Abstract:

Jiama copper polymetallic deposit is one of the large deposits that gained prospecting breakthrough in recent years in
Gangdese metallogenic belt, Tibet. By comparing with    typical SEDEX deposits,  this article means  to point out  the genetic  type
of Jiama deposit as porphyry-skarn  type, related  to porphyry metallogenesis. The porphyry system of Jiama hosts many different
mineralization  types owing  to different wall  rocks,  including porphyry Mo-Cu deposits centered  in  intrusions, skarns, hornfels,

and Au deposits in independent peripheral locations. The differences between Jiama and typical SEDEX deposits are as follows.
① Geologic  features show that the magmatic hydrothermal solutions not only formed layered skarns in the space of K1 l  and J3 d,
but also small-scale, lensing and cystiform skarns in the pelitic limestone interlayers of K1 l . There are no exhalative sedimentary
rocks in Jiama mining area, and the so-called exhalative sedimentary rocks in the previous papers, in fact, are skarnized hornfels.
The mineralization with an age of 14~16 Ma might be related to the delamination and large-scale crust stretching due to Gangdese
collision orogenesis in Miocene epoch, and the porphyry intrusion event in later period. The copper polymetallic deposit of skarn
has no double-layer structure,  typical of SEDEX. The upper skarn with  the wall  rock  roof has a gradual  transition  relationship.
Skarn with coarse grained texture is the result of minerals, self-growth because of long time magmatic supplies. ② Geochemical
characteristics. The combination of ore-forming elements is mainly of Cu, Mo, Au, Ag association, and in less amount Pb, Zn. Mn
contents in limestone of J3d show negative anomaly compared with the regional background. The Zn / (Pb+Zn) ratio in Jiama Pb-
Zn deposit mainly concentrates in two ranges of 0-0.1 and 0.2-0.45. Mineralization  of the deposit presents a zoning Mo→Cu+Mo
→Cu→Pb+Zn. Isotopically, in contrast to SEDEX deposits, the δ 34S values of Jiama deposit are negative and concentrated, and its δ 30
Si values are positive. Besides, an ore-prospection model is set up in this article based on the magmatic-hydrothermal theory
of metallogenesis to search for the areas in Nyainqentanglha region with geological conditions similar to Jiama deposit, and to find
the Pb-Zn ores in shallow, peripheral parts of porphyry bodies and Cu-Mo-W ores in their deeper, central parts.

Key words: peripheral , lead-zinc ore bodies, geological and geochemical , features, exhalative sedimentary deposits , (SEDEX),
porphyry system of Jiama deposit, Tibet