REGIONAL SIGNATURES AND METALLOGENIC MODELS OF SANDSTONE HOSTED URANIUM DEPOSITS IN NORTHERN

Z. LI, M. QIN, Y. CAI, Y. SUN, C. YI, Q. GUO, Y. XIA Beijing Research Institute of Uranium Geology, Beijing, China

1. INTRODUCTION

Since the beginning of the 21st century, uranium exploration in northern China has been focused on sandstone hosted uranium deposits. These represent a major deposit type in addition to granite and volcanic- related uranium deposits and their economic importance is increasing. Owing to progress made in metallogenic theoretical innovation and exploration, new deposits have been discovered and existing resources/reserves expanded in Mesozoic–Cenozoic basins. Examples are, moving west to east, Kujieertai in the Yili Basin; Zaohuohao, Nalinggou and Daying in the northern Ordos Basin; Basaiqi in the Erlian Basin and Qanjiadian in the Songliao Basin.

Sandstone hosted uranium deposits form in different geotectonic settings and have different mineralization and regional signatures which can be used to select targets and evaluate the uranium potential in exploration areas. Besides the traditional interlayered oxidation–reduction (redox) metallogenic model, some new models have been established for the sandstone hosted uranium deposits in northern China, such as the metallogenic superposition model and the tectonic activated metallogenic model, which have been of great importance to exploration and to the discovery of new uranium resources.

2. REGIONAL SIGNATURES

In northern China, the mineralization characteristics and regional signatures of these deposits are summarized below.

2.1. Diversity of metallogenic sedimentary basins

Sandstone hosted uranium deposits have been found in different types of basin. The northern China basins are generally subdivided into western, middle and eastern, based on their tectonic mechanisms. The western part is dominated by intermontane basins such as the Yili and Tuha basins within the Tianshan Mountains, where a number of sandstone hosted uranium deposits have been discovered. A middle foreland basin such as the Ordos Basin contains large uranium deposits and a continental margin rifted basin such as the Songliao Basin has deposits in its eastern part.

2.2. Diversity of the metallogenic sedimentary beds

Sandstone hosted uranium deposits can also be found in different sedimentary beds. The major mineralized host rocks are Early–Late sediments to the west, Late Jurassic–Early sediments in the central area and Late Cretaceous sediments to the east. It is obvious that uranium rich sedimentary beds are younger moving from west to east, indicating a higher degree of erosion to the west due to stronger Himalayan neo-tectonism, especially in north-western China during the Cenozoic era.

2.3. Diversity and multiple stages of metallogenic age

Systematic geochronological studies have revealed that uranium mineralization usually exhibits multiple stages in one deposit and a younger age in the front of roll-front orebodies. Different uranium deposits often

234 have different ages in different areas, in spite of their predominantly Cenozoic age. However, obviously the uranium mineralization age is younger than the age of the host rocks, and in general, the mineralization ages are younger, from east to west, being associated with geotectonism and showing an opposite tendency to the age of the host rock. In addition, the age data of the uranium deposits indicate that they have been affected by several phases, further complicating the formation story in the middle region, such as in the Ordos Basin.

2.4. Diversity of metallogenic fluids and processes

Uranium mineralization processes are dominated by meteoric fluids which form typical redox zones controlling the orebodies to the west. These processes are related not only to meteoric fluids but also to oil– gas and hydrothermal fluids in the middle area to the east, which make the formation processes and signatures of the uranium deposits more complicated to decipher, such as alteration, variable compositions, e.g. pitchblende dominant in ores to the west and both pitchblende and coffinite in the middle and to the east [1].

The formation of these sandstone hosted uranium deposits and their regional metallogenic signatures in northern China are closely related to Himalayan geotectonic movements, which are due to the subduction of the Indian Plate towards the north-west and leads to continent–continent collision and the rise of the Qinghai–Tibetan Plateau. This process started around 55 Ma and is ongoing [2]. The collision resulted in the present signatures of the basins, especially in north-western China and central Asia, and has had a major impact on sedimentary formations, hydrogeological processes, palaeoclimate changes and movements of oil–gas fluids during the Cenozoic period. Furthermore, it has had a fundamental impact on the metallogeny of sandstone hosted uranium deposits in north-western China and central Asia, leading to the formation of world-class sandstone hosted uranium provinces.

