Mercury has been recognized to be an indicator element for both noble and base metal deposits; however, previous investigators have used mobile and total mercury analysis indiscriminately, obtaining mixed results. A close examination of the various forms of mercury containing compounds reveals distinct relationships between the occurrence of these compounds and the mineral zonation associated with the ore forming process, including the development of both primary and secondary halos.

A new extraction and analysis technique has been developed which employs differential thermal analysis (DTA) for the characterization of these various mercury forms. This DTA technique yields superior results as compared to the conventional analysis of making a total mercury measurement which combines all the mercury liberated from its different compounds. The superior performance of the DTA method also allows one to determine the most diagnostic low temperature forms of mercury which have the highest potential as a pathfinder for buried metallic ores.

A unique variation of this new DTA method has been developed for extraction and analysis of mobile mercury which is rapid, highly reproducible, and very cost-effective.
Brief case studies of the application of this DTA exploration technology are presented.

INTRODUCTION

The use of mercury as a pathfinder for buried metallic ore bodies was first proposed by Saukov in 1946. His initial concept was based on mercury's high volatility, supported by studies which showed that the mercury content of metallic ore bodies is almost always higher than in their associated gangue rocks. During the formation of ore bodies, the highly volatile mercury (aided by temperature gradients) would develop primary aureoles, while in later stages the mercury liberated by supergene processes would generate secondary halos in the host rocks.

Saukov (1946) proposed the analysis of mercury vapor in soil-gas as an exploration method for buried or blind ore bodies. Since this pioneering work of Saukov, applications of mercury as a pathfinder for ore bodies has received numerous trails. However, it was found that the content of mercury vapor in the soil-gas is strongly affected by climatic and atmospheric factors which frequently produced unreliable data.

As an alternative, the analysis of total mercury content in soils and rocks was postulated as a way to generate more reproducible and reliable results. This method of analysis, although stable and repeatable, often yields poor results as a pathfinder for blind deposits. The major problem with the analysis of total mercury content is the low anomaly/background ratio, which results in poor resolving power when exploring for buried ore bodies.

The difficulty in applying mercury as a pathfinder lies in an oversimplification in the understanding of the various modes of occurrence of mercury, and in the differentiation of these mercury species in the natural environment. Volatile, and more soluble mercury forms are capable of traveling significant distances from buried deposits. Many higher temperature forms occur in, and very near, the ore zone which is very useful in predicting the proximity to the ore. In fact, by examining the mercury liberated by heating over a wide temperature range, it is often possible to vector the direction of an ore body from a set of borehole samples. Analysis of the different mercury forms also allows recognition of the ore zone, or zones, in proximity to an ore.

Failure to recognize the importance of these various mercury forms is compounded by mercury analysis techniques, which use high temperature retorting (up to 600°C) to release the mercury from the solid phase. This type of total mercury analysis mixes the indigenous and migratory populations during analysis and destroys the information conveyed by the presence of each of these individual mercury compounds. The purpose of this paper is to focus on the pattern of distribution of these various mercury forms, and to explain their usefulness in prospecting for concealed ore bodies.

In order to use these concepts, new differential thermal analysis techniques have been developed which allow accurate and repeatable measurements of thermal mercury liberation forms. These techniques are rapid, highly reproducible and cost-effective.

Extensive case studies have demonstrated the relationships of the individual mercury compounds to known mineralization, and will be the subject of future publications. Brief case study examples are included in this paper in order to demonstrate the diagnostic capabilities of this newly developed technology.

BEHAVIOR OF MERCURY AND ITS ASSOCIATIONS WITH GOLD ORE BODIES

A review of mercury properties suggest that this element has the potential to be one of the bet pathfinders for blind or buried ore bodies. It is a chemically stable element with high ionization potential and its fugacity under the temperature effect is unparalleled among other metals. Because of the high ionization potential, it coverts easily to atomic form and reduces itself to metal from many associations. Mercury mobility is greatly increased under the effect of temperature, and it exists in gangue rocks in very low (in average 50-150 ppb) and uniform (without any predilection for rock type) concentrations. In contrast, in mineralized rocks mercury is preconcentrated to levels which are several orders higher than in gangue rocks. A general correlation between mercury and the type of ore bodies was established by Saukov in 1976. He noted the low temperature ore bodies contain the highest amount of mercury, while in high temperature deposits, the mercury content decreases markedly (Figure 1).

