PRACTICAL AND PREDICTIVE MODELLING OF ORE DEPOSITS IN HYDROTHERMAL SYSTEMS

Over the pastfive years,we have been making efforts to develop a practical and predic- tive tool to exploreforgiantore deposits in hydrothermal systems. Towards this goal,a sig- nificant progress has been made towards a better understanding of the basic physical and chemical processes behind ore body formation and mineralization in hydrothermal systems. On the scientific developmentside,we have developed analytical solutions to answerthe fol- lowing scientific questions:(1) Can thepore- fluid pressure gradientbemaintained atthe val- ue of the lithostaticpressure gradientin the uppercrustof the Earth?and(2 ) Can convective pore- fluid flow take place in the uppercrustof the Earth ifthere is a fluid/mass leakage from the mantle to the upper crustof the Earth?On the modelling developmentside,we have developed numerical methods to model the following problems:(1) convective pore- fluid flow in two- dimensional hydrothermal systems;(2 ) coupled reactive pore- fluid flow and multiple species transport in porous media;(3) precipitation and dissolution of minerals and rock al- teration in the upper crust of the Earth;(4 ) double diffusion driven reactive flow transport in deformable fluid- saturated porous media with particular consideration of temperature- de- pendentchemical reaction rates;(5 ) pore- fluid flow patterns neargeological lenses in hydro- dynamic and hydrothermal systems;(6 ) dissipative structures for nonequilibrium chemical reactions in fluid- saturated porousmedia;(7) convectivepore- fluid flow and the related min- eralization in three- dimensional hydrothermal systems;(8) fluid- rock interaction problems associated with the rock alteration and metamorphic process in fluid- saturated hydrothermal/ sedimentary basins;and (9) various aspects of the fully coupled problem involving material deformation,pore- fluid flow,heattransferand species transport/ chemical reactionsin pore- fluid saturated porous rock masses. The above- mentioned work has significantly enriched our knowledge about the physical and chemical processes related to ore body formation and mineralization in the upper crustof the

Subaerial hot springs and near-surface hydrothermal mineral systems past and present,and possible extraterrestrial analogues

The principal features,geotectonic settings and association with near-surface hydrothermal mineral systems of ancient and present subaerial hot springs,fumaroles and geysers are herein reviewed.Fumaroles and geysers usually occur in volcanic craters and are in most cases,part and parcel of hot spring environments.Subaerial hot springs are characterised by siliceous-and carbonate-rich chemical sediments,such as sinters and travertines,respectively.Sinters are commonly enriched in various metalliferous elements.Hot springs surface discharges are also characterised by pools,which exhibit bright colours due to the presence of microorganisms.Present-day examples discussed in this paper,include the fumaroles and hot springs of the White Island volcano(New Zealand),the world-renowned Yellowstone caldera(USA)and the Afar region of the East African Rift System.The Afar triangle,in the northern part of East African Rift System,provides a good example of hot springs associated with evaporative deposits.The Tuli-Sabi-Lebombo triple junction rifts were formed during the^180 Ma Karoo igneous event in southern Africa,of which the Tuli arm is the failed rift(aulacogen),as is the Afar region.The Tuli rift is effectively an unusual,if not unique,tectono-thermal setting,because it comprises Karoo-age hot springs systems and associated vein stockworks and breccias,which includes the Messina Cu deposit as well as currently active hot springs with sinter deposits probably due to post-Karoo uplift.Fumarolic pipes in ignimbrites of the Erongo Volcano-Plutonic Complex in Namibia are discussed.This is followed by an example of banded chert rocks in the Killara Formation of the Palaeoproterozoic Capricorn Orogen(Western Australia),interpreted as hot spring chemical sediments,which also show evidence of"fossil"microbial filaments.The paper ends with a brief overview of possible analogues of hot springs on planet Mars.

