Silver Bell mine

Producer in Pima county in Arizona, United States with commodities Copper, Molybdenum, Silver

Geologic information

Identification information

Deposit ID 10161929
MRDS ID D000333
MAS/MILS ID 0040190009
Record type Deposit
Current site name Silver Bell mine
Alternate or previous names North Silver Bell pit, El Tiro pit, West Oxide pit, East Oxide pit

Comments on the site identification

  • The Silver Bell Mine is part of the Silver Bell Distric. Also included in this district are the North Silver Bell Mine (10039895), Oxide Mine (10039685), El Tiro Open Pit Mine (10039684)

Geographic coordinates

Point of reference Ore Body
Geographic coordinates: -111.5015, 32.39371 (WGS84)
Elevation 884
Location accuracy 1000(meters)
(click for info)

Geographic areas

Country State County
United States Arizona Pima

Public Land Survey System information

Meridian Township Range Section Fraction State
Gila and Salt River 012 S 008 E 11 S2 Arizona

Commodities

Commodity Importance
Copper Primary
Molybdenum Tertiary
Silver Secondary

Materials information

Materials Type of material
Chalcocite Ore
Chalcopyrite Ore
Molybdenite Ore
Cuprite Gangue

Mineral occurrence model information

Model code 79
USGS model code 21a
Deposit model name Porphyry Cu-Mo
Mark3 model number 2

Host and associated rocks

  • Host or associated Host
    Rock type Plutonic Rock > Granitoid > Quartz Monzonite
    Rock unit name El Tiro and Imperial stocks
    Stratigraphic age (youngest) Early Paleocene
    Stratigraphic age (oldest) Late Cretaceous

Nearby scientific data

Ore Body (1) -111.5015, 32.39371

Geologic structures

Type of structure Local
Structural terms Fracturing

Ore body information

  • Thickness 90M
    Depth to top 60M
    Width 1260M
    Length 1260M
    Area 840HA
    Field Value
    Type of Orebody #1 STOCKWORK
    Shape of Orebody #1 MASSIVE
    Type of Orebody #2 DISSEMINATED
    Primary mode of Origin HYDROTHERMAL
    Secondary mode of Origin OXIDATION
    Primary Ore Control FRACTURING
    Secondary Ore Control IGNEOUS
    Degree of Wallrock Alter. MODERATE
    Type of Wallrock Alter. #1 SERICITIC
    Type of Wallrock Alter. #2 SILICIFICATION
    Type of Wallrock Alter. #3 ADV ARGILLIC
    Avg. Thick. Unconsol. Mat. 0
    Total Surface Area (HA) 840
    Date of Last Modification 830728

Controls for ore emplacement

  • Fracturing:

Economic information

Economic information about the deposit and operations

Operation type Surface
Development status Producer
Commodity type Metallic
Significant Yes
Discovery year 1909
Mining method Open Pit
Year of first production 1954
Milling method Flotation

Mining district

District name Silver Bell

Mineral rights holdings

Type of mineral rights Patented

Land status

Ownership category Private

Ownership information

  • Type Owner-Operator
    Owner Asarco (American Smelting And Refining Co.)
    ID 200134
    Interest 100
    Home office New York
    Year 1972
    First year 1954
  • Type Joint Venture
    Owner MITSUI AND CO LTD
    Interest 25
    Year 2009
    First year 1996
  • Type Operator
    Owner ASARCO INC (GRUPO MEXICO is 100% owner)
    Home office 5285 East Williams Circle Suite 2000 Tucson AZ 85711 United States
    Year 2010
    First year 1999

