Orogenic (Mesothermal) Gold Deposits: Constraining Patterns
of Ore Fluid Flow and Chemical Controls on Ore Deposition
Department of Geosciences, Colorado State University, Fort Collins, CO
That orogenic gold deposits in Archean greenstone belts, slate belts and Cordilleran style orogenic belts are a coherent class of structurally controlled, epigenetic, hydrothermal deposits which formed at depths of a few kilometers in the crust during active tectonism is essentially universally accepted. However, uncertainty and limited consensus over many points of an ore genetic model preclude a full predictive exploration model for the deposits. In this presentation I will outline insights into the nature of these deposits that arose out of compiling data from recent deposit studies, mostly in the Archean cratons of Western Australia, and will discuss new ideas on what controls the siting of ore bodies and gold within the bodies.
Whole ore body geometry and patterns of hydrothermal fluid flow: Although ore-body geometries are in detail complex, enveloping surfaces to ore define, on a gross-scale, narrow tabular to pipe-shaped bodies with vertical continuity, mostly with both a steep dip and a steep plunge. Fluid volumes implied by component geochemical mass balance are such that only a small proportion of total ore fluid volume was in a deposit at any one time. In combination with chemical and fluid dynamic arguments, these characteristics favor models in which one, deeply sourced fluid on single-pass upward migration through the crust is responsible for gold deposition. Zonal distributions of ore and alteration minerals in some large deposits suggest that the ore fluid induced local heating of the rock. For this to have occurred, time periods of fluid flow must have been limited, most probably of order 1000 years.
What does structural control mean? Orogenic gold deposits are the classic 'structurally controlled' deposits, however structural control can result in principle from different hydrodynamic processes. Active structures may promote fluid flow and hence be a prerequisite for deposit formation. Alternatively, fluid flow may be focused to structures where these behave as high-permeability, rheologically weak 'discontinuities'. Structural analysis of a number of deposits shows that fluid flow was generally focused in structures which formed in an earlier stress regime, but that new structures formed during ore-fluid flow where suitable, 'weak' earlier structures were absent. Ore-body siting and geometry can be understood as results of the structural response of a heterogeneous rock mass under a tectonic stress. This is the principle behind 'stress mapping' as an exploration tool.
Controls on ore shoots: Gold grade is heterogeneously distributed within any host structure. The distribution is in some cases a function of channelization of hydrothermal fluid flow within the structure, but can alternatively be a function of local chemical environment. Case studies show the value of considering interrelations between the chemical and fluid flow controls on the distribution of precipitated gold.
Fluid and gold source: Re-evaluation of the mass balance of fluid : wallrock interaction shows that most isotope and chemical markers that have been used to propose ore fluid sources are not reliable source tracers. Our present data set is consistent with two testable ore fluid origins and source to deposit pathways: (a) a metamorphic devolatilization fluids that passed through and interacted with felsic rocks along flow paths; (b) granitoid fluids that were chemically modified by interaction with the orebody hostrock sequence. Observations on granitoids that crystallized at depth in the crust show that at least some exsolved a low-salinity, CO2-bearing fluid that is chemically similar to the orogenic gold fluid.
The Genesis of Carlin-type Gold Deposits -
Current Models and Future Research
Jean Cline, University of Nevada, Las Vegas, Albert Hofstra, U.S. Geological Survey, John Muntean, Placer Dome
Exploration, Inc, Dick Tosdal University of British Columbia, Vancouver, and, Ken Hickey, University of British Columbia, Vancouver
Carlin-type gold deposits, first discovered in northern Nevada in the early 1960's, have enormous gold endowments and have made the Carlin trend one of three gold districts in the world to produce more than 50 million ounces of gold. Forty years of mining and numerous studies have provided a detailed geologic picture of deposits, yet a comprehensive and widely accepted genetic model remains elusive. Currently considered models relate deposits to 1) epizonal plutons that contributed heat and possibly fluids and metals, 2) meteoric fluid circulation resulting from crustal extension and/or widespread magmatism, and 3) metamorphic fluids, possibly with a magmatic contribution, from deep crustal levels.
Difficulties in unravelling deposit genesis are directly related to complications in studying the deposits. Minerals that are part of the Carlin event are fine-grained, volumetrically minor, and common (pyrite, quartz, kaolinite, illite). The regions where the deposits are located have experienced several hydrothermal events, and these common minerals precipitated repeatedly in response to many different processes. As a result, bulk analyses of samples simply produce a signal that is a mixture of several events. Microanalyses can produce a signal from a single geologic event, but require careful petrography to distinguish "Carlin" crystals from pre- or post-Carlin crystals.
