How efficient are they really? A simple testing method of small-scalegold miners’ gravity separation systems

Benjamin Teschner a,⇑, Nicole M. Smith a, Travis Borrillo-Hutter a, Zira Quaghe John a, Tony E. Wong baColorado School of Mines, United StatesbUniversity of Colorado Boulder, United States

a r t i c l e i n f o

Article history:Received 10 November 2016Revised 12 January 2017Accepted 18 January 2017

Keywords:Artisanal and small-scale miningGravity separationMercuryGold

a b s t r a c t

This paper demonstrates a simple, minimally-invasive method of estimating the gold recovery rate ofgravitational separation equipment used by artisanal and small-scale miners at alluvial gold sites inthe Guianas, South America. A local ASM group mining an alluvial gold deposit agreed to allow theresearch team to collect eight samples of ore material immediately before it was to be processed by theirsluice and eight samples taken immediately upon exit from their sluice. Each sample was sieved intothree grain size portions (500 lm) and each portion assayed for gold content.The results indicate that the sluice at this site is capturing approximately 91% of the gold that enters thesluicing system, and the grain size distribution of the gold particles suggests that mercury amalgamationmethods employed by the miners post sluicing are likely to be very efficient as well. Although this studyand sample size is extremely small, and should not be taken as a full replacement of more elaborate geo-logic sampling and metallurgical analysis, this method could provide a useful tool for practitioners whowant to quickly evaluate ASM gold processing methods and consider the effectiveness of introducingalternative processing technologies, especially those that aim to reduce mercury emissions from ASM.

! 2017 Elsevier Ltd. All rights reserved.

1. Introduction

Artisanal and small-scale mining (ASM) employs approximately50 million people worldwide and produces 15–20% of the world’snon-fuel mineral resources (Veiga and Baker, 2004). The sector iswidely credited with supplying an important livelihood to peoplein developing countries that suffer from high rates of ruralunderemployment (Hentschel et al., 2002). Unfortunately, the eco-nomic benefits of ASM are accompanied by environmental damage,occupational health and safety problems and in some cases, humanrights violations. Large-scale mining companies, governments,non-governmental organizations (NGOs) and scholars recognizethe importance of ASM to developing economies, but find the sec-tor unyielding when it comes to curtailing the dangers and some-times illegal activities associated with ASM (Buxton, 2013;Hentschel et al., 2002; Smith et al., 2016).

Although ASM is increasingly active in a wide variety of geo-logic settings, simple, easy to mine alluvial deposits probably stilldominate the ASM sector’s worldwide gold production. Alluvialdeposits were almost certainly where gold was first discoveredand dominated early history’s production (MacDonald, 1983),

and alluvial gold mining technologies have been well understoodfor centuries. Although relatively simple to find and recover, valu-ing alluvial gold deposits and testing the efficiency of alluvial min-ing technologies has been a more complicated matter.

This paper demonstrates a simple, minimally-invasive methodof estimating the gold recovery rate of gravitational separationequipment used by ASM at alluvial gold sites in the Guianas.1 Toaccomplish this goal, a local ASM group, mining an alluvial golddeposit, agreed to allow the research team to collect eight samplesof ore material immediately before it was to be processed by theirsluice and eight samples immediately upon exit from their sluice.Each sample was sieved into three grain size portions (500 lm), and each portion was assayed for goldcontent. The results indicate that the sluice at this site is capturingapproximately 91% of the gold that enters the sluicing system, andthe grain size distribution of the gold particles suggests that mercuryamalgamation methods employed by the miners are likely to be veryefficient as well. This method could provide a useful tool for devel-opment practitioners who want to quickly evaluate ASM gold pro-cessing methods and consider the effectiveness of introducing

http://dx.doi.org/10.1016/j.mineng.2017.01.0050892-6875/! 2017 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: bteschne@gmail.com (B. Teschner).

1 The Guianas include the South American countries of Guyana, Suriname, andFrench Guiana; the latter being an overseas department of France.

Minerals Engineering 105 (2017) 44–51

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alternative processing technologies, especially those that aim toreduce mercury emissions in ASM.

