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Book Chapter

Gilman district

January 01, 1990


A multidisciplinary analysis of the manto-chimney ore complex at Gilman, Colorado, as, well as of the surrounding country rocks, has led to the development of a comprehensive mineralization model. This model is based on previous detailed geologic work (Radabaugh et al., 1968; Lovering et al., 1978) and on the five general topics addressed in the current study: the direction of movement of the ore-bearing fluid, the temperature distribution during the mineralization event, the age of ore deposition, the sources of the ore fluid and sulfur, and the chemical process which resulted in orebody formation. Results are reported in five separately authored sections, followed by a jointly authored discussion.

Direction of fluid movement

The hydrothermal streamlines associated with the movement of the Gilman ore fluid (both into and put of the region of ore deposition) were constrained by comparison of the mineralization and alteration effects in the country rock surrounding the manto-chimney ore complex. The rocks beneath the orebody, the host Leadville Dolomite downdip and adjacent to the orebody, and the rocks overlying the orebody were studied in detail.

Beneath the main Gilman orebody, deep drilling encountered Proterozoic rocks that locally are strongly veined and altered. Based on limited data (11 core holes), these mineralization effects appear to be most abundant beneath the chimneys and the No. 1 manto. No Tertiary intrusive rocks were encountered. The principal sulfide minerals in the deep veins are identical to those in the main orebody.

The Mississippian Leadville Dolomite down dip and lateral to the main orebody (exposed in 12 km of exploration workings and about 300 drill holes) contains more than a dozen small metal sulfide and carbonate deposits. These deposits generally decrease in size away from the chimney orebodies. The deposits have different metal abundances and, when considered together, define concentric metal zoning about the chimneys (successive outward halos of Cu-Ag-Au, Zn or Pb-Zn, and Mn), The structure and vein relationships in several of these deposits clearly suggests feeding from below.

The Laramide Pando Porphyry sill is of regional extent, was intruded along the upper contact of the Leadville Dolomite, and regionally shows weak propylitic alteration. Above the orebody, the base of the Pando is strongly altered locally, but the top is relatively fresh. The most altered rocks are spatially associated with the sulfide deposits in the Leadville Dolomite and are mineralogically zoned away from ore (phyllic to argillic to propylitic). There are no known hydrothermal effects in the Pennsylvanian Belden and Minturn Formations above the Pando sill.

These geologic relations argue that the ore fluid entered the region of ore deposition from below and ascended through the section until it reached the Pando Porphyry sill. Fluid flow was then channeled updip within the Leadville Dolomite, capped beneath the Pando sill.

Temperature distribution during mineralization

Sulfur isotope fractionation between the ore minerals indicates temperatures of about 400° C (chimney ore), 300° to 610°C (No. 1 manto), and 230° to 520°C (mantos in the Sawatch Quartzite). Fluid inclusions in one sample of main-stage sphalerite homogenize at 391° to 413°C, and those in late vug apatite from the chimneys homogenize at 309°C. A large fission-track paleothermal anomaly surrounds the orebody; the anomaly is detectable in apatite and zircon from the Proterozoic basement, the Pando Porphyry, and the Minturn arkoses above the orebody. This anomaly is measurable in both apatite and zircon, extends to a distance of about I km from ore, and indicates heating to more than 150° to 250°C. The region of maximum heating inferred from paleothermal data is located northeast of the chimney deposits rather than beneath the orebody. This is inferred to be the location of an nnexposed stock.

Age of ore deposition

Because the Pando Porphyry is of Laramide age and is hydrothermally altered, the manto-chimney ore complex at Gilman must have formed after the crystallization of that sill. Apparent ages determined by fission-track dating within the paleothermal region at Gilman are as follows: Pennsylvanian arkose above ore (35.9 ± 3.0 Ma), Proterozoic granite below ore (35.9 ± 2.8 Ma), and Laramide Pando Porphyry (30—40 Ma, large uncertainties). Late-stage hydrothermal apatite was dated at 34.5 ± 2.2 Ma. The average of these age determinations is 35.8 ± 2.0 Ma, which is interpreted to be the age of the main orebody.

