ИСТИНА

The published data on the physicochemical parameters and chemical composition of the hydrothermal mineralizing fluids of gold deposits were collected and analyzed in terms of the geological history of the Earth. The sample included 5571 definitions of parameters for 360 gold deposits of various ages, from different regions of the Earth and the various genetic types: orogenic, shear zone related, epithermal, porphyry, “carlin”, intrusion-related, IOGS and others. Mean values of parameters and ranges of variation are shown in Table. The general trends in the changes of physicochemical parameters and the chemical composition of the mineral-forming fluids in the Earth history are revealed. They are partly associated with increasing of the erosion level of hydrothermal systems with increasing of their age, which is consistent with the conclusions of the work [1]. The point is primarily about the increasing of the average quantities of mineral-forming fluid pressure from the Cenozoic deposits to the Proterozoic ones, the absence of low-pressure fluid in the Precambrian deposits and the prevalence of weakly mineralized fluids in young fields. Table 1. The average physicochemical parameters of the gold-bearing ore-forming fluids of various geological ages and the range of their of variations (in brackets). Age, Ma Temperature, ºC Pressure, bar Salinity wt. %-eqv. NaCl Part of low-salinity fluids (<5%), % n 0-65 Cenozoic 257 (50-610) 979 (20-3600) 8.5 (0.01-59.0) 62.4 1233 65-235 Mesozoic 281 (80-957) 1079 (30-3700) 14.2 (0.1-82.0) 31.3 2158 235-540 Paleozoic 262 (48-600) 1278 (4-3500) 10.1 (0.1-66.8) 38.7 895 540-2500 Proterozoic 241 (48-615) 1993 (120-6500) 14.6 (0.1-68.0) 29.7 943 2500-3200 Archean 250 (50-570) 1753 (330-6400) 10.3 (0.1-63.0) 37.9 311 Footnote: n - the number of definitions. However, not all the differences revealed can be explained by an increase in the formation depth of the deposit erosion levels. The highest fluid pressures, the maximal gas saturation and nitrogen enrichment of ore-forming fluids of the Proterozoic, compared with the older Archean ones are associated with the specificity of the endogenous fluid regime of this era. The gas component of the Archean gold deposits is clearly enriched with methane and the Proterozoic one – with nitrogen. Possibly nitrogen entered the atmosphere from the deep layers of the Earth as part of the endogenous fluids. This is consistent with the data of the carbon isotope studies of fluid inclusions in quartz of Archean and Proterozoic African gold deposits. These studies have shown the presence of different geochemical reservoirs of carbon dioxide, from which it enters the Archean and Proterozoic ore-forming systems of African gold deposits [2]. The revealed regularities of the changes of the chemical composition and the physicochemical parameters of the gold-bearing ore-forming fluids in the Earth history require further more detailed study, involving significantly larger data set for understanding the evolution of endogenous fluids in time. References: [1] Kesler S.E., Wilkinson B.H. (2006) Econ. Geol. V. 101. P. 919-922. [2] Lüders V., Klemd R., Oberthür T., Plessen B. (2015) Miner. Dep. 50. P. 449–454.