Production statistics

  • Year 1972
    Period 1954-1972
    Material Probably from old_MRDS Silver Bell District record D000333, source is probably Galey, 1979.
    Ore mined 54,200,000mt
    Accuracy Accurate
    Best? Y
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Primary Copper Copper 1wt-pct
    Secondary Silver Silver 2g/mt
    Tertiary Molybdenum Molybdenum 0wt-pct
  • Year 1978
    Description Fd Grd: 0.65%Cu, 0.017%Mo, 2.5g/Mtag 3239000 Mt Ore/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Silver Silver 3g/mt
    Major Copper Copper 1wt-pct
    Trace Molybdenum Molybdenum 0wt-pct
  • Year 1978
    Description Conc. Grd: 24% Cu, 86.3 G/Mt Ag 69312 Mt Conc/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Conc. Silver Silver 86g/mt
    Major Conc. Copper Copper 24wt-pct
  • Year 1978
    Description Leach Ppt Grd: 72% Cu 3963 Mt Leach Ppt/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Copper Copper 3,963mt 72wt-pct
  • Year 1978
    Description Mos2 Conc Grade: 42.3% Mo 143 Mt Mos2 Conc/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Conc. Molybdenum Molybdenum 143mt 42wt-pct
  • Year 1979
    Description Fd Grd: 0.66%Cu, 0.013%Mo, 2.3g/Mtag 3375000 Mt Ore/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Silver Silver 2g/mt
    Major Copper Copper 1wt-pct
    Trace Molybdenum Molybdenum 0wt-pct
  • Year 1979
    Description Conc. Grd: 24% Cu, 79.5 G/Mt Ag 68536 Mt Conc/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Silver Silver 80g/mt
    Major Copper Copper 24wt-pct
  • Year 1979
    Description Leach Ppt Grd: 75% Cu 4212 Mt Leach Ppt/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major PPT Copper Copper 4,212mt 75wt-pct
  • Year 1979
    Description Mos2 Conc Grade: 43.0% Mo 454 Mt Mos2 Conc/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Conc. Molybdenum Molybdenum 454mt 43wt-pct
  • Year 1980
    Description Fd Grd: 0.67%Cu, 0.012%Mo, 2.0g/Mtag 2000000 Mt Ore/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Ore Silver Silver 2g/mt
    Major Ore Copper Copper 1wt-pct
    Trace Ore Molybdenum Molybdenum 0wt-pct
  • Year 1980
    Description Conc. Grd: 24% Cu, 66.8 G/Mt Ag 41049 Mt Conc/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Conc. Silver Silver 67g/mt
    Major Conc. Copper Copper 24wt-pct
  • Year 1980
    Description Leach Ppt Grd: 75% Cu 2658 Mt Leach Ppt/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major PPT Copper Copper 75wt-pct
  • Year 1980
    Description Mos2 Conc Grade: 40.1% Mo 217 Mt Mos2 Conc/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Conc. Molybdenum Molybdenum 217mt 40wt-pct
  • Year 1981
    Description Fd Grd: 0.71%Cu, 0.012%Mo, 2.7g/Mtag 3346000 Mt Ore/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Ore Silver Silver 3g/mt
    Major Ore Copper Copper 1wt-pct
    Trace Ore Molybdenum Molybdenum 0wt-pct
  • Year 1981
    Description Conc. Grd: 24% Cu, 92.7 G/Mt Ag 74145 Mt Conc/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Conc. Silver Silver 93g/mt
    Major Conc. Copper Copper 24wt-pct
  • Year 1981
    Description Leach Ppt Grd: 75% Cu 4802 Mt Leach Ppt/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major PPT Copper Copper 4,802mt 75wt-pct
  • Year 1981
    Description Mos2 Conc Grade: 41.7% Mo 257 Mt Mos2 Conc/Yr
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Conc. Molybdenum Molybdenum 257mt 42wt-pct
  • Year 1982
    Description Mine & Mill Closed Dump Leach Operating 4720 Mtpy Cu Produced (Leach)
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Copper Copper 4,720mt
  • Year 1982
    Description 342 Kgs Ag Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Silver Silver 342,000g
  • Year 1983
    Description Mine & Mill Resumed Operation In Oct 83 780000 Mtpy Ore Mined
  • Year 1983
    Description Cu Produced From Mill Conc & Leach Ppt 8170 Mtpy Cu Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Conc. Copper Copper 8,170mt
  • Year 1983
    Description 964 Kgs Ag Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Silver Silver 964,000g
  • Year 1984
    Description Mine Has Been Closed Since Aug 1984 2060000 Mtpy Ore Mined
  • Year 1984
    Description Cu Produced From Mill Conc & Leach Ppt 14900 Mtpy Cu Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Conc. Copper Copper 14,900mt
  • Year 1984
    Description 3170 Kgs Ag Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Trace Silver Silver 3,170,000g
  • Year 1985
    Description Cu Produced From Leach Ppt. 4000 Mtpy Cu Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Copper Copper 4,000mt
  • Year 1986
    Description Cu Produced From Leach Ppt. 3300 Mtpy Cu Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Copper Copper 3,300mt
  • Year 1987
    Description Cu Produced From Leach Ppt. 5805 Mtpy Cu Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Copper Copper 5,805mt
  • Year 1988
    Description Cu Produced From Leach Ppt. 3928 Mtpy Cu Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Copper Copper 3,928mt
  • Year 1989
    Description Cu Produced From Leach Ppt. 4544 Mtpy Cu Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Copper Copper 4,544mt
  • Year 1990
    Description Cu Produced From Leach Ppt. 3846 Mtpy Cu Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Copper Copper 3,846mt
  • Year 1991
    Description Cu Produced From Leach Ppt 3656 Mtpy Cu Produced
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Major Copper Copper 3,656mt
  • Year 1995
    Material Asarco 1997 10-K, p. A20.
    Accuracy Accurate
    Best? Y
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Primary Cu Copper Copper 3,100mt
  • Year 1996
    Material Asarco 1997 10-K, p. A20
    Accuracy Accurate
    Best? Y
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Primary Cu Copper Copper 2,200mt
  • Year 1997
    Period July-December
    Material New leach mine and SXEW plant begin production. Asarco 1997 10-K, p. A3, A20.
    Accuracy Accurate
    Best? Y
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Primary SX/EW Cu Copper Copper 8,800mt
  • Year 1998
    Material Asarco 1998 10-K, p. A-15
    Accuracy Accurate
    Best? Y
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Primary SX/EW Cu Copper Copper 19,200mt
  • Year 2005
    Period 1999-2005
    Material Estimate based on SXEW production capacity of between 50 and 67 tpd Cu.
    Accuracy Estimate
    Best? Y
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Primary Copper Copper 100,000mt
  • Year 2009
    Material Grupo Mexico 2009 AR, p. 26.
    Ore mined 347,000mt
    Accuracy Accurate
    Best? Y
    Importance Item Commodity Group Amount recovered Grade Recovery percentage
    Primary Copper Copper 1,000mt

