太阳顶群预富集过程中的生物成矿作用

BIOMINERALIZATION IN THE PRE ENRICHMENT OF TAIYANGDING GROUP

隶属于西秦岭南亚带的拉尔玛金矿床赋存于寒武系太阳顶群中，太阳顶群主要有硅质岩和碳质板岩两类岩石。金与有机碳的关系、矿（岩）石的浸出率与有机碳含量的负相关、微量元素和稀土元素Ｑ型聚类分析及有机质、粘土矿物吸附和释放金的现代模拟实验表明：太阳顶群热水成因的硅质岩构成了拉尔玛金矿床主要的矿源层。太阳顶群硅质岩的有机地球化学研究表明，与金关系密切的有机质的生物先质体为海生低等菌藻类生物。硅质岩中金含量与菌藻生物化石量的正相关关系又进一步说明了拉尔玛金矿床矿源层形成过程中，这些海生菌藻生物起了重要的作用。现代菌藻生物成矿的实验研究表明：（１）菌藻生物对金有明显的富集作用，但不同的生物对金的富集作用强弱不同，蓝细菌较绿藻对金的富集作用强烈，热水环境生物较常温条件下的生物对金的富集作用强烈；（２）蓝细菌生物较粘土矿物、沉积物和生物降解的有机质对金的富集作用强；

The Laerma gold deposit occurs in the Cambrian Taiyanding Group of the Southern Belt of West Qinling. The characteristics of ore geochemistry and regional geochemistry denote that the metallogenic material of the deposit originated from its matrix - the Taiyanding Group. There are two principal rock types in the Taiyanding Group, the siliceous rocks and the carbonaceous slates. The gold and the organic carbon are uncorrelated in the carbonaceous slates, no matter the latter is metallized or not. Instead, the gold and the organic carbon in the siliceous rocks show positive correlatability in unmetallized strata but negative correlatability in metallized strata. The retrieval rate of gold through soaking and retrieving experiments of the metallized and unmetallized rocks show negative correlation with the organic carbon contents, and the retrieval rates of the organic carbon rich slates are generally lower than those of the siliceous rocks. Q mode cluster analysis shows that the microelement composition of ores of both siliceous rock type and slate type demonstrates coincidence and inheritance with siliceous rocks. All these evidences indicate that mobility of gold is much higher in siliceous rocks than in slates, which makes gold in the former ready to remobilize to contribute in the metallogenesis; whereas gold in the slates is mainly inert and difficult to remobilize. Thus, the siliceous rocks are the source beds of the Laerma Gold Deposit. The gold adsorbing and releasing experiment of organic matter and clay minerals show that: (1)the gold adsorbing ability of organic matter is at least 6.89 times higher than that of pure clay minerals; (2)participation of clay minerals decreases the gold adsorbing ability of organic matter; (3)gold releasing ability of both organic and clay minerals decreases remarkably under 300℃. Experimental results also yield the following theoretical estimation: the retrieval of gold from the siliceous rocks of Taiyangding Group makes 99.20 percent of the total retrieval, while that of the slates makes only 0.76 percent. Thus the simulation experiments reveal that these siliceous rocks proved to be of thermal water origin are the main source bed of the Laerma gold deposit during petrogenesis. Researches of organic geochemistry of the siliceous rocks of Taiyangding Group indicate that the biological mother body of the organic matter, which is closely correlated with the gold, is primarily marine bacteria and algae. Metallogenic experiments of bacteria and algae show that: (1)Cyanobacteria and green algae, no matter marine or terrestrial, have remarkable gold enriching ability, and the enrichment ability differs among different taxa and environments; the enrichment ability of cyanobacteria is stronger than that of green algae; the enrichment ability becomes stronger in hot water than in water of normal temperature; (2)The gold enrichment coefficient of cyanobacteria is respectively 7～10, 14～15 and 3 times of those of clay minerals, sediments and organic matter decomposed from organisms; hence the gold enrichment ability of organisms is evidently stronger; (3)Not only is the gold adsorbing ability of clay minerals, sediments and organic matter weaker, but also their absorbed gold becomes more inert in release and remobilization as compared with gold absorbed by organisms. Theoretical estimation from the results of experiments shows that the contribution of organisms alone is sufficient to form a macrosize gold deposit. Thus, the biometallogenesis played a great role in the formation of source beds of the Laerma Gold Deposit.