Genesis of the Xiaotongchang basalt-hosted copper deposit in the Jinping area, SW China: Constraints from geochronology, fluid inclusion and geochemistry

https://doi.org/10.1016/j.oregeorev.2021.104450Get rights and content

Highlights

  • The Xiaotongchang copper deposit mainly formed at 260 Ma and 245 Ma.

  • The metallogenic stage in Yanshanian is proposed by Re-Os analysis.

  • The characteristics of two stages of ore-forming fluid are proposed.

  • A simple metallogenic model of basalt-hosted copper deposit is proposed.

Abstract

The Xiaotongchang basalt-hosted copper deposit in the Jinping area, SW China, is located in the southwestern margin of Ailaoshan orogenic belt. Here, we use H-O isotope, quartz fluid inclusion, sulfide Re-Os and zircon U-Pb isotopic dating of copper ore and element contents of basalts to constrain the age and genesis of the deposit. The basalts in the study area formed in the shallow part of the Emeishan mantle plume by low degree of assimilation-contamination in intraplate environment. Investigations revealed two stages of mineralization in the deposit. The ore mineral assemblage in the early stage was pyrite-chalcopyrite-calcite. The early-stage ore-forming fluid was characterized by medium–high temperature (206.6 °C–279.8 °C) and high salinity (12.28%–22.44%), and it was related to water-rock interaction between the Emeishan sulfur-carbonate-bearing volcanic hydrothermal fluid and gypsum layer in the Carboniferous, and experienced boiling. The ore mineral assemblage in the main metallogenic stage is quartz-chalcopyrite-bornite-calcite. Re-Os dating of chalcopyrite in the main stage in No. 1 ore body yielded an age of 246 ± 17 Ma, and zircon ages of copper ore in No. 3 and No. 4 ore bodies revealed that they formed at 245.6 ± 1.0 Ma and 244.9 ± 2.0 Ma, respectively, suggesting that the age of main metallogenic stage is ca. 245 Ma. The ore-forming fluid in the main metallogenic stage was characterized by high temperature (294 °C–425.4 °C) and low salinity (6.3%–16.05%). Combined with analysis of H-O isotope of quartz samples, we suggested that the main-stage hydrothermal fluid was mainly derived from intermediate-acid magmatic water and experienced degasification. The organic substances may act as the reducing agent during the mineralization process. The main-stage mineralization was related to the continental collision between the Simao-Indosinian massif and the Yangtze massif in the Middle Triassic. Re-Os analysis in No. 3 ore body indicated that the deposit was reformed by later hydrothermal fluid in Yanshanian period.

Introduction

Basalt-hosted copper deposits, known as volcanic redbed copper deposits, were first proposed by Kirkham and were mainly divided into two types: sulfide copper deposits and native copper deposits (Kirkham et al., 1984, Kirkham, 1993). Distributing all over the world, the basalt-hosted copper deposits are mostly associated with mantle plume, such as the Keweenaw-type copper deposits in America, basalt-hosted copper deposits in SW China (Kirkham et al., 1984, Zhu et al., 2002b, Zhu, 2003, Symons and Kawasaki, 2019) and the Brazil native copper hydrothermal deposit in the Paraná volcanic province (Baggio et al., 2017). Previous researches suggested that there are some basalt copper deposits that have no relation with magmatic activity of mantle plume, such as the Dochileh stratiform copper deposit in the Sabzevar Zone of northeastern Iran (Ebrahimi et al., 2019), the native copper deposits with metallogenic age concentrating in 310–270 Ma and sulfide copper deposit with an age of 353 Ma in the Jueluotage belt in East Tianshan area, Xinjiang Province, NW China, which formed in the post-collisional extensional environment (Yuan et al., 2010, Zhang et al., 2013, Wang et al., 2018a). The basalt-hosted copper deposits in the Emeishan Large Igneous Province (ELIP), SW China, were affected both by mantle plume and tectono-magmatic activity, and thus research on the genesis of basalt-hosted copper deposit is important to understand their metallogenic condition.

According to metallogenic factors and geological characteristics, previous studies divided Emeishan basalt-hosted copper deposits into four types including volcanic eruptive type; volcanosedimentary type; tectonic hydrothermal type and fracture-controlled vein-type (Zhao et al., 2005, Zhu, 2011, Zhao and Xu, 2018, Ding, 2019). Many studies have focused on the genesis of basalt-hosted copper deposits, but the relationship between the Emeishan flood basalt and basalt-hosted copper deposit remains controversial (Li et al., 2004, Zhang et al., 2004, Zhao et al., 2005, Wang et al., 2011b). Some scholars suggested that the Emeishan flood basalt was an important metal source for copper deposits (He et al., 2003, Zhang et al., 2006, Hou et al., 2007, Fu et al., 2012, Cai et al., 2013, Xu et al., 2014). Xu et al. (2014) have presented a model to interpret the lack of temporal association between the basalt-hosted copper deposits and the Emeishan basalt. They considered the Emeishan basalt should be the main source of the ore-forming metals and fluids, and basalts would eventually release metal-bearing fluid after several tens of millions of years. However, the existing data, such as metallogenic ages, which are concentrated in 235–228 Ma and 187–162 Ma (Zhu et al., 2005) and metallogenic fluids of native copper deposits that were characterized by mixing of meteoric water, hot brine of the basin and magmatic water (Li, 2009) are hardly to provide direct evidence for the relationship between the Emeishan basalt and the basalt-hosted copper deposits.

