Three-dimensional geochemical patterns of regolith over a

Journal of Geochemical Exploration 164 (2016) 122–135

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Journal of Geochemical Exploration

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Three-dimensional geochemical patterns of regolith over a concealedgold deposit revealed by overburden drilling in desert terrains ofnorthwestern China

Bimin Zhang a,b,c,⁎, Xueqiu Wang b,c, Qinghua Chi b,c, Wensheng Yao b,c, Hanliang Liu b,c, Xin Lin b,c

a School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, Chinab Institute of Geophysical and Geochemical Exploration, CAGS, Langfang 065000, Chinac Key Laboratory for Geochemical Exploration Technology, MLR, Langfang 065000, China

⁎ Corresponding author at: School of Earth Sciences andGeosciences, Beijing 100083, China.

E-mail address: [email protected] (B. Zhang).

http://dx.doi.org/10.1016/j.gexplo.2015.06.0070375-6742/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 22 December 2014Revised 9 June 2015Accepted 13 June 2015Available online 19 June 2015

Keywords:Geochemical patternsConcealed depositOverburden drillingDesert terrains

Desert terrains are widespread in northwestern and northern China, and these areas present particularchallenges for exploration. In recent years, partial extraction techniques have been proven to be effective inthe search for concealed deposits in arid desert terrains in some cases. However, we still lack an understandingof the dispersion patterns of ore-forming elements in regolith. In this study, air reverse circulation drillingswere used to create three-dimensional (3D) distribution patterns of elements in regolith over the Jinwozi golddeposit in China, which is covered by tens of metres of regolith, in order to trace the migration of elementsand to understand the dispersion mechanisms. The 3D distribution maps of elements show that (1) coherentanomalies occur at different depths of transported cover over the ore body, (2) Au tends to be enriched in thetop and bottom horizons and depleted in the middle horizon in the vertical direction, (3) the anomalousdistribution of Au at the bottom is restricted to places at the interface of sediments and bedrock, and (4) theanomaly in the bottom sediments is confined to a width of tens of metres, whereas that in top soils is muchwider and can extend up to several kilometres. In addition, close positive correlations were found between theAs, Hg, and Au distributions.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Desert terrains are widely distributed in northwestern and northernChina, and these areas are covered bywidespread transportedmaterialsthat can mask geochemical signals from ore bodies; such geomorpho-logical structures present major obstacles to mineral exploration(Wang et al., 2007). Over thepast 20 years, partial extraction techniqueshave been developed and proven effective in the search for concealeddeposits in certain terrains (Antropova et al., 1992; Bajc, 1998;Cameron et al., 2004; Clark et al., 1997; Cohen et al., 1998; El-Makkyand Sediek, 2012; Hamilton et al., 2004a,b; Kelley et al., 2003; Mannet al., 1995, 1998; Noble and Stanley, 2009; Wang, 1998; Wang et al.,2007; Williams and Gunn, 2002; Xie and Wang, 2003; Xie et al., 2011;Yeager et al., 1998), while at the same time, some migration modelshave been constructed and employed to explain the formation mecha-nisms of geochemical anomalies (Anand and Robertson, 2012;Aspandiar et al., 2008; Cameron et al., 2004; Garnett, 2005; Hamilton,1998; Hamilton et al., 2004a,b; Kelley et al., 2003; Lintern, 2007; Luzet al., 2014; Mann et al., 2005; Smee, 1998; Wang, 2005; Wang et al.,

Resources, China University of

2007). However, there is still a critical need to study the three-dimensional (3D) distribution of elements in regolith. Such informationis important for further elucidating the potential mechanisms for thetransfer of elements from the ore body upwards through the regolithcover to the surface and for understanding how to conduct successfulexplorations in regolith-dominated terrains, whether for depositsconcealed by the regolith or for those hosted within it.

In this study, we used air reverse circulation (ARC) drilling technol-ogy over the Jinwozi gold deposit in China, which is covered by severalto tens ofmetres of transportedmaterials, to determine the 3D distribu-tion patterns of ore elements in regolith and to investigate themigrationmechanisms.

