Distribution and enrichment processes of cobalt in the Longqiao iron skarn deposit in Eastern China

Longqiao is a cobalt-rich skarn iron deposit in Eastern China. As a typical representative of its type, it provides an opportunity to study the occurrence, distribution, and factors controlling cobalt in these deposits. Cobalt-bearing ores in Longqiao deposit can be classified into two types: cobalt-bearing diopside-magnetite ore (Co-Di-Mag) and cobalt-bearing phlogopite-magnetite ore (Co-Phl-Mag). Systematic whole-rock geochemical analysis, automated mineral analysis (TESCAN Integrated Mineral Analyzer, TIMA), and LA-ICP-MS trace element analysis were conducted on the two ore types. Three independent cobalt minerals(cobaltite, glaucodot, and carrollite)were found in Co-Di-Mag; no independent cobalt minerals were found in Co-Phl-Mag. TIMA and LA-ICP-MS analyses showed that cobalt in Co-Phl-Mag is mainly hosted in pyrite, so the pyrite content has a decisive role in the overall cobalt content. Cobalt in Co-Di-Mag is controlled by the content of magnetite, pyrite, and cobalt minerals. In both the diopside-magnetite stage (Stage I) and phlogopite-magnetite stage (Stage II), the cobalt mainly occurs in magnetite, and its content gradually decreases from 80 to 30 ppm as the system evolved. During the sulfide stage, minor pyrite deposited near the intrusion, and cobalt occurs in the pyrite lattice and also forms numerous independent cobalt minerals. Pyrite is abundant in the distal part of the ore-body, where all cobalt occurs in pyrite, and independent cobalt minerals are absent. Cobalt mainly occurs in pyrite in Longqiao deposit, which is favorable for beneficiation and recovery. Similar skarn iron deposits are widespread in eastern China, and the cobalt in these deposits has potential for recovery.