Elsevier

Lithos

Volumes 382–383, February 2021, 105951
Lithos

Research Article
Magma volume and timescales in the formation of porphyry molybdenum deposits: A case study from the Central Asian Orogenic Belt

https://doi.org/10.1016/j.lithos.2020.105951Get rights and content

Highlights

  • Prolonged hydrothermal processes are not prerequisite for giant Mo mineralization.

  • The formation of porphyry Mo deposit is not sensitive to initial oxygen fugacity.

  • Voluminous reservoir may be a critical factor for large porphyry Mo deposits.

Abstract

Porphyry Mo deposits, as an important repository for molybdenum, are generally considered to have formed from the reworking of Mo-rich ancient crust. However, some of the giant porphyry Mo deposits in the Central Asian Orogenic Belt (CAOB) are thought to have been derived from juvenile crust. Here we evaluate the role of large volume magma chamber,and the timescales of magmatic differentiation and hydrothermal processes in the formation of porphyry Mo deposits. In our case study on the Donggebi porphyry Mo deposit along the southern part of the CAOB, the pressure-temperature conditions of crystallization of the ore-related intrusion are estimated as 1.2–1.8 kbar and 570–727 °C. Based on the sulfur content of apatite grains from the deposit, we estimate that a large magma volume (25–215 km3) was involved in the formation of the Mo resources (0.508 Mt) associated with the Donggebi porphyry Mo deposit. Quartz associated with mineralization in this deposit is identified as two types. Type-1 quartz displays wide and parallel zoning and preserves high Na, K and Ca contents whereas Type-2 displays narrow and subparallel zoning with low Na, K and Ca contents. The Type-1 quartz is likely to retain the magmatic information, whereas the Type-2 quartz possibly formed through metasomatic processes in the hydrothermal stage. We use Ti diffusion patterns in quartz coupled with TitaniQ thermobarometry to estimate the timescales of magma evolution and hydrothermal processes. The quartz residence in the magma chamber is estimated as 10,000 to 40,000 years while that related to hydrothermal fluids is relatively short (800 to 5000 years). Our results suggest that large volume magma chambers might be an essential requisite for the formation of porphyry Mo deposits, as against models that consider prolonged hydrothermal processes.

Introduction

Molybdenum (Mo), with its high melting point and strength, excellent corrosion resistance and semiconductor properties, is one of the critical metals widely used in industry (Lerchbaumer and Audétat 2013). More than 90% of Mo sources in the world come from porphyry Mo deposits (Arndt and Ganino 2012), most of which are distributed within oceanic/continental arc rocks (e.g. Endako, Chalukou and Zhaiwa deposits, Selby et al. 2000; Zhang and Li 2017; Deng et al. 2013), rift-related rocks (e.g. Climax, Urad-Henderson and Caosiyao; Wallace 1995; Seedorff and Einaudi 2004; Wu et al. 2016) and those in continental collision zones (e.g. Qian'echong, Donggou, Donggebi and Baishan; Mi et al. 2015; Yang et al. 2015; Wu et al. 2014; Xiang et al. 2013). These occurrences have been classified as Endako-, Climax- and Dabie-types, respectively. Compared with Cu- and Au-bearing porphyry, the Mo-bearing porphyry intrusions often display high SiO2 contents, which is generally considered to reflect the highly differentiated nature of magma or magma sourced from ancient crust (e.g. Carten et al. 1988). The Mo content of crust is generally higher than that of the mantle, because Mo is highly incompatible during partial melting processes. Multiple events of partial melting would lead to higher Mo content in the ancient crust than that in the juvenile crust, thus favoring the formation of Mo deposits. However, many newly discovered porphyry Mo deposits in the Central Asian Orogenic Belt (CAOB) show positive or near-chondrite εNd(t) and εHf(t)values as well as relatively low initial 87Sr/ 86Sr values, such as the Baishan (εNd(t) of 3.8–7.2, εHf(t) of 4.4–12.2 and (87Sr/86Sr)i of 0.70358–0.70505; Cao et al. 2017) and Donggebi (εNd(t) of 0.60–1.62, εHf(t) of −1.58–4.83 and (87Sr/86Sr)i of 0.70618–0.70821; Han et al. 2014; Zhang et al. 2015; Wu et al. 2017) deposits, suggesting that a juvenile crustal source cannot be explained by the classic ore-forming models.

Two possible mechanisms may account for this inconsistency. The first one is that during the late stage of magma differentiation, Mo may get enriched in the exsolved fluid. However, in a reverse situation, Audétat and Li (2017) noted that the exsolved fluids from barren and ore-bearing intrusions show similar Mo concentration, based on the data of ore-forming fluid inclusions and melt inclusions in different Mo-mineralized occurrences. The second possibility is that a large volume of magma may be involved in the formation of porphyry Mo deposits, although evidence for this has so far not been presented. Furthermore, the high SiO2 contents in the mineralized porphyry systems are generally considered to result from high-degree magmatic differentiation. However, the timescale of magma differentiation associated with porphyry Mo deposits remains poorly constrained. Limited studies suggest that the timescales of formation of medium-giant porphyry deposits could range from a few million years to tens of million years, as inferred from isotopic dating of multiple and long-lived mineralization episodes (Turner and Costa 2007). However, information from quartz diffusion patterns in porphyry Cusingle bondMo deposits reveals a much shorter time scale. For instance, the formation time scales of porphyry Cusingle bondMo deposit at Butte, Montana and porphyry Cu-Mo-Au deposit at Haquira, Peru are inferred to be less than 10,000 and 35,000 years, respectively (Cernuschi et al. 2018; Mercer et al. 2015). Similarly, the precipitation timescale of the Ladolam hydrothermal system in the Lihir Island, Papua New Guinea is proposed to be less than 55,000 years to form 1300 t gold (Simmon and Brown, 2006).

