Noranda District Research Articles

Random forest classification for volcanogenic massive sulfide mineralization in the Rouyn-Noranda Area, Quebec

Random forest (RF) classification was applied to 37 predictor maps (vectors to mineralization) producing a Mineral Prospectivity Map (MPM) for volcanogenic massive sulfide (VMS) mineralization in the Noranda District, Abitibi subprovince, which is host to ∼ 20 VMS deposits and numerous subeconomic occurrences. The predictor maps were created using geological, geochemical, and geophysical data, and the known VMS deposits were used to train the RF classifier. The RF model was applied on two regions of interest (ROI) to investigate the effect of different sized ROIs on the results. Five sets of balanced and unbalanced training data were used in the experiments to examine the sensitivity of RF to different sets of training data. The probability/prospectivity maps showed very high success rate of classification with regard to training data particularly for the training sets with higher ratio of non-deposits/deposits. Accuracy assessment of classification maps using cross-validation also showed very high accuracy when assessed by the training data in all experiments again giving a higher accuracy for the training sets with higher ratios of non-deposits/deposits, however the later showed very low accuracy when applying k-fold cross-validation or when assessed by 15 VMS showings used as the test data indicating an overfitting problem with the unbalanced data. In most of the experiments, principal component 4 (PC4) of geochemical analysis, proximity to synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, lithology, and sericite alteration maps showed higher predictive power. The importance of individual variables changed when the ROI was changed to a smaller area suggesting that RF is sensitive to the study area.

The implications of crustal architecture and transcrustal upflow zones on the metal endowment of a world-class mineral district

Earth’s mineral deposits show a non-uniform spatial distribution from the craton-scale, to the scale of individual mineral districts. Although this pattern of differential metal endowment is underpinned by lithospheric-scale processes the geological features that cause clustering of deposits remains enigmatic. The integration of geological and geophysical (seismic, gravity, and magnetotelluric) features has produced the first whole-of-crust image through an iconic Neoarchean volcanic complex and mineral district in the Abitibi Greenstone Belt, Superior Province, Canada. Observations indicate an asymmetry in surface geology, structure, and crustal architecture that defines deep transcrustal magmatic-hydrothermal upflow zones and the limits of the Noranda District ore system. Here, extreme volcanogenic massive sulfide (VMS) endowment is confined to a smaller area adjacent to an ancestral transcrustal structure interpreted to have localized and optimized magmatic and ore forming processes. Although lithospheric-scale evolutionary processes might act as the fundamental control on metal endowment, the new crustal reconstruction explains the clustering of deposits on both belt and district scales. The results highlight a strong magmatic control on metal and in particular Au endowment in VMS systems. Overprinting by clusters of ca. 30 Ma younger orogenic Au deposits suggest the ore systems accessed an upper lithospheric mantle enriched in Au and metals.

Rhyolite Geochemical Signatures and Association with Volcanogenic Massive Sulfide Deposits: Examples from the Abitibi Belt, Canada

