Abstract
The Tavreh mercury prospect is located in the NW of Iran. This prospect is a listwaenite alteration and mineralization system which is controlled by structural agents and is restricted to shear zones. In this study, the primary geochemical halo was investigated to estimate the level of erosion of mineralization based on the lithogeochemical data. These data were obtained by ICP-MS analysis following an aqua regia decomposition of 64 samples along 13 profiles and 30 samples collected from trenches. The prediction of the erosion level of a geochemical anomaly is usually accomplished by comparing the values of the vertical geochemical patterns with values of the same parameters that are known around similar mineralization. The (As.Sb.Ag/Cu.Co.Ni) (Vz1 index) and (Sb.As/Pb.Zn) (Vz2 index) zonation indices were used as geochemical indicators to detect the present level of geochemical anomalies in the Tavareh region. The average value of the Vz1 index was 1.98 × 10−4, which is higher than the middle part of the recognized listwaenite-type mercury deposits and is lower than the upper part of the supra-ore halo. The average value of the Vz2 index is 3.76, which also indicates a supra-ore geochemical halo. Values of Vz1 and Vz2 indices imply the presence of sub-cropping of the main part of the ore body in the Tavreh area. It is expected that values of Vz1 and Vz2 decrease with increasing depth.
The alteration of ultramafic rocks, formation of listwaenite and associated mineralizations are interesting metallogenic phenomena in the Khoy ophiolite. It seems that the hydrothermal circulations responsible for mercury mineralization were derived from geothermal systems caused by intermediate to acidic magmatism in Neogene–Pleistocene time (Imamalipour 2001; Imamalipour et al. 2013).
Listwaenites are a distinctive alteration of ophiolitic mélange rocks, especially serpentinized peridotite, and are commonly found in shear zones. Reactivation of ophiolitic rocks with hydrothermal fluids led to the hydrothermal alteration and formation of silica-carbonate assemblage (termed listwaenite by Lobochnikov 1936; Bok 1956; Ploshko 1963; Halls & Zhao 1995). Listwanite is a term long used by Soviet geologists working in the goldfields of Russia (Goncharenko 1970; Kuleshevich 1984).
The importance of this type of alteration is due to the associated gold-mercury mineralization. Henderson (1969) reviewed the ‘serpentinite type’ mercury deposits and the associated alterations. In addition to mercury-antimony, arsenic and gold may also accumulate in virtually the same mélange setting (Laznicka 2013). Silica-carbonate alteration of serpentinite is ubiquitous in the coast ranges of northern California, occurring in barren alteration zones and also around mercury-rich, and gold-mercury systems (Sherkock & Logan 1995). Recently, listwaenite drew the attention of geologists because of their worldwide association with gold mineralization (e.g. Barnes et al. 1973; Buisson & Leblanc 1985; Aydal 1990; Uçurum 2000; Zoheir 2011; Azer 2013; Qiu & Zhu 2014). Cox & Singer (1986), in their descriptive model, have described these deposits as New Almaden type, which is formed in contact of serpentinite and siltstone-graywacke above subduction-related thrusts. Cinnabar (HgS) and native Hg are the main minerals of mercury. The geochemical characteristics of these deposits are less known, but anomalies of Hg, Sb, Cu, and Zn are observed around them. Most of the ore bodies are irregularly dispersed and relatively small, between 0.0013 and 0.6 Mt, averaging 0.025 Mt, and reserves greater than 0.6 Mt are most uncommon (less than 10% of these deposits) (Cox & Singer 1986).
Exploration lithogeochemistry is concerned with the detection of primary dispersion patterns around or associated with, mineral deposits (Govett 1997). Each mineral deposit with an economic metal content is enclosed in space wall rocks with decreasing contents that finally approach the local background. This concept of the primary geochemical halo from ore deposits is valuable in prospecting for ores (Solovov 1987). The primary geochemical haloes of mineral deposits are much greater in area and volume compared with an economic ore body, so it will be much easier to locate new deposits with directing geochemical exploratory effort toward a discovery of such haloes (Solovov 1987). It should be noted that non-economic mineralization zones also have primary geochemical haloes but differ in geochemical zonation with the haloes of ore deposits. Economic mineral deposits have primary haloes with a specific vertical geochemical zonality. The values of zonation indices in dispersed non-economic mineralization do not show significant vertical variation and are characterized by constant and substantially low ratios of the vertical zonation indices, while the values of these indices have certain vertical variation in economic ore deposits, they gradually decrease with increasing depth. This important geochemical feature can be used to distinguish dispersed non-economic mineralization from economic mineral deposits.
