Mineralogical-Geochemical Heavy Mineral Concentrate Method for Prospecting Gold Ore Deposits

V. F. Gureev (TsNIGRI)

For prospecting gold-ore deposits, two methods are used primarily: the heavy mineral concentrate method and the metallometric method.

The application of the heavy mineral concentrate method is difficult in those cases where deposits with finely dispersed gold are widespread in the area. When gold-ore deposits with relatively coarse gold, recoverable in heavy mineral concentrates, are identified, distorted results are often obtained because of imperfect and uneven washing of the concentrate and the extremely uneven distribution of gold, which enters the concentrate to a considerable extent by chance. In addition, the transport of gold in alluvial deposits over long distances from the primary source sometimes creates difficulties in interpreting the data of the heavy mineral concentrate method.

Metallometric surveying, as is known, readily detects finely dispersed gold, but it requires a large number of samples and at the same time does not provide complete coverage of the entire area; it is “point-based.”

In order to eliminate these shortcomings and increase the efficiency of prospecting for gold-ore deposits, the author has developed a prospecting method that includes elements of both heavy mineral concentrate surveying and spectrogoldometry. The method was developed in the Kyzylkum gold-bearing province, mainly in the territory where deposits of the type of zones of sulfide dissemination with finely dispersed gold are widespread, and was also tested on the example of certain other gold-ore deposits and ore occurrences of the quartz-vein low-sulfide type.

In most types of gold-ore deposits, auriferous pyrite is present in one quantity or another. The gold contents in pyrites vary widely, from the first g/t to hundreds of g/t. During oxidation of auriferous pyrite, a certain amount of gold leaves the pseudomorphs of iron hydroxides after pyrite, while part remains in the pseudomorphs.

Entering loose deposits, pseudomorphs of iron hydroxides after auriferous pyrite may create dispersion halos of gold, clearly detected by spectral analysis of this mineral for gold. In addition to gold, elements associated with gold are also determined in pseudomorphs of iron hydroxides after pyrite. Comparison of anomal—

... of gold and its associated elements makes it possible to identify prospective areas more accurately and to judge, in a preliminary way, the features of the gold mineralization.

As our investigations have shown, elevated contents of gold and associated elements in pseudomorphs of iron hydroxides after pyrite that have entered alluvial deposits are observed within the first few kilometers from the primary source, increasing noticeably on approach to it. The small size of the dispersion halos of oxidized varieties of gold-bearing pyrite depends, on the one hand, on the relatively low mechanical stability of this mineral during transport and, on the other hand, on dilution of the alluvium due to the influx into it of material from barren areas containing non-gold-bearing pyrite.

The essence of the method consists in selecting from heavy-mineral concentrates pseudomorphs of iron hydroxides after pyrite and in carrying out spectral analysis of them for gold. In addition to determining the gold content of oxidized pyrites, it is also advisable to determine, by semiquantitative spectral analysis, the entire suite of impurity elements, which provides additional data for identifying primary gold deposits. Observations of the proportions of the morphological types of pyrite, which are usually established quite clearly in pseudomorphs of iron hydroxides after pyrite, are also of considerable interest. In some cases the appearance of certain morphological types of pyrite in heavy-mineral concentrates may directly indicate the proximity of gold-ore deposits, and, in combination with analyses of oxidized pyrites, they provide very valuable supplementary information.

The scale of heavy-mineral-concentrate surveying by the proposed method is chosen depending on the intended purpose of the work, the area covered, the degree of dissection of the relief, and the specifics of the gold mineralization. The best results have been obtained in surveys at a scale of 1 : 50000—1 : 25000. In this case, areas of possible development of ore fields and deposits can be identified. Work at a larger scale—1 : 10000—1 : 5000—is advisable within ore fields, with the aim of identifying individual ore bodies.

The relatively large scale of surveying by the proposed method is explained by the fact that the transport of oxidized varieties of gold-bearing pyrite is comparatively limited, and at a distance from deposits only “background” pyrite from various geological formations that do not contain gold is encountered in heavy-mineral concentrates.

