Effects of hydrocarbon generation on fluid flow in the Ordos Basin and its relationship to uranium mineralization

The Ordos Basin of North China is not only an important uranium mineralization province, but also a major producer of oil, gas and coal in China. The genetic relationship between uranium mineralization and hydrocarbons has been recognized by a number of previous studies, but it has not been well understood in terms of the hydrodynamics of basin fluid flow. We have demonstrated in a previous study that the preferential localization of Cretaceous uranium mineralization in the upper part of the Ordos Jurassic section may have been related to the interface between an upward flowing, reducing fluid and a downward flowing, oxidizing fluid. This interface may have been controlled by the interplay between fluid overpressure related to disequilibrium sediment compaction and which drove the upward flow, and topographic relief, which drove the downward flow. In this study, we carried out numerical modeling for the contribution of oil and gas generation to the development of fluid overpressure, in addition to sedi- ment compaction and heating. Our results indicate that when hydrocarbon generation is taken into account, fluid overpressure during the Cretaceous was more than doubled in comparison with the simu- lation when hydrocarbon generation was not considered. Furthermore, fluid overpressure dissipation at the end of sedimentation slowed down relative to the no-hydrocarbon generation case. These results suggest that hydrocarbon generation may have played an important role in uranium mineralization, not only in providing reducing agents required for the mineralization, but also in contributing to the driving force to maintain the upward flow.

An overview of hydrodynamic studies of mineralization

Fluid flow is an integral part of hydrothermal mineralization, and its analysis and characterization constitute an important part of a mineralization model. The hydrodynamic study of mineralization deals with analyzing the driving forces, fluid pressure regimes, fluid flow rate and direction, and their relationships with localization of mineralization. This paper reviews the principles and methods of hydrodynamic studies of mineralization, and discusses their significance and limitations for ore deposit studies and mineral exploration. The driving forces of fluid flow may be related to fluid overpressure, topographic relief, tectonic deformation, and fluid density change due to heating or salinity variation, depending on specific geologic environments and mineralization processes. The study methods may be classified into three types, megascopic （field） observations, microscopic analyses, and numerical modeling. Megascopic features indicative of significantly overpressured （especially lithostatic or supralithostatic） fluid systems include horizontal veins, sand injection dikes, and hydraulic breccias. Microscopic studies, especially microthermometry of fluid inclusions and combined stress analysis and microthermometry of fluid inclusion planes （FIPs） can provide important information about fluid temperature, pressure, and fluid-structural relationships, thus constraining fluid flow models. Numerical modeling can be carried out to solve partial differential equations governing fluid flow, heat transfer, rock deformation and chemical reactions, in order to simulate the distribution of fluid pressure, temperature, fluid flow rate and direction, and mineral precipitation or dissolution in 2D or 3D space and through time. The results of hydrodynamic studies of mineralization can enhance our understanding of the formation nrocesses of hvdrothermal denosits, and can be used directly or indirectly in mineral exnloration.

Hydrodynamic analysis of clastic injection and hydraulic fracturing structures in the Jinding Zn-Pb deposit,Yunnan,China

The Jinding Zn-Pb deposit has been generally considered to have formed from circulating basinal fluids in a relatively passive way, with fluid flow being controlled by structures and sedimentary facies, similar to many other sediments-hosted base metal deposits. However, several recent studies have revealed the presence of sand injection structures, intrusive breccias, and hydraulic fractures in the open pit of the Jinding deposit and suggested that the deposit was formed from explosive release of overpres- sured fluids. This study reports new observations of fluid overpressure-related structures from under- ground workings （Paomaping and Fengzishan）, which show clearer crosscutting relationships than in the open pit. The observed structures include： 1） sand （--rock fragment） dikes injecting into fractures in solidified rocks; 2） sand （~rock fragment） bodies intruding into unconsolidated or semi-consolidated sediments; 3） disintegrated semi-consolidated sand bodies; and 4） veins and breccias formed from hydraulic fracturing of solidified rocks followed by cementation of hydrothermal minerals. The development of ore minerals （sphalerite） in the cement of the various clastic injection and hydraulic fractures indicate that these structures were formed at the same time as mineralization. The development of hydraulic fractures and breccias with random orientation indicates small differential stress during mineralization, which is different from the stress field with strong horizontal shortening prior to miner- alization. Fluid flow velocity may have been up to more than 11 m/s based on calculations from the size of the fragments in the clastic dikes. The clastic injection and hydraulic fracturing structures are interpreted to have formed from explosive release of overpressured fluids, which may have been related to either magmatic intrusions at depth or seismic activities that episodically tapped an overpressured fluid reservoir. Because the clastic injection and hydraulic structures are genetically linked with the mineralizing fluid source, they can be used as a guide for mineral exploration.

