安定同位体比および微量成分分析の塊状硫化物鉱床探査への適用

コロンビア共和国エルロブルレ鉱山周辺地域の例

抄録

El Roble Mine is located at Carmen de Atrato in the northwestern province of Colombia. The ore deposits classified as submarine volcanogenic massive sulfide type are situated along the contacts between the basic volocanic rocks and the sedimentary rocks.
The δ³⁴S values of ore sulfide range from -9.3 to +7.8 per mil with an average value around +3.1 per mil. Moreover, ore sulfides are classified into Type A and Type B based on the occurrence of pyrite. Type A ore is characterized by much amount of pyrite and colloform or framboidal texture, and occurs near the black shales. δ³⁴S values of Type A range from -9.3 to +1.8 per mil. Whereas Type B ore is composed of massive chalcopyrite associated with euhedral to subhedral pyrite crystals. The δ³⁴S values range from +4.3 to +7.8 per mil. Most of sulfurs measured for isopopic compositions are contained in chalcopyrite and pyrite. According to SAKAI(1968) and KAJIWARA and KxousE(1971), if ore is formed from an identical ore solution under an equilibrium condition, pyrite becomes higher in δ³⁴S values than chal-copyrite. But in our research, δ³⁴S values of Type B ore which contains abundant chalcopyrite are higher than those of Type A ore which contains abundant pyrite. Therefore, ore sulfur should have been deribed from two or more sources.
The δ34S values of basic volcanic rocks close to the ore deposits are similar to those of the ore, while in the other areas basic volcanic rocks have various δ³⁴S values. The δ³⁴S values of the black shales near the deposits show wide ranges, whereas those of the part far from the deposits are relatively constant ranging from -20 to -15 per mil.
Each rock facies has a given ⁸⁷Sr/⁸⁶Srr ratio. The average ⁸⁷Sr/⁸⁶Srr ratio of the ore is 0.7063, while those of the basic volcanic rock, the sedimentary rock and andesitic dyke are 0.7055, 0.7081 and 0.7046, respectively. Moreover, the ⁸⁷Sr/⁸⁶Srr ratios of the basic volcanic rocks and the black shales close to the ore bodies range from 0.7057 to 0.7068, which are similar to those of the ore.
The basic volcanic rocks close to the deposits are strongly altered. These rocks have lower Cu content and higher LOI(Loss of Ignition) than the fresh basic volcanic rocks in the other areas:
For the exploration of massive sulfide deposits we consider that the stable isotopic compositions and the contents of minor elements in the host rocks may be good indicaters. In the near future these analytical methods will be an effective exploration tool.