论安徽庐枞盆地大鲍庄式喷气沉积-叠加改造型硫铁矿床

STUDY ON DABAOZHUANG-TYPE PYRITE DEPOSITS——EXHALATION-SEDIMENTATION-SUPERIMPOSITION-TRANSFORMATION SULFIDE DEPOSITS IN THE LUJIANG-ZUNGYANG BASIN,ANHUI PROVINCE

庐枞盆地中发育一些大中型硫铁矿床,曾长期被认为是与次火山岩有关的中低温热液成因。本文从矿床层控特点、矿体形态产状、矿石沉积组构、矿物组合及其生成顺序、矿化分带、包裹体及同位素地球化学等诸方面,证明它们具显著的火山喷气沉积成因特征,并受到后期热液叠加改造。提出了大鲍庄式陆相火山喷气沉积-热液叠加改造型硫铁矿的成矿模式。

Some important pyrite deposits,called“Dabaozhuang-type”pyrite deposits in this paper, are developed in the Lujiang-Zungyang continental volcanic basin.They were considered to be hydrothermal deposits related with subvolcanic rocks and associated with“porphyrite iron deposits”.But our recent study shows that they are substantially of exhalation-sedimentation- hydrothermal superimposition and transformation origin.The main reasons are as followings: 1.Most pyrite deposits are located at the uper horizon of the first volcanic cycle formation, consisting of volcanic-sedimentary sequence.Ore bodies are bedded,bed-like or lenticular,ha- ving many synsedimentary structures.Microbial textures in sulfides can also be found. 2.Host rocks are mainly tuff-siliceous formation and some anhydrite beds with barite, which have been proved to be important prodt,cts of exhalative-sedimentary process. 3.The deposits are clearly zoned in their compositions,ore structures and minor elements. And zoning centers are close to volcanic apparatuses or fracture zones. 4.There exists“reverse”paragenetic sequence from lower to higher temperature minerals: anhydrite→pyrite→magnetite and hematite,being contrary to the hydrothermal sequence. 5.Inclusions in minerals are very small(<3μm)with vapor/liquid ratio less than 10%, their homogenization and decrepitation temperatures are about 100℃ and 105—140℃,respecti- vely.The sulfur isotope composition supports the view that much sedimentary sulfur contrubu- ted to the deposits. 6.In assition,hydrothermal superimposition and transformation are very obvious,such as recrystallization,secondary enlargement and tr nsformation of mineral facies(e.g.pyrite to magnetite)etc..