Peter M. Herzig

Comparative mineralogy and geochemistry of gold-bearing sulfide deposits on the mid-ocean ridges

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 ( 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.

Input from the Deep: Hot Vents and Cold Seeps

The discovery of black smokers, massive sulfides and vent biota at the crest of the East Pacific Rise at 21°N in 1979 (Francheteau et al. 1979, Spiess et al. 1980) confirmed that the formation of new oceanic crust through seafloor spreading is intimately associated with the formation of metallic mineral deposits at the seafloor. It was documented that the 350°C hydrothermal fluids discharging from the black smoker chimneys at this site at a water depth of about 2600 m continuously precipitate metal sulfides in response tomixing of the high-temperature hydrothermal fluids with ambient seawater. The metal sulfides ineluding pyrite, sphalerite, and chalcopyrite eventually accummulate at and just below the seafloor and have the potential to form a massive sulfide deposit. It has also been documented that the circulation of seawater through the oceanic crust is the principal process responsible for leaching and transport of metals in this environment.

Hydrothermal activity in the Lau Basin: First results from the NAUTILAU Cruise

The Lau Basin, a back arc spreading center, is one of the most active hydrothermal areas in the ocean. A scientific team from France, Germany, and Tonga investigated the southern Lau Basin near Tonga in 1989 to study the processes of seafloor ore-mineral formation associated with hydrothermal circulation along the volcanic Valu Fa ridge (Ride de Valu Fa in Figure 1), which lies in back of the Tonga-Kermadec trench. Between April 17 and May 10 scientists on the R/V Nadir used the submersible Nautile to make 22 dives in the southern Lau Basin. The cruise was called NAUTILAU, for Nautile in Lau Basin. In addition to the standard equipment of the submersible (video and photo cameras, and temperature probe), a CTD (conductivity-temperature-depth) instrument was integrated with a “mini rosette” water sampling device used for the first time on the Nautile to obtain correlations between the geological observations and the physical and chemical anomalies measured in the seawater.

Auriferous hydrothermal precipitates on the modern seafloor

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.