The relationship between rhyolite geochemistry and volcanogenic massive sulfide (VMS) mineralization has been proposed as an exploration tool to discriminate prospective felsic volcanic centers. The most widely used classification discriminates between four types of rhyolite: FI, FII, FIIIa, and FIIIb. The FI rhyolites are calc-alkaline, with strongly fractionated REE patterns and strongly negative Ta and Nb anomalies. They are usually considered barren, unless associated with FII or FIII felsic volcanic rocks. The FII rhyolites are calc-alkaline to transitional with moderately fractionated REE patterns and moderate Ta and Nb anomalies. They range from barren to having a high potential to host VMS mineralization. The FIIIa and FIIIb rhyolites are tholeiitic and show weakly fractionated REE patterns and weak to absent Nb and Ta anomalies. They have the highest potential to host VMS mineralization. The FIIIb rhyolites are high-temperature rhyolites with flat REE patterns and no Ta or Nb anomalies. The Abitibi greenstone belt, especially in Quebec, is well known for its abundant and diverse VMS deposits. Representative samples of VMS-associated rhyolites within and outside of mining districts, including the classic Noranda VMS district, were analyzed for major and trace elements to validate their proposed favorability for hosting VMS deposits. Results indicate that all of the rhyolite types are prospective, but mineralization may differ from the classic Noranda-type VMS deposit. The FI-type rhyolites appear to be particularly associated with gold-rich VMS deposits, such as the world-class Laronde deposit, and are more prospective for Cu-Au replacement and vein-type deposits. The FII-type rhyolites account for about 70 percent of rhyolites in the Abitibi belt. Although considered less prospective, some districts dominated by FII rhyolites, such as Val-d’Or and Selbaie, have collectively produced in excess of 100 million metric tons (Mt) of ore. Deposits in these districts mainly consist of sulfide veins and disseminated ore with low Cu and Zn grades and are associated with abundant and highly vesicular volcaniclastic rocks that display a compositional continuum from andesite to rhyolite. Other weakly mineralized FII districts (e.g., Hunter mine, Gemini-Turgeon) are characterized instead by bimodal flow-dome sequences. The FIIIa-type rhyolites occur mainly in the Noranda district and form flow-dome complexes in bimodal sequences with associated Noranda-type VMS mineralization. In small felsic centers (Joutel, Normetal, Chibougamau, Quevillon) that show a volcanic evolution from FIIIa to FII to FI affinities, VMS deposits are directly associated with FIIIa rhyolites, thus demonstrating the usefulness of rhyolite geochemistry for exploration in these areas. The FIIIb rhyolites are rare in the Abitibi belt, with most occurring in the Matagami district where they are associated with Zn-Cu VMS deposits. Based on this analysis, we suggest that a combination of rhyolite geochemistry, volcanic facies, and style of the mineralization may be more meaningfully applied in exploration than rhyolite type alone, particularly in the case of FI and FII rhyolites.

6: Classification of VMS deposits: Lessons from the South Uralides

VMS deposits of the South Urals developed within the evolving Urals palaeo-ocean between Silurian and Late Devonian times. Arc-continent collision between Baltica and the Magnitogorsk Zone (arc) in the south-western Urals effectively terminated submarine volcanism in the Magnitogorsk Zone with which the bulk of the VMS deposits are associated. The majority of the Urals VMS deposits formed within volcanic-dominated sequences in deep seawater settings. Preservation of macro and micro vent fauna in the sulphide bodies is both testament to the seafloor setting for much of the sulphides but also the exceptional degree of preservation and lack of metamorphic overprint of the deposits and host rocks. The deposits in the Urals have previously been classified in terms of tectonic setting, host rock associations and metal ratios in line with recent tectono-stratigraphic classifications. In addition to these broad classes, it is clear that in a number of the Urals settings, an evolution of the host volcanic stratigraphy is accompanied by an associated change in the metal ratios of the VMS deposits, a situation previously discussed, for example, in the Noranda district of Canada. Two key structural settings are implicated in the South Urals. The first is seen in a preserved marginal allochthon west of the Main Urals Fault where early arc tholeiites host Cu–Zn mineralization in deposits including Yaman Kasy, which is host to the oldest macro vent fauna assembly known to science. The second tectonic setting for the South Urals VMS is the Magnitogorsk arc where study has highlighted the presence of a preserved early forearc assemblage, arc tholeiite to calc-alkaline sequences and rifted arc bimodal tholeiite sequences. The boninitc rocks of the forearc host Cu–(Zn) and Cu–Co VMS deposits, the latter hosted in fragments within the Main Urals Fault Zone (MUFZ) which marks the line of arc-continent collision in Late Devonian times. The arc tholeiites host Cu–Zn deposits with an evolution to more calc-alkaline felsic volcanic sequences matched with a change to Zn–Pb–Cu polymetallic deposits, often gold-rich. Large rifts in the arc sequence are filled by thick bimodal tholeiite sequences, themselves often showing an evolution to a more calc-alkaline nature. These thick bimodal sequences are host to the largest of the Cu–Zn VMS deposits. The exceptional degree of preservation in the Urals has permitted the identification of early seafloor clastic and hydrolytic modification (here termed halmyrolysis sensu lato) to the sulphide assemblages prior to diagenesis and this results in large-scale modification to the primary VMS body, resulting in distinctive morphological and mineralogical sub-types of sulphide body superimposed upon the tectonic association classification. It is proposed that a better classification of seafloor VMS systems is thus achievable using a three stage classification based on (a) tectonic (hence bulk volcanic chemistry) association, (b) local volcanic chemical evolution within a single edifice and (c) seafloor reworking and halmyrolysis.

