Search USGS⌕ Search

Geology topics

Lance D. Miller

Publications and source records attributed to Lance D. Miller.

6 recordsLinked to original sources
USGS2004

The late cretaceous Donlin Creek gold deposit, Southwestern Alaska: Controls on epizonal ore formation

The Donlin Creek gold deposit, southwestern Alaska, has an indicated and inferred resource of approximately 25 million ounces (Moz) Au at a cutoff grade of 1.5 g/t. The ca. 70 Ma deposit is hosted in the Late Cretaceous Kuskokwim flysch basin, which developed in the back part of the arc region of an active continental margin, on previously accreted oceanic terranes and continental fragments. A hypabyssal, mainly rhyolitic to rhyodacitic, and commonly porphyritic, 8- × 3-km dike complex, part of a regional ca. 77 to 58 Ma magmatic arc, formed a structurally competent host for the mineralization. This deposit is subdivided into about one dozen distinct prospects, most of which consist of dense quartz ± carbonate veinlet networks that fill north-northeast–striking extensional fractures in the northeast-trending igneous rocks. The sulfide mineral assemblage is dominated by arsenopyrite, pyrite, and, typically younger, stibnite; gold is refractory within the arsenopyrite. Sericitization, carbonatization, and sulfidation were the main alteration processes. Fluid inclusion studies of the quartz that hosts the resource indicate dominantly aqueous ore fluids with also about 3 to 7 mol percent CO 2 ± CH 4 and a few tenths to a few mole percent NaCl + KCl. The gold-bearing fluids were mainly homogeneously trapped at approximately 275° to 300°C and at depths of 1 to 2 km. Some of the younger stibnite may have been deposited by late-stage aqueous fluids at lower temperature. Measured δ 18 O values for the gold-bearing quartz range between 11 and 25 per mil; the estimated δ 18 O fluid values range from 7 to12 per mil, suggesting a mainly crustally derived fluid. A broad range of measured δ D values for hydrothermal micas, between –150 and –80 per mil, is suggestive of a contribution from devolatilization of organic matter and/or minor amounts of mixing with meteoric fluids. Gold-associated hydrothermal sulfide minerals are characterized by δ 34 S values mainly between –16 and –10 per mil, with the sulfur derived from diagenetic pyrite and organic matter within the flysch basin. A smaller group of δ 34 S measurements, which shows values as depleted as –27 per mil, suggests a different local sulfur reservoir in the basin for the later hydrothermal episode dominated by stibnite. Initial ϵ Nd of –8.7 to –3.1 and 87 Sr/ 86 Sr measurements of 0.706 to 0.709 for the ore-hosting dikes also indicate a crustal reservoir for some of the Late Cretaceous magmatism. Overlapping lead isotope data for these intrusive rocks and for sulfide minerals suggest a crustal contribution for the lead in both. Copper- and gold-bearing stockwork veinlets in hornfels occur at Dome, a prospect located at the northern end of the Donlin Creek deposit. These stockworks are cut by the younger auriferous gold veins that define the main Donlin Creek gold mineralization. Highly saline, gas-rich, heterogeneously trapped fluids deposited the stockworks at temperatures approximately 100°C hotter than those of the main gold-forming event at Donlin Creek. The genetic relationship of the Dome prospect to the main Donlin Creek gold resource is equivocal. The epizonal Donlin Creek deposit shows affinities to the gold systems interpreted by various workers as orogenic or intrusion related; it shows important differences from typical epithermal and Carlin-like deposits. The ore-forming fluids were derived by either broad-scale metamorphic devolatilization above rising mantle melts or exsolution from a magma that was dominated by a significant flysch melt component.

Economic Geology↗
USGS2003

Alaska resource data file: Juneau quadrangle

Descriptions of the mineral occurrences shown on the accompanying figure follow. See U.S. Geological Survey (1996) for a description of the information content of each field in the records. The data presented here are maintained as part of a statewide database on mines, prospects and mineral occurrences throughout Alaska.

Alaska↗
USGS2002

Gold deposits of the northern margin of the North China craton: Multiple late Paleozoic-Mesozoic mineralizing events

The northern margin of the North China craton is well-endowed with lode gold deposits hosting a resource of approximately 900 tonnes (t) of gold. The ~1,500-km-long region is characterized by east-trending blocks of metamorphosed Archean and Proterozoic strata that were episodically uplifted during Variscan, Indosinian, and Yanshanian deformational and magmatic events. At least 12 gold deposits from the Daqinshan, Yan-Liao (includes the Zhangjiakou, Yanshan, and Chifeng gold districts), and Changbaishan gold provinces contain resources of 20–100 t Au each. Most deposits are hosted in uplifted blocks of Precambrian metamorphic rocks, although felsic Paleozoic and Mesozoic plutons are typically proximal and host ~30% of the deposits. The lodes are characterized by sulfide-poor quartz veins in brittle structures with low base metal values and high Au:Ag ratios. Although phyllic alteration is most common, intensive alkali feldspar metasomatism characterizes the Wulashan, Dongping, and Zhongshangou deposits, but is apparently coeval with Variscan alkalic magmatism only at Wulashan. Stepwise 40 Ar– 39 Ar geochronology on 16 samples from gangue and alteration phases, combined with unpublished SHRIMP U–Pb dates on associated granitoids, suggest that gold mineralizing events occurred during Variscan, Indosinian, and Yanshanian orogenies at circa 350, 250, 200, 180, 150, and 129 Ma. However, widespread Permo-Triassic (~250 Ma) and Early Jurassic (~180 Ma) thermal events caused variable resetting of most of the white mica and K-feldspar argon spectra, as well as previously reported K–Ar determinations. Compiled and new stable isotope and fluid inclusion data show that most δ 18 O values for ore-stage veins range from 8 to 14‰, indicating a fluid in equilibrium with the Precambrian metamorphic basement rocks; δD values from fluid inclusions range widely from –64 to –154‰, which is indicative of a local meteoric component in some veins; and highly variable δ 34 S data (+7 to –17‰), even within individual deposits, indicate various local country-rock sources for sulfur. Fluid inclusions from all districts show variable homogenization temperatures between 240 and 400 °C, and are consistently characterized by low salinity, H 2 O–CO 2 ± CH 4 , N 2 solutions. Although the data are largely consistent with that from orogenic gold veins, intrusion-related veins and epithermal veins are also recognized. The multiple episodes of mineralization are coincident with episodic tectonic reactivations and associated magmatism along the northern margin of the North China craton.

