Unexpected high-temperature behaviours of sulphate-water systems and their implications fo r rare earth mineralisation

Zhong, R.C.

^1*

, Cui, H.

, Xie, Y.L.

, Brugger, J.

^2,3

, Yuan, X.Y.

⁴

, Chen, H.

, Liu, W.H.

⁵

& Yu, C.

1Civil and Resource Engineering School, University of Science and Technology Beijing, China; ²School of Earth, Atmosphere and the Environment, Monash University, Australia; ³South Australian Museum, Australia; ⁴MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of

Geological Sciences, China; ⁵CSIRO Minerals Recourses, Australia; *zhongrichen@126.com Sulphate salts are known for their two

characteristics: (i) high melting temperatures that would not be significantly changed by the presence of water (884, 1069 and 1460 °C for crystalline Na2SO4, K2SO4 and CaSO4 at ambient pressure, respectively), and (ii) retrograde solubility, i.e., solubility decreasing with increasing temperatures.

This implies that high-temperature geofluids should be expected to be poor in sulphate. However, sulphate-rich fluid inclusions containing sulphate daughter minerals are known to occur. A typical example is the syn-ore inclusions at the giant Maoniuping rare earth deposit in southwest China, which contain up to 70-75 vol% of hydrated K-Na-Ca-Sr-Ba sulphates as daughter minerals (Xie et al., 2014). Furthermore, microthermometric analysis of these inclusions shows that the sulphate daughter minerals melt at ~335 to 350 °C upon heating, a temperature range significantly lower than the expected melting points of sulphate salts with or without water.

In this study, we conducted hydrothermal diamond anvil cell (HDAC) experiments in the Na2SO4-SiO2-H2O and Na2SO4-Nd2(SO4)3-SiO2 -H2O systems to simulate the high-temperature behaviour of sulphate in the presence of excess quartz. The SiO2-saturated systems are more close to the natural crustal geofluids compared to binary sulphate-water systems. Sulphate-oversaturated systems were prepared at room temperature by loading sulphate crystals, sulphate-saturated aqueous solution, and a piece of quartz as starting materials. The experiments were conducted from room temperature to ~550 °C. In the Na2SO4-SiO2 -H2O system, the experimental system successfully reproduced the phase transitions observed in nature. The melting of the Na2SO4 crystal initiated at ~270 °C (solidus temperature), and total homogenisation of the system took place at >~330

°C upon dissolution of sulphate melt into the aqueous solution. Similar phase transitions were observed in the Na2SO4-Nd2(SO4)3-SiO2-H2O system, and prograde dissolution of the Na-Nd-sulphate melting led to the formation an extremely Nd- and sulphate-rich fluid at ~420 °C.

For comparison, a quartz-free experiment was conducted in the Na2SO4-Nd2(SO4)3-H2O system.

Both the Na2SO4 and Nd2(SO4)3 crystals remained in the solid state throught the whole temperature range, and grew progressively upon heating due to

their retrograde solubility, leading to the formation of a REE-poor solution at high temperatures.

References

Xie Y. et al. (2014) Ore Geol. Rev. 70:595-612

Fluid inclusion characteristics of Ma Zhuang lead-zinc deposit in the Yishu fault zone Zhong, S.

^1*

, Chen, Y.

, Liu, T.Y.

, Li, G.H.

, Zhang, S.K.

& Chen, J.J.

1School of Geosciences, China University of Petroleum, China; ²Shandong Institute of Geological Sciences, China;

*s18010052@s.upc.edu.cn

The Yishu fault zone is a large-scale fault zone in eastern China, which is a part of Tanlu fault zone in Shan Dong province. The Yishu fault zone is composed of four faults which strike north-east to north and called respectively Tang wu – Gegou fault, Yishui – Tangtou fault, Anqiu – Juxian fault, Changyi – Dadian fault from west to east. The Ma Zhuang lead-zinc deposit is located in Mazhan Sag which is between Tangwu – Gegou fault and Yishui – Tangtou fault. Ore-bearing fractures parallel the Yishu fault. The mineralization is constrained to the Early-Cretaceous Malangou Formation, comprised of yellow-green pebbly sandstone, tuff sandstone, and fuchsia sandstone. The ore minerals are galena, chalcopyrite, the gangue mineral is quartz, with abundant quartz veins in the host rock. The trend of quartz veins is consistent with the ore-bearing fractures.

With microscopic observation, field investigation and hand specimen analysis, three main stages of fluid activity were identified in study region: (1) the early fluid formed the quartz veins which cut through the wall rock, the quartz crystal are small and the fluid inclusions have irregular outlines with a single contained phase, small volume and distributed along microfractures, (2) the second stage is the metallogenic stage. Quartz cemented the early wall rock and formed breccia. The quartz crystals of this second stage are bigger and euhedral to subhedral. Galena precipitated in the quartz veins of second stage (Fig. 1), (3) the third and final stage is represented by quartz veins cross cutting both of the earlier veins. The fluid inclusions of third stage are secondary and aligned along tensional fractures. The vapor to liquid ratio of the fluid inclusions in metallogenic stage are from 5 % to 10 %, and range in size are from 8 to 30 µm (Fig. 1). Homogenisation temperatures of the quartz-hosted inclusions of Ma Zhuang zinc-lead deposit are 147.8-204.05 °C with salinities ranging from 1.70 to 7.86 NaCl wt%, which indicate the low salinity fluid. Although there is minimal evidence to do so, we postulate, the fluids are the conjugate endmembers of boiling system. Thus, the trapping conditions obtained by PVT simulation show that the trapping temperatures are 179-210 °C and the trapping pressures are between 549 bar to 1571.39 bar (Bodnar et al. 1985; Bodnar and Vityk, 1994).

Because the location of mineral deposit is quite shallow, the metallogenic fluid have been contaminated with meteoric water, resulting in the lower salinity and temperature. Laser Raman spectroscopy on liquid phase of inclusion, indicates that the liquid phases of inclusions are H2O.

Generally, the ore deposit should be classified as an epithermal deposit, with the metallogenic input controlled by tectonic activity along the Yishu fault zone.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No. U1762108) and the Key Research Projects of Shandong Province (No.

2017CXGC1602 and 2017CXGC1608).