## 4 Results  
### Petrography and Mineralogy  
Based on both macroscopic and microscopic analyses, the penetrated interval was found to consist of amphibite or amphibole gneiss (Figure 5a). Meanwhile, each studied amphibolite sample comprised hornblende + plagioclase + ilmenite \(\pm\) quartz. While the amphibolite samples were massive, the few-tens-of-centimetre-thick amphibole gneiss members were foliated and mainly comprised hornblende + quartz + K-feldspar + plagioclase + biotite + ilmenite. The entire penetrated interval was found to be intensely fractured (Figure 5a), with fault breccia layers of several centimetre width at depths of ~2050, 2062, and 2066 m. Furthermore, the results indicated that one brecciated section between 2071 and 2076 m reached 5 m in thickness.  
The ideal sequence of the vein-filling minerals is chlorite/kaolinite \(\rightarrow\) calcite1 \(\rightarrow\) quartz \(\rightarrow\) calcite2 \(\rightarrow\) laumontite (Figure 5b,c), which is typical throughout the region. Numerous entirely mineralised veins were also found to contain only chlorite and calcite, while the closing zeolite phase appeared exclusively in open fractures and brecciated zones (Figure 5c,d). X-ray diffractometry indicates that  
Figure 5: Typical vein-filling mineralogy of the amphibolite. (**a**) Representative fractured amphibolite bore core. (**b**) Sketch of the vein-filling mineral sequence. (**c**) Open vein with brown calcite2 and laumontite. The coin is 16 mm in diameter. (**d**) Vein filled by calcite2 and laumontite.  
laumontite is the only zeolite mineral of the vein-filling sequence. It has a close-to-ideal composition with Na < 0.4 and K < 0.1 pfu (formula calculated for 48 O; [46]. The two alkalis revealed a tight positive correlation, while Na + K increased coevally with the loss of Ca, indicating a Na + K \(\xleftarrow\) Ca substitution (Figure 6).  
To understand the conditions under which potential laumontite-bearing paragenes might be formed, a mineral assemblage that sufficiently mimics the vein composition was chosen. In particular, the behaviour of the theoretical 10 calcite + 6 kaolinite + 3 quartz mineral composition was modelled. We used the THERIAK-DOMINO software for modelling in the _T-X\({}_{CO_{2}}\)_ field [37; 38], with a temperature of 125 \({}^{\circ}\)C \(<\) \(T\) \(<\) 225 \({}^{\circ}\)C and a fixed pressure of P = 400 bars. The CO\({}_{2}\) concentration changed from 0 to being oversaturated. The results indicated that zeolite could not appear at high (>0.5)X\({}_{CO_{2}}\) fluid compositions. Stilbite and heulandite appeared under moderate _X\({}_{CO_{2}}\)_ levels, while wairakite was stable at \(T\) \(>\) 210 \({}^{\circ}\)C. The narrow field in which laumontite became stable was at a low temperature (<200 \({}^{\circ}\)C) and extremely low _X\({}_{CO_{2}}\)_ levels (<0.15; Figure 7). The reaction _calcite_ + _kaolinite_ + _quartz_ + _H\({}_{2}\)O_ = _laumontite_ + _CO\({}_{2}\)_ may be responsible for laumontite formation in the veins.  
Figure 6: Chemical compositions of single laumontite crystals. (**a**) Na vs. K. (**b**) K + Na vs. Ca. Formula calculated based on 48 O.  
Figure 7: Results of the laumontite stability calculations on _T-X\({}_{CO_{2}}\)_ field (THERIAK-DOMINO modelling software; [38].