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3D modelling of granite intrusions in northwest Tasmania using petrophysical and residual gravity data Esmaeil Eshaghi *, Anya M. Reading, Michael Roach and Matthew J. Cracknell, School of Physical Sciences (Earth Sciences) and ARC Centre of Excellence in Ore Deposits (CODES), University of Tasmania (UTAS), Mark Duffett and Daniel Bombardieri, Mineral Resources Tasmania Summary Tasmania in southeast Australia is underlain by basement rocks dating from the Precambrian to the Cretaceous. One important event in the tectonic evolution of Tasmania is the intrusion of Devonian Granites which has resulted in metamorphism and mineralization in some regions. We construct a regional-detailed 3D model for northwest Tasmania including major geological units and estimate density values of each unit to investigate the tectonic setting and geometry of Devonian Granites. Our model contains 20 units with different density properties. The residual gravity data have been used to model the properties of these units and consequently the geometry of Devonian Granites. The new inversion refines the geometry of the Devonian Granites with respect to initial models. A new granite intrusion is also revealed by this study which is the subject of further investigation. Introduction 3D modelling of potential fields may be used for a range of tectonic and geological investigations. Density contrasts between different subsurface units can be traced by gravity data and modelling inversion packages. Recent progress in programming and availibility of powerful computers have provided the ability to model complicated 3D subsurface structures. 3D models give insight into subsurface structures and properties. 3D models can be used for different purposes such as tectonic, environmental, geology, hazard assessment and mining. In 3D potential field modeling, the gravity and magnetic response of the created 3D model of the subsurface is compared to observed gravity and magnetic measurements. The model is refined by changing properties and/or geometries of strcutures to adjust the model with observed data, and hence, construct a robust, improved model. In this study, a 3D geological model of west and northwest Tasmania is constructed, constrained by geological information, geophysical 2D inverted models and geological surface maps. This model is constructed using the Paradigm GOCAD Software Suit and tested against the gravity and magnetic grids by the VPmg package. The main focus of this study is to investigate the geometry of

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Devonian Granites which intrude large regions of the study area. Geology and tectonic structure Tasmania, in southeast Australia, is an area with a complex tectonic and geological evolution. This island encompasses basement rocks from the Proterozoic to Cenozoic, divided into seven number of stratotectonic elements with different tectonic histories and chronostratigraphies including the King Island, the Rocky Cape, the Dundas, the Sheffield, the Tyennan, the Adamsfield-Jubilee, and the Northeast Tasmania elements (Seymour and Calver 1995). The study area encompasses parts of Precambrian Tyennan and Rocky Cape Elements, and Precambrian-Cambrian Dundas, Sheffield and Adamsfield-Jubilee Elements. These main elements contain major Proterozoic and Phanerozoic sedimentary/volcanic/ metamorphic units. Metamorphism was active in different eras from Precambrian to Cenozoic and affected the island through orogenies and intrusions. One of the key tectonic events in terms of mineralisation was the intrusion of Devonian Granites during the Tabberabberan Deformation during the Mid-to Late Devonian (390 Ma; (Black, et al. 2004)). The intrusion of granites across Tasmania began at 400 Ma before the onset of the Tabberabberan Orogeny in northeast Tasmania and continued until the early Carboniferous (400375 Ma; (Black, et al. 2005)). In western and northwest Tasmania, intrusion of Devonian granites occurred 375-350 Ma, with younging towards the west. The Housetop Granite is the oldest of the Devonian-Carboniferous granitoids within the study area with a maximum age range of 343380 Ma (Black, et al. 1997). The Heemskirk Granite (ca. 330.5-362 Ma), Pieman Granite (ca. 338.5-356.5 Ma), the Meredith Batholith (ca. 338.5-336 Ma) and the Granite Tor Granite (ca. 344-375 Ma) are major granite outcrops which affected the study area (Seymour and Calver 1995). Contact metamorphism around the Devonian Granite units in some regions has resulted in geological associations which have the potential for further mineral exploration. Geophysics data Gravity data across onshore and offshore Tasmania were obtained from the Mineral Resources Tasmania (MRT)

