Geochemical Characteristics of Mudstone in the Huanhe Formation in the Southwestern Ordos Basin and Their Paleoenvironmental Significance

Abstract

The trace elements and rare earth elements in sedimentary rocks are closely related to the sedimentary environment, especially mudstone. Conducting trace element analysis on mudstone is an effective means of studying palaeosedimentary environments. The Cretaceous period in the southwestern part of the Ordos Basin is an important period in geological history; however, research on its geochemical characteristics and palaeoenvironment is still relatively weak. The Huanhe Formation in the Zhenyuan area was selected as the research object. Based on geochemical analysis methods, such as major elements, trace elements, and rare earth elements, the indicative significance of geochemical characteristics for its sedimentary palaeoenvironment was explored. The research results show that CIA and C contents, Ga/Rb, and K₂O/Al₂O₃ indicated a warm and humid environment. CaO and MgO contents also support that mudstone experienced dry climatic conditions with strong evaporation. Cr and V/(V + Ni) indicate dysoxic to anoxic conditions. Ni/Co vs V/Cr relationship indicates the oxic condition of the depositional environment. The Huanhe Formation was deposited in an environment ranging from dysoxic to anoxic conditions, and during certain periods, there were also oxic environments present. The ratios of Sr/Ba and Th/U indicate deposition in saline water.

1. Introduction

In recent years, with the application of trace element content and their ratios in mudstone in paleosedimentary environments and paleoclimate changes, the geochemical characteristics of elements in sedimentary rocks have become an effective method for analyzing paleoenvironments. − The content of trace elements and their ratios, such as Li, B, Sr, V and the ratio of Sr/Ba, can be used for the analysis of marine and terrestrial environments. The method of Li, B, Sr and other elements and the ratio of Sr/Ba is a good reflection of paleosalinity. The ratios of elements such as V, Ni, Ba, Zn, and Co to the contents of Sr/Ba and Sr/Cu are good indicators for exploring the paleoclimate changes in lakes. , Rare earth elements have the advantages of stable chemical properties and being less affected by various geological processes. Therefore, they are widely used by geologists to discuss the provenance, paleoenvironment, and paleoclimate of basin sediments. In recent years, many scholars have conducted studies on the source, sedimentary structure and paleoenvironmental restoration of the Ordos Basin using elemental geochemistry. However, most of these studies have focused on the Zhiluo Formation of the Jurassic system, while there are relatively few studies on the Cretaceous system. Based on this, this study intends to take the mudstone of the Cretaceous Huanhe Formation in the southwestern part of the Ordos Basin as the research object, and to constrain the paleoenvironment, paleosalinity, paleodepth and paleoredox environment of the mudstone of the Huanhe Formation through the characteristics of major elements, trace elements and rare earth elements as well as previous research results.

2. Geological Setting

The Ordos Basin is located in the western part of the North China Craton. It is a multicycle superimposed basin with stable subsidence in the Paleozoic Era, subsidence and migration in the Mesozoic Era, and peripheral torsion and faulting in the Cenozoic Era. The basin has undergone different evolutionary stages and eventually formed a structural pattern consisting of one depression (Tianhuan Depression), one slope (Northern Shaanxi Slope), two bulges (Yimeng Uplift and Weibei Uplift), and two belts (West Margin Thrust Belt and Western Shanxi Torsion Fold Belt). The tectonic position of the Zhenyuan area is located on the southern edge of the Tianhuan Depression, adjacent to the slope belt of northern Shaanxi in the east and the thrust zone of the western edge in the west (Figure ). The Tianhuan Depression is 600 km long from north to south and 50 to 60 km wide from east to west. In the Paleozoic Era, it was characterized by a westward inclined slope. From the Late Jurassic period, it began to depression and became the subsidence zone at that time, with the subsidence center located in the Shigouyi–Pingliang area. During the Early Cretaceous, as the thrust zone on the western margin continued to advance eastward, the Tianhuan Depression became its prochasm depression. Its structural feature was an asymmetric syncline with a gentle eastern margin and a steep western margin. At that time, the sedimentary center and subsidence center of the basin overlapped here, accepting the thick sediments of the Early Cretaceous and controlling the sedimentary features of the Lower Cretaceous: The sedimentary system on the western margin is mainly composed of near-source coarse clastic deposits, with alluvial (fluvial) fans, fan deltas and braided rivers as the main forms of development. The gentle slope in the east is mainly composed of lake deltas and eolian sand dunes. During the Cenozoic Era, the depression structure was further strengthened, eventually forming its current tectonic form. The secondary folds from north to south include the Sanyanjing Depression, Yanchi Depression, Huanxian Low Uplift, Zhenyuan Depression and Lingtai Depression, etc. They are distributed in an NNW diagonal pattern and formed under the background of EW compression with the effect of right-directional twisting.

1.

Regional geology-structure unit map of Zhenyuan area in southern Ordos Basin and sample sampling location.

3. Samples and Experiments

Twenty-six mudstone core samples were collected from the Huanhe Formation of the ZKH5 borehole in the southern Ordos Basin and the determination of major and trace elements was conducted at the Analysis and Testing Center of the Beijing Research Institute of Nuclear Geology. The sample should first be rinsed with clean water and then ground to less than 200 mesh in an agate mortar. The ground samples were divided into two parts, which were respectively used for the determination of major and trace elements. The major elements were determined by X-ray fluorescence spectrometer, and the standards followed were GB/T14506.28–2010 and GB/T14506.14–2010. Trace elements were determined by inductively coupled plasma mass spectrometry, and the standards followed were GB/T 14506.30–2010 and GB/T 14506.28–2010. First, place the sample in a muffle furnace and calcine it at 500 °C to remove the organic matter in the mudstone and the crystalline water in the minerals. Accurately weigh 50 mg of the sample again and seal and dissolve it with HF+HNO3. The determination was carried out after standing for 24 h. By comparing the determination results of standard samples and parallel samples, the error is less than 5%.

4. Sedimentary Characteristics of the Huanhe Formation

In the Zhenyuan area, the Huanhe Formation (K₁ h) has developed a set of alluvial fans, braid-shaped rivers, braid-shaped river deltas, and lacustrine deposits. The uranium content of the sandstone in the Huanhe Formation is as high as (2.51–4.47) × 10^–6. According to the stratum structure, lithological assemblage and sedimentary assemblage, the Huanhe Formation is divided into two lithological sections, the upper and the lower: The thickness of the strata in the lower section of the Huanhe Formation (K₁ h ¹) is 220 to 240 m. From west to east, alluvial fans, braided rivers, and braided river delta facies are developed successively. Three to four sedimentary cycles are also developed. The sand bodies are thick-layered, with a thickness of 200 to 240 m. The main development is brownish red, light yellow, gray medium fine sandstone and grayish green medium sandstone, which are the main ore-bearing strata in this area. At the bottom, alluvial fan unequal-thickness conglomerate and sandy conglomerate are developed. The gravel becomes finer radially and has poor sorting. The composition of the gravel is quartzite, gneiss granite, etc. The upper section of the Huanhe Formation (K₁ h ²) is mainly composed of fine clastic deposits, featuring a sedimentary characteristic of being fine at the bottom and coarse at the top. From west to east, braided river deltas, shallow lacustrine facies and semideep lacustrine facies are developed, with 4 to 5 sedimentary cycles. The upper sand body is mainly composed of brownish-red fine sandstone, while the lower part is mainly composed of gray fine sandstone and siltstone, with local intercalation of grayish-green fine sandstone. It is one of the ore-bearing layers in this area.

