ARKANSAS RIVER VALLEY CARBONATITES

By Henry Barwood

Carbonatites were first reported from West-Central Arkansas in 1888;however, they were long considered to be altered lamprophyres (Croneis and Billings, 1930a,b). There are six known outcrops of carbonatite in the Arkansas River Valley area of West Central Arkansas: 1) the Oppello Breccia 2) the Opello Dump site 3) Morrillton Dam spillway site 4) the Arkansas River sills 5) the Perryville breccia and 6) the Brazil Branch breccia (Mitchell, 1977). Many more localities probably exist, but are obscured by weathering or vegetation. A gas test well (Sinclair Oil and Gas #1) drilled to 11,850 feet near Oppello penetrated 41 carbonatites up to an apparent thickness of 30 feet (Boyd Haley pers. Comm. 1989 and Henry Barwood unpublished data). All the exposed carbonatites contain an extensive suite of xenoliths that include fenitized metamorphic rocks brought up from an unknown depth, but estimated at 15 kilometers.

The mode of emplacement of the carbonatites is unusual. At the Oppello Dump, Morrillton Dam and the Arkansas River, the carbonatites are clearly sills (in part dikes where multiple sills have feeders). At none of these sites is any evidence of wall rock alteration apparent, but shale and sandstone xenoliths within the carbonatites range from barely altered to near completely assimilated. Emplacement was apparently both rapid and at a late stage when the carbonatites were quite cool and dry. The Oppello and Perryville breccias are pipe-like bodies that are the apparent result of explosive emplacement. The Brazil Branch breccia is so poorly exposed that the relationship to the wall rocks cannot be determined, but it too is likely a pipe. The breccias also contain xenoliths, but there is much less carbonate groundmass.

The bulk mineralogy of the carbonatites is calcite, mica (biotite, phlogopite and vermiculite alteration) and fluorapatite. Minor diopside and magnetite xenocrysts and zeolites, especially analcime, are also found. Quartz and feldspars dominate most of the xenoliths and there is an extensive associated suite of trace minerals. The carbonate component is exclusively calcitic and no dolomite or siderite has been detected except in replacement rims around xenoliths. Large biotite xenocrysts are characteristic of the carbonatites and indeed, where weathered, may be the only component visible in the field that causes recognition of the bodies as igneous. Fine grained phlogopite and small apatite crystals comprise the rest of the groundmass.

Cathodoluminescent examination of the carbonatites yields the greatest amount of information about the mineralogical relationships within the rocks. The calcite is brightly luminescent orange and exhibits zoning where replacement crystals are developed as rims around xenoliths. The xenoliths show violet quartz grains and blue feldspars typically isolated and corroded by replacement calcite. Shale xenoliths, especially at Brazil Branch, show mustard yellow luminescing rims of feldspar alteration. Apatite grains show the most complex cathodoluminescence and are generally zoned. The earliest apatites show Mn activation and are zoned in shades of yellow. Later enrichment of REE causes violet luminescing apatite, often grading from the Mn activated cores, but more frequently forming distinct rims on earlier formed crystals.

This paper will report on the cathodoluminescence work and present some preliminary data on the deep carbonatite cuttings obtained from the Sinclair Oil and Gas No.1 well.

Croneis, C.G. and M.P. Billings. 1930a. Igneous rocks in central Arkansas. In: Geology of the Paleozoic area of Arkansas. Arkansas Geological Survey, Bull. 3:149-162

Croneis, C.G. and M.P. Billings. 1930b. New areas of alkaline igneous rocks in central Arkansas. J. Geol. 37:542-561
Mitchell, J. R. 1977. The petrology of metamorphic xenoliths in carbonatite intrusions in west-central Arkansas. Unpub. University of Arkansas M.S. thesis. 122 p.

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