Preliminary investigation of the elemental variation and diagenesis of a tabular uranium deposit, La Sal Mine, San Juan County, Utah
Ore in the La Sal mine, San Juan County, Utah, occurs as a typical tabular-type uranium deposit of the-Colorado Plateau. Uranium-vanadium occurs in the Salt…
Public-domain full text preserved in the Mountain Man Mining Library. Original source: pubs.usgs.gov.
UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Preliminary investigation of the elemental variation and diagenesis of a tabular uranium deposit, La Sal mine, San Juan County, Utah By Robert A. Brooks and John A. Campbell Open-File Report 76-287 This report is preliminary and has not been edited or reviewed for conformity with U.S. Geological Survey standards and nomenclature.
Contents Page Abstract-- Introduction Structural setting Stratigraphic setting Location of samples Analytical Results - Chemical
Heavy minerals--
Petrography Composition Di agenesis Discussion of Results Vanadium and uranium Selenium Molybdenum Pyrite Carbon Chromium -- Other elements
Summary of Results
References cited
Illustrations Figure 1.--Index and location maps of the La Sal mine area, San Juan County, Utah 2.--Generalized structural map of the La Sal mine and La Sal Creek Mineral Belt 3.--Uranium and vanadium content of samples 4.--Heavy mineral content of samples 5.--Photomicrographs showing sandstone textures 6.--Distribution of elements in some roll-type uranium deposits 16 7.--Selenium content of samples 19 8.--Molybdenum content of samples 20 9.--Iron content of samples - 10.--Late-stage pyrite formation 23 11.--Carbon content of samples 24 12.--Chromium content of samples 25 13.--Barium and manganese content of samples 27 14.--Lead content of samples -- Tables Table 1.--Chemical analysis of whole-rock samples across tabular ore layer 8 2.--Correlation matrix of whole-rock elemental concentrations -- 3.--Heavy-mineral content of samples 10 4.--Mineralogic composition of samples 13 5.--Distribution of diagenetic events 17
Preliminary investigation of the elemental variation and diagenesis of a tabular uranium deposit, La Sal mine, San Juan County, Utah By Robert A. Brooks and John A. Campbell ABSTRACT Ore in the La Sal mine, San Juan County, Utah, occurs as a typical tabular-type uranium deposit of the Colorado Plateau. Uranium-vanadium occurs in the Salt Wash Member of the Jurassic Morrison Formation. Chemical and petrographic analyses were used to determine elemental variation and diagenetic aspects across the orebody. Vanadium is concentrated in the dark clay matrix, which constitutes visible ore. Uranium content is greater above the vanadium zone. Calcium, carbonate carbon, and lead show greater than fifty-fold increase across the ore zone, whereas copper and organic carbon show only a several-fold increase. Large molybdenum concentrations are present in and above the tabular layer, and large selenium concentrations occur below the uranium zone within the richest vanadium zone. Iron is enriched in the vanadium horizon. Chromium is depleted from above the ore and strongly enriched below. Elements that vary directly with the vanadium content include magnesium, iron, selenium, zirconium, strontium, titanium, lead, boron, yttrium, and scandium. The diagenetic sequence is as follows: (1) formation of secondary quartz overgrowths as cement; (2) infilling and lining of remaining pores with amber opaline material; (3) formation of vanadium-rich clay matrix, which has replaced overgrowths as well as quartz grains; (4) replacement of overgrowths and detrital grains by calcite; (5) infilling of pores with barite and the introduction of pyrite and marcasite.