A number of geochronological studies during the past 5 to 10 years have led to a consensus that the deposits formed during the late Eocene, permitting us to confidently relate the deposits to their tectonic setting. We now recognize that continental rifting followed by compressional orogenies provided a pre-mineral architecture of steeply dipping faults that acted as fluid conduits, high-level shallowly dipping "traps" or aquitards that inhibited fluid ascent to the surface, and reactive calcareous host rocks. Miogeoclinal sequences that formed following active rifting of the continental margin contain reactive silty calcareous rocks, which are the primary host rocks in almost every Carlin-type deposit including all of the >5 million ounce deposits. The main host unit for Carlin-type deposits is the lower plate to the Roberts Mountain thrust. Most giant deposits lie within 100 meters of the thrust or its projection. The thrust is important as it formed a regional aquitard by placing non-reactive, fine-grained siliciclastic rocks with less inherent rock permeability above favorable carbonate stratigraphy and it forced fluids laterally away from conduits and into reactive lithologies. NNW and WNW- striking basement and Paleozoic normal faults were inverted during post-rifting compressional events, resulting in structural culminations (anticlines and domes) that in the Eocene served as depositional sites for auriferous fluids. These culminations are now exposed as erosional windows through the siliciclastic rocks of the Antler allochthon.
Extension during the Eocene in the Great Basin was broadly oriented northwesterly to westerly (280o to 330o). The underlying rifted plate margin and northwesterly oriented Paleozoic faults were subparallel to the extension direction and were reactivated as strike-slip or oblique-slip faults. Northeasterly oriented pre-Jurassic fault fabrics were favorably oriented for extension. Mineralization is associated with the heterogeneous shear and tensional reactivation of these older, variably oriented, structures. Fluid flow and mineral deposition appear to have been fairly passive as there is little evidence for overpressured hydrothermal fluids, complicated multistage vein dilatancy, or significant syn-mineralization slip. Geologic reconstructions and fluid inclusions indicate that deposits formed within a few kilometers of the surface.
Ore fluids were moderate temperature (~180-240oC), low salinity (~ 2-3 wt % NaCl equivalent), CO2-rich (< 4 mole %), and CH4 -poor (<0.4 mole %) with sufficient H2S (10-1 to 10-2 m) to transport gold. The singular occurrence of "invisible" gold in pyrite in unoxidized ore indicates that ore fluids were undersaturated in gold until fluids reacted with wallrocks. Fluid-rock reaction liberated reactive Fe in the wallrock, which reacted with sulfur in the fluid to form pyrite. This reaction reduced the aH2S in the fluid, destabilized the gold-bisulfide complex, and gold and other bisulfide complexed metals were captured as submicrometer structurally bound or native particles in the pyrite. Ore fluids additionally decarbonatized, argillized, and locally silicified wall rocks.
Isotopic studies constrain sources of ore fluid components, but do not provide unequivocal sources or clearly indicate a preferred genetic model. O and H isotopes of minerals and fluid inclusions at the Getchell deposit consistently indicate that ore fluids had a deep magmatic or metamorphic source. However, most similar studies of deposits in the northern Carlin trend and at Jerritt Canyon have identified a meteoric fluid. Sulfur isotopes in ore pyrite from all districts can be derived from a sedimentary sulfur source. However, sulfur in ore-stage pyrites at Getchell exhibits values near 0 per mil, consistent with a magmatic source. Two recent studies at the 30 million ounce Betze-Post deposit in the northern Carlin trend are also consistent with a magmatic sulfur source; however, other studies at this deposit identified higher sulfur isotopic ratios that are not consistent with a traditional magmatic sulfur source. He isotopic studies have been conducted only at the Getchell deposit where inclusion fluids in late-ore stage galkhaite, orpiment and fluorite contain He with an unequivocal but highly diluted mantle signature.
A compilation of data from all trends and districts provides compelling similarities and requires that all Carlin-type deposits formed in response to similar geologic processes. We propose a deep fluid model in which primitive ore-related fluids were generated in response to removal of the Farallon slab, which promoted deep crustal melting, prograde metamorphism, and devolatilization. Primitive fluids travelled upward through the crust, scavenging ore fluid components along the fluid pathway, and were diluted by deeply circulating meteoric water in the upper crust prior to reacting with wallrocks and depositing gold.