Sources of ore fluid and sulfur

The ore-forming hydrothermal fluid at Gilman was both high 18O and high D and is interpreted to be magmatic water. The strongly altered Paudo Porphyry (δ18O = 9—10‰) recrys-tallized in the presence of a fluid with a δ18O value of 5 to 6 per mil. Dolostone in the manto wall rocks has a δ18O value of about 16 per mil, as contrasted to a regional value of about 25 per mil, indicating a δ18Ofluid value of 8 per mil. Vug quartz (δ18O = 14.4‰) and apatite (δ18O = 7.6‰) in the orebody, vug quartz in the wall rock (δ18O = 16.2-17.1‰), and vein quartz in manto ore (δ18O = 14.1‰) all formed from fluids with δ18O values of 7 to 9 per mil. The altered Proterozoic granite beneath the main orebody (δ18O = 11.7-12.5‰) and the veins which cut it (δ18Oquariz = 14.0‰, δ18OdoiomitB = 13.1‰) indicate a δ18Ofuid value 6f 4 to 5 per mil. Mnscovite from phyllically altered porphyry (δD = —93 to —97‰) indicates a δDFLUID value of—45 to —59 per mil. A magmatic hydrothermal interpretation for this fluid is supported by the presence of possible magmatic carbon (δ13C = —7.8‰) in the deep parts of the Gilman system. On the periphery of the high 18O hydrothermal system, meteoric hydrothermal effects are indicated by analyses of jasperoid (δ18O = 10.3‰, δ18OEu!d ≈ — l‰) and kaolinite (calculated δDfluid = -115 to -149‰; δ18Oflllid = -1.5 to -6.3‰).

The δ34S values for pyrite (0.6-2.7%«), sphalerite (1.3-2.4‰), galena (-1.3 to -2.0‰), and chalcopyrite (0.8-1.9‰) show very little variation. Not only is there no measurable geographic variation in δ34S valnes across the deposit, the mean δ34S values of the three paragenetic stages of pyrite are isotopically indistinguishable (pyrite I, 1.5‰; pyrite II, 1.7‰; pyrite III, 1.8‰). Because the Gilman sulfur reservoir is well homogenized and has a bulk system δ34S value of about 1.6 per mil, an unexposed igneous source is strongly indicated.

Chemical process of ore formation

Using three simplifying assumptions (constant temperature, constant fluid pressure, a 12-element fluid), a reaction path model for the deposition of the principal mantos was calculated. Comparison of the predicted mineral paragenesis and deposit zoning with that actually observed constrains geochemically feasible reaction pathways. The model suggests that the ore fluid was initially characterized by intermediate salinity, low pH, low sulfide activity, high activities of iron and carbonate, and moderately low oxygen fugacity. The orebody apparently developed in four stages. Early pyrite deposition resulted from oxidation of the hydrothermal fluid (stage A), probably by mixing with oxygenated meteoric water. Then pyrite and siderite were precipitated by a reaction involving both dissolution of dolomite and oxidation (stage B). Next, previously formed siderite reacted with fresh hydrothermal fluid to produce pyrite (stage C). Finally, additional fresh hydrothermal fluid reacted with previously formed siderite and pyrite to form sphalerite, chalcopyrite, and galena (stage D). Sequential mineralization thus resulted in the observed spatial zoning about points of fluid ingress.

Ag-Pb-Zn ores

Sparse, paragenetically early Ag-Pb-Zn deposits have been identified at several localities in the Gilman area. These deposits have been partially assimilated by the main-stage ores in the center of the district but are well preserved on the periphery (especially at Red Cliff). These deposits are distinctive mineralogically (amber sphalerite, local early white barite) and geochemically (Au free), and in some eases they fit Behre’s (1953) criteria for “Sherman-type” deposits. Some of these Ag-Pb-Zn deposits formed by open-space filling of paleocaves in the upper Red Cliff Member, others formed by replacement of coarse-grained dolostone in the upper Castle Butte Member. The constituent minerals proved unusable for fluid inclusion study, but oxygen and sulfur isotope fractionations correspond to temperatures of about 150°C and 200° to 340°C, respectively. The age of this mineralization event is poorly constrained at Gilman but is known from overgrowth relationships to predate the main mineralization event (35.8 ± 2.0 Ma). These deposits were precipitated from a fluid with a large component of meteoric water (δ18Ofloid ≈-4 to —5 per mil), and they apparently contain sulfur derived from both sedimentary and magmatic sources. Current data are insufficient to prove any single mineralization model for these deposits.

Genetic model

Scattered small Ag-Pb-Zn deposits formed in the Gilman area in unknown abundance at an unknown time early in the geologic history of the area.-These deposits were later overprinted and/or destroyed by a high-temperature (350°-400°C), mid-Tertiary (35 Ma) hydrothermal system which deposited the main orebody. The mid-Tertiary system was genetically related to an unexposed stock, which is inferred on the basis of paleothermal data to be located northeast of the chimneys. The ore fluid was magmatic water which was released from this crystallizing intrusion. Most of the hot, metal-bearing hydrothermal fluid apparently flowed up dip within the basal Cambrian Sawatch Quartzite from the thermal center to the site of the chimneys. There, the fluids turned and flowed up-section to the Leadville Dolomite, where they entered the Leadville aquifer. The fluids then flowed updip within the Leadville Dolomite, capped beneath the Pando Porphyry, and formed the principal manto deposits.

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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
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Tommy B. Thompson
Tommy B. Thompson
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Society of Economic Geologists
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Publication date:
January 01, 1990




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