Origins and applications of size fractions of soils overlying the Beasley Creek gold deposit, Western Australia

8/9/2019 Origins and applications of size fractions of soils overlying the Beasley Creek gold deposit, Western Australia

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ELSEVIER Journal of Geochemical Exploration 66 (1999) 99113www.elsevier.com/locate/jgeoexp

Origins and applications of size fractions of soils overlying the BeasleyCreek gold deposit, Western Australia

I.D.M. Robertson *

Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Division of Exploration and Mining,

Private Mail Bag, P.O. Wembley, Perth, W.A. 6014, Australia

Accepted 24 February 1999

Abstract

Primary mineralisation at the Beasley Creek Au deposit, hosted in black shales within mafic and ultramafic rocks,

was sulphide- and trace element-rich. The deposit subcrops beneath soil within a small window of deeply weathered

Archaean basement in the northeast Yilgarn Craton of Western Australia, surrounded by extensive colluvialalluvial plains.

Regolith overlying the mineralisation and its host sequence is comprised of ferruginous saprolite and some duricrust,

overlain by redbrown soil which is strewn with residual ferruginous lag. The colluvialalluvial wash plains around the

weathered basement window are mantled by a thicker redbrown soil, strewn with polymictic lag. Soil samples and surface

lag were collected along two traverses across the mineralisation with the aim of determining the optimum geochemical

sampling medium for exploration. The 7104000 m soil fraction consists largely of black goethite- and hematite-rich

nodules, red to yellow ferruginous clay granules, minor quartz, calcrete and, close to subcropping mineralisation, scarcegossan fragments. In contrast, the 75710 m fraction, which appears to be largely aeolian in origin, consists mainly of

redbrown hematite-coated sand grains, minor feldspar, and small, ferruginous granules. The largely aeolian 475 m

fraction is composed mostly of quartz, with minor kaolinite and Fe oxides. A clay-rich


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1. Introduction

1.1. Regional regolith history and exploration

implications

The Archaean Yilgarn Craton of Western Aus-tralia (Fig. 1) is a complex granitegreenstone(metavolcanics and metasediments) terrain, whichforms a generally low, flat plain that has been sta-ble and exposed since the Proterozoic and that hasbeen very deeply weathered under seasonally humidconditions from the Mesozoic to the mid-Tertiary.Although there is some relatively fresh outcrop, deepweathered profiles are generally developed on thegreenstones. These consist of thick, clay-rich sapro-lites which pass upward into a clay-rich pedolith, amottled zone, and finally, where the regolith profileis complete, a ferruginous, lateritic residuum form-ing what is regarded as a relict regime (Anand andSmith, 1993). Following the period of deep weath-ering, conditions became progressively arid causingmodification of the weathered profile and partialstripping of the weathered landscape, leaving ero-sional regimes. Concomitant terrestrial sedimentschoked what little relief remained, forming extensivedepositional regimes.

The exploration challenge is to locate Au and

base metal deposits in this deeply weathered land-scape, where outcrop is rare, bedrock is so alteredas to be unrecognisable, and where Au, in partic-ular, may be leached. Despite this, explorationistsin Australia have been very successful at locatingsignificant Au mineralisation in this difficult terrain(see Smith, 1996). Some of this success is due to anunderstanding of the regolith and includes recogni-tion of large geochemical halos developed in lateriticresiduum (Smith and Perdrix, 1983) of the relictregimes which may, in places, even form orebodies

in their own right. Smaller geochemical halos occurin the saprolites of the erosional regimes. This excel-lent ferruginous sampling medium persists, in places,beneath transported material. Careful planning of ex-ploration strategy must begin with an inventory ofthis weathered landscape and sampling strategies(e.g., choice of media and sampling densities) aretailored to the regolith geology (Anand and Smith,1993; Anand, 1993).

