Fingerprinting and tracing the signature of basement-hosted unconformity-type uranium alteration through thick Quaternary tills: an example from the Thelon Basin, Nunavut
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The question of whether or not it is possible to trace the signature of alteration haloes surrounding deep-seated unconformity-type U mineralization through thick Quaternary tills is one of great importance for those conducting exploration in glaciated areas. The geochemical signals associated with alteration are often subtle and subject to numerous sources of noise, and multi-till stratigraphies can completely mask or truncate dispersal patterns. To study glacial dispersal from alteration zones surrounding unconformity-type uranium mineralization, this study focuses on a deep-seated (>100 m) basement-hosted unconformitytype U mineralized body known as Tatiggaq, which is located in the Thelon Basin of Nunavut. The Tatiggaq area presents an ideal opportunity to investigate this problem, since the area is blanketed by a thick (12-34 m) multi-till stratigraphy, mineralization does not intersect the bedrock-till interface, and the mineralization is surrounded by an extensive illitic alteration halo that reaches the bedrock surface. The recovery of till samples from diamond drill core from the multi-till stratigraphy overlying the subcropping alteration halo provides a unique opportunity to model glacial dispersal and identify whether the fingerprint of buried alteration can be traced through three dimensions to the modern day surface. Sampling of till recovered during diamond drilling was combined with the sampling of surficial mudboils in the area surrounding Tatiggaq to trace the dispersal of alteration. The drill core samples were collected from four stratigraphic units that were deposited by early southwesterly and southerly ice flows, followed by a reversal in ice flow towards the northwest and westnorthwest. Knowledge of the till stratigraphy directly above the Tatiggaq alteration zone was used to construct a three dimensional model of the sediments overlying bedrock to allow the tracing of alteration through the subsurface. The geochemical fingerprint of the alteration halo at Tatiggaq was identified by applying univariate and multivariate statistical analysis, including principal component analysis, to geochemical data from altered and fresh rocks in the region. This analysis identified the enrichment of Fe2O3, K2O, Al2O3, P2O5, TiO2, B, Ni, U, Cr, and Sc, and depletion of CaO, MnO, Na2O, Mo, Zn, Ba, and Sr in the altered rocks using total digestion of rock powders (HF-HClO4-HNO3). Partial digestion of the same rock powders using HNO3-HCl identified the enrichment of U and depletion of V, Zn, Y, and Yb in the altered rocks. The observed trends in the bedrock data were then applied to the till geochemistry data to identify alteration signatures that were discernible across the till stratigraphy. These enrichment and depletion trends were used to generate four alteration indices (AI) for use in uranium exploration: - 𝐴𝐼1 = (𝐾2𝑂+ 𝐴𝑙2𝑂3) / [(𝑁𝑎2𝑂+𝐶𝑎𝑂)+ (𝐾2𝑂+ 𝐴𝑙2𝑂3)] (𝑇𝑜𝑡𝑎𝑙 𝐷𝑖𝑔𝑒𝑠𝑡𝑖𝑜𝑛) - 𝐴𝐼2 = (𝐵 + 𝑁𝑖 + 𝑈) / [(𝑍𝑛+𝑆𝑟)+(𝐵 + 𝑁𝑖 + 𝑈)] (𝑇𝑜𝑡𝑎𝑙 𝐷𝑖𝑔𝑒𝑠𝑡𝑖𝑜𝑛) - 𝐴𝐼3 = 𝑁𝑖 / (𝑍𝑛+𝑁𝑖) (𝑇𝑜𝑡𝑎𝑙 𝐷𝑖𝑔𝑒𝑠𝑡𝑖𝑜𝑛) - 𝐴𝐼4 = 𝑈 / (𝑉+𝑈) (𝑃𝑎𝑟𝑡𝑖𝑎𝑙 𝐷𝑖𝑔𝑒𝑠𝑡𝑖𝑜𝑛) A 3D model of the till sequence was constructed with GOCAD® and populated with interpolated values for the four alteration indices in order to trace alteration through the subsurface. Analysis of these trends revealed that alteration near the bedrock surface is pronounced, but rapidly attenuates moving up the stratigraphic sequence. Despite the rapid attenuation of the alteration signal in the subsurface, the signal is detectable at the surface down-ice from the main alteration zone with AI1 and AI2, and above the alteration zone with AI3. Although AI4 was successful at delineating alteration in the subsurface, no patterns were observed in the surficial mudboil data using AI4. This study shows that despite a complex Quaternary stratigraphy and ice-flow history in a region, it is possible to trace glacial dispersal of subtle geochemical signatures related to subcropping alteration zones surrounding buried basement-hosted unconformity-type uranium deposits. However, the detection of such alteration signals requires a detailed knowledge of the alteration signal being sought out and an understanding of the depositional history and stratigraphy of Quaternary sediments. Continuing research into alteration systems surrounding basement hosted unconformity-type uranium deposits will help determine the applicability of the ratios developed here to other regions.