AWA: Academic Writing at Auckland

Brief Descriptions of Rocks and Minerals (with age estimates where possible)

UNIT X:

X-1: Schist – black, shiny (mica). No porphyroblast present. Has distinctive foliation showing preferred orientation of mica grains called schistosity

X-2: Schist (chlorite) – green, shiny, deformed foliations. Medium grade metamorphic rock with evidence of shear strain that produced folds

X-3: Gneiss – high grade metamorphic rock showing distinct black and brown (augen) foliation

X-4: Quartzite – granular, white, glassy lustre. Medium-grained, metamorphic rock. Distinguished from marble with no production of carbon dioxide and high degree of hardness.

X-5: Marble – granular, pink impurities. Medium-grained metamorphic rock producing carbon dioxide on contact with dilute hydrochloric acid. Limestone protolith

UNIT A: Cambrian – Carboniferous

A-1: Sandstone – clastic, ruddy red, soft, well-sorted, visible fine grains

A-2: Conglomerate – clast-supported, occurs near base of unit A due to pebbles (clasts). Grains range from fine-coarse of polymictic lithology (more than one rock type). Coarser clasts are rounded, indicating its textural maturity (long period of transport, deposition at high energy environment) due to river abrasion or coastal erosion

A-3: Gypsum – near top contains lenses of A3, vitreous, tabular/prismatic form, occurred as evaporite deposit on saline environment

A-4: Halite – near top contains lenses of A4, found in association with gypsum as an evaporite deposit within a highly saline environment (ancient salt lakes)

A-5: Order Spiriferida (Cambrian – Carboniferous) – long, straight hinge, biconvex, radial ornament; Phylum Brachiopoda (bilateral symmetry, two valves), Class Articulata (inequivalve – valves different shape and size)

A-6: Class Graptolithoidea (Cambrian – Carboniferous) – graptolites (red, hacksaw blade-like impressions); Phylum Hemichordata (worm-shaped marine invertebrates)

A-7: Class Trilobita (Cambrian –Permian) – trilobites (segmented, three divisions)

UNIT B and UNIT C: Cambrian - Ordovician

B-1: Limestone – (clastic) small, white/yellow fine grains cemented by calcite which is indicated with production of CO₂.

B-6: Marble – granular, white, medium- grained. Produced due to contact metamorphism where baked contact occurs on country rock surrounding igneous intrusion. (locality 6)

B-7: Order Rugosa (Ordovician – Permian) – horn corals (4-fold symmetry, colonial/solitary); Phylum Cnidaria (corals)

B-8: Class Blastoidea (Ordovician – Permian) – sea lilies without arms; Phylum Echinodermata (5-fold symmetry/pentameral)

B-9: Order Mesogastropoda (Ordovician – Recent) – simple, rounded aperture; Phylum Mollusca, Class Gastropoda (coiled, not chambered)

C-1: Greywacke – black, heavy, fine-grained. Deposited in deep ocean water or edge of continental shelf where strong turbidity currents rapidly transport sediments at short distances

C-2: Coal – (anthracite) high grade, light, solid black, not sooty (submetallic)

C-8: Hornfel – intermediate-high grade metamorphic rock, no porphyroblast or foliation. occurred due to contact metamorphism. (locality 8)

UNIT E, UNIT F and UNIT G: (Age: Ordovician – Recent)

E-1: Conglomerate – poorly-sorted, clast-supported, oligomictic lithology (with clasts resistant to erosion)

F-1: Sandstone – well-sorted, very fine grains

F-2: Suborder Ostreina (Triassic – Recent) – oysters; Phylum Mollusca, Class Bivalvia (2 valves/equivalve)

F-3: Order Ammonitida (Jurassic – Cretaceous) – complex, ‘feathery’ suture pattern; Phylum Mollusca, Class Cephalopoda (squids) Subclass Ammonoidea (coiled and straight, chambered, complex sutures)

