BMRlumalf Australian Gelgy & Gephysics. 2 (J977J 149 154 149 Gephysical respnse f heavy-mineral sand depsits at Jerusalem Creek, New Suth Wales D. F. Rbsn and N. Sampath Intrductin BMR has started evaluating hw gephysical methds might assist in explratin fr depsits f the heavy minerals rutile, zircn, ilmenite, and mnazite. Histrically, explratin fr these heavy minerals has cncentrated n the search fr high-grade nshre beach sand depsits, and has emplyed surface sampling and shallw drilling. Hwever there is a grwing awareness that future explratin targets are likely t be lw grade and perhaps either deeply buried r lcated ffshre. Explra tin directed twards these targets will require new explra tin methds, particularly thse which ffer remte sensing capabilities. BMR has begun a prgram f test surveys and labratry investigatins t establish the gephysical respnse f heavy-mineral depsits. As the first stage f these investiga tins, airbrne and grund gephysical surveys were made ver heavy-mineral depsits in the Jerusalem Creek area f NSW during 1975. Jerusalem Creek was chsen fr the initial field investigatins because f the variety and extent f heavy-mineral depsits in the area. The airbrne survey (Fig. 1) was carried ut ver an area f 200 sq km f castal plain suth f Evans Head, and used magnetic and gamma spectrmeter methds. Grund surveys were cnducted ver the Evans West and GL 10 depsits, using magnetic, radimetric and induced plarizatin methds. The lcatins fthe Evans West and GL 10 depsits are shwn in Figure 1. The Evans West depsit is 2 km in length, 3 m thick, 30 m wide, and is cvered by 3 m f verburden. The depsit has a fairly sharp grade cut-ff at its bundaries, and averages abut 10 percent heavy minerals cmprising 36 percent zircn, 35 percent rutile, 25 percent magnetics, minr mnazite and ther minerals. The magnetic fractin is largely ilmenite. The GL 10 depsit has a similar mineral cmpsitin but the depsit cnsists f several parallel leads up t 40 m wide. The average grade fthe depsit is nly 1-5 percent, and the depsit is buried under nly 0.5 m f cver. The magnetic, radimetric and induced plarizatin methds were used because the mineral assemblage f the Jerusalem Creek depsits suggests susceptibility, radiactivity and induced plarizatin cntrasts between re and the sur runding sands. The Evans West and GL 10 depsits were chsen as grund survey sites because extensive mine develupment drilling prvided excellent gelgical cntrl fr the interpretatin f gephysical results. T establish the physical prperties f heavy-mineral res and t assist interpretatin f the gephysical surveys, labratry measurements f susceptibility, induced plarizatin and radiactivity were made f re and sand samples cllected frm the Evans West depsit. The surveys at Jerusalem Creek were carried ut in clse c-peratin with Assciated Minerals Cnslidated (AM A), whse assistance in the planning and carrying ut f the survey is gratefully acknwledged. Jerusalem Creek airbrne survey The main bjective f this survey was t seek brad changes in the radiactivity and magnetic intensity f the sand ridges assciated with heavy-mineral depsits. Figure 1. Lcality map. SURVEY AREA IOkm '-- -'- 1 H56/B7-17A The survey was flwn at an altitude f 100 metres; a traverse spacing f 400 metres and a speed f 180km/hr. The radimetric survey was made with a differential 4- channel 3700 cm' Hamner Harshaw gamma-ray spectr meter; and the magnetic survey used a BMR-designed fluxgate magnetmeter with a sensitivity f 1 nt. The survey specificatins were adequate t lcate the brad surces sught, but were nt suitable fr detecting small islated surces. Radimetric results Figure 2 shws the results f sme ttal cunt radimetric traverses, which are superimpsed n the gelgy (Nichlsn, 1974) f the Jerusalem Creek area. The heavy mineral leads are crss hachured. The radimetric results shw a few large and sme small radimetric anmalies n a lw backgrund f less than 3 IIR per hr. Thrium anmalies with a radiactivity f up t 20llR per hr were recrded ver heavy-mineral stckpiles (area A), and smaller anmalies were bserved ver mining sites (area B) and alng sme sand ridges (area C). The results indicate
