Diagenetic origin

In marbles and skarnoids, the consistent U-Ba-Mo-Pb-V-Zn association and absence of Th, the rare occurence of graphite, and the REE patterns in uraninite are evidences for a possible synsedimentary and/or diagenetic origin of the uranium and associated metals. This primary metal stock located in impure dolostone with possible evaporitic layers (as indicated by the presence of scapolite and kampfite in skarnoids) would have been more or less remoblllzed during regional metamorphism and tectonics. 

Walker T.R. (1976) Diagenetic origin of continental red beds. In Falke, H. (ed.) The Continental Permian in Central, West and South Europe. Reidel Publ. Co., Dordrecht,The Netherlands, 240-282... 

In sediments and sedimentary rocks lMd is the predominant polymorph of illite which indicates a diagenetic origin for more of the material. 

These clays occur in limestones, dolomites, evaporites, shales, siltstones, and hydrothermal deposits. All the sedimentary material appears to have a diagenetic origin. Although the physical environments vary, the chemical environments should be similar. Saline or even super-saline conditions are implied by the presence of evaporite minerals associated with some of the deposits. In the other deposits it is possible that temporary evaporitic conditions (e.g., tidal flats) existed long enough for brucite to precipitate between the layers of expanded-layer minerals. It appears plausible that the parent material was a montmorillonite-like mineral (probably detrital in most cases). 

Schieber L, Krinsley D., and Riciputi L. (2000) Diagenetic origin of quartz silt in mudstones and implications for silica cycling. Nature 406, 981-985. 

Casas, E., T. K. Lowenstein, R. J. Spencer P. Zhang, 1992 Carnallite mineralization in the nonmarine Qaidam basin, China Evidence for the early diagenetic origin of potash evaporites. J. Sed. Pet. 62 881-898. 

Structural clay is of diagenetic origin, i.e., formed within the sandstcuie framework as a deposit of clay clasts. 

Carbonate rocks are not normally transported over long distances, and we find carbonate reservoir rocks mostly at the location of origin, in situ . They are usually the product of marine organisms. However, carbonates are often severely affected by diagenetic processes. A more detailed description of altered carbonates and their reservoir properties is given below in the description of diagenesis . 

This kind of pressure solution / precipitation is active over prolonged periods of time and may almost totally destroy the original porosity. Precipitation of material may also occur in a similar way on the surface of fault planes thus creating an effective seal via a process introduced earlier as diagenetic healing. 

Abnormal Formation Pressures Origin. Four main causes are attributed to abnormal formation pressures compaction effects, diagenetic effects, differential density effects and fluid migration. 

Although the majority of attention in discussions on the origins of BIFs has been on the oxide facies, siderite facies rocks are equally important in many BIF sequences. Reaction of Fe(II)aq and dissolved carbonate with hematite to form siderite and magnetite has been hypothesized to be an important diagenetic process in marine basins during formation of some BIFs if sulfate contents were low (e.g., Klein and Beukes 1989 Beukes et al. 1990 Kaufman 1996 Sumner 1997). In Figure 18 we assume that Fe(II)aq was derived either from MOR sources or DIR, or a combination of the two, which reacted with ferric oxide precipitates to form magnetite or dissolved carbonate to produce siderite. 

These Early works, mainly for ca. thirty years ago, provided information about the nature and the origin (mainly marine planktonic) and the low degree of diagenetic evolution of this OM. Moreover, all these studies concluded the quantitative importance of extractable HC in the OM associated to phosphorites. But these works were performed on bulk phosphorites. Thus, it seems to us that a comparative study of the specific composition and properties of the OM of pellets and of their surrounding sediments... 

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