Reliability
analyses
laboratories
ofgoldandsilver
bycommercial
Methods
A 50-lbsamplewasdonatedto the project from a small gold-silvermine near
NM
byLynnA. Brandvold,
NewMexico
Bureau
of Mines& MineralResources,
Socorro,
Chemist,
Winston in south-centralNew Mexico.
The samplewasapproximately90 percent
quartz with small amouts of chlorite,
Introduction
must agreewith the smelter'sassaywithin
and fine-grainedpyrite.
argentite,
'splitting
The rapid rise in the price of gold and certainlimits termed
limits' (in
The ore wascrushedand groundso that
silver has led to an increasedinterestin the caseof one well-known smelter,to it would passan 80-meshscreen.Standard
prospectingfor thesemetals.Becauseof within 0.02oz/ton of gold and within 0.5
techniquesof ore mixing and splitting
the high price of gold, an ore that con- oz/ton of silver). If assaycomparisons wereusedto insurea homogeneous
samtains0.05to 0.1 oz of gold per ton may be differ by more than theselimits a control ple. The samplewas finally divided into
profitably mined if large tonnages are sample must be submitted to an umpire
32 splits. Each split was placed in a
assay unless one wishes to accept the separatecontainer.
available.
When the price of gold was relatively smelterassay.The losingparty muststand
As a checkfor homogeneity,threeconlow, determining if a rock contained the cost of the umpire assay.A commer- tainerswerechosenat randomand rolled
enoughgold to be profitably mined was cial umpire fire assay, run in quadru- overnight to counteractany settling. A
easy: if the gold could be seen with a plicate, costs two to three times the 14.5833-e sample (%-assay-ton)was
magnifying glass,the ore was probably routine assayprice. Becausemost protaken from each container and the
worth mining. Now with the high priceof spectors must rely on commercial
samples fire-assayedin the Bureau's
gold, ore containinghighly disseminated laboratories, the question arises: how
analyticallaboratory.
gold, invisibleevenunder a microscope, reliableare routine gold and silverassays?
To assureeachlaboratoryan adequate
may be profitably mined. Consequently, To seek an answer to this question and working sample,75 g was sent to each.
prospectorsand srnall-scale
miners must alsoto providean in-housereferencesamSampleswere prepiled in the following
rely on commerciallaboratoriesto deter- ple, the New MexicoBureauof Minesand
manner: Three sample containerswere
mineore values.
Mineral Resourcesundertook this study chosenat random.The bottleswereempWith the exceptionof a few firms that in1977-'18.
tied onto a plasticsheetand the contents
technique,
usea directatomic-absorption
most commercial laboratories in the
southwest determine gold and silver
values by fire assay;a method for determining metalsby using furnaceheat and
Times, dal6, and l@sliorc src publithed 6 rwived.
oJ contilbutoE.
Iterc in this column are the rnporcibilily
dry reagents, used for gold and silver
to
vention in May , 19'19.They needyour assistance
NlNrs lNrnnNertoNer-CoNcnessor
analysisfor thousandsofyears. A routine
help provide papersfor this convention.The theme
CansoxrrEnousSrnerrcnepHveNn GpoLocv,
commercial fire assay is not run in
will be the three R's of re-evaluation,reworking,
May [0-June 2, 1979,
and revival. The convention will be held at the
Washington,D.C. and Urbana,Illinois
duplicateand costsbetween$5 and $10
May 9-ll, 1979.
dependingupon the laboratory.If a pro- Includes: Plenary session,subcommissionmeetings SheratonHotel in Fort Worth,
of InternationalCommissionon Stratigraphy,work- Write to GeorgeH. Weems,Co-Chairman,Techspectoris sellingore to a smelter,his assay ing groups of International Commission on Coal nical Program, BassEnterprisesProduction Co.,
intheSouthwest
a
I
Announcements
Allen, R. S., ed., l9ll, The Mogollon Mines:
Mogollon,NewMexico,v. 2, Februaryl9l1
Ferguson,H. G., 1927,Ceologyand ore depositsof
the MogollonMining District,New Mexico:U.S.
