Mineralized veins and breccias of the Cripple Creek District, Colorado

Colo. State Univ., Dep. Earth Resour., Fort Collins, CO, United States

The Cripple Creek district has yielded nearly 21 million troy ounces of gold since its discovery in 1891. The orebodies occur as narrow veins within Precambrian and Tertiary rocks and as bulk tonnage deposits within tectonic and hydrothermal breccias.The district is localized within and adjacent to a 27.9- to 29.3 + or - 0.7-m.y.-old nested diatreme-intrusive complex. Two magmas, phonolitic and alkali basaltic in composition, generated volcanic flows, subvolcanic intrusions, and phreatomagmatic breccias. Magma mixing is suggested by intermediate composition latite-phonolite and syenite. Subsidence of the diatreme complex rocks is indicated by (1) a thick fluvial-lacustrine sedimentary sequence in the eastern subbasin, (2) the presence of carbonaceous debris, ripple-laminated rocks, and dessication cracks in sedimentary rocks at depths more than 300 m below the present surface, (3) by the fracture systems near the diatreme subbasin margins that reflect basement rock influence, and (4) by flat-dipping veins near intrusive bodies or small breccia bodies (e.g., the Cresson diatreme).The vein deposits as exemplified by those of the Ajax mine cut Precambrian crystalline rocks and Tertiary rocks of the diatreme complex. Within the Precambrian rocks the veins are radial to the margins of the diatreme system and are sheeted zones with rock dissolution and open-space fillings. Where the veins cut the Cripple Creek breccia, they are an irregular anastomosing fracture zone. The major veins exhibit remarkable vertical continuity, extending to more than 1,000 m below the present surface. Vein-related hydrothermal alteration occurs in a narrow selvage that extends outward no more than five times the vein width. Secondary K-feldspar, dolomite, roscoelite, and pyrite occur within an inner zone adjacent to the veins, whereas an outer zone contains sericite, montmorillonite, magnetite, minor secondary K-feldspar, and pyrite. There is no expansion of the alteration zones in the upper level mine exposures.Five stages of minerals are recognized in the Ajax mine veins: (1) quartz-fiuorite-adularia-pyrite-(dolomite-marcasite), (2) base metals-quartz-pyrite, (3) quartz-fluorite-pyrite-hematite-rutile, (4) quartz-pyrite-rutile-calaverite-acanthite, and (5) quartz-fluorite-dolomite. The proportions of each stage vary within and between veins, but the ore mineralogy is consistent throughout the vertical extent of the developed vein systems. Horizontally, gold values ranged between 0.5 and 1.0 oz Au per short ton.Fluid inclusion analyses have documented the presence of early stage 1 saline fluids (33- >40 equiv. wt % NaCl) with the higher salinities found in the upper 300 m of the Ajax mine levels; the fluids were boiling and contained CO ₂ . Stage 2 and 3 fluid inclusions exhibit progressively lower homogenization temperatures, and salinities are markedly lower (0-8.3 equiv. wt % NaCl). The telluride ore was deposited from weakly boiling, dilute fluids (1.4-3.5 equiv. wt % NaCl) with temperatures below 150 degrees C.The bulk tonnage deposits, as exemplified by the Globe Hill area, consist of mineralized tectonic and hydrothermal breccias cutting pyroxene-bearing alkali trachyte. Four structural events occurred at Globe Hill: (I) emplacement of hydrothermal breccia bodies along a north-west-trending 1,800- by 700-m zone; (II) intersecting tectonic adjustments along steep variable-strike zones on the western margin of the stage 1 breccias; (III) intrusive breccia emplacement at the major stage II fault intersection; and (IV) hydrothermal brecciation centered to the immediate east of the Globe Hill pit and characterized by a matrix consisting of anhydrite, carbonate, fluorite, pyrite, and base metal sulfides.Two hydrothermal events generated gold-silver mineralization and associated wall-rock alteration in the bulk tonnage deposits. The precious and base metals occur with alteration products in breccia clasts or in matrix minerals within the hydrothermal and tectonic breccias. The fluids responsible for alteration-mineralization were boiling as indicated by wide ranges of filling temperatures in fluid inclusions of the same mineral grain, extensive development of "explosion" texture in quartz and celestite, and large variations of liquid/vapor ratios in fluid inclusions within individual crystal growth zones. Temperatures were below 200 degrees C as indicated by minimum filling values. Capping of boiling shallow hydrothermal fluids appears to have been enhanced by the alkali trachyte porphyry intrusion at Globe Hill, which acted as a permeability barrier to upward-migrating fluids. Vapor-dominated fluids developed over-pressuring, leading to hydrothermal brecciation and low-grade gold deposits. On the other hand, the vein systems in the Cripple Creek district formed along structures open to the surface; hence, hydrothermal brecciation did not occur.

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