Using electrogeochemical approach to explore buried gold deposits in an alpine meadow-covered area

Panfeng Liu?Xianrong Luo?Meilan Wen?Jiali Zhang?Wen Gao?Fei Ouyang?Xuchuan Duan

1 Introduction

Electrogeochemical prospection,one of the deep-penetration geochemical exploration approaches,effectively works in pinpointing concealed deposits or ones that are hard to locate with other prospecting methods.It is based on a theory proposed by Safronov N.I.,a Soviet researcher,who suggested that an artificial electric field could act on orebodies deeply buried in the Earth and force metal ions contained within these orebodies to transport them to surface.The theory,together with its technology application on mineral exploration,was documented and published first in 1973(Ryss and Goldberg 1973).In 1978,Soviet began to use the method in exploration for Cu–Ni,W–Mo,and gold ores buried at a depth between 600 and 800 m,and also for oil and gas reservoirs of 2000–3000 m deep(Rysset al.1990).It wasnot until 1985 that the method had been introduced to other countries,including India,Canada,the U.S.,and Australia(Shmakin 1985;Salapatra et al.1986;Govett and Atherden 1987;Ryss and Siegel 1992;Smith et al.1991,1993).Later,U.S.expert Leinz R.W.and Israeli scholar A.Levitski made improvements to the extraction device of the method and proposed the socalled ‘‘NEO–CHIM’’and ‘‘Dipole CHIM’’approaches,respectively,to better apply these method to ore deposit survey(Leinz and Hoover 1994,Leinz et al.1998;Levitski et al.1996).

In the late 1970s,several Chinese researchers,including Fei Xiquan and Xu Bangliang,blazed a trail in electrogeochemical exploration in China by introducing the concept into the country and publishing papers on the issue in Chinese journals including Geophysical and Geochemical Exploration and Geology and Prospecting(Fei 1984;Xu et al.1984).Since then,and especially after 2000,electrogeochemical exploration has been applied and advanced in China,gradually forming a technology featuring lowvoltage,dipole,and independent power supply.The technology can be used to find ore deposits or perform assessment of the deposits through metal ion anomalies related to ore deposits.It relies upon an external electric field to move metal ions that were already transported to the Earth’s surface to assigned extraction electrodes,and then collects and analyzes the electrolytes on the adsorbing materials of the electrodes(Luo et al.2002a,b;Kang and Luo 2003).The technology was applied to various regions covered by dry deserts,forests,prairies,and swamps,with positive results achieved(Chen et al.2007;Shu et al.2009;Wang et al.2011;Yan et al.2014).This paper documents an electrogeochemical exploration test in the Bangzhuoma gold deposit covered by alpine meadow in Tibet Plateau and compares the results(sections and planes)with that of conventional soil measurements to further demonstrate the reliability and efficiency of the technology.

The study area is located in a high mountain region with an average elevation of 4800 m,in Longzi County,Shannan Prefecture of Tibet Automonous Region,China.It is higher in the north and west and lower in the south and east.Featuring low temperature,wide-spreading tundra,an annual precipitation of 297.41 mm and less evaporation,the area is mostly covered by 10–20 cm-thick sods of welldeveloped roots(Fig.1).

The area sits on the Himalayan block at the southern part of the Tibet Plateau and the central-eastern part of the Himalayan Tethys orogenic belt.It is defined to the north by the Yarlung Zangbo River structure zone(Fig.2a)(Zheng et al.2004;Wang et al.2013).The area contains complicated structures as a result of intensive magmation and metamorphism asaresult of the Himalayan orogeny.It is a second order tectonic unit of the Kangma-Longzi fold belt and sandwiched by an extension-detachment fault and the Gudui-Longzi fault.The Yelaxiangbo metamorphic core complex developed in the north of the area is the Miocene binary mica and monzonitic granite.The Zhegu-Ridang fold thrust bundle in the south of the area contains Late Cretaceous diabase dyke swams,which provide partial heat source for metallogenic thermal fluidsrising along faults,folds or other tectonic structures(Wang et al.2013).The intensive collision orogenesis between the Himalayan Block and the Gangdise Block and the subsequent largescale extension and detachment are regarded as important factors behind the formation of the Zhegu Au–Sn deposit in the western part of the area and the Mazala gold ore and Zhaxikang lead zinc ore in the southern part of the area,now an important Au–Sn metallogenic belt in the Tibet Plateau(Fig.2b)(Du et al.1993;Nie et al.2005,2006).

