The Geochemical Behaviors Of Sulfate At High Pressure And Temperature And Their Effects On Mineralization

Sulfate is ubiquitous in Earth and other terrestrial planets.As an important mineral,sulfate is closely associated with many kinds of hydrothermal mineralization.Therefore,the geochemical behaviors of sulfate-water system at high pressure and temperature(HPHT)have grabbed many attentions in the past few decades.Sulfate is traditionally known for its retrograde solubility,which means that the hydrothermal geofluids can bear almost no sulfate.Consequently,the investigations of the phase behaviors of sulfate-water system were all carried on for solutions with low sulfate concentrations.However,geological observations in the Bayan Obo and the Maoniuping carbonatite-related rare earth element(REE)deposit proved that sulfate-rich geofluids can stably exist in nature.Based on these,the phase behaviors of sulfate-water system should be investigated to discuss the mechanism of the formation of sulfate-rich geofluids and its implications for REE mineralization.Moreover,the acidity of sulfate solution is another crucial geochemical characteristic of sulfate-water system,but the relative studies were all conducted under low pressure.Considering the existence of sulfate in some high-pressure environments,such as the subduction zone,the acidity of sulfate solution at HPHT is meaningful to understand the geochemical behaviors of sulfate-bearing geofluids and their effects on mineralization.To investigate the phase behaviors of sulfate-water system,we constructed a Na2SO4-SiO2-H2O system using a hydrothermal diamond anvil cell(HDAC),and proved that the existence of quartz can shift the solubility of Na2S04 from retrograde to prograde,which can lead to the formation of sulfate-rich geofluids at high temperature.With the presence of quartz,Na2SO4 can melt at a "low"temperature(?270?),forming the sulfate melt with prograde solubility.Therefore,the solubility of Na2SO4 shows a "V-type" curve during the whole temperature range.To better demonstrate the mechanism of the formation of sulfate-rich fluids,we also drew a P-T phase diagram of the Na2SO4-SiO2-H2O system.Furthermore,the HDAC experiment based on a Na2SO4-Nd2(SO4)3-SiO2-H2O system showed that the sulfate-rich geofluids formed with the presence of quartz can greatly enhance the solubility of REE-sulfate minerals,which is consistent with the relative simulation results.Since quartz is ubiquitous in the crustal environment,the results above demonstrated that the sulfate-rich geofluid is ideal to migrate the REE.Moreover,our HDAC experiments also showed that there are three immiscible phases existing in the Na2SO4-SiO2-H2O system during heating,including the sulfate melt,the sulfate-rich aqueous phase,and the sulfate-poor aqueous phase.At higher temperature,the sulfate melt can totally dissolve,but the immiscible sulfaterich and sulfate-poor aqueous phases can coexist during a wide P-T range.These experimental results confirmed that the immiscibility behavior in sulfate-water system can cause the further concentration of sulfate.Considering that the sulfaterich geofluids have a great ability to migrate the REE,the immiscibility behavior can further concentrate the REE and cause the REE mineralization.In addition,we also investigated the acidity of sulfate solutions at HPHT.In sulfate solutions,H+and SO42-can associate to be HSO4-and,in turn,HSO4-can also dissociate to be H+ and SO42-.Pressure and temperature can put effects on the dissociation and association reaction and thus the acidity of sulfate solutions.Therefore,the dissociation behavior of HSO4-(or the association behavior between H+ and SO42-)should be researched to investigate the acidity of sulfate solutions at HPHT.The fused silica capillary capsule(FSCC)is suitable for in situ quantitative experiment,but the drawback is that the common FSCC cannot withstand high inner pressure.Based on this,we first promoted the common FSCC experimental method,to improve the limitation of inner pressure that FSCC can withstand from 150 MPa to-650 MPa.The further in situ quantitative experiments using the promoted FSCC experimental method showed that the elevated temperature favors the association between H+and SO42-while increasing pressure enhances the HSO4-dissociation.The hydrothermal fluids formed via the dehydration of a subduction slab at the subduction zone is a kind of typical high-pressure geofluids.According to the quantitative experimental results,HSO4-should behave as a strong acid rather than a weak one,which means that HSO4-should greatly dissociate to be H+ and SO42-in the subduction zone.The enhanced concentration of H+can decrease the pH of subduction geofluids to affect the fluid-rock reaction and thus the concentrations of ore-forming elements.On the other hand,the dissociation of HSO4-can also promote the concentration of SO42-,and,as an ideal ligand to transport REE,the increased concentration of SO42-can enhance the ability of sulfate-rich geofluids to migrate REE and cause the REE mineralization.