UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 12)*

(Name of Issuer)

(Title of class of securities)

(CUSIP Number)

(Name, Address and Telephone Number of Person Authorised to Receive Notices and Communications)

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.   ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See § 240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

2

3

Item 1. Security and Issuer

This Amendment No. 12 to Schedule 13D amends and supplements the statement on Schedule 13D originally filed by Rio Tinto plc (“Rio Tinto”) and Rio Tinto International Holdings Limited (“RTIH”) on November 3, 2006, and amended on September 12, 2007, October 26, 2007, January 7, 2008, April 10, 2008, October 30, 2009, March 4, 2010, July 7, 2010, September 13, 2010, December 14, 2010, February 4, 2011, and June 28, 2011 (as amended and supplemented, the “Schedule 13D”) with the Securities and Exchange Commission (the “SEC”), relating to the common shares, without par value (the “Shares”), of Ivanhoe Mines Ltd., a corporation continued under the laws of the Yukon Territory, Canada (the “Company”).

Item 3. Source and Amount of Funds or Other Consideration

Item 3 of the Schedule 13D is amended and supplemented as follows:

On August 24, 2011, RTIH exercised its Subscription Right to acquire Common Shares issued by the Company under the December 2010 Heads of Agreement in respect of 27,896,570 Shares, at CDN$18.98 per Share, being the volume weighted average price of the Shares on the TSX on the five (5) trading days immediately before August 24, 2011.

The funds for the purchase were obtained by RTIH from the working capital of Rio Tinto.

RTIH issued a press release announcing the exercise of the Subscription Right on August 24, 2011, which is filed as Exhibit K hereto and incorporated herein by reference.

Item 4. Purpose of Transaction

Item 4 of the Schedule 13D is amended as follows:

Subject to compliance with the terms and conditions of existing contractual obligations set out herein, Rio Tinto and RTIH have the intention to acquire, directly or indirectly, such number of Shares (or securities convertible or exchangeable into Shares) that would result in them holding in aggregate up to 49.0 per cent of the Shares outstanding. Any such acquisitions or arrangements to acquire Shares (or other securities convertible or exchangeable into Shares) may be made in the secondary market or in privately negotiated transactions.

Except as set out in this Schedule 13D, neither Rio Tinto nor RTIH have any other present intention to acquire securities of the Company; however, they intend to review their investment on a regular basis and, as a result thereof and subject to compliance with the terms and conditions of existing contractual obligations set out herein, depending upon their assessment of the Company’s business, prospects and financial condition, the market for the Company’s securities, general economic and tax conditions, and other factors, may in the future take such actions with respect to its investment in the Company as they deem appropriate including, without limitation, availing themselves of their rights to acquire additional securities of the Company, seek additional representation on the board of the Company, and make proposals to the Company, alone or jointly with a third party, concerning the long-term structure of its existing investment, a direct ownership interest in the OT Project or other changes to the capitalization, ownership structure or operations of the Company.

As disclosed previously in the Schedule 13D, RTIH has confirmed to The Government of Mongolia that it will not sell or transfer to a person controlled by a foreign government any shares it may from time to time hold in the Company, OT LLC or another subsidiary of the Company that is a direct or indirect shareholder of OT LLC without the consent of The Government of Mongolia.

4

Except as otherwise set forth in Item 4 of the Schedule 13D, as amended and supplemented hereby, neither Rio Tinto nor RTIH, has any present plan or proposal that would relate to, or result in, any of the matters set forth in paragraphs (a) through (j) of Item 4 of Schedule 13D. Notwithstanding the foregoing, Rio Tinto and RTIH reserve their rights in full to change their intentions with respect to any and all matters referred to in Item 4 of Schedule 13D as they deem appropriate.

Item 5. Interest in Securities of the Issuer

Item 5 of the Schedule 13D is hereby amended and supplemented as follows:

The responses of Rio Tinto and RTIH to Rows (11) through (13) of the cover pages of this Schedule 13D and the information set forth in Item 3 are hereby incorporated by reference in this Item 5.

Pursuant to the exercise of the Subscription Rights, RTIH will acquire 27,896,570 Shares at an average subscription price of approximately CDN$18.98 per share.

Therefore, each of Rio Tinto and RTIH is deemed to beneficially own 358,158,442 Shares which, assuming the exercise of the Subscription Right, and the 27,896,570 Shares which will be acquired would represent approximately 48.5 per cent of the Company’s outstanding Shares on a fully diluted basis.

The percentage of the class of securities identified pursuant to Item 1 beneficially owned by each of Rio Tinto and RTIH is based on 710,590,047 Shares.

In addition, the Shares deemed beneficially owned by each of Rio Tinto and RTIH with respect to which such person (i) has sole voting power, (ii) shares voting power, (iii) has sole dispositive power and (iv) shares dispositive power are listed in the responses to Items 7, 8, 9 and 10, respectively, of the cover page of this Schedule 13D relating to such person.

Except as disclosed in this Schedule 13D, neither Rio Tinto nor RTIH nor, to the best of their knowledge, any of the persons listed in Schedule A hereto, beneficially owns any Shares or has the right to acquire any Shares.

Except as disclosed in this Schedule 13D, neither Rio Tinto nor RTIH nor, to the best of their knowledge, any of the persons listed in Schedule A hereto, presently has the power to vote or to direct the vote or to dispose or direct the disposition of any of the Shares which they may be deemed to beneficially own.

Except as disclosed in this Schedule 13D, neither Rio Tinto nor RTIH nor, to the best of their knowledge, any of the persons listed in Schedule A hereto, has effected any transaction in the Shares during the past 60 days.

To the best knowledge of Rio Tinto and RTIH, no other person has the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the Shares beneficially owned by Rio Tinto and RTIH.

5

Item 7. Materials to be Filed as Exhibits

6

SIGNATURES

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

Dated: August 25, 2011

Rio Tinto plc

7

SCHEDULE A

The response set forth in Schedule A of the Schedule 13D is hereby amended and restated in its entirety with the information below:

Rio Tinto plc

Directors and Executive Officers

8

9

10

EXHIBIT INDEX

11

EXHIBIT A

JOINT FILING AGREEMENT

The undersigned hereby agree that the Statement on Schedule 13D, dated November 3, 2006, with respect to the common shares, without par value, of Ivanhoe Mines Ltd. is, and any amendments thereto executed by each of us shall be, filed on behalf of each of us pursuant to and in accordance with the provisions of Rule 13d-1(k)(1) under the Securities and Exchange Act of 1934, as amended, and that this Agreement shall be included as an Exhibit to the Schedule 13D and each such amendment. Each of the undersigned agrees to be responsible for the timely filing of the Schedule 13D and any amendments thereto, and for the completeness and accuracy of the information concerning itself contained therein. This Agreement may be executed in any number of counterparts, all of which taken together shall constitute one and the same instrument.

IN WITNESS WHEREOF, the undersigned have executed this Agreement as of the 3rd day of November, 2006.

Cont…/