Heavy-mineral concentrates from alluvial deposits are taken so as to characterize the entire area being surveyed as fully as possible, with one or another degree of detail (depending on the scale of the work). Depending on the detail of the work, concentrates are taken from watercourses of different orders and at different intervals. In the case of route work, concentrates are taken from individual watercourses cutting ore zones or areas prospective for gold; moreover, for interpretation of the results, concentrates should also be taken from watercourses known to be non-gold-bearing. Special attention should be paid to watercourses of small length (1–2 km or less) with small drainage basins. When choosing the frequency of concentrate sampling, the geological situation should be taken into account first of all: in areas most favorable for the presence of gold-ore zones or bodies, concentrates are taken in greater detail.

To obtain comparable results, concentrates should be taken from loose deposits of the same type, preferably from channel deposits.

For comparison of anomalous gold contents in pyrite and in its varieties encountered in alluvial deposits, it is advisable to collect a certain number of heavy-mineral concentrates from eluvial-deluvial deposits over known ore bodies, as well as over zones of hydrothermally altered rocks. Such samples are best collected from the dumps of trenches exposing ore bodies.

On the basis of work carried out in the desert conditions of Kyzylkum, we propose the following method for sampling heavy-mineral concentrates. With certain modifications it may also be applied to other districts.

The volume of alluvial deposits from which the concentrate is washed should be taken as 5–10 m, depending on the coarseness of the deposits. In the case of a large amount of aeolian sand or a coarse pebble fraction, the volume of the concentrate should be increased to 20 l. The collected sample is screened on a 2.0 mm sieve. Preliminary screening considerably facilitates its subsequent processing.

The +2.0 mm fraction is discarded, since, as a rule, it contains almost no pseudomorphs of iron hydroxides after pyrite. In addition, it has been established that in most gold-ore deposits relatively finer pyrite segregations contain larger amounts of gold than coarse ones. If alluvial deposits contain a large amount of aeolian sand, the −2.0 mm fraction is sieved off by several successive screenings.

The weight of the −2.0 mm fraction obtained after screening usually does not exceed 1–3 kg. This sample is washed on a tray or in a pan until the removal begins of pseudomorphs of iron hydroxides after pyrite, which have a specific gravity of 3.4–3.7 and are readily distinguished by their dark color. If unoxidized pyrite predominates in the concentrate, washing is carried out until its removal begins. The weight of the concentrate, depending on the amount of pseudomorphs of iron hydroxides after pyrite, ranges from 10 to 300 g.

Further processing of the concentrate, in the simplest case, is reduced to separating the electromagnetic fraction on a roller electromagnetic separator or with the aid of a Sochnev magnet, and to spectral analysis of the pseudomorphs of iron hydroxides after pyrite for gold and impurity elements. Spectral analysis for gold is carried out by the usual method used in goldometric surveying.

Pseudomorphs of iron hydroxides after pyrite from the electromagnetic fraction of the concentrate are selected under a binocular microscope from the −2.0—+0.5 mm fraction. The weighed portion for determining gold is 120 mg. In addition, for a complete semi-quantitative analysis a weighed portion of 70 mg should be taken.

Because the material analyzed in this case is considerably more homogeneous than in ordinary spectrogoldometry, the results obtained are more precise.

When oxidized pyrite is selected from the electromagnetic fraction of the concentrate, additional data may be obtained which are then used to increase the reliability and accuracy of identifying gold-ore deposits.

Thus, of considerable interest is determination, in concentrates, of the ratios of the contents of concretionary brown iron ores and pseudomorphs of iron hydroxides after pyrite, as well as the total amount of oxidized pyrite in the concentrate. In all cases studied by us this amount increased in the vicinity of gold-bearing ore bodies.

An important characteristic of pyrite is also the ratio of its morphological forms. We have combined all forms into four groups: 1) simple cubes, 2) pentagonal dodecahedra, 3) octahedra, including also

cubo-octahedra; 4) cubes complicated by various faces. The count of morphological forms is made in the \(-2.0\) to \(+0.5\) mm fraction; for this purpose 100 well-faceted pseudomorphs of iron hydroxides after pyrite were selected. In this count, the number of grains of a given morphological variety constitutes the percentage.