Mineralium Deposita↗
USGS2000

The Tintina Gold Belt - A global perspective

The so-called Tintina Gold Belt extends for more than 1000 km along the length of the northern North American Cordillera. Middle to Late Cretaceous Au deposits within the belt have various similar characteristics, among which are a spatial and temporal association with magmatism; Bi-W-Te signatures in deposits hosted by granitod stocks and As-Sb signatures where hosted by sedimentary rocks and dyke systems; and δ180 values consistently > 12 per mil for Au-bearing quartz. Nevertheless significant differences in structural styles, levels of deposit emplacement, ore-fluid chemistry, and Au grades suggest that the characteristics represent a broad range of deposit types. Many of these are best classified as orogenic Au deposits in the Yukon-Tanana terrane, as epithermal and porphyry-style Au deposits in the Kuskokwim region, and as Au-bearing, granite-related veins and stockworks, replacements, and skarns, as well as associated polymetallic lodes, in central Yukon. The diverse types of Au deposits and associated plutons of the Tintina Gold Belt collectively define a 45-m.y.-long period of arc magmatism that migrated northwesterly, for about 1000 km, across the active collisional margin of Cretaceous northwestern North America. The initiation of fluid flow and plutonism in Albian time seems to correlate with the onset of oblique subduction and dextral strike-slip on the Denali-Farewell, Tintina-Kaltag, and related fault systems. Initial Au-vein formation and subduction-related magmatism at about 115-110 Ma (e.g., including the Goodpaster and Fortymile districts), within the seaward side of the Yukon-Tanana terrane, correlate with the arrival of the Wrangellia superterrane off the continental margin. Dextral translation of the allochthonous Wrangellia block was associated with the migration of the thermal pulse to the northwest at about 95-90 Ma. Orogenic (or so­ called mesotherrnal) and granitoid-related Au deposits formed across the width of the Yukon-Tanana terrane (e.g., Fort Knox, True North, Ryan Lode, Kantishna district) and inland into the passive-margin rocks of the Selwyn basin ( e.g., Scheelite Dome, Brewery Creek, Dublin Gulch), respectively. By 70 Ma, the arc had migrated to the vicinity of present-day southwestern Alaska, where it was associated with the formation of additional orogenic Au deposits (e.g., Willow Creek district) and, within still-preserved shallow crustal levels, epithermal Au systems (e.g., Donlin Creek). The Au-bearing deposits of the Tintina Gold Belt are typical of those found in most well-preserved, moderate- to high-temperature Phanerozoic collisional orogens. Around the circum-Pacific region, these would include large areas of Mesozoic tectonism along the Cordilleran orogen, throughout the Russian Far East, and along the margins of the North China craton. Favorable terrain for such Au belts of Paleozoic age worldwide include the active Gondwana margins (e.g., Tasman orogenic system, northern Africa, Telfer district), and the northern margins ( e.g., Caledonian Kazakhstania, Uralian orogen, Baikal orogen, Tian Shan orogenic system) and western margins ( e.g., southern European massifs) to the Paleo-Tethys Ocean. Gold lodes in all of the Phanerozoic belts are dominated by orogenic Au-deposit types; other deposit types are concentrated where relatively shallow levels to the orogens are locally preserved. A significant percentage of the lode-gold resource in many areas was lost to placer accumulation that began forming approximately 100 m.y. after hypogene ore formation, except where continent-continent collision "cratonized" highly mineralized terranes in central Asia.

Book chapter↗
USGS1994

Genetic links among fluid cycling, vein formation, regional deformation, and plutonism in the Juneau gold belt, southeastern Alaska

Gold-bearing quartz vein systems in the Juneau gold belt formed within a 160-km- long by 5- to 8-km-wide zone along the western margin of the Coast Mountains, Alaska. Vein systems are spatially associated with shear zones adjacent to terrane-bounding, mid-Cretaceous thrust faults. Analysis of vein orientations and sense of shear data define a stress configuration with greatest and least principal axes oriented subhorizontally with northeast-southwest trends and subverticaly, respectively. This local stress configuration is compatible with the far-field plate configuration during Eocene time. Isotopic ages of vein formation indicate that fluid cycling occurred between 56.5 and ≥52.8 Ma, and are consistent with a genetic link between veining and a change in plate motion in early Eocene time. Veining was also synchronous with the latter stages of rapid exhumation and voluminous plutonism immediately inboard of the gold belt. We propose a model in which interacting tectonic events facilitated fault-valve action and vein development along now-exhumed shear zones.

Geology↗