Protoliths, diagenesis, and depositional history of the Upper Marble, Adirondack lowlands, New York.

摘要

Major, trace, and rare earth element abundances were used along with Sr isotope and published stable isotope data to constrain the mineral compositions, diagenesis, and depositional history of protoliths of the upper amphibolite facies marbles of mid-Proterozoic age. The mineral compositions of the marble protoliths were estimated by least-squares multiple regression combined with CO{dollar}sb2{dollar} addition, so that the present day chemical compositions of the marbles was accounted for by a set of plausible depositional and diagenetic minerals. Comparison of the results to present day mineral compositions is consistent with isochemical behavior for most of the studied marbles.; Mineral composition, trace element, and isotopic (O, C, and Sr) data show that the largely dolomitic rocks below the main anhydrite horizon (unit 11A of the type-section of Brown and Engel, 1956) of the Upper Marble experienced open-system diagenesis, whereas the rocks above unit 11A experienced closed-system diagenesis. This diagenesis was accomplished penecontemporaneously by sea water-dominated fluids. Differences in the history of the Upper Marble below and above 11A are supported by differences in published Pb data for sphalerite in the Balmat Zn-Pb ores. The distinct estimated Sr isotopic compositions of sea water during Upper and Lower Marble deposition are within the range of published estimates for mid-Proterozoic sea water, and indicate stratigraphic non-equivalence of these two formations. Comparison of preserved premetamorphic Sr isotopic variations in two Upper Marble cores does not support the use of these data for stratigraphic correlation.; Most Upper Marble quartz originated as early diagenetic chert. Increases in aluminosilicate mineral abundances just below unit 11A are related to the tectonic events that caused the basin restriction that resulted in anhydrite deposition. The aluminosilicate minerals may have originated from paragneisses in either the Frontenac terrane or the Adirondack Highlands. A new model for Balmat Zn-Pb ore deposition places {dollar}sp{lcub}34{rcub}{dollar}S-depleted sulfur from the basin brines into the ores to explain increases in {dollar}deltasp{lcub}34{rcub}{dollar}S values of coeval anhydrite without necessitating the formation of undocumented stratigraphically equivalent pyritic schists by the combination of hydrothermal iron with {dollar}sp{lcub}34{rcub}{dollar}S-depleted sulfide from basin brines.