23 research outputs found
Geology, sulfide geochemistry and supercritical venting at the Beebe Hydrothermal Vent Field, Cayman Trough
The Beebe Vent Field (BVF) is the world's deepest known hydrothermal system, at 4960m below sea level. Located on the Mid-Cayman Spreading Centre, Caribbean, the BVF hosts high temperature (âŒ401°C) âblack smoker' vents that build Cu, Zn and Au-rich sulphide mounds and chimneys. The BVF is highly gold-rich, with Au values up to 93 ppm and an average Au:Ag ratio of 0.15. Gold precipitation is directly associated with diffuse flow through âbeehive' chimneys. Significant mass-wasting of sulphide material at the BVF, accompanied by changes in metal content, results in metaliferous talus and sediment deposits. Situated on very thin (2-3km thick) oceanic crust, at an ultraslow spreading centre, the hydrothermal system circulates fluids to a depth of âŒ1.8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies. We suggest hydrothermal interaction with chalcophile-bearing sulphides in the mantle rocks, together with precipitation of Au in beehive chimney structures, has resulted in the formation of a Au-rich volcanogenic massive sulphide (VMS) deposit. With its spatial distribution of deposit materials and metal contents, the BVF represents a modern day analogue for basalt hosted, Au-rich VMS systems. This article is protected by copyright. All rights reserved
Preface: A group of papers devoted to the LaRonde Penna Au-rich volcanogenic massive sulfide deposit, eastern Blake River Group, Abitibi greenstone belt, Quebec
The LaRonde Penna World-Class Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi, QueÌbec: Mineralogy and Geochemistry of Alteration and Implications for Genesis and Exploration
Gold-rich volcanogenic massive sulfide (VMS) deposits consist of synvolcanic banded and concordant massive sulfide lenses and adjacent stockwork feeder zones in which gold concentration in g/t Au exceeds the combined Cu, Pb, Zn grades in wt percent and thus constitutes the main commodity. The Agnico-Eagle LaRonde Penna deposit is a world-class Au-rich VMS (production, reserves and resources of 58.8 Mt at 4.31 g/t Au) located in the eastern part of the Blake River Group of the Abitibi greenstone belt. The deposit comprises four stacked massive sulfide lenses within the upper member of the Bousquet Formation (2698â2697 Ma). The two main ore zones, 20 North and 20 South, are sheetlike, massive to semimassive polymetallic sulfide lenses and stringer zones. Both lenses and sulfide stringers are deformed and transposed by the main foliation. The 20 North lens (Zn-Au-Ag-Cu-Pb) is the main orebody. It is subdivided into two zones: the 20N Au and 20N Zn zones. The 20N Au zone is a transposed and ribbon-textured gold- and copper-rich pyrite-chalcopyrite stringer zone overlain to the south by a 10- to 30-m-thick massive pyrite-sphalerite-galena lens (20N Zn zone). The 20 South lens is an 8- to 10-m-thick gold- and zinc-rich massive sulfide and stringer zone located about 10 to 15 m below the Cadillac Group sedimentary rocks (<2687 Ma). At depth (â„1,900 m below surface), the 20 North and 20 South lenses grade into aluminous zones composed mainly of quartz-pyrite-kyanite-andalusite-muscovite-Znârich staurolite assemblages that host transposed sulfide stringers and local semimassive to massive Au-rich pyrite and chalcopyrite layers.
The synvolcanic hydrothermal alteration now corresponds to mappable upper greenschist-lower amphibo-liteâgrade metamorphic assemblages. The footwall of the 20 North lens is characterized by a large discordant to semiconformable distal quartz-biotite ± garnet assemblage, which transitions laterally into a proximal quartz-garnet-biotite-muscovite zone. The abundance of pink Mn-rich garnet porphyroblasts increases toward the 20N Au zone. The hanging wall of the 20 North lens is characterized by a meter-thick zone of fracture-controlled pink alteration composed of quartz, biotite, rutile and/or anatase, and titanite associated with barren sulfide stringers.
