TREATMENT OF ALASKAN REFRACTORY GOLD ORES

By W. R. McDonald, J. L. Johnson, and R. G. Sandberg

U.S. Department of InteriorBureau of Mines

Salt Lake City Research Center729 Arapeen Dr

Salt Lake City, Utah 84108

Presented and preprinted at the Northwest Mining Association Annual meeting

in Spokane, WA, Dec. 3-5, 1987.

THIS PUBL!3AT' to 0, IN K - ,- LJ Z -_ C; Y ADMUST NOT U4 c K .iA i i t2 LDU` ILGiii

CONTENTS-Page

Abstract ............................................................. 4Introduction ......................................................... 5Materials ............................................................ 6

Crow Creek ......................................................... 6Haines Road Cut .................................................... 6Fern Mine .......................................................... 6E Pluribus Unum .................................................... 7

Identification of refractory ores .................................... 7Cyanide amenability ................................................ 7Target grind size .................................................. 8

Pretreatment of refractory gold ores ................................. 9Gold locked in sulfides ............................................ 9Gold robbers ....................................................... 10

Gold rob test .................................................... 10Gold rob treatment ............................................... 11

Preoxidation with air .......................................... 11Preoxidation with hypochlorite ................................. 11Acid pressure oxidation ........................................ 11Roasting ........................................................ 11Carbon in leach ................................................ 12Gold rob results ............................................... 12

Gold locked in placer sand ......................................... 13Conclusion ........................................................... 13References ........................................................... 14

TABLES

1. Distribution of gold in the magnetic and nonmagneticfractions of Crow Creek placer sand ............................. 6

2. Cyanide amenability results for the four gold ores .............. 8

3. Summary of target grind tests and assay screen analysesof the leach residue for Fern and E Pluribus Unum ores .......... 8

4. Cold extraction from Haines Road Cut as a function ofoxidation method ................................................ 10

5. Gold recovery from Haines Road Cut ore by flotation as afunction of particles size ....................................... 10

6. Gold robbed by ore as a function of oxidation treatmentfor Fern and E Pluribus Unum ores ............................... 12

7. Summary of gold extraction from E Pluribus Unum and Fern Minesamples for cyanide amenability and carbon in leach tests ....... 12

8. Gold and silver recovery from Crow Creek ore with and withoutgrinding ........................................................ 13

9. Gold recovery from Crow Creek ore as a function of particlesize ............................................................ 13

3

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

0c degree Celsiusg gramh hourlb/st pounds per short tonmm millimeteroz/st Troy ounce per short tonpct percentppm part per millionpsig pound per square inch, gaugewt weightwt pct weight pct

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TREATMENT OF ALASKAN REFRACTORY GOLD ORES

By W. R. McDonald,' J. L. Johnson,2 and R. G. Sandberg3

ABSTRACT

The U.S. Bureau of Mines investigated treatment of gold and silverrefractory ores and concentrates that respond poorly to conventionalcyanidation techniques due to the complex mineralogy of the ore. Threetypes of ores were investigated: gold locked placer sands, gold locked insulfides, and gold that is robbed from the pregnant cyanide solutions byminerals in the ore. Ore treatment included grinding, flotation,cyanidation, air oxidation, pressure oxidation, hypochlorite oxidation,roasting, and carbon in leach.

Metallurgist.2Chemical engineer.3Research supervisor.Salt Lake City Research Center, Bureau of Mines, U.S. Department of the

Interior, Salt Lake City, UT.

5

INTRODUCTION

The U.S. Bureau of Mines has joined in a cooperative effort with theState of Alaska to study mineral deposits in the Juneau (AK) MiningDistrict. This study will provide information to economically evaluategold deposits in the district with a final goal of obtaining information ondeposit development and stimulating growth in the area. Similarly theBureau has ongoing site-specific mineral studies throughout Alaska, in thiscase investigating gold recovery problems of the Fern (Willow CreekDistrict) and Crow Creek (Anchorage District). This paper is given as aprogress report and discusses the treatment of representative refractoryores identified by these studies. Refractory gold ores are those ores thatdo not respond to standard gravity and cyanidation processes.

