CHEMICAL SEDIMENTS ASSOCIATED WITH THE IMPACT-GENERATED HYDROTHERMAL SYSTEM, VERMILION FORMATION, SUDBURY STRUCTURE

Hydrothermal activity in the ~200 km diameter Sudbury crater is much more extensive than in other terrestrial craters, due to its very thick (~2.5 km) melt sheet, the Sudbury Igneous Complex, and thick (1.5 km) crater-fill suevite, the Onaping Formation. The highly permeable, andesitic vitric-rich crater-fill reveals vertically stacked, extensive (>6000 km3) alteration zones (Ames, 1999). Above the altered crater-fill Onaping Formation are carbonate and turbiditic sediments of the Vermilion Formation and then post-impact carbonaceous mudstones of the Onwatin Formation, host to a metalliferous plume.

The Vermilion Formation is thin (~14 m) and is composed of the Lower Carbonate (LCM), Grey Argillite (GAM), and Upper Carbonate Members (UCM). The LCM proximal facies is local, banded, mottled, massive, hosts Zn-Pb-Cu mineralization, and consists of 1-2 mm laminae and colloform textures composed of Ca-Mg (Fe, Mn) carbonates, muscovite, hyalophane, celsian, stilpnomelane and minor carbonate. Sulfides include sphalerite, chalcopyrite, minor galena, arsenopyrite and later nickeliferous pyrite, marcasite and anglesite. The laterally extensive distal facies contains beige-brown fine laminae of alternating Mn-Fe-Ca carbonates, barian feldspar, carbon, quartz, phengite, and geochemically is a carbonate-facies iron formation. Spheroidal carbonate with dolomite cores encased by organic carbon and siderite shells, may be related to biologic activity in the cooler stages of hydrothermal activity. Distal turbiditic beds of the GAM lack hemipelagic beds and are thin close to ore deposits (<1 to 5 m) and thicken (~ 50 m thick) away. The 10 cm to 2 m peloidal carbonate mudstone (UCM) is restricted to interpreted vent sites in the ore deposit environment. Syn-crater faulting evident through melt dykes in the suevite, linear distribution of peperite and LCM sites, and variations in thickness and structure of the GAM controls the location of vent sites. The distal low temperature facies and silicified zones are most prospective for biomarkers in the impact-related chemical sediments. Deposits similar to the Vermilion Formation may be present in other large craters, perhaps even on Mars, however to date the Sudbury impact hydrothermal system is unique in the geological record.