It was established that an increase in the amount of complex forms of pyrite in the concentrate occurs with approach to primary deposits and is accompanied by a general increase in the gold content of pseudomorphs of iron hydroxides after pyrite.

The concentrates selected for analysis and study of oxidized pyrite may also be used to determine free gold in them. For this purpose, the remaining non-electromagnetic fraction is washed down to a black concentrate, and the number of colors or the weight content of gold is determined in it. When determining the content of native gold in the black concentrate, attention should be paid, as in the ordinary concentrate method, to other useful minerals as well, and also to minerals that may be indicators of gold.

Our materials showed that in almost all cases when visible gold was encountered in the concentrates, pseudomorphs of iron hydroxides after pyrite contained elevated amounts of gold and of its pathfinder elements. At the same time, very many concentrates near known ore bodies and containing gold-bearing oxidized pyrite were entirely devoid of colors of visible gold.

The principal data of concentrate surveying by the proposed method are plotted as overlays on the geological map. We compiled the following overlays: 1) gold contents in pseudomorphs of iron hydroxides after pyrite; 2) native-gold contents in concentrates; 3) contents of gold pathfinder elements in oxidized pyrites (for the area studied, the gold pathfinder elements are arsenic, antimony, bismuth, and silver); 4) ratios of pseudomorphs of iron hydroxides after pyrite and encrusting iron hydroxides; 5) ratios of morphological forms of pyrite.

It is convenient to plot the results of spectral-analysis data in the form of circles of different colors, depending on the content of the element. Then, over the drainage areas, sectors with elevated contents of gold and pathfinder elements in pseudomorphs of iron hydroxides after pyrite are outlined.

Experimental work by the proposed method for prospecting gold-ore deposits was carried out in one of the gold-ore fields of the Kyzylkum gold-ore province. The ore field is characterized by the presence of a large number of ore bodies of the sulfide-disseminated type, with a generally moderate amount of sulfides (10–12%); among them pyrite is predominant, and arsenopyrite occurs in smaller amounts, approximately 2–3 times less. The gold in the deposit is finely dispersed and is enclosed in the principal ore minerals—arsenopyrite and pyrite. Gold occurs extremely rarely in the form of the finest gold particles, not exceeding 10–20 microns. In concentrates from alluvial deposits, colors of native gold are absent in most cases.

When work was carried out by the proposed method, almost all known ore bodies fell within areas of distribution of anomalous (from 2 g/t and higher) gold contents in pseudomorphs of iron hydroxides after pyrite in the concentrates. In addition, all oxidized pyrites in halos with known ore bodies contained elevated amounts of arsenic, and in some cases also antimony. It should

It should be noted that halos with anomalous contents of gold-associated elements are somewhat broader than those of gold.

In addition to areas with known ore bodies, the survey data revealed several promising sites for gold, where new ore bodies were subsequently exposed.

Overall, the area of the ore field is also clearly distinguished from the surrounding area by other characteristics: the ratio of the morphological forms of pyrite, the ratio of the amounts of nodular and pseudomorphous iron hydroxides after pyrite in heavy-mineral concentrates, etc.

In a small volume, work using the heavy-mineral mineralogical-geochemical prospecting method was also carried out in areas where low- and poor-sulfide quartz-gold deposits are developed. In this case as well, the area of distribution of ore bodies clearly coincided with the contour of anomalous contents of gold and associated elements in pseudomorphs of iron hydroxides after pyrite from heavy-mineral concentrates.

It should be noted that the data obtained by the proposed method make it possible not only to conduct targeted prospecting for gold-ore deposits of different types, but also to clarify certain geological features of the areas in which the survey is carried out.

For example, from elevated chromium contents in pseudomorphs of iron hydroxides after pyrite, it proved possible at one of the ore fields to outline areas of distribution of ultrabasic rocks. Increased quantities of octahedra and cubooctahedra of pyrite in heavy-mineral concentrates, as was verified in several areas, are observed mainly in the near-contact parts of granite intrusions, sometimes even where these have not been exposed by erosion.

Thus, the proposed method, while solving the main prospecting tasks, helps identify a number of geological and geochemical features of gold-ore fields and deposits even under conditions of poor exposure.