APPLICATIONS IN THE SEARCH FOR GOLD
J.B. Maciolek, V.T. Jones III
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
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.
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.
- in close proximity to an
ore body - Pb, Zn, Cu, Ag, Mn,
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.
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,
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.
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.
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.
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.
APPLICATION OF MOBILE AND DIFFERENTIAL MERCURY
ANALYSIS OVER A "CARLIN" TYPE GOLD ORE
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
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