METHODS FOR IDENTIFYING GOLD ORE DEPOSITS UNDER THE CONDITIONS OF NORTHEASTERN TRANSBAIKALIA

V. L. FISHER (ChSU)

The methods used to identify gold ore deposits under the conditions of northeastern Transbaikalia are determined by the geological and metallogenic features of the region.

Northeastern Transbaikalia is located immediately north of the Mongol–Okhotsk belt and is the southwestern part of the Aldan–Vitim Shield [1].

The oldest formations of the region are Archean gneisses and crystalline schists, correlated with the Aldan complex. They were metamorphosed under granulite-facies conditions. A northwestern trend of folded structures is characteristic.

Lower Proterozoic formations, correlated with the Stanovoy complex, are less widespread. They are also represented by gneisses and crystalline schists of varied composition, metamorphosed under amphibolite-facies conditions. A northeastern structural pattern of folded structures is characteristic.

The most extensive development in the region is shown by multiphase batholith-type intrusions, reaching areas of three thousand square kilometers. The rocks of the older phases are characterized by a more basic composition. These intrusions are dated to the Precambrian and Paleozoic.

Hypabyssal intrusions of Late Mesozoic age, confined to systems of differently oriented faults—which accounts for their fissure-like character—are relatively widespread. The Late Mesozoic intrusions have a three-phase structure. The area of the massifs rarely exceeds 500 km².

From the metallogenic point of view, the most interesting is the complex of Upper Jurassic formations, including sheets of effusives, subvolcanic bodies, and small intrusions. The latter form linearly elongated zones traceable for tens and hundreds of kilometers in northeastern and sublatitudinal directions.

Volcanogenic-sedimentary formations of Early Cretaceous age have limited distribution, mainly within graben-like depressions.

Quaternary deposits are developed everywhere, but are characterized by insignificant thickness (up to 15 m). Systems of differently oriented ruptural dislocations are widely developed. The oldest are systems of faults of northeastern and northwestern strike—

...faults that controlled the orientation of batholith-type intrusions and the direction of folding in the Precambrian formations.

Later, probably in Paleozoic time, faults of sublatitudinal and submeridional strike arose. These fault systems were subsequently renewed more than once, as a result of which the area consists of alternating horsts and grabens oriented in a northeastern direction.

The magmatic formations of Late Jurassic age, with which the principal useful minerals of the area—molybdenum and gold—are associated not only spatially but probably also paragenetically, are confined to systems of faults of sublatitudinal and northeastern strike.

An important feature of the area is the wide distribution of gold placers. It is sufficient to note that exploitation of placer deposits has continued uninterrupted for more than one hundred years. In recent years a number of previously known placers have been re-evaluated for mechanical processing. New placers have also been identified.

Alluvial valley placers, which are of the principal industrial interest, are the most strongly developed. Terrace alluvial and deluvial placers are less widespread and of lesser practical importance. Alluvial valley placers are dated to the Middle and Late Quaternary.

For the purposeful conduct of prospecting work for ore gold, the following questions are of great importance: the structural position of the placers, their structure, the character of the gold, the presence of nuggets, intergrowths of gold with quartz and sulfides, etc.

The importance of resolving these questions is due to the fact that there is a clear disproportion between the number of known placers and ore deposits. Far fewer ore deposits have been identified.

Analysis of the distribution of placers indicates that their headwaters are confined to belts of subvolcanic and hypabyssal intrusions of Late Jurassic age. The latter are controlled by systems of faults and zones of increased fracturing of sublatitudinal strike, traceable for tens, more rarely hundreds, of kilometers, with a width of 5–15 km.

Subvolcanic and hypabyssal intrusions form elongated, dike-like, closely spaced bodies, generally oriented in a sublatitudinal direction. The dikes can be traced for hundreds of meters, more rarely for the first kilometers. Intrusions rarely exceed several kilometers in cross-section. In areas where differently oriented fault systems intersect, fields of concentrations of subvolcanic and hypabyssal intrusions are observed. The lengths of the placers vary within fairly wide limits: from 5 to 40 km. As an exception, their length reaches 90 km. The principal feature of the placers is the small size of the gold. It is sufficient to point out that the −0.25 mm class accounts for about 25% of the entire gold content, while the 0.25–0.5 mm class accounts for about 30%. Fineness varies from 700 to 920. Nuggets are not characteristic. Intergrowths of gold with quartz and with sulfides, in particular with bismuthinite, are observed. In the headwaters of placers, bismuth minerals or antimonite are often found in heavy-mineral concentrates.

