Geology, geochemistry and genesis of the Cretaceous and Paleocene sandstone- and conglomerate-hosted Uragen Zn–Pb deposit, Xinjiang, China: A review

Abstract

The Uragen Zn–Pb deposit in Wuqia County, Xinjiang, China, is hosted in sandstones and conglomerates of the Lower Cretaceous Kezilesu Group and argillaceous dolomite of the Paleocene Aertashi Formation in the Kashi sag, northwestern Tarim basin. The deposit is characterized mainly by low grade (Zn+Pb~3wt.%) and large tonnage (~3Mt metals in the southern ore zone, potentially 10Mt metals for the whole deposit), with an average Zn/Pb ratio of about 5.8. The orebodies are tabular and stratiform, mainly distributed in a syncline. The ores are characterized by disseminated sulfides with minor massive, veinlet, patchy structures, and fine-grained, locally colloform and framboidal, textures. The main primary metallic minerals are sphalerite, galena, pyrite and marcasite, and minor supergene minerals include smithsonite, hydrozincite, anglesite, calamine, jarosite, and limonite. The ore minerals mainly occur as replacement of cements, matrix and framework grains of the host rocks and as open space filling to a lesser extent. Based on the fact that orebodies straddle across a hiatus between the lower Cretaceous and Paleocene host rocks and on the replacement and open-space filling textures and structures, the mineralization is interpreted to be epigenetic. The tightly clustered Pb isotopic values of the main-stage ore minerals (206Pb/204Pb ranging from 18.608 to 18.663, 207Pb/204Pb from 15.619 to 15.669, 208Pb/204Pb from 38.677 to 38.839) suggest that a single source supplied the metals, whereas the wide range of S isotopes (δ34S from −27.9‰ to 14.6‰ VCDT) indicates multiple sources of reduced sulfur including those from thermal sulfate reduction (TSR) and bacterial sulfate reduction (BSR). The organic matter in the ores is characterized by relatively low concentrations of saturated hydrocarbons, high concentrations of aromatic hydrocarbons, asphaltenes and non-hydrocarbons, and low maturity compared to that in the host rocks and equivalent strata away from mineralization. Both hydrocarbons in the ores and host rocks show little biodegradation. These characteristics of organic matter suggest that the site of mineralization was buried to significant depths and heated to above 80°C before the charging of hydrocarbons (thus non-development of biodegradation), that some of the hydrocarbons were consumed in mineralization (thus decreased maturity), probably used as reducing agents for sulfate reduction in situ, and that bacteria-derived sulfur in the ores was not generated in situ. Mineralization probably resulted from mixing of a metal-rich fluid with multiple fluids carrying reduced sulfur from different sources (including some reduced sulfur from in situ TSR). However, it remains unclear whether the metal-carrying fluid was derived from a near-surface groundwater system and flowed downward to the site of mineralization at burial conditions, or was derived from deeper parts of the basin and flowed up to the site of mineralization as part of a hydrothermal system; the distinction of the two models requires a better understanding of the thermal profile across the deposit.