Abundant new data about the geology and geochemistry of greenstone-hosted gold lodes have emerged during the worldwide, intensive search for these ores for the last decade. Unfortunately, neither the resulting descriptions of their field relationships nor the data from fluid inclusion, light stable isotopic and radiometric age studies are definitive of a single, unique genetic process. Instead, some of these relationships are best explained by an epigenetic process, others by a synsedimentary one, and many are equivocal and explainable by either. A generally acceptable genetic hypothesis for the many, various forms of these lodes must account not only for all field and laboratory data but, also, must explain the sources of the various components of the ores, and the mechanisms of their mobilization, transport and deposition. A multi-stage, multi-process hypothesis is necessary to provide such a comprehensive explanation. The initial genetic stage involved deuteric reactions between seawater and thick, subaqueous tholeiitic basalts low in the greenstone belt successions. These reactions did not involve deep, sub-seafloor convective circulation of seawater, as in the generation of volcanogenic massive sulfide deposits higher in the successions, but occurred at shallow depths under the strongly reduced conditions of Archean time. They leached gold, silica, carbonate and other components of the ores from the lavas and moved upward, carbonatizing and variably altering the sea floor strata that they penetrated. They were discharged passively over broad areas to the sea floor, where their leached components were deposited as auriferous exhalites, variably sulfidic, ankeritic, magnetitic and siliceous. In places these were pure chemical precipitates, but elsewhere were mixed with variable amounts of clastic or pyroclastic detritus.This accounts for the widespread low, but anomalous gold content of Algoma type iron-formations and for the absence of highly altered rocks marking focused discharge vents beneath the few exhalites that contained enough gold to be ore. Reworking of the auriferous exhalites and enclosing altered lavas along the flanks of growing Archean volcanic complexes shed rounded clasts of auriferous sulfide and ankeritized, fuchsitic rocks into flanking basins, forming pre-metamorphic conglomerates that are essentially coeval with, but locally disconformable or unconformable on their source greenstones. These weakly auriferous strata were important pre-enriched source-rocks during multiple, superimposed stages of metamorphic remobilization, re-deposition and further concentration of gold. Successively, these stages include; burial diagenesis involving compaction, dehydration and lithification; syn-kinematic deformation during subsidence; intrusion of small, sub-volcanic porphyritic plutons that were late but co-magmatic in greenstone belt evolution; orogenic plutonism that accompanied deep-seated granulitization of the belts, and; post-orogenic intrusions. All these formed late, discordant, epigenetic veins, vein systems or stockworks that cut all earlier rocks and one another. Most fluid inclusion and light stable isotopic studies are of minerals from these late, epigenetic veins, not from the earlier, fine-grained syn-sedimentary strata; and their results cannot be generally applied to the latter. The studies show that the late veins were formed by CO 2-rich fluids at 200°–400°C and depths of 3–4 km. The same fluids formed not only mineralized veins in the pre-enriched source-rocks but, also, barren veins widespread throughout most Archean terranes where the source-rocks were absent. The source of these fluids remains problematical. They may have originated from dehydration of rocks undergoing deep-seated granulitic metamorphism, from reactions between fluid and earlier carbonate-rich rocks of the greenstone belts at lower metamorphic rank, by magmatic crystallization-differentiation, or from all of these.
Read full abstract
Oct 18, 2024
Andrea Galli + 12
Open Access
