June 2015 LIP of the Month

Origin of hydrothermal deposits in the Emeishan large igneous province

Dan Zhu, Yingkui Xu, Zhilong Huang, Taiyi luo and Jiaxi Zhou

State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China; zhudan@vip.gyig.ac.cn

15 June 2015

Partly based on the paper published in December 2014 issue of Ore Geology Reviews: Xu, Y., Huang, Z., Zhu, D., and Luo, T., 2014, Origin of hydrothermal deposits related to the Emeishan magmatism: Ore Geology Reviews, v. 63, p. 1-8.

Introduction

In the Emeishan large igneous province(ELIP) (Fig. 1), there are many hydrothermal native copper ore deposits and Zn–Pb ore deposits, including the world-class Huize Zn–Pb deposit with 7 million tons (Mt) of Pb and Zn metals (Huang et al., 2004). These Zn–Pb ore deposits, which form the important Sichuan–Yunnan–Guizhou (SYG) Pb–Zn metallogenic province, contain total Pb and Zn metals of more than 20 Mt at grades of > 15% Pb + Zn, and have been the major source of base metals in China in the past several decades. The Pb-Zn deposits are hosted in the late Sinian, Devonian, Carboniferous and early Permian carbonate rocks (Huang et al., 2004), whereas the native copper deposits are restricted to a transitional zone between the uppermost part of the EFB and the Xuanwei Formation (Bing-Quan et al., 2007). All these native copper and Zn–Pb ore deposits are spatially associated with the Permian Emeishan flood basalts (EFB) (Fig. 1), and stable isotope studies indicate that magmatic-hydrothermal fluids had been involved in the ore-forming processes, such as the Zn–Pb deposits (Huang et al., 2004; Zhou et al., 2013). These led many workers suggest that the EFB were an important source of the ore-forming metals and fluids. However, Pb–Zn (192-226 Ma) and native copper deposits (132-228 Ma) in the SYG province are much younger than Permian EFB (259-263 Ma) (Fig. 2). In addition, after the Emeishan magmatism, there was only minor and local Cretaceous and Cenozoic magmatisms in this region, such as the alkaline ultramafic dykes (88-85 Ma) in southwest Guizhou Province (Liu et al., 2010)(Fig. 2). The maximum time interval of these hydrothermal ore deposits is over 100 Ma, and during this time interval, there is no documented magmatism occurring in this region (Huang et al., 2004) (Fig. 2). Thus, if this giant hydrothermal metallogenic system in the ELIP region is really related to the EFB, the 30-100 myr delay in the timing mineralization with respect to the 259-263 Ma emplacement of ELIP must be explained.

Release of magmatic fluids of the underplated Emeishan basalts and the thermal simulation results

The model presented by Xu et al. (2014) involves the underplated Emeishan basalt at the base of crust (Xu and He, 2007; Zhu et al., 2003). Thermodynamic calculations indicate that when the underplated basalts cooled to about 800 °C, begin to release metal-bearing fluid (Fig. 3). The timing of releasing fluids is not sensitive to the initial H2O concentration in the underplated basalts. For example, if 1.2 GPa pressure is assumed for the crystallisation of the underplated basalts, and 0.05 and 0.4 % of water concentration in the underplated basalts are also assumed, the temperatures for releasing fluids (fluid saturation) are 782 and 810 °C, respectively (Fig. 3). Therefore, when the average initial H2O content is set to be 0.25%, the underplated basalts begin to release fluids, when it cooled to about 800 °C (Fig. 3). In addition, the solidus of the melt is 700 °C. The reason for this is that when the total crystallisation of the underplated melts is over 99.9%, the H2O content in the much evolved melt is over the total silicate components at this temperature (calculated by MELTS); thus, this melt can be looked as a fluid, rather than a silicate melt.

