The alteration characteristic of Cu-Au skarn and porphyry-style alteration in The Deep Mlz, Ertsberg District, Papua, Indonesia

Abstract

The Deep Mill Level Zone (DMLZ) is a mining zone within the lower part of the East Ertsberg Skarn System (EESS), a world class copper-gold skarn deposits that linked to porphyry system. Macroscopic observation along with microscopic analysis such as Petrography, SEM, and Mineragraphic analysis were made on each core samples to determine the zonation of Skarn alteration that superimposed with porphyry style alteration system within the mineralization zone. The DMLZ hosted in the Paleocene Waripi Formation that was intruded by Diorite to Quartz Monzodiorite. The Diorite characterized by its equigranular texture with the appereance of Hornblenda-Biotite phenocrysts, meanwhile later Quartz Monzodiorite shows porphyritic texture with abundant Pyroxene-Biotite phenocrysts embedded in allotriomorphic granular quartz groundmass. The DMLZ is hosted by mixed assemblages of silisiclastic and dolomitized carbonate that associate with the multiphase intrusive complex. The Skarn alteration zonation can be identified based on dominant lithology, such as: 1) Forsterite-diopside Skarn 2)Anhydrite 3)Magnetite Skarn 4)Hornfels 5)Marble and 6)Endoskarn. Meanwhile Porphyry system that identified in the diorite to quartz monzodiorite intrusion consist of: Secondary biotite-K- Feldspar±actinolite±tremolite ±anhydrite-magnetite; Sericite-pyrophyllite-illite-secondary quartz; Chlorite-clay minerals-calcite-epidote-secondary quartz; Calcite ± dolomite ± siderite;. Some alteration zones occur together in several depth indicating overprinting of different alteration episodes, but there is evidence that the carbonate alteration is the latest event. Cu-Au mineralization consists of bornite and chalcopyrite predominantly, both within skarn system and also porphyry copper system.