2. Study area

The Jinwozi gold field is located 200 kmsoutheast of Hami city at theboundary of the Xinjiang and Gansu provinces in northwestern China(Fig. 1). There are two NE-trending mineralized zones in the Jinwozigold field (Fig. 1). In the northern zone, the mineralization is character-ized by an epithermal quartz-vein type. The auriferous quartz veinsoccur at the contact between porphyry and Devonian sequences. Inthe southern zone, the mineralization is characterized by tectonicalterations. The ore bodies occur in a structural shear zone, which is

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Fig. 1. Location and geology of the study area with drill hole sites. Coordinates are UTM Zone 46.

123B. Zhang et al. / Journal of Geochemical Exploration 164 (2016) 122–135

mainly controlled by a NE trending fault that lies within the Devoniansequence. The ores are mainly composed of pyrite, galena, and chalco-pyrite. The pyrite is the primary Au-bearing mineral. The average Augrades of the two mineralized zones are c. 7 g/t and 4 g/t (Wang et al.,2007). The proven total Au reserve of the gold field is c. 10 t.

The northernmineralized zone is located in an outcropping area andhas a relatively high relief. The southern mineralized zone is situated inan area that is covered by the Gobi Desertwith depths of a fewmetres totens of metres. The regolith is composed of windblown sand, alluvium,colluvium, and residuum. The typical zonal structure of the regolithcover is illustrated in Fig. 2. The sequence of regolith materials fromtop to bottom: black gravels (lag), desert crusts, brown sands with

Fig. 2. Sketch illustrating the vertical regolith profiles.

interbedded gravels, brown yellow sands, purple red sands, eluvium,and bedrock. The lags are always covered by a dark and shiny substancecalled desert varnish. Such coatings represent a fine mixture of clayminerals and Fe–Mn oxyhydroxides, which form micrometre-thick

Fig. 3.Golddistribution indifferent fractions of soils along the traverse line that crosses thetwo mineralized zones (Wang et al., 2007).

Fig. 4. Drill hole sites of the ARC drilling located in the southern mineralized zone.

124 B. Zhang et al. / Journal of Geochemical Exploration 164 (2016) 122–135

laminations that parallel the topography of the rock substrate (Potterand Rossman, 1977). It is still debated whether such varnish is formedby (Broecker and Liu, 2001; Krinsley, 1998): (a) slow diagenesis ofdust particles deposited on rock surfaces, (b) leaching from the underly-ing rock substrate, or (c) direct deposition of dissolved constituents inthe atmosphere. The desert crust is created by the breakdown of soilstructural units by flowing water, raindrops, and subsequent evapora-tion. Influenced by the undulating relief of the palaeotopography,Tertiary purple red sand layers are absent in some places.

The area is characterized by an arid climate, low rainfall, highevaporation, vast temperature differences between day and night (thetemperature difference can reach 40 °C), and long hours of sunshine(16–17 h in summer). The annual rainfall is less than 250 mm, andthe potential annual evaporation is c. 1500 mm. July is the hottestmonth of the year (mean daily temperatures range from 25 to 40 °C inJuly), and January is the coldest month of the year (mean daily temper-ature range from−20 to 5 °C). Because of the harsh natural conditions,this area often has sparse vegetation and small amounts of biomass.

3. Previous geochemical work

In the study area, a regional scale geochemical survey over an area of1200 km2 was conducted by Ye et al. (2004); their results showed thatthefine fraction of soil samples (b96 μm)delineates reasonablywell thelocation of the regional tectonic metallogenic belt and mineralization(Ye et al., 2004). Subsequently, Wang et al. (2007) employed a varietyof sample media to a prospect along a transverse line that ran acrossthe twomineralized zones; their results demonstrated that the greatestcontrast between anomalous and background concentrations is in thefine fraction (b96 μm) of soils (Figs. 1 and 3). They advocated that theuse of fine fraction samples from clay-rich horizons or selective leachingof elements adsorbed on clays or oxide coatings would be effective forlocating buried deposits.