By length, character of the gold, and other features, two types are rather clearly distinguished among the placers. The first is characterized by a regular change in the character of the gold along the length of the valley. In the headwaters of such placers, the percentage content of the 0.25–0.5 mm gold class increases somewhat and reaches 35%, while downstream the role of the −0.25 mm class increases. In a similar way, the roundness of the gold increases, as does its fineness. The length of such placers rarely...

exceeds 10 km. Most likely, they are formed by one or several sources located in the area of their headwaters.

Placers of complex structure are more widely distributed; in them an irregular change is established in the shape of the gold grains, their size, fineness, etc. Being characterized by greater extent, such placers are also the principal objects of exploitation. Obviously, a number of spatially separated and, what is very important, qualitatively different sources participate in feeding such placers.

The principal source of metal for the placers of the region is gold-ore deposits, which will be characterized somewhat below.

Gold associated with the cement of Lower Cretaceous conglomerates, zones of pyritization, and gold-bearing terraces also participates in feeding the placers.

Ore mineralization is dated to the Upper Jurassic. But by the beginning of Lower Cretaceous sedimentation, some deposits had been subjected to intensive erosional processes. The liberated gold was deposited in the cement of future conglomerates. During the erosion of gold-bearing strata in Quaternary time, gold from the cement entered valley alluvial placers. The quantitative role of gold from the cement of conglomerates in the overall balance of placer gold is insignificant. But for individual placers originating from fields of development of gold-bearing conglomerates, the role of this source is quite appreciable. The next source of gold is the zones of pyritization. Fire-assay analysis of pyritized rocks and analysis of monomineral pyrite fractions have established that in individual cases pyrite is gold-bearing. Mainly the fine fractions of pyrite are gold-bearing, the size of which does not exceed fractions of a millimeter. Therefore, gold associated with pyrite is microscopic or submicroscopic in size, and for the most part it is carried beyond the limits of gold-bearing valleys. But in extended placers (the length of which amounts to several tens of kilometers), gold associated with pyrite is deposited in their near-mouth parts, increasing the extent of the placers.

Gold-bearing terraces have a somewhat greater significance in feeding valley placers. The latter have been preserved in the lower reaches of gold-bearing valleys. It has been established that where terraces are preserved, interruptions, narrowings, etc., appear in valley placers. Conversely, when they are eroded, the valley placers are enriched. In contrast to ore gold, terrace gold is more rounded, of higher fineness, and gold in a “shirt” is observed.

Thus, in conducting prospecting work for ore gold in northeastern Transbaikalia, the structural position of the placers, their confinement to belts of subvolcanoes and small intrusions, the character of the gold, the types of sources, etc., are of great importance.

One of the serious reasons why prospecting work in the past rarely led to positive results is the fine character of the gold. The fact that 25% of the gold in the placers of the region corresponds to the size class −0.25 mm testifies to the low effectiveness of the panning method of prospecting. It should also be taken into account that relatively the coarsest gold is concentrated in placers. Therefore, in ore deposits the role of the −0.25 mm class will be much greater and will probably exceed 50%.

Under these conditions, the application in the region of lithogeochemical prospecting by secondary dispersion halos of gold and its pathfinder elements proves effective.

In the course of geological surveying and prospecting work at a scale of 1:50,000, lithogeochemical prospecting is carried out according to the generally accepted

method set forth in the relevant instructions. On the basis of the results of semiquantitative spectral analysis, secondary dispersion halos of the indicator elements of gold are delineated.

For gold-ore deposits of various types, the following combinations of the principal indicator elements of gold are characteristic: 1) antimony, arsenic, copper, silver; 2) bismuth, arsenic, copper, silver; 3) bismuth, copper, silver, etc. The secondary indicators of gold include lead, zinc, molybdenum, and tungsten.

