Abstract

Strata-bound Au-Ag-Mo mineralization in the Haile gold mine area is hosted by several ellipsoidal, pyrite-quartz replacement bodies within a laminated phyllite unit that is bounded on both sides by Cambrian volcaniclastic schists. The orebodies occur within two east-northeast trends corresponding to the highly strained limbs of an upright macroscopic F 2 synform and show opposing en echelon distributions along each trend. Although the orebodies are associated with strata-bound and locally stratiform massive pyrite mineralization and have been heterogeneously deformed, detailed textural and structural studies strongly suggest that both the silicic orebodies and massive pyrite lenses formed synchronously with early to late D 2 deformation by syntectonic replacement of parasitic, doubly plunging, antiformal fold hinges.Ore textures include massive to laminated silicified phyllites in low strain domains, silicic mosaic breccias, and lensoidal silicic augen within highly strained, pyrite-sericite schists. Many fragmental textures previously interpreted as primary in origin resulted from brittle-ductile deformation associated with heterogeneous syntectonic alteration. A four-stage syn-D 2 alteration paragenesis involved: (I) fine-grained quartz-pyrite (+ or - K feldspar + or - molybdenite) replacement along bedding and S 2 in some anticlinal fold hinges, (II) extensive development of steeply dipping thin quartz veins, quartz replacement, and local mosaic-slab breccias along S 2 , (III) local replacement of quartz and pyrite by intense sericitization within high strain shear zones developed marginal to and crosscutting silicic zones, and (IV) locally extensive medium- to coarse-grained pyrite (+ or - molybdenite) replacement along sericitic high strain zones. Stages II and III are separated by a generation of barren massive quartz veins. Lowgrade gold occurs with stage I silicification but high-grade gold is locally associated with stage IV pyritization. Some remobilization of gold occurred with pyrite dissolution during stage III sericitization.Controls on fluid-rock interaction at Haile reflect shifting patterns of deformation partitioning at different scales, where replacement dominantly occurred in progressive shortening domains and selective dissolution occurred in progressive shearing zones. The mineralization events were predated by syntectonic feldspar porphyry dikes and postdated by late, retrogressed lamprophyric dikes and unmetamorphosed diabase dikes. Mineralization resulted from hydrothermal alteration by predominantly metamorphic fluids, with a possible magmatic contribution, syn- to postpeak greenschist facies metamorphism, during the later stages of Taconian orogenesis.

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