In exploration geochemistry, geochemical zonation is one of the most important concepts in the investigation of the primary haloes of mineral deposits. Most mineral systems are zoned mineralogically and geochemically (Jackson 2010). Two types of geochemical zoning have been recognized in hydrothermal aureoles (Beus & Grigorian 1977), including axial (vertical) and lateral zonings. Zoning along the direction of flow of the ore-forming fluids is termed axial or vertical zonation, and zoning outward from ore into wall rock, in a direction normal to the hydrothermal flow direction, is termed transverse or lateral zonation (Rose et al. 1979). Lateral zonation halos may exist on a local or regional scale reflecting some primary control on element deposition such as temperature/pressure or a fluid mixing boundary (Jackson 2010).
Vertical zonation is an important vector to ore. The method of geochemical vertical zonation is an effective method for the prediction of the erosion level of mineralization, the distinction between sub-ore and supra-ore primary haloes, and exploration of blind mineral deposits (Beus & Grigorian 1977; Solovov 1987, 1990; Grigorian 1992; Wang et al. 2013). A gradient in vertical geochemical zonation indices (Vz) of multi-elements around mineral deposits allows distinction of levels of mineralization and their primary (supra-ore, upper-ore, ore, lower –ore, and under-ore) haloes (Solovov 1987, 1990; Grigorian 1992, in Ziaii et al. 2011). Although the geochemical zonation method was developed for the prospecting of porphyry-Cu deposits (Grigorian 1992; Dilles 2010; Ziaii et al. 2011; Imamalipour & Mousavi 2017), but it also can be used to explore other deposits having primary haloes. In particular, the geochemical zonality method has been used in the exploration of listwaenitic-type mercury deposits in the former Soviet Union (FSU); for example, Agyatag (Azerbaijan), Ologaoc (central Assia) and Zod (Armenia) deposits (IMGRE 1979; Grigorian 1992; Abedi 1997). The concentration of Au in some of these listwaenites is more important than Hg concentration, such as those in the south of the Urals (Russia). As Belogub et al. (2017) reviewed, the listwaenite-related gold deposits of the South Urals comprise the ore clusters along with other numerous small gold deposits, and constituted the sources for the gold placers exploited in historical time. So, the application of the geochemical zonation method is also important in the exploration of this type of gold deposit. Grigorian (1992) showed that the vertical zonation (Vz) patterns around exposed ore deposits are distinguishable from the Vz patterns around blind deposits.
In the Khoy ophiolite, three types of listwaenites including silica, silica-carbonate, and carbonate have been recognized, occurring in barren alteration zones and also around mercury mineralization systems. Mercury mineralization has been observed just in the silica-type from the mentioned alterations. Alteration zones are controlled by structural agents and mercury mineralization is restricted to shale/serpentinite and conglomerate/serpentinite fault type contacts. Tavreh is located in the province of West Azerbaijan, c. 90 km west of the city of Khoy, near the Iran–Turkey border (Fig. 1). The area of the Hg-bearing alteration zone is c. 0.5 km2.
(a) Distribution of major Iranian ophiolites. BF, Baft; ES, Esfandagheh; KR, Kermanshah; NA, Naien; NY, Neyriz; SB, Sabzevar; TK-IR, East of Iran; KH, Khoy; RS, Rasht; MS, Mashhad; SHB, Shahr Babak; BZ, Band e Ziyarat (Ghorbani 2013). (b) Location of the Tavreh mercury prospect in the Khoy ophiolite.
In this research, we have used geochemical data obtained from whole rock sampling taken along geochemical profiles and also samples collected from pits and trenches. Primary dispersion patterns associated with the Tavreh mercury prospect and their geochemical zonation were investigated. Mineralization in the Tavreh region is listwaenite-type which is associated with extensive alteration and a primary geochemical halo. The purpose of this study is to calculate the geochemical zonation values (Vz) values, identification of their patterns and compare them with prominent listwaenite-type ore deposits. In this way, the erosional surface of mineralization can be largely predicted, and the Vz values of the primary geochemical halo of the ore deposit can be estimated. Such work can greatly contribute to the geochemical evaluation of the exposed mineralization, the extent of its depth extension and the decision to continue exploratory operations, in order to determine the location of drilling targets and the depth of exploration.