Upwelling hot water at a proposed nuclear waste repository?
An economic geology approach to solve an environmental question
University of Nevada, Las Vegas
The origin of secondary calcite-silica minerals in primary and secondary porosity of the host Miocene tuffs at Yucca Mountain has been hotly debated during the last decade. Proponents of the site interpreted the secondary minerals to have formed from cool, descending meteoric fluids in the vadose zone; critics, citing the presence of two- phase fluid inclusions, argued that the minerals could only have formed in the phreatic zone from ascending hydrothermal fluids. Understanding the origin of these minerals is critical in characterizing geologically recent fluid flux at the site, which has significant consequences as to whether or not nuclear waste should be stored at Yucca Mountain. This study addressed questions related to the origin, timing, and temperature of formation of the mineral crusts.
Petrographic and paragenetic studies of 155 samples collected from the Exploration Studies Facility (ESF) and ECRB tunnel indicate that heterogeneously distributed calcite with lesser chalcedony, quartz, opal, and fluorite comprise the oldest secondary minerals. These are typically overgrown by intermediate-aged calcite, often exhibiting distinctive bladed habits. The youngest event recorded across the site is the deposition of Mg-enriched (up to ~ 1.3 wt.%) and depleted, growth-zoned sparry calcite intergrown with U-enriched opal. The cyclical variation in Mg is probably related to climate changes that have occurred during the last few million years. The distribution of secondary minerals is consistent with precipitation in the vadose zone.
Fluid inclusion petrography of sections from the 155 samples determined that 96% of the fluid inclusion assemblages (FIAs) contained liquid-only inclusions that formed at ambient temperatures (< 35o C). However, 50% of the samples (n = 78) contained relatively rare FIAs that contain both liquid-only and liquid plus vapor inclusions (herein termed two-phase FIAs) that formed at temperatures above 35oC. Virtually all of these two phase FIAs occur in paragenetically old calcite; rare two-phase inclusion assemblages were also observed in early fluorite and quartz, and early- intermediate calcite. Homogenization temperatures ( trapping temperatures) across Yucca Mountain are generally 45 - 60oC, but higher temperatures reaching 83oC were recorded in calcite from the north portal and ramp of the ESF. Cooler temperatures of ~ 35 - 45oC were recorded in the intensely fractured zone. Multiple populations of two- phase FIAs from lithophysal cavities in the ESF and ECRB cross drift indicate early fluid cooling with time from temperatures > 45oC in early calcite, to < 35 - 45oC in paragenetically younger calcite. Freezing point depressions range from -0.2 to -1.6oC indicating trapping of a low salinity fluid. The majority of intermediate calcite and all outermost Mg-enriched calcite contains rare all-liquid inclusions and formed from ambient temperature fluids. Depleted hydrogen isotope compositions (-131 to -90 mil) of inclusion fluids from intermediate and the youngest Mg-enriched calcite indicate derivation from surface meteoric fluids.
Opal and chalcedony grains, intergrown with various stages of calcite were dated to constrain absolute timing of the minerals and fluid inclusions. Integrated U-Pb ages and fluid inclusion microthermometry indicate that two-phase FIAs that trapped fluids of >50oC are older than 6.29 +/- 0.30 Ma. Two-phase fluid inclusion assemblages in paragenetically later calcite, which formed from fluids of 35 - 45oC, are older than 5.32 +/- 0.02 Ma. There is no evidence for trapping of fluids with elevated temperatures during the past 5.32 m.y. The youngest Mg-enriched calcite intergrown with opal began to precipitate between about 1.9 - 2.9 Ma and has continued to precipitate within the past half million years. The presence of liquid-only inclusions and the consistent occurrence of Mg-enriched calcite and opal as the youngest event indicate a minor, but chemically distinct, ambient temperature fluid flux during the past 2 - 3 m.y. These results indicate that hydrothermal activity at the proposed Yucca Mountain nuclear waste repository site is more than 5 m.y. old and should not eliminate the site from consideration for a waste repository.
