Volcanology of the Norme´tal volcanic complex

Fig. 3. Schematic stratigraphic column of the Norme´tal vol- canic complex and adjacent units. The Norme´tal volcanic complex includes the 4 km-thick sequence of mafic-felsic vol- canic rocks, the Norme´tal sedimentary rocks and the synvol- canic Norme´tal and Val-St-Gilles plutons. Legend as in Fig. 2. Norme´tal mine 11 Mt grading 5.12 Zn, 2.15 Cu, 0.549 gt Au and 45.25 gt Ag; Teasdale, 1993 and the satellite Normetmar deposit 160 000 t at 12.6 Zn, as well as associated volcanic rocks and intrusions of phase 5, are herein referred to as the Mine Sequence Figs. 2 and 3.

3. Volcanology of the Norme´tal volcanic complex

3 . 1 . Characteristics and terminology The NVC was mapped at scales of 1:5000, 1:1000 and 1:500. For the following descriptions, the NVC is divided geographically into western Fig. 4, central Figs. 5 and 6 and eastern seg- ments Fig. 7, and each segment is separated by major Proterozoic diabase dykes Fig. 2. Each segment has a specific local stratigraphy and a distinct volcanic evolution Fig. 2. Outcrop loca- tions correspond to the volcanic facies symbols plotted in Figs. 4 – 7. Felsic units are defined using phenocryst type, percentage and size Table 2. With the exception of quartz-feldspar-porphyry-3 Qfp-3, which only occurs in phase 4, felsic rock types are not restricted to a specific volcanic phase Table 1. The terminology employed is based on the defi- nition of Fisher 1961 whereby volcaniclastic rocks are ‘deposits composed predominantly of volcanic particles’. Volcaniclastic deposits are de- scribed using the standard granulometric classifi- cation of Fisher 1961, 1966, which is purely descriptive and non-genetic. The general terms volcanic breccia clasts \ 2 mm and vol- canic sandstone grain size B 2 mm are em- ployed for the volcaniclastic sediments and rocks of uncertain origin Fisher and Schmincke, 1984; pp. 91 – 92. Tuff, lapilli tuff and lapilli tuff brec- cia clearly associated with lava flows are referred to as hyaloclastites McPhie et al., 1993. The term volcanic phase referred here to a distinct period of volcanic construction at the edifice scale, by opposition to volcanic cycle which referred to a belt scale Chown et al., 1992. 2 . 2 . Norme´tal 6olcanic complex NVC The 4 km-thick NVC, traceable for 35 km along strike Fig. 2, has been divided into five distinct volcanic constructive phases based on stratigraphic position, crosscutting relationships, and geochemistry Table 1. The basal mafic vol- canic sequence phase 1 is overlain by mafic and felsic volcanic and volcaniclastic rocks phases 2 – 4, and a minor but significant volcaniclastic sedimentary unit. These volcaniclastic sedimen- tary rocks, herein referred to as the Norme´tal sedimentary rocks, represent the principal marker in the NVC. Finally, the uppermost sequence phase 5 is composed of dominant mafic-felsic volcaniclastic rocks and minor massive volcanic rocks, and diorite intrusions Fig. 3; Lafrance et al., 1998. Tops determined from pillows, graded beds, and load casts, as well as general stratigra- phy indicate a south younging direction. The vol- caniclastic rocks of phase 5 which host the former B . Lafrance et al . Precambrian Research 101 2000 277 – 311 Table 1 Volcanic phases of the Norme´tal volcanic complex NVC a a Divisions are based on regional stratigraphy, crosscutting relationships, phenocryst variation in flows and geochemistry. b Qfp, quartz-feldspar phenocrysts; Qp, quartz phenocrysts. B . Lafrance et al . Precambrian Research 101 2000 277 – 311 283 Fig. 4. A Volcanic facies and lithological map of the western and central segment of the Norme´tal volcanic complex see Fig. 2 for location. Large arrows indicate flow direction and areas labeled a – c show inferred lateral volcanic facies transition from proximal to distal in mafic and felsic lava flows. Phases 2a – 2c etc., represent various volcanic constructional phases discussed in text. The location of volcanic facies symbols corresponds to outcrop location. Note superposition of central segment onto the western segment line of stars and Patten fault. Synvolcanic faults are present and have been intruded by late dykes. B Details of the west-closing massive felsic lobes associated with felsic flows. 