The Dal'negorsk borosilicate skarn deposit ( 44° 34 ′ N and 135° 37 ′ E), located in the center of the ore field bearing the same name, is referred to the category of giant deposits. The currently predominant genetic concept assumes that ore mineralization at this deposit is related to a mantle source and that boron and orebearing alkaline fluids are derivatives of a juvenile source as well. The alternative model considered in this paper suggests that sedimentary sequences, probably, evaporites of a local basin, were immediate boron sources and hot subsurface water served as an agent of ore deposition. The authors' conclusions are based on (1) mineralogical and geochemical features of alteration of premineral dikes as evidence for the composition of percolating ore-bearing fluids, (2) results of fluid inclusion study, and (3) boron and oxygen isotopic compositions of datolite. The latite bodies immediately predating deposition of economic datolite ore do not show mineralogical or geochemical attributes of their belonging to alkaline rock series. According to our data, these bodies are composed of Paleogene premineral basalts that intruded into the future borosilicate deposit close to the central channel of ore-bearing fluid, served as fluid conduits, and were altered to ultrapotassic rocks under the effect of such fluid. It is suggested that hot aqueous ore-bearing fluid was enriched in highly soluble compounds of Ba, K, and B and extremely depleted in poorly soluble compounds of Zr, Nb, Ta, La, and Ce. This suggestion does not contradict the properties and composition of primary and pseudosecondary two-phase fluid inclusions in danburite, datolite, quartz, and fluorite from orebodies. Judging from the boron isotopic composition of datolite ( δ 11 B = -9 to -31 ‰), the main amount of boron was extracted from metasedimentary rocks of the Mesozoic basement. The oxygen isotopic composition of datolite from the Dal'negorsk deposit ( δ 18 O SMOW = -1.64 to -2.97 and less frequently up to -5 ‰) indicates a significant contribution of subsurface water to the transport of boron. A model of multistage accumulation of boron in ore of the Dal'negorsk borosilicate skarn deposit is proposed.

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.

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