INTRODUCTION The La Sal mine is located 1.6 km east of the small town of La Sal, in northeastern San Juan County, Utah (fig. 1). It can be reached from Moab, Utah, by taking U.S. Highway 160 38 km south to Utah State Highway 46, then following Highway 46 about 16 km east to La Sal. This mine was examined as part of a reconnaissance study of tabular uranium-vanadium ore deposits of .the Colorado Plateau. Samples were collected along traverses that transected obvious orebodies; however, the orebodies were not mapped in detail. The representativeness of the sample suite described has not been determined; however, the data derived from the sample suite are similar to data derived from other uranium deposits on the Colorado Plateau as well as from other districts. Several people have aided with different phases of this work. Thanks are due to R. P. Fischer and J. Ridgely of the U.S. Geological Survey, Denver, and to C. E. Nichols of Union Carbide Corporation. Structural setting The geology of the area in the vicinity of the La Sal mine has been described by Carter and Gualtieri (1965). The mine is located within a structurally complex area. Salt from the Paradox Member of the Pennsylvanian Hermosa Formation locally has been diapirically injected into a series of northwest-southeast-trending salt anticlines. Faulting accompanying these intrusions and solution features of the salt anticlines have added to the complexity of the area (fig. 2). The La Sal mine lies on the south flank of the Pine Ridge salt anticline and on the north .flank of the East Coyote Wash syncline. Twelve km to the east of the La Sal mine, a considerable amount of uranium-vanadium mining took place along the La Sal Creek Mineral Belt (Carter and Gualtieri, 1965). The La Sal Creek Mineral Belt lies on the north side of the Pine Ridge salt anticline, and very little mining activity has occurred on the south side. The La Sal mine lies along a straight line extension of the La Sal Creek Mineral Belt, but is separated from the belt (as defined by Carter and Gualtieri, 1965) by a large solution-collapse area on the crest of the salt anticline.
Salt Lake City® Provo Price Moab Utah Area of this report 109* 39°- 38°- Utah N 50KilometresL MontjceIIo 30Miles Blending Figure I. Index and location map of the La Sal mine area, San Juan County, Utah.
Figure 2. Generalized structural map of the La Sal Mine and La Sal Creek Mineral Belt (After Carter and Guoltieri, 1965)
pguif (ooshed where approximately tocofedj dotted where concealed. U, upthrown side; D, downthrown side.) Outline of area where bedrock has collapsed by solution of underlying salt moss. Anticline showing aestline and direction of plunge. Syncllne showing troughline and direction of plunge. Area underlain by uranium- vanadium deposit. 0915' 109" O' 109*05' I09°00' 38*25' - 38*20' 38*15'
The possibility of such a western extension of the La Sal Mineral Belt was suggested by Carter and Gualtieri (1965). Stratigraphic setting The ore in the La Sal mine occurs in the fluvial sandstones of the Salt Wash Member of the Jurassic Morrison Formation. The Salt Wash is composed of a sequence of interbedded fine-grained sandstones and si Itstones. Three recognizable sandstone layers within the member form erosion-resistant ledges separated by intervening softer siltstones. The upper layer or "third ledge" is usually the ore-bearing layer. The ore in the La Sal mine occurs as a 25-cm-thick tabular body that parallels bedding within the "third-ledge" sandstone layer. Location of samples Ore occurs in a small raise above the main haulage way and is in the uppermost portion of the third-ledge sandstone. A visibly distinct, stained, horizontal layer is thought to delineate the vertical extent of the orebody. A suite of eight grab samples was collected across the stained layer in order to determine the geochemical and mineralogic nature of the mineralization and alteration. Samples were centered every 15 cm (6 in.) along a traverse perpendicular to the stained layer. Samples 9 through 12 were collected above the stained layer, samples 13 through 15 were collected within the layer, and sample 16 was taken below the stained layer. Sampling was restricted by the dimensions of the mine. ANALYTICAL RESULTS Chemical Figure 3 shows the uranium and vanadium content of the samples. It can be seen that the staining mentioned above indicates the position of the vanadium ore but not of the uranium-rich sandstone. The vanadium concentration reaches 15,000 ppm in the stained zone, but the uranium concentration remains quite low. The uranium is more concentrated above
CD Uranium -o- Vanadium PPM U Figure 3. Uranium and vanadium content of samples. Brockets refer to samples with maximum concentrations of uranium and vanadium. Samples taken over 105 cm. intervals.
