Madagascar is known as the ‘Island of Gemstones’ because it is full of gemstone resources. Apatite from Madagascar is widely popular because of its greenish blue Paraiba-like color. This study analyzes apatite from Madagascar through standard gemological characteristic methods, spectroscopic tests and chemical analyses (i.e., electron probe and laser ablation inductively coupled plasma mass spectrometry). This work explores the gemological and the diagenesis information recorded on Madagascar apatite by comparing them with apatite from other sources and establishes the origin information of Madagascar apatite. The origin characteristics are as follows: Apatite from Madagascar is fluorapatite, with excellent diaphaneity, greenish–blue color caused by Ce and Nd and crystal structure distortion indicated by spectroscopic tests. The F/Cl ratio (16.47 to 21.89) suggests its magmatic origin Cl loss during the weathering processes forming the source rocks, and lg fO₂ (−10.30 to −10.35) reflects the high oxidation degree of magma.

Green diopside is currently popular in the jewelry market due to its attractive color and excellent transparency. Gem-quality diopsides are mainly sourced from Pakistan, Italy, Russia, and other places. The color, geographic origin, and formation mechanism are the main factors affecting the value of gemstones, which can be determined by examining their gemology and composition characteristics. This study systematically characterizes the standard gemology of green diopsides from Pakistan and Russia and compares them with the blue diopsides produced within the skarn process and the diopsides from the nearby region in Russia from previous studies by gemological microscopy, spectral testing (infrared, Raman and ultraviolet-visible spectroscopy), and chemical analyses (electron probe and laser ablation inductively coupled plasma mass spectrometry). The results show that the spectral characteristics and phase composition of the green diopside samples from Pakistan and Russia have excellent uniformity and similarity. The high Cr, Fe, V, and Ni contents are the reasons why they appear as green. Meanwhile, the elemental characteristics of the diopside are effective tools for distinguishing different origins of different diopsides. The Russian green diopsides have higher contents of Sr, Sc, Zr, and rare earth elements (REE), indicating that they are related to alkaline ultrabasic rocks, and the source of the diopside sample from Pakistan is metamorphic rock.

The magmatic process within upper crust encompasses various contents such as the transition between magmatic and hydrothermal systems and changes in oxygen fugacity (ƒO₂), which ultimately play key roles in the formation of porphyry Cu deposits (PCDs). However, tracing these magmatic processes, especially in porphyry systems, is not an easy task. This study reported the detailed process of magmatic fluid exsolution and systematical variation of magmatic ƒO₂ within the upper crust of a Tongchang–Chang’anchong porphyry Cu deposit, based on detailed investigations of mineral crystallization sequences and compositional features of the minerals in the fertile porphyries. Results indicate that the fertile porphyries show a high initial ƒO₂, with ΔFMQ ≥ +3.0 (ΔFMQ is the deviation of logƒO₂ from the fayalite–magnetite–quartz (FMQ) buffer). The magmatic ƒO₂ (ΔFMQ) continued to decrease to ~+2 until fluid exsolution occurred at ~790 °C due to wall-rock contamination. The magmatic fluid exsolution process caused a temporary increase in the ƒO₂ (to ΔFMQ = ~+3.4). The high magmatic ƒO₂ during this process (790–750 °C) resulted in a higher content of ore-forming materials in the exsolved magmatic fluid. When the temperature dropped below 750 °C, the magmatic ƒO₂ began to continuously decrease and eventually reached ΔFMQ = ~+0.6. The lower magmatic ƒO₂ hindered the further migration of ore-forming materials through the exsolved fluid during this process (< 750 °C). Results of this study indicate that the initial magma during the upper crustal magmatic process of PCDs generally has a high ƒO₂, and the contamination of reduced components can significantly decrease the magmatic ƒO₂. The early magmatic fluid exsolution process can maintain a high magmatic ƒO₂ condition, thereby efficiently extracting ore-forming minerals and producing ore-forming fluids, which is the key to the formation of PCDs. The latter continuous decrease in magmatic ƒO₂ during the fluid exsolution process may be the reason preventing the Tongchang–Chang’anchong porphyry Cu deposit to form a giant PCD.

The prevailing view suggests that the formation of porphyry Cu deposits involves differentiation of water-rich, metal-bearing juvenile magmas, with subduction of oceanic slabs supplying the necessary volatiles. However, the occurrence of significant porphyry Cu deposits in continental collision zones, where such volatile sources are absent, challenges this paradigm. We analyze a global dataset of whole-rock platinum-group elements and apatite compositions of ore-forming magmas in both subduction and collision zones, combined with Monte Carlo simulations. The results reveal that ore-forming magmas in collision zones exhibit significantly higher degassing efficiencies compared to those in subduction zones. To explain this, we integrate whole-rock geochemistry, mineral thermobarometry, and thermodynamic modeling, proposing that lower water contents of collision zone ore-forming magmas lead to shallower magma storage depths, significantly enhancing degassing efficiency. In contrast, higher water contents of subduction zone ore-forming magmas result in greater emplacements, limiting degassing efficiency. Despite this limitation, higher magma fluxes in subduction zones compensate for lower degassing efficiencies. These findings highlight distinct frameworks of water content, storage depth, and magma flux between subduction and collision zone porphyry Cu deposits. The synergistic interplay of these factors, rather than simply water-rich magmas, is fundamental to porphyry Cu formation.

New aquamarine deposits have been found around the world in recent years, and how to compare and distinguish aquamarines of different origin has become a significant problem. Aquamarines from Koktokay, Minas Gerais, and Namaqualand were collected for standard gemology tests, spectroscopy, and chemical analysis in this paper. The spectroscopy experiment included infrared spectroscopy and Raman spectroscopy. Chemical composition analysis comprises electron microprobe and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). The results show that infrared absorption peaks related to [Fe₂(OH)₄] and NaH are found in Koktokay and Minas Gerais aquamarines, respectively. Compared with other two origins, Namaqualand aquamarine have strongest type II water Raman peak related to alkali metal content. Compared with aquamarines from other sources, aquamarines from Xinjiang and Minas Gerais are characterized by relatively high aluminum and low alkali contents in chemical composition, while Namaqualand aquamarine have a high scandium content.