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PER-54

A MINERALOGICAL INVESTIGATION OF XENOTIME-FELDSPAR-QUARTZ ROCKS FROM THE VERNON CROOKS RESERVE, NATAL

H.J. Brynard

ATOMIC ENERGY BOARD Pelindaba m PRETORIA 30 Republic of South Africa October 1980

..««until;;, «iiiiliil: ::iiilii: illiiii ATOMIC ENERGY BOARD PER-54

A MINERALOGICAL INVESTIGATION OF XENOTIME-FELDSPAR-QUARTZ

FROM THE VERNON CROOKS RESERVE, NATAL

H.J. Brynard*

*Principal Scientist Geology Division

POSTAL ADDRESS: Private Bag X256 PRETORIA 0001

PELINDABA OCTOBER 1980 ISBN 0 86960 716 2 CONTENTS

SAMEVATTING

ABSTRACT

ACKNOWLEDGEMENT INTRODUCTION PROCEDURE

MACROSCOPIC DESCRIPTION OF THE ROCK

MICROSCOPIC DESCRIPTION OF THE MINERALS URANIUM AND THORIUM ANALYSES URANIUM AND THORIUM DISTRIBUTION GENESIS OF THE ROCK EXTRACTION METALLURGY ECONOMIC GEOLOGY REFERENCE PER-54-2

LIST OF FIGURES

Page

FIG. 1 Autoradiograph of a xenotime-potassium feldspar-quartz 11 rock from the Vernon Crooks Reserve, Natal.

FIG. 2 Photomicrograph of a xenotime-potassium feldspar- 11 quartz rock showing the typical mineral association.

LIST OF TABLES

TABLE I Volumetric analysis of the constituent minerals of a xenotime-feldspar-quartz rock from the Vernon Crooks Reserve, Natal.

TABLE II Uranium and thorium content of fresh and weathered xenotime-potassium-feldspar-quartz rocks from the Vernon Crooks Reserve, Natal. PER-54-3

SAMEVATTING

Twee vars monsters en een verweerde monster van "n xenotiem-kalium veld= spaat-kwartsryke gesteente afkomstig uit die Vernon Crooks Reservaat in Natal is mineralogies ondersoek. Xenotiem (yPOJ is die hoofmineraal met ondergeskikte ortoklaas, mikroklien en kwarts. Die gemiddelde uraan= inhoud van die vars monsters is 12 317 dpm (1,23 %) ILOg en die torium= inhoud is 18 824 dpm (1,88 %) ThCL. Die uraaninhoud van die verweerde monster is 8 969 dpm (0,89 %) lUJg.

Die hoof uraandraende minerale is uraniniet en vandendriesschiet en 'n onge'identifiseerde torium-uraan-silikaatmineraal. "n Klein hoeveelheid uraan kom voor in die kristaltralie van die xenotiem en bykomstige sirkoon. Die uraandraende minerale sal waarskynlik volledig bevry word gedurende die vergruisingsproses en die uraan sal waarskynlik herwin kan word deur die konvensionele suurlogingsproses. Die omvang van die afsetting is onbekend en geen berekening van die tonnemaat kan gemaak word nie.

ABSTRACT

One weathered and two fresh samples of a xenotime-potassium feldspar- quartz rock from the Vernon Crooks Reserve in Natal have been examined mineralogically. Xenotime (yPO,) is the main mineral with subordinate orthoclase, microcline and quartz. The average uranium content of the

fresh rock is 12 317 ppm (1,23 %) U,08 and the thorium content is 18 824 ppm (1,88 %) ThCL. The uranium content of the weathered rock is

8 969 ppm (0,89 %) U30g.

The main uranium-bearing minerals are uraninite and vandendriesschice and an unidentified thorium-uranium-silicate mineral. Some uranium is contained in the xenotime and accessory zircon. The uranium minerals are expected to be easily liberated during comminution and the ore is expected to be amenable to acid leaching. The extent of the deposit is unknown and no estimate can be made of tonnages present. PER-Ó4-4

ACKNOWLEDGEMENT

The author wishes to thank Phelps Dodge Mining Limited for their co-operation and for permission to release the contents of this report. PER-54-5

1. INTRODUCTION

Two fresh samples and a weathered sample of a xenotime-feldspar-quartz- rich rock collected by Dr F.A.G.M.Camisani-Calzolari of the Geology Division of the AEB were received for a mineralogical investigation. The samples were collected during a field trip to the prospecting activities of Phelps Dodge of SA in the Vernon Crooks Reserve in Natal.

The investigation was aimed at establishing the uranium and thorium content of the rock, the identification of the constituent minerals of the rock and the host minerals of the uranium and thorium.

2. PROCEDURE

Autoradiographs of polished surfaces of the rock samples were made by exposing the samples to X-ray film for three days. The intensity of radioactivity was sufficient to result in well-developed prints in this relatively short time (Fig. 1).