There is only one strata-the Nieru Formation of the Triassic(T3n)—outcropped in the study area.In the northwestern and central southern parts,the second member of the Formation(T3n2)contains dark grey to black layers of sericite silty slates with some laminas of greywacke;in the central part,the third member of the Formation(T3n3)has wide-spreading grey-dark grey silty sericite slates interbedded with medium-thick fine-grained metamorphic feldspar and quartz greywacke;and the fourth member of the Formation(T3n4)also exposesa little as grey silty sericite slates interbedded with thick-massive layers of metamorphic feldspar and quartz greywacke.Faults in the area mostly extend in nearly east-westward and north-westward directions.The former is represented by the Sederenjin-Duolazeri fault(F2)—a low angle thrust nappe structure—and a series of secondary faults(F4,F6),which are the main ore-controlling structures and spatial location to most mineralization alteration zonesin the area.The latter is represented by the Xinna fault,a trust fault(F5)(Dong et al.2012;Chen et al.2016).Nine gold veins and five gold ore bodies were delineated in the Bangzhuoma mining area.The major veins are③,④and⑤veins with strike length ranging between 900 and 1300 m and Au grade between 1.02×10-6and 5.47×10-6.They occur mainly in fractured alteration zoneswith altered sandstone/quartz veins.The mineralization types include arsenopyritization,pyritization,and ferritization,and the alterationtypes are largely silicification and carbonatization(Chen et al.2016).

Fig.1 Geomorphic features of areascovered by alpinemeadow

2 Method and technology

2.1 Sampling

A prospecting study wascarried out in areaswith available prospecting lines with a total length of 1000 m and 170°strike angle in the Bangzhuoma mining area and its periphery.Ten sampling lines labeled from 001 to 010 were laid out.Sampling was performed every 40 m inside the mining area and every 200 m×40 m in the periphery.For sampling points with basement crops,sampling was deviated east-westward normally less than 10 m from being perpendicular to the sampling lines.Electrogeochemical samples as well as soil samples were taken at each sampling point.

For electrogeochemical extraction,we used the dipole CHIM extraction device with independent power supply developed by Buried Ore Deposit Research Institute of Guilin University of Technology(Fig.3).The extraction processlasted for 48 h with 1000 mL of extracting solution at a voltage of 9 V.Electrodes were buried in B layer(the illuvium horizon,Fig.3)with a depth of 30–50 cm.Extraction electrodes were placed 100 cm apart,and extraction material was treated in high-density foamed plastic.The soil sampling depth was the same as the electrodes,and samples were sieved with a 20 mesh sifter.

2.2 Analysis method

Electrogeochemical samples(extracted with foamed plastic)and soil samplesweretested with a U.S.-made X-series inductively coupled plasma mass spectrometer in the Test Center of China Nonferrous Metals(Guilin)Geology and Minging Co.,Ltd.

The test was carried out with the following conditions:ICP power:1300 W,cooling air flow rate(Ar):13.0 L/min,auxiliary air(Ar)flow rate:0.90 L/min,atomization air(Ar)flow rate:0.76 L/min,sampling cone size(Ni):1.0 mm,skimming cone size(Ni):0.7 mm,resolution:05–0.1 aum,test mode:peak jumping,analysis mode:pulse counting,sampling depth:60 Step,scan times:50,dwell time:10 ms,sample uptake rate:1.0 m,and quality channel number:3(Shi et al.2009).

3 Results

3.1 Section comparison

Table 1 lists test results of electrogeochemical and soil samples taken from layers above the exploratory lines in Bangzhuoma mining area.Figure 4 shows anomalies of each element with corresponding sections.

1. Original data comparison shows order of magnitude difference between test data of the two kinds of samples.For instance,the electrogeochemical sample measurement yielded the maximum As value of 32.91μg/g,which was equal to the minimum value of As for soil sample measurement.The maximum As value for the soil sample was as high as 85.71μg/g.Analyses of curve shapes and angles indicate that electrogeochemical measurements produce more single-or multiple-peak anomalies with local trapezoid shapes and can be used to reveal the locations of shallow veins and their extension directions.Meanwhile the soil sample measurements yielded mostly single peaks,revealed only the locations of shallow veins(no response to deep veins).The method also appeared to be easily affected by secondary actions.