Lithogeochemistry of altered rocks at the New Insco VMS deposit, Noranda, Quebec

The New Insco Cu-bearing massive sulphide deposit is located in volcanic rocks of Archean age of the Abitibi greenstone belt in the Noranda district. The conformable ore lens lies within a sequence of calc-alkaline andesites (Zr/Y = 6.5; La N/Yb N = 5.3). Hydrothermal alteration has strongly affect these rocks on either side of the ore, converting them to assemblages rich in chlorite near the ore and in sericite ± calcite at the margins. Immobile Ti, Zr and Al were used to calculate mass changes owing to alteration. Net changes of +11 wt% consist of large additions of Fe, Mg, K and S, and depletions Na, Ca and Si. The light- and middle-REE were also mobile in a very regular fashion. Normative alteration minerals, cation bulk compositions, and mobile element mass changes illustrate geochemical and mineralogical trends in the alteration zone. Rocks at the margin of the zone were sericitized, carbonitized and locally silicified. Closer to the ore, Si, Na and Ca were leached, Fe and Mg added, and the rocks were converted to quartz-chlorite and chloritite. These geochemical trends can be used to distinguish hydrothermally altered rocks from other greenstones, and in evaluating the potential for mineralization.

Regional evolution of hydrothermal fluids in the Noranda District, Quebec; evidence from delta 18 O values from volcanogenic massive sulfide deposits

The results of oxygen isotope analysis of hydrothermally altered volcanic rocks are used to define the thermal and isotopic characteristics of the ore-forming fluids which produced several volcanogenic massive sulfide deposits in the Noranda district. These deposits (Corbet, Ansil, Norbec, Horne, and Mobrun) formed during temporally distinct events that span much of the volcanic stratigraphy in the district.The data show that the delta 18 O values of the altered rocks and the size (economic tonnage) of each deposit increase upward through the volcanic stratigraphy from low delta 18 O values at the Corbet (-2.2 to 4.8ppm) and Ansil (-0.8 to 5.0ppm) deposits, to intermediate delta 18 O values at the Amulet (3.6-6.7ppm) and Norbec (3.6-10.5ppm) deposits, to high delta 18 O values at the Horne (4.2-11.6ppm) and Mobrun (6.0-13.8ppm) deposits. A corresponding increase is indicated for the delta 18 O values of the discharging hydrothermal fluids, from -2 + or - 2 per mil at the Corbet deposit to +3.0 + or - 1.5 per mil at the Horne deposit. Alteration in most of the deposits is shown to have resulted from the flow of fluids having temperatures of 300 degrees + or - 50 degrees C, with the exception of the pyrite-rich Mobrun deposit where slightly lower temperatures (200 degrees + or - 50 degrees C) are indicated.These observations are interpreted to indicate that the duration of hydrothermal discharge (as indicated by deposit size) at a given site increased upward through the volcanic stratigraphy. Since the fluid, and thus the altered rock, delta 18 O values normally evolve from low to high with time in a volcanogenic massive sulfide-forming system (e.g., Cathles, 1983), the duration of fluid discharge is a primary factor in controlling the isotopic signature retained at a given deposit. In the Noranda district, the duration of fluid discharge depended strongly on the rate of volcanic accumulation, with short-lived systems (e.g., Corbet, Ansil) forming during periods of rapid accumulation of extrusive volcanic rocks, and longer-lived systems forming during more quiescent periods (e.g., Norbec), or at sites where the accumulation of extrusive material was limited by topography (e.g., Horne).

Oxygen isotope alteration in the Noranda mining district, Abitibi greenstone belt, Quebec

