Hope Bay Volcanic Belt Research Articles

Evidence of upgrading of gold tenor in an orogenic quartz-carbonate vein system by late magmatic-hydrothermal fluids at the Madrid Deposit, Hope Bay Greenstone Belt, Nunavut, Canada

Evidence of secondary gold enrichment due to the addition of new gold into an earlier orogenic quartz-carbonate vein deposit by magmatic-hydrothermal fluids is strongly suggested for the Madrid Deposit, hosted in the Hope Bay Greenstone Belt in Nunavut, Canada. The conclusion is based on an extensive in situ microanalytical protocol (SEM, confocal Raman microspectroscopy, microthermometry, decrepitate mound analysis, LA-ICP-MS, cathodoluminescence, SIMS) not previously applied to gold systems. This approach was used to characterize the mineralogy and fluid inclusion systematics associated with the upgrading event.Mineralization comprised of only Ag-bearing gold (“electrum”; 89.8 at. % Au avg.; n = 8) is present throughout all investigated laminated and brecciated orogenic quartz veins. However, in high-grade vein intersections where gold grades are locally elevated (up to 122 g/t), assemblages containing tennantite-tetrahedrite + chalcopyrite + electrum (80.7 at. % Au avg.; n = 15) ± AgPbAu tellurides occur that are texturally late-stage relative to electrum-only mineralization. Subdomains of quartz coeval with this later assemblage are optically- and texturally-distinct from earlier orogenic quartz. This late mineral assemblage is absent in all low-grade vein intersections (∼1 g/t Au avg.) examined where only electrum is identified. Quartz-hosted fluid inclusions (H2ONaCl ± CO2) of intermediate salinity (16.7 ± 1.2 wt.% NaCl equiv.; n = 93) were identified only in the high-grade vein samples and are present along healed planes associated with tennantite-tetrahedrite + chalcopyrite + electrum ± AgPbAu telluride assemblages. In situ SIMS δ18O analyses of quartz, combined with temperature constraints from mineral equilibria, show that early orogenic vein quartz and late quartz subdomains associated with gold upgrading precipitated from fluids with similar δ18OH2O values of 4.5–12.4‰ (n = 10) and −5.5 to 11.8‰ (n = 13), respectively. Isotopic data suggests that meteoric water was a negligible component in fluids responsible for gold precipitation. Microthermometry and Raman spectroscopy show that upgrading fluids were distinct in composition compared to the earlier metamorphic fluids (H2ONaClCO2 ± CH4 ± N2; 4.6 ± 1.6 wt.% NaCl equiv., n = 33) and later Canadian Shield basement brines (H2ONaCl; 22.4 ± 1.2 wt.% NaCl equiv., n = 12) which have also been identified in the fluid inclusion record at Madrid. Laser ablation ICP-MS analyses indicate the gold upgrading fluids are enriched in AsSbZnPb and LILE (CsBaRbSr); these data are consistent with late fluids derived from an evolved magmatic-hydrothermal system. Trace element mapping of pyrite, coupled with principle component analysis of the data, confirms a strong correlation between Au and AgTeSbBiW(As) in upgraded veins, whereas only Au and As strongly correlate in low-grade veins.The study suggests that gold upgrading, either involving newly introduced gold or remobilization of existing gold, can be linked to the incursion of a late magmatic-hydrothermal fluid that post-dated formation of the main orogenic-type auriferous quartz vein system. The geological setting and mineral-chemical features suggest an intrusion-related (i.e., porphyry), or intermediate-sulfidation epithermal mineralization style for the later event. This work provides another example of the importance of compositionally distinct cumulative hydrothermal events in the development of high-grade gold deposits in orogenic settings.

Volcanic Stratigraphy, Geochronology, and Gold Deposits of the Archean Hope Bay Greenstone Belt, Nunavut, Canada

The Archean Hope Bay greenstone belt is located in the Bathurst Block of the northeastern part of the Slave Structural Province, a predominantly Archean, granite-greenstone-metasedimentary terrane. The greenstone belt is dominated by mafic volcanic rocks with less common felsic volcanic and volcaniclastic products and subordinate ultramafic bodies and metasedimentary rocks. Three main Au deposits have been defined in the greenstone belt, with a cumulative resource of over 10 million ounces (Moz) of Au, as of April 2007. The Hope Bay greenstone belt can be divided into a series of coherent panels of strata that tend to be fault bounded and collectively are used to construct a composite stratigraphic column of the belt constrained by U-Pb geochronology on felsic suites. The oldest felsic suite, the Flake Lake suite (ca. 2700 Ma), is a succession of felsic volcanic rocks that has a tholeiitic geochemical affinity, interpreted as a product of rift-related volcanism. Overlying this suite is a series of well-constrained calc-alkaline, mainly felsic volcanic rocks, which have ages of ca. 2690 Ma (Square Lake suite), ca. 2686 Ma (Windy felsic suite), ca. 2677 Ma (Koignuk suite), and ca. 2662 Ma (Clover Lake suite), all of which are interpreted as products of arc volcanism. The transition from rift- to arc-related volcanism divides the greenstone belt into upper and lower volcanic cycles with the transition occurring at about ca. 2690 Ma. The older cycle of volcanic rocks has a distinctive group of mafic rocks consisting of mafic pillowed flows which have an Fe-rich tholeiitic geochemical affinity which is not recognized in the younger volcanic cycle. These Fe-rich tholeiitic mafic volcanic rocks commonly show a spatial relationship to Au mineralization acting as host strata to the main Au deposits in the greenstone belt. The younger cycle (<ca. 2690 Ma) of mafic volcanic rocks tends to have a tholeiitic affinity with some minor andesitic volcanic rocks near the top of the composite stratigraphic column. Detrital zircon work was carried out for all the main sedimentary successions recognized in the Hope Bay greenstone belt. The detrital zircon results show that in each case the sediments were derived mainly from local sources, as very few anomalously old zircons were recognized. A conglomerate outcropping in the north part of the belt is dominated by locally derived material, with detrital zircon work supporting this interpretation. The three main Au deposits at Hope Bay are all associated with the older cycle of volcanic rocks, interpreted as products of rift-related volcanism. The structural setting of each deposit is different but each is related to D2 strain in structural and/or stratigraphic settings which provided dilational environments to localize auriferous vein-related mineralization or localized alteration and mineralization of favorable host lithologies. The older cycle of tholeiitic felsic and Fe-rich mafic volcanic rocks are rift-related volcanic products, which are time-equivalent to the Kam Group, in the Yellowknife Supergroup. Overlying these rocks are calc-alkaline felsic volcanic rocks and tholeiitic mafic volcanic rocks, which represent the transition to calc-alkaline arc-related volcanism, which correlates to the Banting Group of the Yellowknife Supergroup. Overlying the younger (ca. 2677 Ma) felsic volcanic rocks are a succession of conglomerates consisting mainly of locally derived clasts, representing collapse of the arc into a fluvial-dominated environment. This sedimentary succession is possibly equivalent to the regionally extensive Burwash Basin of the Yellowknife Supergroup.

