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Abstract

To constrain speculation about the character of the host rock at the site of manto ore deposition, we have studied the Leadville Dolomite peripheral to three similar systems of high-temperature, pyritic mantos in central Colorado. The deposits studied are at Leadville, Buckeye Gulch, and Gilman. Comparison of unmineralized sections shows that the Leadville Dolomite has systematic internal stratigraphy, consisting of 15 named beds (from base to top: 6G-8G, 1-12). Many of the mantos are located in one of four stratigraphic positions: the uppermost Leadville, beds 7-8, bed 3, and bed 1. Three of these mineralized beds (containing the largest orebodies) share the characteristics of relatively coarse grain size and porous texture. The fourth mineralized bed (7-8) is the locus of most of the paleocaves in the Leadville Dolomite. In addition, some beds in the Leadville (6G, 4) are selectively unmineralized even in regions of extensive replacement, and these beds are distinguished by their fine grain size and low porosity.

Limited data indicate that the fine-grained beds are six orders of magnitude less permeable than the coarse-grained zebra-textured beds, which in turn are as much as two orders of magnitude less permeable than the manto orebodies. This suggests that the principal control on manto formation was permeability contrast in the dolostone beds of the Leadville Dolomite and that paleocaves were only locally important. The permeability of initial fluid conduits was almost certainly increased by the ore-forming fluid, thereby focusing a higher fraction of the flow into the initial channel. This positive feedback mechanism would result in runaway permeability enhancement and the focusing of essentially all of the flow into narrow conduits. The characteristic shape of the mantos in central Colorado may therefore be an inevitable consequence of the mechanics of fluid flow within carbonate rocks. The general lack of pa-leokarst cave control for the high-temperature pyritic manto deposits is confirmed by comparison of orebody morphology to solution features of hydrothermal and paleokarst origin.

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