Journal of Geochemical Exploration 164 (2016) 122–135
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Journal of Geochemical Exploration 164 (2016) 122–135 Contents lists available at ScienceDirect Journal of Geochemical Exploration journal homepage: www.elsevier.com/locate/jgeoexp Three-dimensional geochemical patterns of regolith over a concealed gold deposit revealed by overburden drilling in desert terrains of northwestern China Bimin Zhang a,b,c,⁎, Xueqiu Wang b,c, Qinghua Chi b,c, Wensheng Yao b,c,HanliangLiub,c, Xin Lin b,c a School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China b Institute of Geophysical and Geochemical Exploration, CAGS, Langfang 065000, China c Key Laboratory for Geochemical Exploration Technology, MLR, Langfang 065000, China article info abstract Article history: Desert terrains are widespread in northwestern and northern China, and these areas present particular Received 22 December 2014 challenges for exploration. In recent years, partial extraction techniques have been proven to be effective in Revised 9 June 2015 the search for concealed deposits in arid desert terrains in some cases. However, we still lack an understanding Accepted 13 June 2015 of the dispersion patterns of ore-forming elements in regolith. In this study, air reverse circulation drillings Available online 19 June 2015 were used to create three-dimensional (3D) distribution patterns of elements in regolith over the Jinwozi gold deposit in China, which is covered by tens of metres of regolith, in order to trace the migration of elements Keywords: Geochemical patterns and to understand the dispersion mechanisms. The 3D distribution maps of elements show that (1) coherent Concealed deposit anomalies occur at different depths of transported cover over the ore body, (2) Au tends to be enriched in the Overburden drilling top and bottom horizons and depleted in the middle horizon in the vertical direction, (3) the anomalous Desert terrains distribution of Au at the bottom is restricted to places at the interface of sediments and bedrock, and (4) the anomaly in the bottom sediments is confined to a width of tens of metres, whereas that in top soils is much wider and can extend up to several kilometres. In addition, close positive correlations were found between the As, Hg, and Au distributions. © 2015 Elsevier B.V. All rights reserved. 1. Introduction 2007). However, there is still a critical need to study the three- dimensional (3D) distribution of elements in regolith. Such information Desert terrains are widely distributed in northwestern and northern is important for further elucidating the potential mechanisms for the China, and these areas are covered by widespread transported materials transfer of elements from the ore body upwards through the regolith that can mask geochemical signals from ore bodies; such geomorpho- cover to the surface and for understanding how to conduct successful logical structures present major obstacles to mineral exploration explorations in regolith-dominated terrains, whether for deposits (Wang et al., 2007). Over the past 20 years, partial extraction techniques concealed by the regolith or for those hosted within it. have been developed and proven effective in the search for concealed In this study, we used air reverse circulation (ARC) drilling technol- deposits in certain terrains (Antropova et al., 1992; Bajc, 1998; ogy over the Jinwozi gold deposit in China, which is covered by several Cameron et al., 2004; Clark et al., 1997; Cohen et al., 1998; El-Makky to tens of metres of transported materials, to determine the 3D distribu- and Sediek, 2012; Hamilton et al., 2004a,b; Kelley et al., 2003; Mann tion patterns of ore elements in regolith and to investigate the migration et al., 1995, 1998; Noble and Stanley, 2009; Wang, 1998; Wang et al., mechanisms. 2007; Williams and Gunn, 2002; Xie and Wang, 2003; Xie et al., 2011; Yeager et al., 1998), while at the same time, some migration models 2. Study area have been constructed and employed to explain the formation mecha- nisms of geochemical anomalies (Anand and Robertson, 2012; The Jinwozi gold field is located 200 km southeast of Hami city at the Aspandiar et al., 2008; Cameron et al., 2004; Garnett, 2005; Hamilton, boundary of the Xinjiang and Gansu provinces in northwestern China 1998; Hamilton et al., 2004a,b; Kelley et al., 2003; Lintern, 2007; Luz (Fig. 1). There are two NE-trending mineralized zones in the Jinwozi et al., 2014; Mann et al., 2005; Smee, 1998; Wang, 2005; Wang et al., gold field (Fig. 1). In the northern zone, the mineralization is character- ized by an epithermal quartz-vein type. The auriferous quartz veins occur at the contact between porphyry and Devonian sequences. In ⁎ Corresponding author at: School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China. the southern zone, the mineralization is characterized by tectonic E-mail address: [email protected] (B. Zhang). alterations. The