5. Geochemistry

5.1. Major Elements

The relative contents of 10 major elements in 26 Huanhe Formation mudstone samples from well ZKH5, upper continental crust (UCC), and post-Archean Australian Average Shale (PAAS) are shown in Table . From Table , we can see that the major elements of the Huanhe Formation mudstone are mainly SiO₂, Al₂O₃, Fe₂O₃, CaO, and TiO₂. The highest content of SiO₂ is from 42.5% to 62.6%, with an average value of 52.31%. Al₂O₃ content takes second place, accounting for 9.37% to 19.1%, with an average value of 14.55%; the average content of another major element is less than 10%. MnO is the least abundant oxide, with contents ranging from 0.03% to 0.18% and an average value of 0.06%.

1. Major Elements of Huanhe Formation Mudstone in Well ZKH5.

		Content (%)
Samples	Depth (m)	SiO2	CaO	Al2O3	Fe2O3	K2O	TiO2	Na2O	MgO	P2O5	MnO	CIA	P/Ti	K2O/Al2O3
H5–N1	834.3	45.2	15.40	9.37	3.24	2.94	0.33	1.11	4.25	0.10	0.06	69.73	0.30	0.31
H5–N2	840.5	42.5	15.50	10.2	3.83	2.68	0.46	1.26	4.58	0.13	0.08	72.03	0.29	0.26
H5–N3	847.9	50.6	7.30	13.4	5.99	4.35	0.54	1.27	4.36	0.14	0.06	70.38	0.27	0.32
H5–N4	855.6	55.6	8.01	12.8	3.93	4.61	0.48	1.49	2.62	0.13	0.07	67.64	0.27	0.36
H5–N5	863.5	53	6.97	13.2	5.17	4.24	0.56	1.28	4.18	0.15	0.06	70.44	0.26	0.32
H5–N6	870	52.4	1.71	17.6	8.28	6.12	0.70	1.34	4.22	0.14	0.05	70.17	0.20	0.35
H5–N7	871.5	52.1	3.93	16.1	6.76	6.09	0.62	1.27	3.79	0.15	0.07	68.57	0.24	0.38
H5–N8	872	53.1	3.09	16.7	7.2	6.02	0.69	1.43	3.60	0.14	0.06	69.09	0.21	0.36
H5–N9	873.5	51.5	2.84	17.3	7.41	6.63	0.65	1.18	3.99	0.14	0.06	68.84	0.21	0.38
H5–N10	874.5	52.4	2.85	17.8	6.35	6.65	0.64	1.28	3.77	0.14	0.05	69.12	0.22	0.37
H5–N11	875.5	54.1	1.75	18.1	6.64	6.68	0.73	1.35	3.72	0.18	0.04	69.21	0.25	0.37
H5–N12	880.6	52.6	10.2	11.6	3.89	3.7	0.55	1.88	2.92	0.15	0.05	67.4	0.28	0.32
H5–N13	891	50.3	11.6	10.7	3.75	2.86	0.47	2.10	4.37	0.15	0.04	68.18	0.33	0.27
H5–N14	898	53.5	8.94	11.6	4.51	2.8	0.54	1.75	3.74	0.15	0.06	71.7	0.27	0.24
H5–N15	913.88	53.6	7.32	11.9	3.86	5.15	0.54	1.32	4.48	0.36	0.05	64.7	0.67	0.43
H5–N16	949.5	55.3	1.15	17.5	7.28	7.28	0.68	0.98	3.40	0.16	0.03	67.9	0.23	0.42
H5–N17	951	62.6	1.22	14.9	5.54	5.98	0.58	0.88	2.77	0.17	0.03	68.43	0.29	0.40
H5–N18	960.3	54.2	0.6	18.6	6.99	7.81	0.70	0.80	3.53	0.15	0.03	68.34	0.22	0.42
H5–N19	988.2	51.3	5.79	14.2	5.36	4.29	0.55	2.03	4.81	0.14	0.06	69.09	0.26	0.30
H5–N20	996.6	53.1	3.65	15.9	6.66	5.39	0.63	1.51	4.16	0.13	0.06	69.66	0.20	0.34
H5–N21	1020.1	54	2.92	16.4	6.89	5.85	0.67	1.46	3.67	0.14	0.05	69.1	0.20	0.36
H5–N22	1031	53.6	3.86	15.3	6.19	6.53	0.62	1.41	3.89	0.16	0.09	65.77	0.26	0.43
H5–N23	1039	62.5	2.79	11.9	5.56	5.16	0.46	0.85	3.62	0.10	0.05	66.38	0.21	0.43
H5–N24	1046.5	51.2	1.24	19.1	7.38	8.69	0.66	0.93	3.66	0.15	0.04	66.47	0.23	0.45
H5–N25	1055.1	44.1	9.91	11.4	4.90	4.52	0.51	0.91	7.00	1.12	0.18	67.67	2.22	0.40
H5–N26	1066.1	45.7	6.27	14.7	6.06	5.71	0.65	0.80	6.37	0.17	0.10	69.27	0.25	0.39

5.2. Trace Elements

Table displays the trace element concentration of the mudstone of the Huanhe Formation. One crucial indicator for element enrichment is the enrichment factor (EF). , By comparing the trace element content of the Huanhe Formation mudstone with the Global Average Shale (AS), the trace element enrichment factors in the research area are computed as follows:

$$X_{EF}=\frac{{(X/Al)}_{\mathit{samples}}}{{(X/Al)}_{AS}}$$ 1

where X and Al denote the concentrations of elements X and Al. , XEF < 1 indicates element X’s dilution concentration compared to the standard composition, XEF > 1 indicates relative enrichment compared to the AS concentration, XEF > 3 indicates detectable autogenetic enrichment, and XEF > 10 indicates moderate to strong autogenetic enrichment. ,

