Supercritical fluids as recorded in quartz megacrysts of the Late Jurassic porphyritic granitic dyke in the giant Dongping gold deposit, Northern

Jiuhua Xu1, Haixia Chu2, Deping Chen1 Hao Wei3 Guorui Zhang4 , 1. University of Science and Technology Beijing, China 2. China University of Geosciences, Bejiing 100083, China 3. GEO university, , 050031, China 4. East China University of Technology, Nanchang, 33000, China

Funded by NSFC of China (41672070, 41372096)1 Outline

 Introduction  Geological Settings  Fluid and Melt Inclusions  Dicussions  Conclusions

2 Introduction The giant Dongping gold deposit, hosted in alkaline igneous intrusions, is the first deposit of this kind discovered among the alkaline complex-hosted gold ores in China. There are two different viewpoints concerning the origin of the deposit： (1) ore-forming process was related to the host Devonian alkaline complex (Li et al., 1997; Song and Wang, 1994); (2) main gold mineralization age of the DP was 157-177Ma and ore-forming process was related to Jurassic granitic magma (Lu et al., 1993; Hart et al. 2002）  A newly found porphyritic granitic dyke along a NEE-SWW striking shear zone and contains abundant quartz megacrysts. Exploratory drillings indicated that gold grades of ores in shear zone are as much as 5.96g/t, and Au reserves are estimated to be more than one tonnage. Here we present a debate viewpoint concerning the origin of porphyritic granitic dyke and quartz megacrysts, and the nature of fluids, according to field geology, petrography, fluid and melt 3 inclusion studies, and geochronology Outline

 Introduction  Geological Settings  Fluid and Melt Inclusions  Dicussions  Conclusions

4 Geological Settings

Fig.1 Simplified geological map of the Shuiquangou Alkali Complex and gold deposits in Chongli area, 5Hebei Province, China (modified after Song et al., 1996; Li et al., 2010; Xu et al., 2018) Fig.2 Sketch map of the Dongping gold deposit (Modified from 1:10000 geological map surveyed by Chongli Zijin Mining Co., Ltd, 2012) 6 Fig.3 Simplified profile of 0# exploratory line in the Zhuanzhilia n area (Simplified from Chongli Zijin Mining Co., Ltd, 2012)

7 Porphyritic granitic dyke and quartz megacrysts

Fig. 4 Porphyritic granitic dyke and quartz megacrysts in the Dongping-Zhuanzhilian area A and B: showing the characteristics of porphyritic granitic dyke and its quartz megacrysts in drilling cores; C: Irregular-shaped quartz megacrysts and stockwork veins near the shear zone; D: Irregular-shaped quartz megacrysts cut by gold-bearing stockwork veins in altered porphyritic granitic dyke near the shear zone (Dark grey, See sample location in Fig.2) 8 Fig.5 Microphotos of quartz megacrysts in porphyritic granitic dyke from the Zhuanzhilian area of the Dongping gold deposit A- Rounded quartz megacryst (Qz) and surrounding secondary quartz grains in intensive altered rock, Sample ZK001- 236; B- Lentoid quartz occurring in orientation in the deformed porphyritic granitic dyke, sample ZK002-309.2; C- Strip-like quartz (Qz) in fractured porphyritic granitic dyke, sample ZK002-287; D- Albite veinlets (Ab) in Rounded quartz megacryst (Qz), sample ZK301b-205; E- A tiny shear zone in fractured porphyritic granitic dyke, sample ZK002-309.2 ; F- Quartz-sulfides (Py) vein filling in the fissures of the quartz megacryst (Qz), 9 sample ZK301b-211 zk301-283.5b Tiny feldspar crystals (10- 50μm) occur as solid inclusions within grains of quartz megacryst.

ZK301A-283.5 ZK301A-283.510 Fig. 6 Back scattered electron images of albite and orthoclase in porphyrytic quartz (After Xu et al., 2018) 11 Outline

 Introduction  Geological Settings  Fluid and Melt Inclusions  Dicussions  Conclusions

12 Petrography of Fluid Inclusions

L-V type, V-Ltype, V- L-V type, L-V-S type

V

V V-L

V-L L-V V 13 Quartz megacrysts and fluid inclusions

ZK102-1

14 Quartz megacrysts and fluid inclusions

ZK102-1

15 ZK102-2

Quartz megacrysts and fluid inclusions

16 Mecrothermometry of fluid inclusions

2mm ZK301A-290.3a

Th=400℃, C

Tm,ice=-5.5~ -1.5 Th=390℃, C Sal. = 8.55~2.57 wt%NaCl eqv. (After Bodnar, 1993) According to critical fluids 20μm (Knight and Bodnar, 1989) Th,c=390℃~400 ℃ Sal. =3.2 wt%NaCl eqv. Th=398℃, C ZK301A-290.3a Ph,c=30MPa 17 Mecrothermometry of2mm fluid inclusions

Th=393℃, L

10μm

Th=390~400℃, C

ZK301A-290.3b

10μm 18 Mecrothermometry Th=361℃, L of fluid inclusions Th=355℃, L

2mm 20μm

Th=376℃, L

ZK301A-311.2

Th>376℃,L 10μm 19 Mecrothermometry of fluid inclusions

Broken 2mm

Th=387℃, L 10μm

Tm,ice=-3.5~ -2.0 Sal. = 5.71~3.39 wt%NaCl eqv. (After Bodnar, 1993) 20 Laser Raman spectrum of ZK301A-290.3-3 fluid inclusions

