Langer Heinrich

The Langer Heinrich uranium deposit is in the Gawib River valley, between the Langer Heinrich Mountain and the Schieferberge, approximately 100km east of Walvis Bay (Figure 1). The geology of the deposit has been described elsewhere [4, 5,6, 7]. A geological map of the area is shown in Figure 5 and the relative mineralogical proportions of the Langer Heinrich Formation are given in Figure 6. The basement rocks of the Langer Heinrich area comprise the Nosib and Swakop Groups (Table 2), which are mainly metaquartzite and schist respectively, into which the late-syntectonic to post-tectonic Bloedkoppie Granite pluton and numerous pegmatites have been intruded.

The Bloedkoppie Granite has importance as it is the probable source of the uranium in the Langer Heinrich Formation. It is a medium-grained, but locally coarse-grained leucogranite with quartz, microcline, plagioclase, and biotite as the main constituents. It has a mean uranium content of 18 ppm and Th/U ratio between 0.9 and 2.6 which gives it a weak radiometric signature [5].

The surficial sediments in the Gawib River valley comprise the Langer Heinrich and Bloedkoppie Formations (Table 2). Headward erosion by the Gawib River initiated by uplift during the Pleistocene, resulted in exposure of the Langer Heinrich Formation. Geophysical investigations, and percussion drilling on the Gawib Flats at the western end of the Gawib River, have shown that the channel narrowed considerably and, at this point, probably

0 QUARTZ 100 l l i i l

10-0 CALCITE 100

l

Drffractometer p«ak 10-height normalized to 100 i————————»•

10-HEMATITE* 10O . , . l

GYPSUM 100 , . , l

* Fe determined by XRF and normalized in the same manner as for minerals

Figure 4

Relative mineralogical proportions of the Jumas Formation.

22°48'-

-22"48-Lag«Ml|

Namib Group Uranium mineralization

| j Pegmatities

|___j Bloedkoppie Granite

\. , j Salem Granite Swakop Group Nosib Group

0 Sola U

— — .^ Drainage channels Roads

Figure 5

Geology of the Langer Heinrich uranium deposit (Gencor).

QUARTZ

IQ-100 I

CALCITE

100 DOLOMITE

ID-

10-

20-100 I

HEMATITE* 100

10-20

10-

20-GYPSUM+ 100

Oiffractometer peak height normalized to 100

*Fe determined by XRF and normalized in the same manner as for the minerals

~t~The gypsum curve was determined from chemical analyses

Xlnterpolation from geological observations

Figure 6

Relative mineralogical proportions of the Langer Heinrich Formation.

first became choked with surficial sediments of the Langer Heinrich Formation. Prior to this event, the Gawib River was probably a tributary of the Tumas River.

The uranium deposit has been almost fully delineated and the distribution of the uranium is shown in Figure 5.

Depths to the base of the palaeochannel are variable and sedimentary thicknesses up to 45 m have been recorded. In general, the uranium mineralization is confined to the Langer Heinrich Formation, with only a small amount in the Bloedkoppie Formation (B. Fletcher, personal communication), and is probably the result of a later, minor redistribution of uranium from the Langer Heinrich Formation. The uranium deposit extends from the Bloedkoppie Flats in the east, westwards along the Gawib River palaeovalley and continues under the Gawib Flats.

The generalized distribution of the uranium is shown in a cross-section (Figure 7) through the Langer Heinrich Formation. The grades tend to be highest in a central core zone, which does not necessarily correspond to the present water table.

Detailed mapping of the eastern wall of a large trench and radiometric total count logs of the centre-line boreholes are shown in Figures 8 and 9. Comparing both sections, it is observed that there are numerous lithological units in the Langer Heinrich Formation and that there is almost no correlation between the uranium

GROUND SURFACE

Legend

Contours of uranium grade which decrease outward Langer Heinrich Formation

k\\ \ \ \J Damara Sequence GWT Groundwater table

Figure 7

Generalized distribution of the uranium grade in a cross section through the Langer Heinrich.

