Nature of ore-forming fluids of the Dal’negorsk deposit: Isotopic and geochemical parameters of the altered host rocks

Abstract

Oxygen isotopic composition was studied in the altered host rocks of the Dal'negorsk borosilicate deposit in order to establish a boron source and the origin of ore forming fluids responsible for deposition of economic borosilicate ore. The relationships between oxygen isotopic composition and geochemistry of the altered igneous rocks occupying various structural and temporal positions in the ore zone were studied, including premineral high potassium minor intrusions located in the zones of datolite mineralization; alkali basalt, gabbro, and breccia from the sedimentary framework of the deposit; and postmineral basaltic andesite, basalt, and dolerite dikes. It was suggested that interaction of aqueous fluid with host rocks brought about not only variation in oxygen isotopic composition but also shifts in geochemistry of these rocks, especially as con cerns the chemical elements contained in ore forming fluid. The disturbance of oxygen isotopic system is typical of all studied rocks: δ 18 O values sharply decrease indicating interaction with aqueous fluid at elevated temperatures. The lowest δ 18 O (from -2.9 to +0.1‰) is characteristic of the premineral high potassium and ultrapotassium minor intrusions from skarn-datolite zone. Igneous rocks from the sedimentary framework of the deposit have δ 18 O of +2 to -0.9‰. The δ 18 O of postmineral basaltic andesite, basalt, and dolerite dikes varies from 0 to +7‰ with increasing distance from the ore zone. The oxygen isotopic composition of aque ous fluid evidences its exogenic origin. The geochemical and isotopic characteristics of ore forming fluid show that it could have been deep seated subsurface water similar to the contemporary water of the Alpine fold zone, which contain up to 700-1000 mg/l B and is distinguished by high K, Li, Rb, Cs contents and high K/Na ratio. Similar geochemistry is characteristic of the fluid inclusions in quartz from ore zones. It cannot be ruled out that continental evaporites were a source of boron as well. The relationships between δ 18 O, KAr age, and geochemical parameters of premineral and postmineral altered intrusive bodies allow us to sug gest that the subsurface B bearing water discharged through narrow channels controlled by premineral basal tic bodies. The discharge was probably initiated by emplacement of basalt and dolerite dikes.

Isotope and geochemical evidence for the influence of ore-bearing solutions on basic dykes of the Dal’negorskii ore region

“…The early stage included the formation of wollastonite-grossular skarns controlled by hydrotherms of the distant magmatic chamber [8], during which danburite mineralization appeared. The next stage of the skarn formation and intrusion of basic and subalkaline dykes included the formation of rich datolite mineralization, which, by our data [1], was controlled by solutions of meteoric origin.…”

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342Altered basalts, alkaline basalts, and gabbro from dykes and fracture bodies intruded before the forma tion of rich datolite ores on the Dal'negorskoe skarn borosilicate deposit located in the center of the ore region of the same name in Primor'e are characterized by extremely low δ 18 О values (from -2.9 to +0.1‰) [1]. Such an oxygen isotope composition is not observed in all types of nonaltered magmatic rocks [2] and, as a rule, is controlled by the interaction of rock during cooling with isotopically light aqueous fluid.

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“…At the stage of the datolite ore formation, the ore bearing fluid was exclusively meteogenic as well (δ 18 О from -2.5 to -5.5‰), and the performed estimation of the chemical composition of ore bearing fluid dem onstrated that it was characterized by high concentra tions of K and Ba, a low K/Na ratio, and other specific features [1,5]. The high temperature interaction of pre ore subalkaline and alkaline basalts with such fluid resulted in an increase in the K and Ba concentrations, change of K/Na ratio in rocks, and violation of the KAr isotope system, which was reflected in their age dat ings [1].The revealed meteogenic nature of ore bearing solutions, which are active at the stage of the forma tion of rich mineralization, distinguishes the Dal'negorskoe skarn borosilicate deposit from most orogenic deposits in this region, for which mixing of fluids of different natures is often established, includ ing participation of meteoric waters; however, the magmatogene component in the ore forming fluid is usually clearly registered [6,7].We studied the oxygen isotope parameters of basic rocks from pre ore and postore dykes located not only within the borosilicate deposit, but on the flanges of the ore region as well in order to establish the scales of abundance of waters of meteoric origin during the for mation of the Dal'negorskii ore region and, if possible, estimate the age limits of hydrothermal system activity. Similarly to altered pre ore rocks occurring directly in the skarn borosilicate ore zone [1], the data obtained for rocks of the whole studied area were compared with the concentration of minor elements and K-Ar age datings.…”

“…At the stage of the datolite ore formation, the ore bearing fluid was exclusively meteogenic as well (δ 18 О from -2.5 to -5.5‰), and the performed estimation of the chemical composition of ore bearing fluid dem onstrated that it was characterized by high concentra tions of K and Ba, a low K/Na ratio, and other specific features [1,5]. The high temperature interaction of pre ore subalkaline and alkaline basalts with such fluid resulted in an increase in the K and Ba concentrations, change of K/Na ratio in rocks, and violation of the KAr isotope system, which was reflected in their age dat ings [1].…”

“…Similarly to altered pre ore rocks occurring directly in the skarn borosilicate ore zone [1], the data obtained for rocks of the whole studied area were compared with the concentration of minor elements and K-Ar age datings.…”