A Structural-Geochemical Investigation of a Mineralized Breccia and Associated Vein System at the Contact of the Strathy Batholith and the Temagami Greenstone Belt, Strathy TWP., Ontario.

Abstract

<p>A 400 by 80 m tabular chalcopyrite-molybdenite mineralized breccia body and associated vein system, are hosted in basic tholeiitic metavolcanics of the Temagami greenstone belt, adjacent to the contact with the Strathy-Chambers batholith.</p> <p>Mapping of the breccia and enclosing volcanics was conducted on a 400 by 600 m grid at a scale of 1:500. Features of the breccia measured were, fragment size, proportion of matrix, aspect ratio of fragments, attitude of tabular fragments and the thickness of plagioclase rims on fragments. Mapping of the remaining 4 km^2 study area at a scale of 1:2000 established the nature of the volcanic-Strathy-Chambers batholith contact zone and the extent of quartz veining and related structural features.</p> <p>Structural orientations measured were primarily of joints, veins, breccia fragments, faults and foliations. Joint densities were also recorded to establish any spatial relation between joint density and proximity to the breccia.</p> <p>Major, trace and rare earth element analysis were conducted on a suite of granitic samples away from the breccia, toward the core of the batholith. A suite of the tholeiitic basalts that host the breccia and a collection of basalt breccia fragments were analysed for major and trace elements.</p> <p>The dominantly quartz cemented breccia varies in style from zones of pervasive breccia, where fragmentation is extensive, and the breccias are matrix supported, to areas of low matrix incipient breccia, where the breccias contain large (> 30 cm) unrotated blocks. Plagioclase feldspar often forms selvages on fragments as well as in veins within close proximity to the breccia. Sulphide mineralization is concentrated in breccias without plagioclase and in areas of relatively small fragment size. All fragmental material is of the basaltic host and these fragments are exclusively angular, and are often tabular. No evidence of fragment abrasion or matrix rock flour were noted. These features, together with the tabular unrotated fragments and a lack of rebrecciation indicates brecciation was relatively instantaneous, and passive in so much as fluid streaming was not important.</p> <p>Two and possibly three steeply dipping joint populations were defined. One set essentially parallels the long axis of the breccia, the other cuts accross the breccia at a high angle. Analysis of joint densities suggests that the density of joints increases with proximity to the breccia. This feature and the observation that tabular fragments are orthogonal to the joint sets suggests that the joints were the failure or tensile fractures along which brecciation occurred.</p> <p>Quartz veins trend toward 071/85NW, parallel to, and perhaps related to a significant shear zone on the east side of Net Lake. Within approximately 150 m of the breccia veins have a much more variable orientation suggesting that the more regional stresses controlling veins were overridden by forces related to breccia formation.</p> <p>Gresens calculations for the identification of element mobility indicated that volcanic breccia fragments associated with sulphide mineralization and biotite alteration had experienced K and Rb metasomatism similar to that found in porphyry coppper deposits.</p> <p>Major, trace element, and REE modelling indicate that compositional variation identified in the intrusive suite resulted primarily from plagioclase fractionation, with minor fractionation of biotite hornblende, ilmenite, magnetite, sphene, apatite and allanite. Granitic rocks are more differentiated and display fluid saturation textures toward the contact zone opposite the breccia, suggesting that this zone represents the apical portion of a differentiated and fluid saturated magma chamber.</p> <p>Brecciation is thought to have been caused by second boiling processes related to differentiation of the Strathy-Chambers batholith that subsequently caused tensile failure and brecciation of the overlying volcanic roof rocks. Based on the similarity of vein-breccia mineral assemblages to contact metamorphic assemblages the breccia and veins likely formed during peak metamorphism (475-550 °C) at a depth of less than 8-10 km.</p>