the vanadium ore layer, where it reaches a maximum concentration of 3,000 ppm. Separation of the uranium and vanadium zones of maximum mineralization is quite common in the uranium-vanadium ores of the Colorado Plateau, and sampling on the basis of color and staining can result in sampling the vanadium and missing the uranium ore (R. P. Fischer, oral commun., 1975). Results of chemical analysis by emission spectroscopy of the sample suite are given in table 1. Most elements analyzed show significant variation across the ore layer. The concentrations of calcium, carbonate carbon, and lead show greater than fifty-fold increases, whereas copper and organic carbon show only several-fold increases. A good deal of variation, especially where the concentration of an element varies directly with vanadium content, is due to the facts that in the vanadium-rich zone the detrital sand grains are corroded and the interstitial pore spaces are filled with a matrix composed of compounds of several elements. In contrast, the vanadium-poor sands have not undergone corrosion, but rather have developed more normal interstitial quartz and calcite cements and overgrowths. Although not statistically significant, some elements seem to vary directly with the vanadium content: magnesium, iron, selenium, zirconium, strontium, titanium lead, boron, yttrium, and scandium. Table 2 gives the linear-regression correlation matrix of all the elements measured. Heavy minerals Table 3 and figure 4 show the content of the heavy-mineral fraction as determined by X-ray diffraction. There is a significant decrease in the relative amounts of heavy minerals from top to bottom in the sample suite. Pyrite is present in all samples, but no other heavy minerals are ubiquitous. The ore minerals uraninite, coffinite,- cuprosklodowskite, and epiianthinite are found in one or two samples. Lead minerals clausthalite and galena are also present. Samples below the ore contain a normal suite of detrital heavy minerals including brookite, zircon, anatase, and rutile, but these minerals are not found in the upper zones.
[All data were determined by analytical laboratories, U.S. Geological Survey. Except as all elements were determined by a L. A. Bradley, analyst. Total Fe Total carbon was determined by a Carbonate analyst. 'N Not noted , semiquantitative six-step spectrographic technique; was determined by atomic combustion-conductometric carbon was determined by a H,PO. Organic carbon was determined by detected or at 2L Detected, but below Sample No U Total Fe Mg Ca T1 percent percent as Fep3 percent percent percent N' (X) Total C as C, percent Carbonate percent Organic C percent Mn Ag B Ba Be Co Cr Cu Mo N1 Pb Sc Se Sr Y Zr C as C, as C, ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm PPm ppm ppm N
limit of limit of N N N N N , N detection. absorption technique; ; W. Mount joy, analyst. V. E. Shaw digestion-gasometric technique; T. L. differences. , analyst. Yager, determination.] N N N N N N N N :
N
N N N N N N
Table 2. Correlation matrix of whole-rock elemental concentrations Mg Mg Ca T1 Mn Ag Ba Co Cr Cu Mo Ni Pb
Sr U Zr Se Fe2°3 TC 1 OC 2 Ca T1 -.51, Mn Ag Ba Co Cr Cu Mo N1 Pb Sr U Zr Se Fe203 TC OC iTC, total carbon. 2OC, organic carbon.
Table 3.--Heavy-mineral content of samples Sample No. -9-11-7 Coffinite RavitQ
Y Apatite y y CuproskloUWvorw 1 lC A Epi-ianthinite Clausthalite Brookite CnkiaT ov* i ~ho oJJria Icr 1 Lc Uraninite £ 1 Dii + ila
Anatase
tr LU CO £o 9h U Figure 4. Heavy mineral content of samples, In percent. Brackets refer to samples with maximum concentrations of uranium and vanadium, Samples taken over 105 cm interval.