3 . 2 . Phase 1: basaltic andesite dominated subaqueous 6olcanism The initial phase of volcanism is represented by a 1 – 2 km-thick sequence of basaltic andesite, andesite and minor dacite. The basaltic andesite and andesite are represented by massive, pillowed and minor pillow breccia flows. Flows are locally porphyritic, with up to 3 1 mm-feldspar and contain 10 – 30 amygdules, 0.3 – 2 cm in diame- ter. Pillowed flows are 4 – 20 m thick with 0.5 – 2 m-wide pillows with local concentric cooling joints. The pillow breccias are composed of 10 – 30 cm-amoeboid pillow fragments. Pillows in the western, central and eastern segments of the NVC display a south younging direction Fig. 2. Local interstratified dacite flows are massive and aphanitic. 3 . 2 . 1 . Interpretation Massive and pillowed flows suggest effusive volcanism in a subaqueous environment Dimroth et al., 1978; Cousineau and Dimroth, 1982, whereas the lateral extent of the mafic unit is consistent with a subaqueous ocean floor setting Chown et al., 1992. The massive dacite remains problematic and it may represent either a flow or intrusion. Fig. 5. A Volcanic facies and lithological map of the western part of the central segment of the Norme´tal volcanic complex see Fig. 2 for location. Large arrows indicate prominent flow direction. Inferred source areas are located in the opposite direction of arrows and lobe closures. The location of volcanic facies symbols corresponds to outcrop location. Aphanitic flow unit labeled a – c indicates lateral volcanic facies transition from proximal to distal segment. B Outcrop map of Qp1 rhyolite of phase 2c crosscut by phase 3 aphanitic rhyolite dyke. Qp1 west-closing lobe showing flow banding is surrounded by autoclastic breccia. Aphanitic phase 3 rhyolite displays endogenic lobes at the contact with the Qp1 rhyolite. B . Lafrance et al . Precambrian Research 101 2000 277 – 311 285 Fig. 6. A Volcanic facies and lithological map of the eastern part of the central segment of the Norme´tal volcanic complex see Fig. 2 for location. Large arrows indicate flow direction. Location of volcanic facies symbols corresponds to outcrop location. B Outcrop map of Qp1 flow-banded flows and lapilli tuffs-lapilli tuff breccia of phase 2c intruded by aphanitic sills of phase 2c. Lobes in flow-banded Qp1 and aphanitic sills are east closing. Qfp3 dyke of phase 4 intrudes phase 2 rocks. Fig. 7. A Volcanic facies and lithological map of the eastern segment of the Norme´tal volcanic complex see Fig. 2 for location. Location of volcanic facies symbols corresponds to outcrop location. B Outcrop map of Qfp1 lapilli tuff and laminated tuff intruded by aphanitic rhyolite lobes and diorite. Large arrow indicates prominent flow direction. Table 2 Petrographic distinction of the Norme´tal volcanic complex rhyolites Aphanitic Qfp1 Qfp2 Qfp3 Qp1 Quartz Quartz and Microfeldspar Phenocryst type a Quartz and feldspar Quartz and feldspar phyric feldspar – B 5 10–25 Quartz phenocryst B 5 10–25 percentage 0.5–1 mm 1–2 mm 3 mm–1 cm Quartz phenocryst size 1–2 mm – a From macroscopic and thin sections observations. 3 . 3 . Phase 2: andesitic-dacitic and rhyolitic subaqueous 6olcanism The 0.8 – 2.2 km-thick andesite-dacite and rhyo- lite of phase 2 Fig. 2, the principal constructive phase of the NVC, is divided into: i a basal andesite unit phase 2a; ii a medial felsic vol- caniclastic unit phase 2b; and iii an uppermost andesite-dacite and rhyolite unit phase 2c which is distributed in the western Fig. 4, central Figs. 5 and 6 and eastern Fig. 7 segments. The basal phase 2a in the western and central segments Figs. 4 and 6, is a 30 – 640 m-thick unit composed of andesitic massive and pillowed flows Fig. 8a as well as pillow breccia passing up-sec- tion and laterally into lapilli tuff over 2 – 3 km Fig. 4; see flows of phase 2a labeled a-b-c. The 0.2 – 1 m-wide pillows have rims rich in amygdules and