Geology of the Tavreh mercury prospect
The Tavreh area is located in the NW of Iran near the Iran–Turkey border. Geologically, this area is situated in the Khoy ophiolite zone. The Khoy ophiolite complex is part of the Tethyan ophiolite belt, and it is one of the largest Iranian ophiolite complexes which covered a widespread area in NW Iran along the Iran–Turkey border. Khalatbari-Jafari et al. (2003, 2004) inferred at least two ophiolites in the Khoy area: (1) the early Jurassic–early Cretaceous Eastern Khoy ophiolite; and (2) the late Cretaceous Western Khoy ophiolite. The second one is a remanent of the Neotethyan oceanic crust. The Tavreh area is situated in the western Khoy ophiolite domain. The Khoy ophiolite has all parts of an ophiolite sequence. This ophiolite is composed of serpentinized peridotite, layered and isotropic gabbro, isolated diabasic dike, pillow basalt, massive sheet flow, and interbedded hyaloclastic breccia and tuffs. Basaltic andesite pillow lavas of the Western Khoy ophiolite accompany inter-pillow siliceous mudstone containing Late Cretaceous radiolaria (Pessagno et al. 2005). Also, the pelagic sedimentary rocks contain Upper Cretaceous (Campanian–Maastrichtian) fossils (Ghazi et al. 2003).
The serpentinized harzburgites and related rocks in the western Khoy ophiolite are intruded by gabbro–diorite intrusions, which appear as a spot inside and/or around serpentinized harzburgites and cannot be a member of the ophiolite sequence (Zaeimnia et al. 2017). Ultramafic rocks of the Western Khoy ophiolite host several podiform chromitite bodies. The chromite deposits have lenticular, tabular and irregular vein shapes and are emplaced in depleted mantle harzburgite (Imamalipour 2011).
Based on the mineral chemistry of chromian spinels, pyroxenes, and mineral inclusions, the chromitites and the host peridotites from the Western Khoy ophiolite were formed in an arc-related setting at two stages during arc growth and are divided into moderately high-Cr chromitites and high-Cr chromitites. The former crystallized from island-arc-tholeiite (IAT) melts during reaction with the host depleted harzburgites, whereas the latter crystallized from boninitic melts (second stage melt) during reaction with highly depleted harzburgite in a supra-subduction-zone environment (Zaeimnia et al. 2017).
Associated with ophiolitic rocks are found flysch-type sediments with Paleocene–lower Eocene age that have syn-orogenic characteristics. After emplacement of the ophiolitic complex at the end of post-lower Eocene time, acidic to intermediate magmatic activity as small granitoid intrusives and andesitic–dacitic volcanic and their sub-volcanic equivalents has occurred. The mafic–ultramafic rocks, especially serpentinite, show extreme alteration along shear zones with the development of listwaenitic rocks. Reactivation of serpentinite with hydrothermal fluids led to the hydrothermal alteration and formation of listwaenites in this tectonic zone. Serpentinite host rock is intensively brecciated. Alteration zones have been controlled by structural agents and are restricted to shale/serpentinite fault-type contacts and developed from these contacts toward brecciated serpentinite (Fig. 2).
Geological map of the Tavreh area (Imamalipour et al. 2013).
Listwaenites in the Western Khoy ophiolite are easily distinguished from other rocks due to their yellow-brown and light colors (Figs 3a and b). Most of the listwaenites in the Khoy ophiolite occur as ridges as well as in the form of irregular lenses along fault zones inside the ultramafic rocks and also between ultramafic rocks and Paleocene shale and conglomerate boundaries. Most of these lenses are thin and reach a few meters in length parallel to the enclosing shear zones, but the Tavreh listwaenite was noted in a 0.5-km2 area. Contacts of the listwaenite alteration zones with country rocks are sharp and regular (Fig. 3a).