'••''•':'•''• •'•':'•'•'•:'"l-'

''• *' i''-"' ! •'•." •

Legend

Gypsiferous alluvium/soil Calcareous conglomerate Hard calcareous grit Soft calcareous clay grit

Calcareous clay Scale 10

Figure 8

Lithology of the Langer Heinrich Formation in the eastern side- wall of a trench at the Langer Heinrich (Gencorj.

content and the lithology of the sediments. Furthermore, the uranium distribution is totally irregular and discontinuous. Boreholes within a metre or less of each other have completely different radiometric grades, demonstrating the patchy nature of the ore, which was formed in pods, veins, lenses and particularly as cavity fillings. The highest uranium grades are in those parts that are least consolidated, i.e. zones low in carbonate cement and which therefore possess the highest porosity. Within the clay portions of the sediments, calcium carbonate-rich nodules and tubules frequently contain high-grade uranium mineralization.

The colour of the sediments exposed in the trench varies from greyish to reddish-brown; intermediate yellowish-brown tones are most common. The clay tends to be slightly greenish-blue, being, in part, the original colour of the micaceous material derived from the schist of the Kuiseb Formation. Within the upper Langer Heinrich Formation, there is no direct evidence from the colour of the sediments of redox control. Determination of iron (ll)/iron (III) ratios shows that the more reduced sections tend to be nearer the surface and that the degree of oxidation increases with depth down to 20 m. This agrees with the observation that when bright yellow carnotite is exposed at the surface, it sometimes tends to become slightly green, probably as a result of the reduction of vanadium (V) to vanadium (IV), the latter having a green colour. Newly excavated carnotite has not been found to be greenish in colour.

WTJJÏÏÏÏi,

Legend ppm eU308

< 100 100-500

>500

10 15

Scale

Figure 9

Equivalent uranium distribution in the Langer Heinrich Formation as determined in the centre — line boreholes in a trench at the Langer Heinrich (Gencor).

The paragenetic carnotite/calcite relationships in the Langer Heinrich Formation indicate that several ages of calcite precipitation took place, probably starting in the Upper Tertiary [4, 5]. Carnotite is associated with one of the earlier, coarse-grained, calcite fractions. Geomorphologically, the carnotite seems to predate the Pleistocene uplift that caused the incising of the Gawib and Swakop Rivers. The last two empirical relationships indicate, therefore, that precipitation of the carnotite took place during Upper Tertiary times.

3.3 Mile 72

Pedogenic uranium occurrences are common in Namibia, the most important being Mile 72. Many calcrete and gypcrete cappings and outcrops contain small amounts of carnotite, which implies that uranium is still mobile.

Occurrences of this nature may constitute the proto-ore forfuture uranium occurrences within fluviatile regimes.

Mile 72 comprises a cluster of uranium occurrences situated along the coast between Henties Bay and Cape Cross (Figure 1), 72 miles (95 km) north of Swakopmund. It lies on the coastal plain of the Namib Desert in the weathered and decomposed basement rocks.

The basement metasedimentary rocks belong mainly to the Khomas Subgroup and consist of marbles, metaquartzites, calc-silicates, and schists, into which various granites such as the syntectonic Salem Granite and post-tectonic pegmatites and alaskites were intruded. Primary uraniferous minerals such as uraninite, betafite, monazite and apatite occur mainly in the alaskite. The thin veneer of Tertiary to Recent surficial material overlying the basement rocks consists of aeolian sand, isolated remnants of fluviatile gravel and desert soil, all of which have been cemented by gypsum and, to a lesser extent, by halite and calcite. These secondary cementing minerals extend down into the basement rocks, filling joints, fissures, and cleavage planes.

During the marine transgression of the late Pleistocene and early Quaternary, most of the surficial sediments, and probably the Langer Heinrich Formation, were eroded west of the "western cutoff line" (Figure 1). Basement rocks were exposed and only remnants of the Langer Heinrich Formation remained in depressions.

The uranium is patchy, and occurs in several pockets associated with metasediments, granite and dolerite.

Secondary minerals, such as carnotite (the dominant phase), and minor amounts of phosphuranylite, occur principally above the water table. Phosphuranylite is closely associated with apatite.

One occurrence in metasediments varies from 1 to 30 m in width and extends to 1 8 m in depth. From trenching operations, it was found that the main uranium deposit occurs nearthe contact between calc-silicates and schist in association with quartz veins and alaskites. The metasediments are, in places, deeply weathered and replaced by crystalline and powdery gypsum and carnotite. Alaskites within this zone tend to be more radioactive than those occurring in less weathered material. However, this could be caused by either primary or secondary enrichments.