Worldwide investigations (1)4 have shown that possible reasons for poorresponse of gold-bearing material to cyanidation include (1) physical lock-up (if the fine gold particles can be liberated by economic grinding thegold is locked, if they can not, the gold is encapsulated), (2)carbonaceous materials (active carbon in the material adsorbs the gold fromsolution), (3) decomposition of host minerals (sulfides decompose to formcyanicides and deplete the oxygen), (4) coated gold (particles associatedwith sulfides in contact with a lixiviant can develop coatings), and (5)insoluble alloys or compounds of gold [gold-bearing tellurides andarsenides, aurostibnite (AuSb2), and maldonite (Au2Bi)].

In addition, some sulfides, such as Sb2 S3 adsorb on gold particles ascharged colloidal particles when in solution. Roasted lead-containing orescan contain lead compounds that coat the gold particles, rendering theminsoluble to cyanidation.

For gold ores to react completely, certain requirements must be met.Gold must be in the form of discrete and clean particles, impurities thatmay inhibit the reaction must be absent, and an adequate supply of oxygenmust be available.

A common technique to treat refractory ores has been roasting offlotation concentrates prior to cyanidation. Other techniques include finegrinding, pressure oxidation of sulfides, bacterial digestion, chemicalpretreatment (preaeration and acid washing), and hypochlorite or chlorineoxidation.

As part of the overall objective of evaluating the potential of golddeposits in the Juneau Mining District, test procedures were developed toidentify the refractory nature of ores and to investigate methods forpretreating refractory ores prior to cyanidation. The three ore types willbe discussed in this paper: (1) gold locked in sulfides, (2) gold robbedfrom pregnant cyanide solutions by minerals in the ore (also referred to aspreg rob), and (3) gold encapsulated placer sands. Four Alaska depositsthat can be classified in one or more of these types of ores are (1) CrowCreek, (2) Haines Road Cut, (3) Fern Mine, and (4) E Pluribus Unum.

'Underlined numbers in parentheses refer to items in the list ofreferences at the end of this report.

6

MATERIALS.

CROW CREEK

The Crow Creek gold placer (3-)9 is located about 30 miles east ofAnchorage, AK. This material is a refractory gold ore because only 41 pctof the jig concentrated gold (28 oz/st Au and 16 oz/st Ag) was recoverableby amalgamation. Mineralogical examination showed that about one-half ofthe sample comprise mica schist. The remainder was largely a mixture ofiron oxides (magnetite, hematite, and limonite). Iron pyrite made upseveral percent of the sample. Other commonly occurring minerals werescheelite, galena, zircon, and native gold. The native gold was abundant.Some of the gold contained inclusions of iron oxide and about 20 pct of thegold reports with the magnetic fraction of the sample as shown in table 1.

TABLE 1. - Distribution of gold in the magnetic and nonmagneticfractions of the Crow Creek placer sand

Product wt pct Analysis, oz/st Distribution, pctAu Ag Au Ag

Magnetic fraction ..... 26 19.5 11.5 17.7 18.2Nonmagnetic fraction..I 74 32 18.3 82.3 81.8

HAINES ROAD CUT

The Haines Road Cut was discovered by personnel from the Bureau in1986, and is located 3 miles south of Haines, AK. Chemical analysis of theore showed it contained up to 0.33 oz/st Au, 0.88 oz/st Ag, and 8 pct Cu.This is the first significant report of such mineralization in the Chilkatvolcanics and opens up a large area to exploration for similarmineralization. A mineralogical examination of the samples of the oreshowed that the major minerals present were chalcopyrite, quartz, andpyrite. Smaller amounts of malachite, chromite, magnetite, ilmenite,pyroxene, K-feldspar, sphalerite, covelite, bornite, rutile, chalcocite,and arsenopyrite were present in the sample. Traces of lead, nickel, andscandium were also detected.

Several gold metallic grains were picked from a gravity concentrate andexamined by scanning electron microscopy (SEM). The grains contained about80 pct Au, 15 pct Ag, and the remainder as iron and chromium minerals.

