Abstract

The Cobalt embayment is a large domain of Paleoproterozoic clastic sedimentary rocks that unconformably overlies the Archean Abitibi greenstone belt. Regionally extensive sills and dikes of Nipissing diabase, emplaced circa 2219 Ma, occur throughout the embayment and are the preferential host to gold-bearing polymetallic vein systems on the Merico-Ethel property, near the northeastern margin of the Cobalt embayment. These gold-bearing, polymetallic veins are predominantly east–west-trending, steeply dipping, discordant calcite-quartz vein systems that formed close to the time of crystallization (within ~15 m.y., based on Pb-Pb ages) of the Nipissing diabase. The ore mineralogy is complex in character, typically comprising sulfides, arsenides, native metals (gold and silver), and specular hematite, preferentially concentrated along the interface between silicate and calcite gangue. A simplified sequence of mineral deposition in the veins is: (1) “Early-stage” pyrite ± chalcopyrite hosted in quartz ± chlorite gangue; (2) “Main-stage” polymetallic (Cu + Co + As + Ag + Au + Bi ± Pb ± Ni ± U) sulfides, arsenides, and native metals hosted in calcite gangue; and (3) “Late-stage” calcite flooding ± galena. Wall-rock alteration in Nipissing diabase is restricted to narrow (<5 cm) haloes of calcite-chlorite-epidote-bearing assemblages, whereas a weak alteration halo of specular hematite, calcite, and allanite-epidote has been observed in the surrounding sedimentary rocks. In terms of their age, geology, mineralogy, paragenesis, and morphology, the gold-bearing vein systems at Merico-Ethel closely resemble the silver-sulfarsenide vein deposits of the historic Cobalt and Gowganda mining camps. These observations indicate that the Au-bearing veins are variants of the Ag-vein systems and as such, have a common genesis belt.

Sommaire

L’Échancrure de Cobalt est un large domaine de roches sédimentaires clastiques d’âge Paléoprotérozoïque qui reposent en discordance sur la ceinture de roches vertes d’Abitibi d’âge Archéen. La Diabase de Nipissing, mise en place vers 2219 Ma et qui constitue un réseau de sills et de dykes d’importance régionale, est présente dans l’ensemble de l’échancrure et constitue la roche-hôte principale des systèmes de veines aurifères-polymétalliques de la propriété Merico-Ethel, près de la bordure nord-est de l’Échancrure de Cobalt. Ces veines à minéralisation aurifère et polymétallique constituent un système de veines de calcite-quartz sécantes d’orientation principalement est-ouest et à pendage fort qui se sont formées peu de temps après la cristallisation de la Diabase de Nipissing (à ~15 Ma près, d’après les âges Pb-Pb). La minéralogie de la minéralisation est complexe; elle consiste typiquement en sulfures, en arséniures, en métaux natifs (or et argent) et en hématite spéculaire, préférentiellement concentrés à l’interface entre les silicates et la gangue de calcite. La séquence de déposition simplifiée des minéraux dans les veines est : (1) ≪Stade précoce≫ pyrite ± chalcopyrite encaissés dans une gangue de quartz ± chlorite; (2) “Stade principal” sulfures polymétalliques (Cu + Co + As + Ag + Au + Bi ± Pb ± Ni ± U), arséniures, et métaux natifs dans une gangue de calcite; et (3) “Stade tardif” calcite massive ± galène. L’altération pénétrative de la Diabase de Nipissing est confinée à des halos étroits (<5 cm) présentant des assemblages marquées par la présence de calcite-chlorite-épidote, tandis qu’un faible halo d’altération en hématite spéculaire, calcite et en allanite-épidote est noté dans les roches sédimentaires environnantes. De par leur âge, leur géologie, leur minéralogie leur paragenèse et leur morphologie, les systèmes de veines aurifères de Merico-Ethel ressemblent beaucoup aux gîtes filoniens d’argent-sulfoarséniures des camps miniers historiques de Cobalt et de Gowganda. Ces observations indiquent que ces veines aurifères constituent une variante des systèmes des veines argentifères et qu’elles partagent une genèse commune.

