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Chapter 10: Comparison of the Allchar Au-As-Sb-Tl Deposit, Republic of Macedonia, with Carlin-Type Gold Deposits

By
Sabina Strmić Palinkaš
Sabina Strmić Palinkaš
1
UiT The Arctic University of Norway, Faculty of Science and Technology, Department of Geosciences, Dramsvegen 201, N-9037 Tromsø, Norway
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Albert H. Hofstra
Albert H. Hofstra
2
U.S. Geological Survey, Denver Inclusion Analysis Laboratory (DIAL), P.O. Box 25046, MS 963, Denver, Colorado 80225, USA
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Timothy J. Percival
Timothy J. Percival
3
Consulting Economic Geologist, 3240 Quartzite Drive, Reno, Nevada 89523, USA
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Sibila Borojević Šoštarkć
Sibila Borojević Šoštarkć
4
University of Zagreb, Faculty of Mining, Geology, and Petroleum Engineering, Pierottijeva 6, HR-10000 Zagreb, Croatia
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Ladislav Palinkaš
Ladislav Palinkaš
5
University of Zagreb, Faculty of Science, Geological Department, Horvatovac 95, HR-10000 Zagreb, Croatia
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Vladimir Bermanec
Vladimir Bermanec
5
University of Zagreb, Faculty of Science, Geological Department, Horvatovac 95, HR-10000 Zagreb, Croatia
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Zoltan Pecskay
Zoltan Pecskay
6
Institute of Nuclear Research, Hungarian Academy of Sciences, Bem tér 18/C, H-4001 Debrecen, Hungary
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Blazo Boev
Blazo Boev
7
Goce Delčev University, Faculty of Natural and Technical Sciences, Goce Delčev 89, MK-2000 Štip, Republic of Macedonia
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Published:
January 01, 2018

Abstract

The Allchar Au-As-Sb-Tl deposit is situated in the western part of the Vardar zone, the main suture zone along the contact between the Adriatic and the Eurasian tectonic plates. It is spatially and temporally associated with a Pliocene (~5 Ma) postcollisional high-K calc-alkaline to shoshonitic volcano-plutonic center.

The Allchar deposit shares many distinctive features with Carlin-type gold deposits in Nevada, including its location near a terrain-bounding fault in an area of low-magnitude extension and intense magmatism. The mineralization is mostly hosted in calcareous sedimentary rocks at intersections of high-angle faults in permeable stratigraphy. The alteration types (carbonate dissolution, silicification, and argillization), ore mineralogy (auriferous arsenian pyrite and marcasite, stibnite, realgar, orpiment, and lorandite), high Au/Ag ratios, and low base metal contents are also typical of Carlin-type gold deposits in Nevada.

However, the Allchar deposit differs from Nevada Carlin-type gold deposits as follows: it is an isolated Au prospect with a close spatial and temporal relationship to a shoshonitic volcano-plutonic center in a mineral belt dominated by intrusion-related Cu-Au porphyry, skarn, and hydrothermal polymetallic deposits. The deposit is clearly zoned (proximal Au-Sb to distal As-Tl), it has a significantly higher Tl content, trace elements in pyrite and marcasite are homogeneously distributed, and synore dolomitization is a widespread alteration type.

Gold mineralization is most abundant in the southern part of the deposit. It occurs mostly as invisible Au in disseminated pyrite or marcasite and as rare native Au grains. Gold mineralization is accompanied by intense decarbonatization and silicification. Fluid inclusions and the hydrothermal alteration mineral assemblage indicate that Au was deposited from hot (>200°C), saline (up to ~21 wt % NaCl equiv), moderately acidic (pH <5) fluids that carried traces of magmatic H2S and CO2. In the calcareous host rocks, mixing of such fluids with cool, dilute, near-neutral groundwater triggered deposition of Au and Fe sulfides. In Tertiary tuff, isocon analysis shows that sulfidation of preexisting Fe minerals was a critical factor for deposition of Au and Fe sulfides.

Antimony mineralization prevails in the central part of the deposit, and it is mostly associated with dark-gray to black jasperoid. Stibnite, the most common Sb mineral in the Allchar deposit, occurs as fine-grained disseminations in jasperoid and as fine- to coarsely crystalline masses that fill vugs and fracture zones lined with drusy quartz. Fluid inclusions entrapped by stibnite-bearing jasperoid, quartz, and calcite crystals suggest that stibnite was deposited from more dilute and cooled fluids (aqueous-carbonic fluid inclusions: 6.0–3.5 wt % NaCl equiv, Th = 102°−125°C; aqueous fluid inclusions: 14.5 and 17.1 wt % NaCl equiv, Th = 120°−165°C).

In contrast to stibnite, As sulfides (orpiment and realgar) and Tl mineralization are associated with argillic alteration. Fluid inclusions hosted by realgar, orpiment, dolomite, and lorandite record deposition from more dilute (2.6–6.9 wt % NaCl equiv) and relatively cold fluids (TH = 120°−152°C) enriched in K. Isocon diagrams show a tight link between Tl and the low-temperature argillic alteration as well as a significant correlation between Tl and K. The spatial relationship of Tl mineralization with dolomite suggests that Tl deposition was also promoted by neutralization of acidic fluids.

The δD and δ18O data obtained from gangue minerals and fluid inclusions indicate that magmatic fluid mixed with exchanged meteoric water at deep levels and with unexchanged meteoric water at shallow levels in the system. The δ13C and δ18O values of carbonate minerals and extracted fluid inclusions suggest mixing of carbonate rock buffered fluids with magmatic and atmospheric CO2. The sulfur isotope values of early disseminated pyrite and marcasite show that H2S was initially derived from diagenetic pyrite in sedimentary rocks. In contrast, Sb and As mineralization indicate a strong input of magmatic H2S during the main mineralization stage. Late-stage botryoidal pyrite and marcasite are depleted in 34S, which indicates a diminishing magmatic influence and predominance of sulfur from sedimentary sources during the late-mineralization stage. Fractionation of isotopically light sulfide species from isotopically heavy sulfates due to oxidation under increased oxygen fugacity cannot be excluded.

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Contents

Reviews in Economic Geology

Diversity in Carlin-Style Gold Deposits

John L. Muntean
John L. Muntean
Ralph J. Roberts Center for Research in Economic Geology, Nevada Bureau of Mines and Geology, University of Nevada Reno
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Society of Economic Geologists
Volume
20
ISBN electronic:
9781629491189
Publication date:
January 01, 2018

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