pH-EVOLUTlON OF ARCHAEAN SEAWATER AND ITS RELATIONS TO ORE DEPOSITION

Based on the simulating experiments on the formation mechanism of banded iron-silica formation in the solution from weathering-leaching basalts, the authors have come to the conclusion that Archaean seawater was an acid solution evolving from strong acid to weak acid, in which a series of sedimentary rocks and ores bodies were formed. For example, banded iron-silica formations found in the ancient shields of the world, according to the simulating experiments ( 1 ) ,had been formed at the strong acid-acid stage of seawater in early geological history. The authors pointed out that primary seawater was a strong acid solution formed by condensation of volcanic gases, with pH of about 0.3. Volcanic degassing reduced the pH value of the seawater and afterwards in the stage of inactivity, pH would be raised as hypergene weathering-leathing went on. Volcanic degassing is in contradiction with hypergene weathering-leathing and it is just the couple of contradictions controlling the pH of the seawater and its evolution. When volcanic degassing was intensive in early geologic history, the pH of seawater was quite acid, but later, when the crust became more stable, and hypergene weathering-leaching gradually occupied a dominant position, the pH of the seawater rose and at last becam e alkaline. The velocity, at which strong acid seawater was neutralized, was also disscused. The authors pointed out that Archaean strong acid seawater was pH-stable"buffer", even in the intermitent period of volcanic activity, it is most likely to take millions and tens of millions years for its neutralization. The long-term acid condition and slow neutralization process were much favourable for the enrichment of ore-forming elements in the seawater and their chemical differentiation and forming sedimentary ores. The characteristics of hypergene weathering-leaching in Archaean were also disscused. The authors suggested that there had been weathering-leaching conditions of alternative strong acid meteoric water polluted by volcanic gases or rain saturated with CO₂ gas. During the first period formed acid ore-bearing seawater and during the second differentiating and precipitating ore-bearing seawater was neutralized by weathering. So the Archaean weathering-leaching conditions were disadvantageous factors for the formation of the ore deposit of -weathering crust, but supplied sedimentary deposits with more ore-forming materials. In the discussion on the problem of minerogenesis in seawater controlled by pH the authors lined up the chemical analyses of sequent products precipitated in the weathering-leaching basalt solutions, in which pH gradually rose from strong acid by adding NaOH. As the simulating experiments showed when pH varied below 1.25, there had precipiteted in the solution only amorphous sillica; when the pH of the solution ranged among 1.25-6.0,thered hadropped the banded iron-sillica formation following the temperatures fluctuating periodically from room temperature to 100℃; when the pH of the solution reached 3.5-4.2 from strong acid, there had precipiteted sediments containing much hydroxides of aluminium; According to the author's calculation the partial pressure of CO₂ in the atmosphere reached about 10-20 atm 2600 million years ago. The equilibrium pH with the atmospheric CO₂ at which carbonates were formed is about 4,5; when the pH of the solution exceeded 5,5, there would precipetate iron silicates instead of amorphous silica. The comparison between these lower limits of pH in which corresponding compounds were precipitated and the earliest minerogenetic ages of corresponding ores shows a general course of pH-evolution of Archaean seawaten The pH was 0.3 about 4000 million years ago when primary seawater had been formed; 3800 million years ago, when the banded iron-silica formations had appeared the seawater' s pH exceeded 1.25 about 3200 million years ago old schists covering some old shields contained appreciable hydroxide of aluminium and corundolite. That means the pH of seawater at that time had reached over 3.5.