Metamorphism and Chromite in Serpentinized and Carbonate-Silica-Altered Peridotites of the Paleoproterozoic Outokumpu-Jormua Ophiolite Belt, eastern Finland

Abstract

The 1.95 Ga Outokumpu-Jormua ophiolite belt of eastern Finland contains numerous mafic-ultramafic, predominantly peridotitic bodies which, despite amphibolite-facies metamorphism and pervasive deformation, retain compelling evidence of a residual mantle origin. These rocks therefore currently represent the oldest documented examples of exhumed mantle lithosphere, so information concerning their primary igneous mineral assemblages and textures and chemical and isotopic characteristics is of considerable scientific value. Although several earlier studies have argued for the preservation of primary mineral assemblages, field and petrographic evidence presented here show that the protolith peridotites had already experienced pervasive low-T serpentinization prior to Svecofennian orogenic deformation, during which they were progressively deserpentinized via antigorite metaserpentinites to olivine-talc-anthophyllite-enstatite-bearing metaperidotites. evidence is also presented to show that the premetamorphic serpentinization event was closely followed by extensive low-T (< 250°C) metasomatic alteration of the marginal parts of the ultramafic bodies to carbonate-silica rocks which, during the subsequent prograde metamorphism, were converted to the distinctive chromite-bearing carbonate-skarn-quartz rocks comprising the Outokumpu rock assemblage. Because these quartz rocks are intimately associated with the Outokumpu Cu-Co-Zn-Ni deposits and have generally been regarded as metamorphosed siliceous seafloor exhalative deposits, the revised interpretation presented here has important implications for ore formation as well. equilibrium mineral assemblages in the interior parts of the ultramafic bodies (low XCO2) define four regional metamorphic zones, expressed as an east to west increase in the peak dehydration temperatures from 500° to 775°C, at 3-5 kbar. Large ultramafic bodies commonly show core to margin zoning from talc via anthophyllite- to enstatite-bearing assemblages, reflecting synmetamorphic core-margin gradients in XCO2, attributed to infiltration of CO2 released by decarbonation reactions in previously formed talc-carbonate and carbonate-silica alteration zones. The only primary igneous phase identified in this study was chromite, which occurs as scattered relict cores within large altered grains. These are relatively common in metaserpentinites, but also occur in metaperidotites and even in carbonate-skarn-quartz rocks. even though Mg, Fe2+, and Zn abundances in these relict chromites may have been somewhat modified, absolute concentrations and ratios of Cr and Al appear to be essentially unmodified. The lobate to amoeboid morphology and observed Cr# range (0.41-0.67) of the best-preserved grains are more consistent with residual depleted lherzolite to harzburgite textures than a cumulate origin. However, during regional metamorphism, most mantle chromites were either pervasively altered to ferrian chromites and Cr-magnetites or (where CO2 and S fugacities were high, such as within smaller ultramafic bodies, or at the margins of larger bodies) to high-Cr (Cr2O3 = 50-70 wt%; Cr# = 0.7-1.0) chromites; with further increase in metamorphic grade, the latter typically recrystallized to mostly spongy/chessboard-textured grains. This interpretation contrasts with previously held views, where high-Cr chromites were considered as residual chromites in ultradepleted residual peridotites. Our study demonstrates that Cr-spinel textures and compositions in amphibolite-facies ultramafic rocks may, to a large degree, be influenced by the metamorphic and metasomatic history of the enclosing host rocks. Clearly, valid application of Cr-spinel as a petrotectonic indicator requires, at least for medium and higher-grade ultramafic rocks, a thorough understanding of the metamorphic and hydrothermal history of the host rocks. Interpretation of the quartz rocks of the Outokumpu assemblage as silicified peridotites demands a reappraisal of the widely accepted concept of the Outokumpu-type sulfide ores as a type example of Precambrian ophiolite—related seafloor hydrothermal sulfide deposits. As an alternative, we tentatively propose a syntectonic hydrothermal origin.