Overall diamond production at the three mines in which Rio Tinto has a stake – Argyle (100%), Diavik (60%) and Murowa (78%) – declined by 15% year on year for the quarter ended September 30, 2014, the company reported while releasing its third quarter operations review. The three mines together produced 3.549 mn carats in Q3 2014 as compared to 4.158 mn carats in the same period a year earlier.

However, cumulative production over the first nine months of the year registered a smaller 4% decline from 11.529 mn cts to 11.031 mn cts.

The major drop during the nine months was seen at the Argyle mine where production fell by 10% to 7.375 mn cts as compared to 8.025 mn cts a year earlier. The statement said that reflected “the move from open pit to underground mining and the processing of lower grade tailings in the first quarter of 2014 as underground production ramped up”. It also noted that a second crusher was successfully commissioned during Q3 which “saw higher throughput offsetting lower grades to keep carat production in line with the previous quarter”.

The company reported strong operational performance in the first nine months of the year at Diavik which saw a 10% rise in production year-on-year from 3.103 mn cts to 3.413 mn cts. It noted that this was “due to improvements in mining rates, with all three pipes in full production, as well as processing plant improvements implemented over the past six months”.

However, it also said that production in Q3 was down compared to Q2 “as a result of lower grades and throughput after the majority of stockpiled ore was processed in the first half”.

Carats recovered at Murowa in the first nine months were 12% higher than in 2013 at 243,000 cts as compared to 217,000 cts a year earlier, while Q3 ’14 production was 29% higher than Q3 ‘13 due to improved throughput and grades.

Looking forward, the statement said that “2014 production guidance for diamonds is 15 million carats ( previously 16 million carats) which reflects a maintenance shutdown at Argyle to implement some improved design modifications to both underground crushers”.