The Mount Morgan geology has been extensively studied and referenced by Taube (1990 and 2000) and reproduced in Unger 2003 (refer to Table 2.2). The area of the MMM site is enclosed by the Mt Morgan Tonalite Sequence, which intrude the mine corridor, the Mundic Sequence, and the Upper mine Sequence. These local rock formations are characterised by the presence of well-bedded series of quartz-feldspar crystal tuff, siliceous ash tuff, derived sediments, chert and jasper, in addition to fragments of chert, limestone and chloritic rock material, and acid andesitic mass flows. The key regional feature in the district is a north-west trending ‘arch’ that separates two Middle-Upper Devonian successor basins to the east and the younger rocks to the west. Mapping produced by MMM indicates the Devonian and Carboniferous have been intruded by NNW-SSE trending dykes, which generally comprise of andesite, dolerite, diorite, and porphyrite.
Table 2.2
Stratigraphic column of the geological formations within vicinity of the Site (extracted from GHD, 2023)
Age
Formation (Symbol)
Description
Gravel, sand and silt; man-made deposits generally associated with landfill or mining (tailings, dumps and rehabilitated areas)
Anthropocene
Fill (Qhh)
Late Triassic to Middle Jurassic
Precipice Sandstone (Jp) White to brown, poorly sorted, thick-bedded, cross-bedded, fine to very coarse-grained, pebbly quartzose sandstone; with only minor white to yellowish brown, laminated siltstone (in upper part),
carbonaceous shale, lithic sublabile sandstone, granule conglomerate Unconformably overlies Triassic or Permian units of the Bowen Basin, Devonian to Permian basement rocks and some granites of the Auburn Arch. Granule to boulder andesitic to dacitic volcaniclastic breccia and conglomerate, locally fossiliferous; lithic to feldspatholithic sandstone, porphyritic andesite, lapilli to ash tuff, tuffaceous sandstone; minor ignimbrite; rare hyaloclastite & limestone Grey medium-grained leucocratic biotite-hornblende tonalite
Late Devonian to Early Carboniferous
Mount Hoopbound Formation (Dh)
Mount Morgan Trondhjemite (Dgmo/t) Mount Morgan Trondhjemite (Dgmo/d) Mount Warner Volcanics (Dcw/q) Mount Warner Volcanics (Dcw/h)
Middle Devonian
Grey medium-grained hornblende diorite, quartz diorite, gabbro
Middle Devonian
Early Devonian to Middle Devonian Early Devonian to Middle Devonian
Silicic alteration zone including gold-copper ore body
Rhyolitic volcaniclastic sandstone and conglomerate
2.5 Hydrogeology Aside from the anthropogenic fill, groundwater storage and permeability is through secondary permeability, such as joints and fracture pathways. The Mount Morgan area is cut by a series of north-west/north-east trending dykes, which compartmentalise the area and inhibit deeper groundwater discharge from the mine site (Water Studies Pty Ltd 2001 referenced in Golder 2016). On a regional scale, groundwater is recharged via infiltration from direct rainfall recharge and previously by seepage from the No. 8 Dam (GHD, 2017). Seepage from No. 8 Dam has since been rectified through DoR rehabilitation and remediation works completed in 2021. Recharge to groundwater is also enhanced at the site due to seepage from the overlying tailings dams to the underlying bedrock (saprolite and/or fractured bedrock), seepage of water held in the Open Cut Pit and Sandstone Gully TSF to the adjacent mine waste and bedrock, and seepage of water held in some of the mine waste dumps to the underlying bedrock (GHD 2017). Much of the shallow groundwater flow through saprolite and fractured bedrock and sub-surface flow through mine waste and tailings is intercepted by the seepage interception system at the various sumps, before being pumped back to the Open Cut Pit.
GHD | Heritage Minerals Pty Ltd | 12638246 | Groundwater Quality Review 2024 9
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