Bulletin of the Geological Society of Malaysia, Volume 19, April 1986, pp. 303 – 314
University of Technology, Lae, Papua New Guinea
Abstract: The chromite deposits of Papua New Guinea occur in the New Guinea ophiolite belt that comprises the peridotite-gabbro-basalt complex. The two most important of these deposits, the Ramu and Hessen Bay chromites occur disseminated in dunites, harzburgites and pyroxenites and are uneconomic in their primary forms. Parts of these deposits, however, had undergone weathering and natural concentration to produce economic grade ores. The Ramu ore estimated at about 225 million tonnes is lateritic and forms two layers; the top layer has a grade of 9%Cr, 1.46% Ni and 0.16%Co, and the bottom layer has a grade of 6%Cr, 1.12%Ni and 0.06%Co. The Hessen Bay chromite ore, however, is a beach ore of a grade in excess of 9%Cr and its reserve is yet to be determined. None of these deposits is presently being mined.
The present work presents the investigation of the Ramu and Hessen Bay chromites by microprobe aided by chemical analysis and microscopic examination to assess the compositional variations, the source of the highly magnetic property of the chromites and the amenability of the chromite ores to upgrading. The results show that both chromites are similar in composition and exhibit characteristics typical of podiform type chromites, namely low Fe3+ and Ti but significant AI substitution for Cr. As a consequent of the high level of Fe2+, the Cr/Fe ratios for Ramu and Hessen Bay chromites are 1.62 and 2.0 respectively. The results are compared with the data for other ophiolitic chromite deposits. By spiral and table concentration of the ores, a concentrate grade of 45% Cr2O3 at 78%Cr recovery for the Ramu ore and a grade of 37% Cr2O3 at 54%Cr recovery for the Hessen Bay ore were achieved.