The garnet-rich assemblage in the footwall records gains in MnO, Fe2O3(total), and MgO and losses of Na2O. In the hanging wall, Fe2O3(total), S, and CO2 were added to the rocks with a slight increase in K2O, and CaO. At depth (â„ 1,900 m), the Au-rich aluminous replacement zone is a (up to 30 m) thick, highly strained zone composed of a quartz-pyrite-kyanite-andalusite-chalcopyrite-gold assemblage. All oxides except Al2O3, SiO2, and Fe2O3(total) were strongly leached. The metamorphosed hydrothermal alteration associated with the 20 South lens is characterized by a pink quartz-biotite-rutile-titanite assemblage very similar to that in the hanging wall of the 20 North lens. Toward the ore zone, the pink assemblage is gradually replaced by a proximal quartz-muscovite-green mica-pyrite assemblage, which hosts the sulfide mineralization.
The aluminous alteration at LaRonde Penna is interpreted to be the metamorphic equivalent of an advanced argillic alteration and has many similarities to that of metamorphosed high-sulfidation systems and particularly a class of Au-rich VMS characterized by aluminous alteration. The LaRonde Penna and Bousquet 2-Dumagami deposits are interpreted to represent one large hydrothermal system in which variable contributions of hydrothermally modified seawater and magmatic volatiles contributed to the different styles of alteration and mineralization.
The study illustrates that diverse styles of Au-rich VMS can coexist within the same deposit. In terms of exploration, almost all sulfide lenses or hydrothermal alterations minerals are located at or near volcanic hiatuses within the Bousquet Formation. These hiatuses represent major exploration targets especially when located in the upper part of the Bousquet Formation. The aluminous alteration zones have accommodated most of the postore strain due to their ductility and are transformed into schists. Consequently, the alteration product coincides spatially with the deformation zones despite the lack of a genetic relationship. Quartz-and Mn-rich garnet-biotite assemblages and/or aluminous schists with anomalous gold and/or zinc in intermediate to felsic transitional to calc-alkaline volcanic or volcaniclastic rocks located underneath a sedimentary cover represent excellent targets for Au-rich VMS in metamorphosed terranes
The LaRonde Penna Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi Greenstone Belt, Quebec: Part II. Lithogeochemistry and Paleotectonic Setting
The Au-rich massive to semimassive sulfide lenses of the LaRonde Penna deposit (58.8 Mt at 4.31 g/t Au) are stacked in a steeply dipping, southward-facing homoclinal volcanic sequence forming a continuous, differentiated volcanic succession composed of two main formations: the ca. 2700 Ma HĂ©bĂ©court Formation and the 2701 to 2698 Ma Bousquet Formation, which corresponds to the uppermost segment of the Blake River Group. The HĂ©bĂ©court Formation is composed of regionally extensive LREE-depleted ([La/Sm]N âŒ0.9) tholeiitic, basaltic to andesitic, massive to pillowed flows that formed a submarine stratum on which the Bousquet Formation was emplaced. The Bousquet Formation is further divided into a lower member and an upper member. The lower member of the Bousquet Formation is composed of feldspar and quartz-phyric tholeiitic felsic (Zr/Y âŒ3.4, Zr/TiO2 âŒ860) sills and extensive effusive and volcaniclastic mafic to intermediate and tholeiitic to transitional rocks. The upper member is mainly characterized by submarine, coalesced dacitic to rhyodacitic autoclastic flows that are cut and/or covered by rhyodacitic and rhyolitic domes and/or partly extrusive cryptodomes and by intermediate to mafic sills and dikes.
Mafic to intermediate and tholeiitic to transitional (Zr/Y âŒ2.3â5) rocks of the Bousquet Formation are characterized by a low Zr/TiO2 ratio (<60), moderately enriched chondrite-normalized LREE and MREE ([La/Sm]N âŒ2.2â2.7) patterns, flat HREE ([Gd/Lu]N âŒ1.2â2) patterns, and negative Nb, Ta, Zr, and Hf anomalies. Felsic transitional to calc-alkaline (Zr/Y âŒ5â8) rocks of the upper member of the Bousquet Formation are characterized by a moderate Zr/TiO2 ratio (âŒ250â615), high incompatible element contents, LREE-enriched patterns ([La/Sm]N âŒ3.2â6.6), flat HREE patterns ([Gd/Lu]N âŒ1â1.4), pronounced negative Nb, Ta, and Ti anomalies, and positive Zr and Hf anomalies. The Nd isotope signature of six separate LaRonde Penna deposit host units (ΔNd âŒ3â3.4) suggests that they were generated by partial melting of depleted upper mantle and/or juvenile material (mafic crust) or by a combination of those two processes. The sequence is interpreted to reflect the progression from diapirism of depleted upper mantle associated with underplating by mafic-ultramafic magma and assimilation and magmatic differentiation (assimilation-fractional crystallization) at midcrustal levels in subsidiary magmatic chambers within a ca. 2721 Ma, relatively thick, juvenile or immature mafic ± felsic arcâback-arc crust in an intermediate setting between back-arc basin and volcanic-arc environments. This setting, compatible with the inferred geodynamic setting for the southern Abitibi belt, could be responsible, at least in part, for the Au enrichment of the volcanic massive sulfide (VMS) deposits of the Doyon-Bousquet-LaRonde mining camp.
