MALACHITE AFTER COPPER

ABOUT THE SPECIMEN

Sharp, distinct, arboreal growths of native copper being replaced by malachite on all sides. Some areas can be seen where unaltered copper is still present. This is a fine miniature that displays well from all sides, and its origin from this classic locality makes is that much better. A former Australian dealer had this one priced at $1,600, and while I’m not sure if that was USD or AUD, this is a great deal either way. 

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The Broken Hill massive sulfide deposit formed around 1685 Ma through submarine volcanic-exhalative processes, hosting primarily galena and sphalerite with minor chalcopyrite. During recent weathering over the past 500 million years, the oxidation zone developed secondary copper minerals including native copper that subsequently altered to malachite. These pseudomorphs typically preserve dendritic or irregular copper crystal forms now completely replaced or coated by bright green fibrous malachite, often found in association with cuprite and azurite in the friable gossan material. What makes these specimens geologically interesting is that they document supergene weathering affecting one of Earth's oldest and largest lead-zinc-silver deposits - the native copper formed through reduction processes in the oxidation zone, then almost immediately began altering to copper carbonates. The preservation of original copper morphology beneath malachite replacement allows these specimens to illustrate textural relationships in secondary mineral formation. While Broken Hill's malachite after azurite specimens are more famous, the copper pseudomorphs remain comparatively scarce since the deposit's primary mineralization contains relatively little chalcopyrite compared to the dominant lead-zinc sulfides.