Exhibit 10.4

Execution Version

JOINDER AGREEMENT

THIS JOINDER AGREEMENT, dated as of February 14, 2014 (this “Agreement”), by and among Barclays Bank PLC (the “Incremental Term Loan Lender”), Fairmount Minerals, Ltd., as borrower representative (in such capacity, the “Borrower Representative”) and Barclays Bank PLC (“Barclays”), as Administrative Agent (together with its successors and permitted assigns, the “Administrative Agent”). This Joinder Agreement shall constitute a “Loan Document” for all purposes of the Credit Agreement (as defined below) and the other Loan Documents.

RECITALS:

WHEREAS, reference is hereby made to that certain Second Amended and Restated Credit and Guaranty Agreement, dated as of September 5, 2013 (as it may be amended, restated, supplemented or otherwise modified from time to time, the “Credit Agreement’’; the terms defined therein and not otherwise defined herein being used herein as therein defined), by and among Fairmount Minerals Holdings, Inc., Fairmount Minerals, Ltd. (the “U.S. Borrower”), certain Subsidiaries of the U.S. Borrower party thereto as guarantors, Lake Shore Sand Company (Ontario) Ltd. (the “Canadian Borrower”), the Lenders party thereto from time to time, KeyBank National Association, as Syndication Agent, Barclays Bank PLC, as Administrative Agent and Collateral Agent, Barclays Bank PLC, as Revolving Administrative Agent and PNC Bank, National Association and Wells Fargo Securities, LLC, as Co-Documentation Agents; and

WHEREAS, subject to the terms and conditions of the Credit Agreement, the Borrower Representative may request an increase to the existing Revolving Commitments or Term Loans and/or the establishment of Incremental Term Loan Commitments, with such increase or establishment becoming effective by the Borrower Representative entering into one or more Joinder Agreements with the Incremental Term Loan Lender and/or Incremental Revolving Loan Lender(s), as applicable, and the Administrative Agent.

NOW, THEREFORE, in consideration of the premises and agreements, provisions and covenants herein contained, the parties hereto agree as follows:

The Incremental Term Loan Lender hereby agrees to commit to provide its Commitment as set forth on Schedule A annexed hereto, on the terms and subject to the conditions set forth herein and in the Credit Agreement.

The Incremental Term Loan Lender (i) confirms that it has received a copy of the Credit Agreement and the other Loan Documents, together with copies of the financial statements referred to therein and such other documents and information as it has deemed appropriate to make its own credit analysis and decision to enter into this Agreement; (ii) agrees that it will, independently and without reliance upon the Administrative Agent or any other Lender or Agent and based on such documents and information as it shall deem appropriate at the time, continue to make its own credit decisions in taking or not taking action under the Credit Agreement; (iii) appoints and authorizes Administrative Agent and Collateral Agent to take such action as agent on its behalf and to exercise such powers under the Credit Agreement and the other Loan Documents as are delegated to Administrative Agent and Collateral Agent, as the case may be, by the terms thereof, together with such powers as are reasonably incidental thereto; and (iv) agrees that it will perform in accordance with their terms all of the obligations which by the terms of the Credit Agreement are required to be performed by it as a Lender.

The Incremental Term Loan Lender hereby agrees to extend Term Loans to the U.S. Borrower on the following terms and conditions and in accordance with the terms and provisions of the Credit Agreement.

a. The parties hereto hereby acknowledge and agree that the term loans made on Increased Amount Date (the “Incremental Term Loans”) shall not constitute a separate Class of Term Loans, but shall instead be part of the same Class as the Tranche B-2 Term Loans made on the Restatement Date. Each of the parties hereto hereby agrees that the Administrative Agent may take any and all action as may be reasonably necessary to ensure that all such Incremental Term Loans may be treated as such Tranche B-2 Term Loans for all purposes under the Loan Documents.

b. The Stated Maturity Date for the Incremental Term Loans shall be September 5, 2019.

In addition, the principal amount of Incremental Term Loans, together with the existing Tranche B-2 Term Loans shall be repaid in consecutive quarterly installments in an aggregate amount equal to $2,315,256.89 with the next payment on March 31, 2014 and then quarterly thereafter as provided in Section 2.12 of the Credit Agreement and Section 2.12 of the Credit Agreement is hereby amended to reflect such amounts as the Installments of Tranche B-2 Term Loans payable on each Installment Date commencing on March 31, 2014 as set forth therein and that the unpaid balance of all Tranche B-2 Term Loans, including the Incremental Term Loans hereunder, shall be due and payable on the Maturity Date.

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IN WITNESS WHEREOF, each of the undersigned has caused its duly authorized officer to execute and deliver this Joinder Agreement as of the date first written above.

[Signature Page to Joinder Agreement]

[Signature Page to Joinder Agreement]

[Signature Page to Joinder Agreement]

SCHEDULE A

TO JOINDER AGREEMENT