Typomorfic features of gold on the Enganepe Ridge (Polar Urals)

Ustyugova K.S.

Syktyvkar State University, Syktyvkar, Republic of Komi, Russia

zelenika@list.ru

In Enganepe-Manitanyrd region of the Polar Urals same-name Cu-Ag-Au mineragenic zone is defined. It consists of isolated Enganepe and Manitanyrd gold-bearing units. Gold-quartz-sulfide (pyrite-arsenopyrite) deposit and several gold occurrences are known on the Manitanyrd Ridge, but on the Enganepe ridge only halos and small placers were found, that’s why prospecting for primary gold here is a topical issue.

The Enganepe Ridge is a brahyanticlinal structure that has Late Riphean-Vendian volcanic and igneous-sedimentary rocks in center and Early Paleozoic sedimentary rocks on wings. District Izyavojski in southern part of the ridge considered being the most perspective for bearing gold, and ZAO “Gold Minerals” pursued here prospecting on load gold at 2007-2009 years. On the first stage of research they executed litho geochemical survey and outlined complex Au-As-Pb-Zn-Cu-Ag anomalies. During confirmation of anomalies by exploring there were found out ones of disseminated pyrite mineralization that contained primary gold.

Sulfide mineralization is connected to submeridional shear zone in volcanic and igneous-sedimentary rocks of enganepe suite (V₂-Є₁en) that are partly cataclastic, milonitized and have metasomatic changes. Petrology of milonitized rocks was difficult to determine, but according to x-ray diffractometric analysis they contain quartz, illite, chlorite (including ferrous), glaucodot, scorodite, pyrite, chalcosine. Disseminated pyrite mineralization is connected with sulfide and rare-earth metal micro mineralization [1]. 22 gold grains were separated from milonitized rocks by geologists of ZAO “Gold Minerals” and are devolved us for research. We used scanning electronic microscope JSM-6400 with energy-dispersive spectrometer «Link» in the Institute of Geology of Komi SC UB RAS (analyst V.N. Filippov) to investigate morphology of gold, its surface relief and mineral inclusions. Chemical composition was determined on surface and in cut of polished section.

Explored gold is of a fine gradation class; its size is within 0.05-0.3 mm. The most common is highly fine (0.1-0.25 mm) – 63%, extremely fine (0.05-0.1 mm) – 32%, fine gold (0.25-1.0 mm) – 5%. Granulometric distribution of placer gold in the same district slightly differs to the enlargement [2].

Morphology of gold is described using only 18 grains, but they include all types – euhedral, subhedral and anhedral: bar-shaped, lump, platy. Platy gold is dominant; its percentage is 39%. There are thick and thin species, often they are characterized by bandy shape and rugged edges. Lump grains (28%) are also of a different shape, they include isometric, extended gold, grains of irregular shape with spikes on edges. Subhedral gold figures up to 17%, it has at least 2 crystal faces, commonly characterized by isometric and slightly extended form. Than bar-shaped grains make up 11%. Euhedral gold is presented by only one grain of rod-like form. In placers of this district all grains are of anhedral class [2], it could be explained by mechanical transformation during transportation.

Gold in most cases has plane compact surface, only in several parts it becomes porous, cellular and spongy. Such element of relief as other minerals’ prints: of cube and hexagon shape, probably, sulfides, parallel extended form – prints of scale or foliated minerals, drills and rollers along print edges, parallel growth step. In summary gold has an ore outlook, and small spaces of partly dissolved surface point out initial stage of gold transformation in supergene process [3]. It’s reasonable, because gold-bearing rocks are located in near-surface environment.

Main impurity elements in gold are Ag и Hg. Content of Ag varies in wide range – from 9.59 to 27.00 mas.% in central part of grains and from 11.28 to 27.34 mas.% at the edge. Spread of Ag content on gold surface is even more – 0.92 to 28.11 mas.%. Ag content tends to grow slightly to the edge of gold that is a characteristic feature of primary gold. Hg content ranges in 0.71-1.67 mas.%, and it doesn’t vary much in different parts of grains. Addition of Cu in quantity of 0.59 to 3.84 mas.% was determined only in several analyses on the surface, it is often defined in parts of high fineness gold.

