Fluid Inclusion Study of the Lu'erba Au Deposit in Gansu Province: Discussion on Fluid Evolution and Metallogenic Mechanism

doi:10.19657/j.geoscience.1000-8527.2021.111

Abstract

Abstract:

The western Qinling Mountains is rich in mineral resources with huge gold reserves. Chinese scholars have carried out a lot of studies on the properties and sources of ore-forming fluids in many Au deposits in the region, but there is no unified view on it. In addition, research on ore-forming fluids in the Lixian-Minxian gold belt, Hezuo-Lu'erba-Yawan Au-Hg-Sb polymetallic ore sub-belt in the region is relatively weak. The Lu'erba is a typical Au deposit hosted by Middle Triassic turbidite in the ore belt. Fluid inclusions in the main mineralization stage (calcite-quartz veins) are dominated by H₂O-rich vapor-liquid inclusions, with minor pure vapor inclusions, liquid inclusions, and CO₂-H₂O three-phase inclusions. Homogenization temperatures of the fluid inclusions range from 81 to 247 ℃, with salinities varying from 1.23% to 10.98%. Vapor phases are mainly composed of H₂O, with minor of CH₄, CO₂, and H₂. The δD_V-SMOW values of the ore-forming fluids are from -84.4‰ to -96.0‰, and δ¹⁸O values are from -4.20‰ to 6.45‰, indicative of magmatic-meteoric fluid mixing. The structural transition may have led to ore-fluid boiling, resulting in extensive gold precipitation.

Key words: Lu'erba Au deposit, fluid inclusion, ore-forming fluid, laser Ramam spectrum, hydrogen and oxygen isotopes

CLC Number:

P618.51

CHANG Ming, LIU Jiajun, YANG Yongchun, ZHAI Degao, ZHOU Shuming, WANG Jianping. Fluid Inclusion Study of the Lu'erba Au Deposit in Gansu Province: Discussion on Fluid Evolution and Metallogenic Mechanism[J]. Geoscience, 2021, 35(06): 1576-1586.

Figures/Tables 11

Fig.1 Geological map of the Lu’erba Au deposit (modified from Lu’erba Gold Company)

Fig.2 Photos showing the characteristics of the Lu’erba Au ore

Fig.3 Thin-section micrographs of the Lu’erba Au ore textures

Fig.4 Petrographic characteristics of fluid inclusions in calcite from the Lu’erba Au deposit

Table 1 Homogenization temperature and salinity of vapor-liquid inclusions from the Lu’erba Au deposit

样号	寄主矿物	阶段	成因类型	数量	大小/μm	气液比/%	均一温度/℃	盐度/%
07LEB-2-2	方解石	Ⅱ	原生	14	3~50	5~40	130~247(166.71)	1.23~8.00(4.39)
07LEB-1	方解石	Ⅲ	原生	13	4~25	5~20	81~161(121.46)	2.74~9.21(5.85)
07LEB-2	方解石	Ⅲ	原生	15	4~12	10~25	111~173(134.60)	3.55~10.98(6.60)

Fig.5 Histograms of homogenization temperatures and salinities of fluid inclusions from the Lu’erba Au deposit

Table 2 Density, trapping pressure and depth data of fluid inclusions from the Lu’erba Au deposit

样号	密度/(g/cm³)	成矿压力/MPa	成矿深度/km
07LEB-2-2	0.82~0.99(0.92)	29.91~64.72(42.86)	1.00~2.16(1.43)
07LEB-1	0.94~1.01(0.98)	21.32~43.37(32.64)	0.71~1.45(1.09)
07LEB-2	0.94~1.00(0.98)	29.64~47.87(36.21)	0.99~1.60(1.21)

Fig.6 Raman spectra of fluid inclusions from the Lu’erba Au deposit (red arrow point to the measurement position)

Table 3 Oxygen and hydrogen isotopic data of samples from the Lu’erba Au deposit

序号	统一编号	样品原号	岩石/矿物	检测结果					资料来源
序号	统一编号	样品原号	岩石/矿物	δ¹⁸O_V-SMOW/‰	δ¹⁸O_流体/‰		δD_V-SMOW/‰		资料来源
1	06LEB	1-1	方解石	17.42		5.89	-96.0	本文
2	06LEB	1-2	方解石	17.98		6.45	-85.0	本文
3	23826	073	石英	20.20		2.63	-84.4	文献[17]
4	23827	093	石英	17.40		-0.17	-89.1	文献[17]
5	23828	096	石英	18.40		0.83	-87.7	文献[17]
6	23829	S-3	毒砂	-4.20		-4.20	-86.8	文献[17]

Fig.7 δ18O-δDV-SMOW plot of the Lu’erba Au ore-forming fluids (base map after ref.[21])

Fig.8 Water-rock hydrogen-oxygen isotopic exchange evolutionary curves at stage Ⅱ and Ⅲ (base map after ref.[22])

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