This study shows that Archean HREE-depleted and high Th, transitional to calc-alkaline dacite, rhyodacite, and rhyolite, referred to as FI and FII type, such as those associated with the LaRonde Penna deposit, can be important hosts for VMS and Au-rich VMS and may be as prospective as the tholeiitic or FIII-type rhyolite-bearing sequences
The LaRonde Penna Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi Greenstone Belt, Quebec: Part I. Geology and Geochronology
The LaRonde Penna Au-rich volcanogenic massive sulfide (VMS) deposit is the largest Au deposit currently mined in Canada (58.8 Mt at 4.31 g/t, containing 8.1 Moz of Au). It is part of the Doyon-Bousquet-LaRonde mining camp located in the eastern part of the Blake River Group of the Abitibi greenstone belt which is host to several of the worldâs most important, present and past, Au-rich VMS deposits (e.g., Horne, Quemont, Bousquet, Bousquet 2-Dumagami).
The LaRonde Penna deposit consists of massive to semimassive sulfide lenses (Au-Zn-Ag-Cu-Pb), stacked in the upper part of a steeply dipping, south-facing homoclinal volcanic sequence composed of extensive tholeiitic basaltic flows (Hébécourt Formation) overlain by tholeiitic to transitional, mafic to intermediate, effusive and volcaniclastic units at the base (lower member of the Bousquet Formation) and transitional to calc-alkaline, intermediate to felsic, effusive and intrusive rocks on top (upper member of the Bousquet Formation). The mafic to felsic volcanism of the Hébécourt Formation and of the lower member of the Bousquet Formation formed an extensive submarine basement or platform on which the intermediate to felsic rocks of the upper member of the Bousquet Formation were emplaced at restricted submarine eruptive centers or as shallow composite intrusive complexes. The submarine felsic volcanic rocks of the upper member of the Bousquet Formation are characterized by dacitic to rhyodacitic autoclastic (flow breccia) deposits that are cut and overlain by rhyodacitic and rhyolitic domes and/or partly extrusive cryptodomes and by intermediate to mafic sills and dikes.
This volcanic architecture is thought to have been responsible for internal variations in ore and alteration styles, not only from one lens to another, but also along a single mineralized horizon or lens. In the upper part of the mine, the 20 North lens comprises a transposed pyrite-chalcopyrite (Au-Cu) stockwork (20N Au zone) overlain by a pyrite-sphalerite-galena-chalcopyrite-pyrrhotite (Zn-Ag-Pb) massive sulfide lens (20N Zn zone). The latter was formed, at least in part, by replacement of footwall rhyodacitic autoclastic deposits emplaced within a subbasin located between two rhyolite domes or cryptodomes. The 20N Zn zone tapers with depth in the mine and gives way to the 20N Au zone. At depth in the mine, the 20N Au zone consists of semimassive sulfides (Au-rich pyrite and chalcopyrite) enclosed by a large aluminous alteration halo on the margin of a large rhyolitic dome or cryptodome.
U-Pb zircon geochronology gives ages of 2698.3 ± 0.8 and 2697.8 ± 1 Ma for the footwall and hanging-wall units of the 20 North lens, respectively. Thus, the formation of the 20 North lens was coeval with other VMS deposits in the Bousquet Formation and in the uppermost units of the Blake River Group. Although deformation and metamorphism have affected the primary mineral assemblages and the original geometry of the deposit, these events were not responsible for the different auriferous ore zones and alteration at LaRonde Penna. Studies of the LaRonde Penna deposit show that the hydrothermal system evolved in time and space from near-neutral seawater-dominated hydrothermal fluids, responsible for Au-Cu-Zn-Ag-Pb mineralization, to highly acidic fluids with possible direct magmatic contributions, responsible for Au ± Cu-rich ore and aluminous alteration. The different ore types and alteration reflect the evolving local volcanic setting described in this study