Abstract:

The Lamasayi copper deposit is a representative copper deposit in the Sailimu massif in Western Tianshan, Xinjiang, China. However, the genesis of this ore deposit is controversial. This study tries to discuss the problem through tracing the origin of ore forming material using stable isotopes of C, O, S and Pb and investigating the geological characteristics of the Lamasayi copper deposit. The δ¹³C values of gangue calcite vary between -1.04‰ and -0.87‰ which are  lower than those of wall-rock limestone (δ¹³C = 3.51‰-5.47‰); the δ¹⁸O values of gangue calcite are from 9.33‰ to 9.61‰ and are quite lower than those of normal marine carbonate (δ¹⁸O =20‰-26‰). The C and O isotopic compositions reflect that the ore-forming fluid may be a mixture of magmatic water and ground water. The δ³⁴S values of sulfides mainly vary from 3.75‰ to 8.64‰ which is close to the sulfur isotope composition of the Hercynian porphyries (for example, the δ³⁴S values of sulfides from the Dabate porphyry copper-molybdenum deposit are from 4.9‰ to 7.9‰) and which reflects that the sulfur may be originated mainly from porphyry. The Pb isotope compositions of chalcopyrite are from 18.264 to 19.544 for  ²⁰⁶Pb/²⁰⁴Pb,from 15.575 to 15.656 for ²⁰⁷Pb/²⁰⁴Pb,and from 38.103to 38.705 for ²⁰⁸Pb/²⁰⁴Pb, respectively, which shows characteristic of two-stage abnormal lead and being rich in radiogenic lead, similar to the Pb isotope composition of the Hercynian porphyries (for example, the Dabate rhyolitic porphyry) and which also indicates that the ore-forming metal may be derived from the porphyry. In summary, the Lamasayi copper deposit may be skarn type deposit related to porphyry.

Key words: ore geology, C, O, S and Pb isotopes, Lamasayi copper deposit, Western Tianshan, Xinjiang