Provision shall be made that the Board shall be the trustees of the funds of the retirement system. The Board shall have full power and authority to invest and reinvest such funds, subject to all terms imposed by the State of Michigan upon life insurance companies in the making and disposing of their investments, except that notes, bonds or other obligations of the City of Garden City shall not be subject to said terms, and except that no money shall be invested in the stock of any company, association or corporation; to purchase notes or other obligations of the City of Garden City at any legally conducted public or private sale; to hold, purchase, sell, assign, transfer, and dispose of any securities and investments in which any of the funds of the retirement system are invested, as well as the proceeds of said investments and any moneys belonging to the retirement system. All funds of the retirement system shall be held for the sole purpose of making disbursements for pensions and other payments authorized under this chapter and the operating ordinance shall be used for no other purpose.