Setting of Zn-Cu-Au-Ag massive sulfide deposits in the evolution and facies architecture of a 1.9 Ga marine volcanic arc, Skellefte District, Sweden

Abstract

The Skellefte mining district occurs in an Early Proterozoic, mainly 1.90-1.87 Ga (Svecofennian) magmatic province of low to medium metamorphic grade in the Baltic Shield in northern Sweden. The district contains over 85 pyritic Zn-Cu-Au-Ag massive sulfide deposits and a few vein Au deposits and subeconomic porphyry Cu-Au-Mo deposits. The massive sulfide deposits mainly occur within, and especially along the top of, a regional felsic-dominant volcanic unit attributed to a stage of intense, extensional, continental margin arc volcanism. From facies analysis we interpret the palcogeography of this stage to have comprised many scattered islands and shallow-water areas, surrounded by deeper seas. All the major massive sulfide ores occur in belowwave base facies associations; however, some ores occur close to stratigraphic intervals of above-wave base facies associations, and the summits of some volcanoes that host massive sulfides emerged above sea level. Intense marine volcanism was superceded at different times in different parts of the district by a stage of reduced volcanism, uplift resulting in subregional disconformities, and then differential uplift and subsidence resulting in a complex horst and graben palcogeography. Uplift of the arc is attributed to the relaxation of crustal extension and the eraplacement of granitoids to shallow crustal levels. A few massive sulfide ores formed within the basal strata of this second stage. The horst and graben system was filled by prograding fluvial-deltaic sediments and mainly mafic lavas, and during this stage the Skellefte district was a transitional area between renewed arc volcanism of more continental character to the north, and subsidence and basinal mudstone-turbidite sedimentation to the south. This whole volcanotectonic cycle occurred within 10 to 15 m.y. We define 26 main volcanic, sedimentary, and intrusive facies in the Skellefte district. The most abundant facies are (1) normal-graded pumiceous breccias, which are interpreted as syneruptive subaqueous mass flow units of pyroclastic debris, (2) porphyritic intrusions, and (3) mudstone and sandstone turbidires. Facies associations define seven main volcano types, which range from basaltic shields to andesitc cones and rhyolite calderas. Despite this diversity of volcano types, most massive sulfide ores are associated with one volcano type: subaqueous rhyolite cryptodome-tuff volcanoes. These rhyolite volcanoes are 2 to 10 km in diameter, 250 to 1,200 m thick at the center, and are characterized by a small to moderate volume rhyolitic pyroclastic unit, intruded by rhyolite cryptodomes, sills, and dikes. Massive sulfide ores occur near the top of the proximal (near vent) facies association. The remarkable coincidence in space and time between the ores and this volcano type indicates an intimate, genetic relationship between the ores and the magmatic evolution of the volcanoes.Many of the massive sulfide ores occur within rapidly eraplaced volcaniclastic facies and are interpreted to have formed by infiltration a...

Actively forming Kuroko-type volcanic-hosted massive sulfide (VHMS) mineralization at Iheya North, Okinawa Trough, Japan

“…An underlying discordant zone of siliceous ore is often associated with the underlying chloritic stockwork with a general zonation from yellow ore (quartz>pyrite>chalcopyrite) in the middle, toward a more sphalerite-galena rich mineral assemblage at the base of the overlying lens, and a quartz>pyrite>>chalcopyrite mineralogy at depth (Eldridge et al, 1983). Classic examples of Kuroko-type VHMS deposits are found in the Honshu arc of Japan in the Hokuroku district, with deposit clusters associated with back-arc rifting (Yamada and Yoshida, 2011), the Mount Read volcanic belt of SE Australia (Large et al, 2001b), Buchans Group of Newfoundland (Piercey et al, 2007), and Skellefte distict of northern Sweden (Allen et al, 1996). At Iheya North, massive sulfides intercepted at the base of North Big Chimney strongly resemble the black ore of the Miocene-age Kuroko deposits of Japan (Sato, 1974(Sato, , 1977, with equivalents to the yellow, gypsum and siliceous ores also recogized by Ishibashi et al (2013).…”

Geology and Age Constraints on the Origin of the Intrusion-Related, Sheeted Vein-Type Åkerberg Gold Deposit, Skellefte District, Sweden

“…The Skellefte district forms a NW-SE trending belt comprising early Proterozoic rocks (Figure 1). The district is delineated by juvenile metavolcanics, and its geology has been dealt with in numerous papers with excellent summaries presented by Allen et al [1] and Kathol & Weihed [25]. The bedrock adjacent to Åkerberg is dominated by 1.9-1.8 Ga old Svecofennian supracrustal rocks and granitoids of different generations.…”

“…The bedrock adjacent to Åkerberg is dominated by 1.9-1.8 Ga old Svecofennian supracrustal rocks and granitoids of different generations. Supracrustal rocks are typically divided into the Bothnian Supergroup, consisting mainly of metagreywackes of the Bothnian Basin, and the Skellefte, Arvidsjaur and Vargfors Groups [1].…”

“…On a regional scale, deformation has been interpreted to comprise two regional fold phases preceded by a possibly synvolcanic event producing a vague, mineral lineation [1]. The first regional (D2) phase resulted in upright, tight to isoclinal folds and related shear zones.…”

Facies architecture of the Early Devonian Ural Volcanics, New South Wales

“…The facies architecture of the Ural Range area is typical of subaqueous volcanic successions that are dominated by felsic lavas and sills (Figure 17) (Allen et al 1996b;Paulick & McPhie 1999;Doyle & McPhie 2000). An important feature of the lavas is the substantial volume of autoclastic facies present and the predominance of autobreccia.…”