2. Characteristics of Trace Elements in Huanhe Formation Mudstone in Well ZKH5.

		Content (ppm)
Samples	Depth (m)	Be	Sc	V	Cr	Co	Ni	Cu	Zn	Ga	Rb	Sr	Mo	Ba	B	Th	U	Zr	Hf	Sr/Ba	Ga/Rb	V/Cr	C
H5–N1	834.3	1.72	9.18	62.4	40.9	10.8	21.1	59.7	50.5	13	98.1	408	3.75	339	83.9	9.35	5.1	112	3.96	1.20	0.98	1.53	0.18
H5–N2	840.5	1.94	10.3	69.6	44.4	9.45	19.2	20.5	53.1	14.5	92.7	401	1.07	364	73	10.2	7.1	137	4.77	1.10	1.15	1.57	0.19
H5–N3	847.9	2.77	11.4	83.7	56	14.4	30.9	29.5	67.6	17.9	130	370	1.47	439	153	13.6	4.1	148	4.68	0.84	1.01	1.49	0.23
H5–N4	855.6	1.81	12.7	63	43.4	15.3	32	35.2	56	15.5	125	514	3.14	1615	115	12.8	4.36	159	4.87	0.32	0.91	1.45	0.07
H5–N5	863.5	2.36	12.8	99.4	51.5	14.5	35.4	17.4	55.4	19.9	136	335	1.41	480	118	13	4.95	171	5.53	0.70	1.08	1.93	0.25
H5–N6	870	3.56	20.9	103	70.3	21.7	54.5	50.2	86.1	28.2	180	272	0.527	498	190	18.7	4.58	219	6.64	0.55	1.15	1.47	0.33
H5–N7	871.5	2.94	17.9	141	85.3	16.6	40.2	29.7	79	25.4	183	571	1.01	683	222	17	3.48	196	6.01	0.84	1.02	1.65	0.23
H5–N8	872	2.9	18.1	96.2	61.7	14.3	38.4	29.3	81.5	25.6	163	310	0.819	495	217	16	3.46	191	6.26	0.63	1.16	1.56	0.26
H5–N9	873.5	3.23	20.3	89.6	76.8	15.9	35.4	20.7	81.4	26.4	203	289	0.786	462	270	18.7	4.22	194	6.19	0.63	0.96	1.17	0.29
H5–N10	874.5	3.64	19.7	101	85.6	19.9	42.1	48.1	71.2	30.5	193	291	0.806	502	211	17.5	5.06	204	6.89	0.58	1.16	1.18	0.31
H5–N11	875.5	3.23	21.1	103	87	22.1	50.1	47.3	73.6	29.4	174	257	0.453	477	195	17.5	4.71	218	6.77	0.54	1.25	1.18	0.36
H5–N12	880.6	2.1	10.6	96.9	54.6	10.5	23.3	26.3	72.2	18.6	100	506	1.03	552	85.8	11.5	5.68	190	5.53	0.92	1.37	1.77	0.18
H5–N13	891	1.95	9.31	89	45.1	11.1	23	22.9	62	14.9	103	583	1.47	450	112	10.3	5.93	154	4.98	1.30	1.07	1.97	0.16
H5–N14	898	1.76	10.1	91.9	52	11.1	25	21.3	55.8	18.5	95.5	372	1.66	487	76.4	11.3	7.6	184	6.01	0.76	1.43	1.77	0.21
H5–N15	913.88	1.93	9.46	80	55.8	11.7	25.9	26	95.8	16.6	95.7	287	1.35	568	92.6	9	3.62	168	4.7	0.51	1.28	1.43	0.20
H5–N16	949.5	3.62	18.6	95.1	64.1	12.6	34.1	40.9	77.3	24.2	181	132	0.48	442	202	16.9	3.27	241	6.26	0.30	0.99	1.48	0.36
H5–N17	951	2.55	12.1	117	84.4	12.8	31.3	17.9	71.6	20.9	133	124	0.674	483	143	20.3	2.76	200	5.22	0.26	1.16	1.39	0.41
H5–N18	960.3	4.14	21.3	170	99.2	16.1	37	119	88.1	28.7	189	128	0.526	481	222	17.4	3.94	249	6.45	0.27	1.12	1.71	0.53
H5–N19	988.2	2.33	13.2	109	60	14.9	32.1	38.5	65.4	21.7	116	833	6.06	495	142	13.8	5.09	146	4.8	1.68	1.38	1.82	0.16
H5–N20	996.6	2.49	16.7	124	88.6	17.8	37.5	44.9	77.1	24.4	143	314	0.828	431	174	14.2	3.45	159	5.24	0.73	1.26	1.40	0.36
H5–N21	1020.1	2.96	17.7	112	83.2	19.9	38.7	26.6	91.8	24.6	172	268	0.451	463	223	16.1	3.39	196	5.38	0.58	1.05	1.35	0.35
H5–N22	1031	2.7	15.9	103	76	22.9	40	23	74.1	22.5	154	268	0.596	466	215	14.5	2.82	182	5.15	0.58	1.08	1.36	0.33
H5–N23	1039	2.06	11.2	129	62.1	19.1	73.3	39.5	132	19.6	113	123	1.26	412	111	12.5	2.84	134	4.89	0.30	1.28	2.08	0.53
H5–N24	1046.5	3.71	19.7	120	91.8	16.4	45.7	32.8	106	26.8	208	113	0.667	556	249	17.6	4.35	199	5.07	0.20	0.95	1.31	0.41
H5–N25	1055.1	2.48	14.3	88.1	51.5	11.5	22	5.48	77.2	15.9	108	232	0.275	306	111	17.7	1.28	146	4.29	0.76	1.08	1.71	0.32
H5–N26	1066.1	2.88	15.2	141	70.8	14	28.6	20.7	67.6	20.4	146	126	0.363	378	152	21.6	3.3	158	4.51	0.33	1.03	1.99	0.50

Figure and Table present the enrichment factors of Huanhe Formation mudstone in the study area. It can be seen from Figure and Table that Sc (1.02–1.61, mean = 1.31), Ga (1.06–1.51, mean = 1.31), Rb (0.96–1.40, mean = 1.17), B (1.10–2.61, mean = 1.78), Th (1.05–2.16, mean = 1.44), U (0.51–3.14, mean = 1.4), Zr (1.04–1.71, mean = 1.30), Hf (1.58–3.09, mean = 2.27) is enriched in the Huanhe Formation mudstone compared with AS, and other elements are not enriched.

2.

AS standardized multielement diagrams of Huanhe Formation mudstone in the study area.

5.3. Rare-Earth Elements

The rare-earth element (REE) content of Huanhe Formation mudstone in the study area is shown in Table . According to Table , the ΣREE content of Huanhe Formation mudstone ranged from 134.685 to 230.965 ppm, with an average value of 183.609 ppm. The light rare-earth element (LREE) content was the highest (mean value = 158.023 ppm), followed by medium rare-earth element (MREE) (mean value = 19.554 ppm) and heavy rare-earth element (HREE) (mean value = 6.031 ppm) in that order. After chondrite standardization, Huanhe Formation mudstone shows a right-dipping REE distribution pattern (Figure ); (La/Yb)_N is 8.587–14.416, with an average value of 11.109.