The role and impacts of the collision and the Tibetan Rise on the metallogeny of sandstone hosted deposits can be summarized as follows.

(a) Deposition and erosion during the Cenozoic period: Cenozoic sedimentary deposition provides possible new uranium-bearing beds and erosion or depositional hiatus facilitates ease of migration of oxidized uranium. In addition, the uplift of the provenance rocks could provide uranium sources and both of these are favourable to the formation of sandstone hosted uranium deposits;

(b) Formation of tectonic slope: The tectonic slope formed by regional tectonic movements can provide a favourable hydrological condition for the formation of the deposits, i.e. oxidized uranium-bearing meteoric fluids can move into the permeable sandstone beds and meet the reductant materials needed for the redox process. The tectonism also formed faults in the discharge area which can improve the hydrological conditions and help channel the necessary reductants into the potential ore beds;

(c) Arid and semi-arid weather conditions: This type of weather, which is favourable for uranium migration, developed during the rise of the Qinhai–Tibetan Plateau, especially in north-western China;

(d) Upward migration and escape of oil and gas: During the rising process of oil- and gas-rich basins, the oil and gas can move upwards to the depressurized areas where they act as reductants and form redox zones for possible uranium deposition. In addition, secondary reduction processes related to oil and gas can protect the existing orebody and lead to secondary metallogenic processes [3].

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3. METALLOGENIC MODELS

In northern China, several metallogenic models have been suggested for the formation of major sandstone hosted uranium deposits and these are summarized below.

3.1.1. Interlayered redox metallogenic model

The interlayered redox metallogenic model is a very popular roll-front type model and is exemplified by the Kujieertai deposit along the southern margins of the Yili Basin. The basement of the Yili Basin comprises two units, a Precambrian crystalline formation and a Late Palaeozoic clastic basement with a relatively high uranium content of 4–14 ppm representing a good potential source of mineralization. The sedimentary cover strata are dominated by –Jurassic strata, together with undeveloped Cretaceous and younger rocks. The uranium-bearing beds are mainly located in Jurassic -bearing clastic formations with frequent interbedded mudstone(coal)–sandstones and usually deposited as fluvio-deltaic sediments with a thickness of 25–40 m. The favourable host rock is a medium- to coarse-grained arkose with good permeability. Uranium mineralization can be found in 7 different ore-bearing horizons, extending over 10 km. The deposit shows complicated roll-front shapes with the ore grade varying in the range 0.01–0.2% U. Uranium exists predominantly as pitchblende, a little coffinite and also as absorbent forms; the associated elements are V, Se, Mo and Re. Mineralization formed during several phases dated at 19, 12, 5, 2 and 1 Ma.

3.1.2. Metallogenic superposition model

The metallogenic superposition model is represented by the Zaohuohao sandstone hosted uranium deposit in the north Ordos Basin. It is located at the southern margin of the Yimeng uplifted block and is adjacent to the Hetao Graben at the northern margin. Only Mesozoic sedimentary strata are exposed. The Upper Triassic Yanchang Formation is essentially composed of gravel-bearing sandstone interbedded with siltstone and mudstones, bearing oil and coal deposits. The Lower–Middle Jurassic Yanan Formation is mainly composed of arkose with coal beds, mudstone and siltstone. The Middle Jurassic Zhiluo Formation is the uranium-bearing bed and comprises grey, grey–green sandstone and mottled siltstone and mudstone, which is parallel to, or locally unconformably underlain by, the Yanan Formation. The Tertiary strata are absent. Sedimentary strata show that the study area underwent multiple tectonic events, which were closely related to uranium mineralization [2].

