Quantifying chemical changes in hydrothermally altered volcanic sequences — silica enrichment as a guide to the crandon massive sulfide deposit,Wisconsin, U.S.A.

A multi-element geochemical study of the wallrocks which host the large Precambrian Crandon deposit (≈ 70 million tons of zinc, copper ore) in Wisconsin, U.S.A. was carried out to identify pathfinder elements which significantly enlarge the size of the deposit as an exploration target. The Crandon deposit occurs in what appears to be a “normal”, subalkaline, intermediate to felsic volcanic sequence. The silica concentration of the wallrocks shows neither a systematic relationship to volcanic stratigraphy nor a distinctive pattern around ore.To determine if silica has been added to the host rocks, a regression analysis was performed on published data from 496 unaltered, subalkaline volcanic rocks from the Abitibi Subprovince of Canada, with silica as the dependent variable and the ratio of “immobile” elements Zr/TiO₂ as the independent variable. Based on the second degree regression curve and the associated standard error of estimate, predicted and residual silica values were calculated and plotted for the Crandon host rocks.The results show that massive sulfide ore was deposited in andesitic rocks with an original SiO₂ content of 52–62%, and that more than 7.4% SiO₂ was added to the rocks stratigraphically beneath the deposit and on strike with the deposit. Silica was added to the hanging wall rocks also, which confirms that hydrothermal activity at Crandon continued after cessation of massive sulfide ore deposition. In contrast to the pattern of silica enrichment at Crandon which significantly enlarges the size of the exploration target, two noneconomic prospects within ten miles of Crandon contain low to moderate levels of silica enrichment and corresponding low base metal values.Silica enrichment occurs in some systems unassociated with mineralization and is therefore not a specific signature of ore-forming processes. However, from the data presented on the Crandon deposit, if silica enrichment does exist in an exploration search area, the chances of finding a massive sulfide ore deposit are improved.