in form and are hosted in the Devonian carbonate rocks. The major two types of ore,beige and gray magnesian siderite,usually have massive,banded or granular textures,and the paragenetic gangue minerals including dolomite,feldspar,pyrite and quartz. The feldspar that is associated closely with quartz shows the typical texture of saddle,which probably implies that hydrothermal superimposed transformation evolved in the process of iron mineralization. In addition,the higher Th( 0. 4 × 10- 6~ 2. 3 × 10- 6,mean 1. 1 × 10- 6and0. 2 × 10- 6~ 3. 2 × 10- 6,mean 1. 3 × 10- 6,respectively),Zr( 2. 7 × 10- 6~ 20. 6 × 10- 6,mean 8. 1 × 10- 6and1. 3 × 10- 6~ 17. 8 × 10- 6,mean 7. 5 × 10- 6,respectively),Sc( 0. 5 × 10- 6~ 3. 9 × 10- 6,mean 1. 3 × 10- 6and0. 1 × 10- 6~ 3. 6 × 10- 6,mean 1. 4 × 10- 6,respectively) and Al2O3( 0. 24% ~ 1. 78%,mean 0. 85% and 0.06% ~ 2. 77%,mean 1. 18%,respectively) content of Shuitang and Xiongfei magnesian siderite ores than Tiekuangshan magnesian siderite ores( Th: 0. 2 × 10- 6~ 0. 8 × 10- 6,mean 0. 4 × 10- 6,Zr: 0. 6 × 10- 6~ 9 ×10- 6,mean 4 × 10- 6,Sc: 0. 1 × 10- 6~ 1. 7 × 10- 6,mean 0. 5 × 10- 6and Al2O3: 0. 01% ~ 1%,mean 0. 38%)indicates that the weathering material from continental source evolved in the process of mineralization of former,while the high Eu / Eu*( 6. 73 ~ 8. 94,mean 7. 68) but low Y/Ho( 26. 67 ~ 33. 33,mean 30. 01) demonstrate that the hydrothermal activities played an more important role in the process of mineralization of later. Combing with the study of isotope sulfur and TOC,the sedimentation——transformation genesis for this deposit has been concluded.