Recent research on porphyry-type copper (Cu) deposits has unveiled significant insights about the ascent and deposition of ore-forming fluids. Conducted by researchers at various institutions, the study focused on 36 magmatic rocks from six mineralized systems located within the Sanjiang region of southwestern China. This region, known for its rich deposits of copper, molybdenum (Mo), and gold (Au), has now provided clarity on the depth and processes involved during the formation of these economically important resources.
The study found melt inclusions - tiny fragments of molten rock trapped within mineral crystals - indicating depletion of chlorine (Cl), sulfur (S), and metals due to the exsolution of aqueous fluids. This exsolution process extracted between 63% and 97% of these components from the magmas, signaling their movement from depths of approximately 10 to 20 kilometers, rather than the previously assumed upper crustal reservoirs of 5 to 15 kilometers. Major phenocrysts, which are the large crystals formed during the cooling of magma, crystallized under high pressures of 0.3 to 0.5 GPa at these depths, highlighting the complex dynamics involved as the fluids transported their mineral endowments up to the eventual sites of ore deposition.
"This study reinforces the idea of mid to upper crustal magma reservoirs being significant contributors to copper mineralization," said the authors of the article. The researchers suggest these ore-forming fluids ascended through the porphyry magmas and then spread via interconnected fluid networks, with significant findings indicating transport distances of at least 10 kilometers for the ore-bearing fluids.
The significance of this research cannot be overstated. Porphyry deposits account for about 75% of the world’s copper supply, making these findings particularly relevant for mining and economic geology. The traditional models had limited depth constraints and suggested different mechanisms than what the Sanjiang deposits reveal. The new data prompt re-evaluations of mineralization theories and the transport of metal-rich fluids, establishing stronger connections between deep magmatic processes and surface-level ore deposits.
Key to the study is the advanced analysis of the melt inclusions using techniques such as laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS), allowing for precise quantification of elements which were previously challenging to measure. The melt inclusions analyzed from Shuangjiao, Jianchuan, Yulong, and Beiya rocks showed variances between the trachydacitic and rhyolitic compositions, as those derived from rhyolitic melts exhibited lowered concentrations of Cl, S, and metals.
With this knowledge, the researchers also examined how effective the fluid exsolution was at various stages of magma evolution. By analyzing rhyolitic melt inclusions from felsic porphyries, they unveiled not just the quantities but the mechanisms of how significant metal deposits formed, including the movements of Cu, Mo, and Au.
To provide more perspective, the volume of mineralized porphyry stocks at Yulong and Beiya is estimated to be less than 10 km³; meanwhile, the metals contained require calculations of upwards of 24 km³ to form economically viable deposits. This discrepancy indicates most ore-forming fluids had to ascend from deep reservoirs rather than being produced by shallow mineralized systems. This aligns with the idea of regional geological processes influenced by collision events, such as the collision of India with Eurasia, which created good conditions for potassic magmatic activity.
Overall, the findings challenge the previous assumptions concerning the formation and timing of porphyry deposits and point toward complex interplays of geological processes at substantial depths. Future explorations of such deposits may benefit from these findings, leading to enhanced methods for discovering and extracting valuable ores. The research opens up exciting new avenues for mineral exploration, potentially reshaping mining strategies worldwide.