Abstract

Auriferous sedimentary rocks at the Dickenson Mine, Red Lake, are characterized by large enrichments of rare elements (Au, Ag, Pd, As, Sb, B) and metals conventionally considered to be relatively immobile (Ni, W), with negligible concentrations of the abundant and mobile base metals. Based on a reconnaissance survey involving 12 rocks analysed for 42 elements, these sediments can be represented in terms of a mixture of two components―mafic volcaniclastic material and hydrothermal precipitates. The volcaniclastic component has a composition closely comparable to mafic volcanic rocks that conformably envelope the sediments, and contributes all of the Al, Ti, V, Sc, Zr, and F. The hydrothermal component donates Si, Fe, Mn, Mg, Ca, K, Si, and C, together with Au, Ag, Pd, As, Sb, B, W, and Ni. Chromium and nickel are contributed from both sources. Alteration of the mafic volcanic rocks that envelope the auriferous sediments involved fixation of Si, K, and CO2, in contrast to the typical patterns of metasomatism in footwall rocks to base metal massive sulphide deposits, which are characterized by Mg, Fe (Si, S) addition.Quartz isolated from chert within the auriferous sediments and volcanic rocks has a maximum δ18O of 19‰, which is within the range of δ values for marine cherts of ~2.8 Ga, and signifies precipitation in equilibrium with ambient marine water of ~−1‰ at ~70–90 °C. Mafic volcanic wall rocks to the sediments have whole-rock δ18O values of 16–17‰ and Δquartz–chlorite ≤ 2‰. The anomalously heavy whole-rock isotopic composition and small quartz–chlorite fractionation may result from the growth of chlorite from precursors such as zeolites and smectite. Alternatively, chlorite or its precursors may have become enriched in 18O by isotope exchange with metamorphic hydrothermal fluids that become ponded when discharge to the hydrosphere was capped by overlying basalts.Rare earth element (REE) distributions in sediments and mafic volcanic wall rocks are characterized by relatively flat normalized patterns up to Sm–Gd, followed by an abrupt continuous decline in abundance. This implies modification of the primary tholeiitic abundances by hydrothermal solutions capable of mobilizing heavy REE's relative to light REE's. The environment that would satisfy the geological observations and chemical data is one of metamorphic hydrothermal fluids emmanating onto the sea floor during a period of relatively quiescent mafic submarine volcanism, with subsequent capping of the discharge during emplacement of the overlying pillow basalts.

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