中下扬子铁帽型金矿氧化剖面及成矿地球化学

ZONING OF OXIDIZATION SECTION AND MINEROGENETIC GEOCHEMISTRY OF GOSSAN-TYPE GOLD DIPOSIT ON REACHES OF MIDDLE-LOWER YANGIZE REVER

铁帽型金矿床氧化剖面自上而下可划分为贫金铁帽亚带、次生金富集铁帽亚带、氧化物-硫化物过渡亚带、原生硫化物亚带。氧化作用诸阶段的主要元素、微量元素、稀土元素、Eh-pH值皆有一定的规律性变化,并与次生金的迁移富集过程具有明显的相关性。

The well-preserved typical oxidization section of gossan-type gold ore deposit is subdivided into four subzones from the upper to the lower : 1. the gold-poor gossan subzone (gold leaching subzone), 2. the secondary gold enrichment subzone, 3. the oxide-sulfide transitional subzone , 4. the primary suifide subzone . Generally , the secondary gold enrichment subzone is always accompanied by gold-poor subzone, unless the later is eroded. Secondary gold and silver are usually concentrated in the middle-lower parts of gossan. The gold and silver content in the secondary gold enrichment subzone is 5-10 times, even 100 times higher than those in the primary sulflde subzone. Zoning of elements in oxidization section indicate the element differentiation during oxidiation process. Cu, Ag, Bi etc. have the same tendency of depletion or enrichment as gold does. Mn, Ni, Co, Pb, Zn etc. have the contrary tendency. In other words the high content of Mn, Ni, Co is contributed to dissolution and migration of Au, while the low content of them is beneficial to precipitation and enrichment of Au. The dissociation level of either Mn , Ni with Fe or Ni with Co is positive correlated with secondary Au enrichment. While (Mn/Fe)× 10-4 > 25, (Ni / Fe) × 10-4 > 1, Ni / Co > 0.7 , it indicates gold-poor gossan, on the contrary ,it probablly belongs to the subzone of secondary gold enrichment. The results of analysis for samples of typical oxidization section of gossan-type gold ore depisits show that the Eh-pH values are different in four subzones. In secondary gold enrichment subzone the pH = 5.13-6 and Eh = 490-634mv. In gold-poor gossan subzone the pH = 6-7 and Eh = 483-700mv. In oxide-sulfide transitonal subzone the pH = 2-3.5 and Eh = 531-641 mv. Summarily , the disolution of Au probablly occurres at early oxidization stage, the precipitation and enrichment of secondary gold takes place at pH > 3.5 and Eh < 600m v.