Metamorphic Skarns in the Yangla Cu Ore Field, Northwest Yunnan Province, China

The Yangla Copper Ore Field in Northwest Yunnan Province, China, is a large region of deposits dominated by copper-bearing skarns whose origin remains debatable despite numerous studies over the past two decades. We have investigated the geological and geochemical characteristics of the skarns using field and microscopic observations combined with chemical analyses. The results show that the skarns fall into two categories. The first category is metamorphic skarn, which constitute the majority (> 90%) of skarns in the deposit and is characterized by stratiform occurrences conformable to Devonian host strata, anhydrous mineral assemblages such as diopside+hedenbergite+quartz, widespread banded structure, fine-grainsize (< 200 mu m) and preserved tuff-like textures. Whole-rock major element compositions, REE and trace-element compositions resemble those of the country rock slates or schists. The skarn layers occur at variable distances (0-2 000 m) from Indosinian plutonic bodies. Fracture-filling veins and/or alteration halos are scarce or absent in or near the skarn layers. This category of skarn probably formed by isochemical contact metamorphism of fine calcareous clastic sediments or impure carbonate rocks during emplacement of the plutonic bodies with no significant material migration by hydrothermal fluids involved during the process, in which case metallic enrichment of the skarn layers was present in the protolith of the skarn. The second category is metasomatic skarn with relatively coarse-grained textures (200-> 1 000 mu m) and volatile-reach assemblages such as diopside+tremolite+scapolite at or near the igneous contact zones, which constitutes only a minor proportion of ore compared with metamorphic skarn. Taking into consideration diverse existing opinions about the genetic type of the deposit, we suggest that the geological and whole-rock geochemical characteristics of the skarns are consistent with a metamorphosed and metasomatized SEDEX (sedimentary exhalative deposit) type rather than a hydrothermal metasomatic skarn type.