ИСТИНА

Magma chamber degassing during cooling and crystallization leads to the release of hot water fluids rich in salts and metals into surrounding porous rocks. Under certain conditions these pro-cesses can lead to generation of porphyry ore de-posits. Our conceptual model [1] involves two-stage degassing. At stage 1, the chamber is filled with silicic magma. Magma degassing results in flow of magmatic fluid rich in water, halite as well as copper in tracer amounts from depth to surface. At shallow depth the magmatic fluid condenses in a compact brine lens. The concentra-tion of salt as well as copper that follows halite becomes very high in the brine lens. At stage 2 there is intrusion of fresh mafic magma that results in changes in the fluid composition. Now, the fluid is rich in SO2 gas that ascends to the shallow depth to the lens level, streams through the lens and triggers copper precipitation that is ore enrichment. Numerical modelling of this two-stage processes faces a lot mathematical challenges associated with multiphase and multicomponent nature of the flow, prediction of the fluid properties in a wide range of pressures and temperatures [2], hal-ite precipitation and permeability reduction. Our work aims at developing methods for modelling these strongly linked and non-linear processes. We work on developing methods that are simple but robust enough in order to capture primary physical effects associated with porphyry deposits formation. PROJECT SUMMARY In order to simulate the process of magma degas-sing we consider an axisymmetric domain corre-sponding to the depth range from 0 to 7 km in-cluding a high-permeability central conduit sur-rounded by low-permeability rocks. The conduit forms a pathway for magmatic fluid along which it ascends towards the surface. Magma degassing is simulated with a point source placed at the bot-tom of the conduit. We developed an extension of our multiphase simulator MUFITS [3] for NaCl–H2O mixture flows. Results of condicted simulations reveal that at the stage 1 the lens formation is caused by phase transitions of two different types undergo-ing at different depths in ascending magmatic fluid [3]. In the shallow zones salt precipitation on the skeleton of the porous medium in the form of a solid phase leads to clogging of pore space and reduction of the permeability. As a result, the magmatic fluid flow towards the surface is blocked and this facilitates the concentrated brine accumulation in a local zone. At stage 2 hot SO2 reach gas interacts with the copper rich brine lens leading to precipitation of copper minerals. We assume instantaneous kinet-ics of the reaction, thus copper mineral concentra-tion is determined only by presence of copper and sulphur in the aqueous phase. This method allows contouring copper deposits during post-processing of porous flow simulations. ALIGNMENT WITH BHPB’S RESEARCH ROADMAP The presented research results aim at justifying the two-stage concept of porphyry copper deposits formation. FUTURE WORK Our future work aims at more accurate represen-tation of H2O-NaCl-H2S-SO¬2 system properties and account for multiple chemical species transport and equilibrium models. REFERENCES [1] Blundy & Mavrogenes (2015), Nat. Geosci., 8, 235–240. [2] Driesner & Heinrich (2007), Geo-chim. Cosmochim. Acta, 71, 4880−4901. [3] Afanasyev (2015), Energy Proc. 76: 427-435. [4] Weis (2015), Geofluids, 15, 350−371.