Geological,physical,and chemical characteristics of seafloor hydrothermal vent fields

Seafloor hydrothermal vent fields(SHVFs) are located in the mid-ocean ridge(MOR),backarc basin(BAB),island arc and hot-spot environments and hosted mainly by ultramafic,mafic,felsic rocks,and sediments.The hydrothermal vent fluids of SHVFs have low oxygen,abnormal pH and temperature,numerous toxic compounds,and inorganic energy sources,such as sulfuric compounds,methane,and hydrogen.The geological,physical,and chemical characteristics of SHVFs provide important clues to understanding the formation and evolution of seafloor hydrothermal systems,leading to the determination of metal sources and the reconstruction of the physicochemical conditions of metallogenesis.Over the past two decades,we studied the geological settings,volcanic rocks,and hydrothermal products of SHVFs and drawn new conclusions in these areas,including:1) the hydrothermal plumes in the Okinawa Trough are affected by the Kuroshio current;2) S and Pb in the hydrothermal sulfides from MOR are mainly derived from their host igneous rocks;3) Re and Os of vent fluids are more likely to be incorporated into Fe-and Fe-Cu sulfide mineral facies,and Os is enriched under low-temperature(<200℃) hydrothermal conditions in global SHVFs;4) compared with low-temperature hydrothermal sulfides,sulfates,and opal minerals,high-temperature hydrothermal sulfides maintain the helium(He) isotopic composition of the primary vent fluid;5) relatively low temperature(<116℃),oxygenated,and acidic environment conditions are favorable for forming a native sulfur chimney,and a "glue pudding" growth model can be used to understand the origin of native sulfur balls in the Kueishantao hydrothermal field;and 6) boron isotope from hydrothermal plumes and fluids can be used to describe their diffusive processes.The monitoring and understanding of the physical structure,chemical composition,geological processes,and diverse organism of subseafloor hydrothermal systems will be a future hot spot and frontier of submarine hydrothermal geology.

Experimental evidence of pressure effects on spinel dissolution and peridotite serpentinization kinetics under shallow hydrothermal conditions

Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs,as well as fluid and element transfer between the Earth’s mantle and crust.However,fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained.Here we present two constant temperature(230℃)time-series of natural peridotite(77.5%olivine;13.7%enstatite;6.8%diopside;2%spinel)serpentinization experiments:at 13.4 MPa;and 20.7 MPa.Al-enriched lizardite was the main secondary mineral in all runs after olivine(olv)and orthopyroxene(opx)serpentinization(without any detectable brucite,talc or magnetite),while primary spinel and diopside partially dissolved during the experiments.Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite,as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine,while the opposite is true for Mg and Fe,with homogenization of serpentines compositions after 40 days.The Ni/Cr ratios of serpentines,however,remain diagnostic of the respective primary mineral.Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day^(-1)(0.26 mmol·day^(-1))and 0.26 wt.%·day^(-1)(0.13 mmol·day^(-1))at 13.4 and 20.7 MPa,respectively.Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al,which accelerates olivine serpentinization while delays orthopyroxene dissolution.Additionally,time-dependent increase in solid products masses suggests rock volume expands linearly 0.37%±0.01%per serpentine wt.%independently of pressure.Mass balance constrains suggests olv:opx react at~5:2 and~3:2 M ratios,resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series(13.4 MPa)and high-pressure series(20.7 MPa),respectively.Elevated starting peridotite olv:opx ratio(7.94:1)therefore indicates orthopyroxene serpentinization is~3.3x and~5.4x faster than olivine at 13.4 MPa and 20.7 MPa,respectively.This contradicts previous assumptions that olivine should dissolve faster than orthopyroxene at experimental conditions.Finally,serpentinization-derived fluids develop pH>10 and become enriched in H_(2),CH_(4),Ca^(2+)and Si within 6 weeks.Aqueous silica concentrations are highest after 5 days(265.75 and 194.79µmol/kg)and progressively decrease,reaching 13.84 and 91.54µmol/kg at 13.4 and 20.7 MPa after 40 days,respectively.These concentrations are very similar to the low-silica(M6)and high-silica(Beehive)endmembers of the Lost City Hydrothermal Field(LCHF).Beyond fluid characteristics,serpentinization products and conditions analogous to the LCHF suggest similar mechanisms between our experiments and natural processes.Our results demonstrate constant temperature serpentinization of a common protolith leads to distinct serpentine and fluid compositions at different pressures.Although additional data is necessary,recent studies and our experiments suggest peridotite serpentinization rates at 230℃rapidly decrease with increasing pressures at least up to 35 MPa.Whether pressure directly influences olivine and orthopyroxene serpentinization kinetics or indirectly controls reaction rates due to spinel dissolution under hydrothermal conditions deserves further investigation.

