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 chloriteepidote (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 pyrophylliteandalusite (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 R1-R2 diagram indicates that R1 [4Si -11(Na + K) -2(Fe + Ti)] increases while R2 [6Ca + 2Mg + Al] decreases with increasing alteration intensity, from least-altered, through early, transitional, to late alteration zones. Rare earth elements-chondrite (C1) 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.

Chemical Composition of Rock‐Forming Minerals in Copper–Gold‐Bearing Tonalite Porphyries at the Batu Hijau Deposit, Sumbawa Island, Indonesia: Implications for Crystallization Conditions and Fluorine–Chlorine Fugacity

“…The composition of the two tonalite porphyries is virtually identical, consistent with its classification as low‐K calc‐alkaline (Idrus, 2006; Fig. 5).…”

“… Affinity discrimination diagram (after Le Maitre et al , 1989; Rickwood, 1989) showing the tonalite porphyries plotted in low‐K calc‐alkaline field (Idrus, 2006). (▪) intermediate tonalite; (□) young tonalite.…”

“…Copper and gold mineralization is closely related to the emplacement of tonalite porphyries (Mitchell et al , 1998; Clode et al , 1999; Garwin, 2002, Arif & Baker, 2004; Imai & Ohno, 2005). During the emplacement and cooling of the tonalite porphyries, the magmatic hydrothermal fluids are exsolved and interacted with the wall rocks, mixing with meteoric waters and formed multiple stages of alteration zones (Idrus, 2006).…”

Spectral characteristics of propylitic alteration minerals as a vectoring tool for porphyry copper deposits

“…Late--stage, structurally--controlled, argillic and advanced argillic alteration are widespread and overprints earlier alteration (Fig. 2;Garwin 2002;Clode et al 1999;Idrus et al 2009). …”

“…2;Garwin 2002;Clode et al 1999). A transitional chlorite--sericite (intermediate argillic zone) developed after the propylitic and potassic alteration stages, overprinting the biotite (potassic) and actinolite zones (Idrus et al 2009). Late--stage, structurally--controlled, argillic and advanced argillic alteration are widespread and overprints earlier alteration (Fig.…”