斑岩型铜(±金-钼)矿床岩浆氧化硫与成矿还原硫转换机制及找矿意义

Mechanisms for transform of sulfate in porphyry high oxidized magmas to ore-forming sulfur of porphyry Cu(±Au-Mo) deposits and their application

斑岩型铜(±金-钼)矿床和高氧化岩浆具紧密成因联系,我国及世界主要斑岩铜钼金矿床岩浆期多发育磁铁矿-赤铁矿组合,个别矿床见岩浆期石膏.这表明斑岩铜(±金-钼)矿床成矿岩浆logf(O_2)>ΔFQM+2,岩浆中的硫主要为氧化硫,斑岩型矿床的硫主要为还原硫.斑岩矿床在岩浆演化晚期及钾化阶段多发育磁铁矿,而岩浆-成矿系统中亚铁(Fe^(2+))和氧化硫反应,可被氧化成为Fe^(3+),形成磁铁矿,氧化硫被还原.因此提出斑岩矿床成矿早期磁铁矿化,使氧化硫被还原形成还原硫,为斑岩矿床硫化物沉淀析出提供充足还原硫,在斑岩矿床形成过程中起着关键作用.据成矿岩体多发育磁铁矿,提出斑岩型矿床找矿中应注意高磁化率中酸性岩带;斑岩矿床主要成矿多发育于黄铁绢英岩化阶段,而在弱酸性环境下的黄铁绢英岩化过程中,磁铁矿会被黄铁矿交代,磁铁矿减少,矿化较强处磁化率会降低.因此,高磁化率带岩体中低磁化率区或高低磁化率变化频繁地段应为寻找富矿的重点地段.

It is well known that porphyry Cu（±Au-Mo）deposits are genetically associated with high oxidized magmas. Magnetite-hematite assemblages have been found in the Yulong in the eastern Tibet, Xiongeun, Duobuza porphyry Cu-Au （Mo）deposits in the southern and the northern Gangdese, respectively, the Dexing porphyry Cu-Au-Mo deposit in Jiangxi and the Luoboling porphyry Cu-Mo deposit in Fujian province; magma anhydrite is found in the Yulong porphyry Cu-Au-Mo deposit and reported in many porphyry deposits all over the world. These features suggest that the magmas of porphyry Cu （q-Au-Mo）deposits are characterized by high oxidized with logf（O2）〉ΔFQM＋ 2 and sulfate dominant. Porphyry Cu（±Au-Mo） deposits are characterized by sulfide assoeiation,suggesting that the reduced sulfur is dominant during main stage of porphyry mineralization. How is the magmatic sulfate reduced to the ore sulfide during the porphyry ore forming processes？ Magnetite alteration is well developed in the silicon-potassium alteration stage of porphyry deposits. Based on that the reaction of ferrous with sulfate could result in the formation of magnetite and reduced sulfur, it is proposed that the early stage magnetite alteration in the porphyry ore forming systems which result in the transform of sulfate to reduced sulfur, play a key role in the formation of porphyry Cu（ ± Au-Mo）deposits. Based on our result it is suggested that felsic igneous belt with high magnetic susceptibility should be the ore prospecting target area of porphyry Cu （±Au-Mo） deposits. Due to magnetite could be replaced by pyrite during sericite-pyrite alteration stage in weak acidic condition and the porphyry ore rich bodies are often located in the overlap zone of potassic-sillicon and sericite-pyrite alteration, the ore rich domains should be characterized by low magnetic susceptibility. It is therefore,that the domain with low magnetic susceptibility in the zone with high magnetic susceptibility should be the key target for high grade porphyry ore exploration.