Introduction

Historically, exploration for hydrothermal mineral deposits within the Cobalt embayment, a ~30 000 km2 domain of Paleoproterozoic siliciclastic sedimentary rocks (the Huronian Supergroup) located in northeastern Ontario, has focused on the silver-sulfarsenide vein deposits of the famous Cobalt and Gowganda regions. These Proterozoic vein systems occur along the eastern and northeastern margins of the Cobalt embayment, where they typically occur in proximity to pre-Huronian faults that were reactivated during emplacement of the Nipissing diabase, circa 2219 Ma (Corfu and Andrews, 1986). In fact, all known silver deposits in the Cobalt and Gowganda camps are hosted within, or adjacent to sills of the regionally extensive Nipissing diabase, in close proximity to the Huronian–Archean unconformity, where the sills and Archean volcanic sequences are juxtaposed (Andrews et al., 1986a). The mineralized veins are characterized by a complex ore mineralogy comprised of arsenides and sulfarsenides of Co, Ni and Fe, together with native Ag and Bi, with minor anti-monides, and sulfides of Pb, Sn, and Cu (Petruk, 1971a, b). The gangue mineralogy is dominated by carbonate minerals (mainly calcite ± dolomite), with silicate minerals (quartz, chlorite, amphibole, epidote, K feldspar, albite) limited in their occurrence to thin selvages attached to and/ or immediately adjacent to vein walls. Where present, the ore mineral assemblage occurs at or near the transition between the silicate and carbonate gangue and, as a result, is often distributed along vein walls (Andrews et al., 1986b). Estimated figures for the total silver production from the Cobalt mining camp are 445 million ounces (Marshall and Watkinson, 2000).

The recent discovery of high-grade gold vein mineralization (grab samples up to 6,222 g/t Au; Temex Resources, 2004) by Temex Resources Corporation (Temex) on the Brett property near Latchford, with mineralogy otherwise very similar to Au-rich veins of the Cobalt and Gowganda camps, has led to a renewed interest in the genesis of these polymetallic vein systems and the metallogeny of the Cobalt embayment. This exploration effort has highlighted the presence of additional gold-bearing (grab samples up to 22.34 g/t Au) polymetallic calcite-quartz vein systems in several areas of the Cobalt embayment (Fig. 1, Table 1). Literature research also highlighted the presence of anomalous Au concentrations in several vein systems from the Cobalt and Gowganda mining camps (listed in Table 1). All of the 29 Au-bearing polymetallic vein systems visited during the present study are characterized by:

  • spatial association with regional fault systems;

  • polymetallic mineralization (Co, Cu, Ni, Fe, Pb, Au, Ag, ±U) in calcite-quartz gangue;

  • steeply dipping, predominantly E–W-trending veins with sharp contacts;

  • limited wall-rock alteration patterns; and

  • Proterozoic (inferred) ages.

Potter and Taylor (2010) presented integrated carbon, oxygen, and strontium isotopic data from the major gangue mineral (calcite) in the Au-bearing vein systems indicating a common hydrothermal origin for the regionally distributed polymetallic vein systems involving mixing of meteoric-dominated fluids with basinal brines. Significant intra-deposit variations in δ34S values of vein sulfides (pyrite, chalcopyrite, galena) suggest that localized sources of sulfur existed in the Huronian and/or Archean rocks, with positive values (δ34S > 0‰) in a number of the mineralized vein systems likely reflecting the introduction of sulfur remobilized from the basement, as a result of the conversion of pyrite to pyrrhotite in the underlying Archean rocks. The authors proposed that metals were transported in oxidized fluids, and that precipitation occurred as a result of oxidation reduction reactions along reactivated fault systems which facilitated fluid mixing.

Of the regionally distributed Au-bearing polymetallic vein systems documented, the Merico-Ethel occurrence is presented as the “type locality” because it is the best exposed and most completely characterized system. As presented in this study, field relationships, mineralogy, and petrography of the vein systems suggest that the gold-rich occurrences might be considered variants of the classic silver-sulfarsenide veins present in the Cobalt and Gowganda mining camps. When considered in a regional context, the veins indicate that basin-wide hydrothermal events were involved in vein formation and that they might represent only one of several potential deposit types produced by these events.

Regional Geology

The Merico-Ethel property is located 5 km northeast of the town of Elk Lake in northeastern Ontario along the northeastern margin of the Cobalt embayment (Figs. 1 and 2). The surface geology is dominated by Nipissing diabase sills and Huronian Supergroup sedimentary rocks, which overlie rocks of the Abitibi greenstone belt of the Superior Province; these units and are exposed roughly three kilometers east-northeast of the property. The Archean greenstone belt in this area is comprised primarily of orthogneiss and large granitic batholiths, which have intruded ultramafic to felsic volcanic and sedimentary rocks. The Round Lake batholith intrudes the metavolcanic sequences of the Abitibi greenstone belt in the vicinity of the property and was encountered at depth (200 m to 345 m) by diamond drilling throughout the property. Only one of the twenty drill holes, located at the most southwesterly corner of the property, intersected mafic metavolcanic rocks at depth. All units of the Abitibi greenstone belt are intruded by the Late Archean Matachewan diabase dike swarm, dated at 2690 ± 93 Ma (Gates and Hurley, 1973).

