-Galkhaite, a complex Tl-Sulfosalt recently found at the Twin Creeks Mine, Humboldt County, Nevada, and its implications in dating Au mineralization-
Philip M. Persson
The University of Colorado
3625 Catalpa Way, Boulder, Colorado, 80304.
philip.persson@colorado.edu
-Abstract-
Galkhaite, a rare Tl-Hg Sulfosalt, was recently identified at the Twin Creeks Mine, a large open-pit gold mine about 45 miles northeast of Winnemucca in Humboldt County, Nevada operated by Newmont Mining Corporation. The Twin Creeks Mine exploits several large disseminated Au deposits hosted in Paleozoic sediments; typical Carlin-type deposits [CTD’s]. Galkhaite at Twin Creeks occurs in an Au-rich hydrothermal sulfide breccia which is controlled and bordered by late Cretaceous and Tertiary tuffaceous equigranular mafic intrusives, which in turn intrude highly faulted, overturned, thinly laminated beds of the middle Comus Formation, a series of decalcilfied detrital marine limestones. Galkhaite found at the nearby Getchell Mine, another very similar CTD, has been used to date the primary Au mineralization at Getchell deposit, as it is the only species thus far identified in a CTD which is unequivocally linked directly to the major period of Au-deposition in the Great Basin ~32-42 Ma. [1] While age dates for the Twin Creeks deposit have been obtained from seritized quartz-andularia veins, it is not clear if these veins are associated with the Carlin-type mineralization period. [Pers. Comm. P. Donovan, 7/06]
The discovery of significant quantities of Galkhaite at Twin Creeks is paramount because this species can hopefully be used to date the major period of Au mineralization at this deposit and establish a more clear relationship between the mineralization at the nearby Getchell deposit and that at other CTD’s across the Great Basin. Galkhaite is a trace mineral in at least four CTDs in Nevada, and contains significant amounts of Rb and virtually no Sr, making it an ideal candidate for radiometric dating. [1] Galkhaite from Twin Creeks occurs as cubic, octahedral, and complex twinned dodecahedral crystals to 8 mm. associated with crystallized stibnite and marcasite/pyrite in fractures and vugs in a highly silicified hydrothermal breccia, composed of angular limestone shards cemented by sulfides. While paragentically late in this assemblage, it is clear from both mineralogical and geochemical data that this Galkhaite would be ideal for dating the major period of Carlin-type mineralization at the Twin Creeks Mine.
-Introduction-
The Twin Creeks Mine, operated by Newmont Mining Corporation (NMC), is located in Humboldt County, Nevada, about 72 km (45 mi) northeast of Winnemucca in the historic Potosi mining district along the northeast flank of the Osgood Mountains. Twin Creeks, which began operating in 1984, is the third largest primary gold producing mine in North America, with past production and reserves of ~500 t (16 Moz) Au from four open pits, which exploit the Mega, Vista, Discovery, and West deposits. The disseminated gold deposits are hosted principally in Paleozoic sedimentary rocks, like other Carlin-type gold deposits in Nevada. Coarsely-crystallized, complex mineralization is rarely seen at Twin Creeks, as with most of the numerous Carlin-type systems in north-central Nevada, the majority of the gold occurs as microscopic inclusion to 3 or 4 nm. in arsenian diagenic pyrite, which form rims around larger pyrite grains in turn disseminated within a complex system of uplifted, faulted Paleozoic strata and later igneous intrusions. These intrusions, of late Cretaceous to early Tertiary age, almost exclusively host any Au mineralization >1 oz/ton at Twin Creeks, and have been interpreted as re-mobilizing and concentrating Au from lower grade disseminated deposits. This appears to be the case in the Galkhaite occurrence.