locally show quartz-epidote-filled intrapillow cavities as well as concentric cooling joints. Lapilli-tuff includes 0.5 – 3 cm aphanitic fragments of andesitic to dacitic composition. The 5 – 50 m-thick felsic volcaniclastic deposits of phase 2b are constrained to the western Fig. 4 and central segments Fig. 6. Western segment deposits are characterized by basal tuff overlain by lapilli tuff breccia Fig. 8b. The 1 m-thick fine to coarse tuff contains 0.5 – 20 cm-thick massive, Fig. 8. Volcanic facies of phases 2a and 2b of the western segment of the NVC see Fig. 4 for location. White arrow indicates younging direction. Pencil 15 cm and hammer 35 cm for scale. A Stratigraphic top indicated by pillows of phase 2a andesite. B Volcaniclastic deposit of phase 2b which are characterized by 1 m-thick turbiditic tuff Tt at the base overlain by 2 – 10 m-thick matrix-supported lapilli tuff breccia Ltb. C Fine to coarse basal turbiditic tuff of phase 2b, characterized by massive graded T a beds laminated T b beds and convoluted T c beds. D Details of load cast structure Lc present at the base of a T a bed that overlies convoluted T c bed. E Matrix-supported lapilli tuff breccia of phase 2b. Subrounded aphanitic and vesicular fragments could be derived from the underlying andesite of phase 2a. Uppermost T c bed of the basal turbiditic tuff is observed at the bottom of the photograph. F Uppermost clast-supported lapilli tuff breccia of phase 2b. Fig. 9. Characteristics of the eastern part of the central seg- ment of the NVC see Fig. 10 for location. Pencil 15 cm and hammer 35 cm for scale. A Subrounded aphanitic Aph and Qfp2 fragments in the lapilli-tuff breccia of phase 2b. B A 5 – 10 m-thick Qfp1 sill of phase 2c with east closing endogenic 2 m-wide lobe intruding Qfp2 lapilli tuff breccia of phase 2b. Top of photograph to the east. C Details of the chilled margin Cm in the Qfp1 sill at the contact with Qfp2 lapilli tuff breccia. tuff breccia, 2 – 10 m-thick, containing 10 – 60 cm subangular to subrounded fragments Fig. 8e as well as 2 – 20 m-thick, clast-supported lapilli tuff breccia with 2 – 8 cm-angular to subrounded Qfp2 fragments Fig. 8f, follow up-section. Phase 2b of the central segment is characterized by 20 – 30 m-thick Qfp2 massive clast-supported lapilli tuff breccia Figs. 6 and 10 composed of abundant subrounded 5 – 20 cm Qfp2 clasts 80, similar to the matrix, and aphanitic clasts 20; Fig. 9a. The lapilli-tuff breccia has a sharp depositional contact with the andesite-dacite and rhyolite flows Fig. 10, which marks the presence of a synvol- canic fault. Phase 2c, 0.8 – 1.5 km thick, is characterized by andesite-dacite and rhyolite flows, dykes and in- trusions. An example of the 90 – 580 m-thick mas- sive and pillowed andesite unit associated with pillow breccia and lapilli tuffs and sills is well exposed in the eastern part of the central segment Figs. 6 and 11. Two 2 – 25 m-thick massive flows grade up-section into 2 – 6 m-thick pillowed flows Fig. 12a, b, 2 m-wide master tubes, or 5 – 23 m-thick pillow breccias with amoeboid fragments Fig. 12c. A 4 m-thick lapilli tuff Fig. 12d with subangular to subrounded clasts follows up-sec- tion. Minor dacites are massive with local, angu- lar breccia-size fragments. The 150 – 600 m-thick felsic units of phase 2c are flows, dykes and intrusions with phenocryst variations permitting identification of individual flow units Table 2. Associated fragmental de- posits tuff, lapilli tuff, and lapilli tuff breccia are composed of angular to subrounded clasts Table 1. Columnar jointing, 5 – 20 cm in diameter is locally observed in massive flows or intrusive bod- ies. Lateral flow facies transitions over a distance of 1 – 2 km are observed in felsic flow units of the western segment. Typically, the flow units display a change from massive to well defined, 1 – 20 m-thick, west-closing lobes with massive centers and a marginal metre-thick flow-banding to tuff, lapilli tuff breccia and laminated tuffs see flows labeled a-b-c in Figs. 4 and 5. Large domal massive bodies, characterized by uniform phe- nocryst