Unconformably overlying the Archean basement are flat-lying clastic sedimentary rocks of the Huronian Gowganda and Lorrain formations (2.5–2.2 Ga; Krogh et al., 1984; Corfu and Andrews, 1986). At Merico-Ethel, the Gowganda Formation is composed of two parts: the underlying Coleman Member diamictite, and the overlying fluvial-deltaic Firstbrook Member sandstone. The Lorrain Formation lies stratigraphically above the Gowganda Formation and comprises arkosic and quartz arenites of alluvial and/or fluvial origin (Young, 1973; Chandler, 1986). These rocks are intruded by sills and dikes of Nipissing diabase, which have been dated at 2219 ± 3.6 Ma by U-Pb methods (Andrews et al., 1986a). The mafic intrusive rocks of the Nipissing diabase are typically massive and occur as large sills (≤200 m thick) over the majority of the property, except in its northeastern corner at the Sauvé occurrence, where they reportedly occur as a northeast-trending dike (Kettles et al., 2006). McCormack (1996) speculated that this dike reflects an underlying structural feature that was reactivated during intrusion of the diabase, and that might have imparted some control on both the Merico and Ethel vein systems. Throughout the property, the intrusive bodies of the Nipissing diabase are highly jointed and fractured near the mineralized vein systems.

The intrusion of the Round Lake batholith caused the development of a nonpenetrative fabric and produced deformation, which resulted in the development of major east-northeast and northeast faults and shear zones in the Archean basement (e.g., the Truax fault, present on the northwest part of the property). The northwest-trending Montreal River fault is situated along Elk Lake and the Montreal River (Fig. 2), immediately to the southwest of the property (Kettles et al., 2006). Although there is little fault offset of lithogical units in the vicinity of Elk Lake, Johns (1986) suggested that the Montreal River fault is a deep-seated structure that: (i) formed as part of the Lake Temiskaming Rift Valley, and is one of many regional-scale faults that were reactivated during the intrusion of the Nipissing diabase; and (ii) might have acted as a major pathway for the movement of mineralizing fluids (see also Born and Hitch, 1990).

Nature and Distribution of the Vein Mineralization at Merico-Ethel

The Merico-Ethel property has an extensive exploration and mining history, with numerous exploration shafts and trenches having been completed in the early to mid 1900s in the search for Cobalt-style high-grade silver mineralization (Silver Jackpot, Paramount, and McManus workings; Fig. 2). The property also hosts the historical Ethel copper deposit (calcite-quartz veins), a short-lived mine fraught with production problems, and the Sauvé uranium vein showing (Kettles et al., 2006). Recent exploration (2004 to 2008) has uncovered the presence of additional calcite-quartz veins with significant gold and base metal mineralization associated with historical workings at Merico. A geological map of the Merico vein system, exposed by backhoe trenching and pressure washing along a strike length of nearly 250 m, is provided in Figure 3. Although exposure is not continuous, several occurrences with similar mineralogy, mineral parageneses, morphologies, and orientations occur throughout the property suggesting that the Merico vein system might have a much greater extent than previously thought (i.e., it might be continuous between the Ethel, Merico, and Sauvé occurrences).

Geological Setting of the Polymetallic Vein Mineralization

The polymetallic vein systems of the Merico-Ethel property are hosted within localized zones of strong jointing and fracturing with minor shearing. These localized zones, although present in the underlying sedimentary rocks, are most prevalent in the Nipissing diabase. The joints and fractures are subvertical and strike in three dominant directions: E–W, NE–SW, and SE–NW. The steeply N-dipping, E–W oriented veins are best characterized by the Merico vein system, the Ethel Mine veins, and the Sauvé occurrence (Fig. 2), that are all located along an E–W-trending structure that branches off the Montreal River fault and that is visible on the most recent regional airborne magnetic survey (Ontario Geological Survey, 2004b). The NE–SW striking and vertically dipping (varies from 75°S to 80°N) orientation is the dominant structure for veins at the Silver Jackpot and Silver Fox occurrences, and the veins located 500 m east of Silver Jackpot on the eastern shore of a large beaver pond. The SE–NW-striking, subvertically dipping (80°S to 78°N) vein orientation is exemplified by the veins at the Paramount occurrence. This orientation is a secondary controlling structure at the Silver Jackpot occurrence and in veins located on the east side of the large beaver pond (Kettles et al., 2006).

