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PT evolution of Sentinel Cu deposit, northwestern Zambia
Meighan, Corey James
Meighan, Corey James
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2015
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Abstract
The Sentinel Cu deposit, located on the eastern margin of Kabompo Dome within the Domes Region of northwest Zambia, is hosted in Neoproterozoic metasedimentary rocks of the Katangan Supergroup. Copper sulfide minerals in the deposit are intergrown with metamorphic minerals formed during the latest Neoproterozoic to early Cambrian Lufilian event, interpreted to be associated with the latter stages of Gondwana assembly. This study examines the interrelated roles of metamorphism and metasomatism in the host rocks to the deposit to help better resolve their P-T history. The results have implications for improved understanding of the tectonothermal evolution of the Domes Region. Whole-rock geochemistry of the Sentinel Phyllite (ore host) indicates that Mg must have been added during syn-metamorphic metasomatism. Quartz veins with copper sulfides also contain kyanite, phlogopite, and Mg-chlorite suggesting these minerals formed by a series of metasomatic reactions requiring the addition of Mg from an aqueous fluid. Based on regional geology, the source of Mg is likely both residual basinal fluids developed during evaporite formation and possibly brines derived from evaporite dissolution. In order to constrain the temperatures of metamorphism/metasomatism at Sentinel three independent thermobarometry approaches including garnet-biotite Fe-Mg exchange thermometry and trace element thermometry (Ti-in-quartz and Zr-in-rutile) were applied. Ti-in-quartz thermometry conducted on quartz from vein and matrix domains within the Sentinel Phyllite, and quartz from matrix domains of rocks from the surrounding stratigraphy record similar temperatures of ~450°C across >500 meters of stratigraphic section. These conditions are interpreted to reflect closure to Ti diffusion during retrograde cooling. Zr-in-rutile thermometry conducted on rutile in quartz veins within the Sentinel Phyllite, and rutile in matrix domains of rocks from the surrounding stratigraphy, records temperature conditions of ~550°C. Rimward increases in Zr concentrations of rutile from veins in the carbonaceous and less carbonaceous phyllites suggest that Zr-in-rutile temperatures record conditions of growth along a path of increasing temperature and not retrograde re-equilibration due to deformation. Temperature estimates from garnet-biotite thermometry are broadly consistent with temperature estimates calculated from Zr-in-rutile thermometers, but give estimates up to and exceeding 600°C. Taken together, the data suggest that metamorphic mineral assemblages in the Sentinel rocks equilibrated at conditions similar to those accompanying vein formation, and that peak metamorphic temperatures were at or above 600°C. Application of Grt-Bt-Pl-Qtz (GBPQ) and Grt-Als-Qtz-Pl (GASP) barometry conducted at Sentinel fall into two groups: ~9-10 kbars and ~7 kbars, with lower pressures for samples with more calcic plagioclase. The most elevated pressures pertain to more albitic plagioclase core compositions from quartz-feldspar-biotite schists, and not necessarily compositions in chemical equilibrium with garnet. Therefore, re-equilibration of more albite-rich plagioclase with garnet to form more anorthitic rims at or below 7 kbars is viable. In the Sentinel area, kyanite appears to be texturally overprinted by talc, and is interpreted to also be related to syn-metamorphic Mg metasomatism. This observation indicates that the talc-kyanite-quartz schist, which is regionally extensive, cannot be considered a “whiteschist” (senso stricto) formed during high-pressure metamorphism. Similarly, high pressure metamorphism associated with kyanite-bearing schists bounding the Sentinel Phyllite is likely related to re-equilibration of a more albite-rich plagioclase at or below 7 kbars. These observations may also have implications for regionally extensive garnet-kyanite bearing schists and gneisses observed in the Domes Region, which indicate ~7 kbars is likely a robust maximum estimate for peak pressures accomopanying metamorphism, rather than the higher pressure estimates of previous studies. The new data suggest that minimal structural thickening during the Lufilian event together with an elevated geotherm could explain the observed mineral assemblages. Copper sulfide minerals are rare in the matrix of the host phyllites at Sentinel, suggesting that Cu in quartz veins is not a remobilization of a pre-existing Cu enrichment within the carbonaceous protolith to the phyllite. Therefore, the Sentinel Cu deposit should be considered a ‘syn-metamorphic’ Cu deposit. Sulfide minerals are intergrown with or form inclusions in rutile. Temperatures of ~550-600°C reflected by rutile growth within the Sentinel Phyllite are likely close to the peak temperatures of ore deposition.
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