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Book Chapter

Breccia Bodies in the Leadville District, with Emphasis on Occurrences in the Black Cloud Mine, Lake County, Colorado

By
J. Scott Hazlitt
J. Scott Hazlitt
Meridian Minerals Company, P. O. Box 1689, Sutter Creek, California 95685
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Tommy B. Thompson
Tommy B. Thompson
Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523
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Published:
January 01, 1990

Abstract

Within the Leadville district and, in particular, the Black Cloud mine, heterolithic breccia bodies cut sulfide replacement bodies. The breccia bodies form dikes and large pipes, generally focnsed along faults around the perimeter of the centrally located Breece Hill stock. The breccia bodies have been informally termed “fragmental porphyry” because of the presence of pseudophenocrysts which are actually crystal fragments.

Postore but prebreecia rhyolite porphyry dikes and plugs were intruded around the perimeter of the Breece Hill stock from a deep stock, The rhyolite porphyry was subsequently incorporated as fragments in the later fragmental porphyry.

The breccia bodies are composed of hydrothermally altered and unaltered rock fragments. Four different types of breccia represent separate developmental stages. Stage 1 breccias are most prevalent and are composed of subangular fragments of every rock type found in the district, set in a dense rock flour matrix. Stage 2 is composed of crosseutting dikes of intrusive fine-grained breccia. Stage 3 breccia forms flat-dipping tabular bodies near the top of the Leadville Dolomite where black shale and/or a Late Cretaceous igneous sill is present above sulfide mantos. The distinctive feature of stage 3 breccia is its black matrix derived from black shale. The stage 4 breccia consists of narrow dikes with well-rounded fragments. Fragments of massive sulfide manto ore are found within all breccias, and veins of similar sulfide minerals crosscut all stages of the breccia.

The fragmental porphyry breccia bodies lack trace metal enrichment except where fragments of sulfide orebodies have been entrained. The presence of local late golden barite cementing breccia fragments indicates the breccia bodies formed later than the main-ore stage. Postore rhyolite porphyry dikes and plugs were intruded around the perimeter of the Breece Hill stock from a deep stock. The breccias are believed to have formed much like diatremes by fluidization of rock along hydrothermal fluid conduits as the ore-forming magmatic hydro-thermal system collapsed and meteoric water encroached. The meteoric fluids were volatilized by residual magma from the deep stock, entraining some of the postore rhyolite porphyry. Some breccia pipes may have vented onto the paleosurface; several increase in diameter near the present surface. The diatreme formation rapidly dissipated heat and fluids, causing the final collapse of the hydrothermal system. Subsidence of a graben, the downdropped block, occurred with the release of hydrothermal fluids and fluidized, comminuted rock material.

The close spatial and temporal relationship of fragmental porphyry breccias with orebodies and the crosseutting late veinlets of ore minerals within the breccias indicate proximity of the breccia bodies to ore fluid conduits; as such, they are excellent guides to blind mineral deposits within the Leadville district.

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Contents

Economic Geology Monograph Series

Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt

David W. Beaty
David W. Beaty
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Gary P. Landis
Gary P. Landis
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Tommy B. Thompson
Tommy B. Thompson
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Society of Economic Geologists
Volume
7
ISBN electronic:
9781629490021
Publication date:
January 01, 1990

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