content, massive facies and local intrusive contacts were also identified Fig. 4. graded, laminated to rippled beds, that are locally convoluted Fig. 8c, d. Matrix-supported lapilli Chronological and geometrical relationships of felsic units defined by facies mapping and phe- nocryst variation reveal lava lobes and complex unit shapes. For example, a chronological rela- tionship is revealed by local lobes with sharp chilled margins and internal flow banding that are compositionally different from the host lapilli tuff breccia or lapilli tuff in eastern and central seg- ments Fig. 7b, Fig. 9b, c. Example of geometri- cal and internal organization of flows is the 100 – 350 m-thick massive to lobate Qp1 flow la- beled I in Figs. 5 and 6, which contain a basal lapilli tuff breccia Fig. 13a and opposing lobe closures that crops out in the central segment. Detailed mapping of this Qp1 flow Fig. 5b, Fig. 6b shows east-closing lobes in the Qp1 flow Fig. 6b, Fig. 13b and in an aphanitic sill Fig. 6b, Fig. 13c that occur in the east, whereas west- closing lobes with lapilli tuff breccia Fig. 13d are found in the west part of the central segment Fig. 5b. 3 . 3 . 1 . Interpretation Andesitic massive and pillowed flows of phases 2a and 2c represent, as in phase 1, calm effusive subaqueous volcanism. Lapilli tuffs, the lateral and up-section continuation of the flows, are hyaloclastites Cas and Wright, 1987; McPhie et al., 1993. Fine to coarse-tuff and lapilli tuff of phase 2b Fig. 8c were deposited by high- to low-concen- tration turbidity currents, and are referred to as turbiditic tuffs Mueller and White, 1992. Mas- sive to graded portions of beds are Bouma 1962 T a or S 3 -beds of Lowe 1982 deposited from massive fallout during transport of high-concen- tration turbidity flows. The laminated and rippled portions of the tuff represent Bouma T bc parts of beds. The matrix-supported lapilli tuff breccia Fig. 10. Outcrop map of Qfp2 lapilli tuff breccia phase 2b overlying massive Qfp2 rhyolite in sharp contact with a massive flow and lapilli tuff of andesite phase 2a from the eastern part of the central segment see Fig. 6 for location. The sharp contact between andesites and rhyolites is interpreted as a synvolcanic fault, which has been subsequently intruded by an aphanitic dyke. The lapilli tuff breccia contains up to 15 colloform pyrite. Fig. 11. Outcrop map of two representative andesitic flows of phase 2c from the eastern part of the central segment crosscut by Qfp3 rhyodacitic dyke of phase 4 see Fig. 6 for location. Fig. 8e is interpreted as a cohesive debris flow deposit Lowe, 1982 and clast-supported lapilli tuff breccia of the western Fig. 8f and central Fig. 9a segments is best explained as a mass flow product transported downslope via laminar or plug flow Lowe, 1982; McPhie et al., 1993. These fragmental deposits may be either pyroclas- tic or autoclastic in origin Cas, 1992 that were subsequently redistributed down-slope via sedi- ment gravity flow processes. Resedimentation of pyroclastic deposits is possible, but remobilization of autoclastic debris or hydroclastic products derived from subaqueous lava flow-water interac- tions Fisher and Schmincke, 1984 appears more probable. Autoclastic or hydroclastic origin is supported by the abundance of lava flows up-sec- tion, breccia-size clasts and the absence of pumice, glass shards, and vesicles in clasts which are related to pyroclastic deposits Fisher and Schmincke, 1984; Dimroth and Yamagishi, 1987. Phase 2c massive to blocky dacite flows and thick massive to lobate rhyolitic flows are typical viscous flow products Kano et al., 1991; Dadd, 1992; Manley, 1992. Fragmental debris associ- ated with the rhyolitic lobes is generated by auto- clastic and thermal granulation processes, and is considered hyaloclastite and carapace breccias Cas and Wright, 1987; McPhie et al., 1993, or lobe-hyaloclastite flow deposits Gibson et al., 1997. Lateral changes from massive flows with columnar jointing to lobes with massive interiors and flow-banded margins, which in turn pass into viscous flow breccias and hyaloclastites Figs. 4 and 5, are strikingly similar to the facies models of de Rosen-Spence et al. 1980 and Yamagishi and Dimroth 1985 for Miocene and Archean analogues. The lobes may be extrusive exogenic; Figs. 4, 5 and 13d, or endogenic and intrude a pre-existing breccia pile Figs. 5, 7b, 9b and 13c. Larger km-scale domal structures with intrusive contacts and a massive appearance are consistent with endogenic domes McPhie et al., 1993. 