Metallogenic time and tectonic setting of the Bainaimiao Cu-Au deposit, Inner Mongolia

Abstract

The Bainaimiao Cu-Au-Mo deposit, Inner Mongolia, is located at the Ondor Sum Caledonian accretionary belt, eastern Central Asian Orogenic Belt. Its metallogenic type, time and tectonic setting have been debated for a long time. To time the mineralization, the authors dated grains of biotites and muscovites separated from Au-Cu-bearing quartz-vein, using high-resolution single-grain laser-ablation 40Ar/39 Ar method. 22 analyses of biotites yielded a 40Ar/39 Ar isochron age of 396 ± 2 Ma (MSWD = 20), and 23 of muscovites yielded a 40 Ar/39 Ar isochron age of 358 ± 2 Ma (n = 23, MSWD = 11). This shows that the ore bodies have been formed in Devonian. In the ore field, the magmatites, such as granitic porphyries, yielded isotope ages ranging from 428 to 466 Ma, which predated mineralization by >40 Ma; while the ore-hosting stratum was suggested to have been formed around 1130 Ma, which predated mineralization by > 700 Ma. This evidenced that the Bainaimiao deposit is neither of porphyry-type nor of submarine sedimentary-exhalative type. Instead, it should be an orogenic-type deposit as revealed by a recently completed comprehensive study in ore geology and fluid inclusions. The mineralization ages consist with both the peak-metamorphism (Ca. 384.5 ± 6 Ma) of the ore-hosting rocks of the Bainaimiao Group and the late Silurian-Devonian collision of the Ondor Sum accretionary magmaitc arc on the northern margin of North China craton. Hence the Bainaimiao Cu-Au deposit is the first case of orogenic-type Cu deposit formed in collisional-orogenic regime.