3. Characteristics of REE in Huanhe Formation Mudstone in Well ZKH5.

		Content (ppm)
Samples	Depth (m)	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	ΣREE	LREE	MREE	HREE	(La/Yb)CN
H5–N1	834.3	31.1	56.3	7.31	25.6	4.73	0.913	4.41	0.713	3.57	0.676	2.06	0.295	2.07	0.285	140.03	120.31	15.01	4.71	10.15
H5–N2	840.5	33	58.7	7.52	28.2	5.23	0.976	4.55	0.735	4.05	0.739	2.22	0.339	2.02	0.227	148.51	127.42	16.28	4.81	11.04
H5–N3	847.9	38.2	66.6	8.67	34.1	5.95	1.24	5.19	0.86	4.59	0.936	2.56	0.408	2.62	0.241	172.17	147.57	18.77	5.83	9.85
H5–N4	855.6	39.9	69	8.19	31.4	5.42	1.18	5.55	0.871	4.11	0.772	2.32	0.397	2.52	0.249	171.88	148.49	17.90	5.49	10.70
H5–N5	863.5	37.9	73	9.13	35.8	6.44	1.27	5.03	0.771	4.39	0.879	2.63	0.377	2.43	0.368	180.42	155.83	18.78	5.81	10.54
H5–N6	870	51.7	93.6	10.8	39.9	7.78	1.67	6.94	1.2	5.61	1.07	3.41	0.534	3.44	0.349	228.00	196.00	24.27	7.73	10.16
H5–N7	871.5	46.8	79.3	9.26	35.9	7.19	1.32	6.21	1.05	5.18	0.94	3.02	0.481	2.95	0.326	199.93	171.26	21.89	6.78	10.72
H5–N8	872	43.9	73.4	8.81	39.8	6.97	1.37	6.27	0.921	4.78	0.971	3.23	0.508	2.86	0.333	194.12	165.91	21.28	6.93	10.37
H5–N9	873.5	48.5	88.2	10.2	38.7	6.72	1.38	6.85	1.05	6.08	1.02	2.78	0.536	3.25	0.342	215.61	185.60	23.10	6.91	10.08
H5–N10	874.5	44.5	84.5	10.6	40	6.96	1.39	6.3	1.05	5.57	1.05	3.22	0.506	3.09	0.365	209.10	179.60	22.32	7.18	9.73
H5–N11	875.5	50.7	85.9	11.7	48.1	8.88	1.74	7.48	1.25	6.15	1.19	3.88	0.561	3.03	0.404	230.97	196.40	26.69	7.88	11.31
H5–N12	880.6	40.4	65.6	8.23	29.4	4.93	1.08	5.03	0.8	3.89	0.659	2.29	0.317	2	0.269	164.90	143.63	16.39	4.88	13.65
H5–N13	891	34.1	58.2	6.9	26.5	4.7	1.13	4.81	0.801	3.86	0.685	2.12	0.326	2.12	0.253	146.51	125.70	15.99	4.82	10.87
H5–N14	898	37.6	61.4	7.71	31.1	5.42	1.13	4.36	0.739	3.39	0.741	2.46	0.352	2.04	0.291	158.73	137.81	15.78	5.14	12.45
H5–N15	913.88	29.9	54.7	6.25	24.8	4.93	1.01	4.09	0.653	3.09	0.603	2.09	0.294	2.02	0.255	134.69	115.65	14.38	4.66	10.00
H5–N16	949.5	43.3	82.5	10.4	37.6	7.17	1.38	5.97	0.958	5.69	1.03	3.1	0.371	2.51	0.393	202.37	173.80	22.20	6.37	11.66
H5–N17	951	37.2	71.5	8.65	34	7.28	1.51	6.39	0.969	4.51	0.789	2.82	0.369	2.03	0.288	178.31	151.35	21.45	5.51	12.38
H5–N18	960.3	43.5	84.1	9.92	35.8	6.31	1.43	6.13	1.08	5.36	1.04	3	0.446	2.7	0.43	201.25	173.32	21.35	6.58	10.89
H5–N19	988.2	40.5	75.8	9.15	33.1	6.09	1.2	5.48	0.797	4.17	0.846	2.57	0.379	2.43	0.35	182.86	158.55	18.58	5.73	11.26
H5–N20	996.6	46.7	72.5	9.45	34	6.02	1.16	5.7	0.851	4.35	0.881	2.64	0.373	2.5	0.38	187.51	162.65	18.96	5.89	12.62
H5–N21	1020.1	45.2	80.4	9.15	36.1	6.92	1.39	5.97	0.956	5.27	0.986	3.12	0.502	3.04	0.466	199.47	170.85	21.49	7.13	10.05
H5–N22	1031	44.8	76.6	9.09	37.3	6.72	1.48	5.37	0.866	4.65	0.943	2.98	0.507	2.94	0.48	194.73	167.79	20.03	6.91	10.30
H5–N23	1039	35.4	60	7.23	26.5	4.5	0.999	4.08	0.673	3.42	0.642	2.01	0.294	1.89	0.286	147.92	129.13	14.31	4.48	12.66
H5–N24	1046.5	51.2	91.1	10.5	38.3	7.26	1.4	6.3	0.956	4.88	0.861	2.76	0.431	2.4	0.351	218.70	191.10	21.66	5.94	14.42
H5–N25	1055.1	33.8	64.5	7.19	27.9	5.38	1.13	4.89	0.834	4.68	0.941	2.9	0.44	2.66	0.439	157.68	133.39	17.86	6.44	8.59
H5–N26	1066.1	47.9	86.8	10	34.8	6.95	1.58	6.34	0.981	5.01	0.832	2.85	0.445	2.61	0.392	207.49	179.50	21.69	6.30	12.40

3.

Standardized map of rare-earth element chondrite in mudstone of the Huanhe Formation.

6. Data Reliability Analysis

Studies have shown that after rocks undergo late-stage diagenesis, the Ce element in the rocks becomes anomalous, resulting in a strong correlation between δCe and δEu in the rocks. In addition, this can also result in a positive correlation between the δCe and ΣREE of the rocks. Therefore, when using the geochemical indicators of rock samples to reconstruct the paleo-sedimentary environment, it is first necessary to analyze whether there is a correlation between δCe and δEu, as well as between δCe and ΣREE in the rock samples, in order to determine whether the rock samples have been affected by later diagenetic processes. Based on the calculation and correlation analysis of δCe, δEu, and ΣREE in the samples (Figure ), the correlation coefficients (R²) between δCe and δEu, and between δCe and ΣREE in the mudstone samples of the Huanhe Formation in the Zhenyuan area at the southwestern margin of the Ordos Basin are 0.16 and 0.12, respectively, which are much less than 1. This indicates that the mudstone samples of the Huanhe Formation were hardly affected by later diagenetic processes. Therefore, the geochemical indices of these mudstone samples can be analyzed to reconstruct the paleo-depositional environment.