In 2003 Twin Creeks produced 25.3 t (812,458 oz) Au from an oxide and sulfide milling and oxide leaching operation. Oxide ores at Twin Creeks are processed at a rate of 2,540 tons/day [t/d] through the Juniper Mill. The leach-grade material is processed on three leach pads. Sulfide ores at Twin Creeks are processed at the Sage Mill, which uses the two largest autoclaves in the gold industry. Start-up of the first 3,629 t /d autoclave occurred in May 1997; the second autoclave, of the same size, began operation in November 1997. The sulfide project enables the mine to process refractory ores that contain approximately 66% of the roughly 186.6 t (6 Moz) of current Twin Creeks’ gold reserves. Current Au production at the Twin Creeks mine is close to 1Moz (~900,000 oz. in 2005), of which approx. two thirds is recovered from sulfide refractory ores; the remainder being from oxidized ores. Current production comes from the southern half of the main open pit, known as the South Mega Pit, but in the near future will be expanded to the North Mega, Vista, and Discovery Pits as new projects are developed to help reach the current mine life estimate of ~2025. [pers. Comm. P/. Donovan, 7/06]
-Background-
In early summer of 2006, Twin Creeks geologists successfully identified and targeted a pronounced and distinct zone of high sulfide enrichment in a lower area of the main production pit. This zone was being exposed in the lowermost levels of the South Mega Pit, in an area known as Cut 18, one of the major mining targets at Twin Creeks for 2006. In late spring of 2006, geologists at Twin Creeks began to notice signs of stibnite, marcasite, pyrite, and other sulfides more frequently in the new benches being driven into Cut 18, with very localized and high-grade gold mineralization hosted predominantly in the middle and lower member of the Cambrian-Ordovician Comus Formation, a silty, locally carbonaceous limestone. These Paleozoic rocks consist of deformed, highly folded and faulted marine detrital sediments that were deposited in a continental-shelf slope environment on the margins of an extensive island back-arc system which was responsible for much of the volcanic activity in what is the now western U.S at the time. It has long been speculated that the origin of the many heavy metals such as Au, Hg, Sb, and Tl found in anomalous quantities in sediments in Carlin-type systems were originally derived from biogenic concentration. This theory has been proposed and several other Au deposits in the Great Basin, including the Mercur Deposit in Utah, the Carlin Group near Elko, Nevada, and the Jerrit Canyon group in the Independence Mountains of Nevada. The identification of Galkhaite, a complex Thallium-sulfosalt hosted in these same Paleozoic sediments lends further credence to this hypothesis. At Twin Creeks, abundant swarms and groups of alkaline igneous sills and dikes intrude silty Comus limestone, and the area has suffered a long and violent tectonic history as well. Between the deposition of the original marine detrital sediments and today, the area has undergone the emplacement of swarms of igneous sills and intrusives, early faulting and silicifacation in micro-fractures in limestones [an important Au deposition period], folding and development of open-space fractures related to the Antler Orogeny and the Roberts Mountain Thrust, decalcification, hydrothermal silicification, introduction of auriferous sulfide mineralization [Carlin Au emplacement period, widespread in region ~44.5 mya], followed finally by massive outpourings of welded Miocene and Oligocene tuffs and ashes and subsequent burial under Pleistocene alluvium and colluvium. [2] It was in this complex, poorly-understood zone that increasingly coarse-grained and complex sulfide mineralization was first noted. The thinly-bedded, carbonaceous silty limestones and siltstones played host to localized, but abundant veins of predominantly massive, colloform marcasite, along with smaller amounts of stibnite and pyrite. The marcasite is often ‘sooty’ and cryptocrystalline, likely of similar diagenetic character to the microscopic FeS2 particles hosting the majority of the Au at Twin Creeks.