FERN MINE

The Fern Mine (5) is located 50 miles northeast of Anchorage, AK. Orefrom the mine was processed in the past by flotation and then treated byamalgamation to recover the gold. Analysis of the ore sample showed asmuch as 7 oz/st Au and 0.19 oz/st Ag. Samples that represent the depositconsist of gold-bearing quartz vein material. The veins contain ankerite,calcite, and considerable clay, probably the result of hydrothermalalteration. The veins are hosted by diorite and locally contain scheelite.Concentrates from bulk sulfide flotation contained visible free gold (30oz/st Au) with much of it being 24 mesh (0.707 mm), plus pyrite,chalcopyrite, and perhaps other sulfide minerals. In addition, the ore

7

contains some carbonaceous material that was determined to be a pregrobber; that is, the carbonaceous material is active so that when it comesin contact with gold in solution it adsorbs the gold from solution. Theapparent gold extraction is lowered by the amount of gold adsorbed.

E PLURIBUS UNUM

The E Pluribus Unum (§) was discovered before 1904 and developed as anunderground mine in southeast Alaska about 24 miles northwest of Juneau.Samples from the deposit contained about 0.24 oz/st Au. Mineralizationconsists of three types of material: (1) a low-grade stringer lode, (2)medium-grade massive quartz veins, and (3) high-grade lenticular pods. Thestringer lode and the quartz veins have pyrite and arsenopyrite. The podscontain arsenopyrite, galena, sphalerite, stibnite, and 4.8 to 14.5 oz/stAu. Carbonaceous material was also found in the samples.

IDENTIFICATION OF REFRACTORY ORES

To evaluate the ores, a testing routine was developed that included (1)mineralogical examination with a SEM on a minus 10-mesh sample to determinethe minerals and their liberation size, and (2) cyanide amenability teststo determine whether or not the ore responds to cyanide leaching.

CYANIDE AMENABILITY TESTS

Tests were initiated by ball mill grinding a sample of an ore at 50 pctsolids to greater than 85 pct minus 325 mesh, adjusting the pH to 10.5 withlime, adding 20 lb/st NaCN, and agitating the 50-pct solids slurry for 72 hin a rolling bottle. The slurry was filtered, washed, and the productsassayed. Cyanide and lime consumption was determined by difference betweenthe amount added at the beginning of the leach and the amount left in theleach solution at the end of the leach. Cyanide concentration in the leachsolution was determined by titration with silver nitrate and the lime withoxalic acid.

The cyanide amenability results shown in table 2 give a low extractionfor the Haines Road Cut, 65 pct. This fact, combined with thedetermination of substantial sulfides by SEM analysis suggest refractorytendencies. The sulfides may be depleting the oxygen and/or the cyanide,and/or the gold is locked in the sulfide matrix.

The extraction percent for the Fern and E Pluribus Unum ores were notextremely low but the residue grade is still high and suggests furthertreatment may improve the extraction. Additional evidence of theirrefractory behavior is seen in the target grind test results discussedbelow.

The Crow Creek sample was treated by amalgamation because of the highconcentration of gold in the sample. Amalgamation is more convenient thancyanidation for recovery of gold from concentrated samples. The Bureauresearch was performed using amalgamation to provide meaningfulinformation. The results of the amalgamation test indicate that the goldis not free and is difficult to extract, and since this is a placer sanddeposit lockup was suspected.

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TABLE 2. - Cyanide amenability results tor the four gold ores

Sample Assay, oz/st Extraction,Head Tail . 1 _PC

Au Ag Au Ag Au AgHaines Road Cut.. 0.39 0.74 0.14 0.72 64.1 2.7Fern ............. 3.01 NA .55 NA 81.7 NAE Pluribus Unum.,. .12 NA .02 NA 83.3 NACrow Creek ...... 53.40 19.80 31.42 j12.04 41.2 39.2NA - Not analyzed.Extraction by amalgamation.

