ALBERT

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 reconnaissance study of 19 volcanogenic massive sulphide deposits in the Notre Dame Bay area indicates Au concentrations of up to 30 ppm (Betts Cove), and elevated gold contents (〉1 ppm Au) have been found in samples from 10 additional past producers and developed prospects. Systematic trends in the occurrence of gold are observed in two principal sulphide assemblages: polymetallic, pyrite–sphalerite–chalcopyrite–galena ± arsenopyrite assemblages (type I) and pyrite–chalcopyrite ± sphalerite ± pyrrhotite assemblages (type II). Type I assemblages occur in deposits with dominantly felsic host rocks, whereas type II assemblages are restricted to deposits in mafic-dominated ophiolite sequences. Free gold grains were observed in samples from eight different deposits in both type I and type II assemblages. X-ray emission spectra and electron microprobe analyses of the gold indicate that most grains are electrum, although a Au-bearing telluride occurs at Point Leamington. Ion microprobe analyses indicate that as much as 50% of bulk gold may be present as "invisible gold" locked in pyrite or arsenopyrite (up to 140 ppm Au at Point Leamington). Well-preserved primary depositional features in gold-bearing sulphides from several deposits suggest that the gold in type I assemblages is syngenetic. A strong correlation between gold and a polymetallic suite of Zn, Ag, Pb, As, and Sb, similar to that observed in Kuroko-type massive sulphides and in modern seafloor sulphides, supports a primary origin for gold in type I assemblages. In type II assemblages primary depositional features have been largely destroyed by deformation and annealing of sulphide grains. Gold is locally enriched in Zn-rich sulphides, and sulphides containing abundant pyrrhotite have the lowest gold contents. However, consistent geochemical associations with other elements are not observed, and this may reflect the strong remobilization of gold during structural deformation. The abundant free gold in some type II assemblages is a product of recrystallization during deformation and was derived locally from primary gold originally present in the host sulphides.