Control of Interior Sources on Time-Bound Characteristics of Mineralization in Southwest Yangtze Massif

Strata-bound ore deposits of Au, Ag, Hg, Sb, As, Sn, Pb and Zn in southwest Yangtze massif are concentrated in Cambrian, Devonian, Upper Permian to Lower Triassic and rarely in other horizons, showing a strong time-bound feature. The metallogenic elements are highly enriched in these host strata and limited in other horizons. Isotope researches indicate that the metallogenic elements in the ore deposits mainly come from the host strata. The studies of trace elements in the host mudstone show lower w(Th)/w(Sc), w(Th)/w(U) ratios, higher mafic elements (such as Cr, Ni, Co) mass fractions than the average values of the same kind of rocks in the upper crust and the research area. The chondrite-normalized REE patterns of mudstone in the host strata are quite different from the common mudstones and shales with relatively steep HREE slopes (relatively high w(Tb)n/w(Yb)n ratios, between 1.48 and 2.00) and no or slightly negative Eu anomalies (high δ(Eu) ratios, between 0.79 and 0.89). The NASC-normalized REE patterns of siliceous rocks in the host strata show the HREE enrichment (the w(La)n/w(Yb)n ratios are less than 1), a little Eu positive anomalies and Ce depletion with high abundances of As, Se, Sb and Ba elements, which shows the hydrothermal deposit features. The NASC-normalized REE patterns of the mudstone in the host strata are very similar to those of the hydrothermal siliceous rocks and to those of the tuffs and basalts except the higher total REE mass fractions. All of these geochemical characteristics suggest the existence of interior sources pouring into basin when the host strata were deposited. The research of basin evolution also indicates that the host strata were deposited in the pulling-apart periods. It can be realized from these facts that the degree of enrichment and deficiency of the elements in the normal depositional strata is very limited, and it is only the particular horizons corresponding to the particular periods of the earth's evolution that can be the significant source beds because only in these unusual horizons can be highly enriched metallogenic elements. The interior sources corresponding to the pulling-apart of basin evolution may be the main cause to make the mineralization of the strata-bound ore deposits in southwest Yangtze massif take on time-bound characteristics.