TARGET GRIND SIZE

The target grind size test follows the cyanide amenability test in theusual evaluation of any gold ore. The purpose is to find the size where anacceptable tail grade will be obtained. In addition, refractory tendenciescan be seen from the results. Target grind size tests were conducted byleaching samples that had been crushed to minus 0.5-in ore, the pH adjustedto 10.5 with lime and 20 lb/st NaCN. The slurry was agitated in a rollingbottle for 72 h. After leaching, the sample was filtered and washed. Theleach residue was screened into size fractions and each size fractionassayed. From the results of this test the required grinding for gold andsilver extraction was estimated. Generally a decrease in the grind sizeshows a decrease in residual gold in the residue. However, in the case ofsome refractory gold ores this is not true. The results of target grindtests for Fern and E Pluribus Unum ores are given in table 3, they showthat for the larger size fractions the gold recovery with cyanide leachingincreased with decreasing mesh size as is normally expected. However, atthe finer size fractions the residue grade begins to rise until at minus325 mesh gold in the cyanide leach residue increased significantly,indicating particle locking or gold robbing.

TABLE 3. - Summary of target grind tests and assay screen analyses ofthe leach residue for the Fern and E Pluribus Unum ores

Leach residue size Fern ore E Pluribus Unumfractions, mesh wt Assay, Au wt Assay, Au

pct Au dist., pct Au dist.,oz/st pct oz/st pct

Plus 20 ............. 34.4 0.67 87 55.1 0.023 53Minus 20 plus 35 .... 19.7 .059 4.4 21.5 .020 18Minus 35 plus 65 .... 11.8 .023 1 10.5 .016 7Minus 65 plus 100 ... 4.5 .032 .5 3.6 .017 2.6Minus 100 plus 150.. 3.3 .045 .6 1.5 .016 1Minus 150 plus 200.. 1.8 .030 .2 .8 .027 .9Minus 200 plus 325.. 1.4 .032 .2 .5 .030 .7Minus 325 ............ 23.1 .065 5.7 6.5 .066 182Gold extraction from the Fern ore was 87 pct with cyanide and 61 pctfor the E Pluribus Unum.

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PRETREATMENT OF REFRACTORY GOLD ORES

GOLD LOCKED IN SULFIDES

Gold ores containing sulfides can be refractory to cyanidation. Theseores typically contain sulfides (2 that include pyrite (FeS2), pyrrhotite(Fe5SO), arsenopyrite (FeSz2FeAs2), realgar (AsS), orpiment (As2S3),stibnite (Sb2 S3 ), galena (PbS), anglesite (PbSO4), and many of the copperminerals. The sulfide minerals can react with oxygen in the cyanidesolutions, thus depleting the solution of oxygen, and others not onlyconsume oxygen but form cyanicides, depleting the cyanide content. Whenarsenopyrite is agitated in a concentrated solution of lime in the presenceof air, alkaline arsenites form as shown below.

4 FeAsS + 4 Ca(OH)2 + 11 02 - 4 FeSO4 + 4 CaHAsO3 + 2 H20. (1)

This is a strongly deoxidizing reaction, which results in a loss ofdissolved oxygen, consequently, gold does not extract. Pyrrhotite is anexample of both cyanide and oxygen consumption as shown in the followingequations:

Fe5S6 + NaCN - NaCNS + 5 FeS (2)

FeS + 2 02 - FeSO4 (3)

FeSO4 + 6 NaCN - Na4Fe(CN)6 + Na2SO4 . (4)

The Haines Road Cut is an example of a sulfide containing gold ore thatis refractory to cyanidation. Direct cyanide leaching of this ore yielded64 pet of the gold being extracted. To improve gold recovery the ore mustbe further treated. Treatments included air oxidation and acid pressureoxidation of the concentrate followed by cyanidation (using test proceduredescribed earlier).

Air oxidation tests were initiated by ball mill grinding to greaterthan 65 pct minus 325 mesh at 50 pct solids. The slurry was thentransferred to a bottle, the pH raised to between 11 and 12 with lime, andthe sample agitated and air sparged for 24 h. Cyanidation of the resultingslurry increased gold extraction to 73 pct.

Acid pressure oxidation tests were conducted on a bulk flotationconcentrate containing 86 pct of the gold [concentrate produced withCyanimide A-2085 (sodium diethyl and sodium di-secondary butyl dithiophosphate), potassium amyl xanthate, copper sulfate and frother] at 20 pctsolids, 220° C, and with 50 psig 02 for 2 h. Cyanidation of the leachresidue resulted in greater than 96 pct gold extraction; total goldextraction was greater than 83.6 pct.