2. Alluvial gold deposits

Gold in alluvial deposits commonly occurs in large, often visiblenuggets and in unconsolidated material (MacDonald, 1983;Youngson and Craw, 1995). This condition makes alluvial gold rel-atively easy to find and recover using simple, inexpensive technol-ogy. Miners usually do not have to break or blast hard rock, nor dothey have to tunnel or remove substantial amounts of overburdenin order to access gold-bearing material. Alluvial gold often occursas ‘‘free gold,” or gold that is not chemically or physically bondedwith other materials, so prospectors and miners do not have toemploy sophisticated milling and roasting processes in order toliberate the gold from gangue minerals and achieve profitablerecoveries. Early alluvial miners developed few formal methodsof valuing deposits or testing the efficiency of their recovery meth-ods because the capital investments required to mine and theresultant costs of inefficiencies were so low (MacDonald, 1983,pp. 1–12).

Alluvial gold mining began to change in the late 1800s, an evo-lution that continued until the outbreak of World War II. Large-scale miners in North America and Southeast Asia began to investin increasingly mechanized mining methods to exploit alluvialdeposits, culminating with large-scale dredging systems in theearly 1900s (Garnett, 1991; Isenberg, 2005). This level of invest-ment soon required more rigorous deposit valuation systems tojustify the investment and mobilization of large machinery. Minersin this period assessed their gold deposits using an ‘‘R/E factor”where R is the amount of actual gold recovered from mining a por-tion of the deposit and E is the amount of gold they predicted theywould be able to recover based on pre-mining sampling. An R/Evalue of 1.00 indicates that the miners recovered exactly the sameamount of gold as pre-mining studies predicted (Garnett, 1991).

Geologists estimate the quantity of recoverable gold in an allu-vial deposit (the ‘E’) by taking bulk samples at a fixed spacing.Because the grades of alluvial gold deposits tend to vary widelyover short distances, it is difficult to estimate with certainty thetrue value of a deposit without prohibitively high quantities oftightly spaced bulk samples. Although advances in the applicationof geostatistical methods, such as Kriging, improved deposit valu-ation in the late 1900s, alluvial gold deposits often contain smallzones of exceptionally high grade – meaning that regular samplingmethods often fail to capture values from the highest-grade por-tions of the deposit. This deposit variability phenomenon and theresulting geostatistical challenge is commonly known as the ‘‘nug-get effect” (Davis, 1987; Garnett, 1991).

By examining dredging records from the height of North Amer-ica’s alluvial mining period in the early 1900s, it becomes clearhow unreliable estimating gold grades and the challenges of the‘‘nugget effect” are in practice. Long-term (multi-year) R/E valuesranged from 0.50 to 2.90; a chilling indictment of the reliabilityof geologic sampling and deposit modeling techniques of the time(Garnett, 1991). As a result of the investment uncertainty and asouring gold market, large-scale mining companies all but aban-doned alluvial deposits after World War II, choosing instead toinvest in exploring and developing larger, more predictable golddeposit systems (Garnett, 1991). As a result, very little publishedwork on improving alluvial deposit estimation has been conductedsince.

Although large-scale companies have been out of the alluvialgold game for decades, the relatively low costs associated withprospecting for, and recovering gold from, alluvial systems is notlost on ASM. While alluvial gold production declined throughout

the developed world, ASM gold production began to increasethroughout Africa, Asia, and Latin America as government reformsand World Bank-driven economic restructuring liberalized devel-oping economies and gave rural individuals access to the risinggold price (Banchirigah, 2006). In rural gold-bearing parts of thedeveloping world, small-scale mining quickly became an attractiveoccupation.

Artisanal and small-scale miners’ success at exploiting alluvialgold deposits in the 1980s and 1990s quickly led to increasedinvestment in ASM projects from successful miners and localinvestors, often outside of the formal mining sector (Banchirigah,2006; Hilson and McQuilken, 2014). Scholars now report that the‘‘small-scale” mining sector is increasingly large, employing rela-tively expensive and highly mechanized mining and processingtechniques which enable them to exploit larger, more geologicallycomplex, and lower-grade deposits (Hilson and McQuilken, 2014;Teschner, 2012, 2014; Verbrugge, 2014; Verbrugge andBesmanos, 2016).

3. ASM gold processing and quixotic western interventions

The ore processing methods used in ASM systems are generallyconsidered to be inefficient and pose serious risks to the environ-ment and human health (Hentschel et al., 2002; Smith et al.,2016). Mercury is often used in ASM gold processing to bind withgold particles to form an amalgam. The creation of this amalgam isa useful method of separating gold particles from other heavygangue minerals, which fail to bond with the mercury. The gold-mercury amalgam is heated to burn off the mercury, leaving thegold behind in a relatively pure form that is easy to value and sell(Hilson, 2006; Veiga and Baker, 2004). During the burning process,miners often inhale the mercury vapors, exposing them and othersin the mining communities to grave health risks. Mercury is apotent toxin that interferes with brain and neurological function;it is particularly harmful to babies and young children (Poulinand Pruss-Ustun, 2008).

