Showing papers by "Richard J. Goldfarb published in 2016"

The conjunction of factors that lead to formation of giant gold provinces and deposits in non-arc settings

Abstract: It is quite evident that it is not anomalous metal transport, nor unique depositional conditions, nor any single factor at the deposit scale, that dictates whether a mineral deposit becomes a giant or not. A hierarchical approach thus is required to progressively examine controlling parameters at successively decreasing scales in the total mineral system to understand the location of giant gold deposits in non-arc environments. For giant orogenic, intrusion-related gold systems (IRGS) and Carlin-type gold deposits and iron oxide-copper-gold (IOCG) deposits, there are common factors among all of these at the lithospheric to crustal scale. All are sited in giant gold provinces controlled by complex fundamental fault or shear zones that follow craton margins or, in the case of most Phanerozoic orogenic giants, define the primary suture zones between tectonic terranes. Giant provinces of IRGS, IOCG, and Carlin-type deposits require melting of metasomatized lithosphere beneath craton margins with ascent of hybrid lamprophyric to granitic magmas and associated heat flux to generate the giant province. The IRGS and IOCG deposits require direct exsolution of volatile-rich magmatic-hydrothermal fluids, whereas the association of such melts with Carlin-type ores is more indirect and enigmatic. Giant orogenic gold provinces show no direct relationship to such magmatism, forming from metamorphic fluids, but show an indirect relationship to lamprophyres that reflect the mantle connectivity of controlling first-order structures. In contrast to their province scale similarities, the different giant gold deposit styles show contrasting critical controls at the district to deposit scale. For orogenic gold deposits, the giants appear to have formed by conjunction of a greater number of parameters to those that control smaller deposits, with resultant geometrical and lithostratigraphic complexity as a guide to their location. There are few giant IRGS due to their inferior fluid-flux systems relative to orogenic gold deposits, and those few giants are essentially preservational exceptions. Many Carlin-type deposits are giants due to the exceptional conjunction of both structural and lithological parameters that caused reactive and permeable rocks, enriched in syngenetic gold, to be located below an impermeable cap along antiformal “trends”. Hydrocarbons probably played an important role in concentrating metal. The supergiant Post-Betze deposit has additional ore zones in strain heterogeneities surrounding the pre-gold Goldstrike stock. All unequivocal IOCG deposits are giant or near-giant deposits in terms of gold-equivalent resources, partly due to economic factors for this relatively poorly understood, low Cu-Au grade deposit type. The supergiant Olympic Dam deposit, the most shallowly formed deposit among the larger IOCGs, probably owes its origin to eruption of volatile-rich hybrid magma at surface, with formation of a large maar and intense and widespread brecciation, alteration and Cu-Au-U deposition in a huge rock volume.

Generation of postcollisional porphyry copper deposits in southern Tibet triggered by subduction of the Indian continental plate

Abstract: Oligocene to Miocene postcollisional porphyry Cu deposits in the Gangdese belt in southern Tibet contain total resources of >20 million metric tons (Mt) Cu and are genetically associated with granodioritic-quartz monzogranitic porphyry intrusions with adakite-like signatures (e.g., Sr/Y >40). The adakite-like magmatic rocks in the southern sub-belt of the eastern Gangdese belt (east of 87° E) range in age from ca. 38 to 18 Ma, whereas those in the northern sub-belt range in age from ca. 21 to 10 Ma. Mineralization ages of the porphyry Cu deposits in the eastern Gangdese belt also show a decreasing trend from south to north, with the deposits in the southern sub-belt being ca. 30 Ma and the deposits in the northern sub-belt, 21 to 13 Ma. Many more of the adakite-like intrusions in the northern part are associated with porphyry copper deposits, compared with those in the southern part. The adakite-like intrusions exhibit high SiO2 (>60 wt %), Al2O3 (mostly >15 wt %), K2O (>2 wt %), and Sr (>300 ppm); low Y ( 80 km) into the lower part (~60–70 km) of the lower crust, together with direct input of fluid liberated from the subducting Indian continental plate, resulted in H2O-added melting of the Tibetan lower crust, generating H2O-rich adakite-like magmas in the eastern Gangdese belt. The adakite-like rocks in the western Gangdese have very similar geochemical compositions to those in the eastern Gangdese, and their generation can also be explained by the melting of subduction-modified mafic lower crust with input of ultrapotassic melt. However, in contrast, colder (5°–8°C/km geotherm) subduction of the Indian continental plate and the opposite younging trend from north to south for the postcollisional adakite-like and ultrapotassic rocks in the western Gangdese belt suggests that the generation of the adakite-like rocks in the west was triggered by a different geodynamic process, which is most likely roll-back and gradual break-off of the northward subducting Indian slab from north to south. We suggest that H2O in the postcollisional ore-related magmas originated from dehydration reactions in the upper parts of the subducting continental plate. Thermal structure of the continental subduction zone and the amount of continental crust subducted to depth seem to be two critical controls on the generation of porphyry Cu deposits in the Tibetan postcollisional setting.

Mineral information at micron to kilometer scales: Laboratory, field, and remote sensing imaging spectrometer data from the orange hill porphyry copper deposit, Alaska, USA

Abstract: Using imaging spectrometers at multiple scales, the USGS, in collaboration with the University of Alaska, is examining the application of hyperspectral data for identifying large-tonnage, base metal-rich deposits in Alaska. Recent studies have shown this technology can be applied to regional mineral mapping [1] and can be valuable for more local mineral exploration [2]. Passive optical remote sensing of high latitude regions faces many challenges, which include a short acquisition season and poor illumination due to low solar elevation [3]. Additional complications are encountered in the identification of surface minerals useful for mineral resource characterization because minerals of interest commonly are exposed on steep terrain, further challenging reflectance retrieval and detection of mineral signatures. Laboratory-based imaging spectrometer measurements of hand samples and field-based imaging spectrometer scans of outcrop are being analyzed to support and improve interpretations of remote sensing data collected by airborne imaging spectrometers and satellite multispectral sensors.