Stephen G. Peters

Formation of oreshoots in mesothermal gold-quartz vein deposits: examples from Queensland, Australia

Abstract Mesothermal gold/quartz vein deposits in granitoid-hosted and melange/sediment-hosted goldfields, contain oreshoots in areas of structural dilation. The environment of formation is localized at depths of 2–5 km in brittle to brittle-ductile shear zones which have stick-slip seismic movement. Lithologic contacts and fissure intersections affect the shape and geometry of the oreshoots. Mineralizing fluids in these shear zones range between 3 and 10 wt% NaCl equiv. with δ 18 O values of about 5–8%. Fluids are locally CO 2 -bearing with formation temperatures of between 220° and 350°C. Fluid source is either of deep magmatic or metamorphic origin and can rarely be correlated directly with nearby plutonism. Fluid transport is via single pass flow in high permeability conduits, such as mylonites in granitoids or melange cleavage in sediments. Complex mixtures of mineralized quartz, gouge, fault rocks and altered wall-rock reflect the environment of formation. Comb, ribbon, buck and breccia quartz and microscopic textures indicate different processes and stages of oreshoot formation. Quartz deposition is due to changes in silica solubility resulting from temperature and pressure fluctuations. Local pressure changes are due to reduced velocity of the fluid in dilated zones according to Bernoullis equation. At restricted or dilated portions of the fissure, throttling and adiabatic cooling are common and result in quartz deposition and channel choking which lead to pressure and temperature build-ups and faulting. The long dip-lengths in many mesothermal oreshoots may also account for substantial pressure and temperature reductions in the fluid from bottom to top. The venturi effect in connecting fissures may also affect quartz deposition. Microscopic textures indicate that multiple generations of quartz have been involved in cracking, stress corrosion and dissolution due to porosity changes and dilation prior to and during faulting. Gouge and clay seams stabilize fault movement and may have acted as impermeable barriers and pressure seals which channelled fluid flow, and if hydrated, may have expelled concentrated brines when compressed by faulting. Hydrothermal alteration indicates early diffusive transport and chemical and pH gradients away from the oreshoots. Four main stages of oreshoot formation occur from the outside to the inside of oreshoots as: (1) ground preparation and nucleation, (2) reinjection and sheeting, (3) major fault movement, channeling of fluid flow, local dissolution and cracking, and (4) consolidation and oreshoot growth, including fluid stagnation and ponding.

Nomenclature, concepts and classification of oreshoots in vein deposits

Abstract Oreshoots are discrete hypogene masses usually hosted within a planar channel, surface, lode or conduit which may be either a shear zone, fissure, fault zone, or lithologic bed or unit such as a contact. Oreshoots are characterized by breadth, strike (>1000 m) and dip, and plunge (100–500 m) lengths and have higher metal contents than the adjacent parts of the host conduit. The mass of most oreshoots ranges between 1 × 10 6 and 2 × 10 4 tonnes. There is a tendency for oreshoots to be thicker and richer in the center, rather than to have uniform grade distributions. The thickness of the oreshoots may be between 0.25 and 1.75 m in shear-zone-hosted deposits, to up to 60 m in replacement deposits. Several conduits may connect to form vein systems. Vein systems have common fluid sources which result in general homogeneity of alteration, mineralization types and oreshoot control, and, therefore, commonly share the same plumbing system. The internal constituents usually reflect unique episodes relating to ore formation. The main intern constituents in oreshoots are mineralization, gangue and alteration. These constituents usually mix with each other in complex patterns, the relationships between which may be used to interpret the processes of oreshoot formation. The term “ground preparation” represents the effect of various events in the geologic history of an ore district or oreshoot area that have assisted in enhancing the rocks so that oreshoots can preferentially form in certain areas or geometries. Several types of ground preparation can be recognized: (1) sequential deformation that produces a grain in the rock, (2) severe faulting and jointing which augments permeability and areas where ore minerals can precipitate, and (3) interplay between ore fluid and deformation to produce an oreshoot. Controls of oreshoot location and shape are usually due to dilatant zones caused by changes in attitude, splays, lithologic contacts and intersections. In addition, conceptual parameters such as district fabric, magic distances and stacking are also used to describe the geometry of oreshoots. Controls in vein systems and the location and geometry of oreshoots within vein systems can be predicted by a number of qualitative concepts such as internal and external plunges, district plunge, district stacking, conduit classification, gradients and warps. These concepts have a practical and empirical application in most districts where they are useful in the exploration for ore, but are of such broad and general application that they can rarely be explained definitively.

