Supergene Gold Mobility: A Textural and Geochemical Study from Gold Placers in Southern New Zealand
Donna M. Falconer, Dave Craw, 2009. "Supergene Gold Mobility: A Textural and Geochemical Study from Gold Placers in Southern New Zealand", Supergene Environments, Processes, and Products, Spencer R. Titley
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Scanning electron microscopy was used to observe a variety of gold remobilization textures and authigenic gold overgrowths on detrital gold particles associated with quartz pebble conglomerates (QPC) from two localities in eastern Southland, New Zealand. Gold spheroids, spheroids with budlike protrusions, polyspheroidal aggregates, budded mass structures, and pseudohexagonal plates are common on detrital gold particles at these two sites. There has been no anthropogenic introduction of mercury or amalgam at either site. Some gold precipitation textures can be distinguished from some gold dissolution textures. Precipitation textures are characterized by surface textures and pure gold rims that occur on the outermost margin of sharply defined silver (± mercury)-depleted rims in most detrital particles. In contrast, dissolution textures are characterized by varying degrees of dissolution occurring preferentially along subgrain boundaries, thereby revealing the polycrystalline makeup of placer gold. Surface textures reflect a close association between gold precipitation and gold dissolution which may lead to the ambiguity inherent in interpreting gold-remobilization textures from placer environments.
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At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years.
Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting.
As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.