Abstract

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 18 O and 13 C isotope fractionations between malachite (super *) and water and between CO 2 gas and malachite were determined by slow precipitation experiments over the temperature range 0 to 50 degrees C: 1000 ln alpha Oxygen mal AK -water = 2.66 (10 6 /T 2 ) + 2.66 and 1000 ln alpha Carbon CO 2 -mal = -1.85 (10 6 /T 2 ) + 10.51 where temperature (T) is in Kelvin, and where mal (super *) represents the acid liberated CO 2 corrected as total calcite oxygen. This internally consistent method renders the acid correction unnecessary for application of the mal (super *) -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 CO 2 and carbonate rock-derived carbon. A strong correlation between the delta 18 O values of malachites and local modern meteoric waters suggests that malachites are relatively young, or alternatively, that they are easily exchanged. Using the mal (super *) -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.

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