(a) An outcrop of listwaenite (List.) adjacent the serpentinized ultramafic rocks (Ubsr) in the Tavreh area. (b) Silica listwaenite (List.). (c) Brecciated serpentinite which altered to listwaenite. (d) Cinnabar (Cin.) veinlet within the listwaenite. (e) and (f) Photomicrographs of cinnabar mineralization in listwaenite, as open space filling texture, containing opaline silica (opa), microcrystalline quartz (mqz), opaque mineral (op) and cinnabar (Cin.).
Alteration and mineralization
The serpentinite of the Khoy ophiolite is anomalous in Hg. Geochemical distribution of Hg in different rock types and its increasing trends from unaltered fresh serpentinite to the altered zone (listwaenite) and finally mineralized zone suggest that the probable origin of Hg is ultramafic rocks (Imamalipour 2001). It seems that the hydrothermal circulations responsible for mercury mineralization were derived from geothermal systems caused by intermediate to acidic magmatism in post-Neogene–Pleistocene time. These solutions circulated along fault zones among the tectonized serpentinite, caused the leaching and mobilizing of Hg. The fertile hydrothermal ascending solutions carried Hg and other elements from depth toward the surface and finally to the surface as hot springs whereupon through reaction with host rocks and the changing physicochemical character, cinnabar and other minerals were deposited. This event probably occurred during the Pleistocene time (Imamalipour 2001).
Three types of listwaenites including silica, silica-carbonate, and carbonate have been recognized in the Khoy ophiolite. The Tavreh mercury prospect has formed in relation to silica-type listwaenite. Mercury mineralization occurred along steeply structures (Fig. 3b). Serpentinite host rock has extensively brecciated along the fault zone (Fig. 3c).
Mineralogically, silica-type listwaenite mainly consists of opal, quartz, cristobalite, chalcedony, and relicts of serpentine, Cr-spinel and secondary iron hydroxides. Magnesite, dolomite, calcite and clay minerals are minor phases. Toward silica-carbonate and carbonate listwaenites, silica minerals decrease and carbonate minerals increase. Sulfide minerals include pyrite, marcasite, cinnabar, and stibnite in this listwaenite. Pyrite and marcasite are abundant but stibnite is found as a rare mineral. In addition, chromite fragments and iron oxides are found in ore. Chromite is one of the primary components of host serpentinite and has no relation to hydrothermal alteration and Hg mineralization. Ore textures are characterized by filling of open space in the existing opaline silica, fissure veins within subsidiary dilatant structures, or matrix to breccias (Figs 3d–f).
By moving away from the fault zones that control the alteration and mineralization, the intensity of alteration decreases. The alteration zone has a southeastern direction. Cinnabar veins are found in the minor faults and have often a northwestern direction. Breccia structure and texture are common features of the rocks in the alteration zone and the mineralization host rock is intensively brecciated. In the alteration zones, two brecciation phases are recognizable: the first phase is related to the tectonic event before the mineralization and to the structural development of the region, and the second phase coincides with the alteration as a result of hydrothermal activity. The eruption in the hydrothermal system has caused brecciation of rocks. Eruption breccias, which form by the rapid expansion of depressurized hydrothermal fluids, are characterized by the intensely silicified matrix. The abundant thin silica veins have cut off the initial alteration products at the last stage of hydrothermal activity. Figure 4 illustrates the schematic cross-section showing the relationship between the listwaenites and their serpentinite host rock in the study area.
Schematic cross-section showing the relationship between the listwaenites and their host rocks in the Tareh area: (1) Neogene volcanic rocks (andesite-trachyandesite); (2) Paleocene shale and siltstone; (3) Silica- listwaenite; (4) altered shale and siltstone; and (5) brecciated serpentinite.
