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Applying non-traditional stable isotopes as tools to study the petrogenesis of Precambrian cherts

Cherts have long been sought after as marker units for discovery of economically viable volcanogenic massive sulfide (VMS) deposits.

The relationship between chert formed via chemical precipitation versus replacement silicification of existing volcanic rocks, however, has remained unclear. The objective of this project is to characterize the δ30Si signatures of a suite of silicified volcanic rocks and related exhalative cherts associated with Precambrian VMS-style mineralization to determine the validity of using silicon isotope signatures as a novel method for differentiating between chert types.

The lack of a universal tool to distinguish between these two chert types has prevented the identification of clear chert marker units to be used as an exploration tool for recognizing some massive sulfide deposits in the rock record. Preliminary data suggests that newly developed silicon isotope proxies can be used to differentiate chemically precipitated cherts from silicified cherts formed during replacement (called silicification) due to their distinct silicon isotope (δ30Si) signatures (see figure to the right). The broad purpose of such work is to ultimately determine the precise fractionation mechanisms of δ30Si during quartz precipitation in both silicified rocks and chemical sedimentary rocks.

At present, we are working on the geochemical and isotopic characterization of the Amulet Rhyolite and associated exhalite sample suite from Rouyn-Noranda, QC, chemical sedimentary rocks from the Deloro Assemblage near Timmins, ON, and age-equivalent units in Northern Minnesota. 

Undergraduate student Dani Stolze’s research project involves mineral identification and comparison of the ~2.7 Ga Deloro Assemblage (Canada) and Soudan Iron formation (Minnesota) rocks using petrography and XRD.

A part of this work is funded to PI Brengman through the Grant-in-Aid program at the University of Minnesota, “Application of a newly developed silicon isotope proxy to sulfide mineral deposit exploration”, $27,109, July 1, 2018 – January 20, 2020.

Collaborators (past and present) include Dr. Christopher Fedo at the University of Tennessee, and Dr. Neil Banerjee at the University of Western Ontario.

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