GeologicalSurvey,Bull. 787
Heinen,H. J., Peterson,D. G., and Lindstrom,R.
8., 1976,Gold desorptionfrom activatedcarbon
with alkalinealcohol solutions:U.S. Bureauof
Mines, Reno Metallurgy ResearchCenler, Reno,
Nevada
Heinen,H. J., Peterson,D. C., and Lindstrom,R.
C., 197'1
, Heap leachingprocessingof gold ores:
U.S. Bureauof Mines, Reno Metallurgy Research
Center.Reno,Nevada
Jones, F. 4., 1904, New Mexico Mines and
Minerals:SantaFe, New Mexico
Lindgren,W., Graton, L. C., and Gordon, C. H.,
1910, The ore depositsof New Mexico: U.S.
GeologicalSurvey,Prof. Paper68
Thompson,A. J., 1962,Silver In New Mexico: New
Mexico BusinessMagazine,July
Zadra, J.8., 1950,A Processfor the recoveryof
gold from activatedcarbon by leachingand electrolysis:U.S. Bureauof Mines, Rept. Investigations 4672
Zadra,J. B., Engel,A. L., and Heinen,H. J., 1952,
Processfor recoveringgold and silver from activated carbon by leaching and electrolysis:U.S
Bureauof Mines, Rept. Investigations4843
I
Petrology, technicalsessionsand symposia,and 24
field trips in easternand westernUnited States.Field
Trip No. 12, Carboniferousof southernNew Mexico, May 27-June2, is beingled by J. L. Wilsonand
F. E. Kottlowski.
Inquiriesshouldbe gddressed
tol
Ellis L. Yochelson,Secretary-General
x-rcc, 1979
Museumof Natural History
Washington,D.C. 20560
Announcement and call
for papere
Fort Worth National Bank Building, Fort Worth,
TX76t02.
SYmPoslum
The Energy Minerals Division of the Auertcex
Assoct.lTroxon Pnornsstolel Georoclsrs eNorge
New Mpxtco Bunreu or MInes ,c.NoMINnRAT
RrsouncBswill be co-sponsoringa symposiumon
the Grants Uranium Region to be held at the AlbuquerqueConventionCenterMay l3-16, 1979.
Papersdealingwith the uranium geologyand mining technologyof the N.W. quadrant of New Mexico will be.featured.For further information, please
contact: Dr. Christopher Rautrnan, Shell Development Company,BellairResearchCenter,P.O' Box
481, Houston,TX 77001.
TsE Foun ConNtrs Gror-ocrclr- SocIErv will
conduct their 1979field trip, "Field Symposiumon
the Permian System of the Colorado Plateau"
September
27-30,1979.The areasto be coveredinclude Canyonlands,Monument Valley, Crand CanSanta Fe Geologlcal Soclety
yon, and the Mogollon Rim. A guidebook will be
Luncheon Meetingsresumedin January 1979at
preparedfor the occasion.If you or your colleagues
the ForgeRestaurant,SantaFe.
or studentshavebeenresearchingthe Permianrocks
February16, 19?9-Dr. Doug Brookins, Universof the Colorado Plateau and wish to contribute to
ity of New Mexico, Professor of Geology,
the guidebook,pleasecontact D.L. Baars, Fort
GeochemicolStudy of Crants Mineral Belt
Lewis College, Durango, CO 81301, phone
March 16, 1979-Ben Bader, Sandia
303-24'l-7767.Tentative titles were to be submitted
Laboratories,In-Situ Coal Gasification,- Hanna
by Januaryl; manuscriptdeadlineis Junel, 1979.