1.2. Mineralisation

Gold mineralisation at Beasley Creek was discov-ered by the Western Mining Corp., 12 km WNW

of Laverton at 122180

E, 28340

S in a small, deeplyweathered window of basement. Proven and prob-able ore reserves, prior to mining, were 2.1 Mtat 2 g=t. The weathered mineralisation, which wasprobably originally rich in sulphides and pathfinderelements, is hosted in a NS-striking black shale,some 1540 m thick, which dips at 45E. The shaleis phyllitic, weathered to over 200 m depth, and theAu is associated with ferruginous zones. The phylliteis enclosed in amphibolite schist that is less intenselyweathered (40 m) and is, itself, enclosed in komati-ites of the Mt. Margaret Anticline. Small granitic andmetadolerite lenses associated with NWSE-strikingfaults and shears, intrude the sequence.

1.3. Research objectives

The geomorphology and regolith geology wereinvestigated and surficial samples were collected atBeasley Creek to: (1) establish regolithlandformrelationships for this arid area; (2) investigate sur-ficial sample media, fitting them into the regolithlandform model; (3) establish the multi-element sig-

nature of Beasley Creek; and (4) formulate optimumexploration methods for the area.

1.4. Regolith and geomorphology

Beasley Creek is typical of the arid NE part ofthe Yilgarn Craton (Fig. 1), well to the north ofthe Menzies Line (Butt et al., 1977), and charac-terised by summer rainfall, acacia vegetation, valleycalcretes, and fresh ground waters. Soil and lag arethe preferred surficial sampling media in the relict

and erosional regimes. The area south of the Men-zies Line is dominated by winter rainfall, eucalyptusvegetation and saline ground waters, where soil car-bonates are the preferred exploration sample mediumfor Au (Lintern and Butt, 1993).

A detailed study of the exposed regolith, coupledwith soil and lag sampling, was completed immedi-ately prior to mining (Robertson and Churchward,1989). Dispersion into the saprolite was investi-gated from drillspoil sampling (Robertson, 1991).


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I.D.M. Robertson / Journal of Geochemical Exploration 66 (1999) 99113 101

Fig. 1. Location map of soil geochemistry orientation site in the arid part of the Yilgarn Craton of Western Australia. Contours of average

rainfall (mm) are shown with the Menzies Line.

The mineralisation lies beneath a low hill, only a

few metres high, comprising saprolite of Archaeanrocks. This is flanked by wash plains of colluviumalluvium, beneath which lie weathered Archaeanrocks. To the east, an ancient channel is filled withdeeply weathered Permian fluvioglacial sediments.The wash plains (Fig. 2) were developed largelyby unchannelled fluvial flow (sheetwash). To thenorth and south lie broad drainage floors in which

ephemeral streams are incised.

The phyllitic host rock follows the crest of thehill (Fig. 3). The deeply weathered profile has beenslightly eroded to leave a partial cap of lateritic duri-crust to the east and saprolites to the west. There arepatches of calcrete on the higher ground (Fig. 2) andmassive gypsum is developed within the top of thesaprolite, indicating modification of the profile by in-

creased aridity. This profile would be classified asB 1 Ca, Gy [1,3] by Butt and Zeegers (1992). Thebasement window is covered with a few small trees

on 100200 mm of neutral to alkaline, organic-poor,brown soil, strewn with ferruginous lag. The sur-rounding wash plains have smaller trees, shrubs andWanderrie banks (Mabbutt, 1963) which have devel-oped on a thicker (300500 mm), acidic, redbrownsoil, strewn with polymictic lag. This washplain soil,the soil on the hill, and some near-surface saprolite,have been slightly silicified to hardpan (Bettenay andChurchward, 1974) below about 200300 mm.

2. Sampling strategy

Samples of soil and lag were collected beforethe ground was significantly disturbed by pre-min-ing activity. These were sampled along two linesacross the mineralisation (Fig. 3), extending wellinto background to the east. Results from the lagsampling have been described elsewhere (Robertson,1989, 1996a). Soil samples of 1 kg were taken from


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Fig. 2. The Beasley Creek mine site, before mining disturbance,

showing the window of Archaean rocks (AR), now saprolite

covered with a thin soil, dotted with exposures of calcrete ( CC)surrounded by wash plains (WP) and Wanderrie banks. Airphoto

by Kevron Air Surveys, published with permission of Metex

Resources NL.

undisturbed locations at 25250 mm depth. The top25 mm was discarded to avoid dust contaminationfrom adjacent drill sites. The sampling interval wasadjusted to ensure adequate resolution; 25 m close

to mineralisation, extending to 50 and 100 m furtheraway. Four background samples straddled the area

some 700 m from mineralisation (Fig. 3), but thesecame from the colluvial wash plains.

