Abstract

Hydrocarbon migration pathways and organic mineral matter associations were used to identify brine pathways in Paleoproterozic to early Mesoproterozoic rocks from the Lawn Hill platform, Mount Isa. Several types of organic matter are identified, and their thermal imprints are used to reconstruct the thermal history of the northern to central parts of the Isa superbasin. Three major thermal hydrothermal episodes are recognized from the organic maturation studies. Isotherm plots on a 175-km-long structural-sedimentological north-south section of the Isa superbasin highlight specific fault systems that acted as hot fluid conduits during the geologic history of the basin. Some of these systems indicate continuing activity into the south Nicholson basin, supported by the presence of low reflectance (type B) bitumen. This bitumen has not been overprinted by later hydrothermal episodes and therefore represents the latest thermal event. Along the north-south profile a general southward increase in temperature is evident. The lowest temperatures are recorded in proximity to the basin margin on the southern flank of the Murphy inlier.

Thermal processes and their sequence of events in the basin are recorded by organic maturation, subsequent hydrocarbon generation, its migration and destruction coincident with transport and precipitation of minerals. As some timing and trapping mechanisms for minerals may have analogues with hydrocarbon entrapment, relative timing of processes leading to organic maturation, hydrocarbon generation and migration are utilized in this study to enhance understanding of ore-grade mineralization. In the Proterozoic successions of the Mount Isa basin multiple hydrocarbon generation events are recognized. These events record the transient passage of potential metal-bearing fluids rather than background conductive heat flow from the basement. Such hydrothermal fluids are responsible for inverse maturation profiles in the vicinity of the Termite Range fault and extreme maturation (reflectance values) up to 6 percent Ro at the Grevillea prospect. At Century, intermediate Ro values of <2 percent document the passage of fluids of less than 200°C. Interaction between these hydrothermal fluids and organic matter resulted in the creation of transient microporosity that was subsequently filled with sulfide minerals, principally sphalerite (porous sphalerite texture of previous workers). Pervasive destruction of organic matter in the host sediments at Century records high water/rock ratios during this mineralizing event. Previous models for Century mineralization interpreted an in situ gas and oil accumulation at the time of mineralization. This study shows that initial maturation of organic matter at Century coincided with the influx of hydrothermal fluids responsible for mineralization. There is no evidence for the existence of an earlier hydrocarbon accumulation.

In this study samples were only collected from organic matter-rich intervals, mainly shales and siltstones. Interbedded sandstones were not sampled. Therefore, it is not possible to determine which siliciclastic lithologic units transmitted the hydrothermal fluids. However, elevated reflectance values consistently occur adjacent to faults, both supersequence boundaries, surfaces of major lithologic contrast, suggesting that these features controlled fluid migration. Carbonate lithologic units sampled in this study are primarily from the Loretta supersequence (Walford Dolomite). These rocks preserve the residue of a primary migrated hydrocarbon overprinted by a hydrothermal fluid event responsible for destruction of the oil and the formation of mesophase textures characteristic of Mississippi Valley-type deposits.

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