ISO GEOPHYSICAL RESPONSE OF HEAVY-MINERAL SANDS Line~IO~I~O.. ~... ~.. "~IIII.. ~... ~.. IIII~... ~.. ~~.J~... --'~ Ttal Cunt 200 t 100 Line 1290 LEGEND Mined areas L:?)J Outer barrier sands Estuarine and f luvial depsits r :::,',,::':' :,1 Inner barrier sands Line 1560 ~ Fluvial and paludal depsits Bedrck 5km H56/87-19 Figure 2. Airbrne radimetric results. that the heavy-mineral depsits at Jerusalem Creek are radiactive but that the radiactivity is easily masked by verburden. Magnetic results The results f the airbrne magnetic survey shw a psitive, magnetic gradient f abut 3 t 5 nt per km. The surces f all magnetic features are deep, and there is n indicatin f magnetic surces within r at the base f the beach sands. Grund surveys at Evans West Grund gephysical surveys emplying magnetic, radi metric and induced plarizatin methds were made ver
GEOPHYSICAL RESPONSE OF HEAVY-MINERAL SANDS lsi the Evans West depsit. Samples were als cllected fr labratry measurements_ Figure 3 shws the results f averaging the heavy mineral cncentratins in the Evans West area ver a depth f 10 metres. The gap in the rebdy between 600 Nand 300 N is due t a large sand dune which has prevented drilling fthe depsit in this area. loow looe -- 1500N Magnetic survey Magnetic surveys emplying intensity, gradient and sus ceptibility measurements were made. The surveys were designed t detect small anmalies caused by thesuscepti bility cntrast between the re and the surrunding sands. The Evans West depsit cntains abut 0.01 percent magnetite and wuld prbably have a susceptibility cntrast f arund 0.0005 SI units. Depending n the size and gemetry f the rebdy such a cntrast might prduce intensity anmalies f up t 5 nt if the depsit were buried n deeper than ne r tw metres. The intensity survey was made with a prtn-precessin magnetmeter f 1 nt sensitivity, and invlved abut fifty traverses acrss the rebdy.. Traverses were up t 300 min length, were frm 10 t 50 m apart, and used a statin interval f 5 m. The results f the intensity survey shw a variatin f less than 15 nt ver the survey grid, and indi cated that there were n magnetic trends r anmalies assciated with the re depsit. Because f the lw suscepti bility and the cnsiderable depth f verburden these results are nt surprising. A magnetic gradient survey was made alng tw traverses using sensrs with a 4 m vertical separatin and a prtn precessin magnetmeter f 0.1 nt sensitivity. This survey was designed t enhance the magnetic respnse f the re bdy by measuring the rapid attenuatin in intensity with distance frm the magnetic bdy. N anmalies were detected n any f the traverses. In situ susceptibility measurements n sand and re were made thrughut the survey area with a Bisn susceptibility meter. N cntrasts in susceptibility were bserved. -- 1300 -- 1100 -- 900 -- 700 Radimetric survey The grund radimetric survey was carried ut with a Gemetrics DISA 400A, 4-channel gamma-ray spectr meter. Abut 30 traverses were made acrss the rebdy, including several traverses where the verburden had been remved. The radimetric data were prcessed by remving nn-gelgical backgrund, and 'stripping' t separate the cntributins t cunt rates frm each fthe radi-elements thrium, uranium and ptassium. The results shw that the radiactivity in the area is predminantly caused by thrium; anmalies up t 50 times backgrund ccur ver expsed mineralizatin. The results nly reflect the re zne where the verburden was stripped. Mnazite accunts fr 0.2 percent f the heavy minerals and is presumably the radiactive surce. Althugh the re is highly radiactive, 3 metres f verburden cmpletely blankets the respnse. Electrical survey The electrical surveys used magnetic induced plariza tin (MIP) and electrical induced plarizatin (EIP). They were designed t detect a pssible electrical prperty cntrast between the re and silica sands. MIP (Seigel, 1974) was used as the main technique because it ffered easier reslutin f small shallw surces and culd ver cme the prblem f the high cntact resistances expected. Few EIP traverses were made ver the rebdy. Figure 3. AVERAGE HEAVY-MINERAL CONCENTRATION >8% _ H56/B7-20A 4-8% ~ 2-4% ~ 1-2% bs;j -- 500 -- 300 -- loon Average heavy mineral cncentratin Evans West. MIP. Figure 4 shws stacked prfiles f the MIP chargeability. Anmalus values are shaded in black, and nn-anmalus znes dtted. Black lines shw the utline fthe re bdy as knwn frm auger hle sampling. Overall there is gd crrelatin f anmalus MIP chargeability with cncentratins f heavy minerals. Westerly extensins f fur nrthern lines and easterly extensins f fur suthern lines shw typical backgrund respnses.