Artisanal and small-scale mining is the largest source of anthro-pogenic mercury emissions worldwide (United NationsEnvironmental Program, 2013). The United Nations EnvironmentalProgram (UNEP) estimates that the ASM sector consumes 640–1350 metric tons of mercury a year – roughly one third of the totalglobal consumption (United Nations Environmental Program,2008). Unlike other industrial uses of mercury, nearly all of whatis used by ASM ends up in the environment. Approximately 40%is released into the air, while most of the remaining 60% is lost intowaterways and soil (Telmer and Viega, 2009). Because mercury cantravel globally, mercury released by ASM often ends up pollutingair, water, and fish all around the world (United Nations Institutefor Training and Research, 2005).

Development organizations have spent considerable time andresources attempting to reduce or eliminate mercury use at ASMsites. The UNEP has a goal of reducing mercury use in ASM by50% by 2020 (United Nations Environmental Program, 2011), andaid organizations, scholars, and a handful of small companies haveendeavored to develop gold recovery technologies that reduce oreliminate the need for mercury. These interventions are commonlyaimed at persuading miners to adopt new technologies and aredeployed under the assumption that alternative technologies willimprove miners’ gold recoveries and reduce the negative impactson the environment and human health of mercury-based technolo-gies (Aubynn, 2009; Hylander et al., 2007a,b; Teschner, 2013).

As early as the late 1990s, development practitioners attemptedto introduce new gravity separation techniques including cen-trifuges, shaking tables, and improved sluices. They believed thatthese systems would draw miners away from more traditional

B. Teschner et al. /Minerals Engineering 105 (2017) 44–51 45

systems and increase recovery rates while reducing mercury use(Hentschel et al., 2003). More recently, the ‘‘Clean Sluice” technolo-gies developed by Cleangold, LLC have been demonstrated in Peru(Veiga et al., 2006a,b), and programs at larger-scale ASM sites haveexperimented with centralized milling and leaching plants mod-eled after conventional large-scale mining technology (Veigaet al., 2009). Isolated projects have been shown to improve goldrecoveries and reduce mercury use (Hylander et al., 2007a,b;Veiga and Baker, 2004), but these technologies have had limitedsuccess in widespread implementation. It is not always clear whythis is the case, but some scholars suggest that these improvedconcentration methods are often difficult to master and requireequipment, parts, and electricity that are sometimes difficult forminers to acquire (Hinton et al., 2003).

Despite the fact that these interventions are billed as boons tominers’ gold recovery, only a limited number of studies on recov-ery rates of miners’ existing technologies have been documentedin the literature. It is worth noting that Hentschel et al. (2003)opine that ‘‘Local techniques have frequently undergone evolution-ary optimization processes over decades of use. [Therefore,] therecovery rates of artisanal miners are often underestimated.” Thisis somewhat of a contradiction (or perhaps correction) of theirgroundbreaking report a year earlier (Hentschel et al., 2002) wherethey defined ASM, in part, by its lack of efficient recovery methods.Unfortunately, current scholars are more likely to reference theolder of the two documents.

A study at the Mount Diwata mining area in the Philippines isone of the only reports that extensively compares the use of mer-cury amalgamation, cyanide leaching, shaking sluices and Clean-gold sluices. In the study, the authors tested sluicing andmercury amalgamation processes on the ore from the area andnoted that they were relatively ineffective, recovering only 25–35% of the gold. Therefore, it is not surprising that most of the min-ers in the area had moved away from mercury amalgamation to amilling and cyanidation process even before the researchers con-ducted their tests. The study showed that the cyanidation processthat the miners were actually using had the potential to recover90–95% of the gold (Hylander et al., 2007a,b).

Studies like this one show that mercury-based processing canbe an inefficient process if used on some ore types. However, thegeologic and metallurgical conditions at every site are unique. Inthe case of the Mount Diwata study for example, the miners wereworking on in situ hard rock deposits (Hylander et al., 2007a,b).The authors note that the deposit had metallurgic complexities,especially multimineralic grains containing gold in chemical andphysical bonds, including gold contained within pyrite. Althoughthe Mount Diwata study uncovers important findings for the min-ers there, it would be entirely inappropriate to apply the results ofthat study to many other locations, especially the alluvial depositslike the one examined here.

