In the Shadow of Large Gravity Anomalies - Recovering Useful Gravity Data for Geologic Exploration

The Nash Creek area in Northern New Brunswick, Canada, is home to a blind Zinc Lead Silver sulphide deposit hosted within a complex Ordovician-Devonian rift system. Extensive exploration of the area over several decades has resulted in a varied set of data at multiple scales, including – but not limited to – magnetic, radiometric, and high resolution LIDAR DEM data at the regional scale and TEM, resistivity, and high resolution terrestrial gravity data at the local scale. Additionally, approximately 400 boreholes have been drilled and logged over and around the known extents of the mineral deposit. This has also resulted in a comprehensive petrophysical database, including the densities of different rock units. Despite this wealth of data, many questions about the geology of the area remain unresolved, and the deposit is poorly constrained. This is in large part due to a thick layer of overburden blanketing the region, rendering existing geological maps ambiguous. Moreover, the gravity data has been difficult to incorporate as it shows a steep gradient over the region.

 

In order to best characterize this gradient, large scale regional gravity data must be analysed. Unfortunately, geodetic control stations (GCSs) are sparse within the region. Thus, we turn to a satellite derived global gravity model (GGM) to get a picture of the regional gravitational field. Using the GGM, we see that a large positive gravity anomaly of approximately 20 mGal lies directly to the west of the study area, and a similarly sized negative anomaly lies just to the east. Over the study area itself, the regional field changes at a rate of roughly 1.62 mGal/km. The primary goal of this study is to investigate the optimal method to remove this gravity gradient from our high resolution dataset. We characterize the regional field by combining the GCS data with the GGM, and then use this regional field approximation to isolate the residual field in our high-resolution gravity dataset. We then compare the results to those attained using other methods, such as the upward continuation method of regional field approximation.

 

Another goal of this study is to study the source(s) of this distinctive gravitational feature. We do this by using mass excess/deficiency calculations, and by incorporating regional magnetic, radiometric, and geologic data into our interpretation. We also compare this feature to other similar large scale gravity anomalies. Studying the cause of the gravity anomaly at Nash Creek can help to achieve a better understanding of the regional geologic history, and potentially help to identify the most promising approaches for geophysical exploration in this complex geological setting.