Of all the known mineralized systems within the property, the Merico veins comprise the most extensive and prospective veins. The Merico vein system consists of a series of anastomosing, polymetallic (Cu-Co-Au-Ag-As), calcite-quartz veins hosted in medium-grained Nipissing diabase (Fig. 3), accompanied by a narrow zone of brittle deformation and minor carbonate alteration. The main exposed vein of the Merico system varies from 0.1 m to 1.0 m in width, and displays a fairly consistent overall westerly strike (260º) with a steep, northerly dip (≥75º) over a continuous length of approximately 250 m. Additional trenching 50 m northeast and 250 m south of the main Merico system suggests the veins might have a much greater extent. Historical diamond drilling reportedly intersected the mineralized zone at 50′ and 100′ depths on the western edge of the main vein system (Kettles et al., 2006). Furthermore, historical reports from the nearby (and possibly connected) Ethel copper mine indicate that mineralization was intersected in the inclined adit at the 125′ level for 500′ striking E–W, and that the vein systems continued into the Gowganda Formation sedimentary rocks underneath the diabase sill (MacKean, 1968). Recent diamond drilling undertaken by Temex Resources Corporation indicates the surface exposure of these veins are located approximately 300 m above the regional unconformity between the Gowganda Formation sedimentary rocks and the underlying Archean granitic basement. As with most of the property showings, the main vein system at Merico is hosted within a zone of intensely fractured Nipissing diabase up to 2.5 m in width. Despite the intensity of fracturing, chemical alteration of the host diabase is very limited in extent (centimeter-scale) and is typically manifested by secondary chlorite + quartz + epidote ± hematite ± calcite alteration, and veinlets developed along fractures. Traces of bright red, carbonate-sodic alteration (secondary calcite and albite) were also noted in the main vein system, but their abundance and extent is negligible. This is in direct contrast to the historical silver occurrences in the area, such as the Silver Jackpot mine, where zones of intense carbonate-sodic alteration completely replace the host rock for up to 12 cm on either side of the veins. Textures within the Merico vein system transition from brittle to ductile shearing in the western portion of the vein system (Fig. 4), to extensional, open-space filling textures in the eastern extent of the vein system (Fig. 5). These textures are virtually identical to the textures observed in the historic Cobalt silver vein systems. Although not observed by the authors, in the central portion of the exposed vein system a few late-stage vugs are reportedly lined with bladed calcite (Kettles et al., 2006), a feature considered indicative of boiling zones in epithermal gold deposits (Browne and Ellis, 1970).

Ore Mineralogy, Geochemistry, and Paragenesis

The polymetallic vein mineralization on the Merico-Ethel property is dominated by chalcopyrite, specular hematite, pyrite, cobaltian pyrite, cobaltite, and cobalt-rich sulfide minerals of the linnaeite group (viz. linnaeite and carrollite). Gold mineralization occurs primarily as small anhedral grains (5 μm to 20 μm in size) of native gold hosted in calcite, sulfides, and quartz. All of the recorded occurrences of gold mineralization are polymetallic in nature, and preliminary results suggest that the chalcopyrite-rich portions of the veins generally have higher gold and lower silver concentrations than the arsenide-bearing (cobaltite) sections, which tend to have higher silver values (Table 2 and Fig. 6). As summarized in Table 2, assays of high-grade vein mineralization from exploration pits and trenches on the property demonstrate the highly enriched nature of the vein mineralization in terms of Cu, Co, As, Pb, Bi, Ag, and Au. The geochemical assay data also permit the discrimination of a gold-rich variety of polymetallic mineralization, where the grade of gold averages 5.05 g/t and has a maximum recorded value of 22.34 g/t. The goldrich mineralization is typically recognized in the field by increased abundance of specular hematite and chalcopyrite, which both exceed 20 modal percent in some parts of the veins. Decreased gold values were observed in As-rich, Co- and Ni-bearing (cobaltite) portions of the vein systems. In terms of their Ag-Co-As-rich character, the mineralized portions of the Merico-Ethel vein systems closely resemble the ore-bearing sections of the silver-arsenide veins from both the Cobalt and Gowganda camps (cf. Petruk, 1971c).

Blocky-to-granular calcite is the dominant gangue mineral in both chalcopyrite- and hematite-rich veins, with lesser amounts of quartz also present. However, both calcite- and quartz-rich sections can occur along strike in the same vein or vein system. With the exception of early pyrite and traces of an early chalcopyrite, the majority of the sulfide minerals are hosted in calcite. The sulfides are typically associated with chlorite and are preferentially concentrated along the transition from quartz- to calcite-dominant gangue. This change from quartz to calcite gangue is also marked by the presence of significant hematite mineralization, followed by the main stage of sulfide precipitation. The hematite is predominantly specular in variety and typically forms large, centimeter-scale rosettes in the vein system along the quartz-to-calcite transition (Fig. 7). When not deformed, chalcopyrite and pyrite are subhedral in habit and are very coarse-grained (up to 1.5 cm in diameter). As shown in Figure 8a, almost all of the chalcopyrite grains record some evidence of replacement of earlier pyrite, either forming rims around pyrite grains or hosting relict inclusions of pyrite. Chalcopyrite also contains inclusions of cobaltian pyrite, some of which are euhedral in habit, and anhedral inclusions of linnaeite-group minerals [CoCo2S4–Cu(Co,Ni)2S4]. The linnaeite-group minerals represent a solid solution comprised of 75% linnaeite and 25% carrollite end-members, calculated by stoichiometry. Where not hosted within chalcopyrite, the linnaeite-group minerals occur as euhedral grains set in calcite gangue. The main-stage of pyrite compositions display varying levels of cobalt substitution (0–21 atomic % Co), but only trace amounts of nickel.

