Textural characterization of unidirectional solidification textures related to Cu–Au deposits and their implication for metallogenesis and exploration

Abstract

Ten rock samples with unidirectional solidification textures (USTs) from nine Asian and Australian porphyry and intrusion-related Au deposits and prospects were investigated in terms of mineralization sequences, textural and paragenetic relationships, melt and fluid inclusion inventory, cathodoluminescence, and trace elements of UST quartz. Despite general similar structural appearance of the investigated UST sequences, each UST locality has its own specific textural features. We provide a structural classification of observed USTs that distinguishes equigranular and heterogranular USTs, which are subdivided according to the UST layer structure into straight, wavy, or contorted/brain-like type. In addition, there is the group of disturbed USTs comprising incompletely developed UST layers, and ductile- or brittle-fragmented UST sequences. Ti-in-quartz-based calculations revealed crystallization temperatures of UST quartz between 590 and 770 °C. Primary silicate melt inclusions found in four samples indicate that quartz crystallized initially from a silicate melt. In five samples, fluid inclusion “boiling” assemblages were observed, documenting brine-vapor phase separation due to pressure drop. In one case, the “boiling” assemblage is in primary inclusions and in the other cases the assemblages have been trapped in pseudosecondary inclusions in a later stage. Primary brine inclusions without coeval vapor-rich inclusions were identified in one sample only. Our results permit UST formation in either an open-system setting where fluids exsolved from UST crystallization are periodically released by fracturing (pressure drops) after the formation of each individual UST layer, or closed-system setting where the exsolved fluids are trapped because of lack of fracturing as long the UST sequence forms. Here, the closed-system model is introduced for the first time to describe the formation of layered quartz UST sequences in porphyry systems where the open-system setting seems to be not applicable. Nonetheless, quartz UST sequences are very important exploration vectors for porphyry systems. Six out of the ten UST samples are high-grade with up to 12 g/t Au. However, in the investigated cases, the Au mineralization postdates UST formation. At Oyut Ulaan and Tampang, the UST quartz formation is co-genetic with major Cu–Fe sulfide mineralization, whereas in the other localities the UST formation predates the sulfide mineralization. In any case, UST layer sequences clearly functioned as an outer carapace and, thus, as a chemo-physical trap of passing mineralizing fluids entering the UST structure and may offer a vector to the main stockwork mineralization, which might occur 100 to 200 m above the UST zone.