Black Shale Series-hosted Silver-Vanadium Deposits of the Upper Sudan Doushantuo Formation, Western Hubei Province, China
The lower and the upper parts of the Doushantuo Formation, Hubei Province, China, contain silicate-type K-rich beds (illite and feldspar), and intraclastic phosphorites, as well as Ag-V and V deposits in a "Black Shale Series". The K-rich beds formed in an anoxic basin whereas the phosphorites formed in a more oxic and higher energy depositional environment. The Black Shale Series occurs in the lower and upper parts of the Doushantuo Formation; that in the upper part consists of black illite shale and dark gray dolostone, and contains Se-rich Ag and V deposits. These deposits are of more industrial significance than the previously discovered Ni-Mo polyelement sul-fide deposits which occur in the Lower Cambrian Black Shales Series of South China. Studies of lithotype, sedimentary sequence, and geochemistry of the Doushantuo Formation have led to more attention being focused on the uppermost Black Shale Series and its Ag-V and V deposits. There are three ore types: black, Ag-V-rich, illite shale; black, V-rich, illite shale; and dark gray, Ag-V-rich, finely crystalline dolostone. The first two ore types contain 3% to 5% C^ and the last, 1.25%. The Ag-bearing minerals are naumannite, argyrodite, aguilarite, acanthite, and native silver that occur as inclusions in pyrite associated with barite. Vanadium substitutes for Al in illite and is also related to organic-matter contents. Because (1) the pre-Sinian Kongling Group contains one to three times greater than the Clark Ag value; (2) black, illite shale has a negative Ce anomaly; and (3) there are large occurrences of black, K-rich, tuffaceous, fine siltstone, the sources of Ag are thought to include subaerial and submarine volcanic materials. It is proposed that Ag, V, and Se were absorbed by clay particles and organic matter during syngenetic deposition in an anoxic environment; Ag subsequently was reconcentrated into sulfides and selenides, whereas V was incorporated structurally into illite during early and late diagenesis. These types of Black Shale Series-hosted Ag-V and V deposits are, therefore, considered to be sedimentary-diagenetic in origin.