Peer Reviewed Article Lost Secrets of Carlin Gold Exploration John Wood, CPG-10580 - Consulting Mineral Discovery Specialist
In the modern era of 3D models, artificial intelligence, and database-centric exploration planning, sometimes obvious exploration vectors go unnoticed. Mineralogy and alteration zonation are often portrayed as alteration types 1, 2 and 3 without deep thought of geologic process, fluid flow or min- eral depositional environments. Using 3D mineralogy models is often more enlightening than geochemistry. Having a rudimentary knowledge of geologic processes and a vision of where gold should deposit can reduce exploration costs and the timeline to discovery.
The various types of gold deposits have differing genetic
origins of fluid chemistry, source rocks, host rocks, mineral- ogy and process of formation. Even similar deposits within the same district may have very different attributes due to changes in geologic parameters of formation. Irrespective of deposit type and structure, fluid-flow pathways are impor- tant controls of gold deposition. Host rock reactivity, fluid conditions, depth, pressure and temperature of formation can all be important controls on the solubility of gold and depositional environments. Most Carlin-like gold deposits occur as relatively small high-grade deposits occurring along folds and contacts with reactive rocks, bedding parallel faults and other structures where fluid flow may be impeded long enough to deposit gold. Breccia structures along larger faults may be mineralized where fluids experience a pressure drop entering an open structure. Flat structures and bedding can deflect ore-bearing fluids great distances from source areas, and barren altered rocks may extend several kilometers from a gold deposit, hindering any quick discovery.
During the 1980s, jasperoid bodies were considered closely associated with gold deposits in Nevada and Utah; many were extensively drilled, and a few gold deposits discovered. In later decades gold deposits were discovered more distal from jasperoid bodies, and complex alteration patterns were studied. Hydrothermal fluids that formed Carlin-like deposits consisted of more than 95% meteoric water (Hofstra, 1997) that circulated several kilometers deep in the upper crust, probably at the base of the paleo-water table. These fluids cir- culated through many cubic kilometers of upper crustal rocks. Chemistry and rock textures hint that relatively barren pro- grade fluids were weakly alkaline to weakly acidic, buffered by host rocks. Advancing fluids leached moderate quantities of carbonate, silica, barite and other soluble constituents from host rocks, increasing permeability of calcareous sedimentary and igneous host rocks. Distal fluids deposited leached mate- rial up hydrothermal gradients from gold deposits, creating a mineralogical zonation of pathfinder minerals. In most cases, jasperoid, calcite and barite, more rarely realgar or stibnite,
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and other minerals formed alteration zones around gold deposits locally extending more than a kilometer from the environment of gold deposition. These distinctive alteration zones can be very subtle and go unnoticed and unrecorded by geologists. Experienced workers often recognize the alteration but have difficulty putting it into the proper 3D context to aid discovery of the gold deposit.
The complex physical controls of Carlin-like gold deposits by host rocks and structure cause most of the exploration difficulties. The process of gold transport and deposition has been illuminated by mineralogical studies, which outline important hydrothermal conditions of gold deposition. Primary fluids reached temperatures up to 350°C and deposited gold at temperatures around 240-220°C with about 6 weight percent NaCl (Emsbro et al., 2003). It is not surprising that gold in the form of Au(HS)2 reaches its maximum solubility at 350° in weakly alkaline fluids (Renders and Seward, 1989). In the gold depositional environment, destruction of the gold carrier com- plexes initiates gold deposition. Depending on environmental conditions, the HS molecules may form hydrogen disulfide (H2S2), hydrogen sulfide (H2S) and/or sulfuric acid (H2SO4). In many deposits, the mineral marcasite forms in close association with gold deposition. It forms in great abundance in the presence of aqueous HS molecules where sulfuric acid forms and fluid pH falls below 5.5. If gold deposition and fluid movement is slow due to high-pressure wallrock reactions, sul- fidation or high temperature, marcasite may not form. Fluids may flow a considerable distance from a gold deposit before strongly acid conditions occur and marcasite forms. At higher elevations and lower temperatures, the HS molecules may
quickly form H2S and/or sulfuric acid, creating strongly acid fluid conditions with extensive marcasite formation. If aque- ous HS molecules are depleted rapidly, by formation of new
compounds or H2S vapor, strongly acid conditions can occur without formation of marcasite. In intermediate areas fluids may go through cycles of strong acidity periodically neutral- ized by wallrock carbonate or less acid fluid inflow, resulting in extensive banding of pyrite and marcasite.
Marcasite is the best indicator mineral of fluid and depo- sitional conditions in Carlin-like gold deposits. Marcasite precipitates in very restrictive depositional environments: pH <5, temperatures below 240°C, with abundant bisulfide
molecules like H2S2 in solution (Murowchick, 1992). Marcasite is unstable at prolonged temperatures above 160°C and tends to go back into solution or revert to pyrite (Nash et al., 1989). This explains a common sulfide texture of alternating bands of pyrite and vuggy pyrite that was formerly marcasite. In some deposits, marcasite is not present, but the banded and botry-
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