Multiple and prolonged porphyry Cue Au mineralization and alteration events in the Halasu deposit, Chinese Altai, Xinjiang,northwestern China

The Halasu area is located in the southeastern margin of the Chinese Altai in Xinjiang, China. It is part of the Altaid orogenic collage where a number of porphyry-type Cue Moe Au deposits have been discovered in recent years. Geological mapping and drilling indicate the presence of various mineralized porphyritic intrusions in the Halasu Cue Au deposit, which is currently under exploration. Ue Pb dating of zircon crystals from four different mineralized porphyries reveals three significantly different ages of magmatic intrusion, i.e., ca. 372e382 Ma granodioritic porphyry and porphyritic granite, ca. 266 Ma quartz monzonitic porphyry, and ca. 216 Ma quartz dioritic porphyry. Ree Os dating of molybdenite from veinletdissemination ores in the granodioritic porphyry yields an age of mineralization of ca. 377 Ma, and Ar e Ar dating of K-feldspar from K-feldsparequartzechalcopyrite veins produces ages of ca. 269 and ca.198 Ma. The mineralization（and alteration） ages correspond broadly to the three episodes of magmatic intrusion, suggesting three overprinting porphyry mineralization events that are significantly separated in time. The first episode of porphyry intrusion and mineralization may be related to the magmatic arc being above a plate subduction zone, and the second was formed in a late-collisional environment during the closing of the Junggar Ocean, whereas the third episode of mineralization took place in the postcollisional stage. This case study suggests that in orogens where major porphyry deposits have been found in magmatic arc environments, the potential of discovering late- to post-collisional porphyry deposits cannot be neglected; conversely, in orogens where most porphyry deposits have late- to postcollisional ages, more attention should be paid to porphyries that were formed earlier in magmatic arc environments.

Expansion of the South China Sea basin:Constraints from magnetic anomaly stripes,sea floor topography,satellite gravity and submarine geothermics

The widely distributed E-W-trending magnetic anomaly stripes in the central basin and the N-Etrending magnetic anomaly stripes in the southwest sub-basin provide the most important evidence for Neogene expansion of the South China Sea.The expansion mechanism remains,however,controversial because of the lack of direct drilling data,non-systematic marine magnetic survey data,and irregular magnetic anomaly stripes with two obvious directions.For example,researchers have inferred different ages and episodes of expansion for the central basin and southwest sub-basin.Major controversy centers on the order of basinal expansion and the mechanism of expansion for the entire South China Sea basin.This study attempts to constrain these problems from a comprehensive analysis of the seafloor topography,magnetic anomaly stripes,regional aeromagnetic data,satellite gravity,and submarine geothermics.The mapped seafloor terrain shows that the central basin is a north-south rectangle that is relatively shallow with many seamounts,whereas the southwest sub-basin is wide in northeast,gradually narrows to the southwest,and is relatively deeper with fewer seamounts.Many magnetic anomaly stripes are present in the central basin with variable dimensions and directions that are dominantly EWtrending,followed by the NE-,NW- and NS-trending.Conversely such stripes are few in the southwest sub-basin and mainly NE-trending.Regional magnetic data suggest that the NW-trending Ailaoshan-Red River fault extends into the South China Sea,links with the central fault zone in the South China Sea,which extends further southward to Reed Tablemount.Satellite gravity data show that both the central basin and southwest sub-basin are composed of oceanic crust.The Changlong seamount is particularly visible in the southwest sub-basin and extends eastward to the Zhenbei seamount.Also a low gravity anomaly zone coincides with the central fault zone in the sub-basin.The submarine geothermic distribution demonstrates that the southwest sub-basin has a higher geothermal value than the central basin,and that the central fault zone is defined by a low thermal anomaly.This study suggests that NW-SE expansion of the southwest subbasin is later than the N-S expansion of the central basin with the sub-basin extending into the central basin and with both expansions ending at the same time.The expansion of southwestern sub-basin,similar to the Japanese Sea,is likely caused by left-lateral strike slip on the central fault zone in the South China Sea,which may have significance for finding oil and gas in this region.

An Updated Database and Spatial Distribution of Landslides Triggered by the Milin, Tibet M_(w)6.4 Earthquake of 18 November 2017

The M_(w)6.4 earthquake on November 18, 2017 in Milin County, Nyingchi City, Tibet triggered thousands of landslides. By comparing visual interpretation of satellite images acquired shortly before and after the earthquake and field survey, we have created a new landslide database which includes 3 130 coseismic landslides, each with an area of 0.01 to 4.35 km^(2). Six factors(elevation, slope angle, slope aspect, lithology, distance from the epicenter and distance from the seismogenic fault) were selected to correlate with the coseismic landslides. In addition, the area and density of landslides were counted as indicators. Results show that most landslides occurred where the elevation is between 2 000–3 000 m, with a 40°–50° slope angle and S, E or SE slope aspect, schist or gneiss lithologies, 10–15 km from the epicenter, and 5 km within the seismogenic fault. Most of the landslides, triggered by the M_(w)6.4 earthquake, are concentrated near the seismogenic fault rather than at the epicenter, indicating that the seismogenic structure is more influential than the location of the epicenter. Our findings may differ from other landslide database due to temporal image acquisition, interference from weather, and image resolution.