The Dal’negorsk borosilicate skarn deposit, primorye, Russia: Composition of ore-bearing solutions and boron sources

Abstract

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.

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

“…On the basis of these and other data, it was sug gested that premineral high potassium minor intru sions are closer to the dike complex of the Late Creta ceous-Paleogene basalts rather than to the late phase of the alkaline magmatic complex (Baskina et al, 2009). The dikes located in the center of the deposit, in the channel of fluid activity, could have been con duits for ore bearing solutions, because they are altered up to the high potassium rocks classified as latites.…”

“…Irrespective of morphology and volume of rock, orthoclase has con stant Na 2 O content (0.3-0.9 wt %) and high Ba con tent (2000-7000 ppm). Geochemical, mineralogical, and petrological study of labites led to the conclusion that the alteration of them occurred at the stage of ore formation under effect of hot aqueous solutions enriched in Ba, K, B, Rb and depleted in high charged Zr, Nb, Ta, and REE (Baskina et al, 2009). To avoid terminological confusion, these rocks will be referred to as high potassium premineral minor intrusions.…”

“…The rocks are unlike alkali basalts in mineralogy and geochemistry (Baskina et al, 2009); they are devoid of olivine, phl ogopite, kersutite, and accessory minerals typical of potassium alkali rocks. We failed to find earlier described orthoclase, leucite or epileucite phenoc rysts.…”

“…Notes: * Calculated after (Zheng, 1993a(Zheng, , 1993b The oxygen isotopic composition of quartz coexist ing with datolite was determined in two samples with maximal and minimal δ 18 О values of datolite (Table 2). Sample B 100 was used as a standard in the fluid inclusion study (Baskina et al, 2009) and sample B M is a typical coarse crystalline drusy quartz-datolite aggregate (Fig. 2).…”

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

“…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].…”