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3D modelling of granite in NW Tasmania

online database. More than 81500 gravity observations are compiled in this work across onshore Tasmania and merged with surrounding marine gravity observations to establish a reliable database. This database is processed for free-air and Bouguer anomaly correction. The residual gravity anomaly is calculated using the MANTLE-09 crustal gravity model (Leaman 2009). Petrophysical measurements Accurate modelling of the potential field data can be achieved based on knowledge of the physical properties of rocks such as density. Petrophysical properties of units across Tasmania have been investigated previously (e.g. Keele, 1992; Roach, 1994; Webster, 2003; Leaman, 2003). In order to improve available databases on the density of rocks across the study area, we collected additional core samples from different rock units from >100 drill holes. Drill holes were selected to have a reasonable geographic distribution, and intercept major units. Samples were collected from depths deeper than 100 m to minimize the effect of weathering. A total of 443 samples were obtained across the study area. The improved database includes 14 major rock units and determines their average density and its variation (Figure 1). Devonian Granites displays a low density, 2.62 g cm-3, with a good contrast compared to other units. Model construction Murphy et al. (2003) constructed a 3D geological model across Tasmania including major geological units to a depth of ca. 10 km. We use this model to construct the initial 3D model. We also use this model and elements from the Mineral Resources Tasmania high precision Rosebery-Lyell 3D model (Bombardieri et al, manuscript in preparation). The granite surfaces imported from model presented by Leaman and Richardson (2003) to improve the certainty of the model. The offshore model is constructed using WORMS, seismic sections (Drummond et al., 2000, Kennett et al., 2013) and sedimentary thickness from the SEEBASETM project (Frogtech pty Ltd, 2005). Study of offshore units is beyond the scope of this research and units are estimated offshore with low certainty. The model in this study encompasses the west and northwest of the island (157.5 km × 216 km). The final model contains 20 geological units extending down to 10 km depth. The model is constructed using the Paradigm GOCAD Package and discretized into 500 m × 500 m vertical prisms. The topography- bathymetry grid with 250 m resolution is used to construct the upper surface. Figure 2 shows the initial model.


Inversion 3D forward modeling and inversions were undertaken using VPmg (Fullagar et al., 2008, Fullagar, 2013). VPmg is developed to invert geologically-constrained potential field data by independently performing two main gravity inversion modes: density optimization (property inversion), and geometry optimization (contact elevation). Inversion is performed to achieve an acceptable degree of fit to the data, subject to the geological and petrophysical constraints on the model.

Figure 1) Boxplot analysis of density (g/cm3) measurements individually represented by lithology. The vertical axis show density (g cm-3) and the horizontal axis shows lithology. Minimum, quartile1, median, quartile3 and maximum of density values for each lithology are presented. CMUC represents the Cambrian Mafic-Ultramafic Complexes and is divided into two sub-groups of low (unit 1) and high (unit 2) densities.

The inversion procedures in VPmg solve the perturbation equations using the Method of Steepest Descent and do not perform matrix inversions. This results in a fast inversion speed due to the abandonment of singular value decomposition (SVD). During inversion, degree of fit is judged according to the magnitude of the chi-squared data norm, L2, and the L1- data norm and RMS misfit. The model is deemed “acceptable’ if L2 ≤ 1 and/or if L1 ≤ 1 (Fullagar, 2013). Inversion stalls or finishes, if it reaches to the maximum allowed iterations, or the RMS misfit reaches to the determined uncertainty, or further iterations do not reduce the RMS misfit. In this study, we performed both styles of inversion, homogeneous inversion of units (property inversion) and geometry inversion, to adjust the model to fit with the observed data.

