The gold—carbonate association: Source of CO 2, and CO 2 fixation reactions in Archaean lode deposits

Abstract

Abundant carbonate is a ubiquitous feature of Archaean lode gold deposits — both in domains of alteration enveloping veins, and in rocks enclosing auriferous sediments. Detailed studies of chemical mass balance in metabasalts, progressively altered towards gold-bearing quartz carbonate veins disposed within shear zones at Yellowknife, reveal massive additions of CO 2, K, Si and Fe accompanying mineralisation, with concomitant depletions of Na. Coherent behaviour of Al, Sc, Zr (also V and Nb) provides a reference for constraining the volume relations during hydrothermal alteration. In the peripheral regions of alteration, depletions of Ca and Mg result in overall volume reduction, but these elements are added to veins and their immediate alteration envelopes where there is a large positive volume change. Precious metals, together with Cr, Ni, Cu, Zn, Pb, Rb, Cd and Ba, have been added to the veins and altered wall rocks. Whereas quartz, noble metals and other trace elements have been precipitated from hydrothermal fluids, solution of Ca, Mg (and some Fe), indigenous to peripheral alteration regions, combining with CO 2 from the mineralising reservoir, appears to be the process for forming the abundant ferri-dolomite gangue. Observations of many Archaean Au deposits reveals that carbonate chemistry reflects the nature of wall rocks, with wall rocks donating the bivalent metal cations, and hydrothermal fluids the CO 2. Alteration reactions at Yellowknife involved hydrolysis of albite accompanied by fixation of K aqueous to produce muscovite, with Na loss; and hydrolysis of chlorite + epidote with CO 2 fixation to form ferri-dolomite. Studies of oxygen isotopes, Fe 2+ ( Fe 2+ + Fe 3+) , and structure reveal that Au, quartz and carbonate were precipitated in the presence of fluids of probable metamorphic origin ( δ 18 O + 8–9‰ ), at low redox potential, and at ambient temperatures of 400–450°C, during episodes of hydrofracturing. The abundant CO 2 and K, required for extensive carbonate—muscovite replacement alteration, could be supplied by fluids released during prograde metamorphism under greenschist or greenschist—amphibolite facies conditions, where the relative proportion of CO 2 H 2 O is in the order of 0.2 to 0.5, and K Na ⋍ 1 . Given high CO 2CO in the hydrothermal reservoir, these molecules may act as complexing agents for transport of Au and other rare elements (e.g., W, Pd, Ni, Cr).