At the same time, semiquantitative spectral analysis of lithogeochemical samples for gold is carried out. To identify secondary dispersion halos of gold, an electrolytic method of analysis is used, which is a certain modification of the absorption method [2]. Let us note the advantages of the electrolytic method:

1. Relatively high sensitivity for production purposes — 0.01 g/t.

2. High productivity — 200 samples per shift — for a crew of 3 people.

For analysis for gold it is not necessary to take an additional sample. From an ordinary sample, 20 g of material is taken. It should be noted that in the course of prospecting work at a scale of 1 : 50000, for 100 km² of territory it is quite sufficient to analyze 1000 samples for gold.

When work is carried out at a scale of 1 : 50000, it is rarely possible to identify individual ore bodies or deposits. Usually ore fields or a number of ore fields are outlined.

According to data from geological surveying and prospecting work at a scale of 1 : 50000, areas are selected for prospecting at a scale of 1 : 10000.

In doing so, the geological structure of the area, its structural position, the features of the placer and the character of the gold, and the dimensions and intensity of secondary dispersion halos of gold and its indicators are taken into account.

Prospecting work at a scale of 1 : 10000 is carried out along previously topographically laid-out profiles. The work program includes prospecting traverses at a scale of 1 : 10000 with the collection of grab samples, the collection of lithogeochemical samples analyzed for gold and a narrow suite of indicator elements, as well as geophysical investigations by magnetic and electrical prospecting methods.

This set of methods makes it possible to compile a preliminary geological map of the area at a scale of 1 : 10000, as well as maps of secondary dispersion halos of gold and its indicators.

Let us note the substantial differences between secondary dispersion halos of indicator elements and of gold:

1. Indicator elements usually form clearly expressed halos. Other conditions being equal, the halos are compact. Conversely, gold halos are less distinct. The epicenters of the halos often do not coincide spatially with the ore bodies. Over one ore body a number of local halos are established that are not spatially connected with one another, etc.

2. At the epicenter, the content of indicator elements is tens of times greater in comparison with the minimum contents in the halos. In gold halos these ratios are much greater and reach many hundreds of times.

The results of prospecting work at a scale of 1 : 10000 serve as the basis for a preliminary assessment of the prospects of individual deposits within ore fields.

Heavy-mineral-concentrate sampling, as indicated above, is of low effectiveness. Therefore it is used in prospecting work at scales of 1 : 50000 and 1 : 10000 as an auxiliary method for solving narrower and more specific tasks: determining the sizes of individual grains of gold-

its roundness, fineness, etc., which makes it possible to judge more confidently the nature of the source and its prospects.

Using the above-mentioned methodology, in recent years a number of gold-ore deposits have been identified in northeastern Transbaikalia. Their spatial position is characteristic within belts of subvolcanic and hypabyssal intrusions of Upper Jurassic age, confined to zones of increased fracturing of latitudinal strike. The placers of the district also originate from these areas, which indicates a direct connection between the gold-ore deposits and valley placers.

Ore fields, deposits, and individual ore bodies are confined to areas of junction between systems of latitudinal faults and disjunctive structures of other directions. In such areas a concentration of bodies of subvolcanic and hypabyssal intrusions is observed.

Among the gold-ore deposits the following types are distinguished:

1. Quartz veins and vein zones.
2. Stockwork zones in potassic metasomatites.

3. Skarn deposits.

4. Quartz veins and vein zones are characterized by lengths of hundreds of meters; more rarely their length is 2.0 km. The thickness of the veins varies widely and averages 1.0 m. Clear and even contacts are more often characteristic of veins. Uneven, patchy contacts are typical of vein zones. The thickness of vein zones is variable and reaches 10 m. The amount of sulfides rarely exceeds the first few percent. In individual ore bodies and deposits the following sulfide associations are established: 1. stibnite, arsenopyrite, pyrite, chalcopyrite; 2. arsenopyrite, pyrite, chalcopyrite; 3. bismuthinite, chalcopyrite, pyrite; 4. pyrite, chalcopyrite.

