Fluid flow and metasomatism in the genesis of the amphibolites-facies, pelite-hosted Kanmantoo copper deposit, South Australia

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ID: 303153
1998
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Abstract
During Ordovician low pressure regional metamorphism, pelitic and psammitic rocks of the Tapanappa Formation around Kanmantoo underwent changes in bulk chemistry and ^18^ O/ ^16^ O ratios inferred to be a product of fluid infiltration. Three scales of isotopic and/or geochemical changes are recognized: broad oxygen isotope changes at greater than 10 km scales in the region, isotopic and geochemical changes over 1 km around the orebody (alteration envelope), and centimeter- to meter-scale changes related to veining within the alteration envelope. Over a distance of 20 km, the metamorphic grade increases from biotite zone (350 degrees -450 degrees C) to sillimanite zone (600 degrees -650 degrees C). Corresponding with this lateral metamorphic gradient, the delta ^18^ O of metapelitic schists decreases from 13.5 per mil at the low-grade end to 8.3 per mil at the high grade end. Gradual depletion in ^18^ O was most likely caused by the penetrative flow and progressive equilibration of fluid throughout the rock mass, in the direction of increasing temperature. Using a one dimensional model for fluid-rock interaction which incorporates provision for modal abundances of minerals, time integrated fluid fluxes in the pelites were on the order of 5X10 ^5^ moles.cm (super -2) , consistent with other estimates of fluid fluxes in fluid-dominated regional metamorphic systems. The chalcopyrite-magnetite ore deposit occurs in the andalusite-staurolite zone ( approximately 550 degrees C). In an area of approx 1 km diam, muscovite-bearing pelitic and psammitic host rocks are converted to Fe- and Al-rich muscovite-poor metasomatic rocks, which surround the now mined-out chalcopyrite-magnetite ore deposit. Delta ^18^ O values of the rocks are substantially depleted relative to the regional gradient, with both pelitic and psammitic rocks attaining values 9.6+ or -2.0 per mil (total range), in equilibrium with water in the range 5.9 to 9.0 per mil. These results are interpreted as reflecting the equilibration of the rocks with an externally-derived fluid derived from a crystallizing magma. Along with the orebody shape that lies parallel to the regional S ~3~ fabric and clearly crosscuts bedding at 100 m-scales or broader, these data suggest that ore genesis occurred late during the metamorphic cycle. Geochemical data for the transformation of host rocks to the alteration envelope were determined with a reference frame involving little net mass change and approx constant values of Mg, Ti, Zr, Al, and Y. Large gains in Fe are matched by depletions in Ca and Na, and the rocks are also enriched in Cu and S. Quartz veins within the alteration envelope display prominent highly aluminous selvages, but these show different mass transfer patterns to those of the alteration envelope and do not represent the conduits for the ore-forming fluid. The temporal and spatial progression from 20 km-scale regional ^18^ O depletion, to kilometer-scale metasomatism around the deposit, to predominantly centimeter-scale mass transfer around veins, may reflect progressive fluid flow channelization with time during the metamorphism, accentuated by the localized input of fluid from late metamorphic granitoids. Ore genesis most likely occurred by mixing of regional metamorphic fluids with granite-derived fluids, although the isotopic and geochemical data suggest that the iron and oxygen budgets of the deposit were mainly granite related.
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openalex_W2037079956 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Nicholas H.S. Oliver, Gregory M. Dipple, Ian Cartwright, Jeffrey C. Schiller
Journal american journal of science
Year 1998
DOI
10.2475/ajs.298.3.181
URL
https://ajs.scholasticahq.com/article/60780.pdf
Keywords Keywords not found

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