Revisiting the age and emplacement process of the Huangshandong Ni–Cu deposit in the Central Asian Orogenic belt, northwestern China: Implications for multiple magma extractions from a short–lived staging magma chamber

Abstract

Magmatic Ni—Cu deposits that associated with mafic–ultramafic intrusions in convergent settings could form from multiple magma pulses with age difference varying from 1 Ma to >10 Ma years. The Huangshandong deposit in NW China of the southern Central Asian Orogenic Belt was formed in such a setting. The intrusion is composed of four units formed by four magma pluses: a gabbroic sequence (Phase I, crystallized from an evolved magma), a sheet–like ultramafic body (Phase II, crystallized from a relatively primitive magma), a dike–like gabbronorite body (Phase III, crystallized from an evolved magma), and an irregular ultramafic unit (Phase IV, crystallized from a relatively primitive magma). Our zircon SIMS U—Pb dating shows that the Phase I and Phase IV magmas of the Huangshandong deposit were emplaced at 280.3 ± 1.9 Ma and 281.4 ± 2.2 Ma, respectively, indicating that the four magma pulses were intruded over a very short period and cannot be distinguished within the uncertainty of ±2 Ma. Moreover, the zircon crystals in the four units record identical ɛHf (t) values (average of 12.1–13.5) and δ18O values (average of 6.3–6.5‰). However, the intrusive relationships of the four units illustrate that the Huangshandong complex cannot form by in–situ fractionation. Therefore, the indistinguishable Hf—O isotopic values, together with their contemporaneous emplacements, indicate that the four magma pulses could originate from a same staging magma chamber. The overall enrichment of ɛHf (t) value relative to that of the depleted mantle (16) and elevation of δ18O values relative to the mantle value (5.3‰) suggest 15–20% crustal material incorporation. Additionally, the magmas may have been contaminated prior to their arrival at the staging magma chamber, where the magma has subsequently experienced variable olivine fractionations and relatively primitive magma replenishment. For most mafic–ultramafic intrusions which are characterized by long–duration magmatism (>10 Ma) in convergent settings, e.g., intrusions in the Kalatongke area, Tulaergen intrusions, Xiarihamu intrusions, the most important Ni—Cu mineralization are closely related to the last stage magmatism. This finding suggests that the intrusions emplaced at the late stage of subduction and post–collisional period of the orogenic process may have more potential for Ni—Cu mineralization exploration.