The crystallization of chalcopyrite was followed by that of cobaltite, with the latter mineral commonly rimming or occurring along fractures within chalcopyrite (Fig. 8). Subhedral to euhedral grains of cobaltite were also noted within calcite. Trace amounts of native gold, galena, wittichenite (CuBiS3), and aikinite-bismuthinite solid solutions (PbCuBiS3–Bi2S3) are hosted in both quartz and calcite gangue and within fractured grains of chalcopyrite. The minute inclusions of aikinite-bismuthinite (≤5 μm) occur in trace amounts in fractured and brecciated chalcopyrite grains that are rimmed by cobaltite (Fig. 8b). Representative analyses of the dominant ore minerals are presented in Table 3. Unlike many of the known Proterozoic polymetallic vein systems hosted in the Cobalt embayment, the Merico veins contain only trace amounts of galena, most of which is paragenetically late.

To summarize, the general sequence of ore mineral deposition in the mineralized veins was: pyrite → Co-pyrite → linnaeite → native gold → chalcopyrite → cobaltite → Cu-Pb-Bi-sulfides → galena (Fig. 9).

Representative electron microprobe analyses of native gold from the Merico- Ethel property and Brett occurrence are presented in Table 4 and the range in gold fineness is illustrated in Figure 10. In general, the native gold from the Proterozoic vein systems is characterized by very low concentrations of trace metals (e.g., Hg, Sb). Silver contents in native gold from the Merico- Ethel vein systems show a considerable range of values from approximately 17 to 37 wt.%, whereas native gold from the Brett occurrence exhibit a much narrower range (8 to 13 wt.% Ag). It is noteworthy that this range encompasses values that are characteristic of volcanogenic and mesothermal type deposits (Morrison et al., 1991). Also, it is striking that the native gold from the Brett occurrence, whose isotopic characteristics indicate derivation of gold from an Archean mesothermal source (Potter and Taylor, 2010), is quite different from that of the Merico- Ethel vein systems (volcanogenic).

Gangue Mineralogy of the Veins

Calcite is the dominant gangue mineral, with lesser amounts of quartz, although their modal abundances vary considerably over the strike length of the main Merico vein system. For example, at the eastern extent of the vein system, calcite comprises approximately 95 modal % of the gangue, whereas in the western branch in the central portion of the vein system quartz comprises approximately 70 modal % of the gangue. Calcite is essentially stoichiometric throughout the vein systems, with very low concentrations of magnesium, iron, and manganese (<0.25, <0.50, and <0.50 wt.%, respectively). However, a few calcite grains analyzed from the main Merico vein system contained elevated iron and manganese concentrations (up to 0.63 wt.% FeO and 2.47 wt.% MnO). Textural relations suggest that these grains are examples of a brecciated, early calcite that is hosted in later calcite, which contains lower concentrations of these elements. After calcite and quartz, chlorite is the next most abundant gangue mineral in the veins and is preferentially concentrated in the early quartz-rich phase of vein formation. Chlorite (in addition to calcite and trace diopside and andradite) also occurs in the center of the majority of the rosettes of specular hematite that are typical of the Merico veins (Fig. 7a), a relationship that suggests these minerals acted as a nucleation site for the subsequent growth of the iron oxide mineral. Other commonly occurring gangue minerals in the veins include: epidote, allanite-Ce, andradite, diopside, apatite, rutile, xenotime-Y, and titanite. Representative analyses of these minerals are presented in Potter (2009). Andradite garnet occurs in trace amounts and is preferentially associated with chalcopyrite, where it occurs as discontinuous rims (10 μm to 20 μm thick) around isolated sulfide grains and within the centers of hematite rosettes. The rare earth element-rich minerals allanite-Ce and xenotime- Y are preferentially hosted in calcite but also occur in quartz, indicating they formed during the transition from a silicate- to carbonate-dominant gangue.

