
<PAGE>
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                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549

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                                 SCHEDULE 14D-1
                               (AMENDMENT NO. 6)

                             TENDER OFFER STATEMENT

      PURSUANT TO SECTION 14(D) (1) OF THE SECURITIES EXCHANGE ACT OF 1934
                                      AND
                              AMENDMENT NO. 10 TO
                                  SCHEDULE 13D
                   UNDER THE SECURITIES EXCHANGE ACT OF 1934

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                              ASARCO INCORPORATED
                           (Name of Subject Company)

                               ASMEX CORPORATION
                                      AND
                           GRUPO MEXICO, S.A. DE C.V.
                                   (Bidders)

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                           COMMON STOCK, NO PAR VALUE
                        (INCLUDING THE ASSOCIATED JUNIOR
                 PARTICIPATING PREFERRED STOCK PURCHASE RIGHTS)
                         (Title of Class of Securities)

                            ------------------------

                                    04341310
                     (CUSIP Number of Class of Securities)

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                            Daniel Tellechea Salido
                Managing Director for Administration and Finance
                           Grupo Mexico, S.A. de C.V.
                              Baja California 200
                                Colonia Roma Sur
                           06760 Mexico City, Mexico
                          Telephone: 011-525-574-2067
            (Name, Address and Telephone Number of Person Authorized
          To Receive Notices and Communications on Behalf of Bidders)

                                    COPY TO:
                            Lori Anne Czepiel, Esq.
                                Brown & Wood LLP
                             One World Trade Center
                            New York, New York 10048
                           Telephone: (212) 839-5300
                           Facsimile: (212) 839-5599

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    This amendment constitutes Amendment No. 6 to the Tender Offer Statement on
Schedule 14D-1 as the same may have been amended from time to time (as amended
hereby, the "Statement") and the Amendment No. 9 to Schedule 13D and relates to
the offer by Grupo Mexico, S.A. de C.V., a Mexican corporation ("Parent"),
through its wholly owned subsidiary, ASMEX Corporation, a Delaware corporation
("Purchaser"), to purchase all of the outstanding shares of common stock,
without par value (together with the associated junior participating preferred
stock purchase rights, the "Common Stock"), of ASARCO Incorporated, a New Jersey
corporation ("ASARCO"), at a purchase price of $29.75 per share of Common Stock,
net to the seller in cash, without interest thereon, upon the terms and subject
to the conditions set forth in the Offer to Purchase dated September 27, 1999,
as amended (the "Offer to Purchase"), a copy of which is attached to this
Statement as Exhibit (a)(1), as amended and supplemented by Supplement No. 1
thereto dated October 8, 1999 ("Supplement No. 1"), a copy of which is attached
to this Statement as Exhibit (a)(10), as further amended and supplemented by
Supplement No. 2 dated October 26, 1999 ("Supplement No. 2"), a copy of which is
attached to this Statement as Exhibit (a)(13) (which, as may have been amended
or supplemented from time to time, together with the Offer to Purchase
constitute the "Offer"). Capitalized terms not defined herein have the meanings
ascribed in the Offer to Purchase, Supplement No. 1 and Supplement No. 2. The
item numbers and responses thereto below are in accordance with the requirements
of Schedule 14D-1.

ITEM 4. SOURCE AND AMOUNT OF FUNDS OR OTHER CONSIDERATION.

    On November 11, 1999, Parent, Purchaser and Chase executed definitive
financing agreements to provide funding for the Offer. The terms and conditions
of the executed agreements are substantially similar to the terms and conditions
of the Credit Facilities previously disclosed in the Statement, the Offer to
Purchase, Supplement No. 1 and Supplement No. 2.

ITEM 10. ADDITIONAL INFORMATION.

    Item 10 is hereby amended by adding the following:

    On November 11, 1999, Parent issued a press release relating to the Offer, a
copy of which is included herein as Exhibit (a)(20) and incorporated herein by
reference.