In this study, we focus on the magma evolution processes in the mush reservoir of the Donggebi Mo deposit in CAOB. The crystallization temperature, pressure, oxygen fugacity as well as the volumes of ore-forming melts and fluids are estimated based on the chemistry of biotite, apatite and quartz. We attempt to constrain the timescales of magma differentiation and hydrothermal activity through the application of chemical diffusion modelling based on Ti diffusion patterns in quartz obtained through scanning electron microscope-cathodoluminescence (SEM-CL) images coupled with trace element analysis. The results from our work provide a better overview of factors controlling the formation of intermediate-giant porphyry Mo deposits, as compared with the previous models that propose initial ore-forming melts/fluids contain high Mo contents.

Section snippets

Regional geology

The Central Asian Orogenic Belt (CAOB), which is also termed as the Altaid tectonic collage, is one of the largest accretionary orogens in the world spreading between the Siberian Craton in the north and the Tarim and North China Cratons in the south. The evolution of this orogenic belt was accomplished through the collision between the Siberia and Tarim-Sinokorea plates along the Solonker-Kumishi Suture that gradually sealed from the Late Carboniferous to Early Triassic (Chen 1997; Xiao et al.

Ore deposit geology

The Donggebi porphyry Mo deposit is located at the northern part of Aqikuduke fault, covering an area of 16.8 km2 although most places are concealed by Quaternary sediments (Fig. 2). The deposit is hosted in the Lower Carboniferous Gandun Formation, which can be subdivided into three parts: the lower metamorphic sandstone, fine-grained quartz sandstone and thin-layered graywacke; the middle coarse-grained sandstone interlayered with phyllite and spilite; and the upper medium-coarse grained

Intrusive rocks and paragenesis

The biotite-bearing porphyritic granite samples selected for this study are gray and pink, displaying phaneritic texture. Coarse-grained phenocrysts (70%) comprise quartz (30–35%), albite (30–35%), K-feldspar (35–40%) and biotite (~5%). Fine-grained groundmass (30%) is mainly composed of quartz as well as a small amount of muscovite, magnetite, titanite, zircon and some opaque oxides. Quartz phenocrysts are subhedral to anhedral, ranging from 0.5 to 2 mm in diameter and shows equilibrium

Mineral major element analysis

The major element content of different minerals was measured by Electron probe microanalysis (EPMA) at the Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS) in Beijing by using the equipment of JXA-8230. The analytical conditions for measuring albite, K-feldspar, biotite, apatite, rutile, xenotime and monazite were set as an acceleration voltage of 15 kV, current of 20 nA, beam spot diameter between 1 and 5 μm. The peak counting time for Na was 10s and other elements

Quartz

The major and trace element analysis of quartz were performed at domains that are away (> 200 μm) from apatite and K-feldspar, and thus the fluorescence effect of Ca and K is negligible. Representative major and trace elements concentration of quartz are listed in Table 1. The major element contents do not exhibit much variation, but significant variations are seen for Ca, Mg, Fe and Ti concentration (515–10,122, 3–2901, 2–857 and 57–204 ppm respectively).

Alkali feldspar

The feldspars are alkali feldspar,

Crystallization pressure, temperature and oxygen fugacity

Magma crystallization pressure is estimated primarily based on TAl content of biotite geobarometer in granitic rocks (Uchida et al. 2007) by using the following empirical equation:Pkbar=3.03×TAl6.53×±0.33in which TAl represents the overall Al concentration in biotite upon the parameter of O = 22. Calculated results are shown in Table 3, which indicate that the granitic magma was formed at relatively low pressure, ranging from 1.2–1.8 kbar, with an average of 1.5 kbar. The corresponding depth

Discrepancies of the timescale of two groups

The discrepancies of the timescales for the two types of quartz crystals far exceed the permitted interval of errors, which likely represent the modification of quartz crystals during different geological events and under different physico-chemical conditions. It is generally believed that the cooling of granitic plutons and formation of porphyry deposits are simultaneous (e.g. Schöpa et al. 2017). In addition, a recent study indicates that high magma influxes are necessary to provide steady

Conclusions

This study presents estimates from thermobarometry, magma chamber volume, and timescales of magmatic and hydrothermal processes in understanding the origin of porphyry Mo systems. Our results show that the Donggebi granitic porphyry within the Central Asian Orogenic Belt formed at pressures of 1.2–1.8 kbar (corresponding to a depth of 4–6 km) and temperatures of 570–727 °C. About 25–215 km3 magma volume was required to provide sufficient Mo resources (0.508Mt) in this deposit. Diffusion

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors would like to thank Changhong Wang, Linghan Liu, Hengxu Li, and Yuqi Jiang for their invaluable support during field work. We are grateful to Zhenyu Chen, Xiaodan Chen, Dongjie Tang for technique support. Editorial handling by Prof. Xian-Hua Li is very much appreciated. The comments from two anonymous reviewers greatly helped to improve the manuscript. This work was funded by the National Key Research and Development Program of China (2016YFC0600502), the National Natural Science

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