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 ( 1 000 μm) 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.