F-4: Order Belemnitida (Jurassic – Cretaceous) – bullet-shaped internal skeleton; Phylum Mollusca, Class Cephalopoda, Subclass Coleoidea (coiled and straight, endoskeleton squid)

G-1: Limestone – (bio-clastic), shelly (large, almost complete shell fragments)

G-2: Sandstone – very fine-grained, well-sorted. (irregular bodies near top)

G-3: Order Neogastropoda (Ordovician – Recent) – posterior margin of aperture extended into siphonal canal; Phylum Mollusca, Class Gastropoda

G-4: Order Veneroida (Ordovician – Recent) – cockles/pipis (equivalve); Phylum Mollusca, Class Bivalvia

G-5: Class Echinoidea (Jurassic – Recent) – irregular, heart-shaped, offset mouth and anus; Phylum Echinodermata, Class Echinoidea

UNIT H:

H-1: Diorite – plutonic; black/white interlocking grains, phaneritic, coarse-grained, batholith. Produced from partial melting of mafic rock above a subduction zone (either in mountain building or volcanic arcs)

H-2: Breccia pipe – large, angular clasts, siliceous matrix, shiny speckles

H-3: Pegmatite – green, vitreous, foliated

H-4: Galena, pyrite, sphalerite – sulfide minerals amalgamated into this one, heavy specimen most probably in a hydrothermal vein deposit. Pyrite and galena both have metallic lustre. Galena has cleavage in 3 directions forming cubes. Sphalerite and pyrite are both yellow in colour

H-5: Gossan (limonite) – cubic, red, iron cap, highly oxidised ore vein

H-6: Gossan – same as H-5

UNIT I:

I-1: Dolerite – black, shiny, heavy, visible individual crystals, shallow intrusions (hypabyssal). Medium grained equivalent of basalt, rapid solidification (recrystallization and hardening of unit A at locality 2)

UNIT J and UNIT K:

J-1: Basalt – olivine inclusions, aphanitic, few (tiny) vesicles, black

J-2: Basalt – part of cone rather than flanks, flow patterns indicate high viscosity (a’a lava flow)

K-1: Scoria – basaltic, reddish brown, vesicular, light, highly vesicular, no phenocrysts, tuff ring

K-2: Scoria bomb – heavier, smaller vesicles, elongated, oxidised

Brief Description of the Adit

Diorite can be found predominantly within the adit. It is an intrusive rock of intermediate mineralogy between gabbro and granite. Its phaneritic texture can be attributed to the slow cooling of plagioclase and amphibole at the earth’s crust, which results in its speckled black and white appearance.

The adit passes through the entire laccolith of dioritic composition. The northern wall passes through a rock of similar composition found in the igneous sill of unit I, which is dolerite. The southern wall cuts through a contact aureole that produced hornfels.

A breccia pipe (H2), which consists of angular clasts with silica matrix, has been encountered at 250m. Often, it initially develops good permeability and porosity at its formation. Therefore, it may have served as conduits for sulfide minerals to precipitate out and pass through.

The adit also encounters pegmatite, which results from partial melting. It is a holocrystalline, phaneritic intrusive rock that could possibly be enriched with rare earth elements and minerals.

Brief Proposal on the Viability of the Survey Area

The survey in Hailsudston has shown numerous, interspersed ore deposits and economically significant rocks and minerals in the area. Quartzite is mostly a silica source for metallurgical processes. Basalt, diorite, greywacke, sandstone, schist, marble and hornfel are widely used in construction, roading, dimension stone for buildings, aggregates, paving, fills, and sea walls. Limestone is utilised as a base for cement, lime fertiliser production and petrochemicals. Evaporites, halite and gypsum, are common economic minerals used for thermal insulation and table salt, respectively. Pegmatite is also economically significant as it is not only the primary source of lithium, beryllium and caesium, but also potentially rare earth elements and gemstones.

