K.M. Scott

Weathering and its effect upon geochemical dispersion at the polymetallic Wagga Tank deposit, N.S.W., Australia

Abstract The Wagga Tank Zn-Pb-Cu-Ag-Au deposit comprises multiple, steeply dipping and structurally controlled sulfide lodes within Early Devonian turbidites and volcaniclastics. Complete weathering at the prospect extends to 100 m below the current land surface. Detailed geochemical studies of drill-derived regolith material were supplemented by extensive mineralogical investigations and enable the major features of the weathered profile to be delineated. The profile consists of soil with reworked portions of a ferruginous lateritic capping within a largely transported overburden. Below the soil, some residual ferruginous capping (characterized by maghemite) occurs above a kaolinite-rich zone which passes into goethitic saprolite containing alunitejarosite family minerals. This lateritic profile was developed during the Tertiary period prior to the Late Miocene onset of more arid conditions. Within the goethitic saprolite, Pb and some Au have remained in their Miocene positions. However, the distributions of Cu and Zn largely reflect the effect of post-Miocene aridity. Thus, these elements (and some Au) are concentrated as supergene minerals at the current water table, with isolated zones higher in the weathered profile reflecting former levels of the water table. Preferential sampling of ferruginous lateritic material in the overlying transported soil can enhance geochemical anomalies and indicate the presence of concealed mineralization at Wagga Tank.

Weathering of the zinc-lead lode, Dugald River, northwest Queensland: II. surface mineralogy and geochemistry

Abstract Gossans associated with the Dugald River zinc-lead lode contain anomalous concentrations of Zn, Pb, Ag, As, Cd, Cu, Sb, Se, Tl and Ba and differ from those on the more pyritic Western Lode (Zn, Pb, Cu, As, Tl) and those associated with copper mineralization in the hanging wall (As, Bi, Co, Cu, Mo, Ni, Sb). Mineralogical and geochemical variations in gossans along strike reflect changes in primary ore and gangue mineralogy, particularly towards the north, where the Dugald River lode and hanging wall copper mineralization merge. Leaching of more soluble elements from the surface and re-precipitation below have resulted in large geochemical variations in the top metre of the profile. Dispersion into wall rocks has occurred over two distinct periods: hydromorphic dispersion, before erosion removed much of the gossan and surrounding Corella Formation, has resulted in very high Zn contents (up to 9%) in the footwall, whereas a more even dispersion of target and pathfinder elements into hanging and footwall rocks is from recent weathering of the slightly elevated gossan.

Spectrally mapping the compositional variation of white micas in hydrothermal systems and the application in mineral exploration

White micas are commonly formed in hydrothermal mineralisation systems. Their compositional variations reflect physical and chemical conditions, and therefore are very useful for reconstructing extinct hydrothermal systems and guiding mineral exploration. Airborne hyperspectral sensing technology and field infrared spectroscopy are operational tools for identifying and mapping the compositional variations of white micas within a hydrothermal system.

The oxidized profile of BIF-associated Pb-Zn mineralization: Pegmont, Northwest Queensland, Australia

Abstract Lead-zinc mineralization with a low Fe-sulphide content occurs in association with a quartz-magnetite-garnet-apatite banded iron formation (BIF) at Pegmont, northwest Queensland. A study of the ore horizon and wall rocks at different levels in the oxidized profile has revealed considerable depletion of Mg, Ca, Na, K, S, Ag, Cd and Zn during weathering. The least mobile elements have undergone residual concentration up to the profile, whereas concentrations of other elements are either unaffected by weathering or vary irregularly, in many cases reflecting local variations within the ore horizon. On oxidation, galena changes either through a pyromorphite ± cerussite assemblage to plumbogummite/corkite or directly to coronadite. The Pb content of outcropping gossan is the same as, or slightly higher than that of unweathered ore. Although Zn is strongly depleted at the surface (×10), its greater abundance (×2 depletion) only 15 m below the surface reflects the truncated profile and immaturity of the gossan. Comparisons with gossans overlying similar mineralization at Broken Hill, New South Wales and at Gamsberg and Aggeneys, North West Cape Province, South Africa, indicate that Pb, Zn, Ag and Cu contents can be used to evaluate BIF-associated ironstone outcrop.

Integrated mineralogical and geochemical exploration for tin in the bygoo region of the ardlethan tin field, Southern N.S.W., Australia

Abstract In the Bygoo region of southern N.S.W. (Australia), the Ardlethan Granite is commonly deeply weathered and has sparse outcrop. It contains small, structurally controlled tin lodes and buried disseminated mineralization which is associated with intense alteration (silicification/greisenization). Although disseminated Sn mineralization may be associated with F and peripheral As, Pb and Zn anomalies in bedrock, these geochemical associations are not consistently developed. The intensely altered zones, however, which are characterized by topaz and abundant muscovite development, can be readily mapped mineralogically. Comparison of geochemical and mineralogical data suggested that the distribution of F reflects its occurrence in the alteration minerals (topaz and muscovite) and the primary igneous minerals (biotite and muscovite) and does not necessarily indicate alteration/mineralization. Therefore, mapping of mineralogical zonation can be important when direct observation of alteration is difficult, e.g. in areas of deep weathering and/or transported overburden.