A regional whole-rock oxygen isotope survey carried out in the Noranda district shows how oxygen isotope alteration can be used to guide exploration for massive sulfide deposits. The survey is based on 588 outcrop whole-rock oxygen isotope determinations from 478 sites in a approximately 35- by 50-km area between the Destor-Porcupine and Cadillac-Larder Lake breaks from east of the Horne mine to west of the Magusi River mine. The mean isotopic composition of all samples is 7.4 + or - 2 per mil (SMOW). Nested analysis of variance of 478 isotopic determinations from 371 of these sites shows that 74 percent of the total variance in whole-rock delta 18 O r (r = whole rock) is associated with outcrop location, 21 percent with outcrop sampling, and 5 percent with sample analysis. An analysis of 259 determinations indicates that rock type is statistically no more important than outcrop sampling. The statistical analysis indicates that isotopic variations greater than + or -1 per mil can be used to map the hydrothermal circulation responsible for massive sulfide mineralization. Specific traverses visually confirm the lack of 18 O dependence on rock type and the utility of a 2 per mil interpretive band. The large-scale pattern of isotopic alteration in the Noranda district is dominated by a nearly continuous 2- to 3-km-wide annulus at the margins of the Flavrian pluton of delta 18 O r 18 O r 18 O r depletion is surrounded up section by a approximately 10-km-wide band of patchy but intense 18 O enrichment (8.5-14ppm). The combined pattern of concentric light and heavy alteration has a diameter of approximately 30 km. The Clericy intrusion shows a similar pattern of isotope alteration on a smaller (10 km diam) scale. The Lake Dufault pluton seems to have been isotopically altered by continuing convection driven by the Flavrian hydrothermal system. Modeling suggests that the 18 O depletion is produced by water circulating toward and along the edges of an intrusion, and that the surrounding heavy alteration is produced either by seawater recharge or limited hydrothermal discharge. The large scale and coherency of the alteration pattern centered on the Flavrian pluton is remarkable and indicates that this pluton drove a major, long-lived ( approximately 10 m.y.) hydrothermal system in which convection penetrated to an >8-kin depth. The intensity and coherency of the 18 O depletion correlates with the tonnage of associated massive sulfide mineralization. Oxygen isotope depletion may in general mark the margins of plutons that have driven sufficient hydrothermal circulation to produce economic massive sulfide deposits up section. It may be possible to screen plutons for up-section massive sulfide exploration potential by collecting and isotopically analyzing approximately 50 samples from five traverses across a pluton9s margins. Additional samples may provide details (light fingers) that guide exploration to particular deposits.

Plutonism in the southern Abitibi Subprovince; a tectonic and petrogenetic framework

Studies of plutonic rocks in Archean terranes such as the southern Abitibi subprovince have been an important component in the development of an accretionary tectonic model for the Superior province. In the southern Abitibi subprovince, pretectonic ( approximately 2.75-2.70 Ga), low K, tholeiitic calc-alkaline, dioritic to tonalitic plutons (e.g., Bourlamaque batholith; Flavrian pluton) are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc-alkaline basalt-andesite. Pretectonic, synvolcanic tholeiitic intrusions (e.g., Dundonald sill, Kamiskotia gabbroic complex: 2.72-2.70 Ga) show in situ fractionation trends, and locally, have mixed magma textures with contemporaneous calc-alkaline granitoids. They are believed to be derived from a midocean ridge basalt (MORB)-like depleted mantle, with little evidence for contamination by an older crustal component. Pre- to syntectonic ( approximately 2.72-2.69 Ga) porphyry and tonalite plutons (e.g., Timmins porphyries, Round Lake batholith) have geochemical characteristics typical of Archean tonalite-trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades. Late to post-tectonic (<2.69 Ga) diorite-monzonite and syenite plutons (e.g., Watabeag batholith, Lamaque stocks, Otto stock) have large ion lithophile element-enriched calc-alkaline to sho-shonitic compositions. These intrusions and associated Timiskaming-type volcanic rocks are attributed to melting of large ion lithophile element-enriched mantle in the late stages of subduction. Late to post-tectonic biotite-muscovite granite intrusions contain a significant contribution of evolved crust (e.g., S-type characteristics of the Preissac-Lacorne batholith).In the southern Abitibi subprovince the tectonic setting of mantle- and crust-derived intrusions is broadly comparable to magmatic processes in Phanerozoic arcs and accreted terranes. The plutonic rocks record a transition from pre- to synaccretionary subduction-related plutonic suites to late to postaccretionary suites derived from enriched mantle and crustal sources. This framework provides a basis for understanding mineral deposits associated with intrusive rocks of the southern Abitibi subprovince which range from Ni-Cu sulfides in layered synvolcanic tholeiitic intrusions (e.g., Kamiskotia gabbroic complex, Ontario) to volcanogenic sulfides in early tholeiitic and calc-alkaline sequences (e.g., Flavrian pluton, Noranda district, Quebec) to the spatial association of gold with late large ion lithophile element-enriched intrusions (e.g., Lamaque plugs, Val d'Or district, Quebec).

