Stone Ridge Quarry Project
Geological Mapping & Petrography
Background
Preliminary geological mapping of the Stone Ridge Quarry Project Area commenced on 27 March 2017, with the purpose of confirming the general character of the dominant Eagleton Volcanics rock types based on examination of surface outcrop exposures. Several representative samples of outcrop were collected for detailed hand specimen and thin section analysis, which was subsequently undertaken by Dr Paul Ashley (Paul Ashley Petrographic and Geological Services). The locations from which these samples were collected are illustrated on Figure 5.
A program of detailed geological mapping was subsequently undertaken across the Project Area with the objective of confirming the lateral extent and contact relationships of major Eagleton Volcanics rock types identified from the preliminary investigations. Mapping activities were ongoing throughout the period of the subsequent resource assessment program, with information generated by diamond drilling and ground magnetic surveys guiding ongoing surface mapping.
Given the excellent outcrop exposure of rhyodacite on Stone Ridge, mapping has tended to focus northwest and southeast of the ridge, where outcrop exposure is more limited and geological complexity is greater.
Methodology
Geological mapping was undertaken using mobile mapping software (initially Locus Map Pro and later Datamine MapInfo Discover Mobile) with the GPS-controlled location of outcrop, subcrop and float recorded for different rock types. Mapping traverse routes where geological information was not available due to the absence of outcrop or subcrop, were also recorded.
Geological information was later plotted as a series of colour-coded points to generate a geological outcrop, subcrop and float map
(Figure 5), with traverse routes devoid of rock exposure also plotted.
Preliminary geological mapping of the Stone Ridge Quarry Project Area commenced on 27 March 2017, with the purpose of confirming the general character of the dominant Eagleton Volcanics rock types based on examination of surface outcrop exposures. Several representative samples of outcrop were collected for detailed hand specimen and thin section analysis, which was subsequently undertaken by Dr Paul Ashley (Paul Ashley Petrographic and Geological Services). The locations from which these samples were collected are illustrated on Figure 5.
A program of detailed geological mapping was subsequently undertaken across the Project Area with the objective of confirming the lateral extent and contact relationships of major Eagleton Volcanics rock types identified from the preliminary investigations. Mapping activities were ongoing throughout the period of the subsequent resource assessment program, with information generated by diamond drilling and ground magnetic surveys guiding ongoing surface mapping.
Given the excellent outcrop exposure of rhyodacite on Stone Ridge, mapping has tended to focus northwest and southeast of the ridge, where outcrop exposure is more limited and geological complexity is greater.
Methodology
Geological mapping was undertaken using mobile mapping software (initially Locus Map Pro and later Datamine MapInfo Discover Mobile) with the GPS-controlled location of outcrop, subcrop and float recorded for different rock types. Mapping traverse routes where geological information was not available due to the absence of outcrop or subcrop, were also recorded.
Geological information was later plotted as a series of colour-coded points to generate a geological outcrop, subcrop and float map
(Figure 5), with traverse routes devoid of rock exposure also plotted.
Results
Mapping and petrographic assessment confirmed the presence of five (5) major rock types, which are summarized below in decreasing order of abundance. Detailed petrographic descriptions of the samples referred to below are provided in Dr Paul Ashley's report #1028, via the link at the bottom of this section.
Rhyodacite - the flanks and crest of Stone Ridge are characterised by extensive outcrop, subcrop and float of a pink to pinkish grey, medium grained, quartz and feldspar dominated volcanic rock, confirmed by petrographic assessment as rhyodacite. Outcrop of rhyodacite varies in character from extensive areas of blocky rubble to areas of broad sheet-like outcrop interpreted as dip-slope bedding surfaces (refer Plates1-3).
Dacite - in contrast to the main ridge line, outcrop exposure on the lower gradient slopes to the north is more sporadic in nature. Initial mapping and sampling revealed that outcrop beyond the limit of outcropping rhyodacite is dominated by a medium grey, medium grained, feldspar dominated volcanic rock, confirmed by petrographic assessment as dacite (refer Plate 4).
Rhyolitic Vitric-Crystal Welded Tuff - several linear, but often discontinuous ouctrops of finer grained volcanic rocks occur within the Project Area that have been confirmed as ‘vitric-crystal welded tuff (ignimbrite)’ of rhyolite or rhyodacite composition (refer Plate 5). The most prominent and most altered of these occurs southeast of Stone Ridge, beyond the limit of rhyodacite outcrop and at the interpreted top of the Eagleton Volcanics stratigraphy. In contrast, less continuous and less altered outcrops of rhyolitic tuff occur lower in the stratigraphy to the northwest of Stone Ridge (refer Plate 6).
Volcanic Breccia - several highly weathered exposures of coarsely fragmental volcanic breccia were observed in gullies on the lower northwest slopes of Stone Ridge, directly west of the Stone Ridge saddle. The most prominent of these (refer Plate 7) occurs immediately below the base of the rhyodacite and is a clast-supported fragmental unit comprised of abundant large sub-rounded to rounded volcanic boulders / clasts. The weathered and unconsolidated nature of the breccia in surface samples prevented the collection of a sample suitable for petrographic assessment.
Andesitic Lithic-Crystal Tuff - the most mafic rock types identified during initial mapping were associated with minor subcrop and float confined to the lower northwest flank of Stone Ridge, directly west of the Stone Ridge saddle. Thin section analysis confirmed these to be lithic-crystal tuff of andesitic composition. The andesite occurs in the same general vicinity as the coarse volcanic breccia and may represent a coarse fragmental component of the latter.
Plate 1 – Blocky rubble outcrop of rhyodacite on Stone Ridge
|
Plate 2 – Sheet-like rhyodacite outcrop on southern side of Stone Ridge
|
Plate 3 – Typical rhyodacite outcrop sample from Stone Ridge (Sample 3)
|
Plate 4 – Dacite outcrop sample from immediately northwest of Stone Ridge (Sample H3)
|
Plate 5 – Altered vitric-crystal welded tuff of rhyodacite composition from outcrop south of Stone Ridge (Sample H2)
Plate 7 – Exposure of volcanic breccia in washout gully directly west of the Stone Ridge saddle and immediately below the base of rhyodacite
|
Plate 6 – Vitric-crystal welded tuff of rhyolite composition from outcrop north of Stone Ridge (Sample 15)
Plate 8 – Lithic-crystal tuff of andesitic composition from subcrop west of the Stone Ridge saddle (Sample H6)
|
| Petrographic Report 1028 - Dr Paul Ashley.pdf |