New data on the gold-antigorite association of the Urals

Abstract

Many placers of the Urals spatially associated withmassifs of serpentinized ultramafic rocks contain gold–magnetite associations [7]. The bedrock analogs of thisassociation are rare, small, and interpreted as deriva-tives of granitoid magmatism. Gold–magnetite occur-rences in the Kagan alpinotype ultramafic massif in theSouth Urals, for example, served as a basis for distin-guishing the Au-productive serpentinite (antigorite)metasomatic association [9]. In addition to the spatialassociation of gold mineralization with antigorite ser-pentinites, the existence of this association is substanti-ated by the fact of significant redistribution and mobili-zation of Au during the development of antigorite,which allowed us to suggest that gold mineralization inthe antigorite serpentinites was formed during crustalregional or local metamorphism of the ultramafic mas-sifs under crustal conditions [6]. The present study ofgold–magnetite ores and altered rocks of the Kaganmassif revealed mineralogical and isotopic–geochemi-cal evidence for the metamorphic origin of mineraliza-tion. In this model, fluid and ore components (Fe, Cu,and Au) were mobilized from ultrabasic rocks.The Kagan Massif is located in the Vishnevogorsk–Il’menogorsk metamorphic complex in the SouthUrals. Lenslike ultrabasic bodies of the complex arecontrolled by deep-seated faults and traditionallyascribed to the Riphean riftogenic ophiolites, whichunderwent Late Cambrian regional zoned metamor-phism and Early Paleozoic siliceous metasomatism [2].According to these concepts, the ultramafic rockstogether with host volcanosedimentary rocks weretransformed with decreasing temperature into olivine–enstatite, talc–olivine, olivine–antigorite, and antigoriteserpentinites during high-grade metamorphism, whilelater metasomatism was responsible for the formationof enstatite, anthophyllite, and talc–carbonate rocks.According to Varlakov, rocks of the Kagan Massifunderwent zoned metamorphism. Its southern part iscomposed of talc–olivine and olivine–antigorite rocks,while the northern part consists of antigorite serpen-tinites with relicts of olivine–antigorite rocks (Fig. 1).The silicic metasomatism is weak and manifested in thedevelopment of anthophyllite in talc–olivine rocks andthe development of talc in olivine–antigorite rocks andantigorite serpentinites. The occurrences of massiveand vein–schlieren magnetite ores containing up to 2–3% sulfides are confined to the tectonic zone extendingover 2 km along the eastern contact of the northern partof the massif. Magnetite lenses up to 5–6 m long and0.2 m thick form chains rapidly pinching out along thetectonic zone. The ores are contained in reticulate ser-pentinite with abundant fine magnetite, which empha-sizes the affiliation of serpentine to chrysotile. In theimmediate contact with ore, the reticulate serpentinitecontains spots and veinlets of antigorite with large mag-netite, talc, chlorite, and amphibole.The gold–magnetite ores were developed by twosmall mines in the mid 20th century. Gold assayaccounts for 0.2–1.2 g/t, sharply increasing in the areaswith visible gold particles. The chemical–spectral anal-ysis of individual samples showed that the magnetiteores contain Au (up to 160 mg/t), Pd (up to 770 mg/t),and Pt (up to 20 mg/t) [5]. The contents of Rh, Ir, Os,and Ru are less than 10–20 mg/t.Magnetite of massive and disseminated ores is char-acterized by a ubiquitous admixture of Mg (1.0–2.4 wt % MgO). Magnetite from the wall rocks, inaddition, contains 0.4–2.0 wt % Cr