Stone Ridge Quarry Project
Magnetic Susceptibility & Ground Magnetic Survey
Background and Objectives
Initial visual analysis of different rock types obtained from surface outcrop and preliminary diamond drilling revealed that the magnetic mineral magnetite (Fe3O4) is a common accessory mineral within most of the different rock types examined. The magnetite generally occurs as fine disseminations within the volcanic rocks and is of a primary igneous nature. In contrast, magnetite present within the sedimentary units was observed to be of a secondary nature, occurring as thin bedding-related bands.
During the initial stages of the resource assessment work program, magnetic susceptibility measurements were undertaken on outcrop and core samples collected for petrographic work. This confirmed that the magnetic susceptibilities of different rock types generally contrast as a result of differing magnetite contents, making the use of magnetic geophysical survey methods worth considering for mapping the lateral extent of the different rock types within the Project Area.
A high-resolution ground magnetic survey was subsequently undertaken over the Eagleton Volcanics within the Project Area during September 2018, with the objective of more accurately mapping the lateral extent of different geological units defined by surface mapping, as well as any cross-cutting structural zones (e.g. faults, shears) defined by contrasting magnetic signatures.
Magnetic susceptibility measurements were also undertaken on all diamond core, with measurements taken at 1 metre depth intervals for the full length of each drill hole.
Methodology
Fender Geophysics was commissioned to undertake the ground magnetic survey and correct the survey data to account for diurnal drift in the Earth’s magnetic field.
The ground magnetic survey commenced on the 17 September 2018 and was completed on the 23 September 2018. The survey covered an area of approximately 125 hectares between Italia Road and Nine Mile Creek, centred on Stone Ridge. East-west oriented survey traverses were nominally spaced at 20m intervals, and survey data was collected using two Geometrics 859AP Mineral MagTM caesium vapor mobile magnetometers with a cycle time of 0.2 seconds and resolution of 0.1 nT resolution. A Geometrics 856 base magnetometer with a cycle time of 15 seconds and resolution of 0.1 nT was established to record the diurnal fluctuation in the Earth’s magnetic field. All data was positioned by internal TallysmanTM GPS.
A total of 348,836 magnetic readings were recorded by the mobile magnetometers along approximately 52 survey traverses (Figure 6). Survey data was subsequently corrected to remove the diurnal fluctuation in the Earth’s magnetic field recorded by the base magnetometer, thereby generating a data set that reflects the magnetic character of the underlying geology. The corrected data was then imaged by ARDG.
Initial visual analysis of different rock types obtained from surface outcrop and preliminary diamond drilling revealed that the magnetic mineral magnetite (Fe3O4) is a common accessory mineral within most of the different rock types examined. The magnetite generally occurs as fine disseminations within the volcanic rocks and is of a primary igneous nature. In contrast, magnetite present within the sedimentary units was observed to be of a secondary nature, occurring as thin bedding-related bands.
During the initial stages of the resource assessment work program, magnetic susceptibility measurements were undertaken on outcrop and core samples collected for petrographic work. This confirmed that the magnetic susceptibilities of different rock types generally contrast as a result of differing magnetite contents, making the use of magnetic geophysical survey methods worth considering for mapping the lateral extent of the different rock types within the Project Area.
A high-resolution ground magnetic survey was subsequently undertaken over the Eagleton Volcanics within the Project Area during September 2018, with the objective of more accurately mapping the lateral extent of different geological units defined by surface mapping, as well as any cross-cutting structural zones (e.g. faults, shears) defined by contrasting magnetic signatures.
Magnetic susceptibility measurements were also undertaken on all diamond core, with measurements taken at 1 metre depth intervals for the full length of each drill hole.
Methodology
Fender Geophysics was commissioned to undertake the ground magnetic survey and correct the survey data to account for diurnal drift in the Earth’s magnetic field.
The ground magnetic survey commenced on the 17 September 2018 and was completed on the 23 September 2018. The survey covered an area of approximately 125 hectares between Italia Road and Nine Mile Creek, centred on Stone Ridge. East-west oriented survey traverses were nominally spaced at 20m intervals, and survey data was collected using two Geometrics 859AP Mineral MagTM caesium vapor mobile magnetometers with a cycle time of 0.2 seconds and resolution of 0.1 nT resolution. A Geometrics 856 base magnetometer with a cycle time of 15 seconds and resolution of 0.1 nT was established to record the diurnal fluctuation in the Earth’s magnetic field. All data was positioned by internal TallysmanTM GPS.
A total of 348,836 magnetic readings were recorded by the mobile magnetometers along approximately 52 survey traverses (Figure 6). Survey data was subsequently corrected to remove the diurnal fluctuation in the Earth’s magnetic field recorded by the base magnetometer, thereby generating a data set that reflects the magnetic character of the underlying geology. The corrected data was then imaged by ARDG.
