"Rock of the Month #175, posted for January 2016" ---

A dense, banded sulphide ore

from the Silverton mining camp in San Juan county, southwest Colorado, U.S.A. Hardwick (1984a) observed that certain of the mines near Silverton occur along a radial fracture system related to the Silverton caldera. In the San Juan Mountains, mineralization is associated with Tertiary-age volcanic structures such as the Silverton caldera. There is a regional pre-ore propylitization, and more restricted mineral-stage alteration within a few feet to tens of feet of ore. There are sulphides and native gold, plus such typical gangue phases as quartz, barite, calcite, ankerite and fluorite (Burbank and Luedke, 1968). Vein and carbonate replacement deposits are largely hosted in a caldera collapse breccia with large blocks of Carboniferous limestone. The "zebra ore" is found where feeder veins intersect the carbonate blocks. It is composed of quartz with bands of sulphides (pyrite, and then sphalerite, chalcopyrite and galena), which appear to replace a depositional or diagenetic fabric in the original limestone (Hardwick, 1984b). Certain bands in limestone of the intertidal facies of the Leadville Formation, containing stromatolitic laminae, may be preferentially infilled by "zebra" style mineralization (Hardwick, 1984b). The general model for the origin of the "zebra"-style ore is a replacement of the permeable Carboniferous strata after structural preparation (caldera formation) and magmatism in Miocene time.

In the Leadville district, uplift of the Sawatch Range in particular generated paleovalleys in a karst landscape, with solution cavities and rubble breccias (De Voto, 1983). Brines from overlying Carboniferous evaporites could have played a role in ore formation. Beaty et al. (1990) recognized dolostone permeability contrast as an ore control. Fine-grained beds in the Leadville district appear to be six orders of magnitude less permeable than coarse zebra textured beds, which in turn may be up to 2 orders of magnitude less permeable than manto ore bodies. The term "zebra texture" or "zebra ore" has been applied around the world to several lithological packages. However, in the present context, it refers to banded, dolomitic rocks.

One feature of associated alteration is zebra-banded dolomite (ibid., p.470). Emmons (1882) described the geology of the Leadville region in great detail, a work he continued, with further publications, for the rest of his life. The rich Pb-Ag ores of the area were first recognized in 1874, and active prospecting across the region began in the spring of 1877. In just 3 years, from 1877 to 1880, Leadville grew from a nucleus community (Agassiz) of <200 to a veritable city of 15,000. Argentiferous galena is by far the most important ore mineral, although cerussite, cerargyrite and anglesite are important secondary minerals. Other phases include pyromorphite, litharge and rarer minium, plus some native gold and sphalerite, plus many other species. Emmons undertook further work in the district until his death in 1911. A synthesis eventually emerged, prepared by his colleagues (Emmons et al., 1927), including a long list of minerals. Some of the local limestone displays zebra texture, white streaks and patches of coarse dolomite in bluish host rock (ibid., p.33), often found near orebodies, as on Carbonate Hill, Iron Hill, and in the Red Cliff district to the north. The white secondary dolomite in the zebra ore is explained by a recrystallization during ore deposition, eliminating carbonaceous matter present in the darker host (ibid., pp.176,219). The Leadville mining district has been described in detail, in terms of both geological and historical interest, by Emmons and his successors (e.g., Rickard, 1932, 1944).

There is an irregular karst surface on the Leadville limestone (Williams and Chronic, 2014). The dolomitization of Leadville limestone to produce zebra rock is well-etablished (e.g., Tweto 1968, pp.696-697). Manto or replacement bodies were known as "contact ore" because they occur in dolomite on the underside of porphyry sills or quartzite beds (Tweto, 1968). At Leadville,the zebra texture consists of alternating layers of diagenetic dolomite and coarse, white sparry dolomite. Petrography indicates that zebra texture at Leadville is intimately connected to the karst dissolution features: the sparry crystals formed during karst dissolution, from material dissolved from adjacent dolomite (Horton, 1989)