Petrography The petrographic relations within the Morrison Formation and within selected areas of the Salt Wash Member have been described by several authors (Hausen, 1959; Cadigan, 1967; Shawe, 1968; Hafenfield and Brookins, 1975). The average mineral composition of the Salt Wash Member sandstones, based on 202 samples throughout the Colorado Plateau, is 71.4 percent quartz, 4 percent quartz overgrowth, 5.8 percent chert, 5.8 percent potassium-feldspar and plagioclase, and 13 percent calcite (Cadigan, 1967). Composition Table 4 shows the mineral composition of the La Sal mine suite of samples. The data are derived from point counts of 100 grains for each sample. This suite is similar to the average Salt Wash Sandstone composition except that it is generally depleted in calcite. In the vanadium-rich samples, the detrital grains have been corroded and the original grain composition is hard to determine. The greatest variation in composition is in the amount of matrix material. Matrix material here is rather loosely defined as an interstitial clay, which in plain light is dark brown to black. This matrix contains vanadium mica, roscoelite, and illite, with minor amounts of kaolinite, chlorite, and mixed-layered illite-montmorillonite. The presence of several of the elements in table 2 also correlates with the abundance matrix material. The process of emplacement of the matrix has been quite corrosive to the original grains, having removed most of the smaller grains, embayed larger quartz grains, and removed secondary overgrowths (fig. 5).
Table 4.--Mineralogic composition of samples [/ Minerals present in amount less than 1 percent] Sample No. Common quartz-- Metaquartzite Quartz overgrowths Sedimentary rock fragments- Volcanic Calcite i jr i i lc Heavies
Opal Void Other Total / / / / / / / / / / / / / / / / / / / /
Figure 5. Photomicrographs showing sandstone textures. a Sample 16-11-7 shows detrital texture only slightly modified by alteration (IOX). b. Sample 13-11-7 shows destruction of detrital components by ore emplacement (IOX).
Diagenesis Table 5 shows the general distribution of diagenetic events as determined from thin sections. A similar sequence of events is visible in all samples; however, not all of the events are apparent in all samples. The sequence begins with the formation of secondary overgrowths on the quartz grains. This secondary quartz is the primary cementing agent in most of the samples. The second event is the introduction of an amber material, which is primarily opal but contains significant amounts of Fe, Ti, and other elements, and which fills or lines voids left after cementation. The composition of this material is presently being analyzed and will be reported later. The third event is the formation of the dark clay-matrix material containing montroseite and other vanadium minerals. Some minor clay minerals, including illite in some samples and kaolinite in others, may have formed earlier. In samples 13, 14, and 15, the dark-clay matrix is so abundant that it obscures evidence of previous events. This matrix material is followed by the introduction of calcite, which grows in the amber material and replaces all other grains and overgrowths. Filling of remaining pores by varite occurs next. In the samples below the ore, a final event is the introduction of pyrite and marcasite. DISCUSSION OF RESULTS Harshman (1974) discussed the distribution of elements in roll-type uranium deposits in Wyoming, Texas, and South Dakota. In addition to uranium, four other elements were particularly significant in terms of the roll model. Figure 6 shows the distribution of the four most significant elements in roll-type deposits plus the distribution of pyrite, which is interpreted to represent all ferrous iron compounds. Selenium and vanadium occur on the altered or oxidized side of the maximum uranium peak, and molybdenum and pyrite occur on the unaltered or reduced side of the uranium ore zone. Shawe (1966) noted the similarity of distribution of eU, U, V, Cu, Pb, Zn, As, and Se in both tabular and roll-type uranium deposits and concluded that both types of deposits were formed by precipitation at the interface between two solutions of different composition. In the La Sal suite
Unaltered facies Karnes County Karnes County Selenium Vanadium Molybdenum Uranium Pyrite Shirley Basin 40 Feet 10 Metres Gas Hills Live Oak County Horizontal Distance Figure 6 Distribution of elements in some roll-type uranium deposits. (Harshman, 1974).