Oxygen isotope zonation and alteration in the northern Noranda District, Quebec; evidence for hydrothermal fluid flow

Volcanic rocks of the New Vauze-Norbec area, central Noranda volcanic complex, consist of andesites and rhyolites that have been chemically and isotopically altered by synvolcanic semiconformable alteration processes including spilitization, chloritization, silicification, and epidote-quartz alteration. Andesites have whole-rock delta 18 O values of 1.9 to 9.7 per mil; those with megascopic mineralogical evidence of hydrothermal alteration have ones of 5.1 to 9.7 per mil. Rhyolites also show a broad range in delta 18 O values: 4.4 to 13.7 per mil. All of the volcanic rocks are either enriched or depleted in 18 O with respect to fresh rhyolite (8ppm) or fresh andesite (6ppm). A marked upward stratigraphic increase in delta 18 O values and the strata-bound occurrence and regional zoning of alteration minerals provide evidence of district-scale hydrothermal alteration. These alteration patterns formed under the conditions of an increasing thermal gradient and a downward decrease in water/rock ratio controlled by the cooling of the Flavrian Lake synvolcanic intrusion.

Hydrothermal alteration and oxygen and hydrogen isotope geochemistry of the D-68 zone Cu-Zn Massive Sulfide Deposit, Noranda District, Quebec, Canada

Pervasive hydrothermal alteration zones in quartz-feldspar porphyry domes underly all massive sulfide lenses in the D-68 Zone Cu-Zn deposit, Noranda. Alteration pipes are mineralogically zoned and contain chloritic cores consisting of stringer sulfides, enveloped by sericitic haloes. Silicified rocks are found locally. Alteration took place at nearly constant volume. Na depletion, and K enrichment relative to the least altered rocks, are found in all alteration zones. Fe and Mg have been added to the chloritic zone and subtracted in the sericitic and silicic zones. Ca and Si are enriched mainly in the silicic zone. Al, Ti and Zr were the least mobile of the elements studied. Whole-rock δ18O values vary from +5.6 to +6.2 per mil in chloritized rocks, +5.8 to + 7.3 per mil in sericitized rocks and + 7.2 to + 8.3 per mil in silicified rocks. δD values for two chloritized samples are − 63 and − 70 per mil whereas in two sericitized samples they are close to −62 per mil. Quartz from the chlorite alteration zone is isotopically heavier (δ18O = 8.6 per mil) than that from the sericite alteration zone (δ18O = 6.4 per mil), suggesting equilibration with different hydrothermal fluid or different temperature of alteration. Assuming an alteration temperature of 300° + 50°C the fluid in equilibrium with quartz and chlorite had δ18O and δD values of about 1.5 ± 2.0 per mil and −23 ± 5 per mil, respectively. The fluid in equilibrium with quartz and sericite had δ18O and δD values of about −0.5 ± 2 per mil and −30 ± 5 per mil, respectively. On the basis of isotopic data, seawater was probably the major constituent of the hydrothermal fluids.

Distribution of gold, arsenic, antimony and tungsten around the Dest-Or Orebody, Noranda district, Abitibi, Quebec

The Dest-Or epigenetic Au deposit occurs in a breccia zone within gabbro, basalt and andesite of the Archean Upper Deguisier Formation. It is located approximately 30 km NE of Noranda. Quebec and 2.5 km N of the Porcupine-Destor Fault, an important vertical shear zone that extends east-west for more than 100 km. The known orebody contains 2.44 Mt of ore at 4.29 g/t Au. Host rocks of the Upper Deguisier Formation typically contain 3.6 ppb Au, 0.8 ppm Sb and 4.5 ppm As. The Au values are comparable to those of tholeiitic mafic rocks elsewhere in the world. but Sb and As values are a little higher. Gold values on approximately 30% of the area of the Dest-Or and Bassignac properties define a log-normal distribution with a median at 9.4 ppb Au (P 16 at 3.1 and P 84 at 27). These are referred to as ore zone halos: they envelop orebody halos which in turn envelop orebodies. An orebody halo can best be defined by close sampling in the immediate vicinity of a known orebody. Around the Dest-Or orebody, this halo is approximately 100 m wide (60 m on the hanging wall and 40 m on the footwall), and it has a median value at 37 ppb Au (P 16 at 17 and P 84 at 74). Gold enrichment in the orebody is 1900 times background value. There are also lesser but significant Sb and As enrichments (20 /sX each). High W values occur in the ore ( > 30 ppm W), but background values were too low ( <5 ppm) to be established with confidence. Gold analyses in the 0.2–100 ppb range can be gainfully used in the search for blind gold ore deposits: As, Sb and W can also be used, but anomalies are less extensive and enrichment is also less pronounced.