Project, Honna, WY
April 20, 1979-(?) Andy Livingston, Chief
Announcement and call
DepositionalModel
Geologist,Bokum Resources,
for papers
For UtOr Morcus, NM
Tne Fonr Wonrs Gsor-ocrcnI-Soctrrv plans to
(continuedon page13)
host the SouthwestSection Regional AAPG Con-
February1979
NewMexicoGeoloSy
ll
price of silveris $4.92per troy oz-a dif- silver loss determined' The corrected
ference in dollars between the highest silver value was 17'46 oz/ton-which
in
assayand the lowestassayof $52.11per agreeswith the atomic absorptionmean
laboraton. The standarddeviationof the gold table 3. Although commercial
assayrepresents
$31.05per ton assuming tories are certainlyawarethat silver is lost
during fire assay,the low reportedsilver
the price of gold is $207 per troy oz-a
valuei indicate that the loss is probably
highest
the
differencein dollars between
in the results.A check
and lowestgold assayof $99'36per ton. not compensated
eight laboratoriesconthe
Thus the margin for error in determining with three of
lossesare disregardassaying
that
firmed
actual valueson an ore shipmentcould
value'
The reasongiven
reported
the
in
ed
a
small
for
result in substantiallosses
operator.Betterto rely on more than one is that smeltersdo not acceptcorrected
assaysas a basisof paymentfor oresand
assay.
sincethey losethe sameperAn argument could be made that this concentrates,
the ore.
in
smelting
silver
bY
cent
difference in values was caused
gold
obtained from
value
mean
The
samPles.
nonhomogeneity between
is much
(0.24
oz/ton)
absorption
atomic
check
homogeneity
in-house
the
However
commerthe
by
obtained
on three random samplesresulted in an lower than that
(0.33oz/ton). However,
averagedeviationof 0.02 oz/ton for gold cial fire assayers
assay values for gold
fire
the
of
one
and0.11oz/ton for silver.
Furthermore,if the samplewere non- (laboratory No. 8, table 2) is unusually
valuesobtainedby atomic high, more than 2 standard deviations
homogeneous,
absorptionwould show much larger de- from the mean.If this valueis discarded,
viations because the sample size is the resultantmeanis 0.28 ozlton. The inResults
(Vz
housefire assaymean for gold was 0'23
The in-househomogeneitycheck pro- smaller-l5 or 30 g for fire assay or
oz/ton.
ducedvalueslistedin table l. The average full-assay-ton),whereasfor atomic abMany factors could accountfor the difg.
3
Table
is
l-5
size
the
sample
sorption
for silver is 16.42 oz/ton with a mean
in fire assay values between
ference
deviationof 0.11. The averagefor gold is lists the atomic absorptionresults from
different analysts,different
laboratories:
minthe
0.23 oz/ton with a mean deviation of the two commerciallaboratories,
different methods,
slightly
equipment,
assays
four
and
laboratory,
0.02. Theseresultsindicatethat the sam- ing company
chemicals.Thesevariables
ple is homogeneous.Table 2 lists the run by various studentsin the Bureau "nd diffetent
if samplesare sent to
eliminated
can be
resultsfrom the commerciallaboratories laboratory. The standarddeviationsfor
Two more samples
laboratory.
same
the
that usedthe fire-assayingtechnique.Two both gold and silver are much less than
laboratoriesto
four
of
each
to
sent
were
deterfire-assay
of the original ten laboratoriesdetermin- thoseof the commercial
to checkthe
and
variables
these
eliminate
of
nonhomogeneity
Therefore
ed gold and silver by atomic absorption; minations.
commerthe
of
some
of
the sampleswas not an apparentfactor in reproducibilities
later.
theseresultswill be considered
2 was
No.
Laboratory
laboratories.
ciil
the differencebetweenthe determinations
gold
silver
and
their
because
excluded
from the commerciallaboratories.
valueswere more than 2 standard deviaTABLE l-REsuLTs oF IN-HousE FIRE AssAY TEsr
FORHOMOCENEITY
tions from the mean. This was done to
ABso
3-AroMtc
TABLE
Deviation
Silver
Oeviation
avoid a bias in the reproducibility results.