Methods of investigation
Based on the 1:20 000 scale geological map of the Tavreh area, the direction of the listwaenitic alteration zone is NE–SW. Sixty-four lithogeochemical samples along 13 geochemical profiles were collected from the altered and mineralized zone. Geochemical sampling of bedrock along profiles was oriented across the strike of assumed altered zone. The spacing between profiles in this ground survey was 30–50 m based on decreasing alteration intensity from south to north (Fig. 5). About 10 chip samples of rocks with a size of 3–4 cm2 were combined into one sample from an interval of sampling. Furthermore, 30 lithogeochemical samples in mineralized zones with thick cover (residual soil) were obtained by trenching. The samples were crushed, reduced in volume, and pulverized to <200-mesh size. Samples were digested in HNO3/HCl (aqua regia) and then analysed by inductively coupled plasma mass spectrometry (ICP-MS) at the Amdel laboratory in Australia. This digestion uses a mixture of hydrochloric and nitric acids to dissolve sulphides, some oxides and some altered silicates. Often base metals will be totally dissolved but this depends on their mineralogy (e.g. not silicate Ni or Zn in gahnite). Detection limits for the elements Hg, As, Sb, Ag, Cu, Co, Ni, Pb, Zn, Ba, Mo, Bi and Au were 0.01, 0.5, 0.1, 0.01, 0.2, 0.2, 1, 0.2, 0.2, 0.2, 0.1, 0.1 and 0.001 µg/g, respectively. Chemical analyses of listwaenites from the Tavreh area are given in Table 1. The concentrations of elements below detection limit were replaced by ¾ times the detection limit of the instrument according to Sanford et al. (1993). Nineteen duplicate samples were analysed and the precision was then calculated. Arsenic, Ba, Be, Bi, Cu, Co, Hg, Ni, and Zn show precision (RSD) of better than 10%. Precision was 16% for Pb, Sb, and Au, and 13% for Mo.
Location map of lithogeochemical samples in the Tavreh area.
Representative chemical analysis data of silica-type listwaenite from the Tavreh area (Au in ppb, other elements in ppm)
The values of vertical geochemical zonality indices (Vz) were calculated on the surficial topographic level. The elements As, Sb and Ag were considered as supra-ore elements whereas Co, Cu, Ni, Pb, and Zn were considered as under-ore elements. Two vertical geochemical zonality indices (Vz1 and Vz2) were calculated to estimate the level of erosion of the listwaenitic-type mercury prospect using the following indices:
(1)
(2)Where As.Sb.Ag and Sb. As denote multiplicative geochemical anomalies of As, Sb, and Ag, and Sb and As, respectively (supra-ore) and Cu.Co.Ni and Pb.Zn refer to multiplicative geochemical anomalies of Cu, Co, Ni and Pb and Zn, respectively (under-ore).
Furthermore, an additive ratio of the supra-ore elements to the sub-ore elements was also used as below:
(3)
Discussion
Lithogeochemistry
Statistical results show that Hg, As, Sb, Ag, Cu, Pb, Zn, Co, Ni, and Ba mean values are 177, 254, 9.6, 0.04, 24.2, 13, 61.5, 42.8, 792, and 42.6 ppm, respectively. The mean content of Au is 28.4 ppb. Variation between maximum and minimum for most of these element concentrations shows a wide range; for example, minimum and maximum Hg concentrations are 0.36 and 10 500 ppm, indicating a high standard deviation (464 ppm). Based on the lithogeochemical data published on the distribution of Hg in Hg deposits of Siberia, its geochemical background in lithogeochemical haloes varies from 0.06 to 0.13 ppm, and geochemical anomalies vary in the range from 200 to 20 000 ppm (Kovalevskii 1986). Therefore, the average concentration of Hg in the study area is close to its anomalies in Hg deposits.
The summary statistics of 94 rock samples for Hg and 15 other elements are listed in Table 2. The frequency distribution of Hg in logarithmic and normal values is shown in Figures 6a and b, respectively. These figures show that the Hg distribution is not normal and appears to more closely approximate a logarithmic pattern. The Hg and Au distribution maps around the Tavreh listwaenite are illustrated in Figures 7 and 8, respectively. These maps show that the maximum amounts of Hg and Au are consistent in this region. The mean contents of Hg, Au, As, Sb, Zn, Pb, Co, Cu and Ba in the silica–listwaenite of the Tavreh area is compared with their crustal mean (Clarke values) and also the ultramafic rocks in Figure 9; however, it is important to note that the aqua regia decomposition can be partial for some elements so this comparison is not entirely justified.
Histogram depicting the distribution of (a) normal and (b) log-transformed rock sample Hg (ppm) values, respectively.
Lithogeochemical distribution map of Hg around the Tavreh listwaenite (concentrations in ppm).
Lithogeochemical distribution map of Au around the Tavreh listwaenite (concentrations in ppb).