Origins of building blocks of life: A review

How and where did life on Earth originate？ To date, various environments have been proposed as plausible sites for the origin of life. However, discussions have focused on a limited stage of chemical evolution, or emergence of a specific chemical function of proto-biological systems. It remains unclear what geochemical situations could drive all the stages of chemical evolution, ranging from condensation of simple inorganic compounds to the emergence of self-sustaining systems that were evolvable into modern biological ones. In this review, we summarize reported experimental and theoretical findings for prebiotic chemistry relevant to this topic, including availability of biologically essential elements（N and P） on the Hadean Earth, abiotic synthesis of life＇s building blocks（amino acids, peptides, ribose, nucleobases, fatty acids, nucleotides, and oligonucleotides）, their polymerizations to bio-macromolecules（peptides and oligonucleotides）, and emergence of biological functions of replication and compartmentalization. It is indicated from the overviews that completion of the chemical evolution requires at least eight reaction conditions of（1） reductive gas phase,（2） alkaline pH,（3） freezing temperature,（4）fresh water,（5） dry/dry-wet cycle,（6） coupling with high energy reactions,（7） heating-cooling cycle in water, and（8） extraterrestrial input of life＇s building blocks and reactive nutrients. The necessity of these mutually exclusive conditions clearly indicates that life＇s origin did not occur at a single setting; rather, it required highly diverse and dynamic environments that were connected with each other to allow intratransportation of reaction products and reactants through fluid circulation. Future experimental research that mimics the conditions of the proposed model are expected to provide further constraints on the processes and mechanisms for the origin of life.

Sicilian serpentinite xenoliths containing abiotic organics with nanodiamond clusters as key model for prebiotic processes

Rock fragments from the deepest parts of a buried hydrothermal system belonging to the Mesozoic Tethys Ocean entered as xenoliths in a Miocenic diatreme,hence brought to the surface,in the Hyblean Plateau(Sicily).Some xenoliths consist of strongly serpentinized ultramafic rocks bearing blebs of abiotic organic matter,where clusters of amorphous carbon nanoparticles,including nanodiamonds,are immersed.Such an occurrence conjures up established hypotheses that diamond surfaces are suitable catalytic platforms stimulating the assemblage of complex bio-organic molecules relevant to the emergence of life on Earth.The appearance of bio-organic molecules under primitive Earth conditions is one of the major unsolved questions on the origin of life.Here we report new micro-Raman spectra on blebs of abiotic organic matter from a selected xenolith.Diamond bands were related to hydrogenated nanocrystalline diamonds,with size of nearly 1-1.6 nm,formed from organics at low pressures and temperatures.In particular,diamond surfaces can give rise to crystalline interfacial water layers that may have played a fundamental role in the early biosphere evolution as a good medium for rapidly transporting positive charges in the form of hydrated protons.Nowadays,proton gradients in alkaline hydrothermal vents along oceanic ridges are generally viewed as key pre-biotic factors.In general,serpentinites span the entire geological record,including prebiotic times.These hydrous ultramafic rocks often display evidence of abiotic carbon species,both organic and inorganic,including nanodiamonds,being also capable to give rise to chemiosmotic processes and proton gradients necessary to the organisms,such as the"Last Universal Common Ancestor"(LUCA),in the prebiotic Earth.