152 D. F. ROBSON AND N. SAMPATH 100W 100E -1500N -1300 similar resistivity t the un mineralized sands. This deduc tin was substantiated by subsequent labratry measure ments. There was n infrmatin n the nature r lcatin f heavy-mineral cncentratins in the shape fthe MIP decay curve. EIP. EIP traverses were made acrss the Evans West depsit using the diple-diple array, with a diple spacing f frm 5 ni t 10 m. T, few traverses were made t allw a satisfactry cmparisn with the MIP results, but small anmalies were recrded ver the heavy-mineral zne which were less well defined than the MIP anmalies. -1100-900 Physical prperty measurements Samples f re, verburden and unmineralized sand were cllected frm the Evans West area, and labratry measurements f susceptibility, resistivity, IP effect and radiactivity were made. The results f the measurements are summarized in Figure S. Susceptibilities f all samples were lw; resistivities shwed n apparent cntrasts between mineralized and un mineralized sand. The mst significant physical prperty was the IP effect and the largest effects were assciated with the magnetic fractin f the re which cntains 70 percent ilmenite. The re.was highly radiactive, and spectral analysis shws that thrium daughter-elements were the main surce f radiactivity. PHYSICAL PROPERTY MEASUREMENTS OF BEACH SANOS JERUSALEM CREEK. NSW DESCRIPTION SUSCEPTIBILITY RESISTIVITY IP EFFECT RADIOACTIVITY (mv/v) (hm-m) (}LRfh ) AT 50 ms DUNE SAND LOW 300-400 <5 2-5 mt T -10 0 +10-500 -300 ORE LOW 300-500 10-30 40-60 MAGNETIC FRACTION LOW 100-300 80-100 <5 OF THE ORE H56/ B7-22A Figure 5. Physical prperty table. Grund surveys at GL 10 Grund gephysical surveys using magnetic. radimetric and MIP methds were made alng fur traverses t the nrth and fur traverses t the suth fthe GL 10 depsit. H56/B7-2IA Figure 4. ChargeabUlty. -loon The anmalus respnses average -3 mt/ T and the backgrund level is frm 2 t 3 mt/t. The chargeability respnse indicates that the re zne is cntinuus between 600 Nand 300 N, where auger sampling was nt pssible. At every chargeability measurement, the nmialized magnetic field strength is als determined. This parameter is similar t a measurement f relative cnductance and can be used t detect changes in the resistivity f the grund. At Evans West this parameter was fairly unifrm and the results indicate that the heavy-mineral depsits have a Magnetic and radimetric survey As at Evans West the magnetic results shw n trends r anmalies assciated with the heavy-mineral leads. The radimetric survey results were als similar t thse recrded at Evans West. Where heavy minerals were expsed at the surface a high thrium cunt rate was detected, but in undisturbed areas a lw unifrm cunt rate was recrded. MIPsurvey Figure 6 shws the results f the MIP survey at GL 10 as stacked chargeability prfiles superimpsed n the heavy mineral cncentratins. The utline f the heavy-mineral cncentratins has been determined by averaging the heavy mineral grade ver a 3 m interval; bundaries f the heavy mineral cncentratins are defined as O.S percent. Small but
GEOPHYSICAL RESPONSE OF HEAVY-MINERAL SANDS 153 persistent MIP anmalies appear t crrelate with the knwn heavy-mineral leads. 240W 120 120E +40-6400 S - 6800 mt T ~ -7200 ~ ~ ------.'... REMOVED:.. -..:..:... -.:-.-..:...:..-~.-.-.-.-.:....:.,.....:...:...: :.......::...