4.

Diagram of Ce/Ce*-Eu/Eu* and Ce/Ce*-ΣREE in the mudstone of the Huanhe Formation (a) Diagram of Ce/Ce*-Eu/Eu*; (b) Diagram of Ce/Ce*-ΣREE.

7. Results and Discussion

7.1. Eu Negative Anomaly Indication Significance

There are mainly two reasons for the abnormality of Eu in sedimentary rocks. The first one is the oxidation–reduction condition. Under oxidation conditions, Eu²⁺ turns into Eu³⁺, which will lead to the loss of Eu. The second reason is the inherited source area characteristics, as Eu is not or less affected by sedimentation and diagenesis. Therefore, Eu anomalies in clastic sedimentary rocks can indicate Eu anomalies in their source regions. Sediments with negative Eu anomalies mainly originate from feldspar crust, while positive or no Eu anomalies indicate the weathering and deposition of magnesia–iron protorocks. The Eu anomaly of sedimentary rocks is slightly affected by sedimentation, but the oxygen fugacity in shallow marine, subtidal, tidal flat, semideep and other sedimentary environments is not sufficient to enrich Eu³⁺ in the environment. There are no obvious Eu anomaly characteristics in seawater and river water, and in continental and marine sediments. Therefore, the obvious negative Eu anomaly in the study area is mainly inherited from the weathering products of the feldspar crust.

7.2. Paleoclimate

Environmental circumstances influence the chemical characteristics of elements and compounds. Sediment geochemistry changes can reveal paleoenvironmental variations during deposition and diagenesis. The primary constituents in sediments (SiO₂, K₂O, CaO, MgO, Na₂O, Al₂O₃, Fe₂O₃, and TiO₂) are redox sensitive markers of paleoenvironmental conditions and easily understood environmental indications. The paleoclimate influences not only the weathering of the parent rock, but also the transit distance of sedimentary detritus and the transfer of nutrients.

The CaCO₃ ratio also shows climate conditions. Arid climates are indicated by high values in closed or semiclosed sedimentary systems, while humid climates correspond to low values. Calcium and magnesium ions generally substitute for one another, but calcium’s ionic radius promotes its greater mobility. Consequently, higher calcium concentrations suggest drier conditions than higher magnesium concentrations. In the Huanhe Formation, calcium content ranges from 0.6 to 15.5 (average 5.65), and magnesium content ranges from 2.62 to 7 (average 4.06). Higher CaO than MgO concentrations in the mudstone suggest formation under dry climatic conditions with strong evaporation. ,

There are many other evaluation indices for paleoclimate, such as the chemical-alteration index (CIA) and the climate index (C). CIA values of 50–65 and C values less than 0.2 suggest a dry and cold climate with low chemical weathering. CIA values of 65–85 and C values between 0.2 and 0.8 indicate a warm and humid climate with moderate chemical weathering. CIA values of 85–100 and C values greater than 0.8 reflect a humid and hot climate with high chemical weathering. The calculation formula for CIA and C is as follows:

$$CIA=\frac{A{l}_2{O}_3\times 100}{A{l}_2{O}_3+N{a}_{2}O+\mathit{Ca}{O}^{*}+K_{2}O}$$ 2

$$C=\frac{Fe+Mn+Cr+Ni+V+Co}{Ca+Mg+Sr+Ba+K+Na}$$ 3

In eq , CaO* only refers to CaO in silicate minerals. Due to the lack of direct measurement means, it is often calculated indirectly by the content of P₂O₅, namely

$${CaO}^{*}=\mathit{mol}(CaO)-\frac{10}{3}\mathit{mol}(P_2{O}_5)$$ 4

where mol (CaO) and mol (P₂O₅) are the mole numbers of CaO and P₂O₅, where when mol (Na₂O) ≤ mol (CaO*), mol (CaO*) = mol (Na₂O); in contrast, when mol (Na₂O) > mol (CaO*), mol (CaO*) = mol (CaO). The CIA values of the Huanhe Formation mudstone in the study area were calculated based on eqs and , ranging from 64.7 to 72.0, with a mean value of 68.67. The climate index (C) is 0.073–0.529 (average = 0.297; Table ). The overall paleoclimate was warm and humid (Figure a).

5.

Paleoclimate of the Huanhe Formation: (a) CIA characteristics of Huanhe Formation mudstone (revised by H.W Nesbitt et al.), (b) cross-plot of K₂O/Al₂O₃ and Ga/Rb. Adapted with permission from [H.W Nesbitt, G. M. Y. Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations. Geochimica et Cosmochimica Acta 1984, 48(7), 1523–1534.] Copyright [1984] [Geochimica et Cosmochimica Acta].

Furthermore, the Ga/Rb vs K₂O/Al₂O₃ cross-plot can analyze the paleoclimate characteristics during the formation of sedimentary rocks. As shown in the cross-plot of Ga/Rb and K₂O/Al₂O₃ (Figure b), almost all points are in the warm/wet area, which indicates that Huanhe Formation mudstone was deposited in a warm and humid paleoclimate. Based on the above analysis, the Huanhe Formation mudstone in the study area was deposited in a warm and humid paleoclimate.

These characteristics indicate that the mudstone of the Huanhe Formation has undergone a transition from warm and humid to dry and hot climatic conditions. Consistent with previous studies suggesting that the primary sedimentary environment of the Huanhe Formation mudstone in the Zhenyuan area was a semiarid to semi humid climate, with a relative lack of organic matter. ,

7.3. Paleoredox Condition

Geochemical indices and isotopic signatures in sedimentary rocks reveal paleo-redox conditions of depositional basins. ,− Redox sensitive trace elements (RSTEs) in sediments exhibit variable enrichment or depletion during deposition contingent upon prevailing redox conditions; consequently, they serve as proxies for reconstructing paleoredox fluctuations. − In stagnant water masses, restricted circulation impedes oxygen distribution, exacerbating anoxia. Consequently, the enrichment of Redox Sensitive Trace Elements (RSTEs) and their ratios serve as valuable proxies for reconstructing oceanic anoxic events throughout geological history. The concentrations of redox sensitive trace elements (RSTEs), including Ni, Cr, Mo, V, U, Cu, Cd, Re, Sb, Tl, and Mn, within fine-grained sediments provide valuable proxies for the reconstruction of paleoredox conditions. ,,,,− Elevated concentrations of uranium, vanadium, and molybdenum suggest anoxic conditions in bottom waters. , Therefore, enrichment factors for these elements, in conjunction with chromium concentrations and ratios of redox sensitive trace elements (RSTEs)specifically V/Ni, V/Cr, Co/Ni, and V/(V + Ni)are utilized in this study to determine the redox conditions prevailing in the depositional basin.