ore bodies occur in a structural shear zone, which is http://dx.doi.org/10.1016/j.gexplo.2015.06.007 0375-6742/© 2015 Elsevier B.V. All rights reserved. B. Zhang et al. / Journal of Geochemical Exploration 164 (2016) 122–135 123 Fig. 1. Location and geology of the study area with drill hole sites. Coordinates are UTM Zone 46. mainly controlled by a NE trending fault that lies within the Devonian interbedded gravels, brown yellow sands, purple red sands, eluvium, sequence. The ores are mainly composed of pyrite, galena, and chalco- and bedrock. The lags are always covered by a dark and shiny substance pyrite. The pyrite is the primary Au-bearing mineral. The average Au called desert varnish. Such coatings represent a fine mixture of clay grades of the two mineralized zones are c. 7 g/t and 4 g/t (Wang et al., minerals and Fe–Mn oxyhydroxides, which form micrometre-thick 2007). The proven total Au reserve of the gold field is c.10t. The northern mineralized zone is located in an outcropping area and has a relatively high relief. The southern mineralized zone is situated in an area that is covered by the Gobi Desert with depths of a few metres to tens of metres. The regolith is composed of windblown sand, alluvium, colluvium, and residuum. The typical zonal structure of the regolith cover is illustrated in Fig. 2. The sequence of regolith materials from top to bottom: black gravels (lag), desert crusts, brown sands with Fig. 3. Gold distribution in different fractions of soils along the traverse line that crosses the Fig. 2. Sketch illustrating the vertical regolith profiles. two mineralized zones (Wang et al., 2007). 124 B. Zhang et al. / Journal of Geochemical Exploration 164 (2016) 122–135 of 50–100 m. At the bore holes, samples were collected continuously every metre from the ground surface to the bedrock. Every sample was well mixed and sieved in the field through a b100 mesh, and the finer fraction (150 μm) passing through the mesh was retained for fur- ther analyses. The choice of fraction size was based on the experimental findings and previous geochemical work in the region. In total, 1046 soil samples were collected from 63 bore holes. It is undeniable that mining contamination occurs in this study area. Hence, drilling sites were selected very carefully to avoid drilling dust and mining contamination. In consideration of the possibility of cross-hole contamination from the drilling activities, drilling equipment and the sampling system were cleaned after use at every hole. The cleaning procedure involved the use of high-pressure air generated by an air pump, which was blown onto the drilling equipment and sampling tools to dislodge any residual debris. After that, we used a clean cloth to wipe off all of the equipment. In the southern mineralized zone, four soil samples were collected and combined to form a composite sample at a depth of 15–30 cm above the buried deposit to confirm the use of the appropriate sampling fraction in the drilling work. The choice of soil sample depth was based Fig. 4. Drill hole sites of the ARC drilling located in the southern mineralized zone. on considerations of the sandy clay enrichment in the study area and previous sampling depths used in this region (Wang et al., 2007). The composite sample was sieved into seven fractions in the field with laminations that parallel the topography of the rock substrate (Potter the following grain size fractions: 830–1700 μm, 380–830 μm, and Rossman, 1977). It is still debated whether such varnish is formed 250–380 μm, 180–250 μm, 150–180 μm, 120–150 μm, and b120 μm. by (Broecker and Liu, 2001; Krinsley, 1998): (a) slow diagenesis of During the process of drilling, the different regolith types (Quaternary dust particles deposited on rock surfaces, (b) leaching from the underly- alluvium, Tertiary red strata, eluvium, and bedrock) could be recognized ing rock substrate, or (c) direct deposition of dissolved constituents in on the basis of the bore hole cutting colour, granularity, and mineral the atmosphere. The desert crust is created by the breakdown of soil composition. Regolith profileswereconstructedforeachboreholethat structural units by flowing water, raindrops, and subsequent evapora- was sampled, and the results are shown in Fig. 5. tion. Influenced by the undulating relief of the palaeotopography, Tertiary purple red sand layers are absent in some places. 4.2. Sample preparation and analyses The area is characterized by an arid climate, low rainfall, high evaporation, vast temperature differences between day and night (the All the samples were ground to less than 200 mesh (75 μm) for temperature difference can reach 40 °C), and long hours of sunshine analyses. A 0.25-g sample was digested in a hot mixture of acids (HCl, (16–17 h in summer). The annual rainfall is less than 250 mm, and HF, HNO3, and HClO4). Inductively coupled plasma-mass spectrometry the potential annual evaporation is c.