- -7600 > 120ms +80 =-:;=v~~""-.. - 8000 S ~ Heavy-mineral cncentratins H56/B7-23A +60 Figure 6. GL 10 chargeability results. IP survey ver heavy-mineral stckpiles The surce f the IP anmalies detected by the MIP and EIP surveys at Evans West and GL 10 was investigated by IP surveys ver three heavy-mineral stckpiles at the Jerusalem Creek prcessing plant. The surveys were carried ut using an ElP, diple-diple array with an electrde spacing f 5 m. The first stckpile (a) cntained heavy mineral cncentratins after the remval f silica; the secnd (b) cnsisted f heavy-mineral cncentrates after the remval f silica and magnetic prducts; and the third (c) stckpile cntained the magnetic fractin f the heavy minerals and cnsisted f ver 70 percent ilmenite. Results are shwn in Figure 7. The traverse acrss the mineral dump with the silica remved recrded 'a peak chargeability respnse f abut 32 ms with backgrund f abut 7 ms; and ver the dump f heavy minerals with bth silica and magnetics remved a lw chargeability respnse f abut 5 ms. Hwever chargeabilities in excess f 120 ms were measured ver the dump cntaining the magnetic fractin. The results f the shrt traverses ver these stckpiles shw that the IP effect recrded is due t the magnetic fractin f heavy-mineral sands. This cnclusin was supprted by labratry measurements. Figure 7..' '. : > 70 % ILMENITE. + 20 -~ :.-: -.:..._....,;_:-:...:.. :...-. -. -.-.:...:.~..:...- H56/ B7-24 IP results ver stckpiles. Discussin and cnclusins Gephysical surveys ver heavy-mineral depsits in the Jerusalem Creek area, NSW, were made with the magnetic, radimetric and induced plarizatin methds.
154 D. F. ROBSON AND N. SAMPATH 100E I - 10 ~T 0 + 10 Figure 8. lode I rs:::sj Heavy-mineral cncentratins Lanherne Beach MIP results. 500E I 1650N -~-~-"'- --~7'--""" ;;;;::::? 0::::::::::: H56/B7-26A t -5 +5 t:5 '"--TT +5 Induced plarizatin and in particular the MIP methd is capable f detecting and utlining bth high and lw grade depsits. Labratry and field measurements shqw that the IP respnse f the heavy-mineral depsits in the Jerusalem Creek area is due t the ilmenite cntent f the depsits. Magnetic surveys did nt prduce a respnse which culd be directly related t heavy-mineral cncentratins. This bservatin can be explained by the lw susceptibility f the re at Jerusalem Creek. Measurements made in the labratry and in mine wrk ings shw that the heavy-mineral depsits are highly radi active, but wing t the blanketing effects f verburden n rad iactivity anmalies were recrded ver in situ depsits. Induced plarizatin, in particular magnetic induced plarizatin, and radimetric methds, may prve t be useful tls fr prspecting fr heavy-mineral depsits. Hwever further wrk is hecessary t establish in what envirnments, and in what peratinal mdes, the methds culd be used. Fllwing the MIP surveys in the Jerusalem Creek area, similar surveys have been made ver a variety f heavy mineral depsits by private cmpanies. AMDEX Mining Ltd have kindly made available the results f MIP test surveys ver heavy-mineral depsits at Lanherne Beach, King Island, Tasmania, and the results are shwn in Figures 8 and 9. Figure 8 shws the results f the MIP survey super impsed n the heavy-mineral cncentratins at Lanherne Beach. The average f this depsit is abut 1.5 percent; the hachured area utlines a zne f greater than 0.5 percent heavy minerals, averaged ver a depth f 5 m. A reasnable crrelatin exists between MIP chargeability and the heavy mineral cncentratins. Figure 9 is als frm Lanherne Beach anj shws that althugh the mineralizatin in crss-sectin is patchy and spradic, the MIP chargeability gives a clear indicatin f the presence f heavy-mineral sands. HEAVY - MINERAL CONTENT _ >5% _ 2-5% _1-2% Figure 9. Lanherne Beach MIP results. 0 VERTICAL SCALE (m) [ 10 References NICHOLSON, D. A., 1974-Selected nshre heavy mineral depsits-jerusalem Creek Area; in THE MINERAL DEPOSITS OF NEW SOUTH WALES (Editrs, M ARKHAM, N. L., & BASDEN, H.,) Gelgical Survey f New Suth Wales. Sydney. 615-6. SEIGEL, H. 0., 1974-The m agnetic induced plarizatin (MIP) Methd. Gephysics, 39,321-39.