Fig.2 Simplified mapsof relevant geotectonic structures and thestudy area.a Approximate location of relevant geotectonic elements,b outline map of structure elements in the study area,c outline map of the mining area.1.Quaternary glacier,alleviation and lacustrine deposits,2.cretaceous marine clastic rocks,3.Jurrasic marine clastic rocks,4.second to fourth members of the Upper Triassic,5.binary and monzonitic granite,6.quartz diorite,7.sillite,8.diorite 9.extension-detachment fault,10.ductile shear zone,11.plate stylolite,12.fault line,13.deduced fault line,14.geological boundary,15.river,16.revealed veins and their serial numbers,17.exploratory trench,18.location of the study area

2. Analyses of correspondence between anomalies and locations of veins showed that samples from No.8 to No.14 prospecting lineshad their elements Au,Ag,As and Bi corresponding well to the locations of cropped veins,samples from No.3 to No.5 prospecting lines had their Au and Ag indicating the locations of veins buried in deeper layers,and samples from No.17 to No.19 prospecting lineshad their Biand Mo providing clues by forming three element combinations through giving signals of Au–Ag,Au–Ag–As–Bi,and Bi-Mo perpendicular to veins.Only soil samples from No.10 to No.14 prospecting lines were measured to have elements of Au,Ag,As,Bi and Mo,corresponding well to outcropped shallow veins,and the anomalies had smaller widths than those in the electrogeochemical samples.

Fig.3 Diagrams showing sampling methods and the dipole CHIM extraction device with independent power supply.1.Controllable power source,2.extraction electrodes,3.connecting wire,4.adsorptive foamed plastic material,5.fixed wire,6.filter paper,7.graphite carbon rod

Table 1 Test results of electrogeochemical and soil samples above the No.4 prospecting line in Bangzhuoma mining area

Fig.4 Comparison of sections derived from measurements of electrogeochemical and soil samples.1.Quaternary,2.second member of the Nieru Formation of the Upper Triassic,3.veins,4.exploratory trench and serial number,5.perforation and serial number,6.electrogeochemical anomalies,7.soil measurement anomalies,8.background value line

3.2 Plane comparison

Section comparisonscan be used to assessthe effectiveness of exploration and determine indicative element combinations through aspects of element content and curve shape.However,information such as element characteristics,the abnormal spatial distribution,and intensity and range of elements,is acquired through statistics of element content and abnormal element planes based on plane comparison.

3.2.1 Statistic comparison of electrogeochemical and soil measurements

Statistics of geochemical elements can be analyzed and used as indicators to ore deposits.Table 2 lists statistics of soil and electrogeochemical measurementsin theperiphery of Bangzhuoma mining area.It shows that:

1. Background values are important indicators of geochemical anomalies.However,element contentsof the samples are sometimesextremely high or low,causing the background values for both measurements to be lower than the average element content values.

2.The difference between the maximum and minimum values was more obvious in electrogeochemical samples than in soil measurements.For example,the content of Au in electrogeochemical samples ranged between 0.03 and 105.40μg/g,which was 3515 times apart.In contrast,the soil samples had an Au content rangeof 1.2 and 119μg/g,only 99 timesapart between the maximum and the minimum.The maximum and minimum content values of element Bi in electrogeochemical samples(56 times)were also many more times apart than that in soil samples(5.8 times),indicating a higher dispersion degree in the former samples than the latter,thus better representing the distribution of elements in the area.

Table 2 Statistics of element content in electrogeochemical and soil samples from the periphery of the Banzhuoma mining area

3. The average element content values of electrogeochemical sampleswere orders of magnitude lower than that of soil measurements,which was consistent with the results of section analyses of the two kinds of samples.To reduce the effect of measurement scale and dimension upon the differentiation of elements in the area,we calculated the differentiation coefficients(standard deviation/averagevalue)of each element and listed them in Table 2.From Table 2 and Fig.5,we have differentiation coefficients of elements for electrogeochemical and soil samples in an ascending orders of Ag(0.67)＜Mo(0.85)＜Bi(0.97)＜Au(1.51)＜As(2.35)and Bi(0.33)＜Mo(0.42)＜Ag(0.87)＜Au(1.03)＜As(1.87),respectively.The differentiation coefficients of Au and As for both kindsof sampleswerebigger than 1,which meant high differentiation and possible mineralization of gold in certain parts of the area.The differentiation coefficients of Ag,Mo and Biin electrogeochemical samples ranged between 0.5 and 1,meaning medium differentiation.Meanwhile those in soil samples were all less than 0.5,indicating low differentiation or no differentiation at all.This shows that electrogeochemical samples yield more differentiated element results.