What the Mount Diwata study illustrates is that artisanal min-ers are entrepreneurial when it comes to new technologies. Minersthere were quick to adapt their processing methods to the charac-teristics of the ore they are mining. This type of technology adop-tion at ASM sites has been widely documented throughout theworld, including in Ghana (Hilson, 2002; Hilson and McQuilken,2014; Teschner, 2012), Mali (Hilson, 2013; Teschner, 2014), Suri-name (Heemskerk, 2011), and elsewhere in the Philippines(Verbrugge, 2014). Although scholars have not followed up theseobservations with formal recovery studies like the one conductedat Mount Diwata or the one discussed here, it is evident that min-ers will quickly adopt new technologies when these technologiesare able to improve their yields and profitability.

Despite these realities, scholars and development practitionerscontinue to claim that ASM is inefficient by definition (PeruSupport Group, 2012; Hentschel et al., 2002; United Nations

Economic Commission for Africa, 2011, pp. 70–71; Veiga et al.,2006a,b), even though there is overwhelming evidence that someASM activity is quite efficient and that relatively inefficient opera-tors are constantly improving. The unverified assumption that aspecific ASM process, group, or site is inefficient can lead to thefalse conclusion that new technology represents a silver bulletfor solving ASM’s litany of problems: If only well-educated devel-opment practitioners and western scholars could develop a moreefficient method of mining and processing gold ores that is envi-ronmentally friendly and safe, then they will be able to addresschallenges such as mercury pollution, and occupational healthand safety problems; and formalize ASM by attracting miners tothe low grade, government-established, formalized mining zones(Aubynn, 2009; Hilson, 2006, 2009; Teschner, 2013; Veiga et al.,2009).

4. Study area

Artisanal and small-scale mining is the primary economic activ-ity for rural populations in the gold-rich interior regions of the Gui-anas (Guedron et al., 2006; Heemskerk, 2011; Vieira, 2006).Approximately 80,000 people work in the three counties as minersor in occupations that directly support mining activities and min-ing camps such as equipment and fuel suppliers, sellers of drygoods, commercial sex workers, restaurant workers, and fuel sup-pliers (Heemskerk et al., 2015; Legg et al., 2015; McRae, 2014). Thiseconomic activity primarily operates informally, but it is neverthe-less a critical piece of each country’s economy; the unregulatedASM sector in Suriname alone is valued at an estimated $1 billionannually (Gurmendi, 2012).

Foreign migrants play an important role in the ASM sector inthe Guianas. Perhaps the most visible are migrants from Brazil.Some scholars estimate that 75% of the ASM workers in the Gui-anas are Brazilian (de Theije and Heemskerk, 2009; Heemskerk,2011), and anecdotal evidence from this study suggests that Brazil-ians have a substantial, if not majority, presence in the ASM prop-erties in the greater study area. Brazilian mining groups commonly‘‘lease” mining land from local land bosses according to informalland tenure systems. Local community residents interviewed forthis study characterized Brazilian miners as highly skilled in allu-vial gold prospecting and recovery and credited the Brazilians fortechnological advances in the ASM sector, such as the introductionof hydraulic mining (see Fig. 1).

The miners in the study area mine alluvial deposits that arealmost certainly the eroded reflection of a large epithermal goldsystem located in the immediate proximity.2 The primary deposithas been subjected to extensive lateritic weathering, as is typicalin tropical regions throughout the world. The result is that the entiresurface of the landscape is covered with an iron-oxide-stained claythat is often tens of meters thick. Lateritic weathering of gold bear-ing zones causes an important transformation of the character ofgold grains relative to the unaltered rock in which they were origi-nally deposited. Colin and Vieillard (1991) observed that lateriticweathering liberated gold from multimineralic grains resulting inrounded ‘‘free gold” grains that could be easily eroded and rede-posited in alluvial systems, thus setting the geologic stage for ASMin the study area.

Hydraulic mining, a mechanically assisted form of gravity sepa-ration, is the dominant mining method reported across the Guianas(Heemskerk, 2011) and is the method the miners use in the studyarea. Fig. 1 shows the major components of this mining system.Mining groups use excavators to remove overburden and pile ore

2 At the time of this study, a large-scale multinational mining company had startedconstruction on a mine to exploit the primary, epithermal deposit.