Rare earth element mineralogy, geochemistry, and preliminary resource assessment of the Khanneshin carbonatite complex, Helmand Province, Afghanistan

The Khanneshin carbonatite is a deeply dissected igneous complex of Quaternary age that rises approximately 700 meters (m) above the Neogene sedimentary rocks of the Registan Desert, Helmand Province, Afghanistan. The complex consists almost exclusively of carbonate-rich intrusive and extrusive igneous rocks, crudely circular in outline, with three small hypabyssal plugs of leucite phonolite and leucitite outcropping in the southeast part of the complex. The igneous complex is broadly divisible into a central intrusive vent (or massif), approximately 4 kilometers (km) in diameter, consisting of coarse-grained sövite and brecciated and agglomeratic barite-ankerite alvikite; a thin marginal zone (<1 km wide) of outwardly dipping (5°–45°) and alkali metasomatized Neogene sedimentary strata; and a peripheral apron of volcanic and volcaniclastic strata extending another 3–5 km away from the central intrusive vent. Small satellitic intrusions of biotite-calcite carbonatite and rare leucite phonolite, no larger than 400 m in diameter, crop out on the southern and southeastern margin of the central intrusive vent. A zone of prospective light rare earth element (LREE) enrichment was delineated by Soviet geological teams in the mid-1970s. The area of LREE-enrichment is situated in extensively veined and dike-intruded barite-ankerite alvikite in the outer part of the central vent near its northeast contact with Neogene sedimentary rocks. In addition to having very high concentrations of LREE, the barite-ankerite alvikites are also highly enriched in barium and strontium. Three reconnaissance scoping missions to the Khanneshin carbonatite were led by scientists of the U.S. Geological Survey (USGS). Two of these were to LREE area of interest which is the primary subject of this report. Two types of LREE mineralization occur. Type-1 LREE mineralization consists of semiconcordant, symmetrically banded veins and discontinuous seams, as much as 0.5–0.7 m thick and several tens of meters long. These occur throughout a vertical thickness of at least 150 m. Type-1 banded veins and seams are yellow-weathering zones, symmetric about a dark central zone, that are enriched in khanneshite-(Ce), barite, strontianite, and secondary LREE minerals (synchysite-(Ce) and parisite-(Ce)). The dark central zone, consisting primarily of ankeritic dolomite, barite, apatite, and strontianite, also has trace khanneshite-(Ce). These type-1 veins and seams alternate with dark, meterthick layers of ankerite-barite alvikite (wall rock) over a vertical distance of approximately 150 m. In some veins LREE carbonate minerals form dense spherically shaped aggregrates (100 micrometers diameter), presumably crystallized from immiscible droplets, which constitute as much as 30 percent (by volume) of the vein. Type-1 veins and seams average 19.92 weight percent (wt. percent) Ba, 3.61 wt. percent Sr, and 2.78 wt. percent total LREE. The values of ∑ LREE (∑ LREE is the sum of La, Ce, Pr, and Nd) for eight average whole-rocks range from 6.23 to 1.83 wt. percent.

APPLICATIONS OF A GLOBAL MINERAL-RESOURCE ASSESSMENT FOR ADDRESSING ISSUES OF SUSTAINABLE MINERAL RESOURCE DEVELOPMENT 1

The future of mining depends on balancing global demands for minerals with societal demands for sustainable development. The U.S. Geological Survey, in collaboration with a variety of international cooperators, is assessing the undiscovered global resources of copper, platinum-group elements, and potash at a scale of 1:1,000,000. Assessment products include maps that show significant identified deposits and permissive areas for undiscovered deposits, as well as probabilistic estimates of contained metal. Derivative products applicable to sustainability issues include maps showing the spatial relationship of permissive areas to infrastructure development, protected areas, threatened ecosystems, seismically active areas, and watersheds.

The global mineral resource assessment project in the southeast Asia region

In response to the growing need for minerals information, the U.S. Geological Survey (USGS) is conducting a cooperative international project to assess the world’s undiscovered nonfuel mineral resources. The Global Mineral Resource Assessment Project (GMRAP) has met with member countries of the Coordinating Committee for Geoscience Programs in East and Southeast Asia (CCOP) to assess undiscovered copper, PGE and potash resources of the region. Tracts permissive for undiscovered porphyry copper and sedimentary-hosted copper deposits have been delineated and probabilistic estimates of the amount of copper resources for each tract made. Preliminary data also have been collected for PGE and potash in Southeast Asia.