Gold
from average
(ozlton)
(oz/lo[\
from avcrage
Smple
(oz'lton)
Silver
are shownin tables4 and 5. Stan(oz,/ton)
Results
Gold
Laboralory
0'17
r6.2s
0.02
0.21
|
18.50
0.21
were not calculated
9,
deviations
dard
16
16'58 0'
0.03
0.26
2
19.00
0.ZS
l0'
0.02
becausethere were only three valuesin
16.4
0.01
0.22
3
17.51
=
eachset,None of the laboratorieswould
Average= 0.23 Average 16'42
r7.60
0.28
havemet smelterlimits on all three assays
16.37
Averagemeandeviation(gold) = 0.02
0.22
Averagemeandeviation(silver) = 0' I I
16.62
0.22
for gold or silver, but laboratories3, 4,
16.62
0.24
and 7 would have met smelter limits on
mean= 0.24 mean: 17.46
two out of three of the gold assaysand
Standarddeviation(gold) = 0.026
3, 4, and 6 would have met
REsuLrs FRoM coMMERcIAL
laboratories
TABLE 2-Frns-asslv
Standarddeviation(silver) = l'01
LABORATORIES
on two out of three of the
limits
smelter
'Commerciallabs
Silver (ozlton)
Cold (oz./ton)
'Mining companylaboratorY
silver assays.Eliminating the variables
Laboratory
16.84
0.36
I
'BureaulaboratorY
between laboratories and analysts im10.87
0.70
2
but not appreciably.
provedthe analyses,
mixed by rolling back and forth. The
material was coned and quartered;oppositequartersweremixedand then combined. Samplesof 75 g werethenweighed
into envelopesuntil all the material had
beenused.This produced24 samples,all
of which were sent to commerciallaboratories. Samples were mailed to 10
laboratoriesunder the name of the New
Mexico Bureau of Mines and Mineral
After all the resultshad been
Resources.
returned.two additionalsplitsweremailed to eachof four laboratoriesunder different names and from different towns.
This was done to comparereproducibility. A raudom split (not one of the
aforementioned24 samples)was sent to
the chemicallaboratory of a New Mexico
mining companywhosechief chemisthad
offered to determinegold values by fire
assayand silvervaluesby both fire assay
and atomicabsorption.
J
0.25
0.27
o.22
1 8t.0
16.10
The mean silver valuesobtained by fire
assayfrom the in-houseanalysesareboth
0.26
lower than the mean silver value obtained
0.30
8
by atomicabsorption.This is understandl6Jl
0.33
Mean:
a6le, considering the recognized silver
2.81
Standard deviation =
lossesduring fire assay.When the mining
companylaboratory fire assayedthe samStandarddeviationof the silver assay ple, a secondsampleof a known amount
represents
$14.12per ton assumingthe bf silner was assayedsimultaneouslyand
)
16.30
16.40
6'
1
12
Februaryl9T9
NewMexicoGeolog!'
TABLE 4-fRrPLIcArE
Laboratory
3
4
6
1
FIng
Gold (ozlton)
.25 .23 .3r
.27 .25 .32
.lr .21 .44
.26 .21 .48
Sct
Mean
.26
.28
.32
.32
Overall mean (gold) = .39
Sct
Range
.08
.07
.21
.27
TABLE
VER
Laboratory
3
4
6
7
5-Tnrpt-rc,tts
FrRE-AssAyvALUESFoR srI--
Silvcr (ozlton)
1 8 . 1 01 6 . 8 01 7 . 9 0
16.1016.9017.10
1 6 . 3 01 4 . 4 51 5 . 9 5
t6.40 15.6012.55
Overallmean(silver)=
Set
Mean
Set
Range
17.50 I . 3
16.70 1 . 0
15.57 l.85
14.85 3 . 8 5
16.16
Table6 listsall the fire assayresultsfor
silver and gold. A "reasonable" mean
was calculatedin the following manner:
the means and standard deviationsfor
gold and silveranalyses
werecalculatedin
the usualmanner.Thosevalueswhich differed by more than I standarddeviation
from the meanswere rejectedand new
meanswere calculated.The silver mean
valuesuncorrectedfor losses.