3 . 4 . Phases 3 and 4: rhyodacitic-rhyolitic subaqueous 6olcanism The 600 m-thick, aphanitic rhyodacite-rhyolite of phase 3, present in the western and central segments Figs. 4 and 5; Table 1, display two laterally continuous 10 km long and 75 – 275 m- Fig. 12. Characteristics of the eastern part of the central segment see Fig. 11for location. Large white arrow indicates younging direction. Pencil 15 cm and hammer 35 cm for scale. A Crosscutting relationship between Qfp3 dyke of phase 4 and andesite of phase 2c. The dyke with flow banding Fb at the margin intrudes lapilli tuff Lt, pillows P, and pillow breccia. B South-younging pillows P and lapilli tuff Lt with sub-angular 0.5 – 5 cm-size fragments small white arrow. Lapilli tuff is interpreted as hyaloclastite. Right side of the photograph to the south. C Decimeter scale amoeboid fragments in pillow breccia of flow 2. D Lapilli-tuff hyaloclastite composed of sub-angular 0.5 – 5 cm-size altered fragments. Fig. 13. Characteristics of phase 2c Qp1 rhyolite in the eastern and western parts of the central segment locations for photos A, B, C are shown on Fig. 6b and photo D to Fig. 5b. Large white arrow indicates younging direction to the south in B – D. Pencil 15 cm and hammer 35 cm for scale. A Clast-supported Qp1 lapilli tuff breccia with subangular fragments with arrow showing 80 cm-size clast. B Meter-scale east-closing lobe with flow banding. C East-closing endogenic lobe in aphanitic sill intruded in Qp1 lapilli tuff breccia. D Meter-scale west-closing exogenic lobe with flow banding. Pencil displays massive part of lobe and black arrow shows brecciated portion. thick flow units, as well as decametre-scale sills and dykes. Flow units display a change over 2 – 3 km, from massive to 3 – 30 m-thick massive or flow-banded lobes to lapilli tuff breccia, which grade up-section and laterally into massive, 1 – 10 m-thick lapilli tuff 2 – 5 cm-size and 1 – 10 m- thick laminated tuff composed of 2 – 10 cm-thick beds Fig. 5a. Extensively developed 8 – 15 cm- wide columnar joints are prominent in the basal parts of flows Figs. 4a and 14a. Sills and dykes crosscut the previous phase 2 flows and feature local lobes and centimetre-scale flow banding along chilled margins Figs. 5b and 14b. Simi- larly, massive to flow-banded units and lapilli tuff breccias alternate in the central segment Figs. 2 and 15. Meter-scale flow banding composed of mm-cm sericite-rich bands characterizes the con- tact between massive facies and lapilli tuff breccia Fig. 14c. The lapilli tuff breccia contains subangular to subrounded flow-banded clasts 0.5 – 15 cm size that display a jigsaw-fit Fig. 14d. The 475 m-thick rhyodacite-rhyolite of phase 4, constrained to the central segment, is composed of a massive unit and dykes Figs. 2 and 6 with exclusively large quartz and feldspar phenocrysts Qfp3 in Table 2; Fig. 16a. The 15 – 20 m-thick Qfp3 dykes cut phase 2 rocks Fig. 6b, Fig. 11, Fig. 16b, c. The dykes are massive with 30 cm- thick flow banded margins Fig. 16b and 10 – 15 cm-thick chilled margins at the contact with Qp1 lapilli tuffs of phase 2c Fig. 16c. 3 . 4 . 