Henrik Stendal

Shallow-water, eruption-fed, mafic pyroclastic deposits along a Paleoproterozoic coastline: Kangerluluk volcano-sedimentary sequence, southeast Greenland

Abstract The 200–300 m thick, volcano-sedimentary sequence at Kangerluluk is part of the psammite zone, one of four major zones, which constitute the 1.8 Ga Ketilidian orogen in south Greenland. Three lithofacies are emphasized in the study: (1) the conglomerate-sandstone; (2) the volcanic; and (3) the pyroclastic lithofacies. The 2–40 m thick conglomerate-sandstone lithofacies represents a subaerial to subaqueous fan-delta deposit. Matrix- and clast-supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits. Erosive-based conglomerate channel fills attest to stream incision. Trough crossbedded sandstone, interpreted as lunate megaripples, planar-bedded sandstone indicative of upper flow regime bar-top sands, and small-scale trough crossbeds reflecting ripples follow up-section, form collectively with the conglomerate, 0.40–2.50 m thick fining-upward sequences. The sandstone-dominated unit, up-section from the conglomerates and composed of planar and low-angle crossbeds, minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base. The clastic sedimentary rocks are suggestive of a fan-delta setting. The 100–200 m thick volcanic lithofacies, composed of pillowed and pillow brecciated lava flows, is consistent with shallow-water deposition. Interstratification of lava flows with both conglomerate-sandstone and pyroclastic lithofacies, intrusion of dykes into volcaniclastic rocks, and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs. The 1–50 m thick, pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate-sandstone lithofacies, was emplaced in a subaqueous setting. The lithofacies is divided into a planar- to crossbedded tuff-lapilli tuff and a bedded lapilli tuff breccia, whereby both deposits are inferred to result from shallow-water surtseyan-type eruptions. The 5–15 m thick, bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes. The deposits are considered proximal to the vent. The 2–50 cm thick, planar- to crossbedded tuff-lapilli tuff featuring abundant euhedral and broken crystals of feldspar (≤2cm) and minor pyroxene (≤1 cm), are massive, graded, crossbedded and stratified. The planar but laterally discontinuous beds, characterized by abundant low-angle scours, are interpreted as low- to high-concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion. Local breccia-size pyroclasts disrupting beds are interpreted as bomb sags. The mafic, eruption-fed, Surtseyan-type deposits, postulated to be a subaqueous counterpart of cold, subaerial base surges, originate from subaqueous tuff cones formed along a rugged volcanic-dominated shoreline featuring high-energy fan-deltas.

Gold occurrences and lead isotopes in Ketilidian Mobile Belt, South Greenland

Abstract Gold occurrences have been found in the Palaeoproterozoic Ketilidian Mobile Belt of South Greenland within the past ten years. The mineralization occurs in various settings with different element associations: Au in the Archaean; Au in the Palaeoproterozoic border zone; the association Au-Bi-(Ag-As-W-Cu-Mo) in the Julianehåb batholith; the association Au-As in supracrustal rocks; and the association Au-Cu within a volcano-sedimentary sequence at the edge of the Julianehåb batholith. The emplacement of the juvenile I-type Julianehåb batholith lasted from 1850 to 1800 m.y. with late-stage intrusions until 1770 m.y. Emplacement of the gold mineralization is considered to have taken place in the late stage of formation of the batholith (1800-1770 m.y.) and during the regional deformation and metamorphism before the intrusion of rapakivi granites. Local remobilization of metals, however, was caused by hydrothermal activity around some rapakivi granitoids. In essence, therefore, gold deposition in South Greenland is restricted to a quite narrow time interval. Lead isotopes from different occurrences in South Greenland indicate two main stages of gold emplacement. The first was related to the Palaeoproterozoic regional deformation and metamorphism (1792-1785 m.y.), during which sediment-hosted gold was epigenetically remobilized into shear zones and vein systems. Pb in these occurrences is indicative of a ca 2000-m.y. source, which is compatible with the direct basic host rocks to these occurrences. The second stage of gold deposition appears to have been temporally related to late stages of emplacement of the Julianehåb batholith. The source of the slightly more evolved Pb in these occurrences is difficult to assess, but a mixture of juvenile Pb from the batholith with some contributions from the host rocks may explain the scatter of data around a 1780-m.y. reference line.