Magnetic Domains
The Total Magnetic Intensity (TMI) image (Figure 7) was generated from the corrected magnetic survey data. Figure 8 presents a magnetic domain and lineament interpretation of the TMI imagery, whereas Figure 9 overlays geological mapping data over the TMI image and interpretation.
The Total Magnetic Intensity (TMI) image (Figure 7) was generated from the corrected magnetic survey data. Figure 8 presents a magnetic domain and lineament interpretation of the TMI imagery, whereas Figure 9 overlays geological mapping data over the TMI image and interpretation.
The magnetic character of the Eagleton Volcanics within the Project Area can be divided into five (5) contrasting domains that directly reflect different geological units. These domains are cut and/or offset by several prominent, structure-related magnetic lineaments (i.e. faults).
Magnetic Domain 1 (MD1) comprises two separate areas (MD1A, MD1B) of relatively high magnetic intensity that correspond with mapped areas of outcropping dacite northwest of Stone Ridge. These sub-domains are separated by Magnetic Domain 2 which appears to underlie MD1A and overlie MD1B. MD1A is disrupted by two magnetic lineaments (ML1 and ML3), with ML1 truncating the southern end of this sub-domain. The northeast end of MD1A appears truncated by magnetic lineament ML4.
The magnetic signature of MD1A is particularly uniform and coherent, suggesting the dacitic rocks in this location are massive and unaltered. Several areas of lower magnetic intensity occur within MD1B and in part can be attributed to outcrop of non-magnetic rhyolitic vitric-crystal tuff. The highest magnetic intensity areas within MD1 occur where MD1A and MD1B are in contact with overlying rhyodacite of Magnetic Domain 3. The magnetic gradient across the contact supports the interpretation that dacitic rocks dip beneath the rhyodacite.
Magnetic susceptibility measurements on petrographic samples of dacite (surface outcrop and drill core) from Magnetic Domain 1 reflect the typically high magnetic signature of this domain. The dacite samples typically have magnetic susceptibilities in the range 1800-2600 x 10-5 SI units. In contrast, occasional narrow zones of variably altered crystal-vitric rhyolitic tuff have magnetic susceptibilities in the range < 10-70 x 10-5 SI units.
Magnetic Domain 2 (MD2) is an area of moderately high magnetic intensity that separates MD1A and MD1B. This domain is associated with gently undulating topography to the north of the central ridgeline and is coincident with surface exposure of volcanic breccia and andesitic volcanics. The magnetic signature of this domain is relatively flat and is cut by three (3) structure-related linear magnetic anomalies (ML1, ML2, ML3). Minor outcrops of crystal-vitric rhyolitic tuff also occur within the footprint of this domain, the most significant of which is associated with a prominent north-south trending magnetic ‘low’ between ML1 and ML3.
Magnetic susceptibility measurements on petrographic samples of volcanic breccia and andesitic volcanics (surface outcrop and drill core) from this domain are consistent with the TMI image, with MD2 rocks being less magnetic than those from MD1, but significantly more magnetic than rocks from all other domains.
Samples of volcanic breccia with a significant matrix component typically have magnetic susceptibilities in the range 900-1200 x 10-5 SI units. Samples of massive andesite (that possibly represents large clasts within the volcanic breccia, up to several metres in size) as well as andesitic volcaniclastics, typically have much higher magnetic susceptibilities in the range 1500-2400 x 10-5 SI units. This contrasts with very low magnetic susceptibilities associated with rhyolitic crystal-vitric tuff.
Magnetic Domain 3 (MD3) is an area of generally much lower magnetic intensity that is directly associated with outcropping rhyodacitic volcanic rocks on Stone Ridge. The noisy, stippled magnetic signature of this domain on the TMI imagery is a direct influence of extensive surface outcrop and absence of a significant weathering profile. The northwest side of MD3 is in contact with MD1 and MD2, and with the exception an area to the immediate west of the Stone Ridge saddle, the TMI image suggests the contacts between these domains are stratigraphic, rather than structural, in nature. The northwest contact of MD3 with MD1A and MD1B is characterised on the TMI image by strong negative dipolar anomalies caused by the significantly higher magnetic intensity of rocks in MD1. As indicated previously, the steep magnetic gradient along this contact suggests the rhyodacite is underlain by dacitic rocks of MD1. MD3 is impacted in the vicinity of the Stone Ridge saddle by three (3) structure-related linear magnetic anomalies (ML1, ML2, ML3). The northeast limit of this domain is truncated by magnetic lineament L4.
