The Origin of the Proterozoic Bidjovagge Gold-Copper Deposit, Finnmark, Northern Norway, as Deduced from Rare Earth Element and Nd Isotopic Evidences on Calcites

摘要

The Bidjovagge gold-copper deposit, northernNorway occurs in the lower Proterozoic Kautokeino greenstonebelt, and consists of ten ore bodies in albite felsite, graphite schistand meta-diabase sills, along with a series of syenodiorite dykes.Calcite is one of the most important gangue minerals occurringwithin the ore veins. The early primary calcite samples from A, Cand D ore bodies (copper-rich) show almost flat chondrite-normalised REE curves, and slightly positive Eu anomalies. A Sm-Nd isotope age of 1858±72 Ma (2?) has been given based onfifteen primary calcite samples. The Sm-Nd age of calcite is in wellagreement with previously published U-Pb davidite age of1885±18 Ma (2?) and Sm-Nd davidite age of 1886±88 Ma (2?).All these isotopic age data indicate that the gold-coppermineralisation may occur during or slightly after the peak of theSvecokarelian orogeny. Two late primary calcite samples from Cand D ore bodies have flat chondrite-normalised LREE curves,positive HREE curves, and negative Eu anomalies. In contrast,most of the early primary calcite samples from B ore body (gold-rich) show an overall negative chondrite-normalised REE slopeand negative Eu anomalies. Sm-Nd data on seven primary calcitesamples from the B ore body are relatively scattered, and gave anisochrone age of 1809±150 Ma (2?), identical to the age of thecalcite samples from A, C and D ore bodies. Two late primarycalcite samples from the B ore body have almost flat chondrite-normalised REE slope and negative Eu anomalies.REE patterns and ?Nd (T) values of calcites suggest that theore fluids for the calcites from the A, C and D ore bodies derivedtheir metals from mafic igneous rocks (diabase sills). For thesecondary calcites from the C ore body, their relative high initial?Nd (T) values of 1858 Ma imply that much more radiogenic Nd-component were incorporated during calcite crystallisation whenanother ore fluid emplaced. Combined Nd isotope data and REEgeochemical features suggest that the isotopically more evolvedsource such as albite felsite and syn-genetic high level syenodioritedykes were probably metal source for these calcites in the B orebody. Three samples of secondary calcite from the B ore body,which obviously deviated from 1809 Ma isochron are easilyexplained as products of post-ore hydrothermal events.