Comments on the production information

  • 1978-1981 Production Data From U. S. Bureau Of Mines Production Statistics. Production In 1982 - 1984 From 1984 Asarco Ann Rept. P. 10. Mine & Mill Have Been Closed Since Aug 1984. Dump Leach Operation Has Continued To Operate. Molybdenum Has Not Been Recovered Since 1981. Production In 1985 - 1986 From Asarco 1986 Ann Rept. P. 12.

Reserves and resources

  • Type In-situ
    Estimate year 1977
    Demonstrated 22,400,000mt ore
    Measured 22,400,000mt ore
    Total resources 22,400,000mt ore
    Commodity Subtype Grade units Group Importance Year
    Copper Cu 0.66 wt-pct Copper Major 1977
    Molybdenum Mo 0.016 wt-pct Molybdenum Trace 1977
    Silver Ag 2.3999 g/mt Silver Minor 1977
  • Type In-situ
    Estimate year 1982
    Demonstrated 20,600,000mt ore
    Measured 20,600,000mt ore
    Total resources 20,600,000mt ore
    Commodity Subtype Grade units Group Importance Year
    Copper Cu 0.68 wt-pct Copper Major 1982
    Silver Ag 2.3999 g/mt Silver Minor 1982
  • Type In-situ
    Estimate year 1987
    Demonstrated 19,010,000mt ore
    Measured 19,010,000mt ore
    Total resources 19,010,000mt ore
    Commodity Subtype Grade units Group Importance Year
    Copper Cu 0.68 wt-pct Copper Major 1987
    Silver Ag 2 g/mt Silver Minor 1987
  • Type In-situ
    Estimate year 1994
    Demonstrated 91,900,000mt ore
    Indicated 91,900,000mt ore
    Total resources 91,900,000mt ore
    Commodity Subtype Grade units Group Importance Year
    Copper Cu 0.47 wt-pct Copper Major 1994
  • Type In-situ
    Estimate year 1996
    Demonstrated 179,000,000mt ore
    Total resources 179,000,000mt ore
    Commodity Subtype Grade units Group Importance Year
    Copper Cu 0.39 wt-pct Copper Major 1996
    Silver Ag 3.4 g/mt Silver Minor 1996
  • Type In-situ
    Estimate year 2009
    Reserves 141,400,000st
    Remarks Grupo Mexico 2009 AR, p. 26.
    Commodity Subtype Grade units Group Importance Year
    Copper 0.33 wt-pct Copper Primary 2009

