Multiple tectonic-magmatic Mo-enrichment events in Yuleken porphyry Cu-Mo deposit, NW China and its’ implications for the formation of giant porphyry Mo deposit

Highlights

• The Yuleken deposit preserves multistage tectonomagmatic evolution and mineralization.

• The progressive Re depletion in molybdenite records records multiple tectonomagmatic events that mobilize Mo and produce overall higher ore grades.

Abstract

The Yuleken porphyry Cu-Mo deposit located in the Chinese Altay-East Junggar, a section of the Central Asian Orogenic Belt, the world's largest Phanerozoic subduction-accretionary orogen, witnessed the multistage tectonic-magmatic evolution and mineralization from arc to syn- and post-collision environment over a time span of about 52 Ma. Four types of molybdenite have been distinguished: (1) primary magmatic-hydrothermal, undeformed molybdenite in porphyry, (2) recrystallized molybdenite associated with ductile deformation, (3) recrystallized, intrafoliational molybdenite, and (4) fracture-related hydrothermal molybdenite, and these are characterized by decreasing Re abundances from magmatic to hydrothermal conditions. Individual molybdenite types are non-stoichiometric and exhibit the following negative correlation between the Mo and Re contents:

Re (apfu) = −0.0026 Mo (apfu) + 0.00268.

This trend reflects successive stages of tectonic-hydrothermal events from a magmatic island-arc to a post-collisional setting. These repetitive mineralization events produced Mo enrichment and higher overall Mo grade of the Yuleken deposit, and are recorded in progressively decreasing Re concentrations in molybdenite. Statistical evaluation of Mo and Re abundances in giant to small porphyry deposits worldwide also indicates that decreasing Re in molybdenite will lead to lower overall Re resource (in a single deposit) and to greater overall Mo resource (in the same deposit). Multiple mineralization processes related to repetitive tectonomagmatic episodes in large porphyry Mo deposits that display Re depletion in molybdenite suggest that reworking of mantle-derived porphyry systems by successive events of crustal growth may be the responsible for Mo enrichment and higher overall Mo ore grade, and eventually produce giant porphyry molybdenum deposits.

Introduction

The majority of the world’s molybdenum resources is derived from porphyry Mo and Cu-Mo deposits. Giant (Mo reserve > 0.5 Mt) and large (Mo reserve of 0.1–0.5 Mt) porphyry Mo deposits with low concentrations of Re supply nearly 60 % of the global Mo demand (Chen et al., 2017a, Chen et al., 2017b, Chen et al., 2017c, Gao et al., 2018). Porphyry Cu-Mo deposits with high concentrations of Re are also its primary source (Sinclair et al., 2016). Re is almost invariably concentrated in molybdenite because the ionic radius of Re⁴⁺ is very close to that of Mo⁴⁺ and it allows the substitution of Re for Mo in molybdenite (Morris and Short, 1969, Shannon, 1976, Sinclair et al., 2016). Large to giant porphyry Mo deposits, which formed in continental collision orogens or in intracontinental rifts through partial melting of Precambrian crust (Chen et al., 2017a, Gao et al., 2018) usually have low Re concentrations in molybdenite (Sinclair et al., 2016), whereas porphyry Cu-Mo deposits formed in ocean island arcs typically have small Mo reserves but high-Re molybdenite (Audétat, 2010, Zhu et al., 2010, Richards, 2011). As accretion of island arcs is one mechanism for the formation of continental crust (Suyehiro et al., 1996), porphyry Mo deposits that formed in the continental crust could potentially be derived from the remnant arcs. The role of molybdenum formed in ocean island arcs, pre-enrichment in porphyry Cu-Mo deposits and the origin of giant porphyry Mo deposits found in late to post-collisional continental tectonic settings are therefore poorly understood.

Most giant porphyry Mo deposits have undergone multiple tectonic and magmatic events (Li et al., 2014, Mi et al., 2015, Li and Pirajno, 2017), and the relationship between tectonic-magmatic events and the formation of low Re molybdenite is also critical for the understanding of the formation of porphyry deposits. Textural and compositional variations in molybdenite are well preserved in porphyry Cu-Mo deposits in the Central Asian Orogenic Belt (CAOB; Xue et al., 2016, Gao et al., 2018) and can be used to decipher the key factors affecting the Re content of molybdenite.

The Yuleken (also called Yulekenhalasu) porphyry Cu-Mo deposit (PCMD) in China is one of several subduction-related deposits in the Kalaxian'gar porphyry copper belt (KPCB), in the central part of the Central Asian metallogenic domain (CAMD; Gao et al., 2018, Wu et al., 2019). The Yuleken deposit preserves evidence of syn- to post-collisional metallogenic overprint (Hong et al., 2015, Xue et al., 2016, Hong et al., 2018a, Hong et al., 2018b), making it a unique target to investigate the behavior and evolution of molybdenite induced by superimposed mineralization events. We report textural and compositional of molybdenite from different ore-forming stages in order to investigate: (1) the Re and Mo variation in molybdenite during superimposed ore-forming processes, (2) if the Re variation in molybdenite can be used as an indicator of multiple mineralization stages, and (3) the Re contents in a wide spectrum of Re-Mo deposits and their global implications.