H2O

300

250 ZK301A-290.3-1 200

150

100 H2O 50

0 1000 1500 2000 2500 3000 3500 4000 21 Melt inclusions in quartz megacrysts

Photographs showing a melt inclusion in room temperature (A) and after quenching at 800℃ (B) and 1000℃ (C) (Note: there is a V-L inclusion near the melt inclusion) 22 Fig. 40Ar-39Ar ages of orthoclase in porphyritic granitic dyke from Dongping gold deposit (A and B: Plateau ages; C and D: Isochronal ages 23 Outline

 Introduction  Geological Settings  Fluid and Melt Inclusions  Dicussions  Conclusions

24 The nature of fluids forming quartz megacrysts We inferred that the original fluid during formation of quartz megacrysts could be a supercritical fluid or a melt-fluid mixture (Xiao et al., 2015). Under high temperatures and pressures, water contents in silicate melt will

increase as the pressure rises (Eggler and Urnham, 1984), so do the SiO2 contents in aqueous solution (Manning, 1994; Stalder et al., 2001; Mibe et al., 2002). When the temperatures and pressures in the system reach a certain critical point (the second critical point, or upper critical point), aqueous solution and silicate melt become one phase of complete miscibility, which can be called as supercritical fluids. The supercritical fluids contain much more of various elements than aqueous solution , but

lower than H2O-rich melt (Hermann et al., 2013). Previous studies show nd that the 2 critical point of SiO2-H2O system has pressure of 1.0GPa and temperature of 1080℃ (Kennedy et al., 1962. It can be seen that the homogenization temperature of melt inclusion in quartz megacryst was around 1000℃ and close to the 2nd critical points of mentioned acid aqueous-melt systems. According to zircon U-Pb dating and Ar-Ar ages of orthoclase, the ZZL porphyritic granitic dyke might be originated from

residual supercritical fluids of Jurassic granite in the south of mine area.25 Fig. Three-dimensional phase diagram of supercritical fluids (Modified after Stalder et al., 2000; Mibe et al., 2004) Supercritical fluids are located in the area above the critical curve. Path A shows that a supercritical fluid of one phase become two fluids (melt and H2O fluid), then two phases (solid and H2O fluid) when temperature decreases at certain pressures. Path B shows that a supercritical fluid of one phase become directly into two phases 26 (solid and H2O fluid). The circle in diagram is the second critical end point. The origin of porphyritic granitic dyke A model for the formation of porphyritic granitic dyke and gold deposits in the Dongping area is shown in Fig.15. (1) From Early Devonian to Late Permian, the Siberian Plate collided with the North China Plate, resulting in emplacement of plutonic magmatic rocks widely in regional scale. In the Dongping area, alkaline magma invaded the Archean Chongli group and formed SQG alkaline complex in Middle Devonian. Both the alkaline complex and metamorphic Chongli group might be the source rocks of gold mineralization (Fig.15A). (2) From Triassic, the westward seduction of the Pacific Plate underneath Siberian Plate gave rise to intensive emplacement of granitic magma in east China. During Late Jurassic to Early Cretaceous, granitic magma intruded into the SQG complex and Chongli group to the south of the Dongping area, resulting in the formation of SSQ K-feldspar-rich granite and migration of separated melt-fluid mixtures (supercritical fluids) (Fig.15B).

27 (3) The supercritical fluids moved to fractures and replaced syenite/monzonite of SQG alkaline complex, which brought about the formation of porphyritic granitic dyke near a major shear zone within SQG. As T-P dropped, supercritical fluids became H2O-NaCl-CO2 fluids and leached Au from country rocks (SQG complex and metamorphic Chongli group), forming gold-bearing veins in the fractures in the mine area (Fig.15C). Fig.15 A model for the formation of porphyritic granitic dyke and gold deposits in the Dongping area (Modified from the scientific report of USTB, 2014) 28 Conclusions (1) We conclude that the quartz megacrysts in the ZZL porphyritic granitic dyke are not magmatic “snowball” originated from residual melt of the host Devonian alkaline complex, but are metasomatic metacrystals which were related to Jurassic Shangshuiquan granitic pluton.

(2) The nature of fluids that formed the quartz megacrysts was H2O- silicate supercritical fluids (or melt-fluid mixtures) separated from the late Jurassic K-feldspar-rich granitic magma. Petrography of the porphyritic granitic dyke shows that deformed and mylonitic granite are usually observed near shear zones, accompanied by intensive silicification, which indicates late hydrothermal replacement after the formation of porphyritic granitic dyke. (3) The porphyritic granitic dyke was coeval with the Late Jurassic Shangshuiquan K-feldspar-rich granitic pluton. The supercritical fluids

became H2O-NaCl-CO2 fluids and leached Au from SQG complex and metamorphic Chongli group, as the temperatures and pressures dropped, resulting in the formation of gold-bearing veins in mine area. 29 Thank you for your attenton !

Acknowledgements The program was funded by NSFC(41672070). 30