Comments on the reserve resource information

  • M1c1=R2. Mo Grade Is Estimated To Be Grade Of Mill Feed Given In Bumines Production Statistics For 1981 (Last Year Of Cu-Mo Production). M9c4=R2. 2 Matrix 9, Col. 5 Represents Dump Leach Rock - M9c5=M2c1. Matrix 2 Quantity Equals Estimated Amount Of Waste Rock That Is Leached. (Company Officials In 1978 Stated 30% Of Waste Rock Mined Is Leached. Therefore, Reserve Equals 20,601,000 Mts X 1.9 (Strip Ratio) X 30%.) Calculated Assay Grade For Dump-Leach Waste Rock (Matrix 2) Is 0.286% Cu (Based On Known Cement Copper Output 9.07 Mtpd 75% Cu, Mill Recovery Of 40%, And Estimated Amount Of Waste Rock Leached Per Day, 5950 Mtpd) M3c1=R3. Asarco 1986 Ann. Rept., P 11.
  • Two Entries Are Made Under The Same Suquence Number In Order To Account For Both The Sulfide And Oxide Ore Bodies. Oxide Resource Is 11.7 Million Mt @ 0.29 % Cu. Mine Is Presently Under Development To Mine For Leach Oxide Copper Ore Using An Sx-Ew Circuit

Workings at the site

  • Type of workings Surface
    Area 80HA
    Field Value
    MAS Development Schedule # 1
    Mining Record # 1
    Status of Mining Method ESTIMATE
    Mining Method BENCH (BERM)
    Swell Factor .75
    Percent Waste Rock 65.5
    Avg. Overburden Thickness 30
    Primary Material Cover M-HARD ROCKS
    Percentage 100
    Hardness of Ore HARD ROCKS
    Avg. Bench Height (meters) 12
    Max. Pit Slope (degrees) 56
    Capacity 10430
    Capacity Units MT ORE/DAY
    Preprod. Stripping Vol. 8184
    Unit Production Cost 2.79
    Units of Production $/MT ORE
    Operating Days per Year 350
    Operating Shifts per Day 3
    Year of Information 1983

Comments on other economic factors

  • Waste Rock Feed Grade For Leach Was Calculated To Be 0.286% Rounded To 0.29% Cu. This Grade Was Calculated In The Same Manner That The Dump-Leach Reserve (Matrix 2) Was Calculated.
  • Mine Trans Cost Included In Mine Operating Costs.

Reference information

Links to other databases

Agency Database name Acronym Record ID Notes
U.S. Bureau of Mines Minerals Availability System MAS 0040190009
USGS Mineral Resources Data System MRDS D000333 MAS references MRDS
Mine Safety and Health Administration MSHA MSHA 0200134

Bibliographic references

  • Age

    Kendrick, M.A., Burgess, R., Pattrick, R.A.D. and Turner, G., 2001, Halogen and Ar-Ar age determinations of inclusions within quartz veins from porphyry copper deposits using complementary noble gas extraction techniques; Chemical Geology, v. 177, p. 351-370.

  • Development

    Asarco's Silver Bell Mining webpage has timeline history and operation information. Accessed 8/30/2010. http://www.asarco.com/silverbell.html

  • General

    Grupo Mexico Annual Report for 2009, 88 p. Accessed on 8/30/2010. http://www.gmexico.com/investors/reports.php

  • Geology

    Richard, K.E., and J.H. Courtright, Structure and Mineralization At Silver Bell, Arizona, Aime Trans., 1954, Pp. 1095-1099.

  • Production

    GALEY, 1979, SOC ECON GEOL, PORPHYRY CU FIELD CONF, 18 P.

  • Reserve-Resource

    Long, K.R., 1995, Production and reserves of Cordilleran (Alaska to Chile) porphyry copper deposits; in Pierce, F.W. and Bolm, J.G., eds., Porphyry Copper Deposits of the American Cordillera, Arizona Geological Society, Digest 20, p. 35-68

  • Age

    Barra, F., Ruiz, J., Valencia, V.A., Ochoa-Landin, L., Chesley, J.T. and Zurcher, L., 2005, Laramide porphyry Cu-Mo mineralization in northern Mexico: age constraints from Re-Os geochronology in molybdenite; Economic Geology, v. 100, p. 1605-1616.

  • Geology

    Geology Of The Porphyry Copper Deposits, Southwestern North America, 1966 Pp. 157-163.

  • Reserve-Resource

    Browne, Q. J. and Miller, M. A., 2002, Use of geologic parameters for development of low-grade copper reserves, Silver Bell Mine, Arizona; In Society of Mining Engineers Annual Meeting and Exhibit, 2002, Technical Program, Society of Mining Engineers, p. 68.

  • Geology

    Watson, Barry N., Updating The Geology And Structural Ore Controls At Silver Bell, Arizona, Pres. At Arizona Aime Spring Meeting, Mining Geology Div., May 20, 1968, 11 Pp.

  • Deposit

    Asarco Inc. 1986 Annual Report, P. 11.