4. Methods

4.1. Sampling

An ARC drilling programme was systematically conducted alongfour transverse lines that ran across the southern mineralized zone(Figs. 1 and 4). The intervals between the transverse lines were c.250 m. Each line had 13–20 bore holes that were spaced at an interval

of 50–100 m. At the bore holes, samples were collected continuouslyevery metre from the ground surface to the bedrock. Every samplewas well mixed and sieved in the field through a b100 mesh, and thefiner fraction (150 μm) passing through the mesh was retained for fur-ther analyses. The choice of fraction sizewas based on the experimentalfindings and previous geochemical work in the region. In total, 1046 soilsamples were collected from 63 bore holes. It is undeniable that miningcontamination occurs in this study area. Hence, drilling sites wereselected very carefully to avoid drilling dust and mining contamination.In consideration of the possibility of cross-hole contamination from thedrilling activities, drilling equipment and the sampling system werecleaned after use at every hole. The cleaning procedure involvedthe use of high-pressure air generated by an air pump, which wasblown onto the drilling equipment and sampling tools to dislodge anyresidual debris. After that, we used a clean cloth to wipe off all of theequipment.

In the southern mineralized zone, four soil samples were collectedand combined to form a composite sample at a depth of 15–30 cmabove the buried deposit to confirm the use of the appropriate samplingfraction in the drilling work. The choice of soil sample depth was basedon considerations of the sandy clay enrichment in the study area andprevious sampling depths used in this region (Wang et al., 2007). Thecomposite sample was sieved into seven fractions in the field withthe following grain size fractions: 830–1700 μm, 380–830 μm,250–380 μm, 180–250 μm, 150–180 μm, 120–150 μm, and b120 μm.

During the process of drilling, the different regolith types (Quaternaryalluvium, Tertiary red strata, eluvium, and bedrock) could be recognizedon the basis of the bore hole cutting colour, granularity, and mineralcomposition. Regolith profiles were constructed for each bore hole thatwas sampled, and the results are shown in Fig. 5.

4.2. Sample preparation and analyses

All the samples were ground to less than 200 mesh (75 μm) foranalyses. A 0.25-g sample was digested in a hot mixture of acids (HCl,HF, HNO3, and HClO4). Inductively coupled plasma-mass spectrometry(ICP-MS) was used for the determination of Ag, Cu, La, Pb, Sb, Sr, Th,U, and Zn concentrations. Additionally, a 10-g sample was digested inaqua regia and analysed by graphite furnace atomic absorptionspectrometry (GF-AAS) to obtain the Au concentration. Furthermore, a0.5-g sample was subjected to an aqua regia digest and analysed byhydride generation atomic fluorescence spectrometry (HG-AFS) todetermine the As and Hg concentrations.

In addition to the above analyses, 24 soil and sediment samplescollected at different depths from one of the drill holes over the miner-alization were analysed for SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, and K2Oby X-ray fluorescence (XRF).

Analytical accuracy and precision for the laboratory quality werestrictly controlled by laboratory replicate samples and Standard Refer-ence Materials (SRMs). Five laboratory replicates were inserted intoeach batch of 50 samples for precision control. The relative deviation(RD%) of determination values of the replicates was listed in Table 1and calculated based on the equation:

RD% ¼ jC1−C2j= C1þ C2ð Þ=2½ � � 100%

where C1 is the first determination and C2 is the second determination.Four standard reference materials (GAU9aGSS1, GAU10aGSS2,

GAU11GSS3, GAU12GSD1a) were inserted blindly into each batch of50 samples and analysed simultaneously with the samples for accuracycontrol. Accuracy was controlled by the logarithmic difference (ΔlgC) ofthe determination value (lgCi) and standard reference value (lgCs) foreach standard (Table 1).