Composition-volume changes during hydrothermal alteration of andesite at Buttercup Hill, Noranda District, Quebec

Abstract Hydrothermally altered andesites in the upper member of the Amulet formation at Buttercup Hill, Noranda, Quebec represent part of the aquifer and cap of a self-sealing geothermal system that focussed the discharge of hydrothermal fluids during the formation of massive Cu-Zn sulfide deposits. Five alteration facies are recognized 1) pervasive greenschist facies regional metamorphism (least-altered andesite) 2) epidotization-silicification 3) albitization-silicification 4) chloritization 5) sericitization-silicification. Alteration is localized on permeable zones such as amygdules, fractures, flow tops, discordant breccia dikes, and conformable breccia horizons. Epidotized-silicified andesite is enriched in Ca-Sr-Eu and depleted in Mg and first transition series metals (FTSM) relative to least-altered andesite. Albitized-silicified andesite is significantly enriched in Na and depleted in most FTSM relative to least-altered andesite. The abundances and inter-element ratios of the rare-earth elements (REE) and most high field-strength elements (HFS: Y, Zr, Th, U, Hf, Ta) are similar in least-altered, epidotized-silicified and albitized-silicified andesites. The most silicified andesites are strongly enriched in Na-Si, strongly depleted in Mg and divalent FTSM and slightly but systematically depleted in REE and most HFS elements. Serialized andesites were previously silicified; they are very strongly enriched in K-Rb-Cs-Ba, very strongly depleted in Na-Ca-Sr-Eu and slightly depleted in light REE relative to silicified andesite. Chloritized andesitic rocks exhibit heavy REE and HFS element ratios similar to those of leastaltered andesite, but are relatively strongly enriched in Mg and divalent FTSM, strongly depleted in Si and large ion lithophile (LIL) elements and slightly depleted in light REE. The coupled behavior of the heavy REE and most HFS elements during epidotization, albitization, silicification, chloritization and serialization suggests that they were inert during hydrothermal modification of the andesite. Mass balance calculations suggest that volume was conserved during epidotization-silicification and albitization-silicification, but that intense silicification was accompanied by volume increases up to 30 percent.

Archean mafic metavolcanics from the Rouyn–Noranda district, Abitibi Greenstone Belt, Quebec. 1. Mobility of the major elements: Discussion

Archean mafic metavolcanics from the Rouyn–Noranda district, Abitibi Greenstone Belt, Quebec. 1. Mobility of the major elements: Discussion

Archean mafic metavolcanics from the Rouyn–Noranda district, Abitibi Greenstone Belt, Quebec. 1. Mobility of the major elements: Reply

In a suite of Archean mafic pillows from the Rouyn–Noranda region of Quebec's Abitibi Greenstone Belt, including both tholeiitic and calc-alkaline varieties spanning the prehnite–pumpellyite to upper greenschist metamorphic facies, three types of alteration can be defined: (I) chlorite–epidote–actinolite; (II) chlorite–epidote; and (III) chlorite ± sericite; the number of mineral phases decreases as a result of progressive hydration from type I to type III alteration. Albitization, resulting from substitution of , in calcic plagioclase, is highly variable in type I alteration, but in types II and III the plagioclase is totally albitized and in some cases silicified. Chloritization is closely linked to increasing hydration and Ca leaching with MgO and FeO substituting for CaO in ferromagnesian minerals.Calcium was mobilized and carried by solutions, as evidenced by the variable concentration of epidote at the margins of pillows. This calcium leaching generated an excess of Al2O3 with respect to the combine...

