PRE-ORE HYDROTHERMAL ALTERATION IN AN UNCONFORMITY-TYPE URANIUM DEPOSIT

Hydrothermal alteration associated with the formation of unconformity- type uranium deposits is manifested mainly by intense chloritic replac ement of metamorphic mineral assemblages. At the Koongarra deposit (no rthern Australia), three stages of pre-ore alteration were identified in chloritized metamorphic muscovite using optical microscopy, the ele ctron microprobe, and transmission and analytical electron microscopy (TEM and AEM). During the first stage metamorphic muscovite was altere d to a mica with very low interlayer occupancy (K content as low as 0. 35 compared to 1 for ideal muscovite), high octahedral cation content (up to 2.3 whereas ideal muscovite has 2) and high Si content. High re solution TEM images show that these changes in composition were accomp anied by frequent layer terminations in contrast to the metamorphic mu scovite, whereas selected area electron diffraction patterns indicate that the altered muscovite remained a two-layer polytype. The alterati on of muscovite appears more extensive in the outer alteration halo an d has been almost completely eradicated in the ore zone where chloriti zation is more complete. During the second stage of the pre-ore altera tion the altered muscovite was replaced by Mg-rich serpentine (lizardi te) and clinochlore. High resolution TEM images show two 10 Angstrom m ica layers coalescing to one 14 Angstrom chlorite layer. This type of coalescence appears to involve transformation of one dioctahedral laye r of the mica structure to one trioctahedral brucite layer in the chlo rite structure through dissolution of the tetrahedral sheets and catio n influx in the octahedral site. The final stage of the pre-ore altera tion resulted in replacement of lizardite and clinochlore by Fe-rich c hlorite. Comparison of the alteration observed in metamorphic muscovit e with the alteration observed in metamorphic biotite and garnet shows clearly that chloritization of these phases was also a pre-ore altera tion event. Preore alteration was principally characterized by the evo lution of chlorite from a Mg-rich phase to an Ferich one.