The mineralogy of seven samples in different fractions and 24samples from one of the drilling holes were further analysed semi-quantitatively by X-ray diffraction (XRD).


5. Results

5.1. Concentrations and variations of elements in different soil fractions

The distributions of elements in different soil fractions are controlledby mineralogy. The mineralogy of the study area is shown in Table 2.The coarser fraction (N830 μm) consisted largely of quartz and feldspar,followed by gypsum and calcite. The intermediate fraction (150–830 μm)was dominated by quartz, gypsum, feldspar, and calcite, with small

Fig. 5. Structure of the regolith p

amounts of clay minerals. The fine fraction (b150 μm) consisted largelyof calcite and clay minerals including illite, kaolinite, and chlorite.

The analytical results of different soil fraction samples sieved fromthe composite sample are shown in Fig. 6. It is obvious thatAu-concentrations of the fine fractions (b150 μm) were higher thanthose of the other fractions (N150 μm). Some other ore elements (Ag,As, Hg, Pb, Zn, Ni, Co, Cr) displayed the same distribution. Consideringtheweight percentage of fine fractions in soils (Table 2), the proportionof the total Au in the fine fractions reached up to 48.93%.

rofiles presented by drilling.

Table 1Calculated values of relative deviation and logarithmic difference for precision and accuracy control.

Parameters Analytical method Unit Detection limit RD% ΔlgC

Au GF-AAS ppb 0.2 24.53 0.05Ag ICP-MS ppb 20 13.35 0.04As HG-AFS ppm 1 7.03 0.03Cu ICP-MS ppm 1 9.19 0.02Hg HG-AFS ppb 2 7.17 0.02La ICP-MS ppm 1 7.06 0.03Pb ICP-MS ppm 2 5.99 0.02Sb ICP-MS ppm 0.05 6.33 0.03Sr ICP-MS ppm 5 2.21 0.02Th ICP-MS ppm 1 8.57 0.03U ICP-MS ppm 0.2 6.19 0.03Zn ICP-MS ppm 2 7.02 0.02SiO2 XRF % 0.1 1.86 0.005Al2O3 XRF % 0.1 2.21 0.01Fe2O3 XRF % 0.1 0.65 0.01MgO XRF % 0.05 1.96 0.02CaO XRF % 0.05 1.05 0.02Na2O XRF % 0.1 1.79 0.03K2O XRF % 0.05 0.95 0.01

Notation: GF-AAS: graphite furnace atomic absorption spectrometry; HG-AFS: atomic fluorescence spectrometry; ICP-MS: inductively coupled plasma mass spectrometry; XRF: X-rayfluorescence spectrometry; RD: relative deviation; ΔlgC: logarithmic difference.

Table 2Mineral composition of different soil fraction samples sieved from the composite sample.

Fraction(μm)

Quartz(%)

Feldspar(%)

Calcite(%)

Dolomite(%)

Hematite(%)

Hornblende(%)

Gypsum(%)

Illite(%)

Kaolinite(%)

Chlorite(%)

Weight percentage(%)

830–1700 32.4 33.0 8.3 – 3.1 0.9 14.6 5.4 1.9 0.4 21.9380–830 19.7 24.2 15.5 – 4.8 – 24.3 7.6 3.3 0.6 4.1250–380 18.7 24.4 9.4 – 5.7 – 30.6 7.1 3.5 0.6 18.5180–250 23.9 13.5 11.4 – 5.0 – 31.0 10.6 3.7 0.9 14.1150–180 15.4 13.3 15.3 4.3 5.3 1.5 24.7 13.8 5.2 1.3 15.9120–150 13.9 14.1 21.9 – 3.5 0.7 16.4 18.8 8.6 2.2 6.0b120 13.6 9.4 19.8 4.8 2.3 0.8 9.0 26.2 11.2 2.9 19.5

Fig. 6. Au, Ag, As, and Hg concentrations in different sieve fractions in the composite soil sample from the southern mineralized zone.


