Genesis of Euhedral Pyrites in the Upper Cretaceous Mengyejing Formation in Simao Basin, Yunnan and Its Geological Significance

doi:10.19657/j.geoscience.1000-8527.2018.01.07

Abstract

Abstract:

The Simao Basin is located in the southeastern part of the Tethyan metallogenic belt. The Mengyejing Formation containing evaporite successions in the Upper Cretaceous is widely distributed in this region. The Mengyejing Formation is currently the only pre-Quaternary solid potash deposit in China. Samples of gypsum and pyrite were collected from the Mengyejing Formation in Jiangcheng County in Simao Basin, and the analysis of major and trace elements and sulfur isotopes of these samples were performed by electron probe micro-analyzer (EPMA) and mass spectrometer. The results show that the S/Fe value of pyrite is 2.02, showing that it belongs to a sulfur-concentration type pyrite. The Co/Ni values range from 3.01 to 10.74, and the varying range is larger.The δ³⁴S_V-CDTvalues of pyrite are in the range of -8.36‰ to -3.36‰ with an average of -6.33‰, while the δ³⁴S_V-CDTvalues of gypsum are in the range of 6.6‰ to 10.7‰ with an average of 9.48‰. The values of S/Fe, Co/Ni and sulfur isotope of pyrites and the values of sulfur isotope of gypsums suggest that the pyrite in the Mengyejing Formation in Jiangcheng County is formed in hydrothermal processes. The hydrothermal alteration causes the gypsum to be reduced to form S^2-. S^2- formed the granulated pyrite combined with Fe from the hydrothermal fluid. Through this study of the genetic mechanism of pyrite and gypsum of the Mengyejing Formation,it comes to the conclusion that the primary evaporites of the Mengyejing Formation in the Upper Cretaceous might be experienced obvious alterations by deep hydrothermal fluids accompanied by several kinds of metals, which is controlled by the Indian and Asian collision,and the hydrothermal activities in Simao Basin recrystallized the potash minerals of the Mengyejing Formation, but might have favorable contribution to the metal deposition.

Key words: Mengyejing Formation, Simao Basin, pyrite, gypsum, genesis, sulfur isotope

CLC Number:

P578.2

DONG Haowei, WANG Licheng, LIU Chenglin, WEI Yushuai, WANG Yanlu, BO Ying. Genesis of Euhedral Pyrites in the Upper Cretaceous Mengyejing Formation in Simao Basin, Yunnan and Its Geological Significance[J]. Geoscience, 2018, 32(01): 77-85.

Figures/Tables 8

References 37

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样品号	Se	As	S	Pb	Cu	Bi	Fe	Co	Zn	Ni
Py1	0.002 9	0.022 1	53.133 3	0.007 7	0.005 0	0.014 2	45.600 4	0.060 9	0.020 0	0.000 9
Py2	0.005 3	0.016 3	52.936 2	0.012 3	0.010 0	0.001 2	45.657 7	0.062 4	0.015 4	0.000 6
Py3	0.003 1	0.007 8	52.997 6	0.043 2	0.007 3	0.029 7	45.532 5	0.055 6	0.020 0	—
Py4	0.003 1	0.012 9	52.905 5	0.005 6	0.010 1	0.001 2	45.776 6	0.058 7	0.017 6	0.003 9
Py5	0.003 1	0.013 9	53.006 7	0.007 1	0.004 6	0.009 8	45.799 9	0.062 0	0.011 6	—

样品号	Se	As	S	Pb	Cu	Bi	Fe	Co	Zn	Ni
Py1	0.002 9	0.022 1	53.133 3	0.007 7	0.005 0	0.014 2	45.600 4	0.060 9	0.020 0	0.000 9
Py2	0.005 3	0.016 3	52.936 2	0.012 3	0.010 0	0.001 2	45.657 7	0.062 4	0.015 4	0.000 6
Py3	0.003 1	0.007 8	52.997 6	0.043 2	0.007 3	0.029 7	45.532 5	0.055 6	0.020 0	—
Py4	0.003 1	0.012 9	52.905 5	0.005 6	0.010 1	0.001 2	45.776 6	0.058 7	0.017 6	0.003 9
Py5	0.003 1	0.013 9	53.006 7	0.007 1	0.004 6	0.009 8	45.799 9	0.062 0	0.011 6	—

样品号	Co均值/10^-6	Ni均值/10^-6	Co/Ni
Py1	609	57	10.738 2
Py2	624	120	5.200 0
Py3	556	—	—
Py4	587	195	3.007 7
Py5	620	—	—

样品号	Co均值/10^-6	Ni均值/10^-6	Co/Ni
Py1	609	57	10.738 2
Py2	624	120	5.200 0
Py3	556	—	—
Py4	587	195	3.007 7
Py5	620	—	—

采样位置		岩性	样品号	δ³⁴S_V-CDT /‰	⁸⁷Sr/⁸⁶Sr	参考文献
思茅盆地南部(江城)	张家寨	黄铁矿	20160401JC04-05-1	-8.11		本文
	张家寨	黄铁矿	20160401JC04-05-2	-6.94
	张家寨	黄铁矿	20160401JC04-05-3	-8.36
	张家寨	黄铁矿	20160401JC04-05-4	-8.07
	张家寨	黄铁矿	20160401JC04-05-5	-6.97
	张家寨	黄铁矿	20160401JC04-05-6	-3.80
	张家寨	黄铁矿	20160401JC04-05-7	-5.28
	张家寨	石膏	20120503-G1		0.708 559	本文
	张家寨	石膏	20120503-G2	10.70	0.706 729
	张家寨	石膏	20120503-G3	10.40	0.707 445
	国庆乡	石膏	20120503-G4	6.60
	国庆乡	石膏	20120503-G5	10.20
	勐野井	石膏		20.58	0.707 121~0.711 069	文献[21]
思茅盆地北部	会泽	铅锌矿			0.716 295	文献[22]