Integrated petrographic and geochemical characterization of rare earth element and uranium mobility in Llandeilo-Caradoc formation Black Shales, Wales, United Kingdom.

摘要

Rare earth element (REE) redistribution is recognized in a sequence of Ordovician Llandeilo-series black shales from the southwest margin of the Welsh Basin, U. K. This redistribution is controlled by the dissolution and/or precipitation of REE and U-rich diagenetic phosphates (e.g. apatite and monazite). An integrated petrographic (cathodoluminescence, backscattered electron imaging, fission track and microprobe elemental mapping), trace element, mineral chemistry, and U-Pb and Sm-Nd isotopic study of these black shales and their diagenetic minerals provides a means to address the question of how phosphates and petrographically related phases (e.g. calcite) influence trace element distribution during diagenesis. The paragenetic sequence from the Llandeilo-Caradoc formation black shales is as follows: (1){A0}pyrite formation, (2a){A0}apatite in the form of rims on pyrite and small grains, (2b){A0}poorly crystalline fine grained carbonate, (3){A0}apatite (REE-rich, U-bearing) nodules, (4a){A0}monazite (REE-rich, U-rich) replacing apatite and occurring in the shale matrix, (4b){A0}a second stage of carbonate growth replacing early carbonate and present in cm- to mm-sized veins and, (4c){A0}chalcopyrite and metal-rich (Au, Ag, U, Th, W, Pb) grains as overgrowth on pyrite and chalcopyrite. The minerals and textures observed require the redistribution of REE and U during the formation and dissolution of diagenetic apatite, monazite and late diagenetic ore-type metal-rich phases.; The redistribution of REE is evident as a large range in the Llandeilo-Caradoc whole rock Sm/Nd ratios (0.14 to 0.20), and a range in Nd-model ages (1.4 to 2.25 Ga) that is outside the expected range (1.6 to 1.8 Ga) for Welsh Basin shales. These changes in whole rock REE chemistry are controlled by the formation and dissolution of REE-rich diagenetic apatite and monazite on a scale greater than an individual whole rock sample at about the time of deposition (460 Ma) at least in part during early diagenetic sulfate reduction. Despite the expectation that organic-rich sediments concentrate U during deposition, the U-Pb whole rock isotopic system records a diagenetic disturbance at about 193 Ma. This disturbance is related to the redistribution (i.e. gain or loss of up to 90%) of U on a scale greater than an individual whole rock sample. This requires a late diagenetic process, such as thermochemical sulfate reduction, unrelated to the early diagenetic processes involved with the redistribution of REE.; The results of this study characterize the timing of and controls on REE and U-redistribution with in the Llandeilo-series black shales. This study is the first attempt to integrate petrography, trace element geochemistry and radiogenic isotope chemistry (Pb, Nd) of both whole rocks and individual diagenetic phases in order to evaluate the diagenetic history of a sequence of black shales.