Correspondingly, dysoxic conditions are reflected by a Cr content greater than 40 ppm, and suboxic conditions are shown by a Cr content less than 20 ppm. Furthermore, their suggestion is that Cr amounts between 40 and 20 ppm denote a transitional zone where dissolved oxygen is scarce (Figure ). Studied Huanhe Formation mudstone samples have Cr contents between 40.9 and 99.2 ppm (average 67 ppm), with V + Ni values between 83.5 and 207 ppm (average 138.26 ppm, Table ). Studied Huanhe Formation mudstone samples plot on dysoxic fields, indicating low dissolved oxygen in the depositional environment.

7.

Cross-plot of the studied samples on the redox potential diagram based on the redox sensitive elements of Cr against the V + Ni (revised by Edress, N. et al.). Adapted with permission from [Edress, N. A. A.; Abdel-Rahman, E. A.; Abdel-Wahab, M. G. F. Geochemical significance for the composition and depositional environments of the Campanian carbonate-rich phosphorite, Abu-Tartur plateau, Western Desert, Egypt. Journal of African Earth Sciences 2023, 202.] Copyright [2023] [Journal of African Earth Sciences].

By employing V/Ni ratios, depositional redox conditions are delineated into four primary forms: an environment is deemed oxic when V/Ni falls below 0.8, oxygen-lean when V/Ni is between 0.8 and 1.5, anoxic when V/Ni lies between 1.5 and 6, and sulfidic when V/Ni surpasses 6. Our investigations reveal V/Ni ratios in the samples under study fall between 1.76 and 4.93, which points to anoxic environmental circumstances. The V/Cr ratios also serve as indicators for the redox conditions of the depositional medium, where ratios less than 2, between 2 and 4.25, and greater than 4.25 have been correlated with oxic, suboxic, and anoxic conditions, respectively. The studied samples display V/Cr ratios ranging from 1.17 to 2.08, with an average of 1.57. As depicted in Figure , these samples plot within an oxic area on the Ni/Co versus V/Cr diagram, indicating the oxygenated state of the depositional setting.

6.

Cross plots of trace-element ratios used as paleoredox proxies V/Cr vs Ni/Co (revised by J.R. Hatch et al. and Bryn Jones et al. , ). Adapted with permission from [J.R. Hatch, J. S. L. Relationship between inferred redox potential of the depositional environment and geochemistry of the Upper Pennsylvanian (Missourian) Stark Shale Member of the Dennis Limestone, Wabaunsee County, Kansas, U.S.A. Chemical Geology 1992, 99(1–3), 65–82.] and [Bryn Jones, D. A. C. M. Comparison of geochemical indices used for the interpretation of palaeoredox conditions in ancient mudstones. Chemical Geology 1994, 111(1–4), 111–129.] Copyright [1992 and 1994] [Chemical Geology].

Redox sensitive elements like Co, Mo, U, Th, V, Ni, and Cr are frequently used to determine the redox conditions in ancient waterbodies because they are essential to the preservation of organic matter in sedimentary rocks. U/Th < 0.75, V/Cr < 2, and V/(V + Ni) < 0.45 are indicative of oxic conditions, while 0.75 < U/Th < 1.25, 2 < V/Cr < 4.25, and 0.45 < V/(V + Ni) < 0.84 are indicative of dysoxic conditions, and U/Th > 1.25, V/Cr > 4.25, and V/(V + Ni) > 0.84 are indicative of anoxic conditions, according to prior data. , There is no significant correlation between V, U, and Th and Al₂O₃ contents in the Huanhe Formation mudstone samples, indicating that V, U, and Th contents in Huanhe Formation mudstone are mainly controlled by authigenic deposition under redox conditions. The U/Th, V/Cr, and V/(V + Ni) of the Huanhe Formation mudstone range from 0.07 to 0.9 (mean = 0.31), 1.17 to 2.08 (mean = 1.57), and 0.64 to 0.83 (mean = 0.75). In light of U/Th and V/Cr, Huanhe Formation mudstones were deposited in an oxic environment, and according to V/(V + Ni), Huanhe Formation mudstones were deposited in a dysoxic environment (Figure ).

In order to determine the paleo-redox conditions of depositional basins, Ce/Ce* ratios of chemically precipitated sediments have been used extensively. ,,,− Ce exists in two valence states (Ce³⁺ and Ce⁴⁺) and is redox sensitive. Ce³⁺ is far more soluble in natural water than Ce⁴⁺. Due to its higher particle reactivity, the dissolved Ce³⁺ in natural water oxidizes to Ce⁴⁺ and is eliminated under oxygen-rich conditions, resulting in Ce-depletion in comparison to the nearby REEs. On the other hand, a positive Ce-anomaly results from the reductive dissolution of benthic particles, such as Fe–Mn oxides, in an oxygen-deficient environment. These signatures are inherited by the chemical sediments that precipitate from the water column; as a result, the positive Ce^– anomaly denotes a reduced condition of deposition, while the negative Ce^– anomaly reflects deposition from oxidized water mass. ,,− As demonstrated in Figure , the Chondrite Normalized REE plot exhibits negative Ce anomalies in the studied samples, attributable to the presence of positive La anomalies. The calculated Ce/Ce∗ ratios in the samples range from 0.77 to 0.93 (mean = 0.86), indicating suboxic to anoxic conditions of deposition. Further verification of the nature of the anomalies can be achieved through analysis of the Ce/Ce* vs Pr/Pr* cross-plot. In this diagram (Figure ), none of the sample plots in the true negative Ce-anomaly field III a, and the majority of the samples plotted in field II a showing apparent negative Ce-anomaly caused by La enrichment. The Pr/Pr* values close to 1 reflect a lack of redox cycling of Ce, indicating deposition in anoxic to suboxic conditions. The positive Eu anomalies in a majority of the samples also indicate anoxic depositional conditions and likely hydrothermal supply of reduced Eu²⁺.

8.

Ce/Ce* vs Pr/Pr* diagram of Huanhe Formation mudstones, where Ce/Ce* = Ce_N/(0.5La_N + 0.5Pr_N) and Pr/Pr∗ = Pr_N/(0.5Ce_N + 0.5Nd_N). Field I, no anomaly; Field IIa, apparent negative Ce anomaly caused by positive La anomaly; Field IIb, apparent positive Ce anomaly caused by negative La anomaly; Field IIIa, real positive Ce anomaly; Field IIIb, real negative Ce anomaly (revised by Michael Bau). Adapted with permission from [Michael Bau, P. D. Distribution of yttrium and rare-earth elements in the Penge and Kuruman iron-formations, Transvaal Supergroup, South Africa. Precambrian Research 1996, 79(1–2), 37–55.] Copyright [1996] [Precambrian Research].