Thin sections were prepared for selected areas of the rock. Represent tative portions of the rock were ground and submitted for uranium and thorium analysis. Thin sections were also prepared for fission-track study of the uranium distribution in the rock.

Qualitative element identification was done by energy dispersive X-ray analysis (EDAX). The uranium minerals were identified by X-ray dif= fraction using the Debye-Scherer method.

3. MACROSCOPIC DESCRIPTION OF THE ROCK

The rock is dark-grey to black, medium-grained and consists of abun dant xenotime and reddish potassium feldspar crystals (nrthoclase), the latter being crystallographically continuous, as is evident from the reflection of light from cleavage planes over large areas in the hand specimen. Large crystals exceeding 5 mm of quartz are visible.

The weathered rock shows the presence of a bright yellow, secondary uranium-bearing mineral which has not been identified, but could be uranophane, a common alteration product of uraninite. A PER-54-6

whitish uranium-bearing mineral in sheaflike aggregates which fluoresces bright yellow-green is present in insufficient quantities for powder X-ray identification. Uranium was leached from the rock during weather ing as shown by the much lower uranium content.

A mineralogical volumetric analysis of the unweathered specimen is given in Table I. TABLE I : VOLUMETRIC ANALYSIS OF THE CONSTITUENT MINERALS OF A XENOTIME-FELDSPAR-QUARTZ ROCK FROM THE VERNON CROOKS RESERVE, NATAL

Mineral Volume \',

Xenotime 52,0 Orthoclase 21,8 Microcline 6,8 Quartz 16,0 Uraninite, vandendriesscnite 2,1 +U-Th-Si minerals Zircon and accessory minerals 1,3

4. MICROSCOPIC DESCRIPTION OF THE MINERALS

Xenotime is the most abundant mineral, constituting more than 50 ; of the rock (Table I). It is a phosphate of yttrium, essentially yPO,, in which U, Th, Ce and other rare-earth elements may substitute for yttrium. Erbium, Tb and Dy were detected by energy-dispersive X-ray analysis.

Mineralogically it is euhedral with minor subhedral forms (Fig. 2). The crystals are normally large, exceeding 2 mm in length but on average 1,5 mm long.

Where the xenotime is associated with microcline, the crystals have been found to be frequently smaller in size, having an average length of 0,2 mm. PER-54-7

Large xenotime crystals are sometimes distinctly zoned. Radial cracks originating from xenotime crystals are abundant in both quartz and orthoclase.

Potassium feldspar occurs as both orthoclase and microcline (Fig. 2). Orthoclase, the most abundant phase, occurs as large crystals or groups of crystals which are optically continuous and which enclose the xenotime and accessory minerals poikilitically. Groups of crystals exceeding 5 mm in length are not uncommon, though the average grain-size is of the order of 1 - 2 mm.

Fine-grained microcline occurs in isolated groups (Fig. 2) where it also encloses xenotime and other accessory minerals poikilitically. Xenotime crystals associated with microcline are frequently smaller in grain size and it is thought that this association represents the last interstitial liquid in the crystallising magma.

The microcline commonly shows polysynthetic twinning. The orthoclase and microcline together constitute about 28 % of the rock by volume (Table I).

Accessory zircon commonly occurs as euhedral prismatic crystals (Fig. 2) which are frequently zoned. Minor sulphide minerals, muscovite and chlorite occur. The ore minerals are mainly uranium-bearing oxides

which have been identified as uraninite (u0?) and vandendriesschite, a hydrated oxide of hexavalent uranium and lead, the latter mineral being an alteration product of uraninite.

Uraninite is the main mineralised phase occurring essentially in three modes:

(a) interstitially to quartz, orthoclase and xenotime and averaging 0,1 mm in diameter;

(b) poikilitically enclosed in quartz or orthoclase; and

The presence of uraninite is apparent on the autoradiograph where the crystals show up as discrete bright spots.

An unidentified uranium-thorium-bearing silicate mineral occurs as sub= hedral to round crystals, 0,2 to 0,4 mm in diameter. It is inter= stitial to quartz, feldspar and xenotime or enclosed by the latter (Fig. 2). The mineral is isotropic or possibly metamict and could not be identified by means of X-ray diffraction. However, the elements present in the mineral have been determined by energy-dispersive X-ray analysis as U, Th and Si with Th being the major element. The mineral may be uranothorite, a silicate of uranium and thorium, (U), (Th),

(Si04).

5. URANIUM AND THORIUM ANALYSES

Finely ground portions representative of both the fresh and weathered rock were submitted for uranium analysis and a portion of the fresh rock only for thorium analysis. Uranium was determined by delayed 208 neutron counting and thorium by analysing Th with Ge-Li gamma-ray spectrometry. Analyses are given in Table II.

