Description of drill-hole T129V core from the Jabiluka unconformity-type uranium deposit, Northern Territory, Australia

Abstract

The study of unconformity-type deposits major sources of uranium that were discovered in the past 15 years in Australia and Canada was part of the U.S. Department of Energy National Uranium Resource Evaluation (NURE) program. Pancontinental Mining Limited kindly gave Dick Grauch and I access to Jabiluka core and made their geological and geophysical data available for inclusion in our reports. Samples were collected from 19 cores. Data and interpretations from the study of Jabiluka should aid in defining characteristics and settings of these world class deposits and guild exploration for similar deposits in the United States. This paper is a description of drill-hole T129V, located in Jabiluka 2 (fig. 2). Nutt and Grauch (1983) gave a description of regional and mine geology at Jabiluka; that paper provides the regional geologic setting of drill-hole T129V. Figure 3 shows the general stratigraphy of the mine sequence. T129V drill hole was drilled to a depth of 232 meters through the Kombolgie and Cahill formations. Figure 4 shows the north-south 129 cross section through Jabiluka 2 that contains drill-hole T129V. Both ore and barren zones were cored. Description of the core consists of a stratigraphic column from 60 to 232 m (fig. 5), compiled from Pancontinental Mining Limited and my logging record; brief descriptions of minerals, identified by microscope, microprobe Energy Dispersive System (EDS), and powder camera; and thin-section descriptions from selected samples (table 1). Uranium distribution and veins are also described. Rock types The rocks in T129V are typical of the Jabiluka sequence (Nutt and Grauch, 1983, gave detailed descriptions of the rock types). The metamorphic rocks, consisting of quartz, chlorite, muscovite, and graphite in a matrix of chlorite and (or) sericite, are: chlorite schist, chlorite-muscovite ± sericite schist, chlorite-sericite ± muscovite schist, and graphite schist. These schists are fine grained with occasional pseudomorphs (most commonly chlorite after garnet) and coarse-grained muscovites and chlorites as large as 2 mm. Tourmaline-bearing pegmatite is the only recognizable igneous rock in T129V. Hybrid zones of mixed pegmatite and metamorphic rocks occur along pegmatite contacts. The Kombolgie Formation is a quartz-rich sandstone; above the unconformity, the sandstone has interstitial chlorite and crosscutting veins of chlorite and hematite.

Description of drill-hole V111V core from the Jabiluka unconformity-type uranium deposit, Northern Territory, Australia

“…In part, this may be because rocks in V111V have less chloritic alteration; coarse-chlorite 2, commonly associated with light-green cryptocrystalline chlorite in T129V, is thought to be an alteration of coarse-chlorite 1 (Nutt, 1983).…”

“…Seven types of matrix and vein chlorite were recognized in T129V (Nutt, 1983); most chlorite in V111V has no unique characteristics and was therefore classified as undifferentiated. Septechlorite and tourmaline (dravite) alterations are widespread in breccia matrix, veins, and schist matrix of T129V, but are only rarely observed in V111V.…”

“…The division of coarse chlorite into coarse-chlorite 1 and coarsechlorite 2 that was used in describing T129V core (Nutt, 1983) is not applicable in the description of V111V core. In part, this may be because rocks in V111V have less chloritic alteration; coarse-chlorite 2, commonly associated with light-green cryptocrystalline chlorite in T129V, is thought to be an alteration of coarse-chlorite 1 (Nutt, 1983).…”

“…In the vicinity of Jabiluka, as well as the other unconformity-type deposits, extensive chloritic alteration has affected the rocks, which now have predominantly greenschist-facies mineral assemblages. Nutt and Grauch (1983) described the regional and mine geology at Jabiluka; that paper was the first in a series of open file reports on Jabiluka (Nutt, 1983) and was written to provide information on the regional geologic setting and on the geology of the ore deposit as background for detailed petrographic reports on samples from the drill holes.…”

“…Also included are brief descriptions of rock types and minerals (identified by petrographic microscope, microprobe Energy Dispersive System (EDS), powder X-ray diffraction, and powder camera), thin-section descriptions of selected samples (table 1), and discussion of the distribution of veins and uranium. Nutt (1983) described the core from drill-hole T129V that penetrates ore zones in Jabiluka 2. Comparison of the rocks in the two holes shows both similarities and differences between ore-bearing and barren rocks.…”