Diagram showing the mean contents of Hg, Au, As, Sb, Zn, Pb, Co, Cu and Ba in the silica–listwaenite of the Tavreh area, crustal mean and ultramafic rocks (Au concentrations in ppb, other elements in ppm).
The summary statistics of 94 rock samples for Hg and 15 other elements, based on raw data
Geochemical zonation and recognition of erosional surface
Interpreting the results of geochemical sampling of ore bodies and their primary geochemical haloes in Hg deposits is very difficult. This is due to the complex geological structure of such rocks and, above all, because of the high spatial variability shown by mineralization. It is also important to note that elements associated with Hg produce weak primary geochemical haloes, due to their low contents in deposits. Therefore, the recognition and determination of the primary geochemical haloes formed by Hg-associated elements is a very complicated problem.
Determining the position of the erosion level of a geochemical anomaly (or an ore zone) relative to the possible mineralization level is of particular importance. This allows geochemical evaluation of a mineralized outcrop and exploration of blind mineral deposits. The horizons of the erosional surface are defined by quantitative identification and evaluation of geochemical anomalies reflecting the presence of hidden ore bodies (Ziaii et al. 2011, 2012).
Vertical geochemical zonality provides the possibility of determining the erosion level of geochemical anomalies and this is one of the most important practical exploration advantages of geochemical zonality. Similar values of a Vz index imply similar depths of mineralization and primary haloes within an ore field. Thus, primary haloes of mineral deposits at different depths are characterized by specific values of a Vz index (Ziaii et al. 2011, 2012). The assessment of the relative position of the erosion level of a geochemical anomaly is usually accomplished by comparing the value of the parameters mentioned above, with values of the same parameters that are known around the orebody and of the same mineralization type. Since the use of single-element geochemical haloes in the exploration for Hg deposits is difficult due to the low intensity of the haloes, the use of multiplicative geochemical haloes can be useful (Beus & Grigorian 1977).
Grigorian (1974) presented the geochemical zonality series of the principal indicator elements for hydrothermal ore deposits. He stated that there is a similarity between the zonality series of the haloes, not only for those of the same type but also for those with different compositions of formation of the deposits.
The vertical geochemical sequence of listwaenite–Hg deposits has been established from the top (supra-ore elements) to bottom (sub-ore elements) as follows (IMGRE 1979; Abedi 1997) (this sequence is also similar to the generalized zonality series provided by Grigurian (1979) for hydrothermal ore deposits):
We used Vz1 and Vz2 indices (equations 1 and 2) for recognition of erosional surface representing vertical levels of geochemical anomalies in the Tavreh mercury prospect. To determine the level of erosion, data from the Ologaoc mercury deposit in Russia (Grigorian 1992; Abedi 1997, 1999) were used as a standard deposit because of similarity in mineralization in two regions.
Values of the Vz1 index (equation 1) decrease with increasing depth in listwaenite–Hg deposits (Grigorian 1992; Abedi 1997), and there is a significant difference between the gradient of the sub-ore and supra-ore primary haloes so its values in the supra-ore halo are c. 10 000 times those of the sub-ore halo (Fig. 10). This large difference in the zonality index can be used for geochemical evaluation of the present erosional surface of the Tavreh mercury prospect. For this purpose, mean values of the sub-ore and supra-ore elements in the lithogeochemical haloes of the mineralized zone were used. Considering the sufficient number of samples taken from the study area, it seems that the obtained elemental index coefficients are sufficiently accurate to determine the current position of the erosion level of mineralization. The average value of the Vz1 index is 1.98 × 10−4. Compared with the values of the Vz1 index in the primary geochemical haloes of the Ologaoc deposit, this value is higher than the middle part of the deposit and is lower than the upper part of the supra-ore halo.
Variation with the depth of the multiplicative vertical zonality values (As.Sb.Ag/Cu.Co.Ni (and (Sb.As/Pb.Zn) for listwaenite–Hg deposits, data from the Ologaoc mercury deposit (middle Asia) (Grigorian 1992; Abedi 1997).
Values of the Vz2 index (equation 2) also decrease with increasing depth in the Ologaoc deposit; its values in the supra-ore halo is c. 100 times those of the sub-ore halo (Fig. 10). The average value of the Vz2 index is 3.76. Compared with the values of the Vz2 index in the primary geochemical haloes of the Ologaoc deposit, this value indicates the supra-ore geochemical halo.