Uranium in Iron Oxide-Cu-Au Deposits
Murray W Hitzman
Department of Geology and Geological Engineering,
Colorado School of Mines,
Golden, CO 80401
The world's largest uranium mine is the Olympic Dam IOCG deposit. However, none of the other IOCG deposits currently in production produce uranium. What causes the Olympic Dam deposit to have so much uranium and, conversely, why do other IOCG's lack it? The Gawler Craton, host for Olympic Dam, is composed of granitoids with highly enriched U-Th-K (so called "high heat flow granites"). Large-scale hydrothermal leaching of this host rock mobilized uranium which was then precipitated with Cu and Au also leached from the host rock. The host rocks for most IOCG deposits are volcanic and sedimentary rocks with normal uranium contents. Leaching of this material may form ores that are geochemically weakly anomalous in U but do not reach ore grades. Several other IOCG prospects in areas of high heat flow granites also show elevated U values. These data suggest that IOCGs concentrate metals available within the host rock sequences that are altered. Finding other U-rich IOCG deposits requires targeting areas of crustal rock with elevated U values, such as high heat flow granites.
Secondary Geochemical Dispersion Through Transported Overburden
D.L. Kelley1, E.M. Cameron2 and G. Southam3
1WMC Exploration, 8008 E. Arapahoe Ct, #110, Englewood, CO 80401 USA
2 Eion Cameron Geochemical Inc., 865 Spruce Ridge Road, Carp, Ontario, Canada K0A 1L0
3 Biological & Geological Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7
As the discovery rate of world class ore deposits continues to decline, increased attention is being focused on geochemical exploration methods designed for covered terranes. For companies that manage their exploration from a risk-based perspective, the development of reliable methods for the detection of ore deposits covered by transported overburden is critical to their future success. For the exploration methods to be reliable, an understanding of the dispersion processes is required. Numerous published case studies have documented secondary geochemical anomalies in transported overburden above buried mineral deposits in many different environments using a variety of methods (e.g. Cameron et al. 2004, Kelley et al. 2002, Wang et al. 1999, Smee 1998; Mann et al. 1998). There is much debate regarding the factors that form these anomalies but research conducted in the past 10 years has clarified some of the processes involved. In simplest terms, secondary geochemical mobility trough transported overburden can be related to dispersion of dissolved solids in water, of soil gases, and by living systems, such as plants and microbes.
Dispersion of dissolved solids in water
Groundwater surrounding oxidizing sulfide deposits contains
a mixture of dissolved components, colloidal material, and secondary
reaction products that form precipitates. The aqueous mobility
of metals is controlled by pH, Eh (oxidation potential), the speciation
of the metals, e.g., anions or cations, presence of other dissolved
species in solution, the composition and reactivity of solid phases
in contact with the aqueous solution, and the interaction with
microbes. The groundwater provides a media for the dispersion
of ore and related pathfinder elements away from the sulfide deposit.
If the sulfide deposit and groundwater table are near the surface, electrochemical processes may dominate, resulting in the redistribution of redox-active elements and carbonate (Hamilton 2000). In arid environments, an external force, such as seismic activity, is required to move metal-rich groundwater vertically through the vadose zone (Cameron et al. 2002). In the case where oxidizing sulfide deposits are buried gradually by successive depositional episodes of alluvium, dissolution of water soluble phases followed by infiltration and evapotranspiration of surface water concentrates metals in the newly deposited alluvium (Kelley et al. 2002).
Self-potential currents associated with oxidizing sulfide deposits (Sato & Mooney 1960) form the basis of electrochemical dispersion models in which the upward movement of electrons in the sulfide body results from electrochemical gradients between the underlying reducing to overlying oxidizing environments (Govett 1973, Bolviken & Logn 1975, Smee 1983). The mass transfer of ionic species facilitates charge balance and creates the geochemical anomalies in the overburden. The model of Hamilton (1998, 2000) takes into account the upward propagation of reduced species to the water table forming a reduced column over the mineralized zone. The oxidation of Fe2+ produces H+, causing dissolution of carbonate, which precipitates at the edge of the reduced column. Theoretical ion migration rates in electrochemical fields are much faster than diffusion rates and are consistent with the formation of geochemical anomalies in young(ca. 8000 years), thick glacial drift (Hall et al. 2004 this volume).