The results of thermodynamic calculations for the fluid saturation of the underplated Emeishan basalts successfully change a fluid releasing problem to a problem related to the temperature of the underplated basalts, i.e., a geological problem to a geodynamic problem. The geodynamic of basaltic underplating is well developed and can be easily simulated by numerical methods (Annen et al., 2006). The key for the thermal simulations is the initial and boundary conditions, which can be well constrained by the petrological and geochemical studies in the ELIP (Xu et al., 2014)

Thermal simulations indicate that the releasing of fluids occurs about 30 Ma after the onset of the underplating, consistent with age the oldest hydrothermal deposits in this region, such as the Huize Pb-Zn and native Cu ore deposits. The modelling also indicate that the release of ore-forming fluids from the crystallizing underplated basalts can persist over 100 Ma. The modelled 30-100 myr age delay  almost covers the all age data available for the hydrothermal deposits (Fig. 2), and thus successfully explains the “lack” of temporal association between the hydrothermal deposits and the EFB (Fig. 4, an animation for the thermal evolution of the underplated Emeishan basalts).

Model for the Emeishan hydrothermal ore deposits

Xu et al. (2014) suggest that the underplated Emeishan basalts would become saturated in sulfur before generation of a deuteric aqueous fluid during their cooling, due to the relatively lower solubility of sulfur in silicate melts (Liu et al., 2007). Thus an immiscible sulfide liquid would be formed before a deuteric fluid. Since Cu, Zn and Pb can strongly partition into sulfide liquid, so they would be enriched in this late-forming sulfide liquid. This Cu, Zn and Pb-rich sulfide liquid would be re-dissolved in the deuteric fluid, and the metals that had been scavenged by the sulfide liquid from the silicate melts would also go into the fluid to produce a S and metal-rich ore-forming fluid. This model successfully explains why fluids released by the underplated Emeishan basalts are rich in Cu, Zn and Pb.

The model for the hydrothermal deposits in the ELIP is that when the underplated Emeishan basalts cools to a temperature (800 °C), the interstitial melt in the cumulate becomes saturated with a deuteric fluid. The emitted hot fluid percolated upward through the cumulate pile, and dissolved the sulfide liquid, which is rich in chalcophile elements such as Zn, Pb and Cu, causing the percolated fluid to be enriched in these chalcophile elements. The chalcophile element-rich fluid moves upward along fault zones, probably underwent boiling(s) as a result of depressurization, and thus caused the formation of ore-forming vapor for native Cu mineralization, which precipitated in the relatively impermeable regions to form the native Cu mineralization. Then when the acidic hydrothermal fluid met the carbonate rocks, it would precipitate sphalerite and galena as a result of cooling and reaction with limestone or by mixing with local meteoric waters, to form the carbonate-hosted Pb–Zn deposits in the ELIP.

Implications

Applying the same principle, the Emeishan plume head is also capable of releasing such fluids, since the trapped melt by the plume head would eventually cool to a fluid-saturated state (Fig. 4). Of course, to evaluate this, further work is needed. Besides Pb-Zn and Cu, numerous hydrothermal Hg, Sb, As, Tl, Cd and Au ore deposits occur in the ELIP region (Hu and Zhou, 2012). At present, it is hard to draw a conclusion that the deposits are related to the underplated Emeishan basalts or the Emeishan plume head, since age data are not available and the isotopic studies of these deposits are sparse.

Magmatic underplating is a common feature in LIPs and rifts (Ridley and Richards, 2010), thus the work presented by Xu et al. (2014) may imply that many hydrothermal deposits in LIPs and rifts may be related to the basaltic underplating. An example is  the Keweenawan native copper deposit in Michigan, which is dated between 1060 and 1047 Ma (Bornhorst et al., 1988), which is about 40 Ma younger than the basaltic flows in the same region (Davis and Paces, 1990); its formation may have a link with the basaltic underplating via a similar model. Also the giant Broken Hill Pb-Zn deposit in eastern Australia probably formed in a similar way (Crawford and Maas, 2009).

The recent seismic tomography survey of deep parts of the Yellowstone volcanic system combined local and teleseismic data reveals an underplated magma body, which has a volume of 46,000 cubic kilometers, and which contains a melt fraction of ~2% (Huang et al., 2015). According to Xu et al. (2014), this huge volume of underplated basalts would release chalcophile element-rich fluid and therefore the Yellowstone is a high potential mineral exploration region for magmatic hydrothermal ore deposits.

Acknowledgements

We thank Prof. Richard Ernst and Prof. Yigang Xu for their suggestions given to this article.

References