46 B. Teschner et al. /Minerals Engineering 105 (2017) 44–51

material at the end of a trench. Miners then wash the ore materialwith high-pressure hoses powered by a diesel water pump creatingan ore-water slurry, which washes down the trench and into asump. A second diesel pump, called a hydraulic elevator, sucksthe water-ore mixture out of the sump and into a sluice. Gold par-ticles and other heavy minerals are captured on the carpets of thesluice. When a mining group is ready to recover the gold, theyremove the carpets from the sluice and wash them into a largebucket. The material is then panned by hand to separate the goldfrom the remaining heavy minerals. This final recovery processcan happen as often as three times per week if the group has founda high-grade zone. However, mining groups working lower gradedeposits may wait two weeks or more to clean the sluice.

5. Experimental method

The research team collected the data for this study on May 23and May 27, 2015 from a single mining group operated by Brazilianworkers. Geologic samples were collected while the mining teamwas actively operating. The researchers interviewed the miners(in Portuguese with the aid of a translator) about their mining pro-cess, mercury use, and gold marketing activities throughout thetwo-day sampling process.

This study recognizes the challenges associated with samplingalluvial gold deposits and the need to collect large-volume samplesin order to represent the heterogeneity of the orebody and mini-mize the nugget effect (MacDonald, 1983, pp. 227–230). Impor-tantly, this study does not attempt to value the orebody, butinstead the efficiency of the gold recovery method used on the ore-body. Nevertheless, the likely heterogeneity of the site must berespected and an analysis of the largest volume samples possibleundoubtedly improves the method’s accuracy. Unfortunately, col-lecting, shipping, and analyzing large-volume samples is impracti-cal, a problem noted by other scholars who have attempted to testalluvial processing systems (Phoon andWilliams, 1991). This studymanages this challenge by collecting approximately eight kilo-grams of material for each sample and splitting each into anapproximately one-kilogram sub-sample at the field site to sendto the lab.

Well-designed gravity separation methods are able to recovergold particles relatively efficiently down to approximately 30 lm.However mercury amalgamation, the process that miners employ

when cleaning the sluice, works best on gold grains larger than75 lm (MacDonald, 1983, p. 389; Mitchell et al., 1997). For thisreason, MacDonald (1983) notes that many alluvial processing sys-tems immediately separate and discard all sub-75 lm sized mate-rial from the ore in a process known as desliming. Miners deslimetheir ores for two reasons: First, efficient recovery of gold grainssmaller than 75 lm often requires specialized processing methods(like cyanidation) that are prohibitively expensive to implement.Second, ores with greater than 5–6% ‘‘slime” can decrease the effi-ciency of gravity separation methods (MacDonald, 1983, p. 387).Therefore, the sub-75 lm portion represents an important fractionof the ore material to understand when considering the recovery ofany gravity separation system.

In addition to very small particles, alluvial gold processing effi-ciency can also be limited by large size material. The most obviouschallenge with large particles is that they are more likely to bemultimineralic. A multimineralic grain may contain gold, but alsoother gangue minerals (in the case of this study area, most com-monly quartz). For example, gold has a density of 19.3 g per cubiccentimeter (g/cm3), while quartz has a density of 2.65 g/cm3. Amultimineralic grain of 5% gold and 95% quartz would thereforehave a density of only 3.5 g/cm3. Despite the fact that this graincontains a large amount of gold, it would likely be washed outthe bottom of the sluice. This study uses 500 lm as the lowerbound of this large sized material modeled after findings inMitchell et al. (1997).

Sixteen samples were collected from the site over a period oftwo days. Eight samples were taken from material that was aboutto enter the hydraulic elevator (referred to as ‘‘ore samples”) usinga post-hole digger, and eight samples were taken from the sluicedischarge (referred to as ‘‘tailings samples”) by placing a samplebag directly under the flow of material exiting the sluice (seeFig. 2). In both cases, the sampler collected approximately eightkilograms of material in large reverse-circulation sample bagsdesigned for retaining fine material.

Immediately after collection, each sample was run through ariffle splitter to reduce the sample to approximately three kilo-grams. During the riffle-splitting process, the sampler split out fourduplicate samples: two ore samples and two tailings samples.Three of the duplicate samples (Ore-8, Tails-7, and Tails-8)returned gold assay results within 5% of their pair. The laboratorywas only able to return partial results from one of the Ore-7 sam-ples due to mishandling of the sample.

Fig. 1. Hydraulic mining and processing method used by the miners in this study.