1 . Interpretation The felsic massive, lobate, and hyaloclastite flows of phase 3 in the central-west segment Fig. 5 show the classical subaqueous morphological facies organization of subaqueous flows de Rosen-Spence et al., 1980. The fragmental units are autoclastic products and together with lobes represent the lobe-hyaloclastite flows of Gibson et al. 1997. The ambient aqueous environment fa- cilitated the brecciation process and the formation of columnar jointing in massive facies of flows. Associated lapilli tuffs and laminated tuffs, trans- ported via high- and low-concentration turbidity flows Lowe, 1982, are either reworked hyalo- clastite breccia or local explosive hydroclastic products transported down-slope Fisher and Schmincke, 1984. The geometry, large-scale change in flow band orientation from NW-SE to NE-SW and intrusive nature of the unit in the Fig. 14. Characteristics of phase 3 rhyolite from western and central segments. Pencil 15 cm and hammer 35 cm for scale. A Columnar joints observed at the base of the aphanitic rhyolite flow located in Fig. 4. B Endogenous aphanitic lobe aph of phase 3 intruded in Qp1 tuff of phase 2c see Fig. 5b for location. C Contact between massive to flow-banded facies and lapilli tuff breccia Br of phase 3 aphanitic rhyolite located in Fig. 15. The contact is marked by millimeter to centimeter flow banding Fb composed of sericite which cumulatively reaches meter scale that passes into 10 cm-thick hyaloclastic material Hy. D Subangular to subrounded, 0.5 – 15 cm fragments of the lapilli tuff breccia located in Fig. 15. Fig. 15. Outcrop map showing phase 3 rhyolite of the central segment of the NVC see Fig. 2 for location. A Massive to flow-banded flows with associated breccia form meter to decameter horizons. B Position of outcrop in A with respect to an endogenous dome modified from Burt and Sheridan, 1987. central segment favor an endogenous dome struc- ture Burt and Sheridan, 1987; Manley, 1996. Flow-banded fragments with a jigsaw-fit texture in a low intraclast matrix is typical of in-situ brecciation and common to dome margins Allen, 1992; McPhie et al., 1993. The massive unit of phase 4 is also interpreted as a high-level dome, also supported by the map scale geometry Fig. 2; Cas et al., 1990. The dykes of similar composition are inferred to be feeders to the dome. The 10 – 15 cm-thick chilled margins of the dykes attest to a significant speed of cooling whereas flow banding is results of viscous magmatic flow and internal friction Kano et al., 1991. 3 . 5 . Sedimentation : 6 olcanic quiescence The Norme´tal sedimentary rocks represent a 20 – 70 m-thick volcaniclastic sedimentary marker horizon, which can be traced for ca. 35 km. The Fig. 16. Characteristics of phase 4 rhyolitic Qfp3 dyke of the central segment of the NVC see Fig. 6b for location. Pencil, 15 cm. A Flow-banded facies of rhyolitic Qfp3 dyke with characteristic large quartz phenocrysts small black arrows. B Crosscutting relationships between Qp1 lapilli tuff, massive aphanitic sill Aph of phase 2c and Qfp3 dyke of phase 4 with flow-banded margin. Top of photograph to the south. C Chilled margin Cm observed at the contact between Qfp3 dyke phase 4 and Qp1 lapilli tuff phase 2c. Fig. 17. Characteristics of Norme´tal sedimentary rocks and phase 5 lapilli tuff of the NVC located in Fig. 5. A Intrafolial folds in the volcanic siltstone-mudstone facies near Norme´tal. Measuring tape, 80-cm. B Turbiditic lapilli tuffs associated with the Normetmar massive sulphide deposit. Strong stretching of the lapilli white arrow defines down-dip lineation related to the Norme´tal deformation zone. Pencil, 15 cm. 3 . 5 . 1 . Interpretation Sharp contacts and normal grading of the vol- canic breccia-sandstone beds are consistent with the S 3 division of Lowe 1982. The thin-lami- nated nature of the siltstone and mudstone repre- sents T de divisions Bouma, 1962. The predominance of mudstone suggests suspension- derived background sedimentation. These charac- teristics coupled with the absence of wave-induced sedimentary structures favors deposition in water depth in excess of 200 m. 3 . 6 . Phase 5: mine sequence Most of the information concerning the 100 – 400 m-thick Mine Sequence comes from diamond drill observations. Massive mafic and felsic flows or intrusions as well as abundant lapilli tuff brec- cias, lapilli tuff and fine tuffs have been identified. A highly altered quartz-sericite-carbonate-chlori- toid felsic tuff traceable for 30 km along strike referred here as the Mine horizon is associated with the VMS deposits. The most representative outcrop of the Mine sequence consists of graded- bedded tuff and lapilli tuffs and lapilli tuff brec- cias exposed at the Normetmar deposit Figs. 5 and 17b. The volcaniclastic unit contains 2 – 5 m-thick fining-upward sequences in which grain size and bed thickness generally decrease up-sec- tion. The tuff, with parallel, undulating and rip- pled laminations, is composed of lithic felsic aphanitic and feldspar-phyric clasts. 