The diagenetic and economic significance of composite grains of monazite and hydrocarbon in Western Australian arenites

Composite grains consisting of detrital monazite and solid hydrocarbon, ranging up to 0.5 mm in diameter, have been found in Phanerozoic sedimentary rocks in three Western Australian basins. The most common form is a monazite core surrounded by a hydrocarbon envelope. The envelopes, which have conchoidal fracture and radial cracks, apparently congealed when hydrocarbons were polymerized and crosslinked by irradiation from the monazite. Some composite bodies contain fractured monazite grains within solid hydrocarbon and a few consist of a microbreccia of hydrocarbon and disseminated monazite fragments. The breakup of the monazite was probably a diagenetic chemical process that began when crosslinked hydrocarbons within cracks and open cleavages swelled as they were infiltrated by fluid hydrocarbons of low molecular weight. Five microprobe analyses of the monazite reveal the following ranges in percent composition: La2O3, 10.88–13.66; Ce2O3, 25.58–28.78; Pr2O3, 3.03–5.38; Nd2O3, 9.15–12.11; ThO2, 7.0–11.49; U3O8, 0–1.44. 2Vγ extends from 12.5–15.5°. In the northern Perth Basin, the composite bodies, which contain cores of monazite derived from local gneissic basement rocks, formed after the growth of siderite cement but before extensive silicification. In the central Perth Basin, the composite bodies seem to have formed after the growth of kaolinite and before silicification. Textures associated with the bodies allow them to be slotted into the diagenetic sequence and they therefore show when the oil was present. Analysis of the bodies has indicated a contribution of oil from the Lower Triassic Kockatea Shale, but some may have grown from more than one generation of petroleum. Given the ubiquity of detrital monazite, similar bodies are likely to be useful diagenetic and economic indicators elsewhere.