Mineralogy, Lithogeochemistry and Elemental Mass Balance of the Hydrothermal Alteration Associated with the Gold‐rich Batu Hijau Porphyry Copper Deposit, Sumbawa Island, Indonesia

Abstract This paper discusses the mineralogy, whole‐rock geochemistry and elemental mass balance of the hydrothermal alteration zones within the Batu Hijau porphyry copper‐gold deposit, Sumbawa Island, Indonesia. The hydrothermal alteration and mineralisation developed in four stages, namely (i) the early stage consisting of a central copper‐gold‐bearing biotite (potassic), proximal actinolite (inner propylitic) and the distal chlorite‐epidote (outer propylitic) zones; (ii) the transitional stage represented by the chlorite‐sericite (intermediate argillic) zone; (iii) the late stages distinguished into the sericite‐paragonite (argillic) and pyrophyllite‐andalusite (advanced argillic) zones; and (iv) the very late stage typified by the illite‐sericite zone. In general, major elements (particularly Ca, Mg, Na and K) and some minor and rare earth elements decrease from the least altered rocks towards the late alteration zones as a consequence of the breakdown of Ca‐bearing hornblende, biotite and plagioclase. Chemical discrimination by means of millicationic R 1 ‐R 2 diagram indicates that R1 [4Si − 11(Na + K) − 2(Fe + Ti)] increases while R 2 [6Ca + 2Mg + Al] decreases with increasing alteration intensity, from least‐altered, through early, transitional, to late alteration zones. Rare earth elements‐chondrite (C 1 ) normalised patterns also exhibit the depletion of the elements through the subsequent alteration zones. These results are consistent with the elemental mass balance calculation using the isocon method which shows that the degree of mass and volume depletion systematically increases during alteration. A decrease of the elements as well as mass and volume from early, through transitional to late alteration stages may imply a general decrease of the element activities in hydrothermal fluids during the formation of the alteration zones.