Cyclical Dilatancy Pumping
The mass transfer (advection) of groundwater and its dissolved components following earthquakes accounts for the redistribution of metals around buried deposits in some environments (Cameron et al. 2002). In a process termed 'cyclical dilatancy pumping' (Sibson 2001), groundwater held in fractures is expelled during and after earthquake events. Changes in water well levels, increased stream flow during drought periods, and the expulsion of groundwater along faults causing surface flooding have been used as evidence for cyclical dilatency pumping (Tchalenko 1973, Tchalenko & Berberian 1974, Nur 1974, Sibson 1981, Muir-Wood, 1994). Secondary dispersion from this process would predictably have 1) characteristic high- amplitude, narrow structurally-controlled response patterns, and 2) chemical associations in soils similar to the groundwater chemistry.
Results of soils and groundwater analyses from an integrated study at the Spence porphyry copper deposit in northern Chile, buried beneath 50 to 100 m of Miocene gravels, are consistent with the vertical movement of metal-rich groundwaters along fractures during earthquakes in this seismically active area (Cameron et al, 2002, 2004). Copper in groundwater is restricted to the mineralized area due to the tendency of Cu2+ released by oxidation of sulphides to adsorb to Fe hydroxide colloidal particles and coatings, whereas elements that dissolve as anions (e.g. As, Mo, Se and Re), are dispersed widely. Field measurement of the conductivity of soil-water slurries showed two zones of salt (NaCl) enrichment, one directly over the deposit and the other 1 km away. Trenching of the soils in these zones revealed vertical fractures in the gravels, whereas trenching in a background area showed no fractures. The fracture zones appear to have formed by reactivation of basement faults. Elements present in the soils above the fracture zones are the same that are enriched in groundwater near the deposit and indicate the pumping of metal-rich groundwater to the surface during earthquakes, followed by evaporation and redistribution of elements by rain. Both fracture zones contain NaCl in addition to anomalous anions, whereas Cu is restricted to the fracture zone above the deposit. Similar anomalies were found in the gravel soils above the boundary faults of the Gaby Sur porphyry copper deposit in the same region.
In arid environments, when oxidized sulfide deposits are buried gradually by alluvium, chemical transference from the underlying source material to the newly deposited alluvium may occur (Kelley et al, 2002). Oxidized sulfide deposits exposed at the surface typically have strongly mineralized residual soil with elements occurring in a variety of forms, including water soluble phases. Deposition of the first layer of alluvium produces a physical mixing of the residual soil with the transported material. Dissolution of the water soluble phases produces metal-rich surface water which infiltrates downward. Evaporation of this water causes metal enrichment within the mixed layer of residual soil and alluvium. Successive wet/dry cycles through time facilitates a gradual chemical equilibrium between the older alluvium and the newly deposited alluvium, allowing the surface signature of an originally exposed deposit to be transferred to successive layers. This process is likely more important in arid environments where the overburden is relatively thin, and soluble saline minerals form from evaporation. The characteristic anomaly pattern is a baseline shift with several samples containing elevated values in sequence.
Dispersion of Soil Gases
Soil gases are of great interest because of their high degree of mobility through transported overburden. The formation, stability and migration of gas species within the near-surface environment govern the usefulness of soil gas as an exploration tool. Soil gases are produced from chemical, biological and physical processes, all of which are strongly dependent on environmental variables. There are no known gases that are uniquely related to mineral deposits, and most species of interest can be generated from a number of other mechanisms not related to mineral deposits. Further complications result from mass flow processes induced by barometric pumping, rainfall and seasonal temperature variations that create high background variations and problems with dilution. Despite the complex and multiple origins of soil gases, numerous soil gas species (e.g. CO2/O2, Hg, Rn, He, sulfur compounds and light hydrocarbons) have been measured over mineral deposits and these appear, at least empirically, to be related to buried mineral deposits (Klusman 1993, Hale 2000).
In arid environments within the vadose zone, vertical movement driven by buoyancy is expected if the overburden is permeable and the gas has significant vapor pressure. Migration of gas below the water table requires that the vapor pressure exceed the fluid pressure in order for a bubble to form, otherwise the gas dissolves in water. Degassing of groundwater may occur by groundwater advection along faults or following seismic activity (Hamilton 2000, Jones et al. 2000). The vertical migration of volatile light hydrocarbons over considerable depths (ca. >1700 m) has been documented by the oil and gas industry by the sharp drop- off of measured hydrocarbons beyond the limits of defined reservoirs and carbon isotope signatures from altered sediments overlying reservoirs (Saunders et al. 1999). Dispersion by rapid streaming of colloid- sized microbubbles driven by buoyancy is the favored interpretation.