represents
MEAN
BETwEENREASoNABLE
TABLE 7-DrrrpnsNct
VALUES
LABORATORY
VALUESANDCOMMERCIAL
Laboratory
I
2
J
4
)
Gold (oz./ton)
+.1
+.M
- .01
+ .01
- .04
+ .05
0.00
+ .04
6
7
8
Smeltersplittinglimits:
Silver (oz./tdn)
+ .39
- 5.58
+ 1.65
- .35
- .15
- .05
+ 4.85
Gold = .02ozlton
Silver= .5 oz/ton
silver. Looking at it another way (table 2
vs. table6), four out of eightlaboratories
are further than I standard deviation
from the meanon gold or silveror both:
two laboratoriesare further than 2 standard deviationsfrom the rneanon gold or
silver.or both. Oneof thesetwo is further
than2 standarddeviationsfrom the mean
on both gold and silver.
Atomic absorptionvalueshaveall been
previouslyshown. Becausethere were so
few values none was discarded-and the
meansare consideredreasonablemeans.
For comparisonthey are shownin table 8
means.
with the fire-assayreasonable
An operatorshouldnot ship ore to the
smelteron the basisof a singlecommercial assayunlesshe has eithera wide profit margin or positive previousexperience
with the laboratory.
Commercial laboratories in general
need to be more concernedwith quality
I
control.
(continuedfrom poge I 1)
A . P . G . S .A n n u a l M e e t l n g
THE AssocIATroNor PnoresstouaLGnoloclceL
ScrsNrtsrsheld its l5th annualmeeting,November
30 through December2, 1978,at the SheratonOld
Town Inn in Albuquerque,New Mexico.The theme
of the conferencewas GovernmentRegulationsBane or Blessing?Featuredspeakerswere J' Allen
Overton, Jr., President of the American Mining
Congress,Charles W. Margolf, Vice-Presidentof
WesternCoal Operationsfor W. R. Grace& Co.,
TABLE 6-Frnn-nss,lv RESULTSFoR c,oLD AND srr-Gen. Richard Bulgin (Ret.), ExecutiveDirector,
VER
AssociatedNuclear Consultantsof America, Ltd.,
RobertD. Gunn. President,AmericanAssociation
Gold (ozlton)
Silver (oz,/ton)
and H. PeterMetzger,Adof PetroleumGeologists,
.36
16.84
ministrator of Environmental Affairs, Public Ser.?0,
10.87
){
viceCompanyof Colorado.
18 . 1 0
All agreed that blessingswere very difficult to
.27
1 6 l. 0
identify. According to Richard W. Everett, Vice'
.22
1 6 . 81
Presidentof ChaseManhattan Bank, a study by a
.31
16.30
group of banksindicatesthat the federaltax burden
.26
16.40
TABLE 8-FInp
AssAY AND AToMlc ABSoRPTIoN of regulationnow totals $103.1billion, and that
.30
21.30*
REASONABLEMEANS
regulation is stifling exploration and development
.22
15.60
balance-of.23
and is aggravatingthe already excessive
t6.80
Atomic absorption
Fire assay
was not so much of the
payments
Criticism
deficity.