3 . 6 . 1 . Interpretation Massive mafic to felsic flows are products of effusive volcanism. Tuffs, lapilli tuffs and lapilli tuff breccias are deposited by high and low-den- sity gravity flows Bouma, 1962; Lowe, 1982 and may either be the reworked counterparts of auto- clastic breccia, a slumped carapace breccia or represent deposition from hydroclastic eruptions Fisher and Schmincke, 1984. 3 . 7 . Geometry and edifice construction Lateral variations of volcanic facies in mafic- felsic rocks and lobe closures in viscous rhyolites delineate flow direction and infer vent location volcaniclastic rocks are composed of graded vol- canic breccia and sandstone, and massive to finely laminated siltstone and mudstone beds, which cap the volcanic sequence Figs. 4 – 7. A fining and thinning upward sequence composed of 10 m- thick volcanic breccia-sandstone to 4 m-thick vol- canic siltstone-mudstone Fig. 17a to 1 m-thick mudstone is recognized near the Norme´tal and Normetmar deposits. Elsewhere in the complex, sedimentary rocks consist of mudstone and minor volcanic siltstone-mudstone. Generally, volcanic breccia 2 – 12 mm fragments and sandstone con- tain 10 – 20 quartz crystals and form 10 – 40 cm- thick beds. Siltstone-mudstone forms 2 – 25 cm and 0.3 – 5 cm-thick beds, respectively. Figs. 4 – 6. The flow direction coupled with dis- tinctive stratigraphy enables the recognition of three probable emission centers, which are located in the western, central and eastern segments Fig. 18. Synvolcanic faults that acted as lava conduits and conduits for hydrothermal fluid circulation, are interpreted from detailed mapping Fig. 10 and 1 changes in flow directions; 2 volcanic facies changes across Proterozoic dykes; 3 pres- ence of Proterozoic dykes; 4 thickness variations and distribution of units; and 5 disruption of geophysical conductors Tessier, 1991. The geometry of the NVC is interpreted as a multivent edifice Fig. 18. The western vent is marked by opposing flow directions on either side of a synvolcanic fault Figs. 4 and 18. With time, effusion of flows decreased whereas flows emanat- ing from the central vent covered the western centre Figs. 4 and 18. Numerous endogenous domes inflated the western volcanic sequence to mark final volcanic activity Fig. 18. The central vent shows opposing flow directions and closure of the Qp1 flow labeled I in Figs. 5 and 6, indicating the location of a major eruptive center probably about 2 km east of the Norme´tal deposit Figs. 5, 6 and 18. Numerous synvolcanic faults and endogenous domes, dykes and sills of phases 3 and 4 as well as phase 3 flows also characterized the central vent. The eastern vent area is charac- terized by an aphanitic flow unit for which the emission centre is poorly defined. Turbiditic sedi- mentary rocks and pelagic mudstone of the Norme´tal sedimentary unit caps the volcanic se- quence with the exception of the phase 4 dome of the central vent, which represent probable resur- gent dome. Renewed volcanism of phase 5 cov- Fig. 18. Reconstructed cross-section of the NVC interpreted from Figs. 4 – 7. The principal emission centers are defined by facies relations and supported by lobe closures of flow units. Synvolcanic faults, thickness and geometric relations of units are used to suggest the presence of a central cauldron structure closely associated with the Norme´tal VMS deposit. Note the overlapping of the central sequence onto western and eastern emission centers line of asterisks. ered the entire NVC. These characteristics argue for the development of a central cauldron struc- ture with minor vents to the east and west on an andesitic edifice phase 1.

4. Geochemistry