From this, it can be seen that the water environment during the sedimentation of the Huanhe Formation mudstone in the study area was a relatively smooth cycle of oxygen rich to sub oxygen rich environment, and the mineralization stage underwent a transition from an oxidizing environment to a reducing environment. The early stage of Yihe Wusu in the Dongsheng area of the northern Ordos Basin was an oxidizing environment, and the mineralization period was a reducing environment, reflecting the similarity of the redox environment in the northern and southern regions of the basin.

7.4. Paleosalinity

Salinity, defined as the concentration of dissolved saline minerals in water, is a pivotal property of water bodies that is utilized reliably as a proxy in paleoenvironmental studies. , The use of geochemical proxies to reconstruct paleosalinity in the mudstones of the Huanhe Formation has yet to be explored in existing literature. Researchers have successfully utilized the ratio of major elements, namely (Fe₂O₃ + CaO + MgO)/(SiO₂ + Al₂O₃), to gain insights into the paleosalinity of water in depositional basins associated with sediments or sedimentary rocks. Research has demonstrated that the ratio of (Fe₂O₃ + CaO + MgO)/(SiO₂ + Al₂O₃) serves as an indicator of aquatic environments, with values less than 0.22 suggesting freshwater conditions and values greater than 0.23 indicating saltwater conditions. The associated rock samples exhibit a ratio of less than 0.22, thereby indicating sedimentation under freshwater conditions. Conversely, the analyzed Huanhe samples present a ratio exceeding 0.23, which implies that the formation of Huanhe mudstone occurred in a saltwater environment. Furthermore, Sr/Ba ratios were initially employed for Cenozoic sequences in China to differentiate water bodies with varying paleosalinities. Previous research has demonstrated that an Sr/Ba ratio of less than 0.2 signifies freshwater, a ratio between 0.2 and 0.5 indicates brackish water, and a ratio exceeding 0.5 represents saline water. The Sr/Ba ratio in the Huanhe samples exceeds 0.5, suggesting deposition in saline water.

Additionally, the Th/U ratio is used to differentiate between terrestrial and marine deposits; terrestrial freshwater, brackish water, and marine saltwater depositional conditions are represented by Th/U ratios of >7, 2–7, and <2, respectively. The Th/U ratio in most of the examined samples ranges from 2 to 7, suggesting that their depositional habitats are brackish to marine saltwater. Furthermore, the deposition of the examined materials in a brackish to saline water medium is supported by the Sr/Ba vs Th/U figure (Figure ). The mudstone of the Huanhe Formation was deposited in brackish and marine saline water conditions, according to the geochemical criteria mentioned above.

9.

Th/U vs Sr/Ba cross-plot showing paleo-salinity sedimentary environments (revised by Fu, J.). Adapted with permission from [Fu, J.; Li, S.; Xu, L.; Niu, X. Paleo-sedimentary environmental restoration and its significance of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin, NW China. Petroleum Exploration and Development 2018, 45(6), 998–1008.] Copyright [2018] [Petroleum Exploration and Development].

Furthermore, a trustworthy indication for determining the salinity of lake waters is CaO/(CaO + Fe₂O₃). In the study area, the mean value of the CaO/(CaO + Fe₂O₃) distribution of the Huanhe Formation mudstone is 0.44, with a range of 0.08 to 0.83. According to the Sr/Ba and B/Ga intersection diagram, the Huanhe Formation mudstones were deposited in freshwater to brackish-water settings, as indicated by the Sr/Ba and CaO/(CaO + Fe₂O₃) intersection diagram (Figure ).

10.

Cross-plot of B/Ga and Sr/Ba (a) and cross-plot of CaO /(CaO + Fe₂O₃) and Sr/Ba (b).

The changes in ancient salinity to some extent reflect the changes in ancient climate, and ancient climate conditions directly control the level of ancient salinity in water bodies through changes in evaporation or rainfall. The mudstone of the Huanhe Formation is generally a brackish water environment with low salinity, and the solubility of uranium in water is high, which can promote the migration ability of uranium in fluids.

7.5. Paleohydrodynamics

Zr is enriched in zircon minerals in sediments, and most of them are deposited in high-energy environments. Rb is basically deposited in low-energy environments and enriched in clay minerals and fine-grained sediments. Therefore, in a high-energy environment, the content of Zr is relatively high while the content of Rb is relatively low, and the larger the Zr/Rb value; Conversely, in a low-energy environment, the content of Rb is higher while the content of Zr is lower, and the Zr/Rb value is smaller. Therefore, Zr/Rb can be used to reflect the changes in paleohydrodynamics (see Table ). Through the statistics of Zr/Rb values of 26 samples from Well ZKH5, the results show that the range of Zr/Rb values is between 0.96 and 1.93, with an average of 1.29. During the sedimentary period of the Huanhe Formation, the Zr/Rb value showed a trend of increasing from small to large and then decreasing, indicating that the water body was relatively stable in the early stage of sedimentation and the hydrodynamic force was relatively weak. In the medium term, the water body is relatively unstable and the hydrodynamic force is strong. Later, it was restored to a sedimentary environment with relatively stable water bodies and weak hydrodynamics (see Figure ).

11.

Geochemical profile of the Huanhe Formation in well ZKH5.

Petrography indicates that there are relatively few interbedded layers in the mudstone of the Huanhe Formation. This is related to the less water disturbance experienced during the sedimentation of the Huanhe Formation, mainly due to weaker hydrodynamic forces during sedimentation.

7.6. Paleobathymetry

Previous research has shown that some elements of the sedimentation process change dramatically with offshore distance. The water depth variation throughout the sedimentation phase can be assessed using these factors. Zr/Al, Rb/K, and MnO content are the most often utilized markers. It is presently thought that the deeper the water and the farther offshore, the lower the Zr/Al ratio or the greater the Rb/K ratio. The Huanhe Formation mudstone’s Zr/Al ranges from 18.89 × 10^–4 to 30.94 × 10^–4 (average = 23.49 × 10^–4), first declining and then increasing with depth; Rb/K varies greatly with burial depth, ranging from 22.39 × 10^–4 to 43.39 × 10^–4 (mean = 33.28 × 10^–4). The ancient water depth during the Huanhe Formation mudstone deposition process has a trend of first falling and then increasing, as indicated by the high-value area of Rb/K being essentially consistent with the low-value area of Zr/Al (Figure ).

Lakes with MnO content below 0.00094% are considered shore lakes, shallow lakes with 0.00094%–0.0075%, intermediate-depth lakes with 0.0075%–0.051%, and deep lakes with >0.051%. The MnO content of Huanhe Formation mudstone ranges from 0.03% to 0.18%, with an average of 0.06%, indicating that it was deposited mostly in an intermediate to deep sedimentary lake environment.

Petrography indicates that the sand bodies of the Huanhe Formation are relatively clean. This is consistent with the sedimentation of the Huanhe Formation in mid to deep lake environments, with deeper depths and less water disturbance.