Mercury in Stratabound copper mineralization in the Mammoth area, northwest Queensland

Abstract Copper mineralization along the Mount Gordon Fault Zone in northwest Queensland contains sufficient mercury to permit mercury pathfinder techniques to be used for exploration for further deposits in the area. At the Mammoth mine, the No. 1 orebody contains 310–14000 ppb Hg, with the highest contents in “sooty chalcocite” which may be of supergene origin. The B orebody contains 100–4300 ppb Hg, with highest concentrations at the top of the deposit. Other deposits in the Mammoth area contain 10–1600 ppb Hg, with mean mercury contents > 200 ppb in fault-related mineralization. There is a strong positive correlation between mercury and copper, sulfur, silver, arsenic, bismuth, lead, antimony and thallium contents in the deposits which suggests mercury was introduced during the mineralizing process. However, most of the mercury occurs on the surfaces of sulfide minerals, indicating its introduction at a late stage of mineralization. Mercury in the No. 1 orebody is partly of supergene origin whereas primary mineralogy may control mercury distribution in the B orebody. The presence near the Mammoth Fault of a lens of pyrite containing high concentrations of mercury (geometric mean 820 ppb) suggests that the mercury content of pyrite encountered in future exploration programmes in the region might be used to indicate proximity to mineralized fault zones. Gossans derived from copper deposits contain more than five times the amount of mercury in ironstones developed over unmineralized or poorly mineralized fault zones. The mercury contents of iron-rich rocks may be used to discriminate gossans from the numerous fault ironstones in the Mammoth area.

Sulphide geochemistry and wall rock alteration as a guide to mineralization, mammoth area, NW Queensland, Australia

Abstract Stratabound mineralization in the Mammoth area of NW Queensland occurs in steeply dipping, faulted Proterozoic arenites and dolomitic rocks overlying basic volcanics. Both syngenetic/diagenetic and epigenetic sulphides are present, with the latter divided into Mammoth- and South Mammoth II-styles. Syngenetic/diagenetic pyrite is distinguished from epigenetic pyrite by higher Co and Ni, and lower As, Mo, Sb and Tl contents. Chalcopyrite is the major copper sulphide associated with syngenetic/diagenetic pyrite and is characterized by low Ag, Bi, Mo, Ni and Tl contents relative to epigenetic chalcopyrite. No substantial wall rock alteration is associated with such sulphides. Disseminated syngenetic/diagenetic sulphides in the mine sequence may have induced deposition of epigenetic Cu mineralization, but alone, even when remobilized, do not reach economic grades. The four epigenetic Mammoth orebodies are all richer in chalcocite and bornite than syngenetic/diagenetic mineralization and have wall rock alteration characterized by alkali depletion and Fe enrichment in the zone between the ore and the fault considered the conduit for the hydrothermal ore-forming fluids. Pyrite associated with Mammoth-style mineralization has high As, Mo, Sb and Tl contents and was formed subsequent to the copper sulphides. Thick, essentially barren, pyritic sequences occur at South Mammoth II where the moderate As, high Mo, Sb and Tl contents reflect their hydrothermal origin but distinguish them from Mammoth-style pyrite. Their low As/Sb ratio and lack of wall rock alteration imply a lower temperature of formation than the Mammoth-style pyrite and possibly such mineralization represents the pre-ore stage of the Mammoth mineralizing hydrothermal system. An essential feature of economic Cu mineralization in NW Queensland is the operation of a hydrothermal system. As all such systems may not necessarily give rise to extensive wall rock alteration, use of the high As, Mo, Sb and Tl contents of hydrothermal pyrite can aid evaluation of mineralization intersected during drilling.

Mapping the effects of regional metamorphism and hydrothermal alteration in the Mount Isa Valley, Queensland, Australia, using airborne hyperspectral data

The Mount Isa Valley in northwestern Queensland, Australia hosts the world-class Mount Isa Cu deposit. Mineral mapping with airborne hyperspectral data has identified a suite of minerals. The spatial representing the effects of regional metamorphism, mineralisation-related hydrothermal alteration and weathering. The spatial occurrences and associations of these minerals reveal the effects of regional-scale metamorphism, hydrothermal alteration and weathering. In particular, it was found that that minor amounts of talc, a hydrothermal alteration mineral closely associated with Cu mineralisation, in the dark and fine-grained sedimentary rocks was mappable from the airborne hyperspectral data. Keywords-Mount Isa, hydrothermal alteration, copper mineralisation, hyperspectral, mineral mapping.