Halogen Chemistry of Hydrothermal Micas: a Possible Geochemical Tool in Vectoring to Ore for Porphyry Copper-Gold Deposit

Abstract

Porphyry copper-gold deposit commonly exhibits an extensive alteration zone of hydrothermal micas particularly biotite and sericite. This study is aimed to analyze and utilize the chemistry of halogen fluorine and chlorine of biotite and sericite to be a possible tool in vectoring to ore for copper porphyry deposits. To achieve the objectives, several selected altered rock samples were taken crossing the Batu Hijau copper-gold mine from inner to outer of the deposit, and hydrothermal micas contained by the rocks were analyzed petrographically and chemically. Mineral chemistry was detected by electron microprobe analyzer, whilst biotite is petrographically classified as either magmatic or hydrothermal types. Sericite replacing plagioclase occurred as fine-grained mineral and predominantly associated with argillic-related alteration types. Biotites in the Batu Hijau deposit are classified as phlogopite with a relatively low mole fraction magnesium (XMg) (~0.75) compared to the “typical” copper porphyry deposit (~0.82). The relationship between the XMg and halogen contents are generally consistent with “Fe-F and Mg-Cl avoidance rules”. F content in biotite and sericite decrease systematically from inner part of the deposit which is represented by early biotite (potassic) zone where the main copper-gold hosted, to the outer part of the deposit. However, chlorine in both biotite and sericite from each of the alteration zones shows a relative similar concentration, which suggests that it is not suitable to be used in identification of the alteration zones associated with strong copper-gold mineralization. H2O content of the biotite and sericite also exhibits a systematic increase outward which may also provide a possible geochemical vector to ore for the copper porphyry deposits. This is well correlated with fluorine content of biotite in rocks and bulk concentration of copper from the corresponding rocks.

Chemical and short‐wave infrared characteristics of white mica associated with the Gajah Tidur porphyry copper system at the deep Grasberg Cu—Au—(Mo) deposit, Indonesia

“…Several MQP alteration white micas plot below the Tschermak line due to high substitution of VI Al 3+ in their octahedral sites (Figure 9c). Fluorine may have been inherited from hydrothermal biotite precursor during white mica alteration event (Munoz, 1984; Parry et al ., 1984; Idrus, 2018; Wallace and Maher, 2019). MQP white micas have strong fluorine and magnesium correlation (Figure 9f) since hydrothermal biotite was pervasively replaced by muscovite.…”

Amphiboles and phyllosilicates in the A-type Mandira Granite Massif, Graciosa Province, SE Brazil: Textures, composition and crystallisation conditions

“…Halogen contents of hydrous minerals, such as biotite, are sensitive indicators of the physicochemical conditions of late-stage or hydrothermal fluids (Ayati et al, 2008;Siahcheshm et al, 2012;Idrus, 2018;Moshefi et al, 2018). Thus, we computed the fugacity ratios f HF /f HCl and f H 2 O /f HF following Munoz (1984Munoz ( , 1992, on the basis of the revised coefficients for F-Cl-OH exchange between biotite and hydrothermal fluids Sverjensky, 1991, 1992).…”

Main mafic and accessory mineralogy in granites and greisens from selected occurrences in the A-type Graciosa Province, S-SE Brazil, and petrological implications

“…The halogen contents of hydrous minerals are sensitive indicators to constrain physicochemical conditions of late-stage or post-magmatic fluids (Ayati et al, 2008;Siahcheshm et al, 2012;Idrus, 2018;Moshefi et al, 2018). The measured F and Cl contents in biotite are shown in Figure 2-11C and compared to those measured in amphiboles as mentioned in section 4.2.…”