Petrography and Mineral Chemistry of Magmatic and Hydrothermal Biotite in Porphyry Copper-Gold Deposits: A Tool for Understanding Mineralizing Fluid Compositional Changes During Alteration Processes

Abstract

This study aims to understand the petrography and chemistry of both magmatic and hydrothermal biotites in porphyry copper-gold deposits, and to evaluate the fluid compositional changes during alteration processes. A total of 206 biotite grains from selected rock samples taken from the Batu Hijau porphyry Cu-Au deposit was analyzed. Detailed petrography and biotite chemistry analysis were performed on thin sections and polished thin sections, respectively, representing various rocks and alteration types. A JEOL JXA-8900R electron microprobe analyzer (EMPA) was used for the chemistry analysis. The biotite is texturally divided into magmatic and hydrothermal types. Ti, Fe, and F contents can be used to distinguish the two biotite types chemically. Some oxide and halogen contents of biotite from various rocks and alteration types demonstrate a systematic variation in chemical composition. Biotite halogen chemistry shows a systematic increase in log (X Cl /X OH) and decrease in log (X F /X OH) values from biotite (potassic) through chlorite-sericite (intermediate argillic) to actinolite (inner propylitic) zones. The y-intercepts on the log (X Cl /X OH) vs. X Mg and log (X F /X OH) vs. X Fe plotted for biotite from potassic and intermediate argillic zones are similar or slightly different. In contrast, the y-intercepts on the log (X Cl /X OH) vs. X Mg and log (X F /X OH) vs. X Fe plotted for biotite from inner propylitic zone display different values in comparison to the two alteration zones. Halogen (F,Cl) fugacity ratios in biotite show a similar pattern: in the potassic and intermediate argillic zones they show little variation, whereas in the inner propylitic zone they are distinctly different. These features suggest the hydrothermal fluid composition remained fairly constant in the inner part of the deposit during the potassic and intermediate argillic alteration events, but changed significantly towards the outer part affected by inner propylitic alteration. High halogen content, particularly Cl, in hydrothermal biotite may portray that copper and gold were transported in mineralizing fluids in the form of chloride complexes CuCl 2 and AuCl 2-, respectively.

Distinctive Features of the Major and Trace Element Composition of Biotite from Igneous Rocks Associated with Various Types of Mineralization on the Example of the Shakhtama Intrusive Complex (Eastern Transbaikalia)

“…In addition, these biotites are distinguished by high X Fe (>0.47) and minimum IV(F/Cl) (3.5-5.5). The increased content of fluorine may be associated with its capture during the evolution of magma [30], which is consistent with Figure 4c, where the values of Log(X F /X OH ) > −0.5 provide evidence of the substantial contribution from the crustal component in the formation of rare-metal-greisen deposits. Similar characteristics are exhibited by the biotites of rocks from the Undinsky complex and the Sredne-Golgotai (Au-Bi-Te-Pb-Sb) deposit: high fluorine content (the Undinsky complex ~1.04 wt.…”

“…Taking into account the published data on the halogen composition of biotites, it may be stated that potentially ore-bearing rocks of the classical porphyry type of mineralization within the boundaries of Eastern Transbaikalia are igneous rocks widely occurring at the Bystrinsky deposit [15,30,32]. The highest values of IV(F/Cl) (4.9-7.1) and IV(F) (2-2.8) were determined for them, which indicates the minimum content of fluorine in biotite (<0.6 wt.…”

Two‐Stage Mafic‐Felsic Magma Interactions and Related Magma Chamber Processes in the Arc Setting: An Example From the Enclave‐Bearing Calc‐Alkaline Plutons, Chinese Altai

“…The other type appears as aggregates of fine-grained flakes, intergrown with opaque minerals and secondary epidote, and usually adjacent to the tabular biotite phenocrysts. It suggests that they are most likely hydrothermal biotites (Idrus, 2018) that have replaced the early primary ones. Unzoned plagioclase, crosshatched K-feldspar, and quartz in the matrix are subhedral, granular, and 0.1-0.5 mm in diameter.…”