Abstract
The Niubiziliang Ni-(Cu) deposit is the first magmatic Ni-Cu sulfide deposit in the North Qaidam Orogenic Belt (NQOB), NW China, and plays a significant role in geological evolution, Ni-Cu mineralization, and exploration in the NQOB. Here, we report on the mineral chemistry, S-Pb-O isotopes, and S/Se ratios of the mafic-ultramafic complex, which provide insights on the parental magma, evolution, and sulfur saturation mechanism. The Niubiziliang mafic-ultramafic intrusion contains four ore blocks and about ten Ni-(Cu) ore/mineralization bodies. Olivines in Niubiziliang belong to the species of chrysolite with Fo values of 88~89, and the pyroxenes are mainly orthopyroxene (En = 79~82) and clinopyroxene (En = 44~40). The olivines and some pyroxenes likely crystallized in a magma chamber at a depth of 35.45~36.55 km at a high temperature (1289~1369 °C) and pressure (9.38~9.67 kbar), whereas the Niubiziliang complex formed at a moderate depth (8.13~8.70 km) with a temperature and pressure of 1159~1253 °C and 2.15~2.30 kbar, respectively. The parental magma was considered to be high-Mg picritic basalt with MgO and NiO contents of 14.95~16.58% and 0.053~0.068%, respectively, which indicated high-degree partial melting of the depleted mantle. The mantle-derived primary magma underwent significant fractional crystallization and crustal assimilation and contamination, which was strongly supported by S-Pb-O isotope data and S/Se ratios, resulting in sulfur saturation and sulfide immiscibility in the magma. Crustal assimilation and contamination contributed more to sulfur saturation than fractional crystallization.
Highlights
The magmatic Cu-Ni-PGE sulfide deposits associated with the mafic-ultramafic complex host major Ni, Cu, Co, and PGE resources, and have significant implications for the evolution of the mantle, magmatism, tectonics, and geodynamic evolution [1]
Naldrett (1995) proposed three key factors for the genesis of world-class Cu-Ni-PGE deposits [3]: (a) The host magma must become saturated in sulfide and segregated immiscible sulfide; (b) these sulfides must react with a sufficient amount of magma to concentrate chalcophile elements to an economic level; and (c) the sulfides must themselves be concentrated in a restricted locality to constitute an ore body
The Niubiziliang Ni-(Cu) deposit is located in the northwestern margin of Qaidam Massif and the southern slope of the Altun Mountains, and is the intersection of the Altun Orogenic Belt (AOB) and the North Qaidam Orogenic Belt (NQOB) (Figure 1a)
Summary
The magmatic Cu-Ni-PGE sulfide deposits associated with the mafic-ultramafic complex host major Ni, Cu, Co, and PGE resources, and have significant implications for the evolution of the mantle, magmatism, tectonics, and geodynamic evolution [1]. The mineralogy and isotope geochemistry of the Niubiziliang mafic-ultramafic complex are relatively poorly understood, so the primary magma composition, magmatic evolution, sulfide saturation mechanism, and their constraints on mineralization remain poorly understood. Our paper attempts to determine the parental magma composition, crystallization conditions, magmatic evolution, sulfur saturation mechanism, and their constraints on mineralization through a systematic study of the mineralogy, O-S-Pb isotope geochemistry, and S/Se ratios in this complex, in order to provide new insights into the metallogenesis of the Niubiziliang
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