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Model Inversion Forward modelling was performed upon the constructed 3D model. Density properties estimated from petrophysical measurements have been used for this model. Forward model shows a 15.17 mgal RMS misfit between observed and calculated model (Fig. 3b). The observed gravity values (Fig. 3a) have an amplitude of ~64 mgal, so the initial forward model shows a ~24.5% misfit. Homogeneous inversion was performed upon this model which reduced the misfit to 7.65 mgal (~12%). Despite the reduction in misfit, Devonian Granites present a very high density (2.65 g cm-3) in the inverted model. While an increase in density of the Devonian Granites compensates the misfit in north of the study area over the Housetop Granites (Fig. 3b (1)), it intensifies the misfit in coastal regions corresponding to the Heemskirk Granites (Fig. 3b (2)). Homogeneous inversion shows that the density variations of other units are in an acceptable range, compatible with petrophysical measurements. Also, the anomalous misfit in some regions shows that the geometry of the model should be refined. Figure 3b shows that the geometry of Devonian Granites accounts for most of anomalous misfit. Misfit across Housetop Region shows that the Housetop Granites have different geometry compared to the initial model and smaller volume of granites account for the negative gravity anomaly. In contrast, the Heemskirk Granites should have different geometry with more granites near the surface and extending to deep units (Fig 3.b (2)). Another anomaly, on the eastern side of the Rocky Cape Group, indicates the location of a less dense unit, likely a volume of the Devonian Granite, at depth beneath this region (Fig 3b (3)).

Figure 2- Created 3D model including 21 geological units. Different colors show different units. The islands’ boundary is shown in the figure. The voxet goes down to 10 km depth (depth exaggeration: 5).

misfit (15.6%). Homogeneous inversion reduces this misfit to 5.84 (9%) with acceptable properties within the estimated range. The remnant misfit should likely be assigned to geometry of other units in the region without granite intrusions. Conclusion

Leclere (1996) modeled the Housetop Granites in 2D sections and showed that this granite body is not as thick as the initial modeled. Hence, geometry inversion of granites in the Housetop region can account for most of the anomalous misfit. In the geometry inversion of the granites in the Housetop region, the top of granite units is fixed. In contrast, misfit across the Heemskirk region indicates that the granite body is closer to the surface in adjacent regions. Due to the high frequency anomalies, the top surface of the granites across Heemskirk region accounts for the misfit and the geometry inversion is performed on both top and bottom of the granites in this region.

We constructed a new, more robust 3D model of Devonian Granites across the northwest Tasmania. Inversion shows that the Devonian Granites across the study area have different geometry than previous models. The Housetop Granites are relatively thin with a maximum thickness of a few kilometers. The Heemskirk Granites displays a thick body with shallow burial depth. Heemskirk Granites continue to the base of model in 10 km and misfit indicates that this unit extends further down at least for a few kilometers. A new granite body is proposed under the Rocky Cape Group. This unit is at depth with no exposure at the surface, and merits further investigation.

New 3D model contains a new granite body close to the Rocky Cape Group, thinner granites across the Housetop region and thicker and shallower burial depths of granites in the Heemskirk region. The new model is used for homogeneous inversion to compare estimated densities with the homogeneous results. Forward modeling using petrophysical estimated properties displays 9.99 mgal

Acknowledgments


We would like to acknowledge research support by UTAS through a Tasmanian Graduate Research Scholarship (EE) and thank N.G. Direen for his generosity towards student research in geophysics at UTAS. We also thank our colleagues at UTAS and MRT for ongoing discussion.

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(a)

(b)

(c)

Figure 3- Gravity data and modelling misfit. a) Observed gravity data. b) Residual gravity data (observed-calculated). c) Residual anomaly after geometry inversion of the Devonian Granites. Region with positive residual (redish) need higher densities and regions with negative residual data (bluish) need less density. Number 1 is the Housetop Region, number 2 is the Heemskirk Region and number 3 shows the anomalous residual in east of the Rocky Cape Group.

(a)

(b)

Figure 4: Devonian Granites in model. a) Devonian Granite in the initial 3D model extending down to 9.5 km. b) Devonian Granites after inversion. The volume of Devonian Granites reduced in the Housetop Region and increased in the Heemskirk Region.