Fig. 7. Scatterplots of Au, Ag, As, and Hg concentrations versus clay content fractions in the composite soil sample from the southern mineralized zone. r, correlation coefficient.


Positive correlations (generally r N 0.8)were found between the claycontent and element concentrations (Fig. 7). Based on the above resultsand previous research, it can be concluded that fine grain-sizedfractions have higher concentrations of target elements and show a

Fig. 8. Vertical variation of mineral contents in the drill profi

greater contrast between anomalous and background concentrationsthan coarse grain-sized fractions. Thus, the selection of the b150 μmfraction from the drilling samples for analysis and research wasappropriate.

le over the mineralization hole (Hole A shown in Fig. 4).

Table 3Mean values for themineralogical composition of samples from different lithological units (Quaternary alluvium, Tertiary red strata, eluvium, and bedrock); these samples were collectedfrom one drill hole over the mineralization.

Unit Clay (%) Quartz (%) Feldspar (%) Calcite (%) Dolomite (%) Hematite (%) Hornblende (%) Gypsum (%)

Quaternary alluvium 27.4 19.1 5.9 10.2 26.1 1.6 2.6 7.1Tertiary red strata 21.5 42.9 5.6 9.1 19.7 – – 1.2Eluvium and bedrock 22.9 45.9 4.2 7.5 19.2 – – 0.3

Fig. 9. Vertical variation of element concentrations, pH, and conductivity of soils in the drill profile over the mineralization hole (Hole A shown in Fig. 4).


5.2. Vertical variation of the mineral composition and element concentra-tions in a drill hole

Soil and sediment samples (b150 μm) at different depths from oneof thedrill holes (Hole A in Fig. 4) over themineralizationwere analysedto determine their mineral compositions (Fig. 8) (Table 3) and elemen-tal concentrations (Fig. 9) (Table 4). The structure of the regolith in thisdrill hole is shown in Fig. 2.

Although the major minerals of the soils and sediments from theground surface to the site of mineralization are similar, i.e., quartz(14–53%), dolomite (15–34%), illite (13–23%), and calcite (5–15%),certain variations were found in regard to mineral compositionwith depth; these variations were especially noticeable in areaswhere characteristic changes in the regolith occurred. Clay minerals(illite, kaolinite, and chlorite) are more abundant in Quaternaryalluvium and low in Tertiary red strata, but are present in higheramounts in weathered eluvium close to the mineralization. Quartzis much lower in Quaternary alluvium than in Tertiary red strataand eluvium. Conversely, gypsum is much higher in Quaternaryalluvium than in Tertiary red strata and eluvium. Feldspar is higherin Quaternary alluvium than in Tertiary red strata and eluvium; butat depths of 17–18 m, feldspar is much higher than at other depths.

Table 4Mean values for the element concentrations, pH, and conductivity of soils from different litholowere collected from one drill hole over the mineralization.

Unit Au (ppb) As (ppm) Hg (ppb) Ag (ppb) Cu (ppm)

Quaternary alluvium 7.8 18.5 8.8 202 28.8Tertiary red strata 3.1 10.9 7.6 225 20.7Eluvium and bedrock 671.8 12.7 10.5 302 18.8

In addition, hematite and hornblende are found to occur only inQuaternary alluvium.

The concentrations of elements also vary with depth. Gold concen-trations are high in the top parts of the bore profile, then they graduallydecrease to low levels in Quaternary alluvium and in Tertiary red strata.Close to the eluvium, Au concentrations are high. In the eluvium, the Auconcentrations reach 2593 ppb. The distribution pattern of Au in theregolith profile is crescent-shaped, i.e., it has a tendency to be enrichedin the lower and upper parts of the profile. The Ag, As, Hg, Zn, and a fewother elements (e.g., W and Bi) have distribution patterns similar to Au.This kind of distribution pattern is similar to the vertical variation of clayminerals, and it is indicative of the positive correlations between thesecomponents. In contrast, Cu is high in Quaternary alluvium and low inTertiary red strata and eluvium. For the major elements, Quaternaryalluvium has less Si and more Fe and Ca than Tertiary red strata andeluvium,which coincideswith the vertical variation of quartz, hematite,and gypsum. Furthermore, the pH levels of the upper half horizons ofQuaternary alluvium are greater than those in the other horizons. Thisis due to the precipitation of some secondary minerals such as gypsum,which is related to long-term evapotranspiration processes. In addition,electrical conductivity is high in Quaternary alluvium and low inTertiary red strata and eluvium.

