Channel Profile

Three principal horizons can be recognised in the drill holes and excavations-viz the overburden, calcrete and clay-quartz (Figure 2). Although the same sequence is present throughout, the horizons nevertheless show considerable variation over short distances and commonly have gradational contacts.

Overburden

The soil is a variable unit, 1 to 2 m in thickness, commonly consisting of a red-brown, sandy loam with a thin, indurated hardpan layer at a depth of 10 to 15 cm. However, on the saline claypans, the surface horizon is a brown, gypseous clay. A local variant of the soil unit is the carbonated loam, marking the transition to the underlying calcrete. Of particular interest m this part of the profile is a thin calcrete gravel, or rubble, composed of either carbonate pisoliths showing a prominent concentric growth pattern, or hardpan peds coated with carbonate. This layer closely resembles the pedogenic calcretes which are widely developed in other parts of the State.

nomr

IRREOULAR «AMTE BASEMENT »T »-20B

CLAY-OUARTZ

rfv°l LOAM rCTl CLAY-OUARTZ

OVERBURDEN J __' ~~

\_gS CARBONATED LOAM CLAY-OUARTZ J O CARBONATED CLAY-QUARTZ CALCRETE I f"l WITICLAY)

CALCHETE-TRANSITION CALCRETE ~J"" DIAMOND DRIILL HOLE

Figure 2

Lithological cross-section

Calcrete

The calcrete at Yeelirrie is of two predominant types with contrasting physical, chemical, mineralogical and genetic characteristics; one is a buff, friable, "earthy" variety, and the other white, hard, nodular"porcellaneous"

and commonly riddled with voids.

Carbonate first appears in the lower part of the overburden as thin, widely-spaced, horizontal seams, becoming more numerous with depth and isolating rafts of soil or hardpan which are progressively reduced in size until they are no more than small inclusions. Thus, the emphasis changes from carbonate seams in soil to soil relics in a soft carbonate matrix. Every stage in the elimination of these inclusions can be seen, from recognisable soil material to the point where their former presence is marked only by slightly darker ghosts, giving the calcrete a mottled texture. With the increase m carbonate downwards, the process eventually leads to the formation of a more or less continuous sheet of earthy calcrete.

In contrast, the porcellaneous calcrete originates in the lower part of the soil horizon as small, discrete spheres covered with a network of contraction cracks (possibly due to dehydration), and in the case of the larger examples, often with an internal cavity. When broken, they show no megascopic evidence of any included soil material. As the spheres become more abundant, either laterally or downwards, they coalesce to form increasingly larger, bulbous masses with the typical porcellaneous character. Further emphasizing the contrast in lithology and structure, the larger bodies commonly truncate the horizontal layering of the earthy calcrete and may be bounded by low-angle, slickensided slip planes. Indurated bands in the overburden can sometimes be seen draped over the upper, convex surface of these porcellaneous calcrete bodies, in extreme cases forming a tight synclme between adjacent masses, or passing down between them as a disrupted septum. The basal zone of the porcella-neou calcrete lenses may be extensively sihcified, and may contain large caverns measuring a metre or more in length.

Where the two calcrete types can be seen m excavations, they demonstrate contrasting modes of formation. The earthy variety appears to develop by an essentially passive process, infiltrating the soil along horizontal planes or transgressive fractures, and gradually incorporating the soil material into a carbonate-rich matrix. On the other hand, the porcellaneous calcrete gives the strong impression of dynamic growth at a number of discrete sites without the assimilation of soil material into the developing carbonate spheres. With the continued precipitation of carbonate, the process can be envisaged as leading ultimately to the formation of upward-moving growth mounds which pierce the overlying carbonated sediment, and probably giving rise to the elongated ridges now seen at the surface along the channel. If it is a dehydration effect, the network of cracks on the surfaces of the porcellaneous calcrete spheres would indicate a decrease in the water content of the surrounding medium, due either to a descending water table, to the upwards movement of the growth mounds above the phreatic zone, or to a combination of the two. Similar anticlinal structures have been observed in calcretes elsewhere m Western Australia [2].

Clay-quartz

The calcrete is underlain by alluvium which continues down to the basement granite. It consists essentially of a red clay containing disseminated detrital quartz grains, and shows little variation other than the presence of occasional quartz-rich bands, thin seams of celestite, or a thin arkosic layer overlying the decomposed granite.