3. Size distribution and fractionation

3.1. Size distribution

Typical soil samples were initially sieved into>4000 m, 7104000 m, 500710 m, 250

500 m, 142250 m, 75142 m and 4000 m, 7104000 m, 75710 mand 4000 m and vegetable matter were discarded. Theferruginous 7104000 m fraction was separated bywet sieving, washed, dried at 95C, milled to


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Fig. 3. Sample sites at Beasley Creek, showing the low relief of the hill, the pit outline, two sampling lines, the mineralisation subcrop,

the varied sample spacing and the location of background samples on the wash plain.

the clay suspension is similar to the pH range of nat-ural soil environments (these soils vary between pH5.0 and pH 8.5 depending on the presence of carbon-ates) so that a minimal loss of pathfinder elements tothe suspending solution would be expected.

4. Characteristics and treatment of key sizefractions

(1) The complete soil (


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Fig. 4. Schematic description of the soil size fractionation process. A combination of dry and wet sieving, clay suspension and clay

flocculation was used. This prepares a wide range of size fractions for geochemical analysis.

some potential for bedrock identification (Robertson,1989, 1996b).

(3) The 75710 m fraction was discarded, asit is largely aeolian. This fraction consists of quartz

Fig. 5. Relationship between particle size and mineralogical composition in a typical soil from Beasley Creek.

and some very fresh microcline, indicating derivationfrom granitic terrain and transport over a significantdistance (Fig. 9). Although some grains are angular,others are rounded and polished or frosted and all


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Fig. 6. The 7104000 m soil fraction contains a large proportion of dark, shiny, goethite- and hematite-rich granules ( GO) and dark,

redbrown to yellow granules of ferruginous clay (FC), with a slightly lesser proportion of quartz (QZ).

Fig. 7. Components of the 7104000 m fraction showing the dark goethite- and hematite-rich granules (A), the clay-rich granules (B)

and quartz (C). Some of the smaller quartz grains are rounded, indicating transport.


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have a very thin coating of hematite. There are alsominor ferruginous granules. This quartz-rich soilfraction is remarkably similar to the materials ofnearby linear dunes of red sand (angular to rounded,

hematite coated quartz with minor microcline) whichoverlie granites and onlap greenstones 5 km to thewest.

(4) The 475 m fraction roughly correspondsto the


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Fig. 10. SiAlFe diagram to illustrate the effects on the bulk compositions of size fractionation.

6.3. Au and Cu

The mineralisation and the halo in the duricrust

are both clearly shown by the lag and the 7104000m soil fraction using Cu (200 m wide reaching 180

ppm) and Au (300 m wide reaching 200 ppb); theanomaly in the complete soil is less obvious (Fig. 11).By analogy with the fine lag (Robertson, 1996a), Cuis probably enriched in goethite-rich gossanous ma-terial in the 7104000 m fraction. Among the finefractions, both Au and Cu are preferentially concen-trated in the


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Fig. 11. Multi-element geochemical profiles across the Beasley Creek Au deposit showing the position of mineralisation and regolith

units.


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m soil fraction; this is less apparent in the completesoil, is weakly evident in the


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Fig. 13. Mineralogical profiles of kaolinite and sericite for the


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posit relatively rich in pathfinder elements, there aresubstantial anomalies in Au, Cu and As, with weakeranomalies in W, Sb, Cd and Zn, reflecting both themineralisation and its substantial halo in the duri-

crust. These results are closely comparable to thosefrom the fine lag, which is to be expected as this soilfraction was the source of the lag.

The 75710 m fraction should be discardedas it consists predominantly of exogenous quartz.Even the 475 m fraction contains significant pro-portions of aeolian quartz so this is also a poorgeochemical medium. However, the


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Origins and applications of size fractions of soils overlying the Beasley Creek gold deposit, Western Australia