Structure And Alteration Related To Hercynian Gold Deposition, Cape Spencer, New Brunswick, Canada

Rocks associated with the Cape Spencer gold deposits near Saint John, New Brunswick were studied to determine the time constraints and the main controls on gold deposition. Aspects studied include regional and detailed structural relationships, geochemistry of the altered rocks, and various details of the gold ores.;The deposits occur in the Avalon Terrane of the Appalachian Orogen, within a largely allochthonous Hercynian fold and thrust belt that is related to dextral transcurrent movement on the adjacent Avalon-Meguma Terrane boundary. Rock units in the Cape Spencer area were juxtaposed by Hercynian faulting. They include Precambrian granitoid and volcanic rocks as well as three sedimentary formations, two of Carboniferous age and one of probable Carboniferous age.;Early effects of Hercynian deformation (D{dollar}\sb1{dollar}) include regional-scale northeast-trending recumbent folds (F{dollar}\sb1{dollar}) accompanied by a penetrative axial-surface cleavage (S{dollar}\sb1{dollar}). A second Hercynian phase (D{dollar}\sb2{dollar}) produced northwest and southeast-dipping thrust faults with associated northeast-trending overturned folds accompanied by a spaced axial-surface cleavage (S{dollar}\sb2{dollar}).;Three distinct types of alteration affected rocks in the Cape Spencer area. The earliest (D{dollar}\sb1{dollar} and/or pre-D{dollar}\sb1{dollar}) was pervasive propylitic alteration of the Precambrian granitoids, involving addition of Na and CO{dollar}\sb2{dollar}, removal of K, and possible redistribution of Fe into the locally abundant, specularite veins. Syn-D{dollar}\sb2{dollar} illitic alteration (illite, ankerite, leucoxene {dollar}\pm{dollar} pyrite {dollar}\pm{dollar} quartz and associated quartz-ankerite and quartz-chalcopyrite veins) was localized along D{dollar}\sb2{dollar} thrust faults and folds. Illitization resulted in removal of Si and Na and addition of K, Ba, CO{dollar}\sb2{dollar}, S, and locally, Au, Ag and base metals. D{dollar}\sb2{dollar} and(?) post-D{dollar}\sb2{dollar} albitic alteration occurred in zones of local extension, involving Na, Si, CO{dollar}\sb2{dollar} and S addition and K and Ba depletion.;Gold deposits occur within pyrite-rich parts of the illitic alteration zones, both in mylonitized granitoid and sedimentary rocks. Ore grades are generally limited to the units richest in iron oxides prior to illitization. Timing of gold deposition is bracketed by the age of the youngest rocks affected by illitic alteration (Westphalian B) and by the {dollar}\sp{lcub}40{rcub}{dollar}Ar/{dollar}\sp{lcub}39{rcub}{dollar}Ar cooling age of illite in the gold ores (277 {dollar}\pm{dollar} 1 Ma)