Hydrothermal alteration zoning and gold concentration at the Kerr-Addison Archean lode gold deposit, Kirkland Lake, Ontario

Abstract

Kerr-Addison, a major Archcan lode gold deposit of the Abitibi belt, with a production of approximately 10 X 10 6 oz Au ( approximately 320 metric tons up to 1983), is localized in a zone of talc-carbonate and chlorite-carbonate rocks, immediately south of a regional structural break--the Kirkland Lake-Larder Lake fault zone. Relict volcanic textures taken in conjunction with the ratios of diag-nostic, relatively less mobile elements collectively indicate that most of the wall rocks of the mine were originally mafic and ultramafic rocks along with their clastic derivatives. Regional greenschist facies metamorphism is locally superposed by a distinct zonation of gold-associated hydrothermal alteration characterized by massive introduction of CO 2 coupled with variable enrichments of alkali metals and calcium, which produced systematic trends of alteration mineral assemblages. Alteration types of the initial stage contain chlorite which is replaced by muscovite or albite in the advanced stage of alteration, with a concomitant increase in the proportion of carbonate minerals. Numerous generations of crosscutting veins are enveloped by alteration borders in which the sequence of the Al-bearing minerals, namely albite, mus-covite, and chlorite, develop from the vein to the wall rocks, always in a systematic sequence. The same sequence is observed at a larger scale from the ore zone to the periphery of the mine area.The intensity of alteration can be monitored by saturation indices: CO 2 /(Ca + Fe + Mg) for carbonates; and (3K + Na)/Al, 3K/Al, and Na/Al for muscovite + albite, muscovite, and albite, respectively (molar ratios). Plots of the moving averages of these indices across the 3,850-ft level of the mine reveal a remarkable correspondence to variations of gold abundance, suggesting a genetic relationship of gold enrichment to hydrothermal alteration. On a broad scale, gold is concentrated to the south of the Kirkland Lake-Larder Lake fault zone and at a more detailed scale in the transition zone between carbonate-muscovite and carbonate-albite alteration types. During advanced alteration, fluid X (sub CO 2 ) was approximately 0.1 and Na/K [asymp], 10.Volcanics of the Kinojevis and Larder Lake groups are 18 O enriched relative to pristine tholeiites and possess distinct mineral delta 18 O values, probably due to different conditions of marine-water alteration during or shortly after extrusion. In domains of advanced alteration these are all shifted down to a common narrow bracket: here 18 O/ 16 O and D/H data indicate fairly constant temperatures (270 degrees -300 degrees C) and a common isotopic composition of hydro-thermal fluids (delta 18 O -- +5ppm; delta D = -30 to -50ppm) across the alteration zoning; these results are in common with many other Archcan lode gold deposits. Fe dolomite has a uniform delta 13 C value of -2.8 + or - 0.42 per mil (1Sigma ) throughout the footwall and ore zones, but it is enriched by approximately 1 per mil in the graphite-free hanging-wall rocks. Fluid delta 13 C value was -4 + or - 1 per mil, reflecting either a magmatic carbon reservoir or mixtures of marine carbonate, carbonaceous, and magmatic sources.The enrichment of gold at one side of the Kirkland Lake fault zone is interpreted to be the result of development of a metasomatic zoning induced by gradients in oxidation state, the precious metal being concentrated in the reducing side. Zonation of alteration types south of the Kirkland Lake-Larder Lake fault zone represents various steps of a continuous process of incremental changes of the rocks to adapt to the composition of an externally generated fluid. Successive generations of fracture arrays and veins have the same alteration patterns and thus the composition of the incoming hydrothermal fluid seems to have remained essentially unchanged through time.Gold mineralization at the Kerr-Addison mine and its contiguous alteration halo are interpreted to be the result of repeated cycles of hydraulic fracturing, fluid penetration, and local reaction with rocks. Fluid-rock interaction produced lateral variations of fluid composition which led to gold concentration, probably due to gradients in pH, preferentially at the interface between the carbonate-muscovite and carbonate-albite alteration types, which is also the site of pyrite concentration. The alteration assemblages and specific site of gold-pyrite concentration at Kerr-Addison are common to several other described Archcan lode gold deposits.