Table 5.--Distribution of diagenetic events Sample No. Quartz overgrowth--- Early clay formation rAI 1 1 U C 1 Matrix opaque ore l Jr I Order of diagenetic event
of samples, abundances of some of the elements vary in a manner similar to their variation in roll-type deposits. Vanadium and uranium Figure 3 shows the maximum content of vanadium to be below the maximum uranium concentration. The concentration of vanadium is 15,000 ppm or 1.5 percent, whereas the maximum vanadium content of the roll-type deposits was found to be only about 1,500 ppm. If Harshman's concept of the migration or movement of elements in the formation of roll-type deposits can be extrapolated to tabular deposits such as the La Sal, the lower side of the ore layer with the high vanadium content would be the oxidized side of the "front," and the upper side of the uranium layer would be the reduced side. Selenium Like vanadium, selenium reaches its maximum concentration below the uranium layer (fig. 7). The maximum selenium concentration is 1,840 ppm in the La Sal suite, but the maximum selenium content in the roll-type deposits is about 1,000 ppm (Harshman, 1974). If the analogy of the roll-type deposits is applied to this tabular deposit, the oxidized side is below the uranium layer. Unlike the toll-type deposits, however, the selenium content does not seem to be an order of magnitude greater in the altered side than in the unaltered side. This variation may be a result of the limited distance away from the ore that samples were available. In the La Sal suite the selenium correlates very well with the lead content, because the selenium is largely tied up in the lead selenide, clausthalite (table 3). Molybdenum -V, Harshman (1974) showed that, in uranium roll-type deposits, the
molybdenum content increases at some distance from the ore in the unaltered or reduced facies. In the La Sal suite, the molybdenum occurs
in and above the uranium layer (fig. 8). This distribution, by analogy with the roll-type model, indicates that the zone above the ore is the
a: UJ CD2 E i l2 o in y U PPM Figure 7.--Selenium content of samples. Brackets refer to samples with maximum concentrations of uranium and vanadium. Samples taken over 105 cm interval.
u PPM Figure 8.--Molybdenum content of samples. Brockets refer to samples with maximum concentrations of uranium and vanadium. Samples taken over 105 cm. interval,
unaltered or reduced fades. The maximum molybdenum content in the La Sal suite is 50 ppm, whereas Harshman found molybdenum concentrations as great as 1,000 ppm; however, this may be attributed to the limited sampling. Pyri te Harshman (1974) also showed that the ferrous iron as pyrite was higher in the unaltered facies than in the altered facies. Figure 9 shows that the iron content in the La Sal suite is greater below the ore, or in the altered facies. However, in the La Sal suite, a late-stage diagenetic event (fig. 10) added iron and makes the iron content undiagnostic. Pyrite of several generations is present in all samples. Carbon The distribution of organic and mineral carbon (fig. 11) shows that the carbonate carbon is significantly greater both above and below the ore layer. Most of the mineral carbon occurs as calcite, and the calcium varies almost directly with the mineral carbon; organic carbon, on the other hand, increases from 0.2 percent above the ore to 0.8 percent within the vanadium zone and decreases to 0.7 percent below the ore zone. Chromium Chromium is low above the ore layer and strongly enriched below (fig. 12). On the Colorado Plateau, chromium-rich zones are quite common in ores of the Entrada and Navajo Formations, but are uncommon in the Morrison Formation (H. C. Granger, oral commun., 1974). The regular distribution of chromium in the Navajo sandstone ores was discussed by Fischer (1960). Anomalous chromium has not been reported in the roll-type deposits of Texas and Wyoming.
-T- LJ CD in K o LJ C/5 U % Iron Figure 9.--Iron content of samples. Brackets refer to samples with maximum concentrations of. uranium ond vcnadium. Samples taken over 105 cm Interval.
Figure 10. Late-stage pyrite formation. (Pyrite is black mineral corroding quartz grain, 25X.)
o Carbonate --o Organic % Carbon U Figure ii.--Carbon content of samples. Brackets refer to samples with maximum concentrations of uranium and vanadium. Samples taken over 105 cm interval.
U PPM Figure 12. Chromium content of samples. Bracket refer to samples with maximum concentrations of. uranium and vanadium. Samples taken over 105 cm interval.
Other elements Barium and manganese are enriched in the upper part of the sample suite (fig. 13). Several other elements, including copper, cobalt, nickel, silver, titanium, boron, beryllium, yttrium, and scandium, are generally enriched in the vanadium-rich portion of the ore. Lead concentration increases greatly in the vanadium-rich samples (fig. 14). This lead increase parallels the increase of selenium and is due to the presence of clausthalite (PbSe) in the vanadium zone.
