Vladimir V. Knauf, NATI Research JSC, St.Petersburg

Introduction

The complex of geochemical investigations directed on revealing of gold deposits, frequently includes an establishment of dispersion haloes of this element in friable (unconsolidated) deposits covering bedrocks. For territory of north-western Russia and Finland the glacial (till) friable or weakly lithificated deposits are most typical.

The specific feature of till deposits is the presence of an essential share of fine fraction material with the size of particles less than 50 µm and concentration of gold on a clarke level.

The experience of the search geochemical investigations in Finland on till deposit shows that the background concentrations of gold in thin fractions are 0.1-10ppb and the concentrations equal to n*10ppb can be considered as abnormally high.

The importance of an establishment of concentrations of gold in till deposits is obvious. However this information appears to be completely insufficient to understand what type of gold deposit was weathered out or to appraise the importance of the received data for a choice of direction and territory for realization of the further searches on bedrocks. The primary deposits of gold of a various types and age are well investigated in Finland. Therefore express reception of mineralogical information for gold in till samples can simplify and speed up the search investigations.

To receive the missing information it is necessary in addition to definitions of concentrations of gold to carry out the mineralogical work for an establishment of the phase forms of an element and mineral paragenesises. It is obvious, that the elucidation of the phase forms and paragenesises at gold concentration in sample equal to n*10ppb is not a trivial problem. It requires the application of the appropriate technique of mineralogical work.

Using "ppm-mineralogy" techniques we studied 10 till samples on a class of < 50 µm. The purpose of the work is to establish concentration of gold in samples and to fulfill mineralogical proving of concentrations.

The first stage of work was the definition of concentrations of Au, Pd, Pt in gravitational concentrates (GC) with a weight equal to 0.5-2.0g, which were received from the parts of initial samples, with a weight of 200g. Such subsample weight (with the account of the FAAS detection limit) provided a gold detection limit in a sample - 1.2ppb. The data on till samples and results of FAAS are given in the table.

As follows from the table, in three samples the concentration of gold turned out to be lower than the detection limit for the sample with a 200g weight. Therefore for mineralogical proving of the obtained concentrations in all samples it was required to increase the weight of samples intended for mineralogical work. Weight of a subsample for mineralogical work was increased up to 300g.

The microprobe study GC has shown, that in each sample there are grains of gold. The measurement of the area of grains and subsequent estimation of volume of grains allows evaluating of the gold concentration in each sample taking into account the density of a mineral (Au-Ag alloys). In the table the concentration of gold obtained by traditional chemistry-spectral (FAAS) and mineralogical methods are given.

The distinctions in concentrations for some samples appear to be significant (three times), however it is necessary to keep in a mind that mineralogical measurements (measurements namely, but not accidental finds of grains!) are made on phase equivalents of concentrations, equal to 2*10^-10(0.2ppb). Anyway, we don’t remember any publications, in which mineralogical work with such sensitivity was carried out.

The results of microprobe study have shown, that some of samples are similar in their mineralogical features, and some have the differences, and the greatest distinctions are established on structure of grains of gold, on mineral paragenesises of gold and on a set of minerals identified in GC. The mineralogical data on some samples are given in the next section.

Minerals. Back Scattered Electron Microscope Images

Sample 1A, Au=8ppb
1	2	3	4		5
6	7	8		Gold and associated minerals: ZR-zircon, SHL-sheelite, CS-cassiterite, Bi-native bismuth.

Sample 2A, Au=0.3ppb
1	2	3		4		5
6	7		Gold and associated minerals: HD-hedleyite (Bi7Te3), JS-b-joseite-b (Bi4Te2S), Bi-native bismuth, CB-cobaltite, MR-marcasite, MGT-magnetite, IL-ilmenite, Ap-apatite, SPH-titanite.

Sample 3A, Au=0.2ppb
1	2		Au-Ag alloy and electrum.

Sample 4A, Au=1ppb
1	2	3		Gold and associated minerals: Ba(SO4)-barite, CP-chalcopyrite.

Sample 5A, Au=3ppb
1	2	3	4	5
Gold and associated minerals: ASB-aurostibite, AL-altaite, HL-chlorite, CP-chalcopyrite, FRB-frohbergite, GN-galena.

Discussion

The given data show, that despite the low contents of gold in till samples (0.2-15 ppb), these concentrations are founded out reliably, if rather large amounts of samples are used.

Extraction of gravitational concentrates from samples and subsequent definition of gold concentrations in gravitational concentrates using traditional methods of analytical chemistry provide the reception of reliable information for the investigation of geochemical haloes, during prospecting of gold and other noble metals.

The definition of a mineral form of gold in till samples allows to solve a number of scientific and prospecting problems: to carry out the control of results of the chemical analysis of samples, to make the mineralogical proving of concentrations, to establish the structure and morphology of grains of gold, to reveal mineral paragenesises of gold by the analysis of gold aggregates with other minerals.

The comparison of obtained data on mineral paragenesises, structure and morphology of grains in till samples with the data on primary deposits of gold in the area being studied allows at a search (prospecting) stage already to predict a genetic type of a primary source of gold, potential reserve and to specify the prospects of search investigations.