A Fluid Inclusion and Stable Isotopic Investigation of the Boston Lode-Gold Deposit, Hope Bay Volcanic Belt, Nunavut

The Boston deposit is a recently discovered Archean lode-gold deposit located approximately 700 km northeast of Yellowknife, in the southern portion of the Hope Bay volcanic belt. Primary H2O-CO2-NaCl fluid inclusions associated with gold mineralization at Boston are of low salinity (4.6 ± 1.4 eq. wt.% NaCl, n = 182) and contain pure CO2. Homogenization temperatures of liquid-phase inclusions average 270° ± 33°C (n = 141), but some fluid inclusion assemblages contain coexisting liquid- and vapor-phase inclusions that homogenize near critical conditions, suggesting phase separation. In these groups, liquid-phase inclusions homogenized at 299° ± 25°C (n = 24), and vapor-phase inclusions homogenized at 338° ± 28°C (n = 18). These temperatures overlap within error, and a median temperature of ~320°C is taken as representative of trapping conditions within the veins. An approximate pressure of 2-3 kbar was estimated from fluid inclusion isochores at this temperature. Stable isotopic compositions of carbonate minerals from the mineralized zones range from -4.7‰ to -3.1‰ in δ 13 C, and from 11.2‰ to 12.8‰ in δ 18 O; quartz ranges from δ 18 O = 12.8‰ to 14.9‰; and pyrite ranges from δ 34 S = 2.1‰ to 3.5‰. The δ 18 OH2O, δ 13 CCO2, and δ 34 SH2S values of the fluid calculated from these minerals at 320°C are 6‰-9‰, -2.4‰ ± 0.6‰, and 1.5‰ ± 0.5‰, respectively, and are intermediate between metamorphic and magmatic compositions. A unique source for the fluids cannot, therefore, be constrained from these data. Petrographic and geochemical data for the Boston deposit are consistent with data from other Archean lode-gold deposits worldwide, implying that the Boston deposit formed in a similar environment and from similar types of fluids. These results suggest that the Hope Bay volcanic belt may represent a major new mesothermal gold belt in this part of Nunavut. ©2006 Canadian Institute of

Gold distribution in glacial sediments and soils at Boston Property, Nunavut, Canada

Geochemical exploration for gold in the Slave Structural Province, Nunavut, Canada is hampered by a complex cover of Quaternary sediments and a lack of information on the effects of glacial dispersion and periglacial conditions on gold distribution. This study investigates distribution of gold in surficial materials on the Boston Property in the southeastern portion of the Hope Bay Greenstone Belt, 650 km northeast of Yellowknife, N.W.T. The principal surficial materials are till with a discontinuous veneer of post-glacial marine sediments. In areas of shallow overburden, frost boils mix weathered bedrock with drift and bring bedrock fragments to the surface. Samples were collected from till and from soil profiles on lines across the mineralized zone. The −212 and −53 μm fractions were separated by wet sieving and heavy mineral concentrates (HMCs) prepared. Gold was determined by fire assay–atomic absorption spectroscopy (FA–AAS) or by a more sensitive aqua regia-column preconcentration–ICP procedure. There are two types of gold anomalies: (1) anomalies in the regional till, and (2) anomalies developed by ongoing frost boil action and colluvial processes. Anomalies are absent in marine sediments. Gold values in the regional till range from 15 to 1030 ppb in −106+53 μm HMCs and from 10 to 65 ppb in the −53 μm fraction, which contains ~60% of the total gold content. Anomalous values extend at least 2 km down-ice from the gold mineralization. In profile, the most recent gold anomaly is mushroom-shaped, up to ∼100 m wide at the surface, and drapes over a ridge of mineralized bedrock. The anomaly in near-surface horizons is 10–50 m broader than the anomaly in the underlying horizons. Maximum gold values (>5000 ppb) are found in areas of weathered bedrock and where frost boils include abundant fragments of mineralized bedrock. For reconnaissance-scale exploration, frost boils in the regional till should be sampled on a 500×40 m grid with lines perpendicular to ice-flow direction. The −53 μm fraction should be analyzed for gold with a detection limit of 1 ppb or lower. At the property scale, with anticipated higher gold values, it is feasible to analyze the coarser −212 μm fraction from near-surface samples collected at a 10–20 m spacing along grid lines 100 m apart.