Ropes Pyritic Gold Deposit In A Dilational Bend, Marquette Greenstone Belt, Michigan (volumes I And Ii)

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Brozdowski, Robert Adam
thesis / dissertation description
The Ropes deposit is in the Late Archean Marquette greenstone belt in Michigan's Upper Peninsula, near a northeast striking transition between basalt flows and dacite tuffs, along which serpentinitic peridotite is emplaced. A tabular trend of quartz-sericite-chlorite rock, which is altered dacite tuff, strikes east northeast from this transition and hosts ore. Carbonate-quartz-chlorite rock, which is sheared and altered peridotite, immediately bounds the quartz-sericite-chlorite rock. This is bound on north and south by carbonate-talc rock, then outward by serpentinitic peridotite.;The deposit is 2.8 million tonnes with 3.7 g/tonne Au; near vertical, 335 m in strike, 600 m down dip, and 12 m average thickness. 95% of the ore is characterized by dispersed sub 100 micron pyrite, with micron gold included and attached, in quartz-sericite-chlorite rock. Ore is zoned longitudinally into three subtypes with differing quartz, sericite, chlorite and pyrite proportions. Silver is with tetrahedrite, galena and electrum. Au/Ag is 0.65. 5% of ore is layered auriferous quartz veins with tetrahedrite, at the south side of the deposit.;The deposit is in a 080{dollar}\sp\circ{dollar} striking dilational bend in a nearly vertical, north side down, oblique dextral, brittle-ductile shear zone which strikes 070{dollar}\sp\circ{dollar} overall. Shear was along a line plunging 60{dollar}\sp\circ{dollar} east, indicated by oblique shear quartz veins and zones of gold concentration with dispersed pyrite which are oriented in right stepping en echelon low angle sigmoidal fashion, and by S-C kinematic indicators. Sericite, chlorite and locally auriferous pyrite along schistosity indicate hydrothermal alteration and gold deposition were contemporaneous with simple shear deformation. Contacts of quartz-sericite-chlorite rock with bounding rocks were weaknesses which facilitated shear. Quartz-sericite-chlorite rock which hosts ore dips less steeply near surface, and narrows up dip and eastward: this may have focussed ascending hydrothermal fluids. Carbonatization of serpentinitic peridotite, and sericite alteration of dacite tuff were sinks for CO{dollar}\sb2{dollar} and K. Simple mineral assemblages and limited compositional ranges of rock forming minerals suggest homogenization by pervasive fluid flow promoted by the dilational characteristics of the shear zone.