[Letterhead of Minera Rio Tinto, S.A. de C.V.]

Sunburst Acquisition IV Inc.
Suite 300 - 541 Howe St.
Vancouver BC V6C2C

Dear Sirs:

Re:      Joint Venture Agreement between Sierra Minerals & Mining Inc. and
         Minera Rio Tinto, S.A. de C.V. (the "Joint Venture Agreement")

This  letter  will  confirm  our  agreement  wherein,   for  good  and  valuable
consideration,  the receipt and sufficient of which is  acknowledged  by each of
us, we have  agreed to amend the Joint  Venture  Agreement  so as to provide the
date of the "Initial  Investment"  (as defined in the Joint Venture  Agreement")
provided  for in Section  2.3(a) of the Joint  Venture  Agreement is extended to
November  10th,  2004. We also confirm that Sunburst will have the obligation to
secure a $2,000,000 line of credit by November 30th, 2004.

If Sunburst is unable to make the mentioned  contribution on time, the provision
stated on clause 3 will apply.

We  confirm  that the  Joint  Venture  Agreement,  amended  as  detailed  above,
continues in full force and effect.

Yours truly,

Dated the 1 day of October

Minera Rio Tinto, S.A. de C.V.


Pre: /s/ Mario Ayub
Mario Ayub - Administrador Unico