Strontium isotope systematics and metallogenesis of skarn deposits in Japan

Abstract

In order to understand skarn petrogenesis, we determined the strontium isotope compositions of 168 skarns and 43 host or surrounding rocks in Japan. Most skarns have low Rb/Sr ratios (<0.1) and formation ages younger than 120 Ma, indicating negligible contents of 87 Sr due to a decay of 87 Rb. Multiple analyses of skarns from three representative base metal deposits demonstrate that their 87 Sr/ 86 Sr ratios vary only slightly (<0.001) within single mines, irrespective of time and position, indicating that even a single skarn sample can give a representative 87 Sr/ 86 Sr ratio on a single mine scale. On the other hand, skarns from 78 ore deposits in Japan reveal regional variations in the 87 Sr/ 86 Sr ratio ranging from 0.705 to 0.726. The variations are best classified by the ore metal type; skarns of the copper-iron type are rich, whereas those of the lead-zinc type are deficient, in Sr derived from the intrusions responsible for skarn formation. In many cases, Sr was derived from the original carbonate and aluminous rocks replaced by the skarns. Given that host limestones have relatively uniform 87 Sr/ 86 Sr ratios (0.7081 + or - 0.0007), the variability in the strontium isotope composition of skarns may be attributed to the difference and/or the amount of the involved aluminous rocks which have a wider range in the 87 Sr/ 86 Sr ratio. This is consistent as a whole with the occurrence of skarn deposits; copper-iron deposits form in geologic settings dominated by and often in contact with cogenetic large plutonic batholiths, whereas most lead-zinc deposits occur where sedimentary or igneous rocks with higher amounts of 87 Sr are common and form nearer and often distant from smaller hypabyssal dike or stock intrusions. An Sr isotope signature derived from the associated magmatic fluid is perceptible only in some skarns of the copper-iron type. The original magmatic fluid may have been modified isotopically when it encountered aluminous rocks and limestones and leached Sr from them.