Derivation of base-line geochemistry, petrography, and isotopic data for the host rocks to the Lucky Strike deposit and comparison with data from other alteration zones, Buchans Mining Camp, Newfoundland

摘要

The Buchans ore bodies of central Newfoundland represent some of the highestudgrade VMS deposits ever mined. These Kuroko-type deposits are also known for the welluddeveloped and preserved nature of the mechanically transported deposits. The depositsudare hosted in Cambro-Ordovician, dominantly calc-alkaline, bimodal volcanic andudepiclastic sequences of the Notre Dame Subzone, Newfoundland Appalachians.udStratigraphic relationships in this zone are complicated by extensively developed, brittledominatedudSilurian thrust faulting.udHydrothermal alteration of host rocks is a common feature of nearly all VMSuddeposits, and the recognition of these zones has been a key exploration tool. Alteration ofudhost rocks has long been described to be spatially associated with the Buchans ore bodies,udmost notably with the larger in-situ deposits. This report represents a base-line study inudwhich a complete documentation of the geochemical variance, in terms of both primaryud(igneous) and alteration effects, is presented from altered volcanic rocks in the vicinity ofudthe Lucky Strike deposit (LSZ), the largest in-situ deposit in the Buchans camp.udPackages of altered rocks also occur away from the immediate mining areas andudconstitute new targets for exploration. These zones, identified mostly by recent andudprevious drilling, represent untested targets and include the Powerhouse (PHZ),udWoodmans Brook (WBZ) and Airport (APZ) alteration zones, as well as the MiddleudBranch alteration zone (MBZ), which represents a more distal alteration facies related toudBuchans ore-formation. Data from each of these zones were compared to those from theudLSZ in order to evaluate their relative propectivity.udDerived litho geochemical data served two functions: (i) to define primaryud(igneous) trends and (ii) secondary alteration trends. Primary trends were establishedudusing immobile, or conservative, elements (i. e., HFSE, REE, Th, Ti0₂, Al₂0₃, P₂0₅).udFrom these, altered volcanic rocks were interpreted in terms of composition (e.g., basalt -udrhyodacite) and magmatic affinity (e.g., calc-alkaline vs. tholeiitic). The informationudsuggests that bimodality is a common feature of all zones, with most rocks plotting asudeither basalt/andesite or dacite (or rhyodacite); andesitic senso stricto compositions areudrare. Magmatic affinities are more varied and complex, but indicate that all units are arcudvolcanic sequences. Rocks from the LSZ/MBZ represent a transitional to calc-alkalicudsequence, however, a slight shift in key geochemical discriminants occurs between theudfoot-wall to the hanging-wall. Specifically, mafic and felsic lavas of the foot-wall are ofudtransitional (or mildly calc-alkaline) affinity whereas the hanging-wall rocks are relativelyudmore strongly calc-alkaline as indicated by enriched LREE/HREE and higher ZrN, NbNudand other ratios in the latter. The geochemical variations also serve as a means to separateudthe units (at least the felsic rocks) into hanging-wall and foot-wall sequences, thereforeudproviding a valuable exploration tool. Volcanic rocks from the WBZ/PHZ (and probablyudthe APZ) are more typical of tholeiitic to transitional suites, yielding flatter mantlenormalizedudREE patterns and lower ZrN ratios. Thus, the relationships between the immediate mining area (represented by LSZ/MBZ) and the Buchans East (PHZ/WBZ)udand the APZ are uncertain.udHost rocks for all zones consist of mafic to felsic volcanic rocks, though theudproportion of pyroclastic and epiclastic rocks, is greatest at the LSZ. Phenocrystudassemblages and textures are common in all zones, with minor exceptions, and are notuduseful for discrimination purposes. Felsic rocks from all zones are dominated by sericiteclay+/-udsilica alteration, whereas mafic rocks are dominated by chlorite- quartz- sericiteudalteration. Pyrite is ubiquitous in all moderately altered rocks and minor associated baseudmetal sulphides occur locally. The exception is at Lucky Strike, where stockwork quartzveiningudcontains abundant base-metal mineralization and barite. Rocks completelyudcomprised of chlorite (chloritite) also occur in the LSZ foot-wall. In addition, K-feldsparudalteration occurs in felsic volcanic rocks at the MBZ associated with Zn-Pb-Ba and,udnotably, without chlorite. This zone represents a peripheral, but proximal, zone ofudalteration induced by lower temperature hydrothermal fluids, presumably with littleudinfluence from seawater.udAlteration geochemistry was interpreted from raw data as well as from massudbalanced (recalculated) data derived from immobile element pairs. The data from theudLSZ/MBZ indicate a range in the degree of alteration from only minor to severeudmodification of precursor compositions. Ba tends to show a strong positive correlationudwith K₂0, although most Ba occurs as barite. With respect to mass changes, Al₂0₃, Ti0₂udand P₂0₅ were shown to be immobile. Nearly all rocks display mass loss of Na₂O, CaO,udand Sr reflecting feldspar destruction. These trends are usually mirrored by K₂0-Rb andudMgO addition, indicating sericitic and chloritic alteration, respectively. More substantialudgains ofK₂0 often occur in rocks with K-feldspar alteration, whereas a few samples alsouddisplayed excessive MgO enrichment and represent chloritites. Fe₂0₃ indicates bothudchlorite and sulphide formation. Si0₂ addition is almost always the case for the alteredudmafic rocks as silica often infills amygdules and replaces the finer tuffaceous material.udThe felsic rocks display more variability in Si0₂. Silicic, sericitic and chloritic alterationudtrends were observed from the other zones, but not K-feldspar, chloritite, or barite.udMicroprobe analysis of chlorites, sericites and carbonates indicate: (i) sericitesudfrom all zones are defined as muscovite and are not phengitic; (ii) at the LSZ, chloritesudranged from Fe-Mg chlorites (pycnochlorite) to Mg-rich chlorite (penninite), with theudlatter occurring in the stockwork zone and more proximal alteration facies; (iii) chloritesudfrom the WBZ were typical of those from the more distal alteration facies of the LSZ,udplotting as ripidolite to pycnochlorite; (iv) conversely, chlorite from the PHZ plot withudMg-Al-rich compositions (chlinochlore to penninite); and (v) carbonate species from eachudzone are also varied, with calcite occurring in each zone, in addition to dolomite andudankerite in the PHZ and WBZ, respectively.udLead isotope ratios for galena separates from the different various zones, whenudcombined with data from older studies, tend to cluster into four distinctive fields.udOverall, the data plot on a broad mixing line and indicate evolution in a relatively low-μ environment. Data from sulphide stringers in altered MBZ rocks, as well as from clasticudsulphides (Sandfill prospect), plot in the Buchans ore field, as do the data for galena fromudaltered rocks in the APZ. Samples from the Buchans East area are even more primitiveudthan the Buchans ores, with lead from the PHZ plotting with the Connel Option prospectudand data from the WBZ matching that of the Skidder prospect. A sample from a newlyuddiscovered debris flow-type sulphide occurrence (Middle Branch East) yields lead isotopeudratios that are slightly more radiogenic than Buchans and plot with the Mary Marchudalteration zone. Data within each cluster are interpreted to represent derivation fromudindividual hydrothermal systems in which metals were derived from a common source.