Numerous analysis of mercury content in sulfide minerals and gold, conducted by Ozerova (1985), indicated that gold, along with certain sulfide (Figure 2) contain the highest amounts of mercury. These two observations suggest that low temperature gold ore bodies should provide the best targets for the use of mercury as a pathfinder.

These conclusions have also been supported by Kunitzyn (1983), who found that Hg is the best pathfinder element for low and intermediate temperature gold deposits. Antimony follows a similar behavior to mercury, while arsenic is a better indicator for the intermediate and higher temperature ore bodies.

Many investigators note, however, that high concentrations of mercury exists, not only in low temperature, but also in higher temperature gold deposits which are quite different in both genesis and/or age. This suggests the emplacement of Hg in such deposits by secondary processes. Omelchenko (1980) found that while mercury concentrations in metasomatic deposits averages more than 10 times higher than the Clarke, in secondary gold deposits derived from these metasomatic ore bodies, the content is over 200 times higher. This secondary enrichment results from liberation of mercury (over geologic time) by the oxidation and destruction of minerals. In addition, non-associated mercury may also migrate from depth along the same reactivated faults and fractures which controlled the deposition of the ore body. This secondary mercury could be trapped effectively by elemental gold (Ryall 1979) resulting in the secondary enrichment of mercury in gold deposits of different ages and origins.

MERCURY IN PRIMARY AND SECONDARY HALOS

During the processes which lead to the formation of an ore body, a primary halo is often developed in the host rocks. The shape and the size of this primary halo depends on the mobility of halo forming elements and the availability of geologic factors such as faults, fractures, and the permeability of the enclosing rocks. The most volatile or soluble elements have the greatest potential to travel significant distances from an ore body. Trofimuk (1978) who studied the distribution of volatile elements in the primary halo over the Sadonskoye polymetallic ore body, detected three bands of halos.

       - in close proximity to an ore body - Pb, Zn, Cu, Ag, Mn,
       - in the central and lower part of the ore body - Co, Ni, Mo, Bi, and
       - in the upper part of an ore body, with continuity to large distances in both the vertical and horizontal plane: I-400 m, Hg-450 m, Sb-400 m, B-600 m.

The results of his findings are presented in Figure 3 and suggest that during the initial stages of exploration for blind or buried ore bodies, the analysis of volatile (Hg) or soluble (I) elements appears to have the greatest exploration potential.

Boldy (1968) investigated primary halos above blind massive sulfide deposits. His analysis was conducted on samples from both surface and drill cores (Figure 4). He concluded that mercury from the primary halo developed above blind deposits and the detection of this mercury could provide a successful exploration technique in areas where the deposit was covered by non-mineralized rocks. Boldy sampled the mercury in minute fracture faces. As illustrated in Figure 4, this anomalous mercury was detected at the surface, as well as in drill holes. Boldy stressed the importance of sampling drill cores so that holes will not be stopped "prematurely". Fursov (1977), who studied the distribution of mercury in primary halos above numerous metallic deposits in detail, stated that the mercury halo could be developed above the ore body to a distance of as much as 1000 meters. The primary mercury halo is developed by native mercury in vapor phase, or its compounds migrating in solution. He concluded that the movement of Hg in the vapor phase is the dominant mode of migration.

While the development of primary halos are stimulated by hypogene processes, the generation of secondary halos are governed by supergene processes. At significant distances, only the volatile or soluble forms of mercury are responsible for the secondary halo development. Mercury gas dispersion is achieved through gas diffusion or effusion along fault and fracture openings. Migrating gas saturates pores and become sorbed by soils and rocks and form organic complexes. Mercury migrating in solutions can be decomposed to metallic mercury, forming gas and occluded mercury halos. Mercury in compounds can be transported in the form of soluble anionic or organic complexes. The soluble forms will be transported in solution along zones of higher permeability until sudden changes in conditions, such as Eh/Ph are encountered.