gical units (Quaternary alluvium, Tertiary red strata, eluvium, and bedrock); these samples

Pb (ppm) Zn (ppm) SiO2 (%) Fe2O3 (%) CaO (%) pH EC (s/m)

31.3 66.5 40.2 4.9 14.9 8 6.219.9 52.3 50.8 3.7 11.6 7.8 2.534.2 63.9 55.9 3.6 9.9 8 2.3

Fig. 10. Three-dimensional spatial distribution of Au.


5.3. Three-dimensional geochemical patterns of ore-forming elements

The distribution of elements in different transported overburdenhorizons is of special significance for geochemical research, especiallythe patterns in the top and bottom layers. The top layer participatesdirectly in the process of hypergenesis and is the highest horizon thatelements can reach during the process of vertical migration. At thesame time, the soil of the top layer offers an economical samplingmedi-um for geochemical exploration. The bottom layer is next to bedrockand contains weathering products from bedrock. Accordingly, some di-rect mineralization information can be exposed by studying this layer.

In order to present the 3D geochemical patterns of ore-formingelements in transported overburden material, the spatial distributionsof Au, As, Hg, Ag, and Cu are shown in Figs. 10–14. The contour plotswere made by Surfer software, and the kriging method was used torepresent the distribution of these elements in the regolith. The statisti-cal values (mean, standard deviation, minimum, and maximum) forthe elemental ARC data from the various lithological units (surface,

Quaternary alluvium, Tertiary red strata, eluvium, and bedrock) arepresented in Table 5.

5.3.1. GoldAt the interface between overburden and basement, regolith Au

concentrations were elevated in the mineralized zone (Fig. 10a). Goldconcentrations in the anomalous centre reach up to 2.59 ppm. At thepositions of 0–1 m, 1–2 m, and 2–3 m, anomalies were found abovethe mineralized zone and in its dip direction (NW trending). The areaof anomalous distribution is larger in the shallower position. The surfacesoil (c. 10–30 cm) has the largest enrichment area, and it is related tothe direction of the tectonic alteration zone and seasonal rainwaterflow. It is obvious that Au tends to be enriched in the lower and upperparts and depleted in the middle parts of the profile in the verticaldirection (Fig. 10b and c). Furthermore, the anomalous distribution atthe bottom is restricted in the lowest places at the interface of theoverburden and basement.

Fig. 11. Three-dimensional spatial distribution of As.


5.3.2. Arsenic, mercury, and silverThe As and Hg distributions were similar to that of Au (Figs. 11–12).

Anomalies occur continuously in the different depths of overburdenover the ore body. There are close positive correlations amongthe As, Hg, and Au distributions. The Ag distribution showed slightresemblance to that of Au, especially the Au distribution at the surface(Fig. 13a).

5.3.3. CopperThe Cu data displayed an irregularly distributed profile (Fig. 14).

There is no obvious enrichment of Cu related to Au mineralization orany close correlation between Au and Cu. In regard to the distributionof Cu and the structure of the regolith profiles, strong Cu anomaliesmainly occur in the lower segment of the transport overburden lyingto the southeast of the mineralization. This portion of Cu-bearingmaterials may have been from past river transportation events. Copperanomalies also occur in the bedrock lying to the northwest of themineralization.

In addition, correlation coefficients among elements (Au, As, Hg, Ag,and Cu) in all soil samples were calculated (Table 6). Gold showed close

positive correlations with As and Hg, a very weak correlation with Ag,and no correlation with Cu.