Zoning of the Polymetallic Veins

As illustrated in Figures 5 and 9, the extensional textured veins typically exhibit a well-defined zoning, consisting of the following mineral assemblages (margin to core): (i) quartz + chlorite ± early-stage sulfides ± albite ± epidote; (ii) calcite + chlorite ± hematite ± main-stage mineralization; and (iii) massive calcite ± late-stage galena. The early-stage mineralization consists of pyrite ± chalcopyrite set in quartz ± chlorite gangue, whereas the main-stage mineralization comprises Cu + Fe+ Co + Ni ± Pb ± Bi sulfides and sulfarsenides and Au + Ag ± U hosted in calcite gangue. Over the strike length of the polymetallic veins, the modal abundance of the base metal sulfides varies considerably, from absent to approximately 30%. Figure 4b is representative of some of the highest grades of mineralization present in the Merico vein system (channel sample: 6.33% Cu, 0.27% Co, 0.03% Ni and 5.99 g/t Au over 0.5 m). Quartz-rich sections of the vein system differ in that they lack late-stage calcite flooding.

Wall-Rock Alteration

Diamond drilling and surface exploration on the Merico- Ethel property has highlighted the presence of a hematite + chlorite + epidote-allanite-Ce + carbonate alteration halo surrounding the mineralized vein systems within the Huronian sedimentary rocks. In Coleman Formation conglomeratic units encountered in diamond drilling located approximately 1.25 km northeast and 1.5 km south of the Merico veins, specular hematite, chlorite, and epidote-allanite-Ce occur as interstitial grains rimming clasts (Fig. 11). On surface, specular to botryoidal hematite + calcite + quartz stringers were noted on the southeastern margin of the property, approximately 750 m northeast of the Paramount occurrence (Fig. 12). Here, the anastomosing stringers crosscut Lorrain Formation arenite and trend 070° and dip 70°–90°S (in the same orientation as the Merico, Ethel, and Sauvé vein systems). Narrow zones (<5 cm wide) of quartz and calcite alteration in the arenite impart a bleached appearance in the vicinity of the stringers. Microfractures within the bleached arenite also contain trace amounts of allanite-Ce. A subsequent examination of Lorrain Formation sedimentary rocks encountered in diamond drilling northeast of the Merico veins revealed additional allanite- Ce in calcite-quartz veinlets and microfractures (Fig. 12c). Interestingly, grains of secondary allanite-Ce were also noted in Coleman Formation sedimentary rocks adjacent to the Little Silver Vein in Cobalt (Venance, 1989), indicating that LREE were remobilized during vein formation in both the hydrothermal systems.

Throughout the property, localized zones of carbonatesodic alteration (fine-grained, granular albite + calcite; often referred to as “aplitic alteration or aplitic dikes” in the historical reports; Fig. 13; MacKean, 1968; Johns, 1986; Kettles et al., 2006) are commonly associated with calcite-quartz veining within the diabase. These fracturecontrolled alterations are generally narrow (≤5cm), sporadic, and, when present, have limited strike lengths (≤1m) along the calcite-quartz veins. More commonly, and in the case of the Merico vein system, chemical alteration of the host diabase is very limited in extent (cm-scale) and is typically manifested by secondary chlorite + quartz + epidote ± hematite ± calcite developed along fractures. This alteration is accompanied by the breakdown of minor magnetiteilmenite intergrowths along the vein contacts.

Age of the Polymetallic Veins

As presented by Potter and Taylor (2009), thermal ionization mass spectrometric lead isotope analyses of pyrite and chalcopyrite concentrates from the Merico-Ethel, Duncan Lake, Capital Mine, Delhi Township, Laura Lake, Fraleck East, and Burda Mine occurrences yielded regressions indicative of vein mineralization ~2.2 Ga, immediately post-emplacement of the Nipissing diabase. The embayment wide data yielded an “errorchron” age of 2236 ± 180 Ma, whereas sulfides extracted from the Merico-Ethel property produced a secondary isochron age of 2204 ± 160 Ma. Both the regional “errorchron” and Merico-Ethel secondary isochron ages fall within error of the Nipissing diabase intrusive event, which has been dated at 2219 ± 3.6 Ma (Andrews et al., 1986b), thus supporting the approximate contemporaneity of the two (i.e., within ~15 m.y.). Although Potter and Taylor (2009) have presented Pb-isotope evidence suggestive of remobilization of certain ore elements circa 1700 Ma in many of the polymetallic vein systems, the Merico vein system does not contain any Pb isotopic evidence of a secondary hydrothermal (mineralization) event. Work is in progress on the geochronology of accessory minerals (i.e., allanite) to better constrain the timing of events in the embayment. Lead isotope evidence from the silver-arsenide vein deposits of the Cobalt area also provides support for the occurrence of these two regional episodes of hydrothermal activity, with the earlier event having produced base metal mineralization in the Cobalt area ~2160 Ma (Thorpe, 1974), and the later event having resulted in significant remobilization of ore elements in many of the Cobalt vein systems ~1650 Ma (Thorpe et al., 1986)

Discussion: Comparison to the Historic Cobalt-Gowganda Silver Veins

Vein Morphologies

Both the Cobalt-Gowganda silver veins and the gold-bearing veins are steeply dipping and anastomosing in nature. Extensional, sheared and brecciated morphologies are commonly present along the same system and contacts with the host rocks are typically sharp. Current exposure limits most of the gold-bearing vein systems to approximately 300 m in strike, depths of 30 m and widths ranging from 0.1 m to 1.0 m, whereas the Cobalt-Gowganda silver vein systems have been traced for greater than 300 m in length and 100 m in vertical extent (Marshall and Watkinson, 2000). Both vein systems are associated with steeply dipping fracture/fault zones which splay off regional fault systems.