5. Quartz-pyrite stockworks in metasomatites are probably one of the most promising types of mineralization. Zones of potassic metasomatites develop along rupture faults. Along strike they are traceable for hundreds of meters. Their thickness reaches the first tens of meters. Quartz-pyrite veinlets are superimposed on the metasomatites, and their thickness rarely exceeds the first centimeters.

6. And, finally, skarn deposits. Skarns are formed both in carbonate and in silicate rocks. The skarn type of deposit is the most complex. The deposits are characterized by irregular outlines, discontinuity of the ore bodies, and significant variations in gold contents are observed along strike.

Analysis of the distribution of gold in rocks subjected to hydrothermal metamorphism and within ore bodies makes it possible to distinguish at least three types of gold. The first is associated with pyrite from pyritization zones, i.e., it is associated with hydrothermal metamorphism. Within ore bodies two types of gold are established: that associated with sulfides and the later free gold. Analysis of monomineralic fractions has established that gold is associated with arsenopyrite, pyrite and, possibly, bismuthinite. Such gold has submicroscopic and microscopic sizes. Free gold has a size of 0.1—0.5 mm. Gold-ore deposits are located among rocks that have undergone hydrothermal metamorphism.

Pyritization is most widely developed, encompassing entirely individual zones of increased fracturing. Pyritization zones are traced for tens, more rarely hundreds, of kilometers with a thickness of 5—15 km and simultaneously encompass a number of ore fields.

More local development is shown by silicification, tourmalinization, sericitization, and beresitization. The latter are mapped in the form of extended zones of relatively smaller area. Such zones are confined to

individual deposits or ore bodies.

The extent and thickness of ore bodies and the amount of sulfides depend on the nature of the enclosing rocks. Other things being equal, Precambrian metamorphic rocks are more prone to fracturing. Therefore, in such rocks the principal mode of deposition of vein quartz is metasomatism. Conversely, in magmatic formations fracturing is relatively less intense. Metasomatic processes are less developed. In this connection, the ore bodies are characterized by lesser extent and thickness. Straight, even contacts are more often observed in the ore bodies.

For the proper orientation of prospecting work for ore gold, the relationship between the location of deposits and the relief is important.

The literature gives examples of deposits being located only within the contours of modern gold-bearing valleys. In particular, such observations were made for the South Yenisei district by E. Ya. Sinyugina [3]. In northeastern Transbaikalia most large valleys are also confined to faults. However, the available materials indicate the absence of a dependence between the locations of gold-ore deposits and the relief. Deposits are situated both on divides and their slopes, and directly in valleys.

It should be taken into account that gold-ore deposits are more often confined not to major faults, with which the principal valleys coincide, but to conjugate small fault structures. Therefore, restricting areas of prospecting work in northeastern Transbaikalia only to near-valley sectors would have an adverse effect on the results obtained.

Thus, the principal methods used in prospecting for gold-ore deposits under the conditions of northeastern Transbaikalia are reduced to the following:

1. Delineation of sublatitudinal systems of faults and zones of increased fracturing that control the position of belts of subvolcanic and hypabyssal intrusions, with which gold-ore deposits are paragenetically associated.

2. Study of gold placers in order to establish their structural position, to identify the sources feeding the placer, and to determine their geomorphological position.

3. Application of goldometric sampling in prospecting at scales of 1:50000 and 1:10000, and of the electrolysis method for sample analysis.

REFERENCES

1. Kirilyuk V. P. Geology and conditions of formation of the Precambrian complexes of the southwestern margin of the Aldan–Vitim Shield. Author’s abstract of dissertation for the degree of Candidate of Geological-Mineralogical Sciences, Lvov, 1966.

2. Safronov I. I., Polikarpochkin V. V., Utgof A. A. Spectrogoldometric surveying as a method of prospecting for gold-ore deposits not accompanied by mechanical halos (placers). Ministry of Geology and Protection of Mineral Resources of the USSR, L., 1957.

3. Sinyugina E. Ya. On the question of the connection of alluvial placers with primary sources (on the example of the South Yenisei gold-bearing district). In: Geology of Placers, Izd. “Nauka,” 1965.