In this way, the geochemical evaluation of Hg mineralization outcrop in this area was carried out based on the calculation of the geochemical zonality indices, and comparison with the standard model provided for the Ologaoc mercury deposit (Fig. 10).
The level of erosion of mineralization in the Tavreh prospect was compared with the standard Ologaoc deposit based on the Vz1 and Vz2 indices in Figure 11. This figure shows that the level of erosion of the Tavreh prospect is much higher than the middle part of the primary geochemical halo of the Ologaoc deposit, and it implies that the main part of the ore body is not eroded and remains at depth. In this study, the additive ratio of the supra-ore elements to the sub-ore elements (equation 3) was also used. The Vz3 index map is illustrated in Figure 12. The values of this index vary from 67 to 398, which is consistent with the Vz1 and Vz2 variables. According to the map, it is clear that the distribution of this ratio is entirely consistent with the distribution of Au and Hg. Therefore, this index can be used to assess the geochemical erosion level of the geochemical anomalies of the listwaenite- type Hg deposits.
Diagram showing the variation of values Vz1 and Vz2 indices in the Tavreh prospect in comparison with the standard Ologaoc deposit. The erosion level of the Tavreh prospect is higher than the middle part of the primary geochemical halo of Ologaoc deposit (from middle Asia).
Distribution map of the additive ratio of the supra-ore elements to the sub-ore elements (Ba + Hg + Sb + Ag)/(Bi + Mo + Co + Be) around the Tavreh listwaenite.
Plotting the values of Vz1 obtained from lithogeochemical samples collected from the Tavreh mercury outcrop on the model of vertical geochemical zonation (Vz1) for typical listwaenite Hg deposits indicates that the erosional level of Tavreh prospect is situated at a level higher than the middle part of the primary geochemical halo of the Ologaoc deposit (Fig. 13), which is consistent with the results of Figures 10 and 11. So, it seems that a small part of the Tavreh prospect has been eroded and the main ore body remains below the surface of the earth.
The erosion level of geochemical halo in the Tavreh mineralization outcrops based on the Vz1 values on the model of the Ologaoc mercury deposit (Grigorian 1992; Abedi 1997), illustrated in the text.
Conclusion
The Tavreh mercury prospect is a listwaenite-type mineralization resulting from the reactivation of serpentinite host rock by low-temperature hydrothermal fluids in shear zones. The alteration zone is restricted to the shale/serpentinite fault-type contact and developed from this contact toward brecciated serpentinite. In this research, we have tried to determine the erosion level of mineralization in the prospect area using a geochemical zonation method. In this way, it is possible to predict blind Hg deposits by the distinction between sub-ore and supra-ore primary haloes. In fact, the vertical zonation method has been developed for the exploration of porphyry-Cu deposits more than for all other types of ore deposits. Therefore, this research can help to develop the application of the geochemical zonation method in the exploration for hydrothermal deposits.
Based on the vertical geochemical zonation (Vz1 and Vz2) model for listwaenite–Hg deposits obtained from the Ologaoc deposit, values of Vz1 and Vz2 of more than 0.001 and 1, respectively, indicate a supra-ore halo. The average value of the Vz1 index was 1.98 × 10−4. This amount indicates a higher level than the middle part of the Ologaoc model deposit, and lower than the upper part of the supra-ore halo. The average value of the Vz2 index is also indicates the supra-ore geochemical halo compared with the values of the Vz2 index in the primary geochemical haloes of the Ologaoc model deposit. Considering the present level of erosion, high values of Vz1 and Vz2 indices imply the presence of sub-cropping of the main part of the ore body in the Tavreh mercury prospect. It is expected that the values of sub-ore elements will gradually increase, with increasing depth, and therefore the values of Vz1 and Vz2 will gradually decrease with increasing depth. Coring drilling is recommended in this area.
Acknowledgments
This research was made possible with the help of the office of vice-chancellor for Research and Technology, Urmia University. We acknowledge their support.
Scientific editing by Gwendy Hall
- Received June 6, 2018.
- Revision received October 16, 2018.
- Accepted October 22, 2018.
- © 2018 The Author(s). Published by The Geological Society of London for GSL and AAG. All rights reserved