A summary of the air oxidation and acid pressure leach tests followedby cyanidation is given in table 4.

5Reference to specific products does not imply endorsement by the Bureauof Mines.

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TABLE 4. - Gold extraction from Hainqs Road Cut as a functionof oxidation method

Assav, oz/st Recovery,Test condition Head Tail Pct

_ Au Ag Au Ag Au AgCyanide amenability.. 0.393 0.74 0.140 0.72 64.4 2.7Air oxidation ........ .329 .88 .089 .88 72.9 0Pressure oxidation'.. J904 NA <.05 NA >95.6 NANA - Not analyzed.lPressure oxidation conducted on flotation concentrate.

Improved overall extraction for the pressure oxidation treatment willresult if the flotation is more efficient. Finer grinding can bebeneficial to a point and then counter productive. Splits of the samplewere ball mill ground so that 62, 80, and 90 pct of the each split passed325 mesh. The gold was concentrated in each split by bulk sulfideflotation. The results of these tests are given in table 5. This tableshows the extraction drop as the slimes begin interfering with theflotation.

TABLE 5. - Gold recovery from Haines Road Cut oreby flotation as a function of particle size

Minus 325-mesh, Assay, oz/st Extraction,pct Head Tail pct

62 .0.336 0.078 76.880 .243 .032 86.889 .431 .072 83.3

GOLD ROBBERS

Ores containing minerals that cause the gold to redeposit from theleach solution are considered gold robbers. Carbonaceous or carbon-bearingmaterials are usually the cause of this problem and must be deactivated byoxidation or by some other approach. Even though 80 pct of the gold isrecovered during cyanide amenability and testing is continued, it isimportant to beware of gold robbing indications. The E Pluribus Unum andthe Fern ores are examples of materials that exhibit this behavior.

Gold Rob Test

The purpose of the gold rob test is to determine if the ores suspectedof being a gold robber will adsorb gold from the cyanide leach solution. Agold-bearing solution, which was prepared by dissolving AuC13 in a cyanidesolution and then neutralizing the excess cyanide, was contacted with aground sample of each ore. The AuC13 solution was made basic with sodiumhydroxide prior to cyanide leaching. The excess cyanide in the goldrobbing solution was destroyed to prevent the leaching of any gold from oreand thus biasing the test results. The results are given in ounces of goldthat is robbed from the known solution per short ton, a decrease in thegold that is robbed shows the degree that the gold robbing material isneutralized. Test results showed that samples of the Fern Mine and the E

Pluribus Unum Mine contained gold robbers..

The gold robbing material in the Fern Mine and the E Pluribus Unum oresis carbonaceous matter. Early operations at the Fern mine recovered thegold by flotation and amalgamation and thus avoided the gold robbingproblems.

Gold Rob Treatment

A gold robbing ore can be treated either by neutralizing the robbingportion by oxidation or by competing with the robbing portion during theleaching of the gold by adding carbon to the leach slurry. Tests conductedto reduce the gold robbing effects of the ore included preoxidation withair, preoxidation with hypochlorite, acid pressure oxidation, roasting, andcarbon in leach (CIL).

Preoxidation With Air

Preoxidation with air tests were conducted by grinding the sample inthe laboratory ball mill at 50 pct solids to 82 pct minus 325 mesh,modifying the pH with lime to about 11, heating the pulp to 45° C, and thensparging air into the pulp for 16 h. After air oxidation the sample wasfiltered and the gold rob test was repeated.

Preoxidation With Hypochlorite

The hypochlorite tests were conducted by first repeating the airoxidation test to the end of air sparging, and then adding hypochlorite instages (5 to 20 lb/st OCI-) until the concentration of hypochlorite insolution stabilized at 20 lb/st OCl-. The OC- concentration wasdetermined by KI addition and titration of the formed iodine with sodiumthiosulfate. The sample was then filtered, washed, and repulped. Theresidual hypochlorite was destroyed with hydrogen peroxide. The sample wasrefiltered and the gold rob test was conducted.

