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Utilizing silicon isotopes (δ30Si) as a tool for volcanogenic massive sulfide mineral deposit exploration

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 replacement-silicified cherts due to their distinct silicon isotope (δ30Si) signatures (see figure to the right).

The figure here is a plot showing (δ30Si) verses δ18O for progressively silicified volcanic rock samples from Group 1, 2, and 3 of the Hunter Mine Group (represented by different shades of green squares), and chert in BIF formed from chemical sedimentation from the Deloro Assemblage (represented by gray triangles). The gray shaded region represents the total range of values expected for (δ30Si) values of silicified volcanic rocks ((δ30Si) ~ 0 ‰), while chemical sediments are expected to plot in the un-shaded region ((δ30Si) < 0 ‰).

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.

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