Around the third week in May of 2006, Twin Creeks geologists along with the author [whom was interning with the geology department] ventured down to the lowermost workings of Cut 18 in the South Mega Pit to map the increasingly rich sulfide mineralization being exposed. Vuggy and fractured thinly to thickly bedded strata of the middle Comus Formation limestone was being encountered, and in these often overturned, anticlinal beds, flattened radial sprays of stibnite crystals to 10 cm. were found in open fractures. Stibnite, usually occurring with marcasite and pyrite, had been noted at Twin Creeks before as a fairly common accessory mineral in the sulfide ores, but the quantity of stibnite seen was unprecedented at the deposit, suggesting localized re-mobilization of sulfide-rich fluids through hydrothermal conduits during intrusion. The overturned, thinly laminated beds of the middle Comus Formation are bordered to the east by a paragenetically related, blocky buff-gray calcareous limestone that hosts numerous coarsely-crystallized calcite veins to 1.5 m in width and several hundred feet in length, with frequent open space development suggesting oblique stress from anticlinal folding during emplacement of the intrusives. [2] Galkhaite at Twin Creeks occurs as isolated crystals in open fracures in highly silicified hydrothermal sulfide breccia, and the crystals are generally isolated on matrix and show only the simple cubic form; occasionally with octahedral and dodecahedral modifications, up to 8 mm. in size. Twin Creek galkhaite is similar to the occurrence at the nearby Getchell deposit in that the crystals are strongly zoned with Hg-rich cores rimmed by a Tl-rich outer zone, seen as distinctive blood red interiors of broken crystals. [2] The initial Twin Creeks galkhaite specimens had been collected almost a mid May 2006 in the northern highwall of the 3800-foot Bench in Cut 18, near an inferred contact with a large, vertical sulfide zone and a series of felsic sills.
The mineralogy of the sulfide breccia hosting the galkhaite at Twin Creek is somewhat anolomous with respect to the deposits, and merit further discussion as well. The bulk of the material was up to 85% sulfide species by weight, with diagenetic colloform marcasite being by far the most abundant sulfide, followed by stibnite, which can account for up to 30% of the bulk of the rock in localized areas. As seen in most Au-rich sulfide ores in CTD’s, this assemblage was fairly devoid of base metals, excepting the Fe in the marcasite/pyrite, an anomaly that has been used as evidence for the biogenic concentration theory. [1] The proposed rheology of this sulfide zone is a fairly brittle, easily sheared, thickly-bedded limestone sequence, which was fractured extensively through the intrusion of the EMI’s and felsic sills, and then cemented by late-stage hydrothermal fluids rich in the heavy metals which crystallized into Galkhaite and other sulfides. Therefore, while Galkhaite is obviously late-stage in the development of the sulfide enriched zones, it appears to have crystallized contemporaneously with the marcasite/pyrite, and therefore can be interpreted as accurately representing the major period of Au deposition in this system. Au is these sulfide zones is restricted to microscopic inclusions in strongly zoned marcasite/pyrite, and does not differ significantly from its occurrence in the disseminated deposits.
Figure 1: Octahedral Galkhaite crystals to 3 mm. with Stibnite and Marcasite on silicified limestone: Field of view 6 cm. Northing 44130, Easting 18050, 3780 Bench, Cut 18, South Mega Pit.
The main sulfide ore zone hosting the Galkhaite is bordered on both sides by narrow, elongated intrusive igneous units, which consist of an equigranular, mafic basalt which may belong to the HGO member of the basalts exposed at Twin Creeks. Geochemically and petrologically the intrusions that confine the sulfide zone are of typical equigranular mafic intrusion [EMI] character, but are strongly tuffaceous, probably resulting from late
Figure 6: Dark Blood-Red Octahedral crystal of Galkhaite 4 mm. across (center) with massive Galkhaite (Red material) on Marcasite with Stibnite: Northing 44130, Easting 18050, 3780 Bench, Cut 18, South Mega Pit.
Cretaceous outpourings of welded tuffs and rhyolitic basalts that occurred in the region at that time. The sulfide zone contains some of the highest average gold values thus far found in the South Mega Pit, with an average of over .25 ounces/ton. With a defined strike length continuity of over 200 m. and an average width of 10 m. and depth of over 50 m., this zone is one of he larger high grade sulfide assemblages encountered thus far at Twin Creeks.