Acid Pressure Oxidation

Bulk sulfide flotation concentrates were used for acid pressureoxidation. The ore sample was ball mill ground at 50 pct solids to 82 pctminus 325 mesh, then a bulk sulfide flotation concentrate was made usingCyanimide A-208, potassium amyl xanthate, and copper sulfate. The sulfideconcentrate was placed directly in the reactor vessel, diluted to 20 pctsolids, and oxidized at 195° C with 50 psig 02 overpressure for 2 h. Theresidue was filtered and washed. The gold rob test was conducted on theresidue.

Roasting

Flotation concentrates were used for roasting. The ore sample was ballmill ground at 50 pct solids to 82 pct minus 325 mesh, and a bulk sulfideflotation was conducted with Cyanimide A-208, potassium amyl xanthate, andcopper sulfate. The sulfide concentrate was dried at 95' C and then deadroasted at 450° C for 2 h. The gold rob test was performed on the roastedconcentrate.

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Carbon In Leach

The sample was ball mill ground at 50 pct solids to 82 pct minus 325mesh, the pH was adjusted to 10.5 with lime, then 20 g of 6X16 Westatesactivated carbon, and 20 lb/st NaCN were added. The sample was agitated ina rolling bottle for 72 h, then filtered, and the products assayed. Thegold loading on carbon was determined by ashing the carbon and then the ashfire assayed.

Gold Rob Results

The results of the gold rob treatment are listed in tables 6 and 7.They show that the Fern ore required only minimal treatment to preventrobbing of the gold, while the E Pluribus Unum required severe treatmentsuch as acid pressure oxidation or roasting. The gold robbed by the Ferndecreased to 0.06 oz/st with air oxidation; whereas, a pressure oxidationleach was required for the E Pluribus Unum to achieve similar results, 0.02oz/st.

TABLE 6. - Gold robbed by ore as a function of oxidationtreatment for Fern and E Pluribus Unum ores

Feed RobbedTest solution solution Gold

Wt. Assay, Wt. Assay, robbed,9 I pPM |PPM oz/st

E PLURIBUS UNUMFeed ...................... 500 25 687 0.5 0.71Air oxidation ............. 500 25 592 7.0 .43Hypochlorite oxidation .... 500 25 525 17 .25Pressure oxidation leach.. 38 25 230 4 .02Roast ...................... 90 25 170 .8 .7

FERNFeed ...................... 500 25 628 17 0.25Air oxidation ............. 500 25 603 19 .06Hyp ohlorite oxidation ..... 500 25 79 1 15 .03P ressure oxidation ........ 27 25 200 3.1 .07Roast ...................... 49 25 150 .9 .6

Table 7 compares results of CIL tests with cyanide amenability tests.The data show CIL improves gold extraction from the Fern Mine sample from81.7 to 89.3 pct. The CIL did not improve gold extraction from the EPluribus Unum mine sample. The gold extraction was 80.6 and 79.8 pct,respectively, for cyanide amenability and CIL tests.

TABLE 7. - Summary of gold extraction from E Pluribus Unum andFern Mine samples for cyanide amenability and carbon inleach tests.

Cyanide amenability CILSample Assay, oz/st Recovery, Assay, oz/st Recovery,

Head Tail pct Head Tail pctFern . 3.012 0.55 81.7 1.871 0.200 89.3E Pluribus Unum... .124 .024 80.6 .119 .024 79.8

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GOLD LOCKED IN PLACER SANDS

In the Crow Creek ore the gold is locked in a placer sand. This is anexample of a material for which particle size reduction is all that isneeded to improve gold recovery. Typically, a placer sand is processed bygravity separation techniques. A sample is screen sorted into sizefractions and then the size fractions that contain the gold are furthertreated by a variety of gravity techniques. This particular sample of CrowCreek placer sand was concentrated with a jig. The jig concentrates fromthe property contained from 2 to 400 oz/st Au. By the nature of the gold,individual splits show a wide fluctuation in assay because the coarseparticle size of the gold made it difficult to assay a representativesample. The initial test on the first Crow Creek sample showed thatgrinding to 67 pct minus 325 mesh increased the gold recovery from 41.2 to97.7 pct. The test results are shown in table 8.