Fig.5 Comparison of element differentiation coefficients of electrochemical and soil samples from the periphery of Bangzhouma mining area

3.2.2 Comparison of plane anomalies in electrogeochemical and soil measurements

To better compare the plane anomalies of electrogeochemical and soil measurements,we used ‘‘single-point anomaly contrast value’’of 1.2 and 2.5 as contour line to map the anomalies with the universal Kring method.

The single-point contrast value(K,dimensionless)refers to the ratio of the original value to the background value of certain elementsin each sampling point.With K larger than 1,we may presume relative enrichment of elements,and with K less than 1,we may infer relative depletion of elements(Zhang et al.2015;Li et al.2016).The value is therefore used to indicate the enrichment or depletion of certain elements.It may be represented by an equation of K=C/C0,in which K is the single-point contrast value,C is the content of certain element,and C0is the background value of certain element.

Anomaly contrast shows how clear an anomaly is.It can be used not only as an anomaly marker but also to eliminate interference caused by background value differences and to compare anomaly scale and enrichment degree of different elements in samples.Figures 6 and 7 are plane graphs of anomaly contrast of elements Au,Ag,As,Mo and Bi,derived from soil and electrogeochemical measurements.It can be concluded from the figures that:

1. It is topographically higher in the south than the north and the east than the west within the study area.A deep V-shaped gully with running water of near E–W direction winds in the north and two V-shaped gullies of near S–N direction run in the south.Soil measurement anomalies are mostly from the lower northwest part(probably affected by the topography).The anomalies of elements Au,Ag and As occupy larger areas and are higher in values.However,the correspondence between these anomalies and revealed veins are not as strong due to secondary actions and topography.Elements Mo and Bi generate small-area anomalies and low values and thus are worthless as indicators.

Fig.6 Plane maps of contrast anomalies of elements from soil measurements.1.Third to fourth members of the Upper Triassic Nieru Formation,2.diabase vein,3.exploratory trench,4.gold grade,5.geological boundary,6.river,7.vein,8.sampling point,9.1.5 isoline of contrast anomalies,10.2.5 isoline of contrast anomalies

Fig.7 Plane maps of electrogeochemical contrast anomalies of elements in the study area.1.Third to fourth members of the Upper Triassic Nieru Formation,2.diabasevein,3.exploratory trench,4.gold grade,5.geological boundary,6.river,7.vein,8.sampling point,9.1.5 isolineof contrast anomalies,10.2.5 isoline of contrast anomalies

2.Anomalies of elements derived from electrogeochemical sample measurements have several high-value areas,showing no clear sign of being affected by topography.Elements Au and Ag not only generate anomalies matching well with revealed veins but also show significant anomalies along lines 003 and 004 and in parts of lines 008 and 009.Element As yields large-area anomalies and higher anomaly values,with anomalies along line 004 corresponding well with revealed veinsand synchronized with(weaker)anomalies of elements Mo and Bi at the third and fourth members of the Nieru Formation at the south part of the line004.Elements Ag,Mo,and Biyield anomalies simultaneously at measure points 14–17 of lines 002,003 and 004;elements Au,As,and Biyield anomalies at measure points 10–15 of lines 008 and 009.

3.The Au grade changesobviously in measurepointsand in ongoing exploratory trenches of known gold mineralization and revealed veins(Fig.7)in the study area.The grade rises from 3.64 to 4.78 g/t from measure points D1 to D2 and is 0.29 and 0.23 g/t for two veins within TC070,respectively.Enrichment of Au is more obvious in TC027,where the Au grade is 0.58 g/t.

Fig.8 Ideal model of electrogeochemical anomaly formation in alpine meadow-covered areas

4 Discussions

Element anomalies derived from soil measurements are mostly single peaks and easily affected by secondary actions.They are only good at defining the locations of shallow veins,showsno obviousanomaliesto deeper ones.Au anomalies are the most common ones in the plane,corresponding well to revealed veins.Ag and Asanomalies are similar.Mo and Bi anomalies are rare and are only present in outcropped veins,indicating a limitation of applying soil measurements in the area.