Geological Settings and Host Rocks

All but one of the currently identified gold-bearing vein systems are hosted in Proterozoic-aged rocks, typically within 300 m of the regional unconformity and in association with Nipissing diabase. The lone exception is the Brett occurrence, which is hosted in both Archean mafic metavolcanic rocks and Gowganda Formation diamictite near the regional unconformity. Likewise, the Cobalt- Gowganda silver veins are hosted in Nipissing diabase, Huronian sedimentary rocks, or Archean mafic basement within 250 m of the diabase. Furthermore, all of the economically productive veins were situated in close proximity to the regional unconformity where Nipissing diabase sills and Archean metavolcanic sequences are juxtaposed (Andrews et al., 1986a).

Ore Mineralogy

Both vein systems are complex in character, containing sulfides, arsenides, oxides, and native metals. The Cobalt- Gowganda silver veins are differentiated by greater concentrations of arsenides and native silver and the presence of anti-monides, whereas the gold-bearing veins are richer in native gold, hematite, and chalcopyrite. Notably, both systems contain cobaltite, pyrite, galena, and rare bismuth sulfides (wittichenite, aikinite-bismuthinite). In both types of vein systems, the ore minerals are discontinuous alongstrike, and when present, are concentrated along the transition from silicate and carbonate gangue. Mineralization occurs in the form of massive pods, bands, dendrites, and zoned rosettes (Andrews et al., 1986a).

Gangue Mineralogy

Both vein systems contain a simple gangue assemblage dominated by carbonates, predominantly calcite and lesser concentrations of dolomite. Silicates (quartz, chlorite, epidote) are ubiquitous but typically comprise less than 20 modal % in the Cobalt-Gowganda silver veins. Some of the gold-bearing vein systems contain significant quartz gangue, but the quartz content can vary greatly along strike length. In terms of precious metal endowment, the quartzrich veins appear to be less prospective.

Accessory Minerals

Both types of vein systems have sporadically developed titanite, K-feldspar, albite, epidote, allanite, garnet (andradite), diopside, apatite, rutile, and xenotime. Perhaps due to greater study, the Cobalt-Gowganda vein systems are also noted to contain actinolite, anatase, apatite, axinite, stilpnomelane, and prehnite (Jambor, 1971; Andrews et al., 1986a).

Mineral Paragenesis

In extensional gold-bearing veins, the following paragenetic sequence is typically present: (i) quartz + chlorite ± early-stage sulfides ± albite ± epidote; (ii) calcite + chlorite ± hematite ± main-stage mineralization; and (iii) massive calcite ± late-stage galena. Similarly, the Cobalt-Gowganda silver veins are characterized by an early crystallization of silicate gangue (mainly quartz, chlorite, actinolite ± K-feldspar) followed by deposition of polymetallic ore minerals (Ag-sulpharsenides, sulfides) along the transition from silicate to carbonate-mineral gangue, and terminated by calcite crystallization (Andrews et al., 1986a).

Wall Rock Alteration

When hosted by Nipissing diabase, both Cobalt- Gowganda silver and gold-rich vein systems have similar alteration patterns: narrow (<5 cm) symmetrical alteration haloes of quartz + chlorite + epidote ± calcite accompanied by the breakdown of magnetite-ilmenite intergrowths. Geochemical studies of the alteration patterns at Cobalt have illustrated that this is driven by Si depletion and Ca, Na, and CO2 enrichment (Marshall and Watkinson, 2000). Localized, narrow (<5 cm) zones of carbonate-sodic alteration (albite + calcite + chlorite ± titanite) have been described from both types of veins. In the surrounding sedimentary rocks, a broad (currently ≤2.5 km) hematite + chlorite + epidote-allanite alteration halo has been noted from the Merico-Ethel property.

