Secondary quartzites and their relation to gold mineralization at the Svetloe deposit, Ul'ya Trough, Okhotsk-Chukotka volcanic belt

Hydrothermally altered rocks and their relation to gold mineralization at the Svetloe deposit in the Ul'ya Trough, western sector of the Okhotsk-Chukotka volcanic belt, are studied. The rock composition and style of mineralization are typical of acid-sulfate type deposits. The hydrothermally altered rocks of the deposit are medium-temperature secondary quartzites with developed alunite facies. The upper structural level in the massif of secondary quartzites is made up of a sheetlike body 30 km(2) in area and up to 300 m thick with gentle (5-10A degrees) periclinal bedding relative to the arcuate axis of the volcanic range. Below the sheet, there are a number of local steep mostly carinate zones and small (a few hundreds of square meters) multilayered trough-shaped structures, the foot of which is located 100-150 m under the secondary quartzite sheet. All structures of the lower level are restricted to the volcanic range pericline, marked by elevated thicknesses of monoquartzites, and have a gradual transition to the sheetlike part of the massif. The massif of secondary quartzites shows symmetrical zoning. The central zone of the sheetlike body is composed of alunite quartzites framed by dickite varieties. Down the section, the latter grade into hydromica and hydromica-montmorillonite argillic rocks. Monoquartzites form axial zones in the structures of the lower level. In the blanketlike part of the deposit, monoquartzites occur as rare thin lenses and interlayers spatially related to local structures of the lower level. Gold mineralization at the deposit associates with porous monoquartzites. Gold precipitated after quartzite formation during infilling of pores and cavities with colloform silica with barite, and hypogene jarosite.