B. Teschner et al. /Minerals Engineering 105 (2017) 44–51 47

The twenty samples (sixteen primary plus four duplicates) weredried in a sample oven, and then split them using a rotary splitterto reduce each sample to approximately one kilogram. The onekilogram samples were sent to a professional laboratory in the Uni-ted States where they were each sieved into the three size distribu-tions: 500 lm. These three sub-sampleswere weighed and separately assayed for gold. The 500 lm portions were pulverized, homogenized,and screen fire assayed. The two largest grainsize portions werealso subjected to a gravimetric finish in order to capture any coarsegold nuggets that may have been present in the sample.

Understanding and verifying miner’s ore-processing activities iskey to appropriate interpretation of this study’s results. For exam-ple, the miners at the site in this study reported that they usedmercury, but that they only used it when they ‘‘cleaned” the sluiceafter a few days or weeks of mining, and there was no mercuryintroduced within the bounds of the sluicing system being tested.However, other miners interviewed in the surrounding areareported a wide variety of mercury use both in quantity and tech-nique. Some groups reported adding large amounts of mercurydirectly to their sluices throughout the sluicing process or to theore sump immediately before the ore material enters the hydraulicelevator, or even to the original ore pile in a process called whole-ore amalgamation.

How mercury is being used changes the interpretation of thisstudy’s results and may change a practitioner’s recommendationsfor possible mercury-reduction technologies. Therefore, theresearch team attempted to verify the claim that the miners didnot add mercury before the sluicing system. The team collectedwater and rock samples of material exiting the sluice and testedthem both for mercury. Both samples returned no detectable mer-cury, seemingly corroborating the miners’ claim (Borrillo-Hutter,2016). Nevertheless, the authors recognize that one-off samplingcould easily lead to false negatives, and other miners in the areaexpressed skepticism that the group was not adding at least smallamounts of mercury to their ore piles before sluicing. It is alsoimportant to note that the area where the miners were workinghad been repeatedly mined and re-mined in the years prior tothe study. Therefore, even if the group is not adding mercury earlyin their current processes, it is possible that the orebody containsresidual mercury from previous mining activities that is assistingin the current miners’ gold recovery processes.

In addition to the geologic samples, the research team collectedenvironmental samples in the immediate and broader study area

(Borrillo-Hutter, 2016), and anthropological data on ASM in thelocal area (Smith et al., 2017) which added a larger context to thisstudy. Over twelve days surrounding and concurrent with the geo-logic sampling in this paper, the team conducted interviews in localvillages, the town center that services the local mining activities,other active ASMmines, and a nearby large-scalemining company’scamp. These interviews included discussions with land bosses,equipment owners, other artisanal and small-scaleminers, commu-nity leaders, other community members, shop owners, commercialsex workers, and the large-scale mining company’s employees.

6. Results and discussion

On average, the ore samples entering the sluice contained0.852 (ppm) gold, and the tailings material exiting the sluice con-tained 0.077 ppm gold, resulting in an average measured recoveryof 91.0% (see Table 1). This recovery rate should be consideredstrong and suggests any attempt to improve the gravity separationmethods employed by this mining group would have only a limitedopportunity to improve gold capture. Nevertheless, the results ofthis study do point to other areas where development practitionersmay have opportunities for beneficial technological interventions.

First, the two sub-500 lm fractions represented approximately25% of the total mass of the sample, yet they contained nearly 100%of the gold. This is not surprising because the deposit had beenmined repeatedly for many years, and the miners reported thatthey very rarely recovered any nuggets, only ‘‘flour gold.” The labresults confirm these reports, most likely because miners hadalready recovered any coarser grained gold years ago. These minerswere probably using inferior technologies to those employed bythe current miners, and as a result, left behind the finer grainedgold that miners are currently finding. These results also indicatethat miners are not losing gold by failing to crush the larger mate-rial, since it does not contain any gold in the first place.

Second, the sluice is effective at recovering both 75–500 lmsized gold grains and also

and the tailings pile, any very fine grained material contained withthe ore stockpile or the tailings piles was probably lost to the sam-pling process.

Because very fine material takes time to settle out of suspen-sion, the authors speculate that the

sample standard deviation, then these data suggest that withgreater than 95% confidence, any material leaving the sluice willhave a grade 500 lm fraction of ore sam-ples and poor recovery of that gold, as indicated by significant goldremaining in the >500 lm of tailings samples, would suggest thatrecoveries could be improved by crushing the ore before sluicing.Alternatively, significant gold contained in the

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