Implications for Mineral Exploration in the Cobalt Embayment

Potter and Taylor (2010) presented a genetic model (Fig. 14) based on the observed isotopic and textural features of the polymetallic vein systems that involves: (i) Regional flow of oxidized, hydrothermal fluids, driven by sedimentary loading and the heat released by the Nipissing diabase intrusive event at ~2200 Ma, focused along the Huronian-Archean unconformity; and (ii) genesis of regionally distributed, discordant, polymetallic vein mineralization through the interaction of the oxidized basinal fluids with both reducing fluids and solid components of the basement, facilitated structurally by fault reactivation and the localized displacement of the Huronian-Archean unconformity. Such a model requires that different basement source lithologies act to control base- and precious-metal endowment in the Huronian polymetallic veins, and we note that the distinction in native gold composition between the Merico and Brett occurrences is consistent with such a model. These features are also indicative that the regional Huronian-Archean unconformity remains a prime exploration target for undiscovered mineral deposits.

Summary and Conclusions

  1. Gold-bearing, polymetallic mineralization in the Cobalt embayment occurs in extensive systems of steeply dipping, calcite-quartz veins that are hosted in Nipissing diabase, Huronian sedimentary and Archean mafic basement rocks. These vein systems formed immediately post-crystallization of the Nipissing diabase sills, ~2220 Ma.

  2. The mineralized vein systems are best characterized by the Merico vein systems, and possibly the Brett occurrence. The Merico, Ethel, and Sauvé vein systems are located along a large curvilinear structure visible on regional aeromagnetic surveys which branches off the Montreal River fault (Ontario Geological Survey, 2004b). Vein morphology is dominated by simple extensional textures, although sheared and brecciated morphologies are also present. All three morphologies can exist alongstrike of one vein system.

  3. The ore mineralogy is complex in character, comprising sulfides, arsenides, native metals (mainly native gold, lesser silver), and hematite (specularite). Sulfide minerals are volumetrically the most important, with chalcopyrite being the most abundant, plus lesser pyrite and galena; Co-rich and Bi-rich sulfides are also present, and cobaltite is the main arsenide mineral.

  4. Polymetallic ore mineralization is typically discontinuous along any given vein; where present, it occurs along the interface between silicate (quartz ± chlorite) and calcite gangue, and therefore is typically distributed adjacent to the vein walls. Such a distribution records: (i) an “Early-stage” Fe ± Cu sulfide mineralization with its associated quartz ± chlorite gangue, followed by; (ii) a “Main-stage” polymetallic (Cu + Co + As ± Ni ± Pb ± Ag ± Au ± Bi ± U) mineralization hosted in calcite gangue; and (iii) “Late-stage” calcite flooding. Wall-rock alteration around the mineralized veins is restricted to narrow (<5 cm) haloes of calcite- and chlorite-bearing assemblages.

  5. The mineralogical, geochemical, and isotopic evidence suggests that the gold-bearing veins are variants of the historic Cobalt-Gowganda silver veins. As such, the veins highlight the basin-wide extent of the hydrothermal event associated with intrusion of the Nipissing diabase. When considered in a regional context, the currently identified veins indicate that significant hydrothermal circulation has occurred along the regional unconformity and that potential exists for additional styles of mineralization.

Acknowledgments

The authors would like to thank management of Temex Resources Corp., for their support and permission to publish the data. N. Pettigrew, N. Sicard, and R. Brett are thanked for their field and logistical support while accessing many of the properties. The reviews of T. Setterfield and A. Sexton and editorial improvements of J. Richards are gratefully acknowledged.

Analytical Techniques

Assay data

All rock samples were sent to Swastika Laboratories Ltd. of Swastika, ON, for Au, Ag, As, Cu, Co, Ni, Pb, Znk, and Te analysis. Samples were crushed to 10 mesh size by a Rolls crusher and split into 350 g aliquots using a Jones Riffle. The 350 g sample was then pulverized to 90% passing through 100 mesh. Gold was analyzed by fire assay/ atomic absorption using a 30 g sample of pulverized material with a detection limit of 2 ppb Au. All samples assaying over 1 g/t were re-assayed by the fire assay/gravimetric method. Analysis for Ag, As, Cu, Co, Ni, Pb, Zn, and U was performed using a 0.5 g sample of pulverized material, digested in warm aqua regia, and then analyzed by atomic absorption spectroscopy. Detection limits were as follows: Ag 0.1 ppm, As 5 ppm, Cu 1 ppm, Co 1 ppm, Ni 1 ppm, Pb 1 ppm, U 2 ppm, and Zn 1 ppm. Selected samples were also sent to Assayers Canada, utilizing a 30 element package comprised of warm aqua regia digestion and analysis by inductively coupled plasma atomic emission spectroscopy.

EMPA

All mineral analyses were performed utilizing a Camebax MBX electron microprobe equipped with four wave-length dispersive spectrometers at Carleton Universy. Operating conditions were 15 kV accelerating potential and a beam current of 20 nA. Peak counting times were 15 to 60 seconds or 30,000 counts, whichever came first. Raw X-ray data were converted into elemental weight percent by the Cameca PAP matrix correction program.