Excess Alumina of Plagioclase Related to Copper-Gold Enrichment: Study Case at the Humpa Leu East (HLE) Porphyry CU-AU Prospect in HU’U District, Sumbawa Island, Indonesia

Abstract

The formation of porphyry copper deposits is controlled by water available in magma to form a deposit since Cu ligands and S have finite solubilities. The high amount of water in magma allows the magma to reach saturation earlier and causes more Au to be drawn from the melt into volatile aqueous phases. High water content in magma is conducive to the migration and enrichment of metal elements, which are very important for mineralization. This study is aimed to examine trace element compositions of plagioclase related to early, intermediate and late causative intrusions of the Humpa Leu East (HLE) prospect by applying the mineral chemistry to assess magma fertility for copper and gold mineralization in term of magma water content. The presence of 'extra' alumina in plagioclase minerals has been linked to high water content in magma. Plagioclase may become enriched because 'excess' alumina excludes copper, enriching the remaining melts. Our findings show that the excess of alumina (Al*) in mineralized/fertile samples of the HLE porphyry Cu-Au prospect exhibits more pronounced (Al*) than the infertile samples of Mt. Kelud. The 'excess' alumina of plagioclase can be linked to high water content in the magma at the HLE porphyry copper prospect. Furthermore, Concentric zones in plagioclase phenocryst may represent fluid injections into the sub-porphyry magma chamber. Plagioclases in the intermediate intrusion shows a higher of 'excess' alumina than those in the early and late causative intrusive rocks, which indicates that the intrusion is genetically related to the high grade mineralization. This study proves that the chemical signatures of plagioclase could be an exploration indicator in term of magma fertility forming porphyry copper-gold deposits.