Uranium ore-forming systems of the - Geoscience Australia

Uranium ore-forming systems of the Lake Frome regionThe content of organic material is high in mineralised samples, resulting in a dark grey to blackappearance in hand sample. Most of the organic matter is represented by finely disseminatedmaterial in the matrix, but larger clasts of organic material are distributed unevenly throughout thesamples. In organic-rich parts of samples, mineral grain size is typically finer, and quartzdissolution is more intense. U-bearing minerals are rare amongst organic clasts, and are of a veryfine grain size where they occur.6.3.2 Post-depositional phasesA wide range of sulfide, oxide, silicate and phosphate minerals were precipitated within the EyreFormation host rocks after sedimentation. The occurrence of secondary pyrite, phosphate anduranium-bearing phases is described below.6.3.2.1 PyritePyrite is a common post-depositional mineral in the Four Mile samples, and is evident on XRDscans. Four types of pyrite are recognised in the sample (Fig. 6.2). The earliest phase of pyritegrowth is represented by framboidal pyrite aggregates. These are most likely bacterial in origin,and formed during or shortly after the deposition of the original sediment. A second phase ofpyrite crystallisation is represented by euhedral pyrite growth, commonly occurring as cement.This is commonly seen to overprint the earlier framboidal pyrite, indicating a later place in theparagenetic sequence. They are generally terminated with a perfect crystal face, suggesting thatthe cement grew in open space between grains. These cements are generally (although notuniversally) associated with euhedral marcasite. The relationship between pyrite and marcasite isdifficult to determine, although it appears that, in general, marcasite occurs as a core to pyrite.Thus, there is a transition in FeS 2 mineralogy with time. A third phase of pyrite is evident asovergrowth rims on the euhedral pyrite. The fourth type of pyrite occurs as tiny euhedral crystalsin the clay matrix. It is not possible to distinguish a chronology between the pyrite in the matrixand the large authigenic pyrite and rims, and the matrix pyrite may have formedcontemporaneously or independently of either.There is a strong association between the authigenic pyrite and ilmenite/Ti-oxide, with authigenicpyrite frequently observed to nucleate around detrital grains of Ti-oxides, and sometimes replacethem. Pyrite is also intimately associated in places with phosphate and uraninite (see below).6.3.2.2 Phosphate mineralsTwo main types of phosphates have been identified:1. Hydrated LREE-rich phosphates (rhabdophane and ningyoite); and2. Aluminous hydroxy phosphates.Hydrated LREE-rich phosphates Phosphates of the rhabdophane group have been detected in 11samples. Minerals of this group have the general formula of XPO4.nH2O where X can be REE, Y,Ca, Pb, Th and U (Jones et al., 1996). Two types of these rare earths phosphates have beendistinguished by their chemical compositions, XRD spectra and habits. The most common is a La-Ce-Ba rhabdophane, which forms micrometric, diffuse, cloudy aggregates scattered amongst thematrix in both the organic-rich and mineralised segments (Figs. 6.3 and 6.4). A second type ofhydrated LREE phosphate occurs as pseudomorphs of a detrital mineral, possibly apatite. It hasbeen identified as ningyoite, a U-Ca-Ce-La-Nd hydrated phosphate (Figs. 6.5, 6.6 and 6.7).Page 69 of 151