7.7. Terrigenous Detritus Input

Due of their relative stability during diagenesis, Ti, Si, and Al are frequently employed as markers of debris flux input. , While Al can be found in feldspar, clay minerals, and other aluminum silicate minerals, Ti is typically found in sediments as rutile (TiO₂) or ilmenite (FeTiO₃). In contrast to Ti and Al, Si originates from a variety of sources, including as biological sources as well as hydrothermal and terrigenous clastic input. Consequently, the source of SiO₂ must be examined when it is used as the evaluation index for terrigenous clastic input. The lack of a clear association between Al₂O₃ and TiO₂ and SiO₂ in well ZKH5 of the research region suggests that their sources are more complicated and not primarily terrestrial debris sources. Thus, in this work, the terrestrial debris input during the deposition of the Huanhe Formation mudstone is indicated by Al₂O₃ and TiO₂.

Well ZKH5 has a greater Al₂O₃ composition, ranging from 9.37% to 19.1%, with an average value of 14.55%; TiO₂ ranges from 0.33% to 0.73% (average = 0.58%). As shown in Table , the Al₂O₃ concentration of well ZKH5 fluctuates more with depth, with an overall growing and subsequently reducing trend, whereas the TiO₂ content fluctuates less with depth, with an overall increasing trend. When combined with the results of paleoclimate analysis in the study area, it is discovered that the terrestrial debris intake during the deposition of the Huanhe Formation strata exhibits an increasing and then declining tendency.

The above research indicates that during the sedimentation period of the Huanhe Formation, there was relatively less foreign debris material, which is consistent with the relatively less reducing carbon debris and other debris material shown by petrology. The results of geochemical reactions are consistent with actual observations.

7.8. Paleoproductivity

Paleoproductivity determines the amount of primordial organic materials in sedimentary strata. ,− Although each element has a specific range of applications, the elements P, Si, Ba, Zn, and Cu are indications of the extent of paleoproductivity. For instance, only the biogenic portions of Si and Ba can indicate productivity, and Zn can only represent productivity change in the sulfide reduction environment. ,

P may be used to describe biological productivity since it is not only a crucial nutritional ingredient in biological metabolism but also a significant component of many organisms. To remove the impact of terrigenous detritus, P/Ti and P/Al are frequently used to represent biological productivity. In the research region, the P/Ti of Huanhe Formation mudstone varies from 0.19 to 2.22%, with an average of 0.34% and a generally low productivity. As shown in Table , paleontological production tended to rise and subsequently fall with depth, according to the analysis of the link between P/Ti and depth.

This is consistent with the lithological characteristics of the Huanhe Formation mudstone strata in the Zhenyuan area of southern Ordos, which show relatively fewer reducing substances. The organic matter generated by the Huanhe Formation itself has little effect on uranium precipitation and enrichment.

7.9. The Indication of Paleoenvironment on Uranium Mineralization

Based on the exploration of the sedimentary environment of the Huanhe Formation mudstone in the Zhenyuan area of the Ordos Basin in the article, it is shown that the early sedimentation of the Huanhe Formation mudstone was under warm humid paleoclimatic conditions. This climate condition is conducive to the weathering and erosion of uranium bearing rocks in the erosion source area, which leads to the activation and migration of uranium elements in the uranium bearing source rocks, and converges to areas where the water flow is relatively stagnant, resulting in an increase in uranium concentration in the water in that area. In addition, the salinity of water can also affect the solubility of uranium, and low salinity water is conducive to the dissolution of uranium. The mudstone of the Huanhe Formation in the study area was in a brackish water environment during the sedimentary period, with a relatively low salinity. The solubility of uranium in water was relatively high, which in turn could promote the migration ability of uranium in fluids.

It is generally believed that the formation process of large sandstone-type uranium deposits is closely related to the large amount of carbonaceous debris and other organic matter that appear in the strata. However, the petrographic characteristics of the Huanhe Formation mudstone strata in the Zhenyuan area in the south of Ordos show that there are relatively few reducing substances in them. This geological phenomenon is significantly different from that of the ore-bearing sandstone in the Dongsheng area in the north of Ordos. However, the Zhenyuan area’s Chang 8 section is rich in oil and gas resources, with huge potential for hydrocarbon generation. Moreover, the Zhenyuan area is located within the Tianhuan Depression, and its west side is the west margin thrust zone, where deep faults exist, providing a channel for the upward migration of deep oil and gas. This can promote the contact between hydrocarbons and sandstone reservoirs and is conducive to the formation of reduction barriers. Existing studies have shown that deep oil and gas play a reducing role in the uranium mineralization process in the Zhenyuan area in the southern Ordos Basin. Based on this, this study holds that the uranium mineralization process in the Zhenyuan area can be divided into two processes: 1. Activation, migration and pre-enrichment stage. Under warm and humid paleoclimate conditions, the weathering and erosion of uranium-bearing rocks in the source area, as well as the activation and migration of uranium elements in uranium-bearing source rocks. Meanwhile, the sandstone deposition period of the Huanhe Formation was in a brackish water environment, and a large amount of activated and migrated uranium dissolved in the water. 2. The stage of upward migration, precipitation and enrichment of reducing fluids. Deep reducing fluids migrate upward, forming a reduction barrier, or directly react with dissolved uranium in water bodies to precipitate and cause uranium enrichment.

8. Conclusions

Through the element geochemistry analysis of the Huanhe Formation mudstone, the following insights have been obtained:

• 1On the basis of various paleoclimate indices like CIA and C contents, Ga/Rb and K₂O/Al₂O₃, a warm and humid environment is inferred. High concentrations of CaO as compared to MgO, also supports that mudstone faced dry climatic with strong evaporation conditions. High values of Cr (40.9–99.2) and V/(V + Ni) indicate that dysoxic to anoxic condition. Ni/Co vs V/Cr relationship indicates the oxic condition of the depositional environment. The Huanhe Formation was deposited in an environment ranging from dysoxic to anoxic condition, and during certain periods, there were also oxic environments present. This facilitates the activation and migration of uranium in source rocks within the weathering zone, as well as the formation of oxygenated and uranium-bearing fluids.
• 2The ratio of Sr/Ba and Th/U indicate Huanhe Formation was deposition in brackish and marine saline water conditions. This facilitates the continuous dissolution of mobilized uranium in water bodies, thereby increasing the uranium content in the water.
• 3Compared to the northern part, the sandstones of the Huanhe Formation in the southern Ordos Basin relatively lack reductive media (such as carbonaceous clastics). This study identifies two stages of uranium enrichment in the southern region: the activation-migration pre-enrichment stage and the upward migration and precipitation enrichment stage by reductive fluids.

All data generated or analyzed during this study are included in this article.

The authors declare no competing financial interest.