The severance tax permanent fund may be invested in revenue bonds issued by the employers mutual company under the authority of the Employers Mutual Company Act N.M. Stat. Ann. § 52-9-1 to 52-9-24, provided that the amount invested shall not exceed ten million dollars ($10,000,000) and provided further that the bonds shall bear interest at a market rate not less than the existing rate of return for ten-year United States treasury bonds on the date of the bond sale.