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Exhibit 4.16

AMENDMENT AGREEMENT TO THE PROFESSIONAL
SERVICES CONTRACT, ENTERED INTO BY AND BETWEEN THE COMPANY "MINERA SAN XAVIER,
S.A. DE C.V.", REPRESENTED BY MR. SERGIO ENRIQUE LOPEZ BRIONES (HEREINAFTER
REFERRED TO AS "MSX") AND BY THE COMPANY "FIREX, S.A. DE C.V.", REPRESENTED BY
MR. JORGE MENDIZABAL ACEBO (HEREINAFTER REFERRED TO AS "FIREX"), ACCORDING TO
THE FOLLOWING ANTECEDENTS, REPRESENTATIONS AND CLAUSES.

A N T E C E D E N T S

[English Translation of Spanish Language
Document]

FIRST.- On October 25th, 2004, MSX and FIREX
entered into a professional services contract, by which the latter committed to
assist the first, during the development of the mining project of Cerro de San
Pedro, located in the Municipality of the same name, for which Mr. Jorge
Mendizabal Acebo was appointed as advisor, acting as General Director of such
project. The term of effectiveness originally agreed for such contract was of
six (6) months.

SECOND.- Since it represents a benefit for
MSX, this has agreed with its counterpart, to modify the term of effectiveness
of such contract, reason that justifies the formalization of the present
amendment agreement.

R E P R E S E N T A T I O N S

> I. Of MSX:
>   UNIQUE.- To ratify each and all the representations
>     established on the contract described in the FIRST antecedent of this
>     document.
>   II. Of FIREX:
>   UNIQUE.- To ratify each and all the representations
>     established on the contract described in the FIRST antecedent of this
>     document.
>   III. Of the parties:

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> a) To ratify the content of their respective representations manifested in
>     this document and to acknowledge the personality and legal capacity to which
>     they formalize the same.
>     b) To accept regulate the legal relationship that binds them, according to
>     the terms, conditions and scope of this amendment agreement, in accordance
>     to the content of the same, granting their free will, not existing deceitful
>     representations, error, bad faith, nor any other act or omission that would
>     induce the parties to enter this agreement that they would not otherwise
>     have made; the parties, therefore, agree the following:

C L A U S E

UNIQUE.- The parties ratify the content and
scope of each and all the clauses container in the professional services
contract, referred to in the FIRST antecedent of this document, with exception
to clauses SECOND and THIRD, which are hereby amended as follows:

> "SECOND.- TERM OF EFFECTIVENESS. The
>     present contract shall be effective for an indefinite term, having the
>     parties the right to terminate it at any time, by giving its counterpart
>     written notice, with at least thirty (30) days of anticipation to the date
>     in it wishes to stop the professional services, and provided that the
>     professional fees agreed in favor of FIREX, shall be paid until the date in
>     which the contract is formally terminated.
>     THIRD.- PROFESSIONAL FEES. MSX shall pay
>     FIREX, for the professional services described in clause first, a monthly
>     fee equal to US$6,250.00 (six thousand two hundred and fifty dollars 00/100
>     USCy) plus the corresponding Added Value Tax, amount that shall be paid in
>     Mexican currency at the rate determined by BANXICO on the day on which the
>     amount is due and payable."

Having read the parties the present agreement, they sign it in
two (2) copies, in the city of San Luis Potosi, on the 15th day of April, 2005.

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|                                     |                           
|------------------------------------ | --------------------------
|By MSX                               | By FIREX                  
|/s/ Sergio Enrique Lopez
    Briones | /s/ Jorge Mendizabal Acebo
|MR. SERGIO ENRIQUE LOPEZ             | MR. JORGE MENDIZABAL ACEBO
|BRIONES                              |                           
|                                     |                           
|                                     |                           
|                                     |                           
|WITNESSES                           
|                                     |                           
|                                     |                           
|                                     |                           
|/s/ Richard J. Hall                  | /s/ Jesús Motilla Martinez
|MR. RICHARD J. HALL                  | MR. JESUS MOTILLA MARTINEZ
|                                     |                           

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