Chromite Composition and Accessory Minerals in Chromitites from Sulawesi, Indonesia: Their Genetic Significance

Abstract

Several chromite deposits located in the in the South and Southeast Arms of Sulawesi, Indonesia, have been investigated by electron microprobe. According to the variation of the Cr# = Cr/(Cr + Fe 3+ ), the chromite composition varies from Cr-rich to Al-rich. Small platinum-group minerals (PGM), 1-10 µm in size, occur in the chromitites. The most abundant PGM is laurite, which has been found included in fresh chromite or in contact with chlorite along cracks in the chromite. Laurite forms polygonal crystals, and it occurs as a single phase or in association with amphibole, chlorite, Co-pentlandite and apatite. Small blebs of irarsite (less than 2 µm across) have been found associated with grains of awaruite and Co-pentlandite in the chlorite gangue of the chromitites. Grains of olivine, occurring in the silicate matrix or included in fresh chromite, have been analyzed. They show a composition typical of mantle-hosted olivine. The bimodal composition and the slight enrichment in TiO 2 observed in some chromitites suggest a vertical zonation due to the fractionation of a single batch magma with an initial boninitic composition during its ascent, in a supra-subduction zone. This observation implies the accumulation of Cr-rich chromitites at deep mantle levels and the formation of the Al-rich chromitites close or above the Moho-transition zone. All of the laurites are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at temperature of around 1200˝C and a sulfur fugacity below the sulfur saturation. Irarsite possibly represents a low temperature, less than 400˝C, exsolution product.

Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia

“…In contrast, the majority of the supra-Moho chromitites are Al-rich and characterized by a variable content of TiO 2 (0.15 wt % to 0.8 wt %). Confirming the previous work by [8,[14][15][16][17]21], the composition of the Alapevsky chromite indicates the coexistence of the Al-rich and Cr-rich chromitites, a common feature of many ophiolitic belts worldwide [22,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50].…”

The Cyclops Ophiolite as a Source of High-Cr Spinels from Marine Sediments on the Jayapura Regency Coast (New Guinea, Indonesia)

“…Black dots: high-Cr spinels, grey dots: high-Al spinels. The composition and origin of chromian spinels is very similar to chromitite from Sulawesi reported by [54]. Spinels were generated in the SSZ environment from a single batch magma with initial high-Cr bonititic composition.…”

“…This proves melt differentiation towards MORB. The change of Cr# and Mg# indexes shows the gradual chemical depletion of original peridotites ( Figure 11A) [54].…”

“…PGMs have been found in several locations in the Indonesian Archipelago: (1) chromite deposits in the South and Southeast of Sulawesi [54]; (2) Alaskan-type ultramafic intrusions in the Kalimantan [65];…”

Geochemistry of Ophiolite Complex in North Konawe, Southeast Sulawesi

“…The biggest ophiolite outcrop in Sulawesi is located on Southeastern Arm; even smaller ophiolite complex occurrences also described in the South Arm [8]. Ultramafic rocks, the important sequence of ophiolite, distributed in many locations in eastern Indonesia which also being explored and exploited for national interest.…”