Rhyodacite samples have a broad range of magnetic susceptibilities (< 10-1870 x 10-5 SI), consistent with the greater range in magnetic intensity reflected in the TMI image. However, the overall average of rhyodacite magnetic susceptibilities is significantly lower than rocks associated with MD1 and MD2.
Magnetic Domain 4 (MD4) is an area of relatively low magnetic intensity that runs parallel to the southeast flank of Stone Ridge. This domain corresponds with the upper extent of the Eagleton Volcanics and extends in width from the downslope extent of rhyodacite outcrop (i.e. approximate upper limit of rhyodacite) through to and including the prominent altered rhyolitic vitric-crystal tuff outcrop that strikes northeast-southwest, parallel to Stone Ridge.
Outcrop within this domain is limited to the altered rhyolitic tuff. The mapped extent of the tuff closely correlates with a linear zone of very low magnetic intensity on the southeast edge of this domain. A magnetic susceptibility measurement on a surface outcrop sample of rhyolitic tuff produced a reading of < 10 x 10-5 SI, indicating the sample is effectively devoid of magnetite.
Diamond drilling has confirmed that the geology between the rhyodacite (MD3) and altered rhyolitic tuff of MD4 is characterised by an interbedded sequence of volcanic sandstone, siltstone, tuff and minor conglomerate.
Magnetic Domain 5 (MD5) is an area of moderate to high and variable magnetic intensity beyond the northern end of Stone Ridge and MD1A and MD3. It is associated with flat topography that drains towards Nine Mile Creek and outcrop within this domain is limited to minor exposures of volcanic sandstone and rhyolitic tuff on a low ridge near the northern edge of the ground magnetic survey area. The southern edge of this domain corresponds with a magnetic lineament (L4) and is therefore interpreted to be of a structural nature.
Magnetic Lineaments
Magnetic Lineament 1 (ML1) is a northwest-southeast (288° grid) oriented feature defined on the TMI image by a combination of negative magnetic anomalism through MD1B and MD2; and more subtle magnetic contrast through MD3 and MD4.
ML1 passes through Stone Ridge in the approximate position of the saddle and can be traced across the Project Area for approximately 1400m, through the entire Eagleton Volcanics sequence. It defines the southwest limit of MD1A and the northeast side of a 95-metre-wide gap in outcrop exposure that extends across the Project Area. This gap is most clearly observed within the rhyolitic tuff horizon at the top of the Eagleton Volcanics.
Diamond drilling has since confirmed that ML1 is associated with a major zone of faulting, shearing, brecciation and dyke emplacement – now referred to as the Central Fault.
Magnetic susceptibility measurements of dolerite core samples from hole that intersected ML1, revealed some of the highest magnetic susceptibilities (2780-5170 x 10-5 SI) of any rocks within the Project Area. Their spatial association with a zone of apparent negative magnetism in the TMI image suggests these dyke rocks display magnetic remanence and have reversed polarity.
Magnetic Lineament 2 (ML2) is located approximately 90 metre south of and parallel to ML1. It is defined on the TMI image by more subtle discontinuous negative magnetic anomalism through MD1B and MD2; and more subtle features through MD3 and MD4.
ML2 can also be traced across the Project Area for approximately 1000m, through the entire Eagleton Volcanics sequence. It defines the southern side of the 95-metre-wide gap in outcrop exposure that again, is best observed within the rhyolitic tuff horizon at the top of the Eagleton Volcanics.
More recent drill testing across ML2 has failed to identify any evidence of faulting, shearing or brecciation. Accordingly, ML2 (the Central Fracture) may be a subsidiary structure related to the Central Fault, but one that has not experienced any major deformation and displacement.
Magnetic Lineament 3 (ML3) is a northwest-southeast (314° grid) oriented feature defined on the TMI image by subtle discontinuous negative magnetic anomalism through MD1A, MD1B and MD2.
ML3 can be traced from the northern edge of the TMI image at the base of the Eagleton Volcanics, through to the Stone Ridge saddle, where it appears to terminate against ML1. ML3 defines an apparent sinistral offset of the southern end of MD1A dacite, as well as the northern edge of rhyolitic tuff outcrop within MD2.
The orientation of ML3 and its relationship to ML1, suggests it may define a splay fault off the Central Fault.
Magnetic Lineament 4 (ML4) is a southeast-northwest (298° grid) oriented feature defined on the TMI image by negative magnetic anomalism defining the northern edge of MD1A with MD5, and a change in magnetic signature between MD3 and MD5.
ML4 can be traced from the northern edge of the TMI image where it defines the southern limit of a northeast-southwest oriented sandstone outcrop of MD5. It approximately defines the orientation of the extent of rhyodacite outcrop at the northeast end of Stone Ridge.