Chemical Composition and Formation of a Massive Sulfide Deposit, Middle Valley, Northern Juan de Fuca Ridge (Site 856)

Abstract

Studies of sulfide cores from Site 856, Middle Valley, northern Juan de Fuca Ridge, established the vertical zonation of a large inactive oceanic massive sulfide deposit. Zone 1 consists mainly of pyritic massive sulfide with minor sphalerite and magnetite (0 to 28 mbsf in Hole 856H and 0 to 40 mbsf in Hole 856G). Compared with the rest of the deposit, this zone is strongly enriched in Zn and a large number of trace metals (in particular Cd, Mn, Sn, and Sb), and in elements contained in gangue minerals (Si, Al, Mg, and Ba). Most of these elements reach their maximum concentrations in the upper part of the zone; the lower part is enriched in Cu, Se, and Ca. Zone 2 consists dominantly of pyrrhotite-rich massive sulfide with minor pyrite and magnetite (28 to 48 mbsf in Hole 856H) and is depleted in minor elements. Zone 3 is almost pure pyritic massive sulfide (48 to 75 mbsf in Hole 856H and 40 to 65 mbsf in Hole 856G) and is relatively enriched in Ge, As, Sb, Pb, and Tl. Zone 4 consists of dominant pyrrhotite with minor chalcopyrite (75 to 95 mbsf in Hole 856H) and shows a maximum enrichment in Cu, Co, and Bi. Correlation analysis shows that elements enriched in zone 1 are associated with different ore-forming and gangue minerals: sphalerite (Cd and Pb), talc and/or chlorite (Sc, Th, and REE), silica (Hg and Ta), dolomite (V), and barite (Sr, Zr, and Hf). Sb, As, Sn, Au, Ag, Mn, Mo, and Tl form a separate association, presumably related to sulfosalt minerals. Enrichment of Zn and minor elements in the upper part of the deposit, which is typical of ancient massive sulfide deposits, is attributed to the zone-refining process of progressive upward replacement of lower temperature minerals by higher temperature minerals during growth of the sulfide body, with the concomitant hydrothermal leaching of trace metals. Later, the deposit was hydrothermally altered, probably due to lateral flow of evolved seawater along turbidite layers. The alteration produced replacement of primary pyrrhotite by pyrite in zone 3 and by pyrite + magnetite assemblages in the upper zones. In the upper levels of the deposit, sulfate-rich water acted as a strong oxidant. At lower levels a hotter and more evolved sulfur-enriched pyritizing solution also introduced additional Pb, Sb, As, and other sediment-derived elements into zone 3. The absence of light rare earth element enrichment and the existence of a positive Eu anomaly in the massive sulfides of this zone is connected with the alteration of plagioclase in the associated turbidites. The secondary convection system that caused alteration of the massive sulfides could have been driven by one of the sills emplaced into Middle Valley sediments subsequent to sulfide formation.

Rare earth element and yttrium analyses of sulfides from the Dachang Sn-polymetallic ore field, Guangxi Province, China: Implication for ore genesis

“…Barrett et al (1990) firstly reported several massive sulfide-sulfate samples from the Southern Explorer Ridge, which have relatively flat REE patterns with positive Eu anomalies and negative Ce anomalies. Such REE patterns are also found in massive sulfides from Middle Valley on the Juan de Fuca Ridge (Krasnov et al, 1994) and oxides from the TAG mound (Mills and Elderfield, 1995). Analysis of hydrothermal plume particles has shown that the particles close to an active vent-field exhibit REE patterns of both seawater and vent-fluid characteristics (i.e., negative Ce anomalies and positive Eu anomalies), whereas at increasing dispersion, seawater characteristics become dominant (German et al, 1999) Mitra et al, 1994).…”

“…End member hydrothermal fluids at active black smoker vents have enriched REE concentrations (10~10,000 × seawater concentrations) and chondrite normalized REE patterns with a large positive Eu anomaly, no Ce anomaly, and enrichment in the LREE compared to the HREE (Klinkhammer et al, 1994;Mitra et al, 1994;Bau and Dulski, 1999). Massive sulfides, anhydrites, barites and oxides collected on the sea floor, such as those from the Red Sea, the Southern Explorer Ridge, the East Pacific Rise, Middle Valley on the Juan de Fuca Ridge and TAG, also show positive Eu anomalies and LREE-enriched patterns (Barrett et al, 1990;Krasnov et al, 1994;Mills and Elderfield, 1995;German et al, 1999;Bach et al, 2003). Positive Eu anomalies are reported in some samples of massive sulfides from ancient massive sulfide deposits, such as the Bathurst district of New Brunswick, Broken Hill in Australia, Sullivan in British Columbia, Kuroko in Japan and so on (Lottermoser, 1989;Jiang et al, 2000;Slack et al, 2000;Terakado and Walker, 2005).…”

Hydrothermal Alteration of Basalts beneath the Bent Hill Massive Sulfide Deposit, Middle Valley, Juan de Fuca Ridge

“…2 and 3) indicate a subhorizontal base to the massive sulfide lens, which is interpreted as a time horizon marking the onset of exhalative hydrothermal activity at this site some 220,000 to 140,000 years ago (Mottl et al, 1994). Textural evidence suggests that the bulk of the sulfide was precipitated at the sea floor, with little evidence for the replacement or intercalation of sediment Krasnov et al, 1994). The primary minerals were dominantly pyrrhotite with less abundant Cu-Fe sulfide (isocubanite) and Zn sulfide (sphalerite ± wurtzite).…”

“…This assemblage together with fluid-inclusion trapping temperatures suggests precipitation from fluids venting at 350°to 400°C, precluding formation from the lower temperature fluids presently venting from the ODP mound . Low As, Sb, and Mo concentrations in the primary sulfides, as well as Pb and He isotope constraints, indicate that the mineralizing fluids attained their chemical and thermal characteristics dominantly through reaction with basaltic basement rather than with overlying sediments Krasnov et al, 1994;Stuart et al, 1994;Stuart et al, 1999;Bjerkgård et al, 2000;Cousens et al, 2002). The deposit has been extensively recrystallized to pyrite ± magnetite by hydrothermal zone refining Krasnov et al, 1994).…”

“…Low As, Sb, and Mo concentrations in the primary sulfides, as well as Pb and He isotope constraints, indicate that the mineralizing fluids attained their chemical and thermal characteristics dominantly through reaction with basaltic basement rather than with overlying sediments Krasnov et al, 1994;Stuart et al, 1994;Stuart et al, 1999;Bjerkgård et al, 2000;Cousens et al, 2002). The deposit has been extensively recrystallized to pyrite ± magnetite by hydrothermal zone refining Krasnov et al, 1994). Conservative estimates suggest a minimum size of about 9 Mt, although this figure includes neither the subsurface feeder zone nor the extensive mineralization associated with the ODP mound, which could potentially double the size of the deposit .…”