Silicate facies iron-formation and strata-bound alteration; tuffaceous exhalites derived by mixing; evidence from Mn garnet-stilpnomelane rocks at Redstone, Timmins, Ontario

Abstract

Silicate facies iron-formation composed of Mn garnet, stilpnomelane, and ferrotschermakitic hornblende mark an exhalative sedimentary stratigraphic horizon located regionally at the break between two volcanic supercycles consisting of lowermost calc-alkaline felsic pyroclastic rocks overlain by komatiitic ultramafic flows. Locally Mn garnet-stilpnomelane rock flanks and in places is footwall to, a strata-bound, volcanic-hosted nickel sulfide deposit at Redstone, Timmins. Mn garnets have normal, reverse, and oscillatory zoning patterns with a coupled substitution of Mn + Ca for Fe + Mg. Stilpnomelane, amphibole, chlorite, and biotite each have elevated MnO contents but do not display any concentric zoning patterns. The Al/Ti and Ti/Zr ratios of Mn garnet-stilpnomelane rock are similar to those of dacitic tuff at Redstone; however, the former rock is enriched in Fe, Mg, and Mn but depleted in Na relative to the latter. Discordant alteration pipes and concordant strata-bound altered rocks, associated with massive base metal sulfide and Archean lode Au, volcanic-hosted deposits are predominated by Fe-Mg-Mn-Al-enriched, alkali-depleted rocks, very similar to silicate facies iron-formation at Redstone.Mn garnet-stilpnomelane rocks represent a mixture of pyroclastic material plus chemical sediment, whereby all of the Al, Ti, Zr, V, and P have a volcaniclastic origin and most of the Fe, Mn, Mg, Ca, Co, and Rb are chemically precipitated, whereas Cr, Ni, Sr, Na, Nb, Ba, and K have been leached from a volcanic precursor. Y is mobile, perhaps due to a preferred partitioning into the Mn garnet structure. Both silicate facies iron-formation and semiconcordant alteration zones are enriched in elements commonly mobile in hydrothermal systems such as Fe-Mg-Mn-Si relative to 100 percent tuff, as well as immobile elements including Al-Ti-Zr-Y-Nb-rare earth elements relative to 100 percent exhalite. The resultant group of aluminosilicate minerals within these altered rocks is a function of the bulk-rock composition which in turn represents a mixture of exhaled chemical sediment and volcaniclastic debris. Strata-bound Fe-Mg-Mn-Al-enriched tuffaceous exhalites typically occur in a mixed volcano-sedimentary setting distal from a volcanic center but proximal to the hydrothermal vent, often flanking or stratigraphically at or near a prospective gold or base metal ore horizon.