Ion microprobe and XRD analysis performed at the UNR Microprobe Lab showed that while having a fairy close-to-idealized composition for Galkhaite, the Twin Creeks material contained virtually no copper but considerable amounts of trace silver; yielding roughly an Arsenic, Antimony, Mercury, Zinc, Cesium Thallium Silver sulfosalt mineral, a most unusual and complex phase. Galkhaite was disovered in 1972 at the Gal-Khaya Au-Hg deposit in Kazakhstan, U.S.S.R, after which it is named, and has since been found at only a handful of locales worldwide, the most prominent being the Getchell Mine approx. 15 km. southwest of the Twin Creeks deposits, where is occurs as blood-red cubic crystals to 1.2 cm. on silicified limestone. It is considered an accessory phase in hydrothermal Au-Hg deposits, and, along with over a dozen other rare Tl-minerals such as Carlinite and Lorandite, is found at a number of other Carlin-type Au systems in the Great Basin.
The otherwise anomalous presence of over a dozen Tl-bearing minerals in Carlin-type Au deposits in the Great Basin region of the western U.S has long been though to possibly be linked to biogenic concentration of Tl and other unusual heavy metals such as Sb, As, and Hg in the detrital marine sediments that were lithified in a continental shelf slope marine depositional environment in the late Paleozoic and host most of the Au mineralization in these systems. [2] The widely disseminated heavy metals such as Tl were then presumably remobilized and concentrated along with Au in hydrothermal solutions associated with widespread plutonic activity in the late Cretaceous and Tertiary ~67 and 44.5 mya. [1] The development of the modern extension tectonic regime in the Great Basin due to current transverse motion of the Pacific Plate is though to be responsible for the widespread hydrothermal activity resulting in pervasive silicification of microfractures in the Paleozoic sediments. [4] However, recent work suggests that the transition to an extensional tectonic regime in the early Tertiary was likely secondary to Eocene magmatism in introducing Au-rich mineralization. [6] This correlates to the rough estimate of 38-44 Ma as the major Au emplacement period for CTD’s in Nevada. [2] The concentration and remobilization of biogenic thallium through conduits created by igneous intrusions likely provided the necessary geochemical conditions for Galkhaite to form at Twin Creeks and other Carlin-type deposits [CDT’s]. In a study conducted in 1997 on wallrock geochemistry of the Twin Creeks deposits, unusual oxygen isotope ratios were found compared to other hydrothermal Au deposits studied, though this was in part because the strata sampled were primarily Pennsylvanian-Permian Etchart Formation, stratigraphically above the sulfide-hosting Comus formation. [5] One conclusion drawn by this study was that the influence of meteoric water in the sediments had a significant influence on directing and concentrating ore-bearing fluids, and this could likely explain the anomalous zoning of Galkhaite at Twin Creeks to the outer 1-2 meters of the sulfide zone, at the intrusive contacts. [5]
A possible explanation for this could be that the initial intrusion of the equigranular mafic basalts provided conduits to channel Tl and other heavy metals out of their disseminated state in the surrounding thinly-bedded, organic carbonaceous limestones and siltstones. Oxygen isotope work has not been conducted on these wallrocks, though the results would likely shed further light on this anomaly. Tectonic activity associated with the Antler Orogeny and the Roberts Mountain Thrust locally overturned the Paleozoic strata and fractured the younger calcareous limestones which had formed between intrusions, allowing secondary hydrothermal activity to create a breccia of angular, deformed limestone shards cemented by sulfides. Differential fluxing and fractionalization of sulfide solutions concentrated the primary sulfide minerals [stibnite, pyrite, marcasite] towards the center of the vein-like structure, while lighter, more complex minerals like the Galkhaite crystallized paragentically late in open fractures in the silicified limestone periphery. [4] While paragenetic and tectonic evidence support this hypothesis, the one piece of evidence lacking is an authoritative date for the major period of Au and sulfide mineralization which deposited the Galkhaite. Similar occurances and evidence at other CTD’s suggest that this period of mineralization and silicification, which provided the majority of the Au at most deposits, was occurring on a massive regional level across the Great Basin, and affecting all the CTD’s we know of today. [3]