The most economically significant minerals can be found in a hydrothermal vein deposit, where sulphide ores originate. Pyrite is an ore of iron, galena of lean and sphalerite of zinc. Two gossan (iron caps) can also be found in locality 5, where there is a distinct, orange, weathered dome. It is a considerable limonite (iron oxide) ore deposit. Furthermore, gossans could also be a secondary ore because of pseudomorph (leaching) weathered minerals, which could also be likely to be economically significant. The coal seam (locality 2) is possibly the most profitable resource in the area because of its high grade and abundance.

However, further in-depth economic and geologic surveys in the area must be carried out. This is to ensure the economic viability of the surveyed area by quantifying the profit-expense costs for the extraction, which at present is not substantiated.

Brief Geologic History of Hailsudston

Unit X represents the oldest rocks present in the surveyed area in Hailsudston. It is characteristically metamorphic, which indicates its requirement for depth. It forms as the basement rocks throughout the area and is found exposed at the valley floor and valley ridges. This means the metamorphic rocks have undergone exhumation. The foliated rocks, schist and gneiss, indicated their protoliths had undergone shear and differential stress; thereby, producing the characteristic folding in the rock samples. The conditions required of these rocks are increasing pressure and temperature as it moves across the earth’s crust during prograde metamorphism; with gneiss achieving the highest grade in the area.

Non-foliated (granular) can also be found in unit X, such as marble and quartzite. Quartzite is the metamorphism of quartz arenite and arkose, which are predominantly quartz. Hence, a large amount of sand is required and suggests a dune-environment. On the other hand, the protolith of marble is limestone, which implies the area is a marine paleoenvironment and a product of calcareous organisms.

Unit A immediately overlies unit X and has the second oldest rocks that are deduced from the fossils imbedded within the unit. A layer of conglomerate is found near the base of unit A, whose poorly-sorted clasts suggest a rapid diagenesis in unit X. Mayhaps due to glacier landslide or talus that abraded rock fragments from unit X. The textural maturity of the clasts resulted from long period of abrasion, possibly in a river or beach.

Evaporites can also be found near the top of unit A, which indicates a highly saline marine environment, possibly an ancient ocean or lake basins. Gypsum and halite precipitated out of a flooded, low latitude basin. Gypsum is a near surface deposit, while halite is more often a basin fill. Additionally, the sandstone found in unit A may have been due to diagenesis of coastal sand dunes or beach sands. The fossils uncovered are indicative of the area being once covered by water and the unit X in mountains has experienced transgressional sea level rise, forming the beach. The marine dwelling fossils found are graptolites, brachiopods and trilobites.

Unit B and unit C support the ancient sea formerly found in the area, both of which are roughly the same age, also deduced from fossils. Rugose corals, echinoderms and gastropods require warm and shallow water, which means unit B was once in the photic zone. The greywacke found, however, is indicative of sudden subsidence and most likely resulted in strong turbidity currents or marine landslides. The high grade coal forms a continuous layer within unit C (coal seam), which indicates the area then becomes a widespread forest during the Carboniferous Period. The hornfel found is a result of a contact aureole surrounding a pluton.

Unit E, unit F and unit G are the most recent sedimentary rocks, all of which still had marine origins but occurred at a much greater rate, probably higher sea level rise. An example is the bio-clastic limestone. Also, the fossils (ammonite, belemnite, oyster, and cockle) aged from Ordovician to Recent and mostly dwell in shallow waters.

The recent additions, unit H, unit I, unit J and unit K, are a result of magmatic activity. Unit J is a remnant of an eruption of a broad and flat shield volcano, with highly viscous mafic lava flowing very slowly. In J1, the lack of vesicles is indicative of the lava flow, as well as the A’a lava properties in J2. In contrast, the vesicular nature of unit K suggests pyroclastic ejecta and tuff ring around the vent.

Unit H is an igneous laccolith of dioritic composition, which characterises orogenic processes at mountain roots. An underlying batholith may also contribute to the magmatic activity throughout Hailsudston. The presence of sulfides (galena, sphalerite, and pyrite) in unit H suggests hydrothermal activity. Unit I forms two sills originating from the pluton and consists of dolerite.