Structural setting and fluid characteristics of the Proterozoic Tartan Lake gold deposit, Trans-Hudson Orogen, northern Manitoba

Abstract

The late-kinematic structural architecture of the Flin Flon domain is dominated by a network of anastomosing shear zones formed by compressional deformation during the Proterozoic Trans-Hudson orogeny (ca. 1900-1800 Ma). A number of structurally controlled mesothermal gold deposits occur throughout the region, associated with this shear zone network. The Tartan Lake gold deposit occurs in the Tartan Lake shear zone, at the contact between a layered gabbroic intrusive complex and metavolcanic rocks of the Amisk group (ca. 1875 Ma).Five phases of deformation have been recognized in the Flin Flon region (P1-P5). Midgreenschist facies peak metamorphic conditions (phase 3) at Tartan Lake are recorded by a mineral assemblage of biotite-actinolite-albite-muscovite-epidote and carbonate in volcani-clastic rocks, along with a well-developed phase 3 schistosity. Early movement of the Tartan Lake shear zone was initiated with phase 3 deformation, during which oblique slip occurred. During phase 4, this shear zone was overprinted by a major regional fold and the shear zone was reactivated by strike-slip movement. Regional phase 3 schistosity is refolded in a series of steeply plunging mesoscopic phase 4 fold closures that were synchronous with the shear zones.A network of conjugate strike-slip shear zones that vary in width from 5 to 35 m comprise the 0.3-km-wide structure at Tartan Lake. Deformation within the shear zones is marked by (1) development of S and C schistosities in which C is dominant, (2) weakly developed sub-horizontal mineral lineations, and (3) folding and kink banding of these schistosities. Within shear zones, sericite, chlorite, quartz, carbonate, albite, tourmaline, and pyrite are the dominant minerals and biotite and actinolite are retrograded to chlorite, such that this deformation and Au mineralization are postpeak metamorphism. Five stages of veining occurred within the shear zones, and these are characterized by distinctive orientations and mineral parageneses. Gold mineralization occurred with two of these stages of veining, along with pyrite, chalcopyrite, tellurides, monazite, scheelite, and rutfie. The timing of this mineralizing event has been constrained by the 40 Ar/ 39 Ar method at 1791 + or - 4 Ma; this is 20 to 40 m.y. postpeak metamorphism.Fluids were of H 2 O-CO 2 -NaCl composition throughout the ore-forming vein events, with modal salinity = 6.9 wt percent equiv, CO 2 density = 0.64 g/cm 3 , and homogenization temperatures ranging from 250 degrees to 390 degrees C (with a median of 295 degrees C). Stable isotope compositions of quartz (delta 18 O = 10.8-13.8ppm), chlorite (delta 18 O = 3.0-4.8ppm), muscovite (delta 18 O = 7.5-9.0ppm), scheelite (delta 18 O = 4.0-5.0ppm), albite (delta 18 O = 9.0-9.6ppm), tourmaline (delta 18 O = 7.9-8.8ppm), pyrite (delta 34 S = 1.7-3.1ppm), and chalcopyrite (delta 34 S = 1.4-1.7ppm) establish the temperature of gold mineralization between 315 degrees and 472 degrees C, with an average at 372 degrees + or - 41 degrees C (1Sigma ). Ore-forming fluids had delta 18 O = 5.2 to 6.5 per mil and delta D = -50 to -86 per mil, and Fe dolomites from mineralized veins exhibit restricted ranges of delta 13 C (-7.0 to -5.2ppm and delta 18 O (9.3 to 10.9ppm),suggesting a uniform fluid composition. These veins formed at a pressure of 1.2 to 2.4 kbars, corresponding to 4.3 to 8.6 km, if lithostatic fluid pressure is assumed.Constraints on the depth at which these veins precipitated and the character of the fluids suggest that this gold deposit formed from advecting metamorphic fluids. A postmineralization stage of brittle fracturing is represented by barren veins that exhibit significant depletion in delta 34 S (-29 to -30ppm). This may be the result of an increase in fluid f (sub O 2 ) in the final stages of crustal deformation during the Trans-Hudson orogeny.