Dispersion Facilitated by Biological Processes
Much of the Earth's biosphere is concentrated within a narrow horizon occurring at the interface between the lithosphere and atmosphere. In this region, eukaryotic ecosystems are supported by photosynthetic carbon fixation (primarily plant- derived). The chemical composition of plant material is related to soil composition and the factors that control element uptake. Elements are classified as essential or non-essential for plant growth. In cases where non-essential elements become toxic (e.g. Au, U, Cu, Pb, Zn), the offending element is 'detoxified' by partitioning to a non-sensitive part of the plant (e.g. bark or twigs) or by chemical binding (Berry 1986). Toxic elements are eventually returned to the ground surface during plant growth or decay, causing natural geochemical recycling. In these near surface environments, bacteria colonize mineral surfaces, growing as biofilms (Southam et al. 1995) or as complex-particle associations of cells (Ferris et al. 1987), and encounter surface- and groundwater as it 'flows' past (a sessile growth phase). Bacteria typically respond to soluble, toxic heavy metals by binding and precipitating these metal ions on their surface, producing fine- grained minerals (Southam et al. 1995). This precipitation is due to the physical and chemical nature of the bacterial cell envelope and can be promoted by dissimilatory metabolic activity. When combined with the ubiquitous nature of bacteria and the enrichment of specific bacterial populations due to mineral and aqueous substrates, bacteria likely have a profound affect on the formation of metal anomalies surrounding mineral deposits.
In addition to these surface processes, the biosphere is active in the subsurface (Lovley & Chapelle 1995) where bacteria serve as active geochemical agents. As a group, bacteria are physiologically diverse and can gain energy from oxidizing inorganic constituents via biooxidation and alternatively, reduce these inorganic constituents using organic carbon as the oxidant. In these systems, the oxidation of metal sulfides is kinetically hindered; however, it can be greatly enhanced via bacterial catalysis, even under neutral pH conditions (Mielke et al. 2003) producing acid and, oxidized iron and sulfur constituents as by-products of metabolism. As these geochemical signatures (including base metals) are released from the sulfide deposit, they will encounter bacterial populations in the subsurface and ultimately the concentrated biosphere at the Earth's surface. Since iron oxidizing bacteria are aerobic autotrophs, the O2 they consume would contribute to the reduced column present above sulfide deposits and may support dissimilatory iron reducing bacteria, which would maintain the iron as a reduced, mobile phase.
Ore deposits are exceptionally rare and their occurrence in the near surface environment disturbs the natural equilibrium established between the shallow oxidized and underlying reduced environments. The dynamic processes of weathering and sulphide oxidation liberates ore and related elements, making them available for the secondary redistribution away from the deposit. A complex interaction between physical, chemical and biological processes respond to this disturbance to re-establish equilibrium conditions.
The processes mentioned here are at least some of the mechanisms that cause geochemical anomalies in transported overburden over buried mineral deposits. The degree of interaction between these processes and their applicability to other environments is largely unknown. Considerable research is needed to understand the interrelations between the physical, chemical and biological process that occur in overburden settings above mineral deposits. The development of reliable remote geochemical exploration methods is critical for the sustainable discovery of blind ore deposits. In order for these methods to be reliable, they must 1) be optimized for the processes that cause geochemical dispersion, 2) provide sufficient contrast in the response by isolating the targeted process, 3) be used with a thorough understanding of background variation, and 4) provide a high level of precision.
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Financing Gold Projects: The Challenges
Research Director & Investment Manager, Veneroso Associates, and President, Celtic Mining LLC
In a simple world the problem of project funding would be simple to solve. Find a gold deposit. Calculate the costs of extraction. Compare the cost to the projected revenues. Demonstrate the profit margin and Hey Presto, financing arrives at your door.
Unfortunately such a simple world only exists in our dreams. The harsh reality is that competition for project finance is extreme and the ability of companies to demonstrate the true value of their "deposit" may be hindered by a variety of factors. The playing field is not flat. Variations in reporting rules allow companies from some countries to report far more ounces of gold as "reserves" and/or "resources" than others. Promoters sell their deposit better than you do. Countries occasionally change the rules without notice. Gold prices change. Investment interest in the sector changes etc..
In this presentation, I will tackle the issue of gold project finance. I will address some fundamental issues of project valuation and then discuss the reporting process and impediments that frequently lead to the public not hearing the "full story". I will address the needs of investors to whom exploration and mining companies go seeking investment. I will explain what they are looking for and why. I will also tackle some macro economic factors that directly affect the gold business. An awareness of these can help understand the direction of the market so as to be in the right place at the right time. I will be using real world examples to illustrate the problems and to provide solutions.
