Breccia Bodies in the Leadville District, with Emphasis on Occurrences in the Black Cloud Mine, Lake County, Colorado
J. Scott Hazlitt, Tommy B. Thompson, 1990. "Breccia Bodies in the Leadville District, with Emphasis on Occurrences in the Black Cloud Mine, Lake County, Colorado", Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt, David W. Beaty, Gary P. Landis, Tommy B. Thompson
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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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Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt
The carbonate-hosted ore deposits at Leadville, Gil-man, Red Cliff, Aspen, Alma, Tincup, Kokomo, and Mount Sherman have enjoyed a long and storied production history. These orebodies, as well as dozens of smaller deposits, are all located in the central Colorado mineral belt and together constitute an important metallogenic province (Figs. 1 and 2).
Recorded metal production of the major districts in this province to date has consisted of 1,630,000 metric tons of zinc, 1,500,000 metric tons of lead, 145,000 metric tons of copper, 15,600,000 kg of silver, and 110,000 kg of gold (Table 1). For several reasons these figures represent only a portion of the metal concentrated by nature in these deposits:
1. Early production records are probably incomplete.
2. Inefficient methods were used to process much of the ore mined during the 1800s, particnlarly for zinc and copper.
3. The ores in the principal mining districts were partially removed by erosion prior to mining.
4. Significant reserves remain in the Leadville district.
In comparison to other mining districts around the world, the carbonate-hosted sulfide deposits of the central Colorado mineral belt have produced relatively low tonnages of high-grade ore (Table 2). The largest of the districts is Leadville, which to date has produced aboul 24,000,000 metric tons of polymetallic ore. By contrast, the Aspen district has produced only an estimated 4,000,000 metric tons of ore (Table 2), but that ore averaged about 1,000 g/metric ton silver. Although all of the deposits in this metallogenic province are polymetallic, the economic significance of the various metals is not equal. The ores at Gilman, Aspen, and Leadville were valuable primarily for their contained Zn-Cu-Ag, Ag-Pb, and Ag-Au-Pb-Zn, respectively (Table 2).
The first discovery of gold in Colorado was made in July 1858, in a stream draining the eastern Rocky Mountains. This led to the “Pike's Peak” gold rush of 1859, during which an estimated 50,000 people moved into the area (Blair, 1980). These so-called “Fifty-Niners” established most of the mining districts in the northeast portion of the Colorado mineral belt during the summer of 1859. By late 1859 the prospectors had penetrated the Continental Divide, and in April 1860, the placer gold deposits at Leadville were discovered.
A rush to Leadville ensued, and as a result of heavy mining pressure, the Leadville placers were essentially depleted by 1868. The much larger and more valuable carbonate replacement ores at Leadville,