The transition from calcrete to clay-quartz is normally gradational, exhibiting similar carbonate seams to those in

the upper part of the profile, but becoming less well developed and more widely spaced as the carbonate content progressively decreases with depth

The principal constituents of the profile are quartz, carbonates (dolomite and calcite), clays (smectite, specifically saponite, ilhte, kaolmite; and occasional sepiohte [5]), and feldspar; gypsum and celestite may be locally abundant m restricted sections.

The clay-quartz is a mixture of kaolmite and quartz in varying amounts—typically 45 % kaolmite and 33 % quartz — together with minor saponite, illite and feldspar In the calcrete, carbonates and saponite become dominant at the expense of quartz and kaolmite, with the proportions of each tending to reflect the different physical characters of the two types- e.g 75 % total carbonate in the porcellaneous calcrete, but with a higher saponite content in the earthy variety Calcite is normally subordinate to dolomite, but it may become important, or even the sole carbonate species, in the transition zones and in the overburden The distinctive nature of these valley (groundwater) calcretes is further emphasized by their appreciable dolomite content, which contrasts with the high calcite content of most pedogemc calcretes Limited chemical analyses indicate an average of 18 8 % MgO m the mam part of the calcrete, but samples of near-surface calcrete gravel with only 3 9 % MgO are more comparable with the world average of 3 1 % [3]

5. URANIUM MINERALIZATION

The main body of ore-grade mineralization is restricted to a zone approximately 9 km long and between 0 5 and 1 5 km wide, elongated along the centre of the channel. It extends from the surface to a depth of 14 m, but is exposed m only one small outcrop, and is mainly concentrated in the interval between 4 and 8 m below the surface The overall form is therefore, of aflat-lying, shallow sheet, averaging approximately 3 m in thickness, and for the most part situated immediately below the present water table (at 4.5 m) Although the mineralization occurs m both the calcrete and clay-quartz, the strongest development is in the transition zone beneath the calcrete Throughout the orebody, the mineralization maintains a general continuity but is highly irregular in detail (Figure 3), as yet, there are insufficient exposures to permit the recognition of any local control on this distribution

The revised estimate of the ore reserves at June 1 982 is as follows Table 1

Revised Estimate of the Ore Reserves at June 1982

Ore

X-ray diffraction analysis has so far failed to identify any uranium mineral other than carnotite At no stage during the exploration or evaluation of the deposit has there been any evidence that it represents either the redistribution of primary mineralization in the basement, or an in-situ alteration product In addition to the normal, canary-yellow colour, the carnotite is not uncommonly light-brown or olive green The possible effect of composition on the colour has not been investigated, but the variations may be due either to a thin film of microcrystallme quartz that commonly coats the carnotite, or to the reduction of vanadium [6]

The carnotite is clearly one of the last stages in the development of the calcrete, only minor quantities of silica or amorphous clays are demonstrably later, usually lining the voids in the porcellaneous calcrete It normally occurs as seams or disseminations in the earthy calcrete, as a gram coating in the clay-quartz, or as a fracture paint —for example, on the slip planes round the margins of the porcellaneous calcrete mounds Disseminated carnotite has not been observed in the porcellaneous calcrete, where it normally forms a thin film lining the voids No systematic determination of uranium m the soils overlying the orebody has been attempted Sporadic sampling during orientation surveys has shown wide variations (1 to890ppm U) in soils taken at depths of between 0 1 and 0 8 m , with mean values along three traverses ranging from 5 to 40 ppm U [7]

6. RADIOACTIVITY

Although there is only one outcrop of ore-grade mineralization, the Yeelirne orebody nevertheless has a strong radiometric signature During the 1 960 s the Federal Bureau of Mineral Resources (Geology and Geophysics) (the BMR) conducted an extensive airborne magnetic and radiometric survey over the eastern part oftheYilgarn Block

Figure 3 Grade distribution

^| >IOOO m «00-1000

260-900

n <*«>

-.———— APPftOX LIMIT OF ORE BOOY

—s— Fence LINE

Figure 4

Ground radiometric survey

at a 1 mile (1.6 km) line spacing and 500 ft (150 m) mean terrain clearance. The radioactivity was recorded by two scintillometers, one with a ten-second time constant mounted inboard to measure the regional level, and the other with a one-second time constant trailed 260 ft (80 m) below the aircraft to locate point-source anomalies.