.25
16.90
(ozlton)
(oz,/ton)
Silver
Gold
Silvcr (ozlton)
Cold (oz,/ton)
.21
regulationsthemselvesasof the mannerin which the
14.45
1'1.46
0.24
16.45
0.26
regulationsare being administeredand applied. A
.21
15.60
Standarddeviation
Standard deviation
principal complaint voiced was that bureaucrats
.22
t6.43
l.0l
0.026
0.043 0.82
.Jl
often lack experienceand understandingof the in17.90
n:7
n=6
n=20 n=20
.32
dustry they are regulating.Federalrequirementsare
17 . 1 0
of assays
n = number
.44t
being applied countrywide without regard to ex15.95
treme variations in local conditions. One paradox
.4gr
I2.55r
cited was the rules regardingsulfur ernissionsfrom
.28
1 7 . I1
Conclusions
coal-burningplants,whereemissionscontrolsmust
.24
16.46
Resultsof gold and silverfire assaysby reducethe sulfur in the stackgasesto a smallpercen'
.22
16.43
commerciallaboratoriesdiffer markedly, tageof the feed. In the caseof westerncoals(which
.22
15.60
.24
t6.42
despitethe homogeneityof ore samples are very low in sulfur) the resulting sulfur fraction
being specifiedis economicallyand metallurgically
: .29
: 16.25 involved. This variation is evident not
Mean
Mean
impossibleto achieve. Ironically, eastern coals,
:
=
Standarddeviation .12
Standarddeviation 2.08
only betweenindividual laboratories,but which meetthe standards,still dump more sulfur inReasonablemean= .26
Reasonablemean = 16.45 within a single laboratory repeatingthe
to the atmospherethan the westerncoals would if
Standarddeviation : .M3 Standarddeviation : 0.82
same test on different days. Atomic ab- burnedwithout controls.
rindicatesvalueswhich werereiectedfor calculationof
Other concernsvoicedwerethe emotionalopposisorption assay results from different
reasonablemeans.
tion
to nuclear power applications, and the diflaboratoriesalso vary, but not to the ficulties
of financing exploration and develoP'
degreeseenin fire-assaycomparisons.
ment-particularly in uranium andcoal.Theultimate
Using these reasonablemeans, some
Insufficient analyseswere obtained by irony is to force minersback into high-risk,high'
further comparisonscan be madewith the atomic absorption methods to draw firm cost undergroundmining at a time when increased
commercial laboratory fire-assaydata in conclusions.Further evaluationis needed coal production hasbeendeclareda national goal.
Harrison H. (Jack) Schmitt, U.S. Senator from
table 2. If the reasonablemeans are of atomic-absorptionmethods, particu- New
Mexico, spokeat the banquet.He describedhis
assumedto be the correctvalues,and the larly for gold. On the basisof the limited and other Congressmen'sefforts to reassertcongressmelter assaysagree, then the difference data provided by this study, gold assays sionalauthority and control over the regulatoryprobetweenthe commercialassaysand the by atomic absorptionappearto be more cess.Congresshas tended to delegatetoo much
agencies.His bill, which
reasonable
meanscan be comparedto the reliablethan fire assayon this particular authority to the executive
will be introducedin Congress,will provide for con'
splittinglimits asshownin table7.
ore sample.
gressionalmonitoring via hearingsand actual recall
Three laboratories (Nos. 3, 4, and,7)
The mean silver value is higher for of regulationsfor reconsiderationshoulddifficulties
meet smeltersplittinglimits on gold, and atomic absorption than for fire assay. develop.
A transcriptof the proceedingswill appearin Profive laboratories(Nos. l, 4, 5, 6, and 7) Thus if an operator is sellingto a smelter,
Geological Scientist,the official publica'
fessional
meet smelter splitting limits on silver. he should have his ores assayedby fire tion of the A.P.G.S. For information,write to P.o.
Only two laboratories,Nos. 4 and 7, meet assay,not atomic absorption(unlessan Box 95?, Golden, CO, 80401.-Clav T. Smith,
Publicily Chairman
smelter splitting limits on both gold and appropriatecorrectionfactor is applied).
February1979
New Mexico Geology