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: The distribution of gold has been examined in sub-seafloor stockworks from two sections of hydrothermally altered oceanic crust beneath the axial zones of fossil spreading centers. Deep Sea Drilling Project hole 504B (Leg 83) contains a narrow zone of stockwork-like sulfides in 6 Ma old basalt from the transition zone between sheeted dikes and overlying pillow lavas (910-928 m). Mineralization occurred as a result of local mixing between ascending hydrothermal fluids (350-degrees-C) and seawater that penetrated the top of the dike section. Previous studies indicate that a significant amount of gold was leached from the sheeted dikes during high-temperature greenschist-facies alteration, but mineralized wall rock in the transition zone is not substantially enriched in gold. Sulfide concentrates from the narrow stockwork average 26 ppb Au, and one sample of As-rich pyrite from a quartz-epidote breccia contains 100 ppb Au. Cyprus Custal Study Project hole CY-2a contains an equivalent section of altered Cretaceous pillow lavas from the Troodos Ophiolite but includes a near-surface stockwork from the ore zone of the Agrokipia B deposit. The combined effects of hydrothermal metasomatism and regional metamorphism are represented by zeolite facies mineralogy above the ore zone (0-154 m), intense silicification and argillic alteration within the stockwork (154-300 m), and propylitic alteration at depth (300-400 m). The sheeted dikes below 400 m are altered uniformly to greenschist-facies mineralogy. Extensive sulfide mineralization in the pillow lavas occurred within a few hundred metres of the seafloor in response to a steep thermal gradient caused by mixing of high-temperature fluids with cold seawater. Pyrite from the stockwork ore contains up to 480 ppb Au and averages 160 ppb Au. A narrow zone of quartz-sulfide veinlets also occurs at the pillow-dike transition. Two samples of As-rich pyrite (up to 0.75 wt.% As) from this zone contain 980 ppb Au, but gold contents in fracture-filling and disseminated pyrite throughout most of the transition zone and sheeted dikes are 〈 20 ppb Au. Despite local enrichment within specific sulfide phases, deep sub-seafloor mineralization does not appear to have been an important sink for gold in either CY-2a or 504B. At higher levels in the crust, as in Agrokipia B, the locus and extent of mixing may be important controls on the intensity of mineralization and the deposition of gold within near-surface stockworks. Without an effective means of interrupting the flow of high-temperature fluids to vents on the seafloor, gold may be carried through the upflow zone at the time when high-temperature stockworks are forming.

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.

Description: The TAG hydrothermal field is a site of major active and inactive volcanic-hosted hydrothermal mineralization in the rift valley of the slow-spreading Mid-Atlantic Ridge at 26[degree]N. The axial high is the principal locus of present magmatic intrusions. The TAG field contains three main areas of present and past hydrothermal activity: (1) an actively venting high-temperature sulfide mound; (2) two former high-temperature vent areas; (3) a zone of low-temperature venting and precipitation of Fe and Mn oxide deposits. The volcanic centers occur at the intersections between ridge axis-parallel normal faults and projected axis-transverse transfer faults. The intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources. Radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 [times] 10[sup 3] yr. The sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow-spreading oceanic ridge. Hydrothermal reworking of a relict hydrothermal zone by high-temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge.

Description: Hydrothermal deposits occur a[ a water depth of 2460 m in Middle Valley, a sediment-fllled railed rift at 48'n' N htitude, on the nordrern Juan de Fuca Ridge. Sites of focused discharge are concentrated above a deep-seated system of fractures paxallel to the regional north-south trend of rift-propagating faults. The Area of Active Venting (AA$ and Bent Hill area are hosts to moderate-temperature (〈276oC) vents. Active chimneys are sulfate-rich and sulfide-poor, consisting of a thick outer wall of anhydrile and a thin clay-rich inner wall with very fine (20 pm) disseminated sulfides. The inner wall contains saponite, pyrrhotite, chalcopyrite, isocubanite, high-iron sphalerite (35-46 mole 7o FeS), pydte, marcasite, galena and arsenopyrite. The active chimneys have low concentrations ofthe base metals but ahigh CulZn value. The fluids discharging through these chimneys have characteristically low a(O2) and a(S2), and locally have deposited significant amounts of hydrocarbon. Hydrothermal mounds associated with active venting are produced by the collapse of chimneys and by mineral precipitation within the pile of sulfide-sulfate talus (i.e., by inflation). The active mounds are composed of anhydrite, amorphous silic4 saponite, serpentine, and minor chalcopyrite, low-iron sphalerite (〈15 mole 7o FeS), lepidocrocite, pyrite, marcasite, galena and barite. Inactive chimneys on or adjacent o the mounds are predominantly barite-rich, with lesser amorphous silic4 hydmcarbons, clay and marcasite. In contrast to the active chimneys, the Bent Hill deposits are sulfide-rich and have higher ZnlCu values. Two older massive sulfide deposits in the Bent Hill area contain pyrite, lepidocrocite, marcasite, moderately iron-rich sphalerite (24-33 mole 7o FeS), chalcopyrite, covellite, galena, silica and barite. Pb-As-Sb sulfosalts occur locally within lowtemperature, silica-rich crusts on the massive sulfides. The mineralogy and geochemistry of the active chimneys suggest hat the present stage of hydrothermal venting at Midclle Valley is botl metaland sulfur-depleted and is likely the product of the moderate-temperature interaction of hydrothermally modified seawater with local sediments. The massive sulfides at Bent Hill are apparently a product of an earlier hydrothermal event dominated by metaland sulfur-enriched fluids, possibly derived from the high+emperature interaction of circulating seawater with basaltic crust in the basement. Recent drilling by the ODP has identified a large pyrite pyrrhotite magnetite deposit beneath the sulfide outcrop at Bent Hill. The present-day mineralogy of this older massive sulfide deposit may be the product of extensive reaction with the cool, sulfur-depleted fluids that are curently venting at Middle Valley.