The results released in 1970 [8] showed an extensive regional anomaly measuring approximately 1 0 x 2 km, elongated along the channel and located directly over the mineralization. Peak values on the final published map are 450 cps, compared with the general background level of 50 to 100 cps over the catchment area. In addition, two significant point-source anomalies were identified up-drainage from the orebody and believed to be over claypans. As this was a total-count system, the radioélément characteristics of the source could not be determined.

A ground radiometric survey using a McPhar TVS intergrating spectrometer on a 500 x 60 m grid showed this to be a strong uranium, but weak thorium, source; the low thorium content was later confirmed by limited chemical analyses during the drilling stage. This ground survey reproduced the airborne regional anomaly, though in greater detail. The orebody is now defined by the twice background contour, corresponding approximately to 400 cpm on the T2 channel (> 1.63 MeV), with several small zones above 10 times background, and a peak in excess of 5 000 cpm (approximately 25 times background) over the only mineralized outcrop (Figure 4). The close agreement between the airborne and ground surveys emphasizes the value of a rapid airborne survey as an exploration technique in this particular environment.

101-200 201-400

>400

Solubility Index

• <-2.0 A -2.0 - -1.0

• -1.0 - 0

• >0 ^kilometres Figure 5

Uranium in groundwater (From : Cameron et al. [13])

The low thorium content of the ore makes the deposit potentially amenable to assessment by y-logging. Detailed studies have shown that although the ore is not in equilibrium, neither the extent nor distribution is sufficiently serious to preclude the application of gamma spectrometry to grade estimation and control [9, 10, 11, 12].

7. RADON

Because of the nature of the mineralization and the local conditions- e.g. near surface position, permeable host rocks; a thin and generally porous soil coven and a stable, arid climate— radon measurement appeared to be potentially useful as an exploration technique. However, events pre-empted its use at Yeelirrie other than in orientation surveys.

An early scintillation-detector survey, sampling soil-gas drawn from a depth of 1.3 m, demonstrated a strong radon flux over the ore, ranging from 10 to 50 times the background level. However, in a study of the comparative merits of scintillation detectors and a-sensitive films, Severne [7] attributed the surface response to the uranium content of the soils immediately around the sampling point, rather than reflecting the underlying mineralization.

Because of the definitive radiometric signature, radon measurement offered no exploration benefit over a more rapid radiometric survey.

8. HYDROLOGY

In the Murchison region the permeable calcrete are important aquifers, providing abundant water for stock purposes and occasional potable supplies; as an example, pumping tests at Yeelirrie produced a yield of approximately 4.5 million litres a day (1 million imperial gallons) from an exploratory excavation measuring 450 x40 x9 m.

Except for the rare occasions when unusually heavy rainfall will produce a short-lived surface flow, the water movement is entirely subsurface. Groundwater recharge is predominantly by infiltration and runoff at the catchment margins and flows through the alluvial plain into the central channel where the water table is at a depth of between 4 and 5 m. Stock bores in the catchment area and some drill holes in the orebody were sampled to provide orientation data for a regional hydrogeochemical survey [13] and for environmental studies [14]. This sampling demonstrated the progressive increase in salinity downstream, from an average of less than 750 ppm TDS around the margins of the catchment area to 5 000 ppm on the edge of the channel, and reaching a maximum of over 25 000 ppm in the calcrete near the orebody, though there are variations, depending on the extent of the lateral recharge. Whilst the concentrations of the major ions (Na+, K+, Mg2+, U, Ca2+, Cl~, SO2."

and HC05) show a sympathetic relationship to salinity, the proportions relative to TDS follow two contrasting trends, with CP, SO2", Na+, K+ and U increasing into the channel, whereas Ca2+, Mg2+ and HCOJ decrease.

The regional hydrogeochemical survey, which ultimately covered approximately 60 000 km2, clearly highlighted the strongly anomalous concentration of uranium in the Yeelirrie groundwater. From a background of 5 to 10 ppb uranium on the alluvial plain, the concentration rises to between 100 and 450 ppb along the channel, and up to 1 200 ppb in watertaken from a drill hole into the orebody [13]. Both the uranium content and the concentration relative to TDS are highest immediately upstream from the deposit. Calculated carnotite solubility indices increase progressively downstream, ultimately becoming positive close to the orebody [1 5].