  • Geology

    Graybeal F T 1983 - Geology of the El Tiro area: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 487-505

  • Geology

    Lopez J A, Titley S R 1995 - Outcrop and capping characteristics of the supergene sulfide enrichment at North Silver Bell, Pima County, Arizona: in Pierce F W, Bolm J G (Eds), Porphyry Copper Deposits of the American Cordillera Arizona Geol. Soc. Digest 20 pp 424-435

  • Reserve-Resource

    World Mining, Sept., 1977, P. 174.

  • Geology

    Richard K, Courtright J H 1966 - Structure and mineralization at Silver Bell, Arizona: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 157-163

  • Reserve-Resource

    Asarco S.E.C. Form 10-K, 1977.

  • Geology

    The Porter GeoConsultancy webpage for Silver Bell contains a description of the geology. Accessed on 8/31/2010. http://www.portergeo.com.au/

  • Reserve-Resource

    Asarco S.E.C. Form 10-K Report, Dec. 31, 1976, Pp. A3-A5.

  • Reserve-Resource

    Asarco 1993 Ann. Rept. P. 5.

  • Reserve-Resource

    Asarco 1981 Form 10-K Annual Report, 1982, P. 20.

  • Reserve-Resource

    Asarco 1986 Sec Form 10-K Report, P. 11.

  • Reserve-Resource

    Asarco Inc. 1995 Annual Report, P.7.

  • Deposit

    Hardwick, W.R., Open Pit Copper Mining And Concentrating Methods and Costs, Silver Bell Unit, American Smelting And Refining Co., Pima County, Arizona, U.S. Bureau Of Mines I C 8153, 1963, 72 Pp.

  • Deposit

    Huttl, John B., Silver Bell A S and R'S New Arizona Copper Project Starts Production, Engr. and Mining J., V. 155, N.7, Pp. 72-9.

  • Deposit

    Silver Bell - Asarco Makes A Mine At A Known Deposit, Mining World, April, 1958, Pp. 40-5.

  • Deposit

    Skillings, David N., Jr., Asarco'S Silver Bell Unit,July 25, 1970. Pp. 3-5.

  • Deposit

    Engr. and Mining Journal Flowsheet Portfolio, Dec., 1957,P.166

  • Deposit

    Sheffer, Herman W., And Lamar G. Evans, Copper Leaching Practices In The Western United States, U.S. Bur. Of Mines I C 8341, 1968, Pp. 10-11, 19-23.

  • Deposit

    Sw Pay Dirt,4-92, P.12a.

  • Deposit

    Schiffer, Craig. American Metal Market, V104 N28, 2-8-96, P.1.

  • Deposit

    Graybeal, Frederick T., Univ. Of Az Press, 1982, P.487.

  • Deposit

    Titley, Spencer R., Univ. Of Az Press, 1982, P.167.

  • Deposit

    Asarco News Letter, 6-30-89.

  • Deposit

    Sw Pay Dirt, 8-90, P,6a.

  • Deposit

    The Mining Record, 9-5-90, P.20.

  • Deposit

    Engineering And Mining Journal, 9-90, P.111.

  • Deposit

    Mining Engineering, 9-90, P.1052.

  • Deposit

    The Northern Miner, 11-11-91, P.17.

  • Deposit

    Asarco Inc. 1993 10-K, P.A15.

  • Deposit

    Sw Pay Dirt, 7-94, P.2a.

  • Deposit

    Asarco Inc., 1994 10-K, P. A18.

  • Deposit

    The Mining Record, Vol 108., #9, Feb. 26, 1997, P14.

General comments

Subject category Comment text
Deposit Original Evaluation Done In 1978. Updated 1983. Location - Silver Bell Consists Of Two Open Pits (Oxide & El Tiro Pit). The El Tiro Pit Is About 10,000 Ft.(3050 M.) Northwest Of Oxide Pit. El Tiro Location: Utm: North 3586800 East 449300 Lat 32 Deg. 25 Min. 8 Sec. North Lon 111 Deg. 32 Min. 21 Sec. West NW Qtr, Sec. 4 T. 12s R. 8e Silver Bell Mine Has Been Closed Since Dec 1981 And Has Not Reopened As Of Jun 1983. This Data Base Represents A Proposed Operation Assuming The Mine Reopened In Jan 1982 And Operated At Design Capacity Thru 1987. Property Updated 7/87.

Reporter information

Type Date Name Affiliation Comment
Reporter 11-MAR-97 Hite, Alan G. U.S. Bureau of Mines
Editor 31-AUG-10 Schruben, Paul U.S. Geological Survey Estimated production 1999-2005.
Editor 31-AUG-10 Schruben, Paul U.S. Geological Survey Revised production and reserves.