September 2005 Technical Presentation
Bulk Rock and Melt Inclusion Geochemistry of Bolivian Tin Porphyry Systems
Department of Geology and Geological Engineering
Colorado School of Mines
The Miocene tin porphyry systems of Llallagua, Chorolque and Cerro Rico have a moderately fractionated rhyodacite to dacite bulk rock composition. Ta, Zr and TiO2 concentrations are close to average upper crustal values. Hydrothermal overprint is reflected by strong enrichment of B, Bi and Sn (>100 times upper crust), and by moderate enrichment of Sb, Pb, Ag, As, Au and W (10-100 times upper crust). Melt inclusions in quartz phenocrysts have been analyzed by electron and proton microprobe techniques. The melt inclusions are characterized by highly fractionated rhyolitic composition with strong depletion of compatible components (0.02-0.14 wt.% TiO2, 15-85 ppm Zr). The trace element pattern with strong enrichment of incompatible elements (5-17 ppm Ta, 7-85 ppm As, 35-643 ppm B, 20-194 ppm Cs, 13-623 ppm Li, 5-43 ppm Sn) is similar to tin granite systems. The compositional gap between melt inclusion and bulk rock geochemistry, and large compositional variations of trace elements among melt inclusions cannot be explained by crystal-liquid fractionation in a closed system alone.
We propose a scenario of selective quartz crystallization in a compositionally zoned magma chamber ranging from intermediate to highly fractionated melt portions. Influx of primitive melt into the magma chamber is thought to result in mixing and to trigger volcanic activity which leads to the intermediate degree of fractionation of the exposed tin porphyry systems. Unexposed tin granitic portions release magmatic vapor phases which follow the volcanic vents and result in hydrothermal alteration and mineralization. Supply of magma and metals from different portions of compositionally zoned magma chambers can explain the exceptional metallogenic association of Bolivian tin porphyry mineralization with only moderately fractionated igneous rocks. It is probably those portions of a general tin granite composition which are chemically linked to tin mineralization whereas the exposed rhyodacitic stocks provide essentially the structural focusing for magmatic vapor phases from a deeper stratified magma reservoir.
October 2005 Technical presentation
The Kupferschiefer and Zambian Copperbelt: Basin Evolution and Mineralisation
David W. Broughton
Department of Geology and Geological Engineering
Colorado School of Mines
The Polish Kupferschiefer and Zambian Copperbelt are located within the world's two largest provincesof sediment-hosted stratiform copper (SSC) deposits. These districts share several fundamental attributes of basin architecture (e.g. redbeds, evaporites, reduced host rocks) and certain aspects of basin evolution (e.g. rift-stage red beds, sag-stage carbonate-evaporite platforms, renewed extensional stage clastics (volcanics), salt tectonics, and basin inversion (relatively modest in Poland).
Both districts also appear to be characterised by multistage mineralisation events linked to successive stages in basin evolution. These include early diagenetic disseminated mineralisation associated with early basin compaction, later
diagenetic disseminated and veinlet mineralisation associated with accelerated extension and heat flow, early orogenic mineralization associated with the onset of basin inversion and hydrocarbon maturationmigration,
and post-orogenic vein mineralization.
In the Zambian Copperbelt, the result was the generation of deposits at mutiple stratigraphic levels. The presentation will explore evidence for this interpretation, and the implication that a better understanding of basin history can aid exploration in these and other prospective basins.
November 2005 Technical Presentation
East Asia: Mineral Deposit Geology/Endowment and Metal Consumption
Laurence P. James
NewWest Gold Corp., Lakewood CO
Japan and South Korea - two "economic miracles" since the l950s - are huge consumers of metals and uranium. South Korea, the economic focus in this talk, is the world's fourth largest steel producer and builds 65% of the world's ships. It has modern copper and zinc and lead smelters plus a nickel plant, and has a huge consumption of many metals by industry and infrastructure building. It is building more nuclear power plants and importing nearly all of its coal supplies. Gold and silver are consumed on a large scale by electronics and jewelry manufacture. Japan has an even larger economy, five copper smelters, and major zinc, nickel and steel industries. Along the eastern coast of the Asian continent several other countries - Taiwan, Malaysia, and (increasingly) Vietnam - have significant metal consumptions. The Philippines, a mining country, now imports concentrates to feed its modern copper smelter.