In contrast,electrogeochemical measurements generate high-to low-temperature anomaly associations of Bi-Mo,Au–Ag–As–Bi,and Au–Ag with vertical veins with obvious zonation.Combining the facts that gold mineralization and arsenopyrite mineralization occur in the second member of the Nieru Formation in the Upper Triassic(T3n2),we suggest that the Formation contains a set of semi-pelagic slope flysch association of dark black–black laminated silt-slate with high content of Au associated with unevenly distributed high-grade Ag(4.25×10-6).We therefore decided to use Au and Ag as major electrogeochemical indicators and As,Bi,and Mo as minor electrogeochemical indicators for defining the locations of veins in the area.In electrogeochemical plane maps,contrast anomalies of elements Au,Ag,Mo,and Bi all show corresponding anomalies to known vein,indicating reliability of the method and better results than that of soil measurements.The Au contrast anomaly with high contrast value is found at the south of lines 002 and 003.Together with synchronized anomalies of elements As,Mo,and Bi in central parts of adjacent lines 004 and 005,they form element combination anomalies.Integrating actual mineralization at the measurement points and gold enrichment at TC026 and TC067,we suggest tendency of downward extension of the vein and the possibility of deeper buried ore bodies.While synchronized anomalies of elements Au,Ag,As,Mo,and Bi in central parts of lines 008 and 009 and Au grade rising from 0.23 g/t at Tc070 to 0.58 g/t at TC027 suggest mineralization in shallow layers in the study area.

The above discussion verifies the applicability of electrogeochemical method in the study area for its effective formation and extraction of anomalies.The electrogeochemical anomalies are generated mainly through dissolution of mineralization bodies, including (1)electrochemical dissolution caused by primary battery as a result of potential difference between different minerals,(2)re-dissolution caused by temperature build-up or surface area enlargement during magmatic evolution,and(3)tectonic pre-solution.Among them,electrochemical dissolution is the most common process,during which an ionic halo is produced around mineralization bodies and then transported upwardly and continuously through diffusion,permeation,evaporation,and soil capillarity,to near surface,where it there unloads the metal ions it has carried all the way up when encountered by some geochemical barriers(such as all kinds of secondary soluble salts,clay,oxides,organic matter,and colloidal substance).The electrogeochemcial method utilizes the external electric field to transport active metal ions to assigned extraction electrodes.By collecting and analyzing electrolytes on adsorbing materials of the electrodes one can discover ion anomaliesconnecting with information of oredeposits.The study area is a very suitable candidate for applying this method,for it has moderate rainfall,high altitude,low air pressure,and a well-developed vegetation root system,all of which are favorable for ion halo migration.The overlying lamina of carbonaceous sandy slate in the area also provides a closed environment for the formation of electrogeochemical anomalies.It is therefore suggested that the establishment of an ideal electrogeochemical anomaly mode for the study area is a necessary theoretical preparation for exploring buried gold deposit in similar areas covered by alpine meadow(Fig.8).

5 Conclusions

1.The electrogeochemical method works more effectively in ore deposit exploration than soil measurement in the study area.It generates data that are more representative,especially in places with complicated topography and no obvious tectonic regions.

2.By combining the fact that electrogeochemical anomalies yield low-to high temperature element combinations of Bi–Mo,Au–Ag–As–Bi,and Au–Ag along vertical veins with geological features of ore deposits in the study area,we use elements Au,Ag,and As as electrogeochemical indicators,and elements Bi and Mo as auxiliary indicators of ore deposits.Apart from revealed veins corresponding well to electrogeochemical anomalies,we also observed anomalies at the southern part of lines 004 and 005 and the central part of lines 008 and 009.Based on the observation of Au grade changing from lean to rich,we suggest the possibility of buried ore deposits in the study area.

3. A three-stage electrogeochemical ideal model was established to guide buried ore deposit exploration in regions similar to this alpine-meadow-covered area,which features moderate rainfall,high altitude,low air pressure,well-developed vegetation and roots as well as an Upper Triassic Nieru Formation carbonaceous sandy slateasoverburden.The three stagesrefer to ore body dissolution,mineralogenetic particle migration,and mineralogenetic particle unloading.

AcknowledgementsThis study was supported by the National Key Research and Development Program of China (Grant No.2016YFC0600603).

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