500 MA K-SPAR MICRO-VEINS IN MID-CONTINENT PRECAMBRIAN BASEMENT: IMPLICATIONS FOR POTASSIUM METASOMATISM

Authigenic K-spar is known to occur across the mid-continental United States in areas intimately associated with the great unconformity, yet much ambiguity remains about the timing and mechanisms of emplacement. Emplacement mechanisms often invoke gravity driven fluids mobilized during orogenesis, or diagenetic reactions in sediments. Measured ages range from Alleghenian-Ouachita (ca. 270 Ma) to Precambrian (ca. 600 Ma). Proposed emplacement mechanisms invoke gravity driven Mississippi Valley-type ore forming fluids when coinciding with orogenic events, and diagenetic reactions when not. Precise age dating of authigenic K-spar is often hindered by the fact that it occurs as overgrowths on primary igneous or detrital sedimentary K-spar; thus, dating methods give ages that are effectively mixtures of the two signals. Here we observe K-spar as micro-veins that are clearly authigenic and devoid of earlier K-spar. Using in-situ UV laser ablation of these K-spar micro-veins we obtained ⁴⁰Ar/³⁹Ar ages of 499.6 ± 1.0 Ma and 499.8 ± 1.2 Ma (i.e. ca. 500 Ma). This age predates tectonic activity for Laurentia during the end Precambrian and earliest Paleozoic and is therefore unambiguous evidence for this occurrence to be unrelated to gravity driven Mississippi Valley-type fluids. A mechanism related to diagenesis in the overlying Sauk sequence sediments may have created overpressures that fractures the basement and injected and emplaced the K-spar micro-veins.

The 500 Ma K-spar micro-veins are observed cross-cutting quartz veins of a unique gold and copper occurrence in central Wisconsin known as the Reef Deposit. The gold bearing quartz veins originally formed in the root zones of a volcanogenic massive sulfide setting during the Penokean orogeny (ca. 1830 Ma). Previous work has identified multiple overprinting geologic events on this gold occurrence, the youngest being gold remobilization and possible enrichment by Paleozoic Mississippi Valley-type (MVT) fluids. Carbonate veinlets examined earlier which cross-cut the K-spar veinlets and newly identified petroleum bearing fluid inclusions in secondary fluid inclusion assemblages are taken as evidence a later gravity driven fluid (MVT) event indeed occurred (i.e. Alleghenian-Ouachita).