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Exhibit 4.21 

LIMITED NOTICE TO PROCEED 

This Limited Notice to Proceed (this"LNTP") is entered into, effective as of the 15th day of November, 2005 (the"Effective Date"), by and between Minera San Xavier, S.A. de C.V. ("MSX") andWashington Group Latin America, Inc. ("WGLA"). MSX and WGLA are each referred toherein as a "Party" and collectively as the "Parties." 

RECITALS
WHEREAS,MSX and WGLA entered into a Mining Contract dated as of December 30, 2003 (the"Mining Contract"), in which MSX engaged WGLA to perform drilling, blasting,excavating, mining, hauling and construction-related services at the Cerro SanPedro Mine (the "Mine") owned by MSX and located in Cerro de San Pedro, Mexico;

**WHEREAS,**for a variety of reasons,including but not limited to MSX's inability to obtain the blasting permit, theParties entered into a standby letter dated May 12, 2004, as extended on August31, 2004; October 29, 2004; and November 29, 2004 (the "Standby Letter"), inwhich WGLA ceased to perform any further Work (as defined by the MiningContract) until such time when MSX notifies WGLA that MSX has secured theblasting permit and authorizes WGLA to recommence the Work; **WHEREAS, on March 29, 2005, the Partiesentered into a revised standby letter (the "Revised Standby Letter") in whichthe Parties revised the terms of the Standby Letter and agreed upon an ExtendedStandby Term (as defined in the Revised Standby Letter); and 

**WHEREAS,**MSX desires that WGLA perform certain limited services during the ExtendedStandby Term, and WGLA is willing to proceed with such services on the basis ofthis LNTP. 

**NOW THEREFORE,** in consideration of theforegoing and the promises and mutual covenants hereinafter set forth, and othergood and valuable consideration, the receipt and sufficiency of which is herebyacknowledged, and intending to be legally bound hereby, the Parties agree asfollows: **AGREEMENT**
1.        WGLA shall proceed with the services at the Mine described in *Attachment A*to this LNTP (the "Services") and other work items as may be mutually agreedupon by the Parties. 

2.       This LNTP shall not affect the standby status of, nor any obligations arisingfrom, the Mining Contract and Revised Standby Letter, except only as may ariseunder section 4 of this LNTP regarding the Mining Contract scope of Work andTarget Budget. 
3.MSX shall pay WGLA for the Services on a unit rate or time and materials basisin accordance with the payment terms set forth in *Attachment B*to this LNTP. Invoicing and payment shall be on a monthly basis. WGLA shallinvoice MSX within the first ten (10) days of each month for the previousmonth's Services. MSX shall reimburse WGLA no later than the last day of themonth in which the invoice is delivered. 
4.        The terms and conditions set forth in *Attachment C*to this LNTP are incorporated herein by this reference. Upon recommencement ofthe Work under the Mining Contract, any Services performed by WGLA under thisLNTP, which are part of the Mining Contract scope of Work, shall be deleted fromthe Mining Contract and the Mining Contract Target Budget revised according tothe Mining Contract terms. 
5.        If the Parties fail to recommence the Work under the MiningContract, and this LNTP is terminated, MSX shall reimburse WGLA for any costsnot previously reimbursed, including any mobilization and demobilizationexpenses and costs. 
6.       This LNTP shall terminate on the earlier of: (a) completion of the Services asmutually agreed by the Parties, (b) full recommencement of the Work under theMining Contract, (c) reasonable written notice of termination for convenience byeither Party, in which case the terminating Party shall pay demobilizationexpenses and other reasonable termination costs, (d) a material breach of thisLNTP by either Party upon the breaching Party's receipt of written notice fromthe non-breaching Party, provided however, that such notice shall provide ten(10) business days for the breaching Party to cure its breach (the "CurePeriod"), or if such breach is incapable of being cured in the Cure Period thenthe breaching Party shall have undertaken, within the Cure Period, good faithmeasures to cure the breach within a reasonable time, or (e) termination of theMining Contract, or (f) termination of the Revised Standby Letter. 
7.        Neither Party mayassign their respective rights, interests or obligations under this LNTP withoutthe express written consent of the other Party. This LNTP shall be binding uponthe Parties and their successors, permitted assigns, and legal representatives.
8.       The failure of either Party to enforce, at any time, any provision of this LNTPshall not constitute waiver of such provision in any way or the right of suchParty at any time to avail itself of such remedies as it may have for any breachof such provision. 
9.       This LNTP shall be governed by and construed in accordance with the laws of theState of Colorado excluding any principles or provisions thereof that wouldrequire the application of the laws of a different jurisdiction. The Partiesshall resolve any performance claim, controversy or dispute between the Partiesarising out of, or in connection with, this LNTP, or anybreach hereof, in accordance with Article 15 of the Mining Contract. 

10.        This LNTP is the complete agreement between the Parties regarding theperformance of the Services set forth in *Attachment A* and supersedes alloral and written proposals and all communication between the Parties relating tothis LNTP, except that the Mining Contract and Revised Standby Letter shallremain intact, operable and enforceable according to section 2 of this LNTP andsubject to section C of *Attachment C*.No modification of this LNTP shall be effective unless in writing and executedby both Parties. 
11.       This LNTP may be executed in any number of counterparts and delivered byfacsimile transmission, all of which taken together shall constitute a singleand complete agreement. **[SIGNATURE PAGE FOLLOWS] 

 

 

**IN WITNESS WHEREOF,the Parties have caused this instrument to be executedby their duly authorized representatives on this 21st day of November, 2005 tobe effective as of the Effective Date. 
  **Minera San    Xavier, S.A.****Washington Group    Latin America, Inc.**  By: /s/ Donald E.    HulseBy: /s/ Robert W.    Zaist  Name: Donald E.    HulseName: Robert W.    Zaist  Title: General    ManagerTitle: President    - Mining******