6. Discussion and conclusions

Fig. 5 shows that palaeotopographic relief is high in the researcharea. The depth of transport overburden in the northwest area isabout 2–12 m. In the middle area, the depth of regolith becomesshallower, but abruptly, the terrain slopes down southeastward alongthe transverse lines. Because of the descent of the ancient landform, athick-layer of Tertiary red strata has been deposited over the bedrocks.The thicknesses of the Tertiary red strata range from several metres tomore than 50 m, over which there is still transport overburden.High-relief palaeotopographic features may influence the mechanicaldispersion of weathering products from mineralization so thattransported regolith containing anomalous trace element concentra-tions can be found in the low-lying areas of ancient landforms. Inaddition, the depth of the regolith cover across the mineralized zonein the middle of lines 1 and 2 is shallow (1–6 m deep), which makes iteasier for the element to migrate.

Fig. 12. Three-dimensional spatial distribution of Hg.


Three-dimensional distribution maps of elements that weregenerated in this study for the Jinwozi gold deposit in China show that(1) coherent anomalies occur at different depths of transported coverover the ore body, (2) Au tends to be enriched in the lower and upperparts and depleted in themiddle parts of the profile in the vertical direc-tion, and the distribution has a ‘C-shaped’ pattern, (3) the anomalousdistribution of Au in the bottom horizon is restricted to places at theinterface of the sediments and bedrock, and (4) the anomaly in thebottom sediments is confined to a width of tens of metres, whereasthat in top soils is much wider and can extend to several kilometres.In addition, close correlations were found between the As, Hg, and Audistributions, but no obvious enrichment of Cu related to Aumineraliza-tion was observed.

Enrichment of elements in the bottom materials was due to theweathering of the ore body, and the elements were mechanicallytransported and deposited in the lowest places by the process ofpediplanation. The contact between the Quaternary alluvium andTertiary strata formed an unconformity along the palaeo-landformsurface, which allows for the possibility that mechanical dispersion ofweathering products from the ore zone onto this surface may havecontributed to some of the higher concentrations present in Quaternary

alluvium relative to the Ternary red strata in the profiles. Themineralizedzone is completely covered by the Gobi Desert, although the coveringlayer is very shallow in some places. Hence, anomalies in top sedimentsformed by vertical migration of elements. In addition, broad patterns insurface soils were driven by continuous vertical migration of elementsfrom the ore body to the surface and further lateral diffusion by windand seasonal rainwater in all directions, providing the theoretical basisfor the use of low-density geochemical mapping when prospecting forconcealed deposits. The possibility may still exist that some anomaloushigh surface values could have resulted frommining activity contamina-tion (despite efforts taken by us to avoid this).

The possiblemodes of verticalmigration in desert terrains have beendescribed by Wang et al. (2007) (Fig. 15). Elements are released fromthe ore body and associated altered rocks during weathering. Becausethe area is so arid, the water-table is hundreds of metres deep andvegetation is sparse. Water and vegetation would therefore seem toplay a very limited role in the transport of elements upwards to thesurface, although evaporation, capillary action, or uptake by plantsmay occur during rainfall events. Hence, gas is regarded as the mainmedium for vertical migration of the elements. Ultra-fine Au particlescan be adsorbed onto the surfaces of gas bubbles and migrate with the

Fig. 13. Three-dimensional spatial distribution of Ag.


bubbles upwards to the surface. Such gases may be derived from theatmosphere and driven to the surface by barometric pumping(Cameron et al., 2004), or they may be released from ore minerals orderived from mantle degassing (Gold and Soter, 1980). Nanoscale Auparticles have been observed in gas over Jinwozi gold deposits using atransmission electron microscope (TEM) equipped with an energydispersive spectroscope (Wang and Ye, 2012); these data provide directevidence of the migration mechanism.