The geology of the New Insco copper deposit, Noranda district, Quebec

The New Insco copper deposit is an Archean volcanogenic massive sulfide orebody situated on the northwestern edge of the Noranda mining district, in the Superior Province of the Canadian Shield. The ore occurs near the base of a thin unit of pillowed and brecciated basalt that overlies rhyolitic volcaniclastic rocks. Both the basalt and rhyolite are heavily chloritized and sericitized and form a zone of alteration that is depleted in SiO2, Na2O, and CaO and enriched in FeOT, MgO, K2O, Cu, Zn, and S. The units are separated from an underlying massive rhyolite by a diorite intrusion and from an overlying series of pillowed basalts by a fault.The orebody is a pyrrhotite-rich lens of massive sulfide, containing conformable bands of disseminated chalcopyrite and pyrite, that is interlayered with lenses of pyrite–chalcopyrite-bearing siliceous ore. The massive ore is interpreted to have been deposited in a distal environment with pyrrhotite as a primary mineral. It was recrystallized during regional metamorphism, which imparted a polygonal–granular texture to the pyrrhotite and formed overgrowths on pyrite crystals. Following Pleistocene glaciation, pyrrhotite was extensively replaced by pyrite, marcasite, iron oxides, carbonates, and hydroxides.

Archean metavolcanics from the Rouyn–Noranda district, Abitibi Greenstone Belt, Quebec. 2. Mobility of trace elements and petrogenetic constraints

Qualitative evaluation of altered mafic pillows from the Rouyn–Noranda district of the Abitibi metavolcanic belt reveals that, for low-grade metamorphism, the high field strength elements (HFS: Zr, Y, Ti) and the rare earth elements (REE) are immobile. However, where alteration is extreme, the REE elements are leached and redeposited by carbonate-rich and K-rich metasomatic fluids whilst Zr/Y and Zr/Ti remain unchanged. Rubidium, Ba, and, to a lesser extent, Sr are mobile at all levels of alteration. However, provided certain filters are applied, the original geochemical values of the lavas for the alkaline elements, the HFS elements, and the REE can be defined.The geochemical variations thus identified for the volcanic series of the Rouyn–Noranda district are ascribed to a combination of contamination and fractional crystallization of a primitive tholeiitic magma.

Archean mafic metavolcanics from the Rouyn–Noranda district, Abitibi Greenstone Belt, Quebec. 1. Mobility of the major elements

In a suite of Archean mafic pillows from the Rouyn–Noranda region of Quebec's Abitibi Greenstoné Belt, including both tholeiitic and calc-alkaline varieties spanning the prehnite–pumpellyite to upper greenschist metamorphic facies, three types of alteration can be defined: (I) chlorite–epidote–actinolite; (II) chlorite–epidote; and (III) chlorite ± sericite; the number of mineral phases decreases as a result of progressive hydration from type I to type III alteration. Albitization, resulting from substitution of [Formula: see text], in calcic plagioclase, is highly variable in type I alteration, but in types II and III the plagioclase is totally albitized and in some cases silicified. Chloritization is closely linked to increasing hydration and Ca leaching with MgO and FeO substituting for CaO in ferromagnesian minerals.Calcium was mobilized and carried by solutions, as evidenced by the variable concentration of epidote at the margins of pillows. This calcium leaching generated an excess of Al2O3 with respect to the combined molecular proportions of Na2O, K2O, and CaO, and is shown by the presence of corundum in CIPW norm calculations. In some pillows showing substitution of [Formula: see text], the fo2 of the invading fluid appears to have remained constant, being buffered by the pillow composition; this would be favored by a low water/rock mass ratio. As a result, the initial pillow Fe2O3/FeO ratio remained constant. In other pillows, the fo2 appears to have been imposed by the invading fluid rather than by the mineral assemblage: the FeO/MgO ratios are thus no longer representative of the magmatic composition whereas the ΣFeO/MgO is still representative of the pristine magmatic value.Two types of substitution of CaO by FeO and (or) MgO have been observed: (1) preferential substitution restricted to type I alteration, of FeO over MgO, similar to low-temperature substitution in modern-day sea-floor alteration; and (2) the more common substitution in type II and III alterations in which MgO predominates over FeO, similar to the high-temperature substitution taking place at great depth on the ocean floor.Although the samples were collected to test mineral heterogeneities caused by chemical degradation, more than 40% of the pillows sampled retained their pristine ΣFeO/MgO ratios. The various alteration patterns are independent of the initial tholeiitic or calc-alkaline lineage; this was confirmed using rare earth elements (REE) and inert trace elements such as Zr, Y, and Ti. The chemical changes in the mafic metavolcanic rocks do not obliterate their tholeiitic or calc-alkaline chemical affinities.