TABLE 8. - Gold and silver recovery from Crow Creek ore, withand without grinding

Product Head, oz/st Tail, oz/st Recover y,' pctI Au Ag Au Ag Au Ag

Without grinding..,. 53.40 19.80 31.42 12.04 41.2 39.2With grinding ...... 46.56 19.92 95.7lGold recovered by amalgamation.

The principle of size reduction to improve recovery was alsodemonstrated with a lower grade sample (1 to 2 oz/st Au and about 1 oz/stAg), as shown by the test results in table 9. Splits of the sample wereball mill ground so that 67, 78, and 89 pct of the each split passed 325mesh. The samples were amalgamated to recover the gold. The results showthat the amalgam tail decreased from 0.29 to 0.044 oz/st when the ore meshsize was decreased from 67 to 89 pct minus 325 mesh, respectively. Therewas poor agreement between head sample assays because of the occurrence ofabundant free gold in the samples.

TABLE 9. - Gold recovery from Crow Creek ore as a function of particle size

Minus 325-mesh, Assay oz/st Recovery,pct He 4 Tail Pti

Au A Au Ag Au AR67 .0.857 1.02 .292 .83 65.9 18.678 .1.523 .73 .172 .25 88.7 65.789.............207 1.19 .044 .76 0.. 9 36.1'Gold recovered by amalgamation.

CONCLUSION

Haines Road Cut and E Pluribus Unum deposits were investigated as partof a study to determine the potential of gold and silver deposits in theJuneau Mining District. The Fern and Crow Creek deposits were investigatedas site-specific studies elsewhere in Alaska (Anchorage and Willow CreekMining Districts). Cyanide amenability tests determined that these oreswere refractory in nature. Tests on these refractory ores included air

14

oxidation, hypochlorite oxidation, acid pressure oxidation, roasting, and

carbon in leach.

Gold recovery from gold locked in sulfides ores, represented by the

Haines Road Cut, was improved by proper grind size and by preoxidation.

Recovery increased from 65.3 pct by cyanide leach to greater than 83.6 pct

by pressure oxidation-leach of a flotation concentrate.

Gold loss by absorption on gold robbing material was reduced by

oxidation. The Fern ore required minimum oxidation treatment to decrease

the gold loss from the preg solution, while the E Pluribus Unum required

pressure oxidation treatment to obtain similar reduced gold loss.

The recovery from gold locked placer sands, as represented by the Crow

Creek deposit was improved by grinding. Gold recovery increased from 41.2

to 97.7 pct.

REFERENCES

1. Dry, M. J., and F. B. Coetzee. The Recovery of Gold From Refractory

Ores. Gold 100. Proceedings of the International Conference on Gold. V.

2: Extractive Metallurgy of Gold. Johannesburg, SAIMM, 1986, pp. 259-274.

2. Nagy, I., P. Mrkusic, and H. W. McCulloch. Chemical Treatment of

Refractory Gold Ores - Literature Survey. Natl. Inst. for Metall., Rep.

38, 1966, pp. 3-25.

3. Hoekzema, R. B., and S. A. Fechner. Placer Gold Sampling In and Near

the Chugach National Forest, Alaska. BuMines IC 9091, 1986, 42 pp.

4. Hoekzema, R. B., S. A. Fechner, and J. M. Kurtak. Evaluation of

Selected Lode Gold Deposits in Chugach National Forest, Alaska. BuMines IC

9113, 1987, 56 pp.

5. Kurtak, J. Results of 1984 Bureau of Mines Site Specific Field

Studies Within the Willow Creek Mining District, Alaska. BuMines OFR 17-

86, 1986, 25 pp.; NTIS PB 86-163904.

6. Redman, E., W. S. Roberts, A. Clough, and J. Kurtak. Preliminary

Mine, Prospect, and Sample Location Maps and Descriptions, Juneau Gold Belt

Area. BuMines OFR 85-86, 1986, 68 pp.

7. The Australian Institute of Mining and Metallurgy. Carbon-in-pulp

Technology for the Extraction of Gold. Clunies Ross House, 191 RoyalParade, Parkville; Victoria, Australia, 3052, 1982, 442 pp.

8. E. M. Hamilton, B. A. (oxon). Manual of Cyanidation. McGraw-Hill

(New York), 1st ed., 1920, pp. 173-187.