This presentation will review the economic geology of nonferrous metals in eastern Asia, with emphasis on large deposits that may be significant in the 2lst century, or as models for exploration elsewhere. Since l960 the region has seen many metal discoveries and new mines. The best triumphs have been for epithermal gold in Japan (e.g. Hishikari) and Indonesia (Kelian), gold-bearing porphyry copper in the Philippines (e.g. Far Southeast; recent Anglo discovery at Boyongan), plus mega gold - copper at Grasberg and Batu Hijau, Indonesia. Compared to most deposits in the Western Hemisphere, all of these deposits are have extremely young ages. A few systems (Hishikari, ~2mA) are associated with igneous rocks which are still cooling. Concepts rising from these discoveries include better understanding of the tectonics underlying and generating mineral belts, and the relationship between epithermal systems and hot springs. Some discussion of the noted "Philippine Fault", and of significant East Asian exploration successes of recent decades will be presented.
Many of the East Asian countries have nearly exhausted their known domestic metal resources. They are heavily populated, and space for mine development and mine/mill waste disposal conflicts with agriculture, marine industry, and living space. Traditionally these countries have purchased raw materials from their neighbors. China, and developing Asian countries in the archipelagoes or island arcs of the adjacent South Pacific have provided metallic minerals. But today, China consumes more than it produces. Which neighboring countries still have the potential to provide future smelter and steel mill feed? Both Japan and Korea have significant, government-funded overseas mine investment and exploration programs. Significant potential for discovery remains in Indonesia, where problems in recent years have driven away most multinational explorers.
Presently, decision - makers of the two most-developed countries, Japan and Korea, believe the Western hemisphere will be the major source of metal resources for the near future. "Going West" into the Asian continent for minerals has proven difficult and complicated. The explosive growth of China, the difficulties of the Russian land and legal systems, and the landlocked character of former Soviet satellites and republics hinder "Going West". The evolving Islamic republics further west are also perceived as difficult, and distances over rugged lands with little infrastructure are large. However, long-term work programs and generative efforts are evident in many interior Asian countries. Mongolia has recently achieved significant success.
North America, with almost no custom smelters, provides a flow of metal concentrates into East Asia and also exports significant gold, some of it in smelter feed. Along with Asian-made automobiles and electrical goods, there is now a perceptible flow of exploration funding coming back across the Pacific, especially to Latin America.
December 2005 Technical Presentation
The Sweet Home Rhodochrosite Specimen Mine, Alma District, Central Colorado: The Porphyry Molybdenum Fluorine Connection
Paul J. Bartos, Colorado School of Mines Geology Museum
Intermediate sulfidation veins containing quartz-sphalerite-tetrahedrite-rhodochrosite-fluorite in the Sweet Home Mine, Alma district, Colorado were originally mined for silver starting in 1873. For the last thirteen years until October 2004, however, the mine has produced world-class rhodochrosite specimens. Some of these specimens are considered to be among the finest mineral specimens ever produced, and certainly the finest of their species, with values well over $1 million U.S. dollars. The extraction, preparation and marketing techniques pioneered at the Sweet Home operation have revolutionized the minerals specimen industry.
The Sweet Home deposit is interpreted here as a failed (single pulse) variant of a Climax-type porphyry molybdenum hydrothermal system. However unlike Climax-type systems, the hydrothermal system at Sweet Home appears to have consisted of a single, relatively small, pulse of magmatic fluid that slowly cooled and diluted with groundwater. This is inferred to have occurred at moderate depths, on the order of 1.5-2.5 kilometers below the surface.
The fluids that formed the Sweet Home veins were dilute (approximately 2-4% NaCl equivalent), high temperature (up to 370 degrees C) and of magmatic origin. Gem quality ruby red rhodochrosite at Sweet Home is very nearly pure manganese carbonate, with minimal solid solution with Fe+2, Ca or Mg. It formed at higher temperatures and salinities in comparison to lower value, pink rhodochrosite. There is a distinct association of gemmy, ruby-red rhodochrosites with highly evolved silica-rich hydrothermal systems; the high fluorine content typical of such systems suggests that Mn was transported in solution as fluorine complexes, which in turn favors rhodochrosite deposition at above average temperatures and with minimal cation contamination.