Oxygen and carbon isotope study of natural and synthetic malachite

Malachite, a common secondary mineral in the zone of oxidation, has recently become a major source of Cu due to the development of solution extraction-electrowinning (SXEW) technologies. Stable isotope characterization can contribute needed information on the formative mechanisms of malachite by establishing the source materials and temperature of Formation. The equilibrium O-18 and C-13 isotope fractionations between malachite degrees and water and between CO2 gas and malachite were determined by slow precipitation experiments over the temperature range 0 to 50 degrees C: [GRAPHICS] where temperature (T) is in Kelvin, and where mal degrees represents the acid liberated CO2 corrected as total calcite oxygen. This internally consistent method renders the acid correction unnecessary for application of the mal degrees-water thermometer. Natural malachites have delta(18)O and delta(13)C values that are similar to those for caliches and speleothems. This similarity supports the field evidence of malachite forming caliche-like crusts and speleothem-like structures, and bears upon the conditions of its genesis. The delta(18)O values of +18 to +31 per mil standard mean ocean water (SMOW) for natural malachites are dominantly controlled by the delta(18)O values of ambient meteoric waters and the surface temperatures. Most malachites have delta(13)C values between 0 and -21 per mil Peedee belemnite (PDB) reflecting a combination of vegetation-respired soil CO2 and carbonate rock-derived carbon. A strong correlation between the delta(18)O values of malachites and local modem meteoric waters suggests that malachites are relatively young, or alternatively, that they are easily exchanged. Using the mal degrees-water thermometer formation temperatures of 5 degrees to 35 degrees C are estimated. These temperatures correlate reasonably well with the modern local air temperatures. However samples from massive sulfide deposits such as Broken Hill, Australia, have estimated temperatures as high as 58 degrees C, probably reflecting the heat generated by sulfide oxidation during weathering.