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Foraminiferal Coloration Index as a Guide to Hydrothermal Gradients around the Porgera Intrusive Complex, Papua New Guinea

Mark Gunson

University of Western Australia, Department of Geology and Geophysics, Nedlands 6907, W.A., Australia

Greg Hall

Placer Exploration Limited, PO Box 764, Cloverdale 6985, W.A., Australia

Mike Johnston

Porgera Joint Venture, PO Box 484, Mt. Hagen, W.H.P., Papua New Guinea, Australia

Corresponding author: e-mail, mgunson{at}geol.uwa.edu.au

The Porgera intrusive complex and gold deposit is hosted in friable black mudstones of Cretaceous age in the deformed northern part of the Papuan basin, Papua New Guinea. Hydrothermal fluids associated with the Pliocene intrusions created thermal gradients in the surrounding sedimentary rocks. These gradients were partitioned by preexisting faults in the sedimentary rocks creating cells of fluid circulation.

Assemblages of agglutinated foraminifera reported from North America have shown color gradients, which are interpreted as being related to the thermal maturation of sedimentary rocks down drill holes. This paper presents the first known application of color changes in foraminifera (marine protistan microfossils) to hydrothermal mineral exploration. The foraminiferal assemblages of the bathyal mudstones from the Porgera region contain abundant agglutinated foraminifera. Color changes, recognized in the agglutinated foraminifera, were used to map the thermal maturation of sedimentary rocks in the Porgera area. Palynological fluorescence, used in petroleum exploration to determine thermal maturation, could not be used owing to the over-maturation of the organicmaterials throughout the area surrounding the Porgera intrusive complex. The agglutinated foraminifera showed a gradation in color, from white to dark gray, in surface samples taken throughout the Porgera area. Mapping of the foraminifera colors using a geographic information system (GIS) illustrated the thermal maturation of the sedimentary rocks around the intrusive complex owing to conductive heating. In addition, the mapping showed patterns consistent with convective heating caused by hydrothermal fluid circulation. The foraminiferal coloration showed pairing of hot and cold areas across major structures, interpreted to be associated with the upflow (hot) and recharge (cold) of fluids from the surrounding host sedimentary rocks.

Illite crystallinity and vitrinite reflectance analyses conducted on a transect of samples from across the Porgera intrusive complex supported the proposed hydrothermal fluid circulation model interpreted from the foraminiferal colors. Paleotemperatures associated with the foraminiferal coloration index from the Porgera region range from 205° to 415°C, and are anomalously high in comparison to other parts of the Papuan fold and thrust belt.

The Foraminiferal Coloration Index (FCI) is a guide to the thermal maturation of sedimentary rocks and provides a useful and low-cost exploration tool for locating and evaluating the size of buried intrusive systems in late Palaeozoic to Cenozoic marine shales. It is an invaluable tool for determining the size and partitioning of a hydrothermal cell associated with hydrothermal/epithermal mineral deposits. The FCI method is also useful on a regional scale in areas where sandstones, shales, or limestones blanket aero-magnetic and resistivity data. The technique is used currently to identify drilling targets for the exploration of the Porgera deposit.

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