A Volcanic Origin for the No. 5 Zone of the Horne Mine, Noranda, Quebec

The No. 5 Zone of the Horne Mine in the Noranda District consists of massive to disseminated sulfides, mainly pyrite, enclosed conformably within steeply dipping, interbedded tuffs, rhyolite tuff breccias, and rhyolite breccias of Archean age. Large sulfide masses are elongate down dip and have oval to roughly tabular cross sections. They are closely spaced and overlapping, due apparently to successive submarine slumping of soft, unconsolidated sulfide masses and rhyolitic pyroclastics on the flanks of a volcano. Textures of the sulfides suggest that they underwent subsequent dynamic and regional metamorphism probably during the Kenoran orogeny (2.5 billion years).Pyrite in the No. 5 Zone contains 12-60 ppm cobalt and 33-88 ppm selenium, whereas that in the adjacent economic H ore bodies contains about 1,000 ppm cobalt and 390-1,000 ppm selenium. These differences suggest an independent formation of the No. 5 Zone.Large fragments of sulfides similar in composition and texture to the sulfides in the massive bodies of the No. 5 Zone occur in the overlying breccias and were apparently derived from the No. 5 Zone during periods of explosive volcanism. The massive sulfides therefore must have formed during the same period of volcanism.

Tectonic Transport of Massive Sulfide Deposits in Submarine Volcanic and Sedimentary Host Rocks

Massive Cu-Zn-Fe sulfide deposits appear to be replacements or syngenetic sediments related to submarine volcanism. Undisturbed kuroko deposits or moderately deformed and metamorphosed orebodies like those of the Noranda district may be considered as models.Massive sulfide deposits in high grade metavolcanic and metasedimentary rocks are postulated to have originated largely in a submarine volcanic environment near the edges of technically active troughs sinking and receiving clastic sediments. Adjacent rising segments eventually expose deep structures almost devoid of base metals. Massive sulfides and volcanics, together with enclosing sediments, are susceptible to transportation by slides and nappes. Thus ores and enclosing rocks may be moved several kilometers and effectively separated from former root structures such as alteration zones and hypabyssal intrusives. The Elizabeth massive sulfide deposits, Vermont thus appear to have moved westward from root zones near the Connecticut River prior to their metamorphism. Possibly some of the metamorphosed massive sulfide deposits of the Sanbagawa terrane, Shikoku, Japan have been transported southward from root zones near the Median Tectonic Line. After initial transposition by sliding, ores and host rocks may be subjected to deep burial, strong metamorphism and one or more episodes of folding.

The discovery of a copper-zinc deposit at Garon Lake, Quebec

An exploration venture that led to the discovery of a small ore-grade Cu deposit in the very active Mattagami Lake exploration area of northwestern Quebec is described. The Garon Lake property was staked to cover a small, isolated E.M. anomaly that was recorded during an airborne survey of a folded post-Keewatin basic intrusion. A ground geophysical survey confirmed the presence of a narrow but good conductor and of a coincident, positive magnetic anomaly. Diamond drilling has indicated the presence of a body of massive sulfides containing a little over 2% Cu. The sulfide deposit is in a group of metamorphosed Keewatin lavas, tuffs, and sediments intruded by older gabbro and by systems of late unmetamorphosed dikes, of which lamprophyre, olivine diabase, and albite granite, are examples. The strike and dip of the mineralization are essentially parallel to the strike and dip of the country rock. The shoot of massive sulfides plunges at about 60 degrees . Pyrrhotite, pyrite, chalcopyrite, and sphalerite occur in varying proportions and show textural evidence of having replaced the pre-existing rock. The mineralization is near the contact between amphibolite (derived from lavas) and quartz-rich schist (derived from sediments). More particularly, the sulfide body is closely associated with an alteration aureole of cordierite-anthophyllite hornfels. Examples of base metal sulfide deposits that are structurally and genetically related to the formation of Mg-rich cordierite-bearing metasomatic zones have been described from other parts of the world. Geographically the closest described example is the Amulet ore body in the Noranda district. The discovery of this type of deposit on the Garon Lake property sheds new light on exploration in the Mattagami Lake area.