HYDROTHERMAL-ALTERED ZONING, FLUID CONDITIONS, AND TYPES OF GOLD MINERALIZATION WITHIN THE ELENA AND EMY DEPOSITS OF THE EPITHERMAL SVETLOE ORE DISTRICT (KHABAROVSK TERRITORY)

Abstract

The relevance of the study is determined by obtaining new data concerning mineral composition, textural and structural features, hydrothermal-metasomatic zoning, fluid regime, and types of gold mineralization of two most contrasting areas of the Svetloe epithermal district (Khabarovsk Territory), formed in different-aged parent rocks of the Late Cretaceous age (K2) of the Khetanian Formation basic composition in the Cognac-Santonian Stage (d. Emmy) and discordant bedding felsic rocks of the Urak Formation of the Campanian-Maastrichian Stage (d. Elena). The selected objects are characterized by different levels of zone erosion and most fully reflect the diversity of altered rocks of the Svetloe epithermal district. Altered rock represented by weakly altered rocks of the Khetanian Formation within the Emmy deposit and strongly altered, including hypergene processes, acidic rocks prevailing on the territory of the Elena deposit. The data obtained play an important role in establishing the type of epithermal deposits, forecasting mineralization at depth, and can also be used to identify search criteria and features of similar objects both in the Okhotsk-Chukotka volcanic belt and beyond. The aim of this work is to study the hydrothermal-metasomatic zoning, material composition, textural and structural features, fluid regime, as well as the forms of gold occurrence in the Svetloe epithermal ore district using the Emmy and Elena deposits as example. The objects of the study are ores, metasomatites and weakly altered parental rocks in the form of core, ore and chipped samples taken as a result of research work from eight wells and two operating open pits. The rocks were studied by optical and electron microscopy methods using energy dispersive spectroscopy, as well as by X-ray diffraction analysis, microthermometry, and Raman spectrometry. As a result of the studies, it has been established that the altered zoning of the Svetloe epithermal district includes an axial zone of residual quartz with various texture and structural features – mosaic, feathered, collomorphic, and crustifying structure and a halo of interbedded linearly elongated zones of altered rocks composed of alunite-and dickite-quartz zone, which are replaced at more distance from the axial zone by sericite and illite-chlorite zones of the propylite formation, with a lesser degree of elaboration of volcanic and volcanic-clastic rocks of the acid composition of the Urak Formation, the ore-containing area of the Elena, and of intermediate and basic composition including the Emmy deposit. The main part of mineralization is associated with the residual quartz zone. The morphology of ore bodies is represented by sub-layer linear keeled deposits with mushroom-like swells in paleo-grooves up to 110 m. The average thickness of ore bodies varies from 12 to 60 m, up to 700 m long, and 100...240 m wide. The fall of the ore zones is gentle, steep near the paleo-grooves with uneven or very uneven distribution of the valuable component. The average grade of gold in ores is 2,5 g/t. The composition of hypogene sulfide mineralization is represented by sulfides of polymetals Cu, Pb, Zn, Fe in the mineral form of pyrite, galena, sphalerite, chalcopyrite, and tennantite-tetrahedrite. Within the ore bodies of the Emmy deposit, the mineral association of sulfides of the hydrothermal stage is more than 5 vol. % and distinguished by the diversity and abundance of tellurides. Gold mineralization is represented by two types: Au-Ag and Au-Ag-telluride. Au-Ag ores are represented by primary ores with an average fineness of ~900 ‰ developed within the Emmy deposit and secondary (oxidized) ores with a fineness of up to 1000 ‰ prevailing at the Elena deposit. According to the results of the study of vapor-liquid inclusions in quartz, the hydrothermal-ore process at the Emmy deposit proceeded at 260...330 °C. Microcryothermometric experiments of the Elena deposit indicate the 200...240 °С range of the hydrothermal process. In terms of the material composition of altered rocks, hypogene sulfide association, and fluid regime, the Svetloe epithermal district belongs to the sulfate-acid and medium-acid (HS-IS) type, showing the prospect of detecting porphyry mineralization at depth.

Keywords

• hydrothermal-altered zoning
• epithermal gold deposits
• textural and structural features
• fluid conditions
• svetloe epithermal ore district
• okhotsk-chukotka volcanic belt