    On November 12, 1999, Parent issued a press release relating to the Offer, a
copy of which is included herein as Exhibit (a)(21) and incorporated herein by
reference.

ITEM 11. MATERIAL TO BE FILED AS EXHIBITS

    Item 11 is hereby amended by adding the following:

<TABLE>
<C>      <S>
 (a)(1)  Offer to Purchase dated September 27, 1999.*

 (a)(2)  Form of Letter of Transmittal.*

 (a)(3)  Form of Notice of Guaranteed Delivery.*

 (a)(4)  Form of Letter to Brokers, Dealers, Commercial Banks, Trust
           Companies and Other Nominees.*

 (a)(5)  Form of Letter to Clients for use by Brokers, Dealers,
           Commercial Banks, Trust Companies and Other Nominees.*

 (a)(6)  Form of Guidelines for Certification of Taxpayer
           Identification Number on Substitute Form W-9.*

 (a)(7)  Form of Summary Advertisement dated September 27, 1999.*

 (a)(8)  Press release of Parent dated September 24, 1999.*

 (a)(9)  Press release of Parent dated September 27, 1999.*
</TABLE>

                                       2
<PAGE>
<TABLE>
<C>      <S>
(a)(10)  Supplement No. 1 to the Offer to Purchase dated October 8,
           1999.*

(a)(11)  Press release of Parent dated October 7, 1999.*

(a)(12)  Press release of Parent dated October 25, 1999.*+

(a)(13)  Supplement No. 2 to the Offer to Purchase dated October 26,
           1999.*

(a)(14)  Form of Revised Letter of Transmittal*

(a)(15)  Form of Revised Notice of Guaranteed Delivery.*

(a)(16)  Form of Revised Letter to Brokers, Dealers, Commercial
           Banks, Trust Companies and Other Nominees.*

(a)(17)  Form of Revised Letter to Clients for use by Brokers,
           Dealers, Commercial Banks, Trust Companies and Other
           Nominees.*

(a)(18)  Press release of Parent dated October 25, 1999 (supersedes
           Exhibit (a)(12)).*+

(a)(19)  Press release of Parent dated November 10, 1999.*

(a)(20)  Press release of Parent dated November 11, 1999.

(a)(21)  Press release of Parent dated November 12, 1999.

 (b)(1)  Commitment Letter from The Chase Manhattan Bank and Chase
           Securities Inc. dated September 24, 1999.*

 (b)(2)  First Amended and Restated Commitment Letter from the Chase
           Manhattan Bank and Chase Securities Inc. dated October 5,
           1999.*

 (b)(3)  Second Amended and Restated Commitment Letter from The Chase
           Manhattan Bank and Chase Securities Inc. dated October 7,
           1999.*

 (b)(4)  Third Amended and Restated Commitment Letter from The Chase
           Manhattan Bank and Chase Securities Inc. dated
           October 15, 1999.*

 (b)(5)  Fourth Amended and Restated Commitment Letter from The Chase
           Manhattan Bank and Chase Securities Inc. dated
           October 22, 1999.*

 (c)(1)  Agreement and Plan of Merger among ASARCO, Parent and
           Purchaser, dated as of October 25, 1999.*

    (d)  Not applicable.

    (e)  Not applicable.

    (f)  None.

 (g)(1)  1997 Annual Report of Parent.*

 (g)(2)  1998 Annual Report of Parent.*
</TABLE>

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*   Previously filed.

+   Exhibit (a)(12) is superseded by Exhibit (a)(18).

                                       3
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                                   SIGNATURE

    After due inquiry and to the best of its knowledge and belief, the
undersigned certifies that the information set forth in this statement is true,
complete and correct.

Dated: November 12, 1999

<TABLE>
<S>                                                    <C>  <C>
                                                       GRUPO MEXICO, S.A. DE C.V.

