Late Mio-Pliocene chemical weathering of the Yulong porphyry Cu deposit in the eastern Tibetan Plateau constrained by goethite (U–Th)/He dating: Implication for Asian summer monsoon
Introduction
Chemical weathering is important in shaping earth's surface and regulating global climate fluctuations and global chemical cycles. Precipitation, temperature, vegetation, and physical erosion are the dominant controls on long-term ( yr) chemical weathering (White and Blum, 1995, Riebe et al., 2004, West et al., 2005). Identifying the primary controls on chemical weathering is important for understanding the interplay between tectonics and climate in the Asian continent (Molnar and England, 1990). Some authors have proposed that regional or local climate is the critical variable influencing chemical weathering (Clift et al., 2008), and others have emphasized that physical erosion, and a consequent supply of un-weathered materials, provides the primary control on chemical weathering rates through time (Millot et al., 2002, Riebe et al., 2004).
Mineralogical, geochemical, and magnetic proxies from loess sections in the China Loess Plateau (An et al., 2005, Sun et al., 2010) and ocean sediments in the South China Sea (Wan et al., 2012, Clift et al., 2014) have been used to infer a record of chemical weathering over the Asian continent. However, changes in the provenance, and erosion and deposition rates of ocean sediments possibly complicate the interpretation of weathering proxies from marine sediments (Clift et al., 2014). In addition, chemical weathering indicators from loess deposits may be affected by the variation of eolian source and mass accumulation rates (Nie et al., 2014). Thus, the timing and history of chemical weathering in the Asian continent since the late Miocene remain controversial.
Weathering profiles can record a complex and protracted history of chemical and physical processes. Supergene minerals in weathering profiles commonly precipitate directly from weathering solutions, thus isotopic geochronology applied to these solids constrains the timing of chemical weathering processes (e.g., Beauvais et al., 2016; Bonnet et al., 2016). Supergene goethite forms mainly under relatively wet climate conditions (Tardy and Nahon, 1985), thus the frequency distribution of goethite ages can serve as a paleoclimatic proxy for past wet climatic conditions (Vasconcelos et al., 2015). Previous studies have demonstrated that goethite is suitable for (U–Th)/He dating; by using the 4He/3He methodology, loss of radiogenic 4He by diffusion through the solid phase can be identified and accounted for to determine the timing of goethite precipitation (Shuster et al., 2005). Goethite (U–Th)/He and 4He/3He geochronology has been used to quantify rates of weathering front propagation (Heim et al., 2006), to determine formation ages of channel iron deposits (Vasconcelos et al., 2013), and to unravel changes of climatic conditions prevailing in the geological past (Vasconcelos et al., 2015).
Cenozoic porphyry Cu deposits are widely distributed over the Tibetan Plateau (Hou et al., 2003). Many of these deposits have been exposed to the surface or subsurface by post-mineralization exhumation processes, leading to prolonged oxidation of the mineralized porphyry and consequent formation of thick (∼5–150 m) weathering profiles, which provide an ideal opportunity to determine the timing and history of chemical weathering and climatic conditions over the Tibetan Plateau. Here we present 4He/3He and (U–Th)/He dating results of supergene goethite from weathering profiles derived from the giant Yulong porphyry Cu deposit on the eastern Tibetan Plateau. These data are used to constrain the timing and duration of chemical weathering of regional significance. We further compare the observed goethite precipitation ages to time series of independent paleoclimatic proxies from the eastern margin of Tibetan Plateau and the South China block to document the fundamental controls on chemical weathering in southeastern Asia and to provide new insights into the Asian summer monsoon evolution since the late Miocene.
Section snippets
Geological background
The Yulong deposit (31°24′30″N, 97°44′00″E) in the eastern Tibetan Plateau (Fig. 1) is the third largest porphyry Cu deposit in China, with proven reserves of >6.2 Mt Cu and 0.4 Mt Mo (Hou et al., 2003). The ore-related granite porphyries have zircon U–Pb ages of 41.2–40.7 Ma, whereas molybdenite from Cu–Mo sulfide ores has a Re–Os isochron age of Ma (Hou et al., 2003). Copper mineralization occurred at ∼2 km beneath the paleosurface (Tang, 2003), consisting mainly of quartz-sulfide
Sampling and methods
Twenty-six iron oxide samples were collected from a 100-m-deep drill core and two weathering sections well exposed by the open pit mining. Thirteen samples were collected from top to bottom of a single weathering profile revealed by drill hole ZK1217 (Fig. 3A). Five in-situ iron oxide samples were taken from the surface of weathering profiles along section a–b, and the sixth is an Fe-oxide pebble contained in transported cover overlying the weathering profile (Fig. 3B). Similarly, six in-situ
Results
Field observations confirm that iron-oxides investigated in this study formed by intensive weathering and oxidation of the primary Cu–Fe sulfides. Petrographic determinations show that goethite grains typically have massive, botryoidal, or banded textures, indicating direct precipitation from weathering solutions (Figs. 4 and S1). Most goethite grains have homogeneous texture (Fig. 4A), indicating a single episode of precipitation. Some grains consist of micrometric bands (Fig. 4B) or are
Caveats to the analysis and age data limitations
The goethite (U–Th)/He ages may be complicated, and their geological significance may be questionable, due to several potential problems: (1) 4He loss from the iron-oxide structures due to diffusion (Shuster et al., 2005), (2) U and Th loss or gain from goethite by exchange with weathering solutions (Reiners et al., 2014); (3) contamination by other hypogene phases (Vasconcelos et al., 2013); and (4) multiple goethite generations present in a single grain (Monteiro et al., 2014). These
Conclusions
Goethites from weathering profiles in the Yulong porphyry Cu deposit in the eastern Tibetan Plateau yield (U–Th)/He ages between 6.73 and 0.53 Ma. Climatic conditions inferred from these goethite ages are consistent with variation in paleoclimatic conditions deduced from supergene Mn-oxides and loess deposits in southeastern Asia. The observed goethite (U–Th)/He ages suggest monsoonal precipitation controlled the chemical weathering in southeastern Asia, and further reveal a relatively strong
Acknowledgments
We thank Nick Fylstra, Jia Chang and Shentai Liu, and Hongqiang Wang for their help during (U–Th)/He analysis, field work, and sample preparation, respectively. This work was jointly supported by the National Natural Science Foundation of China (41325007), the Ann and Gordon Getty Foundation, and International Postdoctoral Exchange Fellowship Program from China Postdoctoral Science Foundation (2013-81), and National Demonstration Center for Experimental Mineral Exploration Education at China
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