The large Zaohuohao deposit corresponds to a special type of sandstone hosted uranium deposit, different from other types of sandstone hosted deposit because of its unique signatures. It is generally controlled by a transitional zone between greenish and greyish sandstones, both sandstones indicating reduced geochemical environments. The greenish colour of the palaeo-oxidized sandstones results from chloritization and epidotization relating to hydrocarbon driven secondary reduction processes [3]. The deposit is genetically different from ordinary sandstone hosted deposits, being of more complex origin, undergoing not only a palaeo-oxidization mineralization process, but also circulating oil–gas fluids and hydrothermal reworking processes. The metallogenic superposition model for this type of uranium deposit has been established, i.e. the deposit underwent multiple mineralization processes and stages, such as tectonic multiphase ‘dynamic–static’ coupling movements, superposition of palaeo-phreatic oxidation and interlayer oxidation mineralization and composite transformation of oil–gas and hydrothermal fluids. The metallogenic stages can be identified as: (i) preliminary enrichment stage at 170 Ma, (ii) palaeo-phreatic oxidation stage at 160–135 Ma, (iii) palaeo-interlayer oxidation stage at 125–65 Ma, and (iv) oil–gas reduction and thermal modification at 20.8 Ma. Analytical data show that thermal modification of the deposit happened after the deposit formed. Coffinite, selenium and sulphide minerals formed under relatively high temperatures, leading to the preferential enrichment of elements such as P, Se, Si, Ti and REE over uranium [2].

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3.1.3. Palaeochannel metallogenic model

The palaeochannel metallogenic model is represented by the Bayinwula deposit which is located in an Early Cretaceous palaeochannel in the Erlian Basin. The exposed crystalline rocks to the north of the mineralized area have a high uranium content of 8–11 ppm U and are thought to provide a good source of uranium for the deposit. The ore-bearing bed is characterized by braided palaeochannel sedimentary systems and the ore is controlled by both interlayer and phreatic oxidation processes. The favourable host rocks are debris sandstone and arkose, with some organic matter and sulphur materials. The orebody usually shows a roll- front or tabular shape and has an average thickness of 6.38 m and ore grades varying in the range 0.0113– 0.2477% U. Uranium exists in predominantly absorbent form and also as pitchblende, and is associated with Re, Se, Mo, Sc and V. Three major metallogenic stages have been defined at: (i) 95 Ma with preliminary sedimentary enrichment, (ii) 65 Ma with dominant phreatic oxidation process, and (iii) 45 Ma with phreatic and interlayer oxidation process [4].

3.1.4. Tectonically activated metallogenic model

The tectonically activated metallogenic model is applied to the Qianjiadian sandstone hosted deposit in the south-western part of the Songliao Basin. It is located within the two wings of the anticline formed by a late activated tectonic event known Renjiang during the Late Cretaceous. The anticline structure which is also termed a ‘window structure’, plays a very important role in the formation of the uranium deposit. The host beds correspond to the Late Cretaceous Yaojia Formation with dominant fine-grained sandstones. Rocks such as Mesozoic granites and acidic volcanic rocks with a uranium content of 7–15 ppm could be potential sources of uranium. The late metallogenic processes are characterized by both oxidized infiltrating and reduced fluids due to oil–gas migration through deep faults to form a large tabular orebody. The Qianjiadian deposit is also affected by hydrothermal fluids related to basic dykes and dated to 53 Ma. Uranium mainly exists in an absorbent form and as pitchblende with an average ore grade of 0.0265%. Several metallogenic phases have been recorded at 96, 67, 53 and 40 Ma.

REFERENCES

[1] LI, Z., et al., Origin and superposition metallogenic model of the sandstone-type uranium deposit in the northeastern Ordos Basin, China, Acta Geol. Sin. 82 4 (2008) 745–749. [2] XU, Z., et al., Collision geotectonics between India and Asia continents, Acta Geol. Sin. 85 1 (2011) 1-33. [3] LI, Z., et al., Origin of gray-green sandstone in ore bed of sandstone type uranium deposit in North Ordos Basin, Science in China, Series D 50, Science in China Press, Beijing (2007) 165–173. [4] LIU, W., et al., Metallogenic paleochannel type uranium deposit in Erlian Basin, north China, Uran. Geol. 29 6 (2013) 328–335.

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