                                                       By:  /s/ DANIEL TELLECHEA SALIDO
                                                            -----------------------------------------
                                                            Name: Daniel Tellechea Salido
                                                            Title:  Managing Director for
                                                            Administration and Finance

                                                       GRUPO MEXICO, S.A. DE C.V.

                                                       By:  /s/ HECTOR CALVA RUIZ
                                                            -----------------------------------------
                                                            Name: Hector Calva Ruiz
                                                            Title:  Managing Director for Exploration
                                                            and Projects

                                                       ASMEX CORPORATION

                                                       By:  /s/ DANIEL TELLECHEA SALIDO
                                                            -----------------------------------------
                                                            Name: Daniel Tellechea Salido
                                                            Title:  Vice President and Treasurer
</TABLE>

                                       4
<PAGE>
                                 EXHIBIT INDEX

EXHIBIT NO.

<TABLE>
<C>      <S>
 (a)(1)  Offer to Purchase dated September 27, 1999.*

 (a)(2)  Form of Letter of Transmittal.*

 (a)(3)  Form of Notice of Guaranteed Delivery.*

 (a)(4)  Form of Letter to Brokers, Dealers, Commercial Banks, Trust
           Companies and Other Nominees.*

 (a)(5)  Form of Letter to Clients for use by Brokers, Dealers,
           Commercial Banks, Trust Companies and Other Nominees.*

 (a)(6)  Form of Guidelines for Certification of Taxpayer
           Identification Number on Substitute Form W-9.*

 (a)(7)  Form of Summary Advertisement dated September 27, 1999.*

 (a)(8)  Press release of Parent dated September 24, 1999.*

 (a)(9)  Press release of Parent dated September 27, 1999.*

(a)(10)  Supplement No. 1 to the Offer to Purchase dated October 8,
           1999.*

(a)(11)  Press release of Parent dated October 7, 1999.*

(a)(12)  Press release of Parent dated October 25, 1999.*+

(a)(13)  Supplement No. 2 to the Offer to Purchase dated October 26,
           1999.*

(a)(14)  Form of Revised Letter of Transmittal*

(a)(15)  Form of Revised Notice of Guaranteed Delivery.*

(a)(16)  Form of Revised Letter to Brokers, Dealers, Commercial
           Banks, Trust Companies and Other Nominees.*

(a)(17)  Form of Revised Letter to Clients for use by Brokers,
           Dealers, Commercial Banks, Trust Companies and Other
           Nominees.*

(a)(18)  Press release of Parent dated October 25, 1999 (supersedes
           Exhibit (a)(12)).*+

(a)(19)  Press release of Parent dated November 10, 1999.*

(a)(20)  Press release of Parent dated November 11, 1999.

(a)(21)  Press release of Parent dated November 12, 1999.

 (b)(1)  Commitment Letter from The Chase Manhattan Bank and Chase
           Securities Inc. dated September 24, 1999.*

 (b)(2)  First Amended and Restated Commitment Letter from the Chase
           Manhattan Bank and Chase Securities Inc. dated October 5,
           1999.*

 (b)(3)  Second Amended and Restated Commitment Letter from The Chase
           Manhattan Bank and Chase Securities Inc. dated October 7,
           1999.*

 (b)(4)  Third Amended and Restated Commitment Letter from The Chase
           Manhattan Bank and Chase Securities Inc. dated
           October 15, 1999.*

 (b)(5)  Fourth Amended and Restated Commitment Letter from The Chase
           Manhattan Bank and Chase Securities Inc. dated
           October 22, 1999.*
</TABLE>

                                       5
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<TABLE>
<C>      <S>
 (c)(1)  Agreement and Plan of Merger among ASARCO, Parent and
           Purchaser, dated as of October 25, 1999.*

    (d)  Not applicable.

    (e)  Not applicable.

    (g)  None.

 (g)(1)  1997 Annual Report of Parent.*

 (g)(2)  1998 Annual Report of Parent.*
</TABLE>

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*   Previously filed.

+   Exhibit (a)(12) is superseded by Exhibit (a)(18).

                                       6
