Element transport and enrichment during propylitic alteration in Paleozoic porphyry Cu mineralization systems: Insights from chlorite chemistry

Chlorite is one of the most widely developed minerals in propylitic alteration around porphyry deposits and can be used to investigate element transport and enrichment processes during hydrothermal alteration. The Paleozoic Tuwu porphyry Cu deposit (metal reserve: 0.7 Mt Cu @ 0.67%), located in the southern margin of the Central Asian Orogenic Belt, is one of the most economical and largest porphyry Cu deposits in NW China. The Tuwu porphyry Cu deposit comprises an early porphyry Cu mineralization (including potassic, propylitic and phyllic alteration stages) and later overprinting Cu mineralization periods with chlorite found in both. Chlorite from the propylitic zone at Tuwu is geochemically similar to propylitic chlorite in younger Cenozoic porphyry systems, but enriched in Mg and K and depleted in Fe and Al relative to the overprinting chlorite. Scandium, V, Ti and Ga are enriched, and Li, Sr, Mn and Zn are depleted in the Tuwu propylitic chlorite proximal to the orebody, consistent with Cenozoic deposits. Compared with metamorphic chlorite from Proterozoic metamorphic terranes in Australia, Tuwu propylitic chlorite has higher Mg and K, lower Fe, Al and As. Our results suggest propylitic alteration in the Tuwu district is probably the product of cooling of fluids derived from the magmatic-hydrothermal system, but not from peripheral waters, and trace element variations in chlorite are mainly controlled by temperature although other factors such as fault development could affect variation trends. The common elemental features of chlorite in both Paleozoic and Cenozoic porphyry Cu deposits indicate it could act as a potential tool for mineral exploration