The following examples illustrate the distribution of mercury in secondary dispersion above gold mineralization. The first example, (Figure 5) shows the distribution of mercury sourced in Au mineralization and migrating upward through cover of gravel in Nevada. The surface samples indicate a weak but detectable migratory mercury anomaly which is about three times background, while the mercury values obtained from the drill hole indicate almost logarithmic decrease of mercury with distance from an ore.

The second example illustrates the migration of mercury through unconsolidated sediments which cover secondary karst gold deposits (Figure 6). As shown, the mobile mercury anomaly is much larger than either As or T1, and appears to have a direct relationship with the Au ore. Both these examples show direct associations between gold mineralization and surface mercury anomalies, suggesting the movement of Hg in vapor phase.

Fursov (1977) presented this research results on the distribution of mercury in secondary halos above both mercury and non-mercury mineralization. These results can be summarized as follows:

       - the ratio of anomaly/background was found to be about eight for mercury deposits, and about two-three for non-mercury deposits.
       - the ratio of the mercury halo diameter to the diameters of typical ore bodies was found to be about five for both mercury and non-mercury deposits.
       - the mercury halo was developed in the vertical direction to a distance of several hundred feet.

These observations strongly suggest that the movement of mercury in the gas phase is the most dominant form of migration and suggest the following concepts:

       - temperature is the most dominant factor stimulating mercury distribution in mineralized environments,
       - the temperature gradients around an ore body may be related directly to the distribution of mercury forms in the primary halos,
       - migration as a vapor and in solution are the two major mechanisms responsible for the buildup of mercury in halos,
       - the movement of mercury in the vapor phase appears to be dominant over solution,
       - therefore, in the entire spectrum of mercury species existing in the environment, only native mercury or forms originated from mercury vapor, and soluble mercury compounds are indicative of the present of primary or secondary halos from distant ore bodies.

MERCURY FORMS AND THEIR DISTRIBUTION IN GANGUE AND MINERALIZED ENVIRONMENT

Mercury exists in the environment in a number of states. It commonly occurs in the vapor state (in macropores, micropores, and in micro and macro fractures) as sorbed mercury, as organo-complexes, as mercury compounds, and within the structure of minerals. This entire spectrum of forms which make up the total mercury content have been separated into two general categories for the purposes of this discussion.

     - syngenetic
     - epigenetic

The syngenetic Hg is the typical background population, while epigenetic mercury was either introduced into the environment by processes or primary or secondary halo formation, migration from depth or transport. Successful geochemical exploration with pathfinder elements requires separating between these background (syngenetic) and anomalous (epigenetic) populations.

In case of shallow ores, the epigenetic population is generally much larger in magnitude than the syngenetic population, therefore, the analysis of total Hg is sufficient for detection of either outcropping or shallow ore bodies.

In the case of deeper ore bodies, separating between migratory and indigenous populations results in much higher diagnostic capability. This point is illustrated schematically in Figure 7, where the level of the average total mercury background in the host rocks is about 100 ppb and the level of mobile mercury background is only 5 ppb. Migration of mobile mercury, from subsurface mineralization in this example increases the mercury content in the primary halo by about 30 ppb. Since the primary halo is developed by mobile mercury forms, the analysis of this mobile mercury yields an anomaly/background ratio of 7 (35 ppb/5 ppb), while the analyses of total mercury content produces a ratio of only 1.3 (ppb/100 ppb).

Pshenichny (1983) analyzed the total mercury content in host rocks (curve 1), Hg within a pyrite ore body (curve 2), and Hg residing in epigenetic pyrite disseminated within the host rocks (curve 3). As shown in Figure 8, all three curves show a higher Hg analyses of the host rocks (curve 1) yields a small contrast, and is or little value in prospecting for this ore body, the analyses of Hg from epigenetic pyrite would clearly indicate the presence of this ore body. Thus, as most of our studies have shown, analysis of epigenetic mercury has a much greater likelihood of identifying a concealed mineralized zone than total mercury techniques.