105 cm. SAMPLE NUMBER O o.
lu 3p o ° I'S 3 w 3. CJI Oo o ff eno : o
IN5 IO £ O LJ CO 16U PPM Figure isb.--Manganese content of samples. Brackets refer to samples with maximum concentrations of uranium and vanadium. Samples taken over 105 cm interval.
u PPM Figure u Lead content ot samples. Brockets refer ID samples with maximum concentrations of uranium and vanadium. Samples taken over 105 cm interval.
SUMMARY OF RESULTS 1. Vanadium is concentrated in the dark-clay matrix, which constitutes visible ore. Uranium content is higher in the zone above the vanadium zone. 2. Calcium, carbonate carbon, and lead show greater than fifty-fold variations across the ore zone. 3. Copper and organic carbon show several-fold increase across the ore zone.
4. Elements that vary directly with vanadium content include magnesium, iron, selenium, zirconium, strontium, titanium, lead, boron, yttrium, and scandium. 5. Molybdenum occurs in and above the tabular layer. 6. Selenium occurs below the uranium zone within the richest vanadium zone. /.Carbonate carbon is more abundant on both sides of the ore layer and depleted within it. 8. Iron is enriched in the vanadium zone. 9. Chromium is depleted from above the ore and strongly enriched below. 10. Barium and manganese are most abundant in the upper part of the zone. 11. The diagenetic sequence, as determined from thin sections, is as follows: (1) formation of secondary quartz overgrowths as cement; (2) the infilling and lining of remaining pores by amber quartz; (3) the formation of the dark-colored vanadium-rich clay matrix, which has replaced overgrowths as well as quartz grains. This event obscures all previous events in the richest vanadium ores; (4) introduction of calcite, replacing overgrowths and detrital grains and growing in amber material. This phase does not occur in the richest ore zones; (5) the infilling of pores with barite and the introduction of pyrite and marcasite. 12. The ore-forming processes produced the following textural and mineralogical changes in the sandstones: (1) marked increase in matrix content; (2) decrease in quartz overgrowths; (3) decrease in detrital grains, especially quartz, but also various rock fragments; (4) decrease in carbonate content.
References cited Cadigan, R. A., 1967, Petrology of the Morrison Formation in the Colorado Plateau region: U.S. Geol. Survey Prof. Paper 556, 113 p. Carter, W. D., and Gualtieri, J. L., 1965, Geology and uranium-vanadium deposits of the La Sal Quadrangle, San Juan County, Utah, and Montrose County, Colorado: U.S. Geol. Survey Prof. Paper 508, 82 p. Fischer, R. P., 1960, Vanadium-uranium deposits of the Rifle Creek area, Garfield County, Colorado: U.S. Geol. Survey Bull. 1101, 52 p. Hafenfeld, S. R., and Brookins, D. G., 1975, Mineralogy of uranium deposits northeast of Laguna District, Sandoval County, New Mexico: AnuAssoc. Petroleum Geologist Bull., v. 59, no. 5, p. 910-911. Harshman, E. N., 1974, Distribution of elements in some roll-type uranium deposits, in Formation of uranium ore deposits: Internat. Atomic Energy Agency, Vienna, Proc. p. 169-183. Hausen, D. M., 1959, Some mineralogical, textural and paragenetic relations in Salt Wash ore deposits in the Uravan area, Colorado; in National Western Mining and Energy Conference: Colorado Mining Assoc., 9 p. Shawe, D. R., 1956, Significance of roll-orebodies in genesis of uranium-vanadium deposits on the Colorado Plateau, jn Page, L. R., Stocking, H. G., and Smith, H. B., compilers: U.S. Geol. Survey Prof. Paper 300, p. 239-241. 1966, Zonal distribution of elements in some uranium-vanadium roll and tabular orebodies on the Colorado Plateau: U.S. Geol.. Survey Prof. Paper 550-B, p. 169-175. 1968, Petrography of sedimentary rocks in the Slick Rock District, San Miguel and Dolores Counties, Colorado: U.S. Geol. Survey Prof. Paper 576-B, 34 p.