In the period from 1936 to 1941, the I-M Mine was in full production and processed through two identical process plants located at the two mining operations within the I-M Mine Property, the Brunswick Mine and the Idaho Mine. Gold recovery from the I-M Mine is reported in detail from the processing of large amounts of mineralized material.
The Idaho mineralization had higher gravity gold recovery of 76% versus Brunswick at 64%. The average flotation concentrate grade for Idaho Mine and Brunswick Mine was 3.6 oz/ton (123gpt) and 4.2oz/ton (144gpt), respectively. Nearly all gold at the I-M Mine is free milling, as demonstrated by the leaching of flotation concentrates and tailings by the Mine during past production.
Total gold recovery from the processing of 2,011,000 tons of mineralized material from 1936 to 1941 is estimated at 96.3%, of which 67.5% is estimated to have been recovered by gravity methods. Gold recovery details for the I-M Mine is summarized in Table 1. More details >>
Table 1: Gold Recovery Summary from 1936-1941*
Detailed production, processing, and metallurgical data is available from the internal records of the Idaho Maryland Mine are available for the period from 1936-1941. Internal mill reports were completed by the operator on daily, monthly and yearly reports. Metallurgical data was reconciled by the operator to bullion sales. The metallurgical data from the internal documents was transcribed from the monthly reports and reconciled to the annual reports. In general, the Idaho Maryland Mines Co. appears to have been a well-run company with excellent record keeping. It is believed the entire record of mill reports is contained in the library of documents. The I-M Mine’s historical financial statements, which recorded the results from bullion sales, appear to reconcile with the historical production and the reported metallurgical recovery rates included in the historical data that the Company has reviewed. The following was used to prepare an estimate during the period from 1936-1941:
- Idaho Maryland Mines Co. Mill Reports (1936-1941)
Detailed records of metallurgical recoveries from the I-M Mine prior to 1924 are not available. From 1924-1930 gold recoveries ranged from 72% to 89% using a similar process to that used in the years prior to 1924. Lindgren (1896) estimated that gold mills in the Grass Valley mines averaged 75% metallurgical gold recovery but noted that the I-M Mine was unique in that it treated the tailings from its concentrates by secondary processes. The Company has assumed a metallurgical recovery of 85% for the pre-1924 processing at the I-M Mine which it believes is the best estimate possible given the information available.