ALBERT

Description: A routine method for the direct determination of Au and associated trace metals in sulfides has been developed for use with a low-flux nuclear reactor (SLOW-POKE II Reactor Facility, University of Toronto). Small samples (100-200 mg) are analysed simultaneously for Au, Ag, As, Sb, Mo, Co, Se, Cd, Fe, and Zn. Sulfides containing finely divided gold can be analysed with a high degree of precision and yield results which agree closely with commercial fire assay-atomic absorption analyses for Au. Favourable comparisons with other analytical techniques (flameless atomic absorption and emission spectrometry) are also indicated for Ag,As, and Sb. Neutron activation has the advantage of providing precise and accurate results for these elements without chemical separation or preconcentration. Analyses of 10 sulfide-bearing reference materials from CANMET, Ottawa are compared with published best values for selected trace metals including Au.

Description: A comparative study of the mineralogy and geochemistry of sulfide deposits on mid-ocean ridges in the Northeast Pacific and the Mid-Atlantic reveals common characteristics associated with primary gold enrichment. Average gold contents of 0.8 to 5 ppm Au occur in sulfides from Southern Explorer Ridge and Axial Seamount (Northeast Pacific) and from the TAG hydrothermal field and Snakepit vent field (Mid-Atlantic Ridge). The enrichment of gold in these deposits is consistently related to a phase of late-stage, low-temperature (〈 300°C) venting. Concentrations 〉 1 ppm Au occur exclusively in pyritic assemblages and commonly with abundant Fe-poor sphalerite and a suite of complex Pb—Sb—As sulfosalts. Amorphous silica and, locally, barite or carbonate are important constituents of the gold-rich precipitates but do not contain gold themselves. High-temperature (350°C) black smoker assemblages, consisting dominantly of pyrite, chalcopyrite, pyrrhotite, isocubanite and abundant anhydrite are uniformly gold-poor (≤0.2 ppm Au). To the extent that individual sulfides can be mechanically separated, chemical analyses by neutron activation indicate that gold is most abundant in sphalerite (up to 5.7 ppm Au) but also occurs in pyrite and marcasite. Samples of sphalerite with abundant inclusions of fine-grained sulfosalts locally contain up to 18 ppm Au, suggesting that sulfosalts may be repositories for gold. No free gold has been observed at 4000 × magnification of polished specimens, indicating that the gold is present only as submicroscopic inclusions or as a chemical constituent within the sulfides. Samples from gold-rich deposits in the Northeast Pacific and Mid-Atlantic are compared with similar but relatively gold-poor sulfides from the Galapagos Rift and 13°N on the East Pacific Rise (EPR), and with barren sulfides from 11°N EPR, 21°N EPR, the Endeavour Ridge, and the Southern Juan de Fuca Ridge. Trace element analyses of more than 170 samples show that gold enrichment in almost all of the deposits is associated with high concentrations of Ag, As, Sb, Pb and Zn, and locally with high Cd, Hg, Tl, and Ga. In contrast, gold is typically depleted in samples with high Co, Se, and Mo. The close association of Au with Ag, As, Sb, and Pb may reflect the common behavior of these metals as aqueous sulfur complexes (e.g., [Au(HS)−2]) at low temperatures. Similar mineralogical and geochemical associations are observed in sulfide deposits from modern back-arc settings and in the ancient geologic record.

Description: Submarine hot springs were probable sources for gold enrichment in a variety of rock types which host mineable gold deposits. These include iron formations, mixed chemical and clastic sediments, tuffaceous exhalites, and disseminated or massive sulphides in both volcanic-and sediment-dominated sequences. Gold-bearing iron formations and interflow metalliferous sediments associated with seafloor hydrothermal activity also have been implicated as potential source rocks for some nonstratabound gold deposits in ancient greenstone belts (Foster and Wilson, 1984; Keays, 1984). Recent studies of gold in volcanogenic massive sulphides indicate a strong genetic relationship between gold and sulphide mineralization in seafloor hydrothermal systems (Hannington and Scott, 1989; Large et al., 1989; Huston and Large, 1989). The total past production and current reserves of gold in massive sulphides world wide amount to nearly 2900 t Au and indicate that modified seawater is capable of transporting and depositing significant amounts of gold. In addition, the discovery of gold-rich sulphides actively forming at hydrothermal vents on the modern seafloor has confirmed the existence of gold-bearing fluids in submarine hot springs and supports a seafloor hydrothermal origin for gold in many preserved deposits now on land. The documentation of fluid chemistry at active vents also has served to constrain the conditions of gold mineralization on the present-day seafloor. In this chapter, we describe the occurrence and distribution of gold in modern hot spring deposits and discuss aspects of gold transport and deposition in seafloor hydrothermal systems with reference to possible implications for the origin of gold deposits in auriferous chemical sediments.