Petrochemical features of the dike complex of the Vorontsovskoye gold-ore deposit (Northern Urals)

Relevance of the work. The Vorontsovskoye deposit has some peculiarities of the Carlin-type deposit and is one of the largest gold fields in the Urals. However, nowadays, it is almost worked out; similar objects in the region have not been identified. The dike complex of the stockwork type was not previously studied at the deposit. The undertaken research can help clarify both the model of formation of ore mineralization and the predictive model. Purpose of the work. Study of the composition and petrochemical features of the dike complex of the Vorontsovskoye deposit as one of the possible factors for the formation of gold-ore mineralization. Methods of research. The measurements of the occurrence of dikes within the current open-pit with the results on the azimuth grid were carried out. A silicate dike sample assay was performed, and available past data was also used. The lithologic study of dikes was accompanied by the study of samples using the JSM-6390LV scanning electron microscope (JEOL) with the IncaEnergy-450 energy-dispersive spectrometer and the CamecaSX100 electron probe microanalyzer. Results of the work. At the first stage of the research, the structural position of dikes was specified; results of lithologic and petrochemical studies and data obtained by microanalyzers were summarized. A significant prevalence of the main dikes with various compositions from picrobasalt to gabbrodiorite (andesibasalt) has been identified. A group of dikes with increased and high alkalinity (including lamprophyres) with compositions from moderately alkaline and alkaline picrobasalt to monzodiorite (trachyandesibasalt) has been identified as well. Occurrences of kalifeldspath metasomatism with the mineral association of chlorite-quartzpotassium feldspar-pyrite (±galena) were recorded in dikes of complete line. Earlier, areas of kalifeldsparization were observed in tuff siltstones. Conclusions. Availability of dike stockwork can be considered as a positive structural factor in the formation of the deposit. Its role as a magmatic factor is unclear; however, it can be assumed that occurrences of kalifeldsparization are allied to dykes of increased alkalinity.

There are no obvious patterns in the distribution of dikes belonging to different groups by chemical composition, which may be due to a lack of data.
Data on the chemical composition of dikes are given in Table 1 (the analyses were performed in the laboratory of FHMI IGG of RAS, analyst is N. P. Gorbunov). Petrographic studies showed that all dikes were altered to some extent by the imposed metamorphic and metasomatic processes. The following Table 1 excludes full metasomatites, intensely sulfidized differences and analysis with values of the indicator "P.p.p. " ≥ 7.  1 -dikes of basic composition (normal alkaline condition); 2 -dikes of medium composition (normal alkaline condition); 3 -dikes of increased alkaline condition; 4 -"neighboring composition" dikes (moderately alkaline / normal); 5 -no available data for chemical composition (according to TAS-diagram, Fig. 3). Рисунок 2. Диаграмма распределения элементов залегания даек Воронцовского месторождения на азимутальной сетке. 1дайки основного состава; 2 -дайки среднего состава; 3 -дайки повышенной щелочности; 4 -дайки «пограничного состава» (умереннощелочные/нормальные); 5 -нет данных по химическому составу. По TAS-диаграмме, рис. 2.  In the classification TAS-diagram (Fig. 3), the dikes of the Vorontsovskoye deposit fall mainly in the area of the main rocks, and only some of them -in the area of the middle rocks. It is noteworthy that there are not only quartz diorites among the dikes but also diorites typical of the area as a whole. In addition, the presence of two trends -normal alkalinity condition and hyperalkalinity -is clearly visible, and in the latter, there are dikes corresponding in composition to ultrabasic formations -picrobasalts.
According to the petrographic characteristics, several main types of dikes are distinguished (the corresponding numbers are given in parentheses for Table 1): 1. Porphyric pyroxene-plagioclase dikes (from essentially pyroxene to essentially plagioclase ones) of basalt, less often andesibasalt composition (Nos. 1,2,3,5,7,8,13,19,20 It should be noted that moderate alkaline and alkalized differences (with the exception of lamprophyres and relatively coarsegrained gabbroids) are quite difficult to identify in conventional petrographic studies; and the presence of potassium feldspar is mainly determined by using more detailed physical methods of research -a scanning electron microscope (SEM). Micrographs of individual species of dikes are shown in Fig. 4, 5.
The relationship of dikes of different composition within the field is still unclear, although there are separate observations, for example, a distinct intersection (pyroxene) -plagioclase porphyry dikes of the average composition of the dike of corniferous lamprophyre of the spessartite type (Fig. 5).
According to the results of studies of polished thin sections carried out using the JSM-6390LV scanning electron microscope (JEOL) with the IncaEnergy-450 energy-dispersive spectrometer and the CamecaSX100 electron probe microanalyzer, the composition was specified of disseminated minerals in the most common porphyritic dikes -pyroxene-plagioclase of basic composition and amphibole-plagioclase of medium composition (studies were conducted on previously selected samples). It is determined that plagioclase of disseminated minerals in dikes of both types is represented by labradorite with small differences in the content of the anorthite minal. The compositions of pyroxene correspond to augite, and amphibole -hornblende ( Table 2).    In addition, in the detailed studies of polished thin sections in dikes similar in composition (samples 4/17, 6/17), occurrences of quartz-kalifeldspath metasomatism were found. They are small plots and irregularly shaped patches composed by the chlorite-quartz-potassium feldspar-pyrite association (Fig. 6, b). Titanite (sphene) usually occurs, and galenite is sometimes intruded in pyrite. Potassium feldspar is characterized by the admixture of a small amount of barium (Ba, 0.32-0.53% w.). In the same thin sections, potassium and sodium-potassium feldspar were observed in the form of uneven discontinuous margins (with intimate intergrowth) at the edges of disseminated plagioclase (Fig. 6, a) and, in some cases, in the form of patches (antiperthites?) in the inner parts of large disseminated minerals. It can be assumed that this potassium feldspar is "petrean" or that associated with some previous process of kalifeldsparization.
In the process of exploratory development in 2009-2010, occurrences of intense kalifeldsparization in tuff siltstones were identified at the Vorontsovskoye deposit within one of the ore blocks opened by the existing open-cut mining [6]. Optically, these kalifeldsparized sites did not stand out and were detected by abnormally high potassium levels (up to 7-10% of weight according to ICP-MS). Potassium feldspar in metasomatites is represented by a fine-grained aggregate (20-50 µm, rarely up to 200 µm) in association with quartz. According to the results of microprobe determinations and X-ray diffraction analysis of rocks, it can be assumed that these are orthoclase and/or sanidine. It should also be noted that earlier V. N. Sazonov with his coauthors [3] identified propylitized rocks in the area of the deposit with ice spar replacing plagioclase.

Discussion of results and conclusions
The importance of dikes in the formation of the Vorontsovskoye gold-ore deposit remains debated. Typically, the "dense" dike ore fold is observed only within the deposit, which confirms the important role of dikes as a factor of structural control [4]. The dike complexes of most Carlin-type deposits of the state of Nevada (USA) [4,7,[8][9][10] and separate objects within the territory of the PRC [11][12][13] have the same importance.