Historically, the Carlin-type deposits of the Great Basin region have been notoriously difficult to date accurately. Because of the geochemistry and petrology of these deposits, age results have always been highly variable. Galkhaite is a trace mineral in at least four CTDs in Nevada, and contains significant amounts of Rb and virtually no Sr, making it an ideal candidate for radiometric dating. [1] Galkhaite from the Getchell Mine in the Potosi Mining District, Humboldt County, Nevada, was analyzed using Rb-Sr isotope dating techniques, and was found to be ~39.5 Ma, which correlates strongly to the major Au-deposition period for most major CTD’s across the Great Basin. [2] Galkhaite at the Getchell deposit is found is a very similar assemblage and environment as at Twin Creeks, in a silicified hydrothermal breccia in an Au-enriched sulfide ore zone structurally controlled by ~92 Ma intrusives related to the nearby Osgood Mountain stock. [1] Analysis of Getchell Mine Galkhaite also showed both the surrounding silicified limestone and the Galkhaite itself to contain significant trace Au, from 17 to 113 ppm, as lattice constituents and sub-micrometer inclusions. [1] This presents further evidence the Galkhaite is unequivocally directly associated with major Au-deposition in these CTD’s. Age dates from sericitic alteration and Andularia veins at Twin Creeks have been obtained, but these usually post or pre-date the major local Au-deposition period of ~39.5-41.5 Ma. [5] Galkhaite is the only species thus far identified in multiple CTD’s which has a direct and conclusive link to the major period of Au-mineralization. Galkhaite from the Getchell Mine indicates a mineralization age of ~39 Ma and that from the Rodeo Deposit (Carlin trend) indicates a mineralization age of 40 Ma. [1] The age dating information obtained from Galkhaite and other species at multiple CTD’s indicates that the major period of Carlin-type mineralization occurred between 32 and 42 Ma. [2]. While radiometric age data obtained through Galkhaite is by no means definitive in determining the paragenetic history of CTD’s such as Twin Creeks, it is certainly an valuable indicator. If an accurate date for the Twin Creeks Galkhaite can be obtained through the utilization of Rb-Sr isotope dating, this will provide a valuable comparison both to Andularia dates from Twin Creeks (~38.5 Ma.), and establish the relationship between Au-deposition at Twin Creeks and at the Nearby Getchell Mine, both world-class, economically important deposits.
-References-
1.) Arehart, G. B, Tretbar, D.R, Christensen, J. N, “Dating Gold Deposition in a Carlin-type gold deposit using Rb/Sr methods on the miberal Galkhaite”, Geology; October 2000; v. 28; no. 10; p. 947-950.
2.) G. B. Arehart, A. M. Chakurian* and D. R. Tretbar., “Evaluation of Radioisotope Dating of Carlin-Type Deposits in the Great Basin, Western North America, and Implications for Deposit Genesis” Economic Geology; April 2003; v. 98; no. 2; p. 235-248.
3.) “ORIGIN OF CARLIN-TYPE GOLD DEPOSITS”, CLINE, Jean S.1, MUNTEAN, John L.2, LONGO, Anthony A.1, and SIMON, Adam3, GSA 2009, Portland, Oregon, Paper 204-5.
4.) “IMPLICATIONS OF STRATABOUND CARLIN- TYPE GOLD DEPOSITS IN PALEOZOIC ROCKS OF NORTH-CENTRAL NEVADA”: SLIDE PRESENTATION. Vladimir I. Berger and Ted G. Theodore U.S. Geological Survey, Menlo Park, California, 2005 Vladimir I. Berger and Ted G. Theodore U.S. Geological Survey, Menlo Park, California, 2005.
5.) “Carbon and oxygen isotope zoning around Carlin-type gold deposits: a reconnaissance survey at Twin Creeks, Nevada”, David P. Stenger1, Stephen E. Kesler* and Torsten Vennemann2 Journal of Geochemical Exploration. Volume 63, Issue 2, September 1998, Pages 105-121.
6.) Eocene magmatism: The heat source for Carlin-type gold deposits of northern Nevada, Henry, Christopher D.; Boden, David R. Geology, vol. 26, Issue 12, p. 1067.
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