The Carboniferous Leadville limestone formation displays uniform beds of limestone, dolomite and chert. The results of a pioneering oxygen isotope study by Engel et al. (1958) are consistent with crystallization of the beds at near-ambient temperatures. In contrast, quartz and carbonates in the hydrothermal alteration halo of dolomite about the sulphide deposits at Gilman, north of Leadville, reveal elevated temperatures in ore and at conduits to the orebody. The formation of zebra rock, dark dolostone replaced by bands of coarser secondary white dolomite, generated stripes which tend to lie parallel to bedding, through they may diverge by 10-30 degrees or more. Vugs may occur, lined by crystals of dolomite, quartz and rarely pyrite. The host dolostone appears to be darkened by carbonaceous matter.

Other occurrences

Zebra rock has also been described at Gayna River in the Proterozoic of the Mackenzie Mountains in the Northwest Territories (Hewton, 1982, p.682). Zebra rock (striped crystalline dolomite) occurs also in the Metaline district of Washington state (McConnel and Anderson, 1968). At the Eugui-Asturreta magnesite deposit in the Western Pyrenees of Spain, the carbonate sequence also displays a zebra structure, with magnesite as a replacement of host dolostone (Lugli et al., 2000: see also Wallace et al., 1994).

Figure 3. Two polished thin sections of "zebra rock", image size 54x46 mm, clearly showing patches of banded mineral zonation.
Viewed in unpolarized transmitted light. The darker (yet not opaque) grains are mostly sphalerite.
Sections ZR-1 and (right) ZR-2, total polished area circa 18 cm².

Figures 4-5. Details from slices ZR-1 (left) and ZR-2. Two photomicrographs in reflected, plane-polarized light, nominal magnification 50X, long-axis field of view 1.7 mm. In thin section, the rock is seen to be roughly, in volume percent: 78% silicate matrix, dominated by turbid quartz, with minor iron-bearing carbonate; 2% subhedral pyrite; and 20% ore minerals of zinc with lesser lead and copper. The latter are opaque, apart from the abundant translucent brownish, isotropic sphalerite. Pyrite is the earliest sulphide in the paragenesis. At right it is seen to be partially replaced by galena (pale grey, with triangular cubic pits), and it may also be veined by chalcopyrite (much stronger yellow than pyrite, when the two are seen together). Sulphides and carbonate may infill open space around euhedral, hexagonal quartz prisms growing into voids, as at left. The sphalerite has also exsolved minor iron and copper from its crystal structure, to yield myriad micron-scale blebs of CuFeS₂ ("chalcopyrite disease", nicely displayed at left). Similar mineral assemblages are common, albeit generally without the zebra texture. An example is documented here from Guanajuato state, central Mexico, at the old mines of Mineral de Pozos.

---

References

Beaty,DW, Thompson,TB and Johansing,RJ (1990) The stratigraphic position, lithologic character, and preore dissolution features of ore-bearing beds in the Leadville Dolomite, Leadville-Gilman area. In `Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt' (Beaty,DW, Landis,GP and Thompson,TB editors), Economic Geology Monograph 7, 424pp., 118-129.

Burbank,WS and Luedke,RG (1968) Geology and ore deposits of the Western San Juan Mountains, Colorado. In `Ore Deposits in the United States, 1933-1967' (the Graton-Sales Volume, Ridge,JD editor). AIME, New York, 1880pp., 714-733.

De Voto,RH (1983) Central Colorado karst-controlled lead-zinc-silver deposits (Leadville, Gilman, Aspen, and others), a late Paleozoic Mississippi-Valley type district. In Proc.Internatl.Conf. on Mississippi Valley Type Lead-Zinc Deposits (Kisvarsanyi,G, Grant,SK, Pratt,WP and Koenig,JW, editors), Univ. of Missouri-Rolla, 603pp., 459-485.

Emmons,SF (1882) Geology and mining industry of Leadville, Lake county, Colorado. USGS 2nd Annual Report, 55+588pp., 201-290.