Since the behavior of mercury is so temperature dependent, it is logical to expect that the temperature gradients around an ore body will have strong influence on the development of mercury in the primary halo. This is illustrated schematically in Figure 9. Inside, and in proximity to the ore body, higher temperature mercury forms should dominate, while more remote areas will contain the most volatile, and to some extent soluble mercury forms. Fursov (1977) confirmed such zonations in the primary mercury forms are developed in proximity to an ore body, and less temperature resistant forms characterize the more remote parts of the halo. Permeability and fracturing has the most pronounced effects on zone II (soluble forms) and must be taken into consideration in order to produce a more realistic picture. The pressure and temperature gradient drives volatile mercury upwards. Thus zone III would be depleted in the lower levels as volatile mercury shifts upwards towards the cooler parts of the mineralized zone.

Therefore, the analysis of mobile mercury forms appears to be the most advantageous in the initial stages of exploration for concealed mineralization, while differential analysis of mercury species during drilling programs may vector the direction of the ore body, and may be of more use in development drilling.

CASE HISTORIES

MOBILE MERCURY IN THE SEARCH FOR CONCEALED GOLD MINERALIZATION

Gold Bug Area

This example illustrates the application of mobile mercury analysis in the search for gold mineralization formed by leaching of gold from volcanics, and it subsequent deposition on the volcanics, and its subsequent deposition on the volcanic/sediment contact. With the exception of a single outcropping gold occurrence in the center of the valley, where an exploratory shaft discovered gold mineralization, the bedrock in the remainder of the area is covered by about 100 feet thick gravel cover. Over 600 mobile Hg analyses were conducted on soil sample collected on 100 foot centers. As shown by Figure 12, this mobile Hg reflects this mineralized contact is confirmed by the exploratory shaft and subsequent drilling in the southern and northern areas. Two anomalies mapped on the flanks of the survey are interpreted as boundary faults.

Quartzite Area

This example in Figure 13 illustrates a large scale survey of over 2000 samples collected on 100 foot centers conducted in the Mojave Desert area. The hydrothermal gold mineralization in this area is associated with large scale (N-W trending) regional faulting. The majority of the survey is covered by gravel which ranges in thickness from several to tens of feet. The mercury data reflects the mineralized trends in a very distinct manner indicating the presence of gold mineralization not only in the outcrop, but also under the gravels. Analysis of mercury over several placer deposits in this area produced high reading of mobile mercury over each placer, demonstrating deposits. It should be noted here that both the Gold Bug and Quartzite areas were previously surveyed using total mercury analysis techniques. These total mercury surveys detected only the outcropping mineralized areas and do not detect the mineralized trends which extend under the gravel cover, as did the subsequent mobile Hg surveys discussed previously.

GOLD HILL

A survey conducted over a gold vein in an Appalachia Area, as shown in Figure 14, contains an order of magnitude higher values of mobile mercury over the gold vein, which sharp distinctive boundaries between anomalous and background zones. The excellent correlation between Hg and other elements including Au and Ag as shown in Figure 15, emphasizes the value of mobile mercury analysis as a screening tool for detailed, more costly multi-element analysis. In addition, this survey demonstrated the positive one to one relationship of the mobile Hg with the gold mineralization.

APPLICATION OF MOBILE AND DIFFERENTIAL MERCURY ANALYSIS OVER A "CARLIN" TYPE GOLD ORE

This final example illustrates a large scale detailed survey of over 3000 samples collected on 100 to 50 foot centers, over an area containing deeply seated Carlin type disseminated gold deposits. The results of this survey show two large N-S trending anomalies detected above the ridges of the adjacent mountains, while the valleys contained mostly background values. As shown in Figure 16, the eastern anomaly can be divided into a southern and northern portion. The southern anomaly appears to be directly related to shallow gold mineralization which has been exploited since the turn of the century.
The northern anomaly outlined the areal extensions of a buried gold ore body located more than 600 feet beneath the surface. As established from over 20 drill holes. The western anomaly, although very promising, has not been tested by drill holes. The results of differential mercury analyses conducted on drill core samples are described in the following section.