Fineness of gold varies from 735 to 889‰. Gold of moderately high fineness (800-899‰) absolutely dominates - 94%, gold of relatively low fineness (700-799‰) is present in small amount – 6%, distinct parts has high fineness (900-1000‰). Fineness of gold in grains and on surface virtually doesn’t differ; exception is for small areas of high fineness gold adjacent to gathering of iron hydroxide that also points out initial stage of supergene gold transformation. Generally argentous gold prevails (70%), considerably rarer argentous-mercurous gold occurs (17%), and copper gold and argentous-copper gold (8 и 4% corresponding).

In placer gold of Izyavojski district small amount of low fineness gold is presented, however mostly gold is of high fineness [2]. Such correlation in fineness of primary and placer gold is well-known and widespread in gold-bearing regions. As well as in newly found primary gold in placer gold of this district argentous gold with Ag content 0-29.1 mas.% is predominant, rarely admixture of Hg (0-1.8 мас.%) is determined in gold. In placer gold in contradiction from primary gold Cu content hasn’t been recognized [2].

During the electronic-microscoping of gold on its surface, inside it and in accretion many mineral inclusions have been found out. They include chalcopyrite and arsenopyrite inside gold and allanite in accretion. Chalcopyrite forms isometric inclusions of less than 1 mkm. Chemical analysis of its composition corresponds formula Cu_0.82Fe_0.8-0.81S₂, lack of cations is due to nanoparticle analysis fault. Arsenopyrite is present by accumulation af nanoinclusions of 50-100 nm size. Analysis results corresponds formula Fe_0.88-0.9As_0.95-0.98S. Allanite is in form of inclusions of 1-3 mkm inside gold, and also larger grains (~15 mkm) are determined on gold surface in accretion with biotite. Calculated formula is (Ca_1.32-1.6Ce_0.2-0.24La_0.1-0.1Nd_0.06-0.11)_1.73-2.05(Al_1.66-1.9Fe_0.95-1.17Cr_0.19-0.34Mn_0.00-0.08)_3.05(OH)O[Si_1.1-1.48O₄][Si₂O₇]. Admixtures of Fe, Cr, Mn are identified. On the surface of gold such inclusions as sulfides (chalcopyrite), nonmetal minerals (albite, micas, peridot (?), garnet (?), quartz) and epigenetic minerals (scorodite, iron hydroxide, bismuth and lead arsenates).

This research showed that gold from milonitized rocks of pyritization zones has ore outlook, characterized by fine gradation class, various morphology with predomination of platy and lump gold, mainly relatively high fineness, common impurity elements are Ag и Hg, rarely Cu. Explored gold from pyritization zone is similar to placer gold of Izyavojski district by a range of features, therefore these mineralization zones could be considered as primary source of placer gold.

Ore mineral association compound of micro inclusions inside gold and in accretion (chalcopyrite, arsenopyrite) and accessory epigenetic minerals (scorodite, iron hydroxide, bismuth and lead arsenates) lead us to suppose of resemblance between gold-sulfide mineralization in milonitized rocks of pyritization zones on Izyavojski district and same mineralization in gold occurrences of Manitanyrd ore unit [4].

The work is supported by UB & FEB RAS projects № 12-C-5-1006 and Presidium RAS project № 12-P-5-1027.

References:

1. Ustyugova K.S. Sulfide micro association in disseminated mineralization zone of the Enganepe Ridge (Polar Urals)//Materials of II International Research-to-Practice Early Career Scientists and Specialists in Commemoration of academician A.P.Karpinski. SPb: RGRI, 2011. P.183–187.

2. Kuznecov S. K., Spirin A. V. Alluvial gold of Enganepe region // Syktyvkar mineralogical digest. Syktyvkar, 2003. № 3. P. 70–80.

3. Petrovskaya N.V. Native gold. M.: Nauka, 1973. 347 p.

4. Noble metals from eastern flank of the North of the Urals and Timan/ Kuznecov S.K., Tarbaev M.B., Mayorova T.P. et al. Syktyvkar: Geoprint, 2004. 48 p.