Emmons,SF, Irving,JD and Loughlin,GF (1927) Geology and Ore Deposits of the Leadville Mining District, Colorado. USGS Prof.Pap. 148, 368pp. plus 70 plates.

Engel,AEJ, Clayton,RN and Epstein,S (1958) Variations in isotopic composition of oxygen and carbon in Leadville limestone (Mississippian, Colorado) and in its hydrothermal and metamorphic phases. J.Geol. 66 no.4, 374-393.

Hardwick,JF (1984a) Epithermal vein and carbonate replacement mineralization in Cunningham Gulch, Silverton, Colorado. GSA Abs.w.Progs. 16, no.4, 224.

Hardwick,JF (1984b) Epithermal Vein and Carbonate Replacement Mineralization related to Caldera Development, Cunningham Gulch, Silverton, Colorado. MA Thesis, University of Texas at Austin, 125pp.

Hewton,RS (1982) Gayna River: a Proterozoic Mississippi Valley-type zinc-lead deposit. In `Precambrian Sulphide Deposits' (Hutchinson,RW, Spence,CD and Franklin,JM, editors), H.S. Robinson Memorial Vol., GAC Spec.Pap. 25, 791pp., 667-700.

Kile,D and Staebler,GA (editors) (2011) The San Juan Triangle of Colorado: Mountains of Minerals. Lithographie No.15, 105pp.

Horton,RA (1989) Origin of zebra texture in dolomite: evidence from the Leadville dolomite (Mississippian), central Colorado. GSA Abs.w.Progs. 21 no.5, Spokane, 95.

Lugli,S, Torres-Ruiz,J, Garuti,G and Olmedo,F (2000) Petrography and geochemistry of the Eugui magnesite deposit (western Pyrenees, Spain): evidence for the development of a peculiar zebra banding by dolomite replacement. Econ.Geol. 95, 1775-1791.

McConnel,RH and Anderson,RA (1968) The Metaline district, Washington. In `Ore Deposits in the United States, 1933-1967' (the Graton-Sales Volume, Ridge,JD editor). AIME, New York, 1880pp., 1460-1480.

Rickard,TA (1932) A History of American Mining. McGraw-Hill Book Company, Inc., New York, 1st edition, 419pp.

Rickard,TA (1944) The Romance of Mining. MacMillan Company of Canada Ltd, Toronto, 450pp.

Tweto,O (1968) Leadville District, Colorado. In `Ore Deposits in the United States, 1933-1967' (the Graton-Sales Volume, Ridge,JD editor). AIME, New York, 1880pp., 681-705.

Wallace,MW, Both,RA, Morales Ruano,S, Hach-Ali,PF and Lees,T (1994) Zebra textures from carbonate-hosted sulfide deposits: sheet cavity networks produced by fracture and solution enlargement. Econ.Geol. 89, 1183-1191.

Williams,F and Chronic,H (2014) Roadside Geology of Colorado. Mountain Press Publishing Company, Missoula, 3rd edition, xvi+399p.

Graham Wilson, 25,26 October 2015, 17-19 November 2015, 16 December 2015, 04-05,10,12,15 January 2016

Visit the Turnstone "Rock of the Month" Archives!

---

Class/Group/Family	List of Colorado "Rocks of the Month" through March 2016
	n=7
Inosilicates - pyroxenoids	---- #22 --- Pyroxmangite (`rhodonite') from the Sunnyside mine (San Juan county)
Oxides - Pb	--- #113 --- Minium from Leadville, Lake county, & Arizona
Quartzite	--- #174 --- Quartzite from Quandary Peak, Tenmile Range
Slags - Durango, CO	--- #176 --- Slag from the Durango mining district
Tellurides - Au Ag Te	--- #158 --- Gold-silver tellurides, Gold Hill area (Boulder county) and Sunnyside mine
Tellurides - Au Ag Te	--- #161 --- Gold-silver tellurides, Cresson mine, Cripple Creek
Zebra ore / zebra rock	--- #175 --- "Zebra rock", banded sphalerite-(galena- chalcopyrite) ore, Sunnyside mine

---