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EDITED REFERENCES Note: This reference list is a copyedited version of the reference list submitted by the author. Reference lists for the 2016 SEG Technical Program Expanded Abstracts have been copyedited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES

Black, L. P., C. R. Calver, D. B. Seymour, and A. Reed, 2004, SHRIMP U-Pb detrital zircon ages from Proterizoic abd Early Palaeozoic sandstone and their bearing on the early geological evolution of Tasmania: Australian Journal of Earth Sciences, 51, 885–900, http://dx.doi.org/10.1111/j.14000952.2004.01091.x. Black, L. P., M. P. McClenaghan, R. Korsch, J. L. Everard, and C. Foudoulis, 2005, Significance of Devonian-Carboniferous igneous activity in Tasmania as derived from U-Pb SHRIMP dating of zircon: Australian Journal of Earth Sciences, 52, 807–829, http://dx.doi.org/10.1080/08120090500304232. Black, L. P., D. B. Seymour, K. D. Corbett, S. F. Cox, J. E. Streit, R. S. Bottrill, C. R. Calver, J. L. Everard, G. R. Green, M. P. McClenaghan, J. Pemberton, J. Taheri, and N. J. Turner, 1997, Dating Tasmania’s oldest geological events. tasmania national geoscinece mapping accord, mineral resource tasmania: Australian Geological Survey Organisation: Record, 1997, 1–57. FrogTech, 2005, OZ SEEBASETM Study 2005: Public Domain Report to Shell Development Australia by FrOG Tech Pty Ltd. Fullagar, P. K., 2013, VPmg user documentation, version 7.1: Fullagar geophysics Pty Ltd, Report FGR01F-4. Fullagar, P. K., G. A. Pears, and B. McMonnies, 2008, Constrained inversionof geologic surfaces pushing the boundaries: The Leading Edge, 27, 98–105, http://dx.doi.org/10.1190/1.2831686. Drummond, B. J., T. J. Barton, R. J. Korsch, N., Rawlinson, A. N. Yeates, C. D. N. Collins, and A. V. Brown, 2000, Evidence for crustal extension and inversion in eastern Tasmania, Australia, during the Neoproterozoic and Early Paleozoic: Tectonophysics, 329, 1–22. Keele, R., 1992, The king river regional cross-section: A transitional through the Dundas Trough., CODES : AMIRA Project P291 - Report 4. Kennett, B., E. Saygin, T. Fomin, and R. Blewett, 2013, Deep crustal seismic reflection profiling: Australia 1978-2011: ANU Press and Geoscience. Leaman, D. E., 2003, Quantitative interpretation of magnetic and gravity data for the Western Tasmania regional mineral program - Part 1: Mineral Resources Tasmania: Tasmanian Geological Survey: Record, 2002, 9–51. Leaman, D. E., 2009, MANTLE-09––A new crustal gravity model for Tasmania. Leaman, D. E., and R. G. Richardson, 2003, A geophysical model of the major Tasmanian granitoids, Mineral Resources Tasmania: Tasmanian Geological Survey: Record, 2003. LeClere, M., 1996, The geophysics of the housetop granite: Bachelor of science with honours: University of Tasmania. Murphy, B., K. Denwer, R. Keele, P. Stapleton, R. Korsch, D. Seymour, and G. Green, 2003, Tasmania Mineral Province Geoscientific database: 3D Geological Modeling, Mines and Mineral Prospectivity; Project T3: Release Notes, pmdCRC, University of Melbourne, Mineral Resource, 1–83. Roach, M. J., D. E. Leaman, and L. M. Richardson, 1993, A comparison of Regional - Residual separation techniques for gravity surveys: Exploration Geophysics, 24, 779–784, http://dx.doi.org/10.1071/EG993779.


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Seymour, D.B. and C.R. Calver, 1995, Explanatory notes for the time-space diagram and stratotectonic elements map of tasmania:, Tasgo NGMA Project: Sub-Project 1: Geological Synthesis, Tasmanian Geological Survey record 1995/01. Seymour, D. B., G. R. Green, and C. R. Calver, 2007, The geology and mineral deposits of Tasmania: A summary, Geological survey: Bulletin, 72, 32. Webster, S. S., 2003, Quantitative interpretation of magnetic and gravity data for the Western Tasmanian Regional Mineral Program, Part 2: Mineral Resources Tasmania- Tasmanian Geological Survey: Record, 2002, no. 15, 52–91.


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This is the title of an example SEG abstract using Microsoft Word 11 ...

This is the title of an example SEG abstract using Microsoft Word 11 ...

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