Volcanic history, mineralization, and alteration of the Crandon massive sulfide deposit, Wisconsin

Abstract

The early Proterozoic Crandon massive sulfide deposit occurs in a 280-km-long, east-trending greenstone belt along the southern margin of the Southern province of the Canadian Shield. The deposit is conformably contained within a sequence of subaqueous andesitic to dacitic pyroclastics, flows, and associated chemical sedimentary rocks which strike N 85 degrees W and dip 70 degrees to 90 degrees N. Regional metamorphism in the area achieved lower greenschist facies. Regional dip of the rocks is near vertical, but smaller scale or penetrative deformation features are rare.The footwall of the deposit consists of a thick sequence of andesitic tuff overlain by a series of volcanic breccias. Some of the breccias resulted from local fumarolic activity, whereas other breccias represent a complex history of phreatic and phreatomagmatic volcanism and silicification some distance from the deposit. Examination of the breccia types, distribution, and thicknesses suggests at least two source areas for the breccias. The breccias served as permeable conduits for the ore fluids, directing them laterally toward a syndepositional graben where the fluids were vented into a topographic depression on the ocean floor during a hiatus in local volcanic activity. Up to 100 m of massive sulfide consisting of laminae of pyrite and sphalerite with minor chalcopyrite, galena, quartz, chlorite, sericite, and dolomite and minor interbedded tuff, chert, argillite, sandy tuff, and dolomite were deposited.Following chemical sedimentation, hydrothermal venting continued producing crosscutting vein mineralization. Vein development is intimately linked to the distribution of breccia in the footwall. Ascending fluids continued to migrate laterally through the permeable breccia and deposited vein mineralization which plugged the original vent areas with fine-grained precipitates of silica and sulfide. Secondary porosity was created by refracturing of tuff and breccia. Vein mineralization, localized in the footwall of the massive sulfides, exhibits a systematic compositional variation with time and space from west to east. Beneath the west end of the deposit the earliest veins consist of quartz and grade eastward into quartz-chalcopyrite-pyrite, quartz-pyrite-sphalerite-chalcopyrite, pyrite-sphalerite, and finally pyrite veins. The distribution and degree of veining suggests two source areas which correspond to the two source areas for the lithic breccias.Recoverable reserves in the deposit are estimated at 61 million metric tons averaging 1.1 percent Cu, 5.6 percent Zn, 0.5 percent Pb, 37 g/metric ton Ag, and 1.0 g/metric ton Au.Alteration of the footwall rocks at Crandon consists primarily of silicification, sericitization, pyritization, and minor chloritization. Interaction of ore fluid with wall rock has resulted in enrichment in SiO 2 , Fe, K, F, S, Cu, Zn, As, Sb, Ba, Au, Hg, Pb, Bi, Se, and Cd and depletion in Al, Mg, Ca, Na, V, and Sr. The morphology of the alteration zone at Crandon is tabular, due to control by primary and secondary porosity.