APPLICATION OF DIFFERENTIAL MERCURY TECHNIQUE

Differential mercury analysis conducted on surface and borehole samples of rocks yielded numerous mercury peaks in the temperature range from 150-700°C. The distribution of these sublimation Hg peaks from these samples show a very coherent spatial distribution, which have been separated into four general groups. These are illustrated on Figure 17 and described below:

1. Far-zone: above the ore body.
Samples that are collected at distances of several hundred feet, but not closer than 100 feet from any mineralization, have been classified as the far-zone. Results from the differential analysis of these samples yields a single large mercury peak in the temperature range from 150-300°C, with much smaller unresolved higher temperature forms. This pattern illustrates that at significant distances from this ore body, mercury vapor is mainly responsible for the development of the mercury halo. This mercury may be bound loosely to rocks, or it may exist in organo-complexed forms.

2. Near-zone: slightly above and containing the upper part of the ore body.
The near-zone includes samples that were collected adjacent to and in the upper part of the ore body. Differential extraction of these samples yields more numerous peaks over a temperature range from 130-650°C. The low temperature mercury peak attains the largest values in this zone depending on the proximity to the ore body, and intensity of secondary processes. Note that higher temperature forms are gaining importance. These higher temperature peaks may be indicative of less mobile more temperature resistant mercury forms, or mercury existing in crystal lattices of some of the disseminated minerals.

3. Ore-Zone: Samples from the ore zone.
The samples collected directly from the mineralized ore zone, exhibit a dominant characteristic peak in the temperature range from 450-550°C, with smaller accompanying peaks in the temperature range from 200-700°C. This direct relationship between the temperature of extraction of 400-500°C and the gold mineralization is interpreted to be an extraction of mercury from gold (mercury can be released from gold amalgam by temperatures from 380-500°C). The release of mercury from this temperature range may also come from the crystal lattice of arseno-pyrite, which is known to be associated with the Au mineralization in this ore body.

4. Low-zone: zone below the ore body, and/or containing the lower portions of this ore body.
Samples which are collected from the lower part of the ore body, or beneath the ore body, are classified as the low-zone. These samples yielded a mercury extraction pattern having a wide range of peaks in the temperature range from. 200-700°C. In this zone, a characteristic high temperature peak (570-650°C) appears to be dominant. This peak was very seldom detected in the upper parts of this ore body and has been interpreted as an extraction of mercury from silica.
A vertical cross-section of the mobile mercury distributions and their relationship to the gold mineralization is shown on Figure 18. As illustrated, the lowest mobile mercury content was detected near the ore body. Larger values were noted at a distance of about 100 feet from the ore body, with elevated concentrations continuing practically to the surface. In contrast, the distribution of mercury forms found to be correlated with gold mineralization is shown in Figure 19. As shown, the largest concentrations of this high temperature form of Hg corresponds very closely with the zone of significant gold mineralization.

BIBLIOGRAPHY

Boldy, J., 1968, Mercury Dispersion Halos as Exploration Targets.
Fursov, V.Z., 1977, Mercury as an Indicator in Prospecting: Niedra, Moscow.
Kunitzyn, V.V., 1983, The Basic Characteristic of Primary Halos From Gold Ores; Nauka, Novosibirsk.
Ozerova, N.O., 1985, Prospecting with Mercury: Geochemical Prospecting for Ore Bodies, Nauka, Novosibirsk.
Pshenichny, G.N., 1983, Mercury in Pyrite ores and Primary Halos and Its Utilization in Prospecting: Geochemical Prospecting Utilizing Primary Halos, Nauka, Novosibirsk.
Ryall, W.R., 1979, Pathfinder and Multielement Geochemistry: Contribution from CSIRO.
Saukov, A.A., 1946, Geochemistry of Mercury: Akad. Nauk. S.S.S.R. Doklady. Inst. Geol. Nauk. no 73 (Mineralogo-Geokhem. Seriya No. 17), 129 pp.
Saukov, AA, 1976, Geochemical Sketches: Nauka, Moscow.
Trofimuk, N.N., 1978, Halos of Wide Disseminated Elements in the Presence of Polimetallic Bodies: Okhrana Nedr. No.4., pp 25-29.