Three-dimensional geochemical patterns of regolith presented byoverburden drilling also provide good evidence for the transportmode of vertical migration. ‘C-shaped’ patterns have been observed inregolith of other landscapes, for example, over ore bodies in the Yilgarncraton of Australia (Gray et al., 2008). This pattern is thought to be theresult of depletion of elements in themiddle of the regolith due to salinegroundwater under an arid climate. In our study area, the groundwatertable is below 200 m and the rain water, with an annual rainfall ofb250 mm, does not extend down 20 cm from the soil surface (sincethe beginning of the Quaternary). Thus, it is impossible for elementsto have been depleted by saline groundwater in this very youngwind-blown soil regolith terrain.

In the future, we plan to conduct additional geochemical mappingresearch on some known concealed deposits in desert terrains and ap-plying our mapping technology to other areas of potential geologicalsignificance. In addition, further studies on the migration mechanismsof elements are needed to gain better understanding of the genesis ofore element anomalies over concealed deposits.

Acknowledgments

This paper presents the results of the “Deep-Penetration Geochemi-cal Detection Technology Project” (201011055, SinoProbe-04-03),which was supported by China's Ministry of Land and Resources. Thisstudy was also financially supported by the Natural Science Foundationof China (41203038) and the ChinaGeological Survey (1212011120206,12120113100900). The authors thank Dr. Ravi R. Anand, who is affiliat-ed with Australia's Commonwealth Scientific and Industrial ResearchOrganization (CSIRO), for providing us with a critical review of anearlier draft of themanuscript.We also thank the reviewers and editorsfor their thoroughwork and very helpful comments on prior versions of

Fig. 14. Three-dimensional spatial distribution of Cu.

Table 5Statistical parameters for elemental (Au, As, Hg, Ag, and Cu) ARC data from the various lithological units (surface, Quaternary alluvium, Tertiary red strata, eluvium, and bedrock).

Elements Sample types N Min. Max. Mean SD

Au (ppb) Surface soil 63 2.78 386.00 48.38 81.43Quaternary alluvium 456 0.58 146.30 9.10 20.93Tertiary red strata 372 0.19 19.07 2.38 2.78Eluvium and bedrock 155 0.31 2593.70 56.99 169.37

As (ppm) Surface soil 63 3.24 64.03 20.89 8.34Quaternary alluvium 456 6.77 78.90 20.12 7.24Tertiary red strata 372 4.64 35.00 12.04 4.50Eluvium and bedrock 155 1.90 213.99 22.41 23.76

Hg (ppb) Surface soil 63 6.00 84.50 21.43 17.81Quaternary alluvium 456 2.50 69.50 8.46 6.17Tertiary red strata 372 5.00 31.50 10.00 3.05Eluvium and bedrock 155 2.50 90.50 12.40 15.75

Ag (ppb) Surface soil 63 58.97 1339.42 116.47 62.62Quaternary alluvium 456 35.97 837.24 128.85 83.26Tertiary red strata 372 45.38 909.72 135.13 106.69Eluvium and bedrock 155 18.52 2239.87 142.89 240.77

Cu (ppm) Surface soil 63 17.73 32.34 25.00 3.18Quaternary alluvium 456 5.96 485.41 32.17 31.31Tertiary red strata 372 10.08 200.51 23.42 14.22Eluvium and bedrock 155 3.96 57.74 23.27 12.32

Notation: N = number of samples; Min. = minimum; Max. = maximum; SD = standard deviation.


Table 6Correlation coefficients between the elements (Au, As, Hg, Ag, and Cu) in all soil samples.

Element Correlation coefficients

Au As Hg Ag Cu

Au 1 – – – –As 0.71 1 – – –Hg 0.82 0.69 1 – –Ag 0.17 0.07 0.06 1 –Cu −0.02 0.05 −0.01 0.03 1

Fig. 15. Conceptual model of migration in desert regolith profile (Wang et al., 2007).


this paper. All of these supportive efforts are acknowledged here withappreciation.

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