The Jinping basalt belongs to the Emeishan flood basalt (Nian et al., 2006, Zhang, 2006, Wang et al., 2007), and the left slip movement of the Ailaoshan-Red River fault in Cenozoic brought it to the present position (Tang, 2010) (Fig. 1a). High copper content of basalt led it to the main copper-bearing formation in the area, and formed many basalt-hosted copper deposits, such as the Yanpo, Xiaotongchang and Longgu deposits (Fig. 1b). The Xiaotongchang basalt-hosted copper deposit in the Jinping area, Yunnan Province, is located in the junction of the Qinghai-Tibet-Yunnan plate and Ailaoshan metamorphic terrane (Nian et al., 2006, Zhang, 2006, Wang et al., 2007, Shen et al., 2010, Shellnutt, 2014, Zhang et al., 2019) (Fig. 1a). Shen et al. (2019) proposed that the early-stage (256 ± 9.4 Ma) ore body in the Xiaotongchang deposit is syngenetic hydrothermal mineralization related to the eruption of Emeishan flood basalt, and the late-stage (240–231 Ma) ore body is magmatic hydrothermal mineralization related to the Late Triassic rhyolite porphyry. Li et al. (2019b) obtained metallogenic age of 230.6 ± 1.1 Ma in the Xiaotongchang deposit by chalcopyrite Re-Os isotopic analysis, which indicated a close relationship with the late collisional process in the Ailaoshan orogenic belt. The previous studies have suggested that the Xiaotongchang deposit was affected by Emeishan mantle plume and later magmatic hydrothermal fluids based on the geological characteristics and metallogenic age of the deposit, but the main-stage mineralization age is not accurate and the characteristic of ore-forming fluids is unclear.

In this study, we analyze the geochemical characteristics of basalt and minerals in ores to trace the metal source and tectonic setting of the deposit. H-O isotopes and fluid inclusions of quartzs were analyzed to elaborate the characteristics of ore-forming fluid. The zircon U-Pb and chalcopyrite Re-Os isotopic dating of copper ore were taken to constrain the metallogenic age. Then, the genetic model of the Xiaotongchang basalt-hosted copper deposit is proposed. Our study will provide direct evidence for association between the Emeishan flood basalt and basalt-hosted copper deposit, and provide directions for further geological exploration in the ELIP.

Section snippets

Regional geology

The Jinping area is located in the southwestern margin of Ailaoshan orogenic belt, which formed by convergence of the Yangtze massif and Simao-Indosinian massif (Fig. 1a). The exposed faults in the area are mainly NW-trending Red River fault, Ailaoshan fault and Tengtiaohe fault. The regional strata are exposed from Lower Silurian to Quaternary. The Lower Silurian, Tertiary and Quaternary are dominated by clastic rocks, and the Middle-Upper Permian is dominated by basalts, whereas others are

Analytical methods

In order to understand the characteristics of basalts in different eruptive cycles in the Xiaotongchang deposit, ten basalt samples were collected from LD-3, LD-7, LD-8 and LD-10 tunnels, which were located from the first cycle to the fourth cycle. Minerals such as zircon, chalcopyrite, pyrite, quartz and calcite were handpicked from copper ores under a binocular microscope. The purity of each single mineral separate was better than 99%. We cleaned all mineral separates in an ultrasonic bath

Major and trace elements

Basalt samples were collected from the first cycle (JM-154, JM-157), the second cycle (JM-159, JM-161, JM-162, JM-165), the third cycle (JM-175, JM-176, JM-179) and the fourth cycle (JM-169). Most of the samples fell in the alkaline basalt region and four samples (JM-159, JM-175, JM-176, JM-179) fell in the subalkaline basalt region on the Silica-Alkali diagram (Fig. 7). The values of (K2O + Na2O)/(SiO2-39) range from 0.02 to 0.71 (Table 1), suggesting that the Xiaotongchang basalts belong to

Assimilation and contamination

According to analysis of the covariant relationship between the ratios of elements, which have similar total distribution coefficients and are sensitive to assimilation and contamination (such as Ce/Pb, Th/Yb, Nb/Ta, Ta/Yb, K2O/P2O5, Ti/Yb, Zr/Nb, etc.), assimilation-contamination can be accurately indicated and the degree of contamination can be determined (Baker et al., 1997, Macdonald et al., 2001). The diagrams suggest that the Xiaotongchang basalts experienced some degree of assimilation

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.

Acknowledgments

We are grateful to Mu Liu for help with major and trace elements testing, to Yi Zhang for help with fluid inclusion analysis, and to Professor Ryan Mathur for Re-Os isotopic dating. This study was financially supported by the National Natural Science Foundation of China (Grant No. 41502064 and 41602110), the Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J18KA213), the Scientific Research Foundation of Shandong University of Science and Technology for

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