TABLE II : URANIUM AND THORIUM CONTENT OF FRESH AND WEATHERED XENOTIME- POTASSIUM-FELDSPAR-QUARTZ ROCKS FROM THE VERNON CROOKS RESERVE, NATAL

Sample U30g (ppm) ThO^ (ppm)

Weathered sample 8 969 not determined VC 1 A) 12 582 not determined VC 1 BJ fresh 12 053 18 824 ppm

6. URANIUM AND THORIUM DISTRIBUTION

The autoradiograph (Fig. 1) of a polished rock face shows the uranium distribution. Radioactive xenotime can be distinguished by the wedge- shaped whitish spots of large diameter and medium intensity. Discrete high-intensity white spots due to the presence of uraninite are PER-54-9 distributed fairly uniformly throughout the rock, however, these are markedly less abundant in the finer-grained phases. Uraninite in cracks is evident in the large black areas which represent quartz crystals. The intensity of the radioactivity due to potassium feldspar on the autoradiograph is negligible.

Subsequent fission-track studies confirmed the autoradiographic analysis of uranium distribution and in addition, uranium was found to be present in the unidentified thorium-uranium silicate and in zircon.

The fairly uniform distribution of fission tracks across xenotime cry= stals suggests that the uranium forms part of the crystal structure. From this preliminary investigation it is difficult to determine the partition of uranium between xenotime and the other uranium-bearing phases. From estimates of the intensity of radioactivity on the autoradio= graph and the fission-track prints (bearing in mind that thorium con= tributes to the radioactivity in xenotime), it is concluded that most of the uranium is present in the form of the uranium oxides. This can possibly be substantiated by leach tests, which were, however, not car= ried out for this investigation.

The volumetric analysis shows that uranium-bearing minerals constitute approximately 2 % (by volume) of the rock (Table I), which implies a uranium content (as calculated from molecular proportions) somewhat higher than that shown by the analysis.

7. GENESIS OF THE ROCK

Uranium, thorium and rare-earth elements are commonly associated with rocks containing quartz and feldspar, i.e. pegmatites or possibly alaskites which are regarded as late-phase magmatic derivatives. In view of this, it is considered that the rock under investigation represents the crystallisation of final-stage granitic magmatic fluids enriched in the rare elements.

The initial magma crystallised fairly slowly with the subsequent forma= tion of large crystals of quartz, orthoclase and euhedral xenotime and interstitial uraninite but crystallised more rapidly to form microcline PER-54-1D and small euhedral xenotime crystals enclosed poikilitically. The possible action of groundwater partly redistributed the uranir.ite, alter= ing it to vandendriesschite and bringing about the formation of secondary minerals in the weathered rock.

8. EXTRACTION METALLURGY

It is concluded that most of the uranium occurs in the form of uranium oxides and that only a small fraction is contained in the xenotime crystal lattice. The ore would therefore be amenable to the conventional acid leaching process. Most of the uranium-bearing minerals are expected to be liberated during comminution.

Xenotime is not readily soluble in the conventional lixiviants. It has been decomposed on a large scale by a method utilising digestion in sulphuric acid. The finely ground ore is preheated to 170 °Z and introduced into hot (190 °C) 93 sulphuric acid, using an acid-to-ore ratio of 2. Exothermic reaction raises the temperature to 240 - 250 C and the attack is complete within 12 h. The sulphates and uranium and thorium are leached with cold water and loaded on to ion-exchange columns (Bril, 1964).

9. ECONOMIC GEOLOGY

The rocks described show a uranium content exceeding 12 kg ^Og/t which represents a very high ore grade. It is not known what the extent of the deposit is from which the samples were derived and there= fore no estimate can be made of the tonnages present.

10. REFERENCE

BRIL, H.J. Mass extraction and separation in Eyring, L.(ed.) Progress in the Science and Technology of the rare earths. Pergamon Press, London (1964), p.30-61. PER-54-11

FIG. 1 Autorai':igraph of a xenotime-potassium feldspar-quartz rock from the Vernon Crooks Reserve in Natal, showing the distribution of radioactivity in the different phases.

The effect due to uranium and thorium in xenotime is evident in the angular white areas which are prominent on the lower right side of the print The discrete bright spots are due to uraninite grains, a mineral which is also present in cracks in quartz (dark spots in the centre of the print). The distribution of uraninite is fairly uniform.

FIG. 2 Plwtomicrograph of a xenotime-potassium feldspar-quartz rock showing the typical mineral association.

Euhedral xenotime (upper left corner) is poikilitically enclosed in quartz (q) with interstitial microcline (m) and uraninite (U1 and an unidentified thorium-uranium-silicate mineral (TUS). Accessory zircon (z), muscovite (mv) and chlorite (cl) are shown to occur intarstitially.