This paper is the result of mineral exploration engineering.

Objective The copper–nickel–cobalt deposit in the Alxa Left Banner is a typical sulfide deposit in western Inner Mongolia, developed in the ultrabasic−medium basic magmatic rock zone. The gabbro is an ore–forming rock, and recognizing the magmatic activity, metallogenic regularity and tectonic background have become an urgent geological problem.

Methods Related samples were collected to study petrography, petrogeochemistry, zircon U–Pb chronology and Hf isotope.

Results Geochemical characteristics shows that the Yagan gabbro has the characteristics of high Al₂O₃ (15.99%–17.47%), sub−alkaline (K₂O+Na₂O=4.94%–5.86%), low TiO₂ (0.81%–1.12%), low P₂O₅ (0.14%–0.21%), high MgO (3.18%–5.64%), low K₂O (1.14%–2.05%), indicating of the calcium alkaline series. The total amount of rare earth (ΣREE) is 71.43×10⁻⁶–94.22×10⁻⁶, presents right–inclined distribution model, the light rare earth is relatively enriched and the heavy rare earth is depleted. The high field strength elements (Nb, P, Ta) are obviously depleted, incompatible elements (U, Sr) are enriched, suggested that the Yagan gabbro originated from the lithospheric mantle and experienced crystallization differentiation in the late magmatic stage. The zircon U–Pb weighted average age of the Yagan gabbro is (268.8±3.1) Ma, indicating the age of diagenesis belongs to Middle Permian. The ε_Hf(t) values is −7.1–2.9 and two staged Hf model age is 1272–2177 Ma.

Conclusions Regional geological data and geochemical characteristics indicate that the Yagan gabbro original magma may be confused by some crustal materials during the migration process. The formation tectonic background may be the